Partitioning of Carboxylase Activity in Nitrogen-Stressed Sugarcane and Its Relationship to Bundle Sheath Leakiness to CO₂, Photosynthesis and Carbon Isotope Discrimination

Abstract

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 (Δ), and bundle sheath leakiness to CO₂ (Φ) in two cultivars of the C₄ 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 C₃ to C₄ cycle activity. Concurrent determinations of Δ 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 Φ 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 C₃ to C₄ pathway activity and its consequences for Φ may play a key role in the photosynthetic performance and growth of C₄ grasses under both favourable and stressful conditions.