Differences between rice and wheat in ribulose-1,5-bisphosphate regeneration capacity per unit of leaf-N content

The photosynthetic rates under saturating CO₂ conditions per unit of leaf-N content were higher in wheat than in rice. This suggested that ribulose-1,5-bisphosphate (RuBP) regeneration capacity is greater in wheat. Therefore, the biochemical factor(s) for this difference were examined between rice and wheat. Soluble protein-N, insoluble-N, and trichloroacetic acid (TCA) soluble-N contents were found not to differ between the two species. The activities of several Calvin cycle enzymes such as RuBP carboxylase, NADP-glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase and chloroplastic fructose-1,6-bisphosphatase (cpFBPase) activities per unit of leaf-N content were all higher in wheat than in rice. Among them, cpFBPase activity was most highly correlated with CO₂-saturated photosynthesis. The V_max activity of sucrose-phosphate synthase (SPS) for UDP-glucose was almost the same between the two species and its K_m value was a little lower in rice. Chlorophyll content and its alb ratio did not differ. Cytochrome (Cyt) f content was greater in wheat, whereas coupling factor 1 content was greater in rice. Cyt f content was highly correlated with CO₂-saturated photosynthesis, irrespective of the two species. The results thus suggested that higher RuBP regeneration capacity in wheat leaves is most closely related to a greater Cyt f content and that another candidate is cpFBPase.