Ascorbate metabolism in potato leaves supplied with exogenous ascorbate

Photosynthesis and leaf ascorbate were measured in potato (Solanum tuberosu m L.) plants grown in low light and then transferred to high light. Total f oliar ascorbate content in low light-grown plants was 4.72 +/- 0.42 mu mol mg(-1) chl. Over 80% of the ascorbate pool was found in the reduced form ir respective of position on the stem. No statistically-significant light-depe ndent effects were observed. Leaf discs supplied with [C-14]- ascorbate in the dark showed significant ascorbate uptake such that after a 16h incubati on over half of the total ascorbate pool in the discs was labelled [C-14]-a scorbate. No ascorbate efflux from the leaves occurred during the period of [14C]-ascorbate uptake. The total amount of ascorbate did not increase, ho wever, implying modified ascorbate turnover. The turnover of the [C-14]-asc orbate in the leaves occurred at similar rates in both light and darkness. Little degradation of labelled ascorbate was observed, suggesting that upta ke of exogenous ascorbate leads to inhibition of de novo ascorbate biosynth esis in potato leaf discs.