Comparison between benzyladenine and metamitron
as chemical thinning agents in ‘Gala’, ‘Kanzi®’, ‘Pink
Lady®’ and ‘Red Delicious’ apple cultivars
Nídia Diana Heleno Rosa
Dissertação para obtenção do Grau de Mestre em
Engenharia Agronómica
Orientadores: Doutora Cristina Maria Moniz Simões de Oliveira
Mestre Wim Verjans
Júri:
Presidente:
Doutor Henrique Manuel Filipe Ribeiro, Professor Auxiliar do Instituto Superior de
Agronomia da Universidade de Lisboa
Vogais:
Doutora Cristina Maria Moniz Simões Oliveira, Professora Associada com Agregação do
Instituto Superior de Agronomia da Universidade de Lisboa
Doutora Mariana da Silva Gomes Mota, Técnica Superior do Instituto Superior de
Agronomia da Universidade de Lisboa
Mestre Anabela Carvalho Nunes Maurício, Técnica de Campo da Frubaça, CRL, na
qualidade de especialista
Dedicatio
To my mother who is certainly the proudest person of all my achievements
Acknowledgements
I am very thankful to my supervisor, Cristina Oliveira, for her outstanding support and friendship during this entire journey. Without her scientific and technic guidelines along with her time and patience during the course of this work, this would not be possible. Thank you for being a mentor, by encouraging me to pursue opportunities that would enrich my knowledge and make me grow and even by creating me new ones.
Special thanks to my supervisor, Wim Verjans, not only for the help and friendship during my stay in Belgium but also for the patience to answer to all my doubts and the knowledge transmitted during the internship. I am also really thankful for all the effort made to create opportunities for me to meet and integrate the scientific group of investigators in Belgium and Europe.
I would like to thank Tom Deckers, for the knowledge and support transmitted during my stay in Belgium and for the wise questions that helped me organize my thoughts.
À minha família, em especial ao meu pai, que ao longo de toda a minha vida me tem orientado, apoiado e incentivado, e à minha tia, a pessoa que mais força e coragem me deu quando decidi dar esta reviravolta no meu percurso académico. To my sister Nélia, for her friendship and support behind the bad temper.
I am thankful to Renato, for being with me during all the good moments and for helping me overcome the bad ones. It has been a pleasure to walk on this road together with you during all these years.
To my friends, Ana, Catarina, Isabel, Silvia Andreia, Silvia Tavares, Laureano, Carlos, Flávia for the friendship moments that last for more than 15 years, for making me laugh and listening when I need you to. To my master colleagues, Miguel, Sara, Inês e Filipe for the companionship and the support given in the most challenging subjects for me.
Thank you for being a great source of energy and motivation in all my achievements. Without every single one of you this would not be possible.
Title: Comparison between benzyladenine and metamitron as chemical thinning agents in ‘Gala’, ‘Kanzi®’, ‘Pink Lady®’ and ‘Red Delicious’ apple cultivars.
Abstract
This experiment was conducted in PCFruit Research Station orchards in Sint-Truiden, Belgium in 2015, in ‘Gala’, ‘Kanzi®’, ‘Pink Lady®’ and ‘Red Delicious’. Four treatments were tested: untreated, manual thinning, two applications of 165 ppm of metamitron at 8 and 12 mm and one application of 150 ppm of benzyladenine between 8 and 12 mm fruit diameter. All cultivars were sprayed on the same days. Fruit drop was counted, phytotoxicity evaluated after June drop, fruit growth rate was assessed and photosynthetic efficiency was measured.
Fruit drop was higher in all cultivars with metamitron application than with benzyladenine (P<0.05). A greater growth rate in fruits treated with metamitron was registered, as well as, a positive correlation between the sum of the PSII efficiency and the thinning percentage (R2=0.98). A relation between the duration of the blocking effect of metamitron and phytotoxicity level in the trees was observed.
At harvest, metamitron and manual thinning showed the best results in average fruit weight, in all cultivars. Despite the size improvements, the only influence in quality was observed in ‘Pink Lady®’ Brix° values. All cultivars thinned with 150 ppm benzyladenine were far from the ideal crop load for fruit size, probably due to the poor weather conditions after application, while with two times 165 ppm metamitron sprays had the same results as manual thinning. ‘Pink Lady®’ was the easiest cultivar to thin, with a slight over thinning effect, followed by ‘Gala’, that achieved the ideal crop load and ‘Red Delicious’ was the most difficult to thin. It was not possible to compare ‘Kanzi®’ with the other cultivars due to the poor fruit-set observed before the product’s application. Metamitron doses can be adjusted to achieve optimum results; in ‘Pink Lady®’ the dose should be lower while in ‘Red Delicious’ higher.
A simpler version of the Greene model was tested the percentage of error was 7.5% for fruits treated with metamitron while in benzyladenine treated fruits was 18%.
Título: Comparação de benziladenina e metamitrão como substâncias de monda química nas cultivares de macieira ‘Gala’, ‘Kanzi®’, ‘Pink Lady®’ and ‘Red Delicious’.
Resumo
O ensaio foi conduzido na estação de investigação PCFruit, em Sint-Truiden, Bélgica, em 2015 em quatro cultivares de macieira: ‘Gala’, ‘Kanzi®’, ‘Pink Lady®’ and ‘Red Delicious’. Foram testados quatro tratamentos: controlo, monda manual, duas aplicações de 165 ppm de metamitrão aos 8 e 12 mm e uma aplicação de 150 ppm de benziladenina entre 10 e 12 mm de diâmetro dos frutos. A aplicação foi feita em todas as cultivares no mesmo dia. A queda de frutos foi contada, a fitotoxicidade foi avaliada depois da queda de Junho, a taxa de crescimento dos frutos foi acompanhada e a eficiência fotossintética foi medida.
A abscisão de frutos foi maior nas quatro cultivares com a aplicação de metamitrão do que com benziladenina (P<0,05). Foi registada uma taxa de crescimento mais alta em frutos de árvores tratadas com metamitrão assim como uma correlação positiva entre a soma dos valores de eficiência do fotossistema II e a percentagem de monda (R2=0,98). Foi observada também uma relação entre a duração do bloqueio fotossintético provocado pelo metamitrão e o nível de fitotoxicidade observado nas árvores.
À colheita, a monda manual e o metamitrão foram os tratamentos que apresentaram os melhores resultados no peso médio do fruto, em todas as cultivares. Apesar das melhorias no calibre, na qualidade, apenas na ‘Pink Lady®’ se observou um aumento dos valores de Brix° nos frutos das árvores tratadas com metamitrão. A carga em todas as cultivares ficou longe do ideal após o tratamento com 150 ppm de benziladenina, provavelmente devido às condições meteorológicas que não foram ideais, ao passo que os resultados obtidos com duas aplicações de 165 ppm de metamitrão foram semelhantes à monda manual. ‘Pink Lady®’ revelou ser a cultivar mais fácil de mondar, tendo a queda de frutos sido até excessiva, seguida da ‘Gala’ que atingiu a carga ideal e da ‘Red Delicious’ que foi a mais difícil de mondar. Não foi possível incluir a ‘Kanzi®’ na comparação com as outras cultivares devido ao baixo número de frutos vingados nas árvores, antes da aplicação da monda química. As doses de metamitrão podem ser ajustadas para obtenção de resultados óptimos, na ‘Pink Lady®’ a dose deve ser menor e na ‘Red Delicious’ maior.
Uma versão simplificada do Greene model foi testada, tendo sido obtida uma percentagem de erro de 7,5% em frutos tratados com metamitrão enquanto que com benziladenina o erro foi 18%.
Palavras-chave: abscisão, benziladenina, metamitrão, monda química, taxa de crescimento dos frutos
Título: Comparação de benziladenina e metamitrão como substâncias de monda química nas cultivares de macieira ‘Gala’, ‘Kanzi®’, ‘Pink Lady®’ and ‘Red Delicious’.
Resumo Alargado
A maçã é um fruto com bastante importância a nível mundial e nos últimos anos o modo de produção tem-se alterado, de forma a satisfazer as exigências do consumidor e a aumentar o valor económico da produção. A competitividade no sector tem vindo a aumentar, levando o agricultor a focar-se mais na qualidade da maçã obtida ao invés da quantidade. Para isso é necessário adoptar técnicas que levem à melhoria da produção esperada, como a monda, que elimina os frutos mais pequenos e melhora a qualidade dos frutos obtidos ao mesmo tempo que controla o vigor da árvore e diminui a alternância, permitindo manter uma produção constante ao longo dos anos (Costa et al., 2013).
Wismer et al. (1995) e Clever (2007) observaram em árvores tratadas com benziladenina, uma redução no número e na massa dos frutos por árvore e, consequentemente, um aumento no seu calibre. Apesar dos resultados positivos, para ter efeito esta substância ativa necessita de temperaturas pelo menos 18°C nos 2 a 3 dias seguintes à aplicação. Estudos realizados por Clever (2007) e Bazak (2011) concluíram que a aplicação de metamitrão também induz efeitos positivos na produção. No entanto, foram observados sintomas de fitotoxicidade nas folhas, tendo a árvore recuperado completamente, sem que quaisquer efeitos negativos na qualidade dos frutos fossem observados. Em todas as situações, os agentes de monda foram aplicados entre 8 e 12 mm de diâmetro dos frutos.
O ensaio foi conduzido na estação de investigação PCFruit, em Sint-Truiden, Bélgica, em 2015 em quatro cultivares de macieira: ‘Gala’, ‘Kanzi®’, ‘Pink Lady®’ and ‘Red Delicious’. Compararam-se quatro tratamentos: controlo, monda manual, duas aplicações de 165 ppm de metamitrão aos 8 e 12 mm e uma aplicação de 150 ppm de benziladenina entre 8 e 12 mm de diâmetro dos frutos. Apesar das pequenas diferenças no diâmetro dos frutos, a aplicação foi feita em todas as cultivares no mesmo dia de forma a verificar o efeito de condições meteorológicas uniformes dos dias seguintes.
Foi registada a evolução do número de frutos por corimbo ao longo da campanha nos vários tratamentos e cultivares, tendo a redução sido mais alta com a aplicação de metamitrão e com a monda manual do que com benziladenina (P<0,05). Assim como Verjans em 2013 (não publicado) foi observada uma queda precoce dos frutos nas árvores tratadas com metamitrão quando comparada com o início da queda de frutos nos outros tratamentos. Os frutos caídos foram medidos e as sementes contadas, de forma a verificar se a eficácia da benziladenina estaria relacionada com a diminuição significativa do número de sementes (Greene et al., 1992, McArtney et al., 1995). Tal não se verificou em ‘Gala’, ‘Kanzi®’ e ‘Pink Lady®’, no entanto, o número de sementes nos frutos caídos de ‘Red Delicious’ revelou ser significativamente mais baixo do que no controlo. Esta análise feita nos frutos caídos permitiu concluir que a aplicação de metamitrão levou a que os frutos à colheita tivessem um maior número de
sementes em ‘Gala’, ‘Kanzi®’ e ‘Red Delicious’. Os requisitos meteorológicos da benziladenina acima descritos não foram cumpridos, levando a que, à colheita, a monda manual e o metamitrão fossem os tratamentos que apresentaram os melhores resultados em termos de número de frutos por árvore, percentagem de peso (kg) nos calibres maiores e peso médio do fruto, ao passo que nas árvores tratadas com benziladenina os frutos revelaram poucas diferenças em relação ao controlo (não mondado). No que diz respeito ao número de frutos por árvore, a ‘Gala’ foi a cultivar que sofreu a maior redução com a aplicação de metamitrão, seguida de ‘Red Delicious’, ‘Pink Lady®’ e ‘Kanzi®’.
Apesar das melhorias no calibre, na qualidade, apenas na ‘Pink Lady®’ se observou um aumento do Brix° nos frutos das árvores tratadas com metamitrão. Na degradação de amido, firmeza, número de sementes desenvolvidas, carepa e rácio altura/diâmetro não foram observadas diferenças em nenhum dos tratamentos e cultivares. Ao contrário do que seria de esperar, dado que de acordo com Meland (2009) uma redução na carga de frutos leva ao aumento da percentagem total de cor vermelha na epiderme do fruto, a aplicação de metamitrão originou frutos com epiderme mais brilhante (Croma) mas não com tonalidade (hue) mais vermelha.
A dinâmica de crescimento dos frutos foi acompanhada durante toda a campanha através de sucessivas medições do diâmetro com uma craveira digital. Assim, foi possível observar a fase de divisão celular até cerca de 4 a 5 semanas após a plena floração, tal como Westood (1978) concluiu. O mesmo autor defende que a redução da carga de frutos da árvore leva a um aumento da taxa de crescimento, logo a monda é uma técnica que influência positivamente o desenvolvimento de frutos de maior calibre, como se verificou pelo aumento da taxa de crescimento diária dos frutos em que metamitrão foi aplicado, em todas as cultivares.
Os valores de eficiência do fotossistema II foram medidos desde a primeira aplicação de metamitrão até um mês depois da segunda aplicação. Assim como McArtney et al (2012), dois dias após a primeira aplicação o aparelho fotossintético das folhas começou a recuperar. No entanto, o mesmo autor concluiu que 8 dias após a segunda aplicação, o efeito do metamitrão cessou, ao passo que neste estudo, a supressão do aparelho fotossintético durou 21 dias na ‘Kanzi®’, 26 na ‘Red Delicious’ e após 29 dias as diferenças entre as árvores tratadas de ‘Gala’ e ‘Pink Lady®’ e o controlo eram ainda significativas. Após a queda de Junho, a fitotoxicidade provocada por esta substância ativa foi avaliada, concluindo-se que foi mais alta na ‘Pink Lady®’ e na ‘Gala’, mas sem quaisquer efeitos negativos para o fruto ou para a árvore, e quase inexistente na ‘Kanzi®’ e ‘Red Delicious’. Conclui-se portanto, haver uma relação entre a duração do efeito do bloqueio do fotossistema da planta e o grau de fitotoxicidade observado.
Considerando o número de frutos à colheita do controlo como 0% de eficiência de monda, a percentagem de eficiência foi calculada para cada cultivar e tratamento. A eficiência da monda variou entre 17,2 e 46,9% com a monda manual nas quatro cultivares, entre 11,3 e 45,8% com a aplicação de metamitrão e entre -27,1 e 3,5% com a aplicação de benziladenina. Concluiu-se existir uma correlação
positiva entre a soma dos valores de eficiência do fotossistema II e a percentagem de eficiência da monda (R2=0,98).
Nenhuma das quatro cultivares atingiu a carga ideal após o tratamento com 150 ppm de benziladenina, provavelmente devido às condições meteorológicas que não foram ideais, ao passo que após duas aplicações de 165 ppm de metamitrão, a ‘Pink Lady®’ revelou ser a cultivar mais fácil de mondar, tendo a abcisão de frutos sido até excessiva, seguida da ‘Gala’ que atingiu o número ideal de frutos e da ‘Red Delicious’, a cultivar mais difícil de mondar. Não foi possível incluir a ‘Kanzi®’ na comparação com as outras cultivares devido ao baixo número de frutos vingados nas árvores antes da aplicação dos agentes de monda química.
Assim, considerando a dose ideal para que a ‘Gala’ atinja resultados óptimos, para atingir os mesmos na ‘Pink Lady®’, a dose deve ser diminuída ao passo que na ‘Red Delicious’ deve ser aumentada.
Um modelo simplificado do conhecido “Greene Model”, aplicado em apenas 40 corimbos e com duas medições de diâmetro (uma antes da aplicação e outra 8 dias depois) foi testado, de forma a calcular a sua precisão na previsão da queda de frutos, auxiliando na decisão de uma segunda aplicação de um agente químico. Foi também aplicado às árvores controlo em duas datas distintas, com o objectivo de perceber se poderia ser útil na previsão da necessidade de monda ou até na severidade da sua dose. Conclui-se que quanto mais perto da queda de Junho, menor a percentagem de erro do modelo. No entanto, na primeira data de aplicação do modelo às árvores controlo, 22/05, apesar da mais alta percentagem de erro (22% contra 8,3%), devido ao menor diâmetro dos frutos, a gama de produtos para mondar à disposição do agricultor é maior. A precisão obtida neste estudo vai de 78 a 92,5% nos vários tratamentos e cultivares, valores que estão de acordo com os obtidos por Greene et al. (2004) em Massachussets e Geneva. Estes resultados indicam que esta versão simplificada do modelo pode ser utilizada pelo agricultor como auxílio na tomada de decisão da monda.
VII Index Acknowledgements ... I Abstract ... II Resumo ... III Resumo Alargado ... IV Index ... VII
Table of Contents ... VII
Index of Figures ... X
Index of Tables ... XII
Abbreviation Index ... XIV
Table of Contents
1. Introduction ... 1
2. Literature Review ... 3
2.1 Apple Production ... 3
2.1.1 Production in the World ... 3
2.1.2 Production in Belgium ... 4
2.1.3 Production in Portugal ... 4
2.2 Malus domestica ... 6
2.2.1 Dormant Period ... 6
2.2.2 Vegetative and Reproductive Growth ... 7
2.3 Cultivars ... 8 2.3.1 Gala ... 8 2.3.2 Kanzi® ... 9 2.3.3 Pink Lady® ... 9 2.3.4 Red Delicious ... 10 2.4 Rootstock ... 10 2.5 Fruit Set ... 11
VIII
2.5.1 External Factors ... 11
2.5.2 Internal Factors ... 13
2.6 Physiology of Fruit Growth and Development ... 14
2.7 Organ’s Abscission ... 14
2.8 Thinning ... 16
2.8.1 Physiological Effects of Thinning ... 16
2.8.2 Blossom Thinning ... 17 2.8.3 Fruit Thinning ... 18 2.9 Thinning Techniques ... 19 2.9.1 Manual Thinning ... 19 2.9.2 Mechanical Thinning ... 19 2.9.3 Chemical Thinning ... 20
2.9.4. Shadowing as a thinning technique ... 27
2.8 Precision Crop Load Management ... 27
2.8.1 Carbohydrate Model ... 28
2.8.2 Greene Model ... 29
3. Materials and Methods ... 30
3.1Plant Material ... 30
3.2 Trial Design ... 30
3.3 Measurements ... 33
3.3.1 Greene Model ... 34
3.3.2 Maximum Potential Quantum Efficiency of PSII ... 35
3.3.3. Seed Count and Phytotoxicity Evaluation ... 35
3.4 Yield and Fruit Quality Analysis ... 36
3.5 Statistical Analysis ... 37
4. Results ... 38
4.1 Phenological Evolution ... 38
4.2 Fruit Set ... 39
4.2.1 Number of Flower Buds per Tree, TCSA and No of Flowers and Fruits per Cluster ... 39
4.2.2 Evolution of the Number of Fruits per Cluster and Influence of the Treatments in the Beginning of Fruit Fall ... 40
4.2.3 Percentage of Fruit Set ... 42
4.2.4 Number of Seeds and Fruit Size of the Fallen Fruits ... 43
4.3 Thinning Efficiency ... 44
IX
4.3.2 Production per Fruit Size Class ... 46
4.3.3 Total Production per Treatment ... 48
4.3.4 Quality Parameters ... 49
4.4 Maximum Potential Quantum Efficiency of PS II ... 51
4.4.1 Correlation with Thinning Efficiency ... 54
4.5 Phytotoxicity ... 55
4.6 Greene Model ... 56
4.7 Evolution in Fruit Growth ... 58
4.7.1 Evolution of the Average Fruit Diameter ... 58
4.7.2 Evolution of the Growth Rate ... 59
5. Discussion ... 61
5.1 Evolution in the Number of Fruits per Cluster ... 61
5.2 Influence of the Treatments in the Beginning of Fruit Fall ... 61
5.3 Number of Seeds and Fruit Size of Fallen Fruits ... 62
5.4 Production and Quality ... 62
5.5 Growth Rate ... 64
5.6 PSII Efficiency ... 65
5.7 Phytotoxicity ... 65
5.8 Greene Model ... 66
5.9 Overall Comparison Between Cultivars ... 66
6. Conclusion ... 68 7. Bibliography ... 70 8. Websites ... 80 Annex A ... 81 Annex B ... 82 !
X Index of Figures
Figure 1 - Evolution of the apple production in the world from 2004 to 2013 (FAO, 2013) ... 3
Figure 2 - Biggest apple producers in the world (FAO, 2013) ... 3
Figure 3- Evolution of apple's production in Belgium (2004-2013) (FAO, 2013) ... 4
Figure 4 - Apple production in Portugal (2005-2013) (FAO, 2013) ... 4
Figure 5 - Commercial trades in portuguese apple sector (GPP, 2013) ... 5
Figure 6 - Apple supply in Portugal in season 2012-13 (GPP, 2013) ... 5
Figure 7 - Kautsky Induction Curve (Hansatech, 2006). The peaks are denoted by the letters O, J, I and P. ... 26
Figure 8 - Combined thinning approach with Greene and CH models ... 28
Figure 9 - 'Gala', ‘Pink Lady®’ and 'Red delicious' trial (Red Color) and ‘Kanzi®’ trial (Blue Color) (Google Earth) ... 30
Figure 10 – Treatment distribution of GL, PL and RD (left side) and KZ (right side) ... 31
Figure 11 - Labelled clusters and numbered fruits on the 25th of May: RD at the left side, GL in the middle and PL at the right side. ... 34
Figure 13 – A sample of 20 RD fruits and the measurements of the diameter, color and firmness ... 36
Figure 14 - Evolution of the development of KZ flower buds in more than one year old wood and one year old wood, during 2015 growing season in Sint-Truiden ... 38 Figure 15 – Mean No of flowers per cluster counted on 23/04 and evolution of the mean No of fruits per cluster since 12/05 until 15/07. Each point corresponds to the mean ± standard error per
treatments/cultivar. Standard error is not visible when the size is the same or smaller than the symbol dimensions. Measurements made in N=40 clusters/treatment in GL, PL and RD and N=36
clusters/treatment in KZ ... 40 Figure 16 - Sample of RD fallen fruits from UNT, BA_150 and MET 165_165 treated trees, collected on 16/06 ... 43
Figure 17 – Fruit size and number of seeds of each treatment collected on the 18th of June from GL's fallen fruits ... 43
Figure 18 – Mean kg distribution ± standard error per fruit size class from <60 to >80 mm in N=8 trees/treatment in GL, PL and RD and N=12 trees/treatment in KZ ... 46
Figure 20 – Mean No of fruits ± standard error in N=8 trees/treatment in GL, PL and RD and N=12 trees/treatments in KZ ... 48
Figure 21 - Mean Kg per tree ± standard error in N=8 trees/treatment in GL, PL and RD and N=12 trees/treatments in KZ ... 48
Figure 22 - Course of the average photosynthetic efficiency in UNT trees and MET 165_165 treated trees. Each point corresponds to the mean and standard error per treatment/cultivar. Standard error is not visible when the size is the same or smaller than the symbol dimensions. Black arrows represent the 1st spray on the 13th of May and 2nd spray on the 21st of May. Measurements were made from the 13th May to the 19th of June in N=32 individual leaves per treatment/cultivar ... 52
Figure 23 - Difference between the photosynthetic efficiency of untreated and treated trees (black arrows represent both spraying dates). Each point corresponds to the mean value of measurements made from 13th of May to 19th of June in N= 32 individual leaves per treatment/cultivar ... 53
Figure 24 – Regression curve modeling the sum of PSII measurements and the thinning efficacy (%) in the four cultivars ... 55
Figure 25 – Phytotoxicity symptoms in sample leaves of the four cultivars collected on the 17th of June ... 55
Figure 26 - Percentage of Type A and Type B errors calculated with two measurements in UNT fruits (22/05 and 29/05) and one in MET 165_165 (29/05) and in BA_150 (25/05), in the four cultivars. Measurements made in fruits of N=40 clusters/treatment in GL, PL and RD and fruits of N=36
clusters/treatment in KZ. Number of measured fruits per treatment/cultivar varied from 35 to 176 ... 57 Figure 27 - Mean fruit diameter measured since 12/05 until 8/09 in the remaining fruits. Each point corresponds to the mean ± standard error per treatments/cultivar. Standard error is not visible when the size is the same or smaller than the symbol dimensions. Measurements made in N=40 clusters/treatment in GL, PL and RD and N=36 cluster/treatment in KZ. Number of measured fruits per treatment/cultivar varied from 35 to 176 ... 59
Figure 28 - Mean fruit growth rate measured since 12/05 until 8/09 calculated with the remaining fruits on the 8/9. Each point corresponds to the mean ± standard error per treatments/cultivar. Standard error is not visible when the size is the same or smaller than the symbol dimensions. Measurements made in N=40 clusters/treatment in GL, PL and RD and N=36 Cluster/Treatment in KZ. Number of
Index of Tables
Table 1 – Trial experimental design used in the four cultivars ... 31
Table 2 - Mean fruit diameter (mm) and standard deviation in the 1st and 2nd applications of
metamitron (13/May and 21/May) and in benzyladenine application (17/May) ... 32 Table 3 - Minimum and maximum temperature (ºC), precipitation (mm), humidity (%) and radiation (W/m2) in the application day and in the 1st and 2nd days after applications of metamitron (13/May and 21/May) and application of benzyladenine (17/May) ... 32
Table 4 - No of labeled clusters per tree, treatment and cultivar ... 33
Table 5 - Dates of fruit diameter measurements made throughout the season ... 33
Table 6 - Dates of the 1st and 2nd Greene model measurements made in UNT, BA_150 and MET 165_165 in the four cultivars ... 35
Table 7 - Phenology followed on more year old wood in the four cultivars according to BBCH scale (Meier et al., 1994), registered in Sint-Truiden, 2015 ... 38
Table 8 - Flower buds per tree, TCSA and flowers per cluster. Counts and measurements made in N=8 trees/treatment in GL, PL and RD and N=12 trees/treatment in KZ and N=40 clusters/treatment in GL, PL and RD and N=36 clusters/treatment in KZ. ... 39
Table 9 – Mean No of fruits per cluster in the four cultivars and treatments from 26/05 until 9/06. Measurements made in N=40 clusters/treatment in GL, PL and RD and N=36 clusters/treatment in KZ .. 41
Table 10 – Mean percentage of fruit set after flower drop and fruit drop per treatment and cultivar calculated with flower count made on 23/04 and fruit count on 12/05 and on 12/05 and 15/07. Counts made in N=40 clusters/treatment in GL, PL and RD and N=36 clusters/treatment in KZ ... 42
Table 11 – Mean size (mm) and respective No of seeds of fallen fruits of UNT, MET 165_165 and BA_150 treated trees on the 18th of June ... 44
Table 12 – Thinning efficiency (%) calculated for the three treatments at harvest: MT, MET 165_165 and BA_150. Values were calculated considering UNT as 0% thinning ... 44
Table 13 – Mean No of fruits per fruit size from <60 to >80 mm in the four treatments and cultivars, in N=8 trees/treatment in GL, PL and RD and N=12 trees/treatment in KZ ... 45
Table 14 – Mean No of fruits per TCSA (cm2) and fruit weight (g) per treatment and cultivar, in N=8 trees/treatment in GL, PL and RD and N=12 trees/treatments in KZ ... 49
Table 15 – Mean values of brix°, firmness (kg/cm2) and No of developed seeds measured N=20 fruits/ treatment and starch index (1-9) measured in N=10 fruits/treatment ... 50
Table 16 - Mean values of Lightness (L), Chroma (C) and hue angle (h°) for red color intensity. Measurements made in N=20 fruits per treatment/cultivar………...51 Table 17 - Photosynthetic efficiency in UNT and MET 165_165 treated trees in the four varieties. The 1st measurement was on the 13/May before the application and the other dates correspond to the last 5 measurements. Measurements made in N=32 individual leaves per treatment/cultivar ………. .54 Table 18 - Sum of the difference of mean values of PSII Efficiency between UNT and MET 165_165 measured with FluorPen FP100 (Photon System Instruments, Czec Republic) from 13th of May to 19th of June in N=32 individual leaves per cultivar and thinning efficacy (%) of MET 165_165 in the four cultivars calculated with the final No of fruits at harvest ... 54
Table 19 - Mean values of phytotoxicity level observed on the 17th of June in N=8 trees per
treatment/cultivar in GL, PL and RD and N=12 trees per treatment/cultivar in KZ ... 56 Table 20 – Daily mean growth rate of the 20 fastest growing fruits (mm/day) in UNT (22/05 and 29/05), MET 165_165 (29/05) and BA_150 (25/05) fruits of the four cultivars. Measurements made in N=20 from 40 clusters/treatment in GL, PL and RD and 36 Cluster/Treatment in KZ ... 56
Table 21 - Overall mean percentage of Type A and B error of Greene Model in UNT fruits (22/05 and 29/05), MET 165_165 (29/05) and BA_150 for the four cultivars. Measurements made in N=40
clusters/treatment in GL, PL and RD and N=36 Cluster/Treatment in KZ. Number of measured fruits per treatment/cultivar varied from 176 to 35 ... 58
Abbreviation Index
6-BA 6-Benzyladenine
ABA Abscisic Acid
ACC 1-aminocyclopropane-1-carboxylic acid
ATS Ammonium Thiosulfate
AZ Abscission Zone
BA_150 Benzyladenine 150ppm between 8 and 12mm fruit diameter
CH Carbohydrates
Cfb Temperate Fully Humid Warm Summer
DAFB Days After Full Bloom
db Broadest Diameter
ds Shortest Diameter
EPP Effective Pollination Period
Fm Fluorescence Maximum
Fo Fluorescence Origin
Fv Variable Fluorescence
Fv/Fm Maximum Potential Quantum Efficiency of PSII
GAP Good Agriculture Practices
GL Gala
IAA Indol Acetic Acid
IFP Integrated Fruit Production
KZ Kanzi®
LLS Liquid Lime Sulfur
LWS Leaf Wall Surface
MET 165_165 Metamitron 165ppm at 8mm and 12mm fruit diameter
MT Manual Thinning
NAA Naphtaleneacetic Acid NAD Naphthaleneacetamide
PDO Protected Designation of Origin PGR Plant Growth Regulators
PL Pink Lady®
PS II Photosystem II
Qa Plastoquinone A
RD Red Delicious
SSC Soluble Solids Content TCSA Trunk Cross Sectional Area
UNT Untreated
WAFB Weeks After Full Bloom
1. Introduction
Apple is a fruit with a major importance worldwide. It is the third most produced fruit in the world and its production has been increasing all over the years (FAO, 2013). While in the past there were less cultivars available in the distribution platforms and the fresh market consumers had almost no requirements, nowadays the situation has changed. There are, not only more cultivars available to the consumers, but also their demands increased: consumers look for a quality product at a fair price.
Agriculture became a highly competitive activity and this evolution changed the grower’s production perspective into more focus in quality and less in quantity, to increase the economic value of the crop. The goal is to achieve the best balance between quality and quantity, using the less economic resources possible. For that, it is more and more necessary for the growers to be aware of the most recent information and technology and to adapt it to their orchard reality. The grower capacity of maximizing these tools and resources along with the natural risks that the orchard is exposed to, determines the success of the year’s production.
The tree has a natural way of controlling crop load, which is not enough to achieve the ideal commercial parameters of quality for fresh consumption. Fruit thinning is an agronomical technique widely used that significantly improves fruit quality with a high yield by enhancing the natural reduction in the number of fruits in the tree. This practice, by reaching the optimal crop load, reduces annual bearing, controls tree’s vigor, it avoids wood damage due to high weight in the tree and save labor costs for harvest. Thinning techniques have been changing over the years. In the past, it was done by removing flowers and fruits manually. Despite the high selectiveness, the high time consumption and high costs of hand labor (Schröder et al., 2013) make manual thinning a non viable option to the grower due to the negative impact on the economy of the orchard. Mechanical and chemical thinning seem to be the most promising options to overcome these disadvantages.
Chemical thinning is based on growing regulators or substances that reduce or block the photosynthetic capacity of the plant. One major problem with chemical thinning is its inconsistent efficacy due to variations in environmental and tree factors (Iwanami et al., 2012). Moreover, chemical thinning is not selective and depends on several factors such as weather conditions, local and cultivar, which makes the task of finding the optimal thinning intensity a major difficulty. Cultivar related differences in the degree of abscission and the cause of these differences have not been investigated (Iwanami et al., 2012). Due to the increasing competitiveness in apple production, it is necessary not only to achieve every year’s ideal doses, but also to define the ideal dose for each one of the cultivars present in the orchard. The objective of this study was to evaluate the efficiency of one application of 150 ppm of benzyladenine and two applications of 165 ppm of metamitron along with the evaluation of the results in fruit quality, in order to identify cultivar differences in the reactions to both compounds. Besides this, the work developed also
2 pretends to compare the growing rate and diameters induced by each one of the chemical compounds and assess if there was any influence in the size and number of seeds of the fallen fruits.
Since metamitron is a photosynthetic blocker, it is possible to evaluate the effect of the compound in the tree using a device that measures the photosystem II efficiency. This work also intends to assess the correlation of the values measured with the actual efficiency of the thinning that occurred and, consequently, assess the viability of this device as a tool that allow the grower to measure the intensity of the thinning that will occur with the doses previously applied.
Another relevant problem concerning fruit thinning is the need of a second application and how high should the dose be. Greene model claims to allow predicting the severity of fruit fall 8 days after the application with just one measurement before the spraying and another one after, in a range of 105 clusters. Due to the high amount of time that this model requires, a simpler version was tested. This simpler model was also done in untreated trees in two different dates, to understand if it can be used to estimate the need and the intensity of thinning.
2. Literature Review
2.1 Apple Production
2.1.1 Production in the World
Apple is the third most produced fruit in the world and its production has reached a total of approximately 81 million tons in 2013 (FAO, 2013). Over the ten past years, the apple’s annual production has been increasing on average 1 822 021 tons per year (Figure 1).
Figure 1 - Evolution of the apple production in the world from 2004 to 2013 (FAO, 2013)
According to FAO (2013), in 2013, China stated once again as the world’s biggest producer, retaining 49% of the world total production, followed by United States of America (5%), Turkey (3,9%), Poland (3,8%) and Italy (2,7%) (Figure 2
).
Figure 2 - Biggest apple producers in the world (FAO, 2013)
Regarding the world trade of apple, European Union and China are the biggest exporters and Russia and Germany have the biggest import percentage.
The most cultivated varieties in Europe, from the most to the less cultivated, are the following: ‘Golden Delicious’, ‘Gala’, ‘Idared’, ‘Red Delicious’ and ‘Jonagold’ (WAPA, 2013).
0 10000 20000 30000 40000 50000
China USA Turkey Poland Italy
Pr o d u cti o n (to n x 1 00 0) Country 60000 65000 70000 75000 80000 85000 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 Pr o d u cti o n (to n x 1 00 0) Year
4
2.1.2 Production in Belgium
Flanders is the most important fruit-growing region in Belgium. The fruit industry has blossomed into one of the most important fruit regions in Europe, being Sint-Truiden its ‘capital’.
In 2013, Belgium was the 10th biggest apple producer in the European Union. However, the production in this country has been decreasing for the last ten years, going from 345 000 tons in 2004 to 235 000 in 2013 (Figure 3). According to FAO (2013), the yield per hectare of the country has also been reduced to half in the period from 2004 to 2013.
Figure 3- Evolution of apple's production in Belgium (2004-2013) (FAO, 2013)
Besides the decrease in apple’s production, Belgium is still one of the biggest producers in Europe, exporting it to the Netherlands (28%), France (22%), Germany (17,5%) and Russia (13%) (VLAM, 2013).
The most cultivated varieties in Belgium are: ‘Jonagold’, ‘Jonagored’ and ‘Golden delicious’ (VLAM, 2013). ‘Belgica’ and ‘Kanzi’ are the most popular local cultivars in development, and suffered an increase of 22,5% in the total production during 2012 (Flandria, 2012).
2.1.3 Production in Portugal
In the last ten years, a different development has been observed in the apple production in Portugal comparing with the rest of the world. While the world production is increasing, in Portugal it has been decreasing, except on 2013, in which it was reached the best production of the past ten years (Figure 4).
Figure 4 - Apple production in Portugal (2005-2013) (FAO, 2013) 150 200 250 300 350 400 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 Pr o d u cti o n (to n x 1 00 0) Year 150 200 250 300 350 400 2005 2006 2007 2008 2009 2010 2011 2012 2013 Pr o d u cti o n (to n x 1 00 0) Year
In Portugal, the balance of trades for this fruit exhibits a positive evolution, despite the negative balance. From 2007 until 2012, the volume of exports doubled in opposition to the imports, which have been decreasing (Figure 5).
Figure 5 - Commercial trades in portuguese apple sector (GPP, 2013)
Regarding apple’s trading importance in national economy, it is important to highlight that, within all the fresh fruit production, this is the most cultivated species in the country, representing about 26.5% of the total volume (GPP, 2007). However, it is a strongly atomized culture, characterized by a large number of growers with acreage of less than two hectares. Ribatejo and Oeste regions possess the biggest acreage percentage, stating both as the main Portuguese apple producing regions.
The most grown cultivars in Portugal are ‘Golden Delicious’, ‘Gala’, ‘Red Delicious’, ‘Fuji’ ‘Jonagold’, ‘Jonagored’ and ‘Reineta’. Recently, local cultivars such as Bravo de Esmolfe and Riscadinha de Palmela have been gaining popularity among the consumers, obtaining the Protected Designation of Origin (PDO) granted by European Union.
As shown in Figure 6, due to storage structures, such as conventional cold storage and controlled atmosphere storage and due to the SmartFresh® with active ingredients as 1-methylcyclopropene, the apple supply lasts for almost a year.
Figure 6 - Apple supply in Portugal in season 2012-13 (GPP, 2013) 0 20 40 60 80 100 2007 2008 2009 2010 2011 2012 Tr ad es (to n x 1 00 0) Year Exports Imports
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2.2 Malus domestica
The apple tree belongs to the Rosaceae family, sub-family Pomoideae and has the botanical name
Malus domestica Borkh. (Webster, 2005). It is cultivated worldwide but its origin is in Asia and Europe,
more accurately in the border between the West China and Soviet Union’s Caspian Sea region (Harris et al., 2002; Jackson, 2003).
Malus domestica is a monoecious species, which leaves are dark green colored and the
inflorescence is called a cyme. The central flower of the inflorescence is the king bloom, is the first one to open and develops a larger fruit (Wertheim and Schmidt, 2005; Pereira-Lorenzo et al., 2009).
The apple tree is characterized for its auto-incompatibility mechanism, which is genetically controlled and occurs when the pollen and stigma present the same S-allele (Broothaerts et al., 2004; Wertheim and Schmidt, 2005). When this situation happens, growth rate of the pollen tube is slow, or even null, delaying its penetration until the ovary (Wang et al., 2012)
Due to the high level of gametophytic self-incompatibility, when installing the orchard, one or more compatible cultivars should be planted, in order to allow cross-pollination and a good fruit set. The maximum fruit quality is achieved and the fruit production is maximized, when cross-pollination occurs, with a suitable pollinator variety. Besides this, it is important not only to be aware of the compatibility between cultivars, but also if the bloom dates of the chosen varieties overlap themselves (Soster and Latorre, 2007; Petri et al., 2011).
2.2.1 Dormant Period
The apple tree is a deciduous tree, which means that, at the end of the season, it stops growing, looses its leaves, and steps into a dormant period. The dormancy is a condition defined by a temporary growth cessation and suppressed metabolism (Marini, 2014). This adaptive mechanism allows them to survive during the winter, when the environmental conditions do not favor growth and protects them from freezing (Petri and Leite, 2004).
During this period, besides not showing any visual growth, specific biochemical reactions occur inside the plant, essential to initiate a growing cycle. This growing cycle is affected by the environmental conditions, more specifically, the number of chilling hours, which both induce and break the dormancy period (Petri and Leite, 2004). These chilling hours are, according to the Chilling Hours Model, the number of hours below 7.2ºC at which the tree is exposed from October to February (Luedeling and Brown, 2010). The chilling requirements depend on the variety, but it can go from 300 up to 1000 hours (Pereira-Lorenzo et al., 2009).
Hereupon, one of the most important things to take in consideration when installing an orchard is the number of chilling hours available in the region. Not fulfilling the variety requirements will result in a series of anomalies that will reduce the productivity and fruit quality, such as irregular bud break and bloom,
paralysis of the shoot growth, development of small leaves, development of small flat shape fruits, low rate of effective fructification and reduction of the flowering-maturation cycle (Petri and Leite, 2004).
2.2.2 Vegetative and Reproductive Growth
Understanding the fruiting habits of the apple tree is essential to define the orchard’s cultural practices, namely, the training and pruning system, and also the use of growth regulators. The amount and quality of the fruit produced by an apple tree is determined by the relationship between vegetative and fruitful growth (Forshey, n.d.).
An adequate canopy management that allows light exposure is essential to guarantee a large, functional leaf surface and for the development of new bearing wood. This means that moderate vegetative growth is necessary resulting the excessive or insufficient vegetative growth in loss of fructification and possible reduction in fruit size. The relationship between vegetative and fruitful growth is influenced by many factors, such as fertilization, weather, and crop load, but pruning plays a major role (Forshey, n.d.; Marini, 2014).
Apple trees have two types of buds: vegetative buds, that only develop to leafy vegetative shoots and develop later in the season and mixed buds, where both leafy shoots and flowers emerge from (Marini, 2014). Mixed buds develop in one-year-old wood or more-year-old wood branches, being the last ones earlier than the ones from one-year-old branches (Francescatto, 2014). Most of these buds are formed terminally on short, less than 15 cm shoots that terminate with a rosette of leaves, which are called spurs (Wünsche and Lakso, 2000). Lateral buds are formed in the axils of leaves and are often referred as lateral buds or axillary buds (Marini, 2014).
The set of flowers is called a cluster and is composed by five to eight actinomorphic and hermaphrodite flowers, each one with a calyx with five sepals and a corolla with five colored petals (Wertheim and Schmidt, 2005; Francescatto, 2014).
The cycle of flower development lasts from 9 to 10 months in which the flower buds pass in the following succession of events: induction of flower bud formation, histological transformation, morphological differentiation and anthesis. This process starts 45 to 60 days after flowering of the previous year (Petri et al., 2011), meaning that the appearance and formation of the floral primordial happens in summer and the final formation of flower parts in spring (Koutinas et al., 2010).
The quality of the flowers is also dependent on the age of the fruit-bearing wood, fruit yield and harvest date in the previous season. As the development of the flower buds occur during the vegetative cycle, a number of nutritional, cultural, physiologic and genetic factors can influence positively or negatively the number of flower buds, being those factors the cultivar, rootstock, number of fruits per tree in the year before, radiation interception, ecological conditions and hormones, among others (Petri et al., 2011).
8
2.3 Cultivars
There are more than 7500 cultivars of apples all over the world, which come from bud sports or from scientific breeding. When these bud strains are just slightly different from the parent cultivar they become new commercial clones, and when they differ sufficiently from the parent tree are considered new cultivars.
In general, depending on the region and year, in the northern hemisphere the harvesting period can occur from late July until October and is one the most commonly used parameters to classify apple varieties. On the other side, Agusti (2004) divided apple cultivars in five commercial groups, according to the epidermis color:
• Yellow epidermis: Golden; • Red epidermis: Red Delicious;
• Bicolor epidermis: Gala, Fuji, Elstar, Braeburn, ‘Pink Lady’, Jonagold, Kanzi; • Green epidermis: Granny Smith, Greenstar;
• Bronzed epidermis: Reinetes.
2.3.1 Gala
Gala is a cultivar grown all over the world, obtained via crossbreeding ‘Kidd’s Orange Red’ x ‘Golden Delicious’, in New Zealand in 1939 by J.H. Kidd.
Belongs to the bicolored epidermis group, but the coloration depends from clone to clone, as well as with the year, environment and fruit positioning in the tree (Trillot et al., 1995).
It is an early cultivar that if harvested in the proper time, generally in August, is crispy, sweet, slightly acidic and moderately fragrant. According to Trillot et al. (1995), the optimum parameters at harvest are: 12 to 14º Brix and 8 to 9 kg/cm2 firmness. Color is also a good harvesting indicator.
The spurs are short and mostly situated in one to three year old wood branches, and brindles are also frequent (Lespinasse, 1977). ‘Golden delicious’ is a good pollinator variety, as long as ‘Braeburn’, ‘Elstar’, ‘Fuji’ and others.
Gala is a cultivar with several clones, namely Mondial Gala®, Royal Gala®, Schniga®, among others, each one better adapted to different climates, producing more and better quality fruit. It is a low sensible to the bearing effect, very fertile and early producer cultivar, if trained under proper conditions (Trillot et al., 1995).
Mondial Gala® is a clone originary from New Zealand, registered in 1988 and is characterized by a deep red striped blush covering 3/4 to 4/4 of the skin on a yellow-orange background, deeper and brighter than the original cultivar (Mondial Fruit, 2003). It is a less sensible to biannual bearing clone. According to a study conducted in Sarajevo by Kulina et al. (2013), from the range of clones available in the market, is one of the earliest in flowering, presents the biggest fruits and the second highest yield.
2.3.2 Kanzi®
Kanzi® is a club variety created and marketed by Better3fruits and Greenstar Kanzi Europe. The cultivar is ‘Nicoter’ and is commercialized under the trade name ‘Kanzi’. It was obtained in 1993 by crossing ‘Gala’ and ‘Braeburn’, in Belgium. By the year of 2002, the first commercial orchards started to be planted in Belgium, and Kanzi® reached the markets for the first time in the selling season 2005/06. Since then, the production has been increasing from 1500 tons to 60 000 tons in 2015 season (Urs Luder, Oral Communication). For this reason and due to its firm, juicy and crispy flesh, sweet and slightly sharp with a mild but really flavored taste, it is considered the apple with the fastest growing area in the world.
In Europe, Belgium, Netherlands and Germany are the countries with the highest production but Kanzi® is also increasing the acreage outside Europe, namely in Chile, USA, South Africa, New Zealand and Australia.
Kanzi® apple has a yellow-green background color with some bright red shades and has a smooth and uniform skin. It is a diploid self-fertile variety, but the production is improved with the use of pollinators, such as ‘Granny Smith’, ‘Braeburn’, ‘Pinova’, ‘Idared’ or ornamental Malus (Kanzi Quality Manual, 2015). It has a uniform production with a good size distribution, in average around 70-80 mm.
According to Kanzi® Quality Manual (2015), ‘Nicoter’ period of harvest is between middle September and middle October, period in which it achieves its maximum quality allowing the harvest of fruits with 7 kg/cm2 firmness, 12º Brix and 3 to 4 grades of starch degradation in Lugol’s scale.
Relating to storage behavior, to maintain its optimal quality the storage period should not exceed eight months, after which the production from the Southern Hemisphere bridges the supply through the whole year (Urs Luder, Oral Communication).
It is a cultivar sensitive to internal breakdown disorder and moderately sensitive to lenticels spots, a calcium related problem that is more problematic in younger trees and manageable with careful nutrition plans. It is also susceptible to Neonectria galligena, causing cankers (Kanzi® Quality Manual, 2015).
2.3.3 Pink Lady®
Pink Lady® was obtained in 1973, by John Cripps, in Bowden, Australia by crossing ‘Golden Delicious’ x ‘Lady Williams’ (Pink Lady Apples, 2015). The name of the cultivar is ‘Cripps Pink’, but it is commercialized under the trademark Pink Lady®. The apple has a particularly pink skin and crisp and juicy flesh. Its taste is sweet, intense and flavored.
According to Benitez and Pensel (2004), it is ready to harvest 186 Days After Full Bloom (DAFB), when the fruits reach the following maturity values: 7.5 to 8.5 kg/cm2 of firmness, 13 to 15º Brix and 30 to 50% of starch degradation. This cultivar has been a great success all over the world in the latter years, even though growers have to pay the royalties. This is mainly due to the rigorous criteria for the trademark quality specifications that guarantee consistent quality (Pink Lady Apples, 2015). It can be stored with no quality degradation during 4 to 5 months without controlled atmosphere.
10 Pink Lady® is an early flowering cultivar whereas it is one of the latest to be harvested, giving the apples an extra time of exposure to the sun that origins its pink color (Pink Lady Apples,2015).
It is a cultivar with low susceptibility to superficial scald and bitter pit. If picking is postponed the risk of flesh browning and other senescence symptoms, like loss of firmness and juice increases (Benitez and Pensel, 2004). It is also sensible to very hot weather with high susceptibility of stem cavity cracking during rainy seasons (Benitez and Pensel, 2004).
As in many other cultivars, there are also several commercialized clones, namely ‘Rosy Glow’, ‘Lady in Red’, ‘Ruby Pink’, and others.
2.3.4 Red Delicious
‘Red Delicious’ is a cultivar obtained in the USA, in 1879 but only adopted this name in 1914 (Benitez and Pensel, 2004). In the 80’s it was one of the most widely produced apples in the world, although in the past years its production has been decreasing.
Due to several improvements to make it more suitable for travel and to improve shelf life, nowadays it is relatively common to find a ‘Red Delicious’ that detains an excellent external appearance while the internal quality is no longer at its best. On the other side, these improvements originated a thick skin with a bright and shiny red color. The flesh is characterized by slightly crisp and dense texture and sweet but not intense flavor. It is easily identified not only for the color but also for its fix distinct crowns on the base. According to Lespinasse (1977), it is a ‘Spur-type’ cultivar, which means that fruit buds are situated on two year old or older wood. Usually, there are no brindles and branching is weak.
Benitez and Pensel (2004) state that ‘Red Delicious’ is a middle season apple and to achieve its optimum quality, fruit should be picked with firmness values between 7.5-8.5 kg/cm2, Brix values higher than 10º and more than 20% of starch degradation (Benitez and Pensel, 2004).
For the same authors, this is an apple quite sensible to disorders such as bitter pit, water stress, sunburn, lenticels spots, internal breakdown, among others, and its open calyx facilitates the entrance of
Alternaria spp. and other fungi that cause rot. ‘Red Delicious’ has been used for breeding for a long time,
so there are several used clones around the world.
2.4 Rootstock
There is an extensive list of rootstocks to choose from when installing an orchard. Depending on the type of orchard the grower wants to install and the existing constraints, it could go from very dwarfing to very vigorous, several diseases resistant and more adapted to different types of soil (Ferree and Carlson, 1987).
According to Ferree and Carlson (1987), M9 was selected as a chance seedling in 1879 in France, being nowadays widely used in apple commercial orchards. It produces a tree 25 to 35% the size of seedling, allowing the grower to profit early, due the precocity of the tree and to maximize yield with high-density plantation (Ferree and Carlson, 1987). Besides that, M9 production is easier to pick due to
the small tree’s height, and also because most of the production is provided by less than two meters branches. In addition to these facts, the less number of branches in the tree allows the radiation to come inside the tree, developingfruits with more color and more sugar content.
Rootstock M9 induces a small dwarf tree and has poor anchorage, so the drawback is that the trees need a support from a stake and the ground should be mulched or kept weed free, mainly in the first years, because of its lack capacity of competing with other plants (Royal Horticultural Society, 2015).
2.5 Fruit Set
A mature tree with a heavy bloom may have around 2000 flowers, but only 8 to 10% of these flowers will develop into fruits (Way, 1995). Even with provision for good cross-pollination, a high proportion of the fruitlets will fall during the June drop because of the competition for nutrients from the host tree, a mechanism of self-thinning that prevents the tree to exhaust all its carbohydrates reserves.
In apple production, the blooming period and fruit set are crucial phases to guarantee a good harvest. Fruit set is characterized by the rather sudden growth of the ovary soon after anthesis (Wertheim et Schmidt, 2005). During this period, the young fruit becomes an important sink to which nutrients flow.
There are several environmental and physiological factors that interact during the fruit set period and consequently, will influence the production. The low effective fructification and formation of small or deformed fruits can be associated with flaws in pollination or genetic incompatibility, and also environmental conditions.
2.5.1 External Factors
Poor weather conditions at the time of flowering are major causes for poor fruit set and yield in temperate climates. External factors do not only affect the flowers and fruits directly, but also the pollinator insect’s activity (Corvallis, 1981). If cross-pollination does not occur, fertilization and seed initiation do not take place, and the apple flower dries up and drops off without setting a fruit (Way, 1995).
2.5.1.1 Temperature
Temperature is the most important factor influencing not only in the apple tree’s development and fruit set (Pasqual et al., 1983; Wertheim and Schmidt, 2005) but also the fruit growth is strongly influenced by the post-flowering environmental conditions (Francescatto, 2014),
As long as the phenology is developing, the sensibility of the flower buds to frost increases (Wertheim and Schmidt, 2005). Late frost during flowering and fruit set period could really compromise the year’s production (Wünsche and Ferguson, 2005; Soster and Latorre, 2007). Warrington et al. (1999) concluded that low temperatures during this period originated smaller fruits whereas temperatures around 20ºC provide a fruit development ten times faster.
Way (1995) concluded that apple blossoms die at about -2.7ºC, depending on the cultivar’s susceptibility. Furthermore, cold periods just before flower opening can injure ovules and prevent fruit set.
12 Young fruits can be killed by cold weather 7 to 10 days after bloom as well, even if they are properly pollinated during bloom. This occurs because pollen tube growth is extremely slow below 10ºC and pollen will fail to germinate bellow 5ºC. In consequence, no fertilization will occur and there is low percentage of fruit set (Way, 1995).
Excessively high temperatures, above 29ºC, also prejudice fruit set. A reduced cell division in the fruitlets was observed when the trees were exposed to temperatures higher than 28ºC 8-28 DAFB (Flaishman et al., 2010). According to the same authors, if the high temperatures were applied at later stages of fruit development, 28-42 DAFB, it did not affect cell division, suggesting that environmental conditions during early fruitlet developmental stages are essential for the appropriate cell division in the apple fruit.
In addition, pollinator insect’s flight is reduced at temperatures below 18ºC (Flaishman et al., 2010) and extremely reduced at temperatures below 10ºC (Wertheim et Schmidt, 2005) preventing a good fruit set.
2.5.1.2 Precipitation
Apple trees’ pollination is mainly entomophilous, which means that apple growers rely on pollinator insects the task of carrying pollen. According to Wertheim and Schmidt (2005) there are several species that participate in this task, namely bumble bees (Bombus terrestris L.), solitary bees (Osmia lignaria S.) and certain flies (Diptera).
Often, low yield or poor fruit quality are attributed to poor pollination performance by bees due to low densities (Garratt et al., 2014) and unsuitable weather conditions during flowering (Boyle and Philogène, 1983). According to Wertheim and Schmidt (2005) and Sheffield (2014), unfavorable weather conditions, particularly precipitation, contributed to pollination levels being less than in other dryer periods, due to decreased pollinator activity. In addition, pollen grains lose viability rapidly once wetted, reducing the fertile period (Forshey, 1986).
2.5.1.3 Radiation
Light is the most important factor controlling the fixation of atmospheric carbon dioxide by the leaves (Corelli Grappadelli et al.,1994; Wünsche and Lakso, 2000). The photosynthetic process is important since more than 90% of the total dry matter produced by apple trees is originated from photosynthesis (Hansen, 1989).
As the fruit set requires a big amount of assimilates, is important that the canopy allows the incidence of radiation in the leaves to optimize this process (Gillapsy et al., 1993). The higher the photosynthetic rate the higher the level of translocate assimilates to the fruitlets, allowing its growth and increase of dry content matter. If fruit demand for assimilates exceeds carbohydrate availability the resulting supply limitation leads to a decreased fruit set and fruit growth, resulting in fewer fruit cells and consequently increased fruit abscission or reduced fruit size at harvest (Wünshe and Ferguson, 2005).
Limiting incident light during the 3 to 5 Weeks After Full Bloom (WAFB), due to natural reasons or cultural practices like shadowing nets, changes the carbohydrates distribution, leading to a poor flower bud formation and limiting the assimilates to the fruitlets (Corelli Grappadelli et al., 1994; Wertheim, 2005).
The implementation of hail nets is one of the cultural practices that may change the microclimate of orchards, reducing the solar radiation over the plants and interfering in photosynthesis and influencing the production and quality of the fruits (Leite et al., 2002; Wertheim, 2005; Bosco et al., 2014).
According to Amarante et al. (2007 and 2009), the use of hail nets, compromising the incident radiation in the trees has a direct effect in the accumulation of carbohydrates and flower bud differentiation, leading to a reduction in fruit set. Bosco et al. (2014) concluded that the anti-hail net reduced incident photosynthetic active radiation by 32%.
2.5.1.4 Wind, Humidity and Water Availability
Wind can affect fruit set by desiccating the stigmas or physically destroying them (Corvallis, 1981; Forshey, 1986). Besides this, bees’ flight is affected by winds greater than 24 to 32 km/h. Corvallis (1981) also concluded that high humidity may prevent proper release of pollen and low humidity can dry out the stigmas and reduce pollen germination, being both extreme situations prejudicial to fruit set.
According to Wünsche and Ferguson (2005), water availability has a huge impact on processes associated with cell division, such as fruit set. During this period, this kind of stress leads to high profound deficit in fruit set and fruit growth rate. Water stress after flowering decreased fruit set in apple trees to about one-third of that in irrigated trees (Kozlowski and Pallardy, 1997).
2.5.2 Internal Factors
2.5.2.1 Nutritional State
Several cultural practices can improve fruit set, and one of them is fertilizing. A tree with its nutritional requirements satisfied, will develop strong spurs with a good supply of stored carbohydrates that will help to ensure a good fruit set (Way, 1995).
Deficiencies of macronutrients, especially nitrogen, reduce the percentage of flowers that set fruits (Kozlowski and Pallardy, 1997). The right amount of nitrogen will help to maintain fruit fill, but an excessive doses will cause excessive vegetative growth and reduced flower bud formation later in the season (Way, 1995). Improving the strength of flower buds by late summer nitrogen applications can improve fruit set by increasing the length of effective pollination period (EPP) in apples (Williams, 1965).
2.5.2.2 Hormonal State
Growth and differentiation of young fruits is controlled by hormones produced in the developing seeds (Kozlowski and Pallardy, 1997). During fruit set the fruitlets are a strong sink in the apple tree and the sink strength is thought to be largely determined by the content and translocation from the fruitlet of certain growth hormones, such as gibberellins and auxins (Tromp, 2005; Wertheim and Schmidt, 2005).
14 According to Tromp and Wertheim (2005) cytokinins are also involved in early fruit development by stimulating cell division. These have a crucial function in the fructification inductive phase and in the partenocarpic fruit development, by that increasing the transport of carbohydrates and mineral nutrients into developing ovaries or fruits (Francescatto, 2014; Kozlowski and Pallardy, 1997). However, if sprayed at a later stage have thinning proprieties. Gibberellins on the other hand, stimulate pollen germination and pollen tube growth and its exogenous application in flowers improve fruit set in the absence of pollination (Gustafson, 1960).
In apple trees the more apple stigmas are pollinated the higher the seed number and fruit set, having the number of seeds in a fruit a positive influence in its sink strength and also improving the symmetric growth of the fruit (Wertheim and Schmidt, 2005).
2.6 Physiology of Fruit Growth and Development
The growth of apple fruits has been described as simple sigmoidal (Tromp and Wertheim, 2005) and occurs generally in two phases: the phase of cell division and cell expansion for approximately 4 to 5 weeks after full bloom, followed by the phase of only cell expansion for the remaining growing season (Bain and Robertson, 1951; Westwood, 1978).
During the first 1-4 weeks after flower fertilization, flesh volume increases rapidly as mainly a result of cell division (Gillapsy et al., 1993). At the end of this first sigmoid phase of fruit development cell division may cease and the fruit growth slows down. This means that practices that increase the fruit’s ability to produce cells right after bloom have greater effect in fruit size than late in the season practices (Wünsche and Lakso, 2000; Wünsche and Ferguson, 2005). Cytokinins have influence during this period, increasing cell division and enlargement.
After canopy closure, the light interception becomes constant, and the growth is a linear function of light availability (Wünsche et al., 1996). This characterizes the second phase when growth is mainly accomplished by a transfer from cell division to cell expansion in longitudinal, radial and tangential planes until it reaches its final size (Gillapsy et al., 1993).
The last phase of fruit development is ripening, which is initiated after seed maturation has been completed (Gillapsy et al., 1993) and is characterized for tissue softening and high accumulation of sugar and acids.
2.7 Organ’s Abscission
Abscission plays a crucial role in health and reproductive success of plants. It is a physiologically active process that enables plants to shed, in a controlled manner, unwanted organs that no longer serve a functional role to the plant (Patterson, 2001). According to the same author, damaged or infected organs may be rapidly shed as a mechanism of defense. Abscission may be observed in most plant organs such as leaves, flowers, petals, petioles, fruitlets and fruits.
