CHANGES IN THE QUANTITIES OF VIOLAXANTHIN DE-EPOXIDASE, XANTHOPHYLLS AND ASCORBATE IN SPINACH UPON SHIFT FROM LOW TO HIGH LIGHT

Zeaxanthin, a carotenoid in the xanthophyll cycle, has been suggested to play a role in the protection against photodestruction. We have stu died the importance of the parameters involved in zeaxanthin formation by comparing spinach plants grown in low light (100 to 250 mu mol m(- 2) s(-1)) to plants transferred to high light (950 mu mol m(-2) s(-1)) . Different parameters were followed for a total of 11 days. Our exper iments show that violaxanthin de-epoxidase decreased between 15 and 30 %, the quantity of xanthophyll cycle pigments doubled to 100 mmol (mol Chl)(-1), corresponding to 27 mu mol m(-2), and the rate of violaxant hin to zeaxanthin conversion was doubled. Lutein and neoxanthin increa sed from 50 to 71 mu mol m(-2) and from 16 to 23 mu mol m(-2), respect ively. On a leaf area basis, chlorophyll and beta-carotene levels firs t decreased and then after 4 days increased. The chlorophyll a/b ratio was unchanged. The quantity of ascorbate was doubled to 2 mmol m(-2), corresponding to an estimated increase in the chloroplasts from 25 to 50 mM. In view of our data, we propose that the increase in xanthophy ll cycle pigments and ascorbate only partly explain the increased rate of conversion of violaxanthin to zeaxanthin, but the most probable ex planation of the faster conversion is an increased accessibility of vi olaxanthin in the membrane.