GROWTH TEMPERATURE VERSUS CO2 UPTAKE, RUBISCO AND PEPCASE ACTIVITIES,AND ENZYME HIGH-TEMPERATURE SENSITIVITIES FOR A CAM PLANT

For the succulent CAM plant Opuntia ficus-indica growing at day/night air temperatures of 45 degrees C/35 degrees C, total daily net CO2 upt ake was 29% lower than at 15 degrees C/5 degrees C and 45% lower than at 30 degrees C/20 degrees C. However, maximal activities of Rubisco ( EC. 4.1.1.39) and PEPCase (EC. 4.1.1.31) per unit surface area for pla nts at 45 degrees C/35 degrees C averaged 43% higher than at 15 degree s C/5 degrees C and 21% higher than at 30 degrees C/20 degrees C. As t he growth temperature was increased by 30 degrees C, the optimal tempe rature for Rubisco activity increased by 5 degrees C and that for PEPC ase increased by 15 degrees C. About 10% less Rubisco was in the activ ated state in vivo for plants at 15 degrees C/5 degrees C compared wit h the other two growth temperatures. The total amount of soluble prote ins was higher at 30 degrees C/20 degrees C than at 15 degrees C/5 deg rees C and 45 degrees C/35 degrees C, as were the amounts of Rubisco a nd PEPCase. The broad temperature range for maximal PEPCase activity ( 25-40 degrees C) suggested the influence of different PEPCase isoforms at the different growth temperatures; moreover, the high-temperature sensitivity for this enzyme decreased as the growth temperatures incre ased. PEPCase was less tolerant of high temperatures than was Rubisco, although substantial decreases in activity did not occur until above 55 degrees C for both enzymes.