Physiological response of soybean to shallow water table depths

The objective of this study was to determine the interrelationships among various factors governing physiological responses of soybean [Glycine max (L.) Merr.] leaves to different water table (WT) depths (0.15, 0.30, 0.45 and 0.60 m below the surface). Photosynthesis, stomatal conductance, transpiration, chlorophyll and leaf temperature were measured weekly on leaflets of soybean (CV. Hobbit 87) grown in lysimeters in controlled environment chambers. A week after the WT treatments were established, the photosynthesis and transpiration rates decreased significantly with the rise in WT depth. However, the results were not found consistent during the WT treatment period of this study. Mean photosynthetic rates and stomatal conductance for 0.15 m WT treatment were 19 and 10% lower than 0.60 m WT treatment. Effects of WT on chlorophyll content was not noticed until later part of the growing season, where significantly lower values of this parameter were observed for 0.15 m WT treatment than 0.60 m WT treatment. Leaf transpiration was highly responsive to WT depth, showing significant differences between WT treatments. Canopies under 0.15 m WT treatment transpired 16% less water compared with 0.45 m WT treatment due to excessive moisture stress. Shallow water table depths significantly reduced the grain yield compared with deeper WT depths. Highest soybean yield was obtained under 0.60 m WT depth and the lowest was for 0.15 m WT depth. Average yield obtained at 0.15 m WT was 48% lower than 0.60 m WT treatment. Relationships between different parameters were developed by various regression models. Linear positive relations were found between photosynthesis and leaf temperature under deeper WT depths. Results of these analysis revealed that leaf temperature was a good parameter to predict photosynthetic rates. The transpiration rate was found to be highly dependent on stomatal behaviour. Seed yield was also increased linearly with the increase in photosynthesis, stomatal conductance and transpiration rate. Overall results of this study show that plant physiological parameters could be used to evaluate the effectiveness of WT management practices for soybean growth.