Evaluation of Methods for Estimating Transpiration Response to Soil Drying for Container-Grown Plants

Physiological responses in plants to drought at various fractions of transpirable soil water (FTSW) are often presented relative to well-watered controls. Theoretically, this method allows for comparison of sensitivity among genotypes for various physiological processes and across experiments. It can be difficult to accurately estimate FTSW for measurements occurring over a relatively long period of time, such as gravimetric measurement of transpiration. This research evaluated the impact of determining the FTSW threshold at which transpiration begins to decline based on pot weights made at the beginning, ending, or average of daily transpiration periods. Pots were weighed once or three times daily during an 8-d soil-drying cycle for four soybean [Glycine max (L.) Merr.]genotypes grown in 2-L pots. There were large artefactual differences in FTSW breakpoints among cultivars when either beginning or ending pot weights were used to estimate daily FTSW. In contrast, the estimated FTSW breakpoint was between 0.36 and 0.41 for all genotypes when the average of the beginning and ending interval pot weights was used to calculate FTSW, whether pots were weighed once or three times daily.