Partitioning of carboxylase activity in nitrogen-stressed sugarcane and its relationship to bundle sheath leakiness to CO2, photosynthesis and carbon isotope discrimination

We studied the effects of external nitrogen (N) supply on ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, phosphoenolpyruvate carboxylase (PEPC) activity, leaf gas exchange, carbon isotope discrimination (Delta), and bundle sheath leakiness to CO2 (Phi) in two cultivars of the C-4 grass, sugarcane (Saccharum spp. hybrid). In addition to reducing overall levels of carboxylase activity and therefore photosynthetic rates, reduced N supply altered the partitioning of carboxylase activity. Under long-term N stress (4 months) Rubisco activity decreased more than PEPC activity causing significant reductions in the Rubisco/PEPC activity ratio, a measure of the ratio of C-3 to C-4 cycle activity. Concurrent determinations of Delta for leaf dry matter and the prevailing ratio of intercellular to ambient partial pressure of CO2 (p(i)/p(a)) during leaf gas exchange suggested that the decreased partitioning of N to Rubisco activity under long-term N stress led to increased Phi and reduced photosynthetic efficiency. The two cultivars studied maintained similar PEPC activities but differed genetically in regard to investment of N in Rubisco. Greater investment of N in Rubisco was associated with higher rates of photosynthesis and growth at similar or slightly lower leaf N content, indicating that greater relative investment of N in Rubisco activity also led to higher N-use efficiency. The results suggest that regulation of the ratio of C-3 to C-4 pathway activity and its consequences for Phi may play a key role in the photosynthetic performance and growth of C-4 grasses under both favourable and stressful conditions.