Plant water deficit or plant water stress produced by drought will disturb plant internal physiological processes when it reaches a critical level. Photosynthesis is especially sensitive to water stress,as it is affected by decreased stomatal conductance and chloroplast destruction when soil relative water content declines.Usually, plants need an inductance protection mechanism or a repair mechanism, or both, to acclimate toenvironmental adversities.Most researchers have reported that plants, in response to summer water stress, depend on a combination of morphologic and physiological adjustments, such as deepening the root system,decreasing the leaf transpiration rate ,improving the water use efficiency, accumulating osmotic materials, and improving dehydration tolerance .In recent years, many studies focused on plant photo-protective mechanisms under both intense radiation and drought stress ,but there have been fewer studies on chloroplast ultrastructure variations of leaf and stem under extreme drought stress, and almost none on the eco-physiological mechanisms of how xerophytes survive after all leaves have fallen in a severe drought event and then revive in a subsequent rehydration process.

Caragana korshinskii is a perennial subshrub with high drought tolerance. It can survive in multiple environmental stresses, including low water availability, extreme temperature fluctuation, high radiation, and nutrient deficit. C.korshinskii is widely distributed in desert and semi-desert zones and the loess plateau in northwestern China .Water resource is the main factor influencing its growth. There are many studies on the classification,community distribution, biological and ecological characteristics,and species diversity of C. korshinskii .In this research the researchers study its stress resistance mechanisms and drought adaptation strategies. In the experiments, the leaves of C. korshinskii wilted and then fell off under induced extreme drought conditions, followed by cessation of plant growth. In the subsequent induced rehydration process,the plants recovered and grew new leaves. Through studying the plant’s response strategies to drought stress on the individual level, our intent was to improve our understanding of the mechanisms with which plants adapt to arid environments.

In order to study the eco-physiological mechanisms of C. korshinskii adaptation to extreme drought stress, the researchers of careeri 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 rehydration 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 ultrastructure and chloroplast configuration in leaves were irreversibly disturbed, as manifested 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 ultrastructure 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 extreme drought stress, and maintains chloroplast integrity and a considerable amount of chlorophyll to enable a rapid recovery of photosynthesis under the rehydration process.

Transmission electron microscopy of mesophyll cells in leaves and stems of C. korshinskii. (a) A leaf after 3 days of dehydration;(b) a leaf after 42 days of dehydration; (c) mesophyll cells in leaves begin to break off; (d) mesophyll cell of a new-growth leaf;(e) mesophyll cell in stem after 3 days of dehydration;(f) mesophyll cell in stem after 53 days of dehydration. Cp: chloroplast;N: nucleolus; V: vacuolar; S: starch grain.

Transmission electron microscopy of chloroplasts in leaves and stems of C. korshinskii. (a) Leaf chloroplasts after 3 days of dehydration (b) leaf chloroplasts after 42 days of dehydration; (c) chloroplasts in leaf begin to break off;(d) chloroplasts in new-growth leaf; (e) stem mesophyll cell after 3 days of dehydration;(f) stem mesophyll cell after 53 days of dehydration.M: mitochondria; S: starch grain.