The role of light in the regulation of ascorbate metabolism during berry development in the cultivated grapevine Vitis vinifera L.

BACKGROUND: The accumulation of L-ascorbate (Asc) in fruits is influenced by environmental factors including light quantity. Fruit exposure to ambient light is often reduced by the surrounding leaf canopy, and can be altered by cultivation practices. The influence of reduced sunlight exposure on the accumulation of Asc and its catabolites was investigated in field-grown berries of the cultivated grapevine Vitis vinifera L. cv. Shiraz.
RESULTS: Growth under sunlight-eliminated conditions resulted in reduced berry fresh weight, chlorosis and a reduced total L-ascorbate pool size. The concentration of the Asc catabolite L-tartaric acid (TA) was reduced in berries grown without light. Conversely, concentrations of oxalic acid (OA), an alternative catabolite of Asc, and malic acid (MA), were unaffected by shading the berries during development. Brief and significant reductions in transcription of the Asc metabolic genes were observed in shade-grown berries after 4 weeks of dark acclimatisation whilst a key TA biosynthetic gene was not regulated by light.
CONCLUSIONS: The results demonstrate that light-regulation of Asc and TA occurs only at brief stages of development and that OA and MA accumulation is light independent. Additionally, the comparable ratios of TA product to Asc precursor under both light regimes suggest that the diversion of Asc to TA is driven by factors that are not responsive to light. These findings suggest that an altered light regime is not the key to manipulating TA or MA levels in the harvested berry.