Abstract
The distribution of 14C in photosynthetic metabolites of two naturally occurring higher plants with reduced photorespiration, Moricandia arvensis and Panicum milioides, in pulse and pulse-chase 14CO2 incorporation experiments was similar to that for the C3 species, M. foetida and Glycine max. After 6 seconds 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 our experiments (310 microliters CO2 per liter, 21% O2, 1100 or 1900 micromoles photon per square meter per second, 27°C).
After a 36-second chase period in air containing 270 microliters CO2 per liter, 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-second chase period in 100 microliters CO2 per liter, the percentage in glycine was about twice that at 270 microliters CO2 per liter in the C3 species and P. milioides, but only 20% more 14C was in glycine in M. arvensis. These data suggest that 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 carbon 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.
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