Heat-stress-induced reproductive failures in chickpea (Cicer arietinum) are associated with impaired sucrose metabolism in leaves and anthers

Chickpea (Cicer arietinum L.), in its reproductive stage, is sensitive to heat stress (32/20°C or higher as day/night temperatures) with consequent substantial loss of potential yields at high temperatures. The physiological mechanisms associated with reproductive failures have not been established: they constitute the basis of this study. Here, we initially screened a large core-collection of chickpea against heat stress and identified two heat-tolerant (ICC15614, ICCV. 92944) and two heat-sensitive (ICC10685, ICC5912) genotypes. These four genotypes were sown during the normal time of sowing (November-March) and also late (February-April) to expose them to heat stress during reproductive stage (>32/20°C). The genotypes were assessed for damage by heat stress to the leaves and reproductive organs using various indicators of stress injury and reproductive function. In the heat-stressed plants, phenology accelerated as days to flowering and podding, and biomass decreased significantly. The significant reduction in pod set (%) was associated with reduced pollen viability, pollen load, pollen germination (in vivo and in vitro) and stigma receptivity in all four genotypes. Heat stress inhibited pollen function more in the sensitive genotypes than in the tolerant ones, and consequently showed significantly less pod set. Heat stress significantly reduced stomatal conductance, leaf water content, chlorophyll, membrane integrity and photochemical efficiency with a larger effect on heat-sensitive genotypes. Rubisco (carbon-fixing enzyme) along with sucrose phosphate synthase (SPS) and sucrose synthase (SS) (sucrose-synthesising enzymes) decreased significantly in leaves due to heat stress leading to reduced sucrose content. Invertase, a sucrose-cleaving enzyme, was also inhibited along with SPS and SS. The inhibition of these enzymes was significantly greater in the heat-sensitive genotypes. Concurrently, the anthers of these genotypes had significantly less SPS and SS activity and thus, sucrose content. As a result, pollen had considerably lower sucrose levels, resulting in reduced pollen function, impaired fertilisation and poor pod set in heat-sensitive genotypes.