Acclimation response of signalling molecules for high temperature stress on photosynthetic characteristics in rice genotypes

Abstract

Three signalling molecules viz. salicylic acid, calcium (calcium chloride) and brassinosteroid (24-epibrassinolide) were exogenously applied at pre-anthesis stage to investigate their role in ameliorating high temperature effects on CO2 assimilation, chlorophyll fluorescence, photosynthesis pigments and their correlation with grain yield in two contrasting rice genotypes (Pusa Sugandh 5 and Nerica L 44). Three different concentrations of salicylic acid (SA) (0.1, 0.25 and 0.5 mM), calcium (Ca) (10, 50 and 100 mM of CaCl2) and brassinosteroid (BR) (0.5, 1 and 1.5 ppm of 24-epibrassinolide) were applied thrice through foliar spray at pre-anthesis stage. After foliar spray, one set of plants was exposed to high temperature stress (36 ± 1.7 C) using temperature tunnel for a period of 2 weeks. High temperature significantly decreased the net photosynthetic rate (PN), photosynthetic water use efficiency (PWUE), stomatal conductance (gs), total chlorophyll content as well as relative efficiency of PSII photochemistry (Fv/Fm) in both the genotypes. Transpirational water loss (E) and internal CO2 concentration (Ci) increased under high temperature stress. Lower concentration of SA (SA1 and SA2) and Ca (Ca1 and Ca2) improved PWUE by enhancing PN and reducing E as well as Ci. Similarly, all three concentrations of BR showed alleviation of high temperature stress effects on photosynthetic activity. The amelioration effect of signalling molecules for high temperature stress effects were more pronounced in PS 5 compared to Nerica L-44. Regression analysis indicated involvement of these molecules to nullify the effects of high temperature stress on PN in relation with grain yield per plant in both rice genotypes and was significant in PS 5. The findings of the study conclude that application of above signalling molecules may negate the high temperature stress induced reductions in PSII efficiency, PWUE, chlorophyll pigments and gaseous exchange in rice at pre-anthesis stage.