Differences between rice and wheat in RuBP regeneration capacity per unit of leaf nitrogen content

Abstract

Light-saturated photosynthesis in air CO2 levels for a given leaf-N content is higher in wheat than in rice. This is caused by a greater Rubisco capacity in wheat (Makino et al., 1988, Planta 174, 30-38). In this study, we found that CO2-saturated photosynthesis is also higher in wheat than in rice. This means that RuBP regeneration capacity is also greater in wheat. Therefore, we examined the biochemical factor(s) for differences in RuBP regeneration capacity between rice and wheat. Insoluble-N, soluble protein-N, and TCA soluble-N contents did not differ between the two species. The activities of several Calvin cycle enzymes such as Rubisco, NADP-triosephosphate dehydrogenase, phosphoglycerate kinase and choroplastic fructosebisphosphatase activities per unit of leaf nitrogen were all higher in wheat than in rice. Although sucrose-phosphate synthase (SPS) is a key enzyme during Pi regeneration, the Vmax activity for UDP-G was almost the same between the two species and its Km value was a little lower in rice. Chl content and Chl a/b ratio did not differ. Cytochrome (Cyt) f content was greater in wheat than in rice, whereas coupling factor 1 content was greater in rice. Among them, Cyt f content was most highly correlated with CO2-saturated photosynthesis, irrespective of the two species. In addition, the regression line between Cyt f content and CO2-saturated photosynthesis in both species fell on the same line. The results, thus, suggest that higher RuBP regeneration capacity in wheat leaves is caused by a greater Cyt f content.