Yield based indices of heat tolerance
Yield-based stress indices TOL, MP, GMP, HM, SSI, STI, YI, YSI and RSI were determined for studied bottle gourd genotypes using yield under both normal (Yp) and heat stress (Ys) conditions (Table 1). The genetic variation among genotypes defined by different heat stress indices are depicted in figures (Fig 3-5). Nineteen bottle gourd genotypes possessed higher TOL, SSI, YSI and RSI and were marked as heat-susceptible genotypes because they recorded high yield under normal environments but zero yield when the plant died under heat stress and hence, these genotypes are only appropriate for normal environments.
The lowest value of TOL, SSI, YSI and RSI was exhibited in genotypes BG-42 followed by BG-18, BG-35 and BG-39 denoted as tolerant to heat stress. These genotypes performed less under both conditions due to less yield differential, implying that low values do not always indicate high performance and emphasising the relevance of yield as a crucial factor.
Lamba et al., (2023) suggested to determined heat tolerant genotypes considering lower TOL, SSPI, YSI and RSI which supported the current findings. Superior genotype selection has also been reported based on lower SSI and TOL in chickpeas
(Jha et al., 2018), common bean (
Porch, 2006) and bottle gourd
(Mashilo et al., 2017). Based on greater values of MP, GMP, HM, STI and YI and lower TOL and SSI combinations genotype BG-02 were considered to be most stable and productive genotypes followed by BG-43, BG-57, BG-01 and BG-45 identified among all the genotypes under both conditions as heat tolerant.
Sofi et al., (2018) suggested classifying genotypes into four groups based on their resilience and productivity in both stress and non-stress environments. These indices were effective in identifying high-yielding cum heat-tolerant, which identical to the findings reported by
Poudel et al., (2024) and
Jha et al., (2018).
Correlation between fruit yield and stress tolerance indices
The indices that showed a strong relationship with fruit yield in both environments were chosen as best in selecting genotypes with high yield in both conditions (Table 2). Fruit yield under non-stress (Yp) was significant positive association (0.369) with fruit yield under heat stress (Ys) suggested that these might be utilized to select high yielding bottle gourd genotypes. Fruit yield (Ys) had positive and significant relationship with MP (0.904), GMP (0.983), HM (0.991), STI (0.968), YI (1) and YSI (0.952) but highly negative significant correlation with TOL (-0.796) and SSI (-0.952). Whereas, Yp demonstrated a significant and positive relationship with TOL (0.268), MP (0.732), GMP (0.405), HM (0.396), STI (0.514) and YI (0.369). Fruit yield (Ys) was negatively correlated with TOL (-0.796) while positively correlated (0.268) under normal conditions; thus, selection based on these indices will yield more fruit under normal conditions but less under heat stress
(Lamba et al., 2023). Yield (Yp and Ys) were determined positive association with MP, GMP, HM, STI and negatively correlated with TOL and SSI for heat tolerance in wheat
(Poudel et al., 2024), chickpeas
(Jha et al., 2018) and water stress in cow pea
(Gull et al., 2019) which matched with current findings.
Kumar et al., (2020) reported that yield (both normal and stress environments) in mungbean was positively associated with all stress tolerance indices except SSI and Superiority measure. Both TOL and SSI had substantial negative associations with all stress tolerance indices, whereas SSI and TOL demonstrated a strong positive connection (0.886). High positive associations with SSI and TOL but negative correlations with other heat stress indices were also reported by
Jha et al., (2018). The findings indicated that these parameters were more effective in selecting genotypes with high yields under different environments.
Principal component and biplot analysis
The first two principal components (PCs) with an Eigen value >1.0 generated variations by 83.89% and 14.50% of PC1 and PC2, respectively, for heat tolerance indices (Table 3) which contributed highest variation (98.39%) of the six components.
Seepal et al., (2025),
Mashilo et al., (2017) and
Farshadfar et al., (2013) reported 97.46%, 99.72% and 99.60% of total variations in field pea, bottle gourd and wheat respectively. PC1 explained significant positive association with Ys, MP, GMP, HM, STI, YI and YSI but had a negative relationship with TOL and SSI. Thus, PC1 regarded as stress tolerance component. PC2 were identified as heat susceptibility component since it explained a greater correlation with TOL, SSI and Yp. This method also utilized by
Poudel et al., (2024) and
Devi et al., (2021) to categorize the components of wheat under heat stress.
Ullah et al., (2022) suggested that most appropriate criteria are to select stable genotypes with low PC2 and greater PC1 values and vice versa. Thus, genotypes BG-45, BG-57, BG-02 and BG-43 were found higher PC1 values with lower PC2 values regarded as superior with stable and tolerant genotypes under both heat stress and non-stress environments and might recommended for cultivation,
Lamba et al., (2023) employed this approach to identify stable heat tolerant genotypes.
The biplot depicted that Yp and Ys were positively connected with MP, GMP, HM, STI, YI and YSI, while Ys was negatively associated with TOL and SSI, as evidenced by the obtuse and acute angles between their vectors respectively (Fig 6).
Lamba et al., (2023) and
Jha et al., (2018) displayed these types of relationship among indices using biplots for heat tolerance in wheat and chickpeas, respectively.
Devi et al., (2021) similarly illustrated positive relationship with MP, GMP and STI as acute angles between their vectors while negative with TOL and SSI as an obtuse angle in Ys.
Cluster analysis
The cluster analysis categorized genotypes into three clusters based on stress tolerance indices trends. Among the clusters, cluster 2 possessed the twenty-one genotypes with highest MP, GMP, STI, YI and YSI, indicating strong performance and desirable traits categorized as tolerant. Similarly, cluster 3 exhibited medium values of heat tolerance indices and consisted 18 genotypes classified as semi-tolerant. Consequently, cluster 1 had the highest TOL and SSI values but least MP, GMP and YI values, with 19 genotypes suited only for normal condition, indicating that they were susceptible genotypes (Fig 7).
Naghavi et al., (2013) categorized genotypes into three clusters for drought tolerance which validated selection through clustering of studied genotypes.
Ranking of genotypes
The ranking system was employed to determine the overall performance of genotypes. In order to identify the most heat-tolerant genotype based on all indicators, mean rank (R) and SDR of all indices were estimated. The genotypes BG-02, BG-45 and BG-57 were identified as most heat-tolerant with low rank values based on all heat stress tolerance indices (Table 1).
Mashilo et al., (2017) and
Naghavi et al., (2013) utilized ranking method for screening corn and bottle gourd genotypes, respectively.