Transpiration Rate and Water Use Effeciency of Soybean Leaves Adapted to Different CO2 Environments1

Abstract

Gas exchange rates were measured during podfill on leaflets of soybean [Glycine max (L.) Merr. ‘Bragg’] plants grown in 330 (ambient) and 660 (high) μmol CO2−1 environments in outdoor growth chambers and rooted in a reconstituted profile of Arredondo fine sand (loamy, siliceous, hyperthermic Grossarenic Plaleudult). Carbon exchange rates (CER), transpiration rates (TR), leaf resistance (rL) and leaf temperature (TL) measured to investigate their diurnal changes. These parameters were also measured during midday periods at various CO2 concentrations (90‐990 μmol CO2 mol−1) to study the effects of adaptation to ambient and high CO2, environments on the subsequent response to CO2 levels. The primary objective was to determine CO2 effects on leaf water use and water‐use efficiency, an important question as global CO2 continues to rise. Diurnal TR of leaflets grown and measured at high CO2 levels was not different from TR of leaflets grown and measured at ambient CO2. Leaf temperature was approximately 1.5°C higher for the leaflets at the high CO2 concentration; also, rL was higher throughout the day on these leaflets than rL of those at ambient CO2 levels. Water use efficiency (WUE = CER/TRf) or leaflets at high CO2 was about twice the WUE of leaflets grown and measured at ambient CO2. Transpiration rates of leaflets adapted to ambient or high CO2 concentrations were not significantly affected by short‐term CO2 changes. However, regardless of CO2 level of adaptation, TL and rL increased as CO2 increased. It was concluded that: (i) TR of the high CO2 leaflets was similar that of ambient CO2 leaflets because the increases in rL caused by high CO2 were partially offset by increases in vapor pressure gradient between leaf‐to‐air caused by increased TL; (ii) Leaves adapted to high CO2 had higher WUE than leaves adapted to ambient CO2, mainly because of a two‐fold increase in CER.