Due to changing climatic conditions heat stress significantly reduces crop productivity particularly during reproductive phase like flowering and grain filling stage in mungbean (Iqbal et al. 2021). In present study promising results were observed for yield traits like PH, NFB, NP, HSW and SY. Grain yield was adversely affected by high temperature treatment which suppressed the growth of plant resulting in reduced number of seeds in pod and low seed weight (Jha et al. 2023; Van Haeften et al. 2023). Likewise, yield is a complex trait which depends on performance of other morphological traits such as PH, HSW, SPP and NSP (Samyuktha et al. 2020). These traits are interlinked with each other. Relationships among these traits can be assessed by correlation analysis. Yield was positively associated with the morphological traits such as PH, SPP, and HSW under heat-stress environment, indicating heat tolerant genotypes have high NP and HSW. Hence, it is suggested that selection could be made based on performance of such traits as rise in these traits can significantly enhance the grain yield of mungbean (Iqbal et al. 2021). As heat stress reduced grain yield due to loss in NP, NSP and HSW; interestingly, findings of this study showed that DM had a negative association with NP, PL, HSW and SY. Thus, it is suggested that early maturing genotypes can handle high-temperature stress effectively and should be the primary target trait to develop heat-tolerant lines (Hanif and Wahid 2018; Iqbal et al. 2021).
Nevertheless, correlation analysis is a useful parameter to examine relationship between various parameters. However, PCA is a more efficient method to assess genetic variability for high yielding genotypes. Therefore, using stress tolerance indices, PCA was conducted to screen out resilient genotypes (Kamrani et al. 2018). To examine the G×E interaction, researchers employed various types of biplot approaches to identify genotypes for varying environmental conditions (Kendal 2019). High STI and GMP indicated better efficiency of plant under both normal and harsh climate. It was clear from PCA biplot that MNH-2225 and MNH-7124 were tolerant genotypes as they showed higher yield under heat stress with high STI and GMP. In contrast, genotypes MN-2011 and MN-98 showed high yield under normal condition but low yield under stress due to low STI and GMP. Comparable results were reported by Kamrani et al. (2018), who proposed that genotypes with greater yields and heat tolerance can be selected based on GMP and STI values. Similarly, PC1 showed high contribution of tolerance indices such as STI, YI, YS, MP and GMP. Correspondingly, the results of Ganta et al. (2022) and Jha et al. (2017) reported similar outcomes for mungbean and chickpea, respectively. In biplot analysis, association between the stress indices and other morpho-physiological traits are indicated by the cosine of the angle. MP, GMP, NSP are close to each other showing positive inter-linkage. Hence, these indices are helpful in screening of tolerant genotypes by indirect selection (Öztürk 2022).
Heat stress significantly affected physiological traits like Chl a, Chl b, CARO, RWC and CMS under heat stress. High temperature stress has a negative impact on chlorophyll content and causes leaf senescence. Stable chlorophyll content under heat stress indicates tolerance against heat stress. In this study, heat tolerant genotypes reserved relatively higher Chl a, Chl b and CARO than heat sensitive genotypes under heat stress. Similarly, earlier studies showed relatively higher chlorophyll content in tolerant genotypes of lentil under heat stress (Jha et al. 2023). Similarly, RWC is a physiological parameter which shows relative hydration status of a plant. It is useful for assessing the level of damage caused by heat stress and water shortage in leaves (Junttila et al. 2022). MN-2011 and MN-98 showed high Chl a, Chl b, CARO, RWC and CMS under heat stress, indicating tolerance against high temperature. Favorable results were reported in mungbean (Ikram 2022), chickpea (Jha et al. 2022) and urdbean (Chaudhary et al. 2022) under heat stress. Various physiological parameters like CMS, SPAD and Chl a exhibited significantly positive association with grain yield. These outcomes suggest that such traits could prove valuable in selection process for improving grain yield and stress tolerance in future breeding programs.
Cluster analysis (Fig. 3) showed that similar genotypes were positioned closer together; whereas, diverse genotypes were placed apart from each other (Ton 2023). A cluster with various genotypes can give an ideal breeding stock to achieve the best possible genetic improvement. Among these clusters the predominate genotypes with high yield efficiency retained in cluster 1. While, highly tolerant genotypes were gathered into cluster 2, which showed high YI and STI along with high CMS, RWC, Chl a and Chl b under heat stress (Iqbal et al. 2021; Lamba et al. 2023). In order to improve mungbean yield via heat tolerance breeding, it may be helpful to combine genotypes from cluster 1 and cluster 2 through hybridization programs.