Extraction of Apoplastic Water during Pressure‐Volume Dehydrations 1

In interpreting pressure‐volume (P‐V) curves, it would be helpful to know the effects of relaxing some of the assumptions generally made in their analysis. More specifically, the consequences of apoplastic water loss during dehydration are of interest. The fraction of water acting osmotically was determined from pressure‐volume curves generated for water and drought‐stressed leaves from field‐grown soybean plants [ Glycine max (L.) Merr. ‘Wilkin’]. All plants showed a seasonal increase in the apparent fraction of water acting osmotically. Occasionally, particularly in the stressed leaves, the osmotic fraction was higher than previously reported and in some cases > 1. During the generation of P‐V curves, the volume of apoplastic water is generally assumed to remain constant as plant tissue dehydrates. A recent analysis of water release curves allowed the volume of apoplastic water to vary during dehydration, but the form of the resulting equation relating water potential to relative water content did not apply directly to P‐V curves. This equation was transformed into the form generally used to interpret the curves. In this form the equation suggests that the anomalously high observed osmotic fractions may be due to the loss of apoplastic water during dehydration.