The relationship between carbon-dioxide-limited photosynthetic rate and ribulose-1,5-bisphosphate-carboxylase content in two nuclear-cytoplasm substitution lines of wheat, and the coordination of ribulose-bisphosphate-carboxylation and electron-transport capacities

摘要

Photosynthesis in two cultivars ofTriticum aestivumwas compared with photosynthesis in two lines having the same nuclear genomes but with cytoplasms derived fromT. boeoticum. The in-vitro specific activity of ribulose-1,5-bisphosphate carboxylase (RuBPCase; EC 4.1.1.39) isolated from lines withT. boeoticumcytoplasm was only 71 of that of normalT. aestivum. By contrast, the RuBPCase activities calculated from the CO2-assimilation rate at low partial pressures of CO2,p(CO2), were the same for all lines for a given RuBPCase content. This indicates that both types of RuBPCase have the same turnover numbers in-vivo of 27.5 mol CO2·(mol enzyme)−1·s−1(23°). The rate of CO2assimilation measured at normalp(CO2),pa=340 μbar, and high irradiance could be quantitatively predicted from the amount of RuBPCase protein. The maximum rate of RuBP regeneration could also predict the rate of CO2assimilation at normal ambient conditions. Therefore, the maximum capacities for RuBP carboxylation and RuBP regeneration appear to be well-balanced for normal ambient conditions. As photosynthetic capacity declined with increasing leaf age, the capacities for RuBP carboxylation and RuBP regeneration declined in p