Eco-physiological mechanisms of Caragana korshinskii Kom. adaptation to extreme drought stress: Leaf abscission and maintaining stem chloroplast integrity

In order to study the eco-physiological mechanisms of C. korshinskii adaptation to extreme drought stress, we investigated the variations of water content in soil, leaves, and stems, the chlorophyll a and b and the carotenoid content in leaves and stems, as well as changes of chloroplast ultrastructure in 2-year-old C. korshinskii specimens during a progressive soil drought process （by ceasing watering until all leaves were shed） and a subsequent rehydmtion process. During the dehydration process, the chlorophyll a and b and carotenoid contents in the leaves decreased, as did the carotenoid content in the stems. During the 4-day rehydration process, the chlorophyll a and b and carotenoid contents in the leaves and stems increased and gradually returned to normal levels. During ongoing drought stress, chloroplasts in the leaves broke away from cell walls and appeared in the center of cells. Under severe drought stress, the mesophyll ultrastmcture and chloroplast configuration in leaves were irreversibly disturbed, as mani- fested by the inner and outer membranes being destroyed; the thylakoid system disintegrated, the starch grain disappeared, and parts of cell tissue were dismantled into debris. However, the mesophyll ultrasWacture and chloroplast configuration in the stems remained complete. This indicates that C. korshinskii utilizes leaf abscission to reduce the surface area to avoid damage from ex- treme drought stress, and maintains chloroplast integrity and a considerable amount of chlorophyll to enable a rapid recovery of photosynthesis under the rehydration process.

A comprehensive framework for seasonal controls of leaf abscission and productivity in evergreen broadleaved tropical and subtropical forests

Relationships among productivity,leaf phenology,and seasonal variation in moisture and light availability are poorly understood for evergreen broadleaved tropical/subtropical forests,which contribute 25% of terrestrial productivity.On the one hand,as moisture availability declines,trees shed leaves to reduce transpiration and the risk of hydraulic failure.On the other hand,increases in light availability promote the replacement of senescent leaves to increase productivity.Here,we provide a comprehensive framework that relates the seasonality of climate,leaf abscission,and leaf productivity across the evergreen broadleaved tropical/subtropical forest biome.The seasonal correlation between rainfall and light availability varies from strongly negative to strongly positive across the tropics and maps onto the seasonal correlation between litterfall mass and productivity for 68 forests.Where rainfall and light covary positively,litterfall and productivity also covary positively and are always greater in the wetter sunnier season.Where rainfall and light covary negatively,litterfall and productivity are always greater in the drier and sunnier season if moisture supplies remain adequate;otherwise productivity is smaller in the drier sunnier season.This framework will improve the representation of tropical/subtropical forests in Earth system models and suggests how phenology and productivity will change as climate change alters the seasonality of cloud cover and rainfall across tropical/subtropical forests.