Effects of irradiance and temperature on photosynthesis in C, C and C/C Panicum species.

Species in the Laxa and Grandia groups of the genus Panicum are adapted to low, wet areas of tropical and subtropical America. Panicum milioides is a species with C photosynthesis and low apparent photorespiration and has been classified as a C/C intermediate. Other species in the Laxa group are C with normal photorespiration. Panicum prionitis is a C species in the Grandia group. Since P. milioides has some leaf characteristics intermediate to C and C species, its photosynthetic response to irradiance and temperature was compared to the closely related C species, P. laxum and P. boliviense and to P. prionitis. The response of apparent photosynthesis to irradiance and temperature was similar to that of P. laxum and P. boliviense, with saturation at a photosynthetic photo flux density of about 1 mmol m s at 30°C and temperature optimum near 30°C. In contrast, P. prionitis showed no light saturation up to 2 mmol m s and an optimum temperature near 40°C. P. milioides exhibited low CO loss into CO-free air in the light and this loss was nearly insensitive to temperature. Loss of CO in the light in the C species, P. laxum and P. boliviense, was several-fold higher than in P. milioides and increased 2- to 5-fold with increases in temperature from 10 to 40°C. The level of dark respiration and its response to temperature were similar in all four Panicum species examined. It is concluded that the low apparent photorespiration in P. milioides does not influence its response of apparent photosynthesis to irradiance and temperature in comparison to closely related C Panicum species. [ABSTRACT FROM AUTHOR]