Grafting Onto Artemisia annua Improves Drought Tolerance in Chrysanthemum by Enhancing Photosynthetic Capacity

Drought stress drastically influences the yield and quality of chrysanthemums, and thus grafting has been widely used to improve tolerance to biotic and abiotic stresses. To explore the mechanisms underlying improvements in drought resistance afforded by grafting, we investigated the changes in growth, gas exchange, ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) expression and activity, sugar components, and the relative expression of photosynthesis-related genes of chrysanthemum (Chrysanthemum morifolium Ramat. 'Hangbaiju') plants grafted onto Artemisia annua rootstocks under drought stress by withholding water for 6 d. The results revealed that the growth declines in the grafted chrysanthemums were relatively lower than those of the non-grafted plants under drought stress, and net photosynthetic rate, stomatal conductance, water use efficiency, and transpiration rate in the non-grafted chrysanthemums were significantly decreased. Moreover, the intercellular CO2 concentrations were significantly increased compared with the grafted plants at 5 and 6 d following drought stress. The grafted plants exhibited higher relative expression of the CmrbcL, CmrbcS, CmpsaB, and Cmcab genes, as well as higher Rubisco activity and chlorophyll content under the drought treatment. Sugar accumulation also increased under drought stress, particularly in the non-grafted plants. This result suggested that non-grafted chrysanthemums were less able to resist dehydration, and repressed the genes encoding the expression of photosynthetic components. In conclusion, using A. annua rootstock could alleviate drought stress in chrysanthemums by improving gas exchange capacity and maintaining CmrbcL, CmrbcS, Cmcab, and CmpsaB gene expression, thereby increasing Rubisco activity and improving photosynthetic performance.