Water is the most essential condition to plant life, but the water resource is becoming more and more shorter in the 21st century. Soybean, as one of the five major crops in the world, is mankind’s important sources of high-quality protein and edible oil. However, soybean is sensitive to water deficit and the water requirement of soybean is relatively high. Soybean in Chinese all three main soybean planting regions often suffer from drought to some degree during its growth season. So, the drought-resistant breeding and drought-resistant culturing of soybeans has been paid high attention since 1980’s. Photosynthesis is an important factor that determines soybean’s yield. At present, the research on soybean’s photosynthetic characteristic mainly concentrates on its relationship with the yield. But it is lacking in studies on soybean’s photosynthetic physioecological responses to drought and rewatering, which is helpful for constructing a theoretic basis for drought-tolerance planting and high-efficiency water use of soybean.
Field experiments were conducted with soybean (Glycine max，yudou 29), a mainly planted soybean variety in Henan Province to study the relationship between the leaves photosynthetic characters and other physioecological parameters under soil drying and rewatering treatments. Soil moisture was controlled by weighing method, and leave water potential was determined by HR-33T dew point microvolt thermometer. The diurnal course of leaf photosynthetic ratio and transpiration ratio of top fully spread leaves were determined by LI-6400 portable photosynthesis measure system in situ (begun at 8:00 a.m). Each treatment had 5 replicates.
It was showed that the dawn water potential of soybean leaves under drying treatment decreases little in comparison with well-watered treatments (CK), when soil moisture was higher than 47% of FWC (field water capacity). But when soil moisture was below 47% of FWC, the leave water potential decreased rapidly, which appeared a significant threshold value reaction. The threshold value of soybean dawn leaves water potential was nearly -1.02 MPa. Below this threshold value, the leave water potential and net photosynthesis ratio dropped rapidly. On the third day, the soil moisture under drought treatment was 47%, leaf water potential of the treatment was 7% lower than that of the CK, transpiration ratio and net photosynthesis ratio were respectively 67% and 90% that of the CK, the WUE was 35% higher than that of the CK. Above results indicated that the transpiration of soybean is more sensitive to drought than photosynthesis. This proved that it was possible to save water and increase yield simultaneously for soybean by cultivating measure. After rewatering, the leave water status of soybean was improved, the net photosynthesis ratio and transpiration ratio increased linearly, and Gs also recovered quickly. The results showed that soybean have a fast-growing characteristic after removing stress. However, whether there is a lagging effect of rewatering on soybean physiological traits and how important it is need further research.