Chapter 1 A Review of Literature
1.10 Selection indices for drought tolerance
stress in the 2010/2011 season and from 39.37 g under non-stress to 31.91 g under drought stress in the 2011/2012 season.
Table 1.2: Drought tolerance selection indices
Indices Formulae References
Drought resistance Index, DR
DR = Ys(Ys/Yp)/Xp Lan (1988) Harmonic Mean of grain
yield, HM
HM = 2(Yp*Ys)/(Yp+Ys) Jafari et al. (2009)
Mean Productivity, MP MP = (Yp + Ys)/2 Rosielle and Hamblin (1981) Stress Susceptibility Index,
SSI
SSI = (1-Ys/Yp)/(1- Xs/Xp)
Fischer and Maurer (1978) Stress Tolerance Index,
STI
STI = (Ys*Yp)/ (Xp)2 Fernandez (1992)
Tolerance Index, TOL TOL = Yp – Ys Rosielle and Hamblin (1981) Yield Index, YI YI = Ys/Xs Gauzzi et al. (1997)
Yield Reduction, YR YR = 1 – (Ys/Yp) Golestani-Araghi and Assad (1998)
Yield Stability Index, YSI YSI = Ys/Yp El-Mohsen et al. (2015)
Key: Ys - yield for each genotype under drought stress, Yp - yield for each genotype under non-stress, Xp - yield for all genotypes under non-stress, Xs - yield for all genotypes under drought stress.
1.10.1 Harmonic Mean and Stress Tolerance Indices
Drought tolerance indices TOL, SSI, MP, STI and HM were successful in grouping maize hybrids in accordance with the above-mentioned grouping system in the study by Jafari et al.
(2009). Correspondingly, significant correlations were detected between the yield under non- stress with HM and STI with coefficients of 0.80 and 0.88, and under water stress also with HM and STI, with correlation coefficients of 0.96 and 0.90, respectively (Jafari et al., 2009).
The significantly high linear relationship between STI and HM reported by Jafari et al. (2009) indicated STI and HM to discriminate drought tolerant accessions. In a study by Dorostkar et al. (2015), the grain yield of diverse wheat genotypes was also significantly correlated with HM and STI under drought stress and non-stress. Dodig et al. (2012) reported a correlation between MP and TOL. Selection for TOL under drought stress led to reduced grain yield under non-stress (Dodig et al., 2012).
1.10.2 Drought Resistance and Yield Indices
The drought stress tolerance indices DR and YI were reported by El-Mohsen et al. (2015) to effectively detect drought tolerant genotypes under both drought stress and non-stress conditions. These deductions were inferred from the significant correlation detected between the indices DR and YI with yield under drought stress and non-stress deficit also (El-Mohsen
et al., 2015). The drought selection indices DR and YI were detected with a high factor loading for the principal component designated for drought tolerant genotypes, along with the indices STI, MP, HM and the grain yield under non-drought and drought stress (El-Mohsen et al., 2015).
1.10.3 Stress Susceptibility and Tolerance Indices
An SSI less than 1 indicated drought tolerant genotype as reported by Fischer and Maurer (1978). Landraces tended to display lower values for TOL and SSI compared to improved germplasm, thus were more drought tolerant than modern cultivars (Dodig et al., 2012). Dodig et al. (2012) further discussed SSI must be used under severe drought stress environments.
Sio-Se Mardeh et al. (2006) reported high TOL values effectively revealed drought susceptibility of accessions under both drought stress and non-stressed conditions.
Essentially such accessions are classified as group D genotypes, as recognised by Jafari et al. (2009) utilising the drought selection indices TOL and SSI. A negative correlation was detected between the grain yield under drought stress for the TOL and SSI with correlation coefficients of -0.51 and -0.329, respectively, by Dorostkar et al. (2015), thus indicating for selection of appropriately drought tolerant wheat genotypes. Low values for TOL and SSI are recommended under drought stress. Moreover, genotypes which also exhibited poor seed yield under drought stress and non-stress, thus belonged to group D, were detected by high TOL and SSI drought selection indices by Dorostkar et al. (2015).
1.10.4 Yield Stability, Yield Reduction Indices and Mean Productivity
A notable correlation was detected between the drought selection index YSI with the grain yield under both non-stress and water stress according to El-Rawy and Hassan (2014). An arguably poor correlation between the seed yield under drought stress and non-stress was detected by El-Mohsen et al. (2015) for the YSI, YR, SSI and TOL. On the other hand, Hohls (2001) reported selecting for MP under drought stress, however, would not lead to limited yield either under drought stress nor non-stress. Dorostkar et al. (2015) reported a significant correlation between MP and the yield under drought stress and non-stress with correlation coefficients of 0.55 and 0.59, respectively.
1.10.5 Ranking of genotypes using selection indices
The rank mean, standard deviation of ranks and rank sum were used for ranking of genotypes by El-Mohsen et al. (2015) and by Farshadfar et al. (2012). Whereby, genotypes with low values for rank mean and rank sum ranked higher, whereas, a low value for standard deviation of rank meant stable rank across different drought tolerance indicators. The standard deviation of ranks and rank sum are given in formula a) and b), respectively.
a) Standard deviation of ranks = ∑mi=1(Rij-Ri mean)
n-1 (El-Mohsen et al., 2015)
b) Rank sum = Ri mean + standard deviation of ranks (Farshadfar et al., 2012; El-Mohsen et al., 2015)
Where, Rij is the in vivo drought tolerance or resistance index rank, Ri mean is the rank mean across all drought tolerance or resistance indicators for each genotype. These ranking procedures ensure drought tolerant, germplasm are selected without bias by considering a single drought selection indicator (Farshadfar et al., 2012).