Leaf photosynthesis and Rubisco activity and kinetics of soybean , peanut and rice grown under elevated atmospheric CO 2 , supraoptimal air temperature and soil water deficit

Soybean (Glycine max L. Merr. cv. Bragg), peanut (Arachis hypogaea L. cv. Georgia Green) and rice (Oryza saliva L. cv. IR-72) were grown for a full season in sunlit, controlled-environment chambers at 350 (ambient) and 700 (double-ambient, elevated) imol CO2 moF' air, and under daytime maximum/nighttime minimum air temperature regimes ranging from 28/18 to 48/38°C for soybean and peanut, or soil water deficit for rice. The objectives were to characterize the interactive effects of elevated [CO2] and high air temperature, or elevated growth [CO 2 ] and soil water deficit, on leaf CO2 exchange rate (CER) and Rubisco activity, and to test whether elevated [CO 2], high temperature or severe drought stress would induce changes in the kinetic behavior [Km(CO2)] Of Rubisco. Leaf CER of soybean, peanut and rice were increased by CO 2 enrichment, but decreased by high temperature and drought. For soybean, despite the deleterious effect of high temperature on CER, CO2-enriched plants not only outperformed ambient-0O2 plants at the optimum growth temperature (32/22°C) for photosynthesis, but also *To whom correspondence should be addressed: Dr. Joseph C. V. Vu, Center for Medical, Agricultural and Veterinary Entomology, Chemistry Research Unit 1600/1700 SW 23°f Drive, Gainesville, FL 32608-1069 USA compensated much better for the adverse effects of high temperatures on CER. In addition, the degree of enhancement induced by elevated [CO2] on soybean CER increased in a linear manner with increased growth temperature. For peanut, however, the degree of enhancement in CER by elevated [CO2 ] as a function of growth temperature differed from that for soybean. For rice subjected to drought stress, elevated [CO 2] delayed by one day the substantial reduction in midday leaf CER. Elevated [CO2], high temperature and drought reduced the initial (non-activated) and total (1-lCO3 7Mg2 -activated) activities as well as the activation state of midday-sampled leaf Rubisco. The Rubisco initial Km(CO2) of soybean and peanut was not markedly altered by elevated [CO2] or high temperature, as neither severe drought imposed at panicle initiation changed the initial Km(CO2) value of the enzyme for rice. However, moderate increases in Rubisco total K,(CO2) were observed for soybean and peanut at elevated [CO 2] and high temperature. The moderate increases in Rubisco total K 3(CO2) suggest that multiple generations of growth under CO2 enrichment might be required for Rubisco of the C 3 species to evolve towards a more effective type enzyme.