LAUSR

Calcium (Ca2+) is involved in mediating anti-stress responses in plants through one or combination of biochemical processes. Thus, the major objective of present study was to assess the protective effect of exogenous Ca2+ (0, 2.5, 5.0, and 7.5 mM) on photosynthetic machinery of cv. Jaguar and SER-16 under heat stress (HS 25–45 °C for 48 h). The applied HS (45 °C) caused substantial reduction in net photosynthetic rate (Pn 73%), photosystem II quantum yield (ΦII 24%), linear electron flow (LEF 13%), and SPAD values (27%) with imbalancing of sugars metabolism and redox potential. At heat stress (40 °C), plants of both cultivars tried to acclimate by regulating leaf temperature through increased stomatal conductance (gs 36%), transpiration rate (E 60%) and by increase in non-photochemical quenching (NPQ 19%). In addition, Ca2+ pre-treatment protected the membrane from oxidative damage by up-regulating the enzymatic activities and down-regulation of malondialdehyde (MDA) accumulation and electrolyte leakage in plant leaf tissues. Pre-treatment with Ca2+ enhanced the accumulation of sugars (glucose, fructose, inositol, and raffinose) in both heat stressed bean plants (45 °C). In conclusion, cv. Jaguar had greater tolerance to HS than SER-16, which is associated with Ca2+-induced photo-protective effect on PSII, the balance between LEF and reactive oxygen species (ROS) generation with concomitant up-regulation of antioxidant enzymes, and increase in compatible solutes. Based on changes in physiological attributes, it is speculated that Ca2+ application might have mediated thermo-tolerance in plants via Ca2+-sensing and signaling pathway, which needs to be verified through molecular studies.