AN EXAMINATION OF THE ADVANTAGES OF C3-C4 INTERMEDIATE PHOTOSYNTHESIS IN WARM ENVIRONMENTS

The photosynthetic responses to temperature in C3, C3-C4 intermediate, and C4 species in the genus Flaveria were examined in an effort to identify whether the reduced photorespiration rates characteristic of C3-C4 intermediate photosynthesis result in adaptive advantages at warm leaf temperatures. Reduced photorespiration rates were reflected in lower CO2 compensation points at all temperatures examined in the C3-C4 intermediate, Flaveria floridana, compared to the C3 species, F. cronquistii. The C3-C4 intermediate, F. floridana, exhibited a C3-like photosynthetic temperature dependence, except for relatively higher photosynthesis rates at warm leaf temperatures compared to the C3 species, F. cronquistii. Using models of C3 and C3-C4 intermediate photosynthesis, it was predicted that by recycling photorespired CO2 in bundle-sheath cells, as occurs in many C3-C4 intermediates, photosynthesis rates at 35-degrees-C could be increased by 28%, compared to a C3 plant. Without recycling photorespired CO2, it was calculated that in order to improve photosynthesis rates at 35-degrees-C by this amount in C3 plants, (1) intercellular CO2 partial pressures would have to be increased from 25 to 31 Pa, resulting in a 57% decrease in water-use efficiency, or (2) the activity of RuBP carboxylase would have to be increased by 32%, resulting in a 22% decrease in nitrogen-use efficiency. In addition to the recycling of photorespired CO2, leaves of F. floridana appear to effectively concentrate CO2 at the active site of RuBP carboxylase, increasing the apparent carboxylation efficiency per unit of in vitro RuBP carboxylase activity. The CO2-concentrating activity also appears to reduce the temperature sensitivity of the carboxylation efficiency in F. floridana compared to F. cronquistii. The carboxylation efficiency per unit of RuBP carboxylase activity decreased by only 38% in F. floridana, compared to 50% in F. cronquistii, as leaf temperature was raised from 25 to 35-degrees-C. The C3-C4 intermediate, F. ramosissima, exhibited a photosynthetic temperature response curve that was more similar to the C4 species, F. trinervia, than the C3 species, F. cronquistii. The C4-like pattern is probably related to the advanced nature of C4-like biochemical traits in F. ramosissima. The results demonstrate that reductions in photorespiration rates in C3-C4 intermediate plants create photosynthetic advantages at warm leaf temperatures that in C3 plants could only be achieved through substantial costs to water-use efficiency and/or nitrogen-use efficiency.