Exogenous Potassium–Instigated Biochemical Regulations Confer Terminal Heat Tolerance in Wheat

Heat stress at spike initiation and flowering is chief constraint hampering the accomplishment of yield potential in wheat crop. The present study was aimed at comparing the thermo-sensitivity of terminal stages, optimizing exogenous potassium to alleviate impacts of heat and to explore biochemical attributes triggered regulations in morphological attributes of wheat. The experiment was performed at the University of Agriculture Faisalabad, Pakistan during winter 2015–2016 and 2016–2017. The experiment was laid out in Randomized Complete Block Design under split arrangement and replicated thrice. The treatments comprised of heat stress in main plots viz. H0 = No heat imposition; H1 = Heat stress imposition from complete emergence of spike to grain filling initiation and H2 = Heat stress imposition from flowering initiation to grain filling initiation and exogenous potassium (K) concentrations in split plots viz. K0 = Control (water spray); K15 = 15 g L−1; K30 = 30 g L−1; K45 = 45 g L−1, and K60 = 60 g L−1. Under “H0,” statistically similar and relatively more activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were quantified with 45 and 60 g L−1 foliar “K.” Contrarily, significantly more SOD, POD, and CAT contents were recorded with 60 g L−1 exogenous potassium under “H1” and “H2” while significantly lesser chlorophyll a, b contents and grain yield were recorded under “H1” compared to “H0” and “H2,” whereas statistically similar and significantly more grain-filling rate (GFR) and statistically similar and significantly lesser grain filling duration (GFD) were observed under “H1” compared to “H0” “H2.” Exogenous application of K at 45 and 60 g L−1 depicted statistically similar and comparatively more chlorophyll a, b contents, GFR, GFD, and grain yield compared to control. Moreover, strong, positive, and significant association of antioxidants and chlorophyll contents with morphological attributes was observed. Conclusively, “H1” proved more deleterious compared to “H0” and “H2” and exogenous K at 60 g L−1 effectively alleviated adversities of heat. Moreover, K-mediated regulations in biochemical attributes improved morphological attributes of heat-stressed wheat crop.