Impaired Photorespiratory Metabolism Underlies the Decline in CO2 Assimilation Induced by Alternative Oxidase Inhibition in Rumex K-1 Leaves.

Photosynthetic carbon assimilation under photorespiratory conditions requires tight coordination between chloroplast electron transport and mitochondrial redox metabolism, yet the contribution of mitochondrial alternative oxidase (AOX) remains unresolved. Using 1 mM salicylhydroxamic acid (SHAM) to inhibit the AOX pathway in Rumex K-1 leaves, we investigated how mitochondrial alternative respiration contributes to carbon assimilation. AOX inhibition imposed a non-stomatal limitation on CO2 assimilation and reduced photosystem II (PSII) electron transport. However, low O2 or elevated CO2 alleviated the decline in CO2 assimilation while PSII photochemistry remained depressed, and AOX inhibition reduced apparent Vcmax without significantly affecting Jmax, indicating that the primary constraint lay downstream of PSII in photorespiratory carbon metabolism. Electron flux through PSII and the electron fluxes supporting the photosynthetic carbon reduction and photorespiratory carbon oxidation cycles all decreased under SHAM treatment, indicating reduced PSII electron transport and electron use associated with carbon assimilation and photorespiration. AOX inhibition caused glycine accumulation and increased the Gly/Ser ratio under illumination but not in darkness, indicating restricted mitochondrial glycine-to-serine conversion during photorespiration. Together, these responses suggest that AOX-dependent ubiquinol oxidation helps sustain mitochondrial NADH reoxidation and NAD+ regeneration, thereby supporting glycine-to-serine conversion and photorespiratory carbon recycling.