Crassulacean Acid Metabolism in the Succulent C4 Dicot, Portulaca oleracea L Under Natural Environmental Conditions 1

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Crassulacean acid metabolism (CAM) was examined under natural environmental conditions in the succulent C4 dicot Portulaca oleracea L. Two groups of plants were monitored; one was watered daily (well watered), while the other received water once every 3 to 4 weeks to produce a ψ of −8 bars (drought stressed). Gas exchange, transpiration rate, and titratable acidity were measured for 24-hour periods during the growing season. CAM activity was greatest in drought-stressed plants during late August which had 13 hour days and day/night temperatures of 35/15°C. Under these conditions net CO2 uptake occurred slowly throughout the night. Diurnal fluctuations of titratable acidity took place in both leaves and stems with amplitudes of 17 and 47 microequivalents per gram fresh weight, respectively. Transpiration data indicated greater opening of stomata during the night than the day. CAM was less pronounced in drought-stressed P. oleracea plants in July and September; neither dark CO2 uptake nor positive carbon balance occurred during the July measurements. In contrast, well-watered plants appeared to rely on C4 photosynthesis throughout the season, although some acid fluctuations occurred in stems of these plants during September.

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