CONICET | Buscador de Institutos y Recursos Humanos

The effects of growth irradiance and respiration on ascorbate (AA) synthesis and accumulation were studied in the leaves of wild type and transformed Arabidopsis thaliana with modified amounts of the mitochondrial alternative oxidase (AOX) protein. Plants were grown under low (LL; 50 µmol photons m-2 s-1), intermediate (IL; 100 µmol photons m-2 s-1), or high (HL; 250 µmol photons m-2 s-1) light. Increasing growth irradiance progressively elevated leaf AA contents. The dark-induced disappearance of leaf AA was also influenced by the day time light level, with values of 11, 55 and 89 nmol AA lost g-1FW h-1, from LL, IL and HL-grown leaves respectively. When HL leaves were supplied with L-galactone-1, 4-lactone (L-GalL; the precursor of AA) they accumulated twice as much AA and had double (or doubled) the maximal L-galactone-1, 4-lactone dehydrogenase (L-GalLDH) activities of LL leaves. Growth under HL enhanced dehydroascorbate reductase and monodehydroascorbate reductase activities. Leaf respiration rates were highest in the HL leaves, which also had higher amounts of cytochrome c and cytochrome c oxidase activities as well as enhanced fluxes through the AOX and cytochrome c oxidase electron transport pathways. Intact mitochondria, from high AOX transgenic lines, had higher capacities to synthesise AA than those from the wild type or antisense lines even though they had similar amounts of L-GalLDH and cytochrome c protein. Leaves of the AOX over-expressing lines accumulated more AA than WT or antisense leaves, particularly at HL. We conclude that regardless of limitations on L-GalL synthesis by regulation of early steps in the AA synthesis pathway, the regulation of L-GalLDH activity via the interaction of light and respiratory controls is a crucial determinant of the overall ability of leaves to produce and accumulate AA.