Short-term Thermal Acclimation Increases Ribulose 1, 5 Bisphosphate Carboxylase/Oxygenase Activity and Content and Enhances Heat Stress Tolerance of Photosynthesis in Cucumber

Abstract

Future global temperature change, with predicted 1.5– 5.8 °C increases in temperatures by 2100, will cause increased heat stress to plants and create threats to agricultural production (Rosenzweig et al., 2001). The increasing threat of temperature change is already having a substantial impact on agricultural production worldwide as heat waves cause significant yield losses posing great risks for future food security for humankind (Christensen and Christensen, 2007). The unfavorable effects of heat stress can be mitigated by developing crop plants with improved thermotolerance using an assortment of genetic approaches. For this reason, it is crucial to have a thorough understanding of the physiological responses of plants to high temperatures and their mechanisms of heat tolerance, as well as to formulate possible strategies for improving crop thermotolerance. Photosynthesis is one of the most sensitive physiological responses in plants to heat stress. Thus, it is important to maintain high photosynthetic activity for heat stress tolerance in plants (Berry and Björkman, 1980). When plants are subjected to high temperatures, carbon dioxide (CO2) fixation, oxygen (O2) evolution, and photophosphorylation are restrained rapidly (Berry and Björkman, 1980). The limit of CO2 fixation by high temperature occurs simultaneously with the inactivation of ribulose 1, 5 bisphosphate (RuBP) carboxylase/oxygenase (RuBisCO) activase, which leads to the activation of RuBisCO (Feller et al., 1998; Salvucci et al., 2004). In the thylakoid membrane, the most sensitive component element to high temperature is photosystem II (PSII). Heat stress may suppress the light-absorption capacity of the plant owing to the dissolution of the O2 evolution apparatus (Mamedov et al., 1993; Nash, et al., 1985; Tompson et al., 1989). Many studies have shown that the instantaneous response of leaf carbon exchange to temperature depends on the temperature experienced by the plant over longer time periods, a response termed temperature acclimation (Atkin et al., 2005; Atkin and Tjoelker, 2003; Berry and Björkman, 1980; Smith and Dukes, 2013; Way and Yamori, 2014; Yamori et al., 2014). Temperature acclimation can be observed through a change in the parameters that define the instantaneous temperature response curve as a result of changes previously experienced by the plant or the acclimated temperature (Atkin and Tjoelker, 2003). Hikosaka et al. (2006) indicated that changes in the photosynthesis-temperature curve with long-term thermal acclimation are attributable to four factors: intercellular CO2 concentration, activation energy of the maximum rate of RuBP carboxylation (Vcmax), activation energy of the rate of RuBP regeneration (Jmax), and the ratio of Jmax to Vcmax. Of these, the activation energy of Vcmax may be the most important factor that influences thermal acclimation. Smith and Dukes (2017) also indicated that “fast mechanism” of thermal acclimation may be attributable to