The Alleviation of Photosynthetic Damage in Tomato under Drought and Cold Stress by High CO(2) and Melatonin

The atmospheric CO(2) concentration (a[CO(2)]) is increasing at an unprecedented pace. Exogenous melatonin plays positive roles in the response of plants to abiotic stresses, including drought and cold. The effect of elevated CO(2) concentration (e[CO(2)]) accompanied by exogenous melatonin on plants under drought and cold stresses remains unknown. Here, tomato plants were grown under a[CO(2)] and e[CO(2)], with half of the plants pre-treated with melatonin. The plants were subsequently treated with drought stress followed by cold stress. The results showed that a decreased net photosynthetic rate (P(N)) was aggravated by a prolonged water deficit. The P(N) was partially restored after recovery from drought but stayed low under a successive cold stress. Starch content was downregulated by drought but upregulated by cold. The e[CO(2)] enhanced P(N) of the plants under non-stressed conditions, and moderate drought and recovery but not severe drought. Stomatal conductance (g(s)) and the transpiration rate (E) was less inhibited by drought under e[CO(2)] than under a[CO(2)]. Tomato grown under e[CO(2)] had better leaf cooling than under a[CO(2)] when subjected to drought. Moreover, melatonin enhanced P(N) during recovery from drought and cold stress, and enhanced biomass accumulation in tomato under e[CO(2)]. The chlorophyll a content in plants treated with melatonin was higher than in non-treated plants under e[CO(2)] during cold stress. Our findings will improve the knowledge on plant responses to abiotic stresses in a future [CO(2)]-rich environment accompanied by exogenous melatonin.