Water relations and non-structural carbohydrate responses to the combined effects of defoliation and progressive drought in a dioecious tree

Increases in drought frequency and insect defoliation are emerging threats that typically affect the plantation production of Populus yunnanensis , which is an endemic dioecious tree species in China. Nevertheless, it is difficult to predict which sex of P. yunnanensis will be more affected by the combined effects of water limitations and defoliation. In this study, we examined the combined effects of progressive drought and defoliation on water relations, growth, gas exchange and leaf carbohydrate reserves in P. yunnanensis females and males to test whether defoliation alters the effects of drought and and how this response varies between genders. The experimental design included two watering regimes (well-watered and drought-stressed) and two levels of defoliation (leaves untouched and 60% defoliated). Defoliation exacerbated drought stress in females by decreasing photosynthesis, dry mass accumulation and leaf non-structural carbohydrate (NSC) concentration. In contrast, the total biomass of males grown under drought combined with defoliation was not significantly altered when compared to those grown under drought alone. These results suggest that defoliation does not aggravate the drought effects on males. The males presented a high capacity to maintain foliar water status through stomatal control under drought conditions irrespective of defoliation treatment. The conservative water use strategy in males resulted in a gradual reduction in leaf water potential, gas exchange rates and NSC. Our data showed that the different responses of the females and males to drought and defoliation were influenced by plant water status and consequent effects on changes in carbon assimilation and leaf non-structural carbohydrate reserves.