The leaf photosynthetic rate, A, of higher plants is known to decrease as the relative water content and leaf water potential decrease. The most sensitive indicator of overall physiological state of plant is often stomata behavior. The controversy continues as to whether drought mainly limits photosynthesis through stomata closure or through metabolic impairment. Evidence that impaired ATP synthesis is the mainly factor limiting photosynthesis even under mild drought has further arose debate. Currently, many transgenic crops, mainly in wheat, have been achieved, and they exhibited a drought resistance to some extent. But they all have some shortage related to agronomical performance and/or development. These results show that systemic, deeper, and comprehensive understanding of physiological mechanism of crops under drought stresses is not enough.
A soybean variety widely cultivated in Henan Province, Glycine max cv. yudou29, was selected as the experimental material. The endeavor is aimed at providing a theoretical basis for high efficient water use in soybean cultivation, by studying the dynamic changes of photosynthesis and various physiological factors in the process of drought and rewatering.
The experiment began at the pre-flower stage of the soybean and lasted 9 days. The research shows that when SRWC (soil relative water content) of group t was higher than 46.5%, leaf water potential of Glycine max cv. yudou29 was unaffected by SRWC. Net photosynthesis rate of the leaf appeared to peak when SRWC was moderate: at 64.3%, net photosynthetic rate of group t was 11.2% higher than that of CK (P < 0.01). On the third day, when SRWC of group t was 46.5%, leaf water potential of this group was 7.2% lower than that of the CK（P＞0.05）, and net photosynthesis ratio of this group was 89.6% of that of the CK（P＜0.05), while gs rapidly dropped to 44.7% of that of the CK（P＜001). This shows that gs was more sensitive to soil water deficiency than photosynthesis and leaf water potential. After rewatering, the leaf water potential, net photosynthesis rate, gs and chlorophyll fluorescence parameters were all recovered quickly, and approached to the level of the CK by the end of the experiment. This indicates that the Glycine max cv. yudou29 leaf photosynthesis has the ability to rapidly recover from drought once the stress is released.