Photosynthesis of Digitaria ciliaris during repeated soil drought and rewatering

The ability of psammophyte photosynthesis to withstand and recover from severe droughts is crucial for vegetation sta- bility in semi-arid sandy lands. The responses of gas exchange and chlorophyll fluorescence of an annual grass, Digitaria ciliaris, were measured through three soil drought and rewatering cycles. Results showed that the net photosynthesis rate （P,） decreased by 92%, 95%, and 63% at end of the three drought periods, respectively, water use efficiency （WUE） decreased by 67%, 54%, and 48%, while the constant intercellular CO2 concentration （Ci） increased by 1.08, 0.88, and 0.45 times. During those three cycles, the trapping probability with no dark adaptation （Fv＇/Fm＇） decreased by 55%, 51%, and 9%, the electron transport per cross section （ET0＇/CS0＇） decreased by 63%0, 42%, and 18%, and the dissipation per cross section （DI0＇/CS0＇） increased by 97%, 96%, and 21%. These results indicated that D. ciliaris was subjected to photoinhi- bition and some non-stomatal limitation of photosynthesis under drought. However, after four days of rewatering, its photosynthetic characteristics were restored to control values. This capability to recover from drought may contribute to making the plant＇s use of water as efficient as possible. Furthermore, the photosynthesis decreased more slowly in the subsequent drought cycles than in the first cycle, allowing D. ciliaris to enhance its future drought tolerance after drought hardening. Thus, it acclimatizes itself to repeated soil drought.

The Stimulating Effects of Rewatering on Leaf Area of Winter Wheat Suffering Water Stress

After water stress at various levels and durations at different growth stages, rewatering could greatly stimulate the leaf area development of winter wheat. The results showed that the stimulation effect changed with water stress time, degree and duration. Rewatering under earlier stress had greater stimulation effect on leaf area than that under later stress. Higher stimulation effect was observed under severe water stress than that under moderate stress. Longer duration of stress resulted in low stimulation effect. In spite of the greater stimulation effect under severe and longer stress, the final leaf area in these situations was lower than that under moderate stress and shorter duration. Whenever the stress occurred, the stimulating effect was due to the increase of the leaf area of the tillers. Once the leaf on the main stem emerged during stress period, rewatering had no effect on its size, and consequently its leaf area. The stimulation of rewateirng on leaf area contributed to the final grain yield by 45% under moderate stress, and 67% under severe stress. Although the stimulation partly compensated for the loss during stress, the final leaf area and the grain yield could not reach the level without water stress.

Effect of Drought Stress and Rewatering on the Photosynthetic Physiological Characteristics of Leaves and Bark of‘84K’Poplar

To study the difference of photosynthetic physiological indexes between leaf and bark of‘84K’poplar under drought stress and rewatering,the cutting seedlings of‘84K’poplar were treated with drought and rewatering for 7 days.The content of chlorophyll,gas exchange parameters and chlorophyll fl uorescence parameters in leaves and bark were measured.The results showed that the total chlorophyll content of bark was obviously lower than that of leaves.Compared with the control,the difference of chlorophyll(a+b)between drought and rewatering treatment was not signifi cant.The chlorophyll(a+b)of drought treatment decreased signifi cantly by 13.48%,and rewatering treatment could restore to the control level.The chlorophyll a/b of the bark was higher than that of the leaves,which indicated that the bark had a strong ability to utilize the weak light.Under drought stress,the actual photosynthetic rate of bark and leaves decreased by 46.71%and 67.59%respectively,and recovered to 74.45%and 57.55%after rewatering.Drought treatment had no signifi cant effect on chlorophyll fl uorescence parameters of bark,but had signifi cant effect on chlorophyll fl uorescence parameters of leaves.The effects of drought stress on gas exchange and chlorophyll fl uorescence of leaves were obvious,but the bark was not sensitive to drought stress.After rewatering,most of the photosynthetic parameters of leaves restored to the control level,showing a superior recovery ability.