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

The atmospheric CO concentration (a[CO]) 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 concentration (e[CO]) accompanied by exogenous melatonin on plants under drought and cold stresses remains unknown. Here, tomato plants were grown under a[CO] and e[CO], 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) was aggravated by a prolonged water deficit. The P 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] enhanced P of the plants under non-stressed conditions, and moderate drought and recovery but not severe drought. Stomatal conductance (g) and the transpiration rate (E) was less inhibited by drought under e[CO] than under a[CO]. Tomato grown under e[CO] had better leaf cooling than under a[CO] when subjected to drought. Moreover, melatonin enhanced P during recovery from drought and cold stress, and enhanced biomass accumulation in tomato under e[CO]. The chlorophyll content in plants treated with melatonin was higher than in non-treated plants under e[CO] during cold stress. Our findings will improve the knowledge on plant responses to abiotic stresses in a future [CO]-rich environment accompanied by exogenous melatonin.