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

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 (r_L) and leaf temperature (T_L) 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, r_L was higher throughout the day on these leaflets than r_L 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, T_L and r_L 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 r_L caused by high CO₂ were partially offset by increases in vapor pressure gradient between leaf-to-air caused by increased T_L; (ii) Leaves adapted to high CO₂ had higher WUE than leaves adapted to ambient CO₂, mainly because of a two-fold increase in CER.

Key Words: CO₂-enrichment • Photosynthesis • Leaf temperature • Leaf resistance • Rectangular hyperbola

² Post-doctoral research associate, Agronomy Dep.; professor, Agric. Engineering Dep.; professor, Agric. Engineering Dep., Univ. of Florida, Gainesville, FL 32611; soil scientist, USDA, Gainesville, FL.

Received for publication July 13, 1984.

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