DETERMINATION OF EFFECTIVE TEMPERATURE MINIMA FOR CAM CARBOXYLATION IN DIVERSE PLANTS BY SCANNING MICROCALORIMETRY

Biochemical and biophysical constants can be obtained from in vivo or in situ samples by calorimetric measurements. In this work scanning mi crocalorimetric measurements of nine Crassulacean acid metabolism (CAM ) species were made in a range between 5 degrees C and 35 degrees C du ring the CAM night cycle. When plotted in the Arrhenius format, the mi crocalorimetric data showed a characteristic energy of activation (E(a )) response to temperature, yielding two major temperature ranges: a h igh temperature range where E(a) is low and the process is metabolical ly viable, and a low temperature range where E(a) is high. In this low temperature range, when E(a) is above certain levels, the process is considered to have ''chill impaired function'' and unlikely to have ph ysiological significance. Temperature range of the transition between high/low E(a) areas was species dependent. Without added CO2, abrupt r ate changes caused fluctuations in usual Arrhenius plot patterns. CO2 additions eliminated these fluctuating rate responses. The level of CO 2 required to reliably eliminate fluctuations was also species depende nt. The data is discussed in terms of using low-temperature-tolerant p hosphoenolpyruvate carboxylase path enzymes from CAM plants to transfo rm C-4 crop plants.