STOMATAL BEHAVIOR AND RELATIONSHIP BETWEEN PHOTOSYNTHESIS AND TRANSPIRATION IN FIELD-GROWN COTTON AS AFFECTED BY CO2 ENRICHMENT

The relationship between photosynthetic (Pn) 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 Pn 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 substomatal CO2 differentials at 350 and 650 μmol mol-1 were estimated as around 60 and 95 μmol 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 Pn in cotton plants with the near-doubling of atmospheric CO2 was estimated as 17%. The close linear relationship between Pn and Tr/VPD1 was found under the high atmospheric CO2, which would be useful for remote sensing of physiological activity of crop plants. © 1990, CROP SCIENCE SOCIETY OF JAPAN. All rights reserved.