Transpiration and Photosynthesis in Saltcedar

Abstract

Factors controlling transpiration and photosynthesis of saltcedar were investigated in the field near Bernardo, New Mexico. Transpiration rates were similar to those for several herbaceous species, but photosynthesis and water use efficiency were significantly lower in saltcedar. Photosynthesis was light saturated at an irradiance equal to 44% of full sunlight, while the stomata were apparently fully open at light levels greater than one-third full sunlight. Optimum leaf temperatures for photosynthesis were between 23° and 28 °C, considerably lower than typical daytime ambient temperatures. Photosynthesis was reduced about 20% at 35 °C. Stomatal resistance increased linearly with increases in leaf temperature between 14° and 50 °C, with relative humidity held constant. The increase in stomatal resistance could have been caused by direct effects of temperature on the stomata, by increases in the absolute humidity gradient from leaf to air, or by both. Increased stomatal resistance at high temperatures and low relative humidities would account for observed afternoon depressions in transpiration and photosynthesis and increases in canopy resistance. Estimates of stomatal resistance for twigs in full sunlight ranged from 2 to 6 sec cm⁻¹, with most values falling between 3 and 5 sec cm-, when leaves were at 30 °C.