The amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), total chlorophyll (Chi), and total leaf nitrogen were measured in fully expanded, young leaves of wheat (Triticum aestivum L.), rice (Oryza sativa L.), spinach (Spinacia oleracea L.), bean (Phaseolus vulgaris L.), and pea (Pisum sativum L.). In addition, the activities of whole-chain electron transport and carbonic anhydrase were measured. All plants were grown hydroponically at different nitrogen concentrations. Although a greater than proportional increase in Rubisco content relative to leaf nitrogen content and Chi was found with increasing nitrogen supply for rice, spinach, bean, and pea, the ratio of Rubisco to total leaf nitrogen or Chi in wheat was essentially independent of nitrogen treatment. In addition, the ratio of Rubisco to electron transport activities remained constant only in wheat. Nevertheless, gas-exchange analysis showed that the in vivo balance between the capacities of Rubisco and electron transport in wheat, rice, and spinach remained almost constant, irrespective of nitrogen treatment. The in vitro carbonic anhydrase activity in wheat was very low and strongly responsive to increasing nitrogen content. Such a response was not found for the other C3 plants examined, which had 10-to 30-fold higher carbonic anhydrase activity than wheat at any leafnitrogen content. These distinctive responses of carbonic anhydrase activity in wheat were discussed in relation to CO2-transfer resistance and the in vivo balance between the capacities of Rubisco and electron transport. ach, clarified the relation between nitrogen nutrition and nitrogen partitioning into the various photosynthetic components and activities. They found that although nitrogen supply increased the ratio of Rubisco activity to electron transport activity, ATPase, Chl, or total leaf nitrogen, the balance between the in vivo activities of Rubisco and electron transport remained constant. They concluded that this difference was compensated for by the presence of a C02-transfer resistance between intercellular air spaces and the carboxylation sites. As a result of this resistance, the in vivo Rubisco specific activity was reduced progressively with increasing amount of enzyme because the partial pressure of CO2 at the carboxylation sites was reduced and kept in a constant balance with electron transport activity. The increase in the ratio of Rubisco to total leaf nitrogen or Chl with nitrogen supply is frequently found for other C3 species, such as tobacco (1), cotton (32), Solanum (11), bean (26), and pea (18).However, in spite of the existence of significant C02-transfer resistance in wheat (5,8,23, 30), the ratio of Rubisco to total leaf nitrogen or Chl in fully expanded young leaves seems to be independent of nitrogen nutrition (5,17,18 Plant Physiol. Vol. 100, 1992 CO2 diffusion to the carboxylation sites remains uncertain. In addition, the response of CA activity to changing nitrogen content is not known.In this study, we used fully expanded, youn...