Nitric oxide secures reproductive efficiency in heat-stressed lentil ( Lens culinaris Medik.) plants by enhancing the photosynthetic ability to improve yield traits

Rising temperatures, globally and locally, would be detrimental for cool- and summer-season food legumes, such as lentil ( Lens culinaris Medik.). Lentil is highly sensitive to supra-optimal temperatures (> 30 °C), particularly during reproductive growth, resulting in flower and pod losses. Thus, suitable strategies are needed to introduce heat tolerance in this legume. Here, we evaluated the efficacy of nitric oxide (NO)—applied as foliar treatment of 1 mM sodium nitroprusside (SNP), twice (one day before final exposure to high temperature, and again five days later)—on heat-stressed (32/20 °C) lentil genotypes, differing in heat sensitivity. As a result of heat stress, endogenous NO increased significantly in heat-tolerant genotypes (46–62% in leaves and 66–68% in anthers, relative to the respective controls), while it decreased in heat-sensitive (HS) genotypes (27–30% in leaves and 28–33% in anthers, relative to the respective controls). Foliar supplementation with SNP markedly increased endogenous NO in leaves and anthers of both the control and heat-treated plants. Heat stress significantly accelerated phenology, damaged membranes, chlorophyll, chlorophyll fluorescence, cellular viability, and decreased leaf water status, carbon fixing and assimilating ability, less so in plants treated with SNP. Heat stress plus SNP significantly improved carbon fixation (as RuBisCo activity) and assimilation ability, (as sucrose concentration (in leaves and anthers), sucrose synthase and vacuolar acid invertase activity, reducing sugars), as well as osmolyte accumulation (proline and glycine betaine) in leaves and anthers. Moreover, SNP-treated plants had significantly less oxidative damage—measured as malondialdehyde and hydrogen peroxide concentrations—in leaves and anthers, relative to the respective control. Reproductive function—assessed as pollen grain germination and viability, stigma receptivity, and ovular viability—decreased markedly in plants exposed to heat stress alone, more so in HS genotypes, but increased significantly with SNP treatment as a consequence of improved leaf and anther function, to significantly increase the pod and seed numbers in heat-stressed lentil plants, relative to heat-stress alone.