Photosynthetic/Photorespiratory Carbon Metabolism in the C3-C4 Intermediate Species, Moricandia arvensis and Panicum milioides

Abstract

The distribution of 14C in photosynthetic metabolites of 2 naturally occurring higher plants with reduced photorespiration, M. arvensis and P. milioides, in pulse and pulse-chase CO2 incorporation experiments was similar to that for the C3 species M. foetida and Glycine max. After 6 s of 14CO2 incorporation, only about 6% of the total 14C fixed was in malate and aspartate in both M. arvensis and P. milioides. The apparent turnover of the C4 acids was very slow, and malate accumulated during the day in M. arvensis. Thus, C4 acid metabolism by M. arvensis and P. milioides had no significant role in photosynthetic carbon assimilation under the conditions of the experiments (310 .mu.l CO2/l, 21% O2, 1100 or 1900 .mu. molphoton/m2 per S, 27.degree. C). After a 36-s chase period in air containing 270 .mu.l CO2/l, about 20% of the total 14C fixed was in glycine with M. arvensis, as compared to 15% with M. foetida, 14% with P. milioides, and 9% with G. max. After a 36-s chase period in 100 NB .mu.l CO2/l, the percentage in glycine was about twice that at 270 NB .mu.l CO2/l, in the C3 species and P. milioides, but only 20% more 14C was in glycine in M. arvensis. Either the photorespiratory glycine pool in M. arvensis is larger than in the other species examined or the apparent turnover rate of glycine and the flow of C into glycine during photorespiration are less in M. arvensis. An unusual glycine metabolism in M. arvensis may be linked to the mechanism of photorespiratory reduction in this crucifer.