Seasonal effects on carbon isotope composition of cactus in a desert environment

Abstract

IN plants with crassulacean acid metabolism (CAM), CO₂ is fixed in the light by ribulose-l,5-bisphosphate carboxylase^1,2 and in the dark by phosphoenol pyruvate (PEP) carboxylase^2,3. Because these two enzymes fractionate the stable isotopes of carbon (¹²C and ¹³C) in the atmosphere and discriminate against ¹³C to different extents⁴, the wide range of values of carbon isotope discrimination ratios (δ¹³C) found in these plants⁵ has been attributed to different contributions of dark and light fixation to carbon gain⁶. The proportion of total carbon fixed at night can be altered by environmental variables such as temperature⁷, water status⁸ and photoperiod⁵. Predictable changes in δ¹³C values have been observed when these environmental conditions have been varied. With one exception, all such studies have been performed in artificial conditions. Because the δ13C ratio of CAM plants has been used as an indicator of environment in palaeoecological investigations⁹, we have examined the sensitivity of these δ¹³C ratios to seasonal environmental influences in a natural system. Using Opuntia acanthocarpa, O. basilaris and O. bigelovii, we found that the carbon isotope composition of the plants did not change during the experiment, indicating the insensitivity of this measurement to short term seasonal influences in this environment.