Temperature effects on the carbon-isotope ratio of C3, C 4 and crassulacean-acid-metabolism (CAM) plants.

The temperature coefficient of the δ(13)C value over the temperature range 14 to 40° has been measured for plants with different photosynthetic pathways. C3 plants (6 species) had a temperature coefficient of-0.0125‰/(o), C3 plants (5 species)-0.046‰/(o), "C4-like" CAM (2 species)-0.13‰/(o) and "C3-like" CAM (2 species)-0.019‰/(o). The relative temperature insensitivity of the δ(13)C value is in contrast with the measured temperature dependence of δ(13)C values for ribulose-diphosphate carboxylase in vitro, and to the temperature response of photosynthesis. The temperature sensitivity of the "C4-like" CAM plants was the relatively greatest; this is consistent with the evidence that high night temperature inhibits dark CO2 fixation. The same inhibition may occur in the "C3-like" CAM plants but it would not have as significant an effect on the δ(13)C value as these plants are primarily dependent on primary fixation of exogenous CO2 via the ribulose-diphosphate carboxylase reaction.