Chapter 5: Yield-Based Selection Indices for Drought Tolerance Evaluation in Selected
5.7 Discussion
The study revealed the presence of genetic variability for drought tolerance in bottle gourd landrace collections. Results of phenology showed that under DS conditions landraces produced male flowers 5 days earlier than under NS condition (Table 5.4). Further, early maturity was observed under DS conditions than NS conditions. This suggests that bottle gourd landraces under DS conditions accelerated flowering time and maturity to avoid terminal drought. Flowering time is an important trait related to drought adaptation, where early flowering can lead to drought escape (Araus et al., 2002). Early maturity is an important drought adaptation mechanism useful in breeding for drought tolerance (Chaves et al., 2003). Developing short-cycle varieties is therefore an effective drought avoidance strategy (Farooq et al., 2009).
The landraces BG-78, BG-52 and GC had the highest fruit yield under both DS and NS conditions and can be considered drought tolerant (Table 5.5; Figure 5.1). According to Blum (1988), screening for drought tolerance among genotypes must be conducted based on high performance in stressed and non-stressed conditions. These landraces (BG-78, BG-52 and GC) would have economic yields in dry years and high performance under optimal rainfall conditions (Figure 5.5).
Further, the landraces (BG-78, BG-52 and GC) had the highest values for STI, GMP, MP, TOL and HARM hence; they may be the most productive landraces under both DS and NS conditions (Table 5.6). Stress tolerance index (STI) and geometric mean productivity (GMP) have been proposed for the selection of genotypes that produce high yields under DS and NS environments (Fernandez, 1992). The higher the value of STI for a given genotype, the higher its drought tolerance and yield potential (Kumar et al., 2014). Also, a relatively higher GMP value of a given genotype suggests its greater grain yield performance under both DS and NS conditions (Fernandez, 1992). Also, selection for higher MP and HARM values should increase yield in both stress and non-stress conditions (Hohls, 2001; Ganjeali et al., 2005). Therefore, selection of bottle gourd landraces based on higher STI, GMP, MP, TOL and HARM will result in drought tolerance and yield improvement as they have been successfully used for selection of drought tolerance in various crop species (Ganjeali et al., 2005; Ganjeali et al., 2011; Belko et al., 2014; Naderi and Emam, 2014).
Drought tolerance is a complex trait and its inheritance is governed by polygenes. Also it is highly influenced by genotype and environment interaction (Bahrami et al., 2014). Therefore, selection under managed drought condition and use of a combination of indices may enhance breeding gains.
Typically, suitable drought tolerance indices should significantly correlate with yield under stressed and non-stressed conditions (Mitra 2001). In this study, drought tolerance like STI, MP, GMP, YI and HARM correlated with yield under non-stressed and drought-stressed conditions (Table 5.7). These suggested that the drought tolerant indices used in the present study are suitable to screen drought-tolerant, high yielding bottle gourd landraces under dryland and supplemental irrigation conditions. Results in this study are in agreement with Golabadi et al. (2006) and Sio- Se Mardeh et al. (2006), Ganjeali et al. (2011) and Ashraf et al. (2015) who reported that these indices are suitable for discriminating the best genotypes under stressed and irrigated conditions.
significant correlation between TOL and Yds suggested that selection based on TOL will result in reduced yield under well-watered conditions. Similar results were reported by Clarke et al. (1992), Rosielle and Hamblin (1981), Sio-Se Mardeh et al. (2006) and Naderi and Emam (2014). A positive and significant association between Yds and Yns (r = 0.68) suggest a relatively consistent yield levels among genotypes. The current results suggested that selection of bottle gourd landraces based on yield potential would improve yield under stressed and non-stressed conditions.
In contrast, Ceccarelli et al. (1992), Abebe et al. (1998), Yadav and Bhatnagar (2001) and Ganjeali et al. (2011) reported low to moderately association in yield performance of genotypes under stress and non-stress conditions. Rosielle and Hamblin (1981) also indicated that under most yield trials, the correlation between stressed and non-stressed yield is smaller indicating that selection for yield potential would only increase yield under non-stressed environments while the selected genotypes would perform poorly under stressed conditions.
Time to 50% male and female flowering were non-significantly associated with all yield-based indices of drought tolerance under both stressed and non-stressed conditions (Table 5.8). These suggest that the reproductive period for male and female flowering may not result in drought tolerance improvement in bottle gourd (Siddique et al., 1999). Days to maturity was negatively correlated with stress tolerance index (STI), yield index (YI), harmonic mean (HARM) and drought-stressed yield (Yds) under drought stress conditions (Table 5.8). The present results suggested that breeding for bottle gourd genotypes with early maturity, increased drought- tolerance and yield potential under drought-stressed conditions is possible.
Principal component analysis indicated that PC1 explained most of the variation. Yield-based indices of stress tolerance index, GMP, MP, TOL, YS, HARM, Yds, and Yns positively correlated with PC1. Stress susceptibility index (SSI) negatively correlated with PC2 while YSI positively correlated with PC2 (Table 5.9). Thus, the first component can be considered in affecting drought tolerance and yield potential (Aliakbari et al., 2014; Bahrami et al., 2014; Naderi and Emam, 2014). These indices are the most important criteria to select best performing genotypes under normal and stress conditions. The second component (PC2) is regarded as drought sensitive and
and non-stress environments (Golabadi et al., 2006; Aliakbari et al., 2014; Naderi and Emam, 2014). In this study, landraces well-associated with PC1 values were BG-52, BG-78 and GC and those with low PC2 values were BG-31, BG-48, BG-67 and BG-79. These landraces were also the most tolerant characterized by low RS values (Table 5.10).