To make clear the effects of air humidity on diurnal patterns of CO_2 exchange rate, leaf conductance and transpiration rate in crassulacean acid metabolism (CAM) plants, pineapple (full CAM), Mesembryanthemum crystallinum (weak CAM) and peanut (non-CAM) growing under field conditions in Fukuoka were examined. Gas exchange measurements were performed under two different leaf-to-air vapour pressure difference (VPD) conditions. Results obtained were compared with the humidity responses of C_3 and C_4 grass species. 1) Induction of CAM in M. crystallinum in response to water stress was observed 12 days after withholding water. The plants exhibiting CAM-like CO_2 exchange reactions accompanied by distinct depression of CO_2 exchange and leaf conductance at the midday. 2) M. crystallinum exhibited its mode of carbon assimilation from C_3 photosynthesis to CAM when plants were exposed to high NaCl content in the soil for 34 days. At constant leaf temperature CO_2 exchange rate and leaf conductance of the plant were depressed during light period when the VPD was increased. So the reduction in CO_2 fixation during the light period was largely due to stomatal closure in response to the increased VPD. 3) M. crystallinum was remarkably induced CAM-like CO_2 gas exchange pattern by treatment with 3% NaCl for 70 days. When VPD increased, the plant showed a marked reduction of the daytime CO_2 fixation, whereas nighttime CO_2 uptake was only slightly reduced. Consequently CAM mode was pronounced at high VPD conditions. From the VPD response pattern of leaf conductance, it was suggested that stomata of the stressed-plant was very sensitive to the air humidity. 4) Pineapple plant performed high CAM ability when VPD increased. CO_2 exchange rate was significantly correlated with leaf conductance not only during light period but also during dark period. However, slope of the regression line obtained from nighttime data were steeper than that from daytime one. On the other hand, VPD had no effects on the slope. The role of the stomatal responses to VPD in determining the patterns and rates of CO_2 exchange and transpiration in the light or dark are significantly different. 5) In peanut plant, leaf conductance was closely correlated with net CO_2 exchange rate. These results suggest that stomata of CAM plants are more sensitive and strictly regulate the CO_2 exchange as compared with C_3 and C_4 grass species.
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