Improving heat tolerance is a targeted breeding trait for wheat improvement in mega-environments (MEs), especially ME1 and ME5 where the crop may also be exposed to boron (B) deficiency. Temperature > 25°C during reproductive crop stages disrupt physiological processes, for instance, anthesis induced restricted embryo development reduces pollen viability, increased anther sterility produce less grain numbers and shortened growth cycle causes reduced grain yields. Nonetheless, the genotypic variation exists in wheat germplasm both for B efficiency and heat tolerance simultaneously. Fang-60 (PIIFDIIPI/MZ/3/MXP) as boron efficient and heat tolerant is an ideal check cultivar. We compared local wheat cultivars (Bhakkar Star-19, Galaxy-13, AARI-11, Millet-11 and Jauhar-16) with Fang-60 under optimal B soil condition and terminal heat stress (±5°C) started from booting stage until maturity. Genotypes differed for morpho-physiological traits under heat stress. Cultivars Galaxy-11 and Jauhar-16 expressed similar grain numbers, increased 100-grain weight (14.5-66.5%) and grain set index compared to Fang-60 associated with increased SPAD-chlorophyll (3%) at anthesis, membrane thermo-stability (33%) and reduced canopy temperature under heat stress. Further, high yielding cultivars (Galaxy-11 and Jauhar-16 vs Fang-60) were phenotyped for pollen viability, flag leaf B contents and morpho-physiological traits when grown with (+B) and without (-B) under heat stress. Earlier heading, anthesis and physiological maturity was observed under heat stress while Jauhar-11 was early maturing compared to Fang-60 and Galaxy-13 with delayed maturity. Heat stress reduced plant dry biomass (20%) and spike length that improved with +B (26.5%) respectively while no difference was observed among genotypes. Heat stress reduced spikelets and grain number per spike (13.57%) which improved with +B supply and among genotypes increased grain numbers was observed for Jauhar-11 (11%) and Galaxy-13 (7%) compared to Fang-60. Likely, heat stress reduced 100-grain weight which improved (3%) by +B while among genotypes, Galaxy-13 produced similar 100-seed yield to Fang-13. High grain yield in Galaxy-13 was associated with reduced canopy temperature during anthesis and grain filling, highest SPAD-chlorophyll contents under +B and heat stress compared to –B condition during grain filling but less than Fang-60. Likely, Galaxy-13 also expressed highest cell membrane thermo-stability (13%) with +B compared to -B condition under heat stress. Among genotypes response varied and Jauhar-16 expressed more pollen viability (2.7%) while Galaxy-13 expressed highest (24%) stem water soluble carbohydrates compared to Fang-60 with +B under heat stress. Nonetheless, reduced spikelets number per spike increased grain number under heat stress was associated with improved pollen viability (13%) and stem water soluble carbohydrates (15%) by +B addition. The improved pollen viability was strongly correlated (R2=0.69) with high flag leaf B contents at anthesis. In crux, Galaxy-13 was found as heat tolerant and B-efficient genotype can be used for pre-breeding evaluation of spring wheat germplasm for B efficiency and heat tolerance.
