The Regulation of CAM and Respiratory Recycling by Water Supply and Light Regime in the C 3 -CAM Intermediate Sedum telephium

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Abstract
The expression of C3 and CAM in the temperate succulent Sedum telephium L. was investigated in response to varying photosynthetically active radiation (PAR) and watering regimes. Plants accumulated malic acid under all light and watering regimes although this was enhanced by drought and higher PAR. The transition from predominantly C3 metabolism to net dark fixation of CO2, however, was not regulated exclusively by plant water status. Under drought stress and high PAR approximately 36% of malic acid accumulated overnight was derived from external sources of CO2 as calculated both directly, in CO2-free air, and indirectly, as the difference between net dark fixation of CO2 and nocturnal accumulation of malic acid. Under all other light and watering regimes recycling of respiratory CO2 accounted for all the malic acid accumulated overnight. Direct and indirect estimates of recycling compared well with the availability of respiratory CO2, with droughted plantes recapturing a higher proportion of respiratory CO2 than well-watered plants. Consequently CAM is initially enhanced in S. telephium through an increase in the amount of respiratory CO2 which is recaptured by the enzyme PEP carboxylase.

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