Triose phosphate utilization in leaves is modulated by whole-plant sink–source ratios and nitrogen budgets in rice.

This article investigates how triose phosphate utilization (TPU), a biochemical process limiting leaf photosynthesis, is influenced by whole-plant sink–source relationships and nitrogen (N) status in rice (Oryza sativa). Using gas-exchange measurements on leaves from different genotypes, nitrogen levels, growth stages, and sink manipulations (including panicle pruning and adaxial versus abaxial leaf illumination), the study found that higher leaf nitrogen content leads to TPU limitation at lower intercellular CO₂ concentrations and increases photorespiration-associated amino acid export. The proportion of glycolate carbon exported as serine (αS), linked to nitrogen assimilation via photorespiration, correlated positively with leaf nitrogen but was reduced in yellower-leaf genotypes, under abaxial illumination, and after panicle pruning. Although panicle pruning altered whole-plant sink–source ratios and increased leaf nitrogen, it did not consistently reduce TPU rates, suggesting that increased leaf nitrogen content can mask feedback inhibition of photosynthesis caused by sink limitation. These findings highlight the complex interplay between leaf-level TPU limitation, whole-plant sink demand, and nitrogen budgets during rice grain filling, with implications for modeling crop responses under elevated CO₂ conditions.