I have read that vitamin C is highly sensitive to light. So, how could parsley, for instance keep its vitamin C as it's flooded with sunlight?
DHA can be reduced to AsA by a specific dehydrogenase using reduced glutathione as reductant. I'm pretty sure that all cells are able to carry out this recycling of ascorbate.
Interestingly, in plants it appears that ascorbate acts as an electron donor to photosystem II (which is equivalent to it being oxidised).