Soybean Transpiration and Leaf Expansion Recovery from Soil Drying.

Drought is a common problem in most areas of soybean production, leading to the continual pursuit of drought tolerance traits.  Under episodic drought, plants are most often exposed to repeated cycles of drying and re-watering.  Transpiration (T) and leaf expansion (LE) are two physiological processes directly related to plant productivity and have been characterized during soil drying.  In this study we examine T and LE in five genotypes of soybean during a five-day recovery phase after plants were first subjected to complete soil drying under three different environmental conditions.  In one greenhouse and two growth chamber experiments, water-deficit stress was imposed over a period of 12-15 days by controlling irrigation until T declined to minimal rates and subsequently re-watered and kept well-watered for five days.  Measurements of whole plant T by a gravimetric approach and leaf expansion by a non-destructive approach were taken during the drying and recovery phases.  Environmental conditions appeared to modify T and LE responses during the drying phase.  Limited genotypic variation in T and LE was detected during the soil drying phase.  Maximum recovery of T and LE in all genotypes occurred on the the third day, and on day one or two in the recovery phase, respectively.  Genotypic differences during the recovery phase were detected in two experiments.  However, relative ranking of genotypes was inconsistent from one experiment to the other.  Genotypic differences for LE recovery were detected in one experiment.