Diurnal Trends in Water Status, Transpiration, and Photosynthesis of Saltcedar

Abstract

Relative water content (RWC), water potential (P), and gas exchange were measured on saltcedar at the Bernardo, New Mexico, lysimeter site. RWC and s were closely correlated; but, water potential measurements, taken with a pressure bomb, were more convenient and reliable. RWC and r decreased sharply from sunup until about 0900, when minimum values of about -26 bars T or 80% RWC were reached. Water status then remained constant or improved slightly through late afternoon. Transpiration rates typically remained high until about noon and then began a steady, gradual decrease that continued throughout the afternoon. The data suggest that water stress may be a factor in initiating stomatal closure; however, transpiration continued to decline despite a constant or improved leaf water status. Maximum net photosynthetic rates occurred by 0900, and depressions throughout the remainder of the day were largely accounted for by increased leaf temperatures. Afternoon depressions in transpiration and photosynthesis occurred in twigs held at constant temperature and relative humidity, suggesting that a diurnal rhythm may be involved in control of gas exchange. Water status of plants growing on the lysimeters was comparable to that of plants in adjacent natural stands; gas exchange rates were slightly higher for the lysimeter-grown plants.