Soybean petiole ureide response to water deficits and decreased transpiration

The sensitivity of [N.sub.2] fixation to drought is greater than many other physiological processes in soybean [Glycine max L. (Merr.)], and, therefore, maintaining [N.sub.2] fixation may be a key process for improving soybean drought tolerance. The ureides, allantoin and allantoate, are the final products of [N.sub.2] fixation in soybean, and ureide concentration in petioles has been correlated wit h nitrogen accumulation in well-watered plants. Petiole ureide concentration may, therefore, be used as a screening tool provided that it is closely associated with [N.sub.2] fixation during drought. The objectives of this study were to determine the relationship between petiole ureides and nodule activity during drought, and to determine the relationship between transpiration rate and petiole ureide concentration in well-watered plants. Soybean was grown in the greenhouse until the V6 growth stage when water was withheld for either 0, 4, 5, or 7 d to create several stress levels on the day of harvest. Plants were harvested at 0700, 1400, and 1900 h. In general, petiole ureide concentration increased with stress severity, and was negatively associated with nitrogenase activity at 1400 h (r = - [0.62.sup.**]) and 1900 h (r = - [0.49.sup.**]) but not at 0700 h (r = - 0.34). In well-watered plants, treatments that decreased transpiration also resulted in increased petiole ureides. The increase in petiole ureide concentration during drought was concluded to be due partially to low transpiration. Petiole ureide concentration holds promise as an indicator of [N.sub.2] fixation during drought, but the consistency of the relationship between petiole ureide concentration and [N.sub.2] fixation across genotypes must be evaluated.