Discussion

Significant differences were observed between two sites for SDMY, SH, SN and LAI at most harvests in both years. Relatively higher values for SDMY, SH, SN and LAI were observed at irrigated site as compared to rain-fed site. This may be due to the fact that optimal amount of water was available for maintaining plant growth under irrigated site throughout the vegetation period through supplemental irrigation. These differences can also be attributed to relatively lower organic carbon contents in 30-90 cm of soil

profile for soils of rain-fed site (Table 3.1) that led to their lower water holding capacity.

Soils in the rain-fed site were slightly better in terms of soil available water as revealed by retention curves data (Table 3.3) but this did not seem to have much effect on better yield performance of lucerne varieties under rain-fed site probably because this was superseded by the effect of irrigation. No supplemental irrigation was provided to rain- fed site thereby creating a water deficit at this site when compared to irrigated site where supplemental irrigation was provided. This water deficit leads to a reduction in SDMY for lucerne as reported in earlier studies (Carter and Sheaffer, 1983). Higher values of leaf to stem ratio are observed usually under rain-fed site as compared to irrigated site.

This may be due to the fact that water deficit causes a cessation of stem growth while leaf growth continues thereby leading to a higher leaf to stem ratio. Increase in leaf to stem ration under water deficit conditions has also been reported in earlier studies (Carter and Sheaffer, 1983; Halim et al., 1989).

Non-significant differences among varieties for RWC depict that varieties had very small differences for this parameter and even site/irrigation effect did not bring major change.

RWC has been shown to correlate well with drought tolerance (Jamaux et al., 1997;

Altinkut et al., 2001; Colom and Vazzana, 2003). Non-significant differences among varieties for their chlorophyll content may explain us that these varieties may not differ in their drought tolerance under stress conditions and it is justified as site is not going to alter chlorophyll content of a variety as this character is in built and fixed for a given variety. On the overall basis, higher values of chlorophyll content for Sitel are justified as it is a local well-adapted variety.

Differences in CID values at two sites can be attributed to differences in water regime of two sites and these findings are supported by earlier studies of Xu et al. (2007) who reported that CID can vary with plant part being studied, stage of sampling, environment and water regime. Non-significant differences among varieties for CID coincide with results of other physiological parameters (RWC and chlorophyll content) in the present study. As CID is also related to drought tolerance (Condonet al., 2004) , it explains that narrow genetic diversity exists among studied varieties for their suitability under drought,

that is why they do not produce significantly different results for drought sensitive parameters.

CID varied with plant parts being studied and these findings are in line with earlier studies (Araus et al., 2002; Xu et al., 2007). Root and stubble have relatively lower values of CID as compared to shoots. These findings partly coincide with those of Johnson and Rumbaugh (1995) who reported lower values of CID for lower parts of plants.

Differences among varieties and differences among sites were found non-significant for root biomass. Differences in root biomass in different depths at different sites were found significant and it can be due to the fact that as the roots grow deeper, their distribution pattern becomes different. Rain-fed site have relatively higher biomass as compared to irrigated site. Under water limited conditions, roots tend to grow more in search of water.

Contrasting and inconclusive results have been reported by earlier workers (Jodar-Karimi et al., 1983; Luo et al., 1995) dealing with research on lucerne roots due to difficulties associated with traditional methods of studying roots in the field as these methods require more labour , time and equipment. Root studies are associated with high variability coupled with the variability in the environment and the age of plant being studied (Sheaffer et al., 1988).

RLD varied significantly only with depth in both years of study. In 2007, RLD is higher in lower soil layers as compared to upper soil layers. Contrasting results have also been reported earlier for lucerne root studies (Jodar-Karimi et al., 1983; Luo et al., 1995).

Possible causes of higher RLD in lower soil layers can be colliding of branches of roots in middle to lower soil layers (30-90 cm) due to relatively narrow row spacing, mixing of on the row and between the row samples, small number of replications and small diameter of augers used in root sampling. Heterogeneity in root distribution studies even with higher number of replicates is well known besides this root distribution itself is highly variable (Bengough et al., 2000). In 2008, varieties have higher RLD in upper soil layer (0-30 cm) as compared to lower soil layer (30-60 cm). These findings coincide with those of Zahid (2009). Results of RLD in second year of studies seem more reliable due

to use of auger of larger diameter (7 cm) and washing and analysis of on the row and between the row samples separately.

ETa of varieties was found non-significant at most of the harvests and it depicts that varieties do not differ drastically in their water requirements for transpiration. The same trend follows for WUEp as it was based on ETa values where differences among varieties were found non-significant. Differences among sites for WUEp seem largely due to differences in irrigation and water holding capacity of the sites. Higher values of WUEp in the irrigated site as compared to rain-fed site are associated with higher SDMY.

In the rain-fed site, significant differences among varieties for SDMY (based on yearly averages) can not be considered as an indicator of variation among varieties as varieties did not differ significantly for SDMY at most of the harvests in both years of study (Table 3.5). Also the varieties did not differ significantly for SH, LAI, leaf to stem ratio, RWC and chlorophyll content at most of the harvest in both years of experimentation.

On the overall basis, varieties tended to produce relatively higher SDMY in 2008 as compared to 2007 due to higher precipitation during vegetation period of 2008 (see Fig.

3.1). Based on total yearly SDMY in the rain-fed site, Ordobad was consistently low yielding variety in both years and not fit for use under rain-fed conditions. NS-banat, Sitel and Niva seem to perform better under rain-fed conditions and can be suitable choice for rain-fed conditions due to their relatively higher yields and WUEp. In the rain- fed site, NS-banat had relatively higher WUEp during all main harvests mainly because it had lower ETa values. Non-significant differences among varieties for their ETa and WUEp indicate that there exists narrow physiological and genetic diversity among varieties under study.

Varieties tended to perform better under irrigated conditions as compared to rain-fed conditions in both years of study. Differences among dry matter yield and associated morphological parameters seem to be caused by irrigation, site and year instead of varietal effect. This is reflected in the physiological parameters like relative water

content, chlorophyll content and relative water content that do not seem to be much different among sites as well as among varieties. This holds true for comparison of varieties in two sites as well as for comparison of six varieties in the rain-fed site.

Based on total yearly shoot dry matter yield from comparison of three varieties at two sites, Sitel is the best variety followed by Niva and Mohajaren. In the rain-fed site NS- banat, Sitel and Niva seem to perform better and can be suitable choice for rain-fed conditions due to their relatively higher yields and WUEp. As roots are equally important especially under organic farming conditions, we found that under irrigated conditions, Sitel is the leading variety as it produced the highest root biomass followed by Mohajaren and Niva. In the rain-fed site, Ordobad had the maximum root yields followed by NS- banat and Sitel.

Chapter 4

Effect of lucerne utilization system on yield, biological nitrogen fixation and water conservation