Nitrogen Availability and Vegetative Development Influence the Response of Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase, Phosphoenolpyruvate Carboxylase, and Heat-Shock Protein Content to Heat Stress in Zea mays L.

We examined the influence of plant nitrogen (N) status and vegetative development on the accumulation of ribulose 1,5-bisphosphate carboxylase/oxygenase large subunit (rubisco), phosphoenolpyruvate carboxylase (pepcase), and the heat-shock proteins Hsp24 and Hsp60 during and after heat stress in corn (Zea mays L.) to explore the possibility that much HSP-N may originate directly or indirectly from photosynthetic proteins in leaves. In general, rubisco and pepcase content decreased in response to a 45⚬C heat stress compared with unstressed controls (28⚬C). Rubisco and pepcase declined relative to total detergent-soluble protein, which was unaffected by heat stress. Plants provided with lower levels of available N during growth and that had lower total protein content (low-N plants) allocated a greater fraction of total protein to rubisco and pepcase in recently expanded leaves and exhibited greater relative decreases in rubisco and pepcase than high-N plants. The decreases in low-N plants became evident earlier during the 20-h heat stress, whereas decreases in high-N plants occurred later during heat stress or during recovery from stress, when levels of some HSPs (e.g., Hsp60) were still increasing. Decreases in rubisco and pepcase were greater in adult, compared with juvenile, vegetative plants that had greater protein content, but less rubisco and pepcase, than adults. Heat stress appeared to delay ontogenetic changes in rubisco and pepcase content in leaves in some instances. These results, consistent with our previous evidence indicating that HSP production may be N costly, indicate that abundant soluble photosynthetic enzymes such as rubisco and pepcase may supply much of the N required for the heat-stress response in plants, particularly in mature leaves.