Stomatal Behavior and Relationship between Photosynthesis and Transpiration in Field-grown Cotton as Affected by CO₂ Enrichment

Abstract

The relationship between photosynthetic (P_n) and transpiration (Tr) rates was investigated for field grown cotton plants under elevated atmospheric CO₂ conditions which were provided by computer-controlled open-top chamber systems during the entire growing season. Linear regressions were obtained between P_n and the ratio of Tr to vapor presure deficit (VPD₁) both in 350μmol mol^-1 and 650μmol mol^-1 CO₂ treatments. The slope in 650μmol mol^-1 CO₂ was 62% greater than that in 350μmol mol^-1 CO₂, which suggested higher water use efficiency under higher CO₂ conditions. An analysis on this linearity revealed that the ambient and substomatal CO₂ differential remains fairly constant under a given ambient CO₂ condition. Intercellular CO₂ concentrations were closely correlated with those outside the stomata and the ratio of the two concentrations was conservative with varying ambient CO₂. The ambient and sub stomatal CO₂ differentials at 350 and 650μmol mol^-1 were estimated as around 60 and 95μmol mol^-1, respectively. Stomatal resistances increased by 38% with near doubling CO₂, i.e., CO₂ enrichment was accompanied by a negative feedback effect on stomatal opening. The substantial increase of P_n in cotton plants with the near-doubling of atmospheric CO₂ was estimated as 17%. The close linear relationship between P_n and Tr/VPD₁ was found under the high atmospheric CO₂, which would be useful for remote sensing of physiological activity of crop plants.