Alleviation of Terminal Heat Stress in Wheat (Triticum aestivum L.) Through Potassium and Selenium Nutrition

Abstract

Coincidence of high temperature at the terminal phenological stages of the wheat crop is a prime constraint to reach full yield potential in Pakistan. The present research work was conducted to determine the thermo-sensitivity of Pakistani wheat genotypes and alleviation of negative implications of heat through exogenous application of potassium and selenium. All research work was performed at the Agronomic Research Area, University of Agriculture Faisalabad, Pakistan from November 2014 to May 2017. In the first year, wheat genotypes were screened for terminal heat tolerance under field conditions. The experiment was laid out in a Randomized Complete Block Design (RCBD) in a split plot arrangement and was replicated 4 times. Treatments were comprised of heat stress in main plots viz. H0 = no heat imposition; H1 = Heat imposition from complete emergence of spike to grain filling initiation (Feekes Scale = 10.50 to 11.00) and wheat genotypes in sub plots viz. Punjab-2011, AARI-2011, Galaxy-2013, Millat-2011, Aas-2011, Fareed-2006, Chakwal-50, Mairaj-2008, Pakistan-2013, NIBGE-NIAB-1 and Kohistan-97. Imposition of heat stress deleteriously impacted the metabolism of all genotypes. The synthesis of antioxidants and osmo-protectants were enhanced in genotypes ‘AAS-2011, Chakwal-50 and Mairaj-2008’ under the high temperature environment compared to ‘no heat stress’. While, in all other genotypes biosynthesis of antioxidants and osmo-protectants was suppressed under heat compared to control. Likewise, adverse impacts of heat on spike growth, stay green trait, grain yield and yield components were relatively lesser in genotypes ‘AAS-2011, Chakwal-50 and Mairaj-2008’ than other genotypes. Statistically similar and relatively more grain yields compared to other genotypes were recorded for ‘Aas-2011’ (3.71 t ha-1), ‘Chakwal-50 (3.36 t ha-1) and ‘Mairaj-2008 (3.04 t ha-1) under heat stress. In the second year, two independent field experiments were conducted with the objective of mitigating heat stress using potassium and selenium as beneficial nutrients. The experimental design for both experiments was randomized complete block design (RCBD) with split plot arrangement having three replications. In both experiments, the main plot factor was comprised of three heat stress treatments viz. H0 = No heat imposition; H1 = Heat stress imposition from complete emergence of spike to grain filling initiation (Feekes scale = 10.50 to 11.0); H2 = Heat stress imposition from flowering initiation to grain filling initiation (Feekes scale = 10.5.1 to 11.0). In the second experiment, potassium was supplied via foliar application in sub plots at 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 to mitigate heat stress. In the third experiment, selenium was foliar applied at Se0 = Control/ water spray; Se25 = 25 mg L-1; Se50 = 50 mg L-1; Se75 = 75 mg L-1 and Se100 = 100 mg L-1 to alleviate heat stress. During the third year, heat stress mitigating experiments were repeated as described in the second year. Negative implications of heat were more pronounced under ‘heat from spike to grain filling’ compared to ‘heat from flowering to grain filling’. Grain yield in second experiment was decreased by 42-45% under ‘heat from spike to grain filling’ and 25-31% under ‘heat from flowering to grain filling’ compared to ‘no heat stress’. While, in third experiment, ‘heat from spike to grain filling’ and ‘heat from flowering to grain filling’ caused decrease in grain yield compared to ‘no heat stress’ by 43-44% and 33-36%, respectively. Whereas, varying concentrations of foliar potassium and selenium differed significantly from each other and remarkably improved response variables compared to control/water spray. Application of potassium at 45 and 60 g L-1 and selenium at 75 and 100 mg L-1 depicted statistically similar and relatively more grain yield, yield components, spike growth attributes, chlorophyll content and quality attributes compared to other concentrations under all treatments of heat stress. Likewise, statistically alike and comparatively more antioxidants, osmo-protectants and water relations attributes and statistically similar and relatively lesser malondialdehyde were observed with 45 and 60 g L-1 foliar potassium and 75 and 100 mg L-1 foliar selenium under ‘no heat stress’. However, application of 60 g L-1 potassium and 100 mg L-1 selenium showed significantly more antioxidants, osmo-protectants and water relations attributes and significantly lesser malondialdehyde under ‘heat from spike to grain filling’ and ‘heat from flowering to grain filling’. Conclusively, genotypes ‘AAS-2011, Chakwal-50 and Mairaj-2008’ displayed terminal heat tolerance while genotypes ‘Fareed-2006’ and ‘Punjab-2011’ exhibited medium tolerance. In contrast, all other genotypes tested did not produce remarkable responses under heat and were characterized as terminal heat susceptible based on recorded parameters. Under ‘no heat stress’ application of exogenous potassium at 45 g L-1 and selenium at 75 mg L-1 effectively alleviated the adverse impacts of heat. Whereas, application of potassium at 60 g L-1 and selenium at 100 mg L-1 provided more promising morphological and biochemical responses under ‘heat from spike to grain filling and ‘heat from flowering to grain filling’. While, foliar applied potassium and selenium proved more important under heat treatments compared to ambient conditions. Moreover, biochemical attributes modulated regulations in growth, yield components and grain yield were significant under varying temperatures.