Influence of Water Availability on Photosynthetic Characteristics of Diverse Soybean (Glycine max) Genotypes.

Increasing water use efficiency (WUE) is one potential avenue to increase crop yields in water limited production environments. Variation in WUE has been demonstrated in a variety of important crop species, but only limited research has been conducted in soybean. Differences in WUE may arise due to stomatal and/or non-stomatal limitations for CO₂ uptake.  Plant introductions (PI) 416997 and 398198 have been identified to possess higher WUE in well watered and water deficit conditions compared to PIs 398965 and 567201D in both greenhouse and field environments. To examine the factors underlying differences in WUE, whether from stomatal factors, or non-stomatal factors (i.e mesophyll or enzyme limitations), or perhaps a combination of both, gas exchange measurements were conducted in the field in well watered and two varying water deficit treatments. No significant differences were observed for instantaneous water-use efficiency and internal CO₂ concentration (C_i) among the PIs. However, PIs 416997 and 398198 were observed to have a significantly lower stomatal conductance and transpiration rate, but only PI 416997 exhibited a significantly lower photosynthetic rate. Sensitivity analyses from A/C_i and light curves suggested that greater stomatal regulation may be the major factor underlying higher WUE. Over the course of a full growing season these differences likely led to a higher whole plant WUE, even though differences were difficult to detect from instantaneous leaf measurements.