The alleviation of photosynthetic damage in tomato under drought and cold stress by high CO2 and melatonin

Abstract

The atmospheric CO2 concentration (a[CO2 ]) 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 CO2 concentration (e[CO2 ]) accompanied by exogenous melatonin on plants under drought and cold stresses remains unknown. Here, tomato plants were grown under a[CO2 ] and e[CO2 ], 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 (PN) was aggravated by a prolonged water deficit. The PN 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[CO2 ] enhanced PN of the plants under non-stressed conditions, and moderate drought and recovery but not severe drought. Stomatal conductance (gs) and the transpiration rate (E) was less inhibited by drought under e[CO2 ] than under a[CO2 ]. Tomato grown under e[CO2 ] had better leaf cooling than under a[CO2 ] when subjected to drought. Moreover, melatonin enhanced PN during recovery from drought and cold stress, and enhanced biomass accumulation in tomato under e[CO2 ]. The chlorophyll a content in plants treated with melatonin was higher than in non-treated plants under e[CO2 ] during cold stress. Our findings will improve the knowledge on plant responses to abiotic stresses in a future [CO2 ]-rich environment accompanied by exogenous melatonin.