Temperature has considerable effects on germination characteristics including the initiation and the final germination percentage, it is therefore the most critical factor in the determination of success or failure of plant establishment (Al-Ahmadi and Kafi 2007). Reduction in germination due to heat stress in wheat has been attributed to impairments in the activity of alpha amylase which hampers starch breakdown and abolish the delivery of nutrients to developing embryos (Essemine et al 2010). Another reason for germination suppression is reduced metabolic efficiency at high temperature than optimum temperature (Thygerson et al., 2002). Heat stress caused inhibition of seed germination through synthesis of ABA in wheat (Hasanuzzaman et al 2013). Heat stress induced decrease in germination has also been documented in other plant species, viz. Moringa peregrine (Sardooi et al., 2019), Brassica juncea (Toosi et al., 2014), wheat (Hakim et al., 2012; Hossain and Teixeira da Silva 2012).
The tolerant as well as sensitivegenotypes exhibited an increase in speed of germination with an increase in temperature from 25°C to 30°C, but germination speed declined with a further increase in temperature (35°C). These results showed agreement with findings of Toosi et al. 2014 in Brassica juncea and Limma et al 2021 in Dalbergia spruceana, who also observed that increase in temperature upto 30°C caused an increase in speed of germination but speed of germination declined with the further increase in temperature to 35°C. With an increase in temperature, the activities of genes for production of GA are upregulated and thus speed of germination is hastened in Brassica (Finch-Savage and Leubner-Metzger 2006).
SVI I determines the fate of seedling establishment and incorporates the combined effects of germination and seedling growth. The higher vigour index attributed to higher germination rate and seedling length for heat tolerant cultivars. On the other hand, the sensitive ones had the lower germination rate and seedling length, ultimately had a lower vigour index. The decline in germination and in turn seedling vigour was observed due to high temperature stress during early seedling growth in wheat (Hasan et al., 2013). Singh et al. (2000) and Mound and Maghsoudi (2008) also observed decline in germination percentage and seeding growth under salinity stress in wheat. Reduction in the seedling dry weight and vigour was a result of reduction in seed reserve mobilization due to heat stress in wheat (Essemine et al., 2010).
High temperature increases the fluidity of membrane lipids causing the subsequent loss of cellular membrane stability indicating the vital role of MSI trait in response to heat stress in crops (Horvath et al., 2012). Dwivedi et al (2017) also observed significant decline in the MSI values in wheat seedlings due to high temperatures. Under heat stress, permeability enhancement and changes in cell differentiation and elongation as well as expansion of cellular membranes or thylakoids cause injuries to cellular processes such as photosynthesis and respiration via alterations in chloroplast proteins and the performance of ion channels (Kalaji et al. 2011, Bita and Gerats 2013). Rapid catabolism of proline may provide reducing equivalents that support mitochondrial oxidative phosphorylation and the generation of ATP for recovery from stress and repair of stress induced damage (Pandhare et al., 2009). Other than being an osmo-protectant, proline can act as a potent non-enzymatic antioxidant and a singlet oxygen quencher and scavenger of hydroxyl radicals (Rejeb et al2014). The greater proline accumulation in the shoots of barley genotypes under high temperature stress observed in this study is in agreement with the finding of Gosavi et al., (2014) who reported higher proline contents accumulated in wild sorghum genotypes under heat stress. Germplasm with higher proline content could be exploited in breeding programs for introgressing heat tolerance in cultivated crop varieties. At 35°C, tolerant genotypes BL1729 had the highest total phenol content while the lowest phenol content was observed in sensitive genotype BL1723. Hussain et al (2019) also observed an increase in total phenol content in barley cultivar cv. Jow-83 under heat stress. The increase in soluble phenolics has been correlated to enhanced phenylalanine ammonia-lyase activity and reduced activities of polyphenol oxidase and peroxidase (Rehman et al., 2006). Accumulation of phenolics has been reported to help in acclimation phenomenon against heat stress in tomato and watermelon (Rivero et al., 2001).Leaf chlorophyll, a vital pigment involved in light-harvesting and energy dissipating functions in plants has high sensitivity to heat stress (Brestic et al., 2016). High temperature induced decline in chlorophyll content has been reported in many plant species (Kumar et al.,2015).At 35°C, tolerant genotype- BL1784 had the highest total chlorophyll content, while the lowest chlorophyll content was observed in sensitive genotype-IBYT-E15. Hussain et al (2019) reported decrease in chlorophyll content under heat stress in Hordeum vulgare L. The reduced chlorophyll content under heat stress could be explained by the reduced expression of Chl synthase (CS) gene as reported by Saha et al., (2016) for C4 plant species Setaria viridis. Under heat stress activities of chlorophyllase and chlorophyll degrading peroxidase increase dramatically, resulting in dramatic reduction of chlorophyll levels (Wang et al., 2018).Principal component analysis has also been used by scientists in wheat seedling traits to determine diversity and grouping (Saima et al., 2012).