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To test the hypothesis that antioxidant systems flexibly ad-rather than light-regulated. In contrast to the enzymatic defences, ascorbate levels increased by about 30% under bright just to short-term, diurnal fluctuations of ambient environmental conditions, ascorbate-related systems were studied sunlight suggesting that protection from excess light is mediated via the adjustment of metabolites. Under these condi-over several day/night cycles in mature sun-acclimated leaves tions the apparent electron transport rate exceeded the of field-grown beech trees (Fagus syl7atica). Light-dependent capacity for assimilation and the dehydroascorbate pool in-increases in the activities of ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate radical reductase creased twofold. Since dehydroascorbate reductase activities were hardly detected, MDAR activities seemed to be the (MDAR, EC 1.1.5.4) and glutathione reductase (GR, EC 1.6.4.2) were not observed. Lowest activities of APX and major enzyme to keep ascorbate in its reduced state. However, MDAR appeared to be insufficient to maintain the MDAR were found on hot, sunny days. A strong negative correlation occurred between APX activities and ambient redox balance of the ascorbate pool under high light intensitemperatures suggesting that this enzyme was temperature-ties in the field.

Green and white variegated leaves of Coleus blumei, Benth. were separated into albino and green sections and used to determine the distribution of vitamin C and L-galactose dehydrogenase activity, an enzyme supposed to be involved in ascorbate metabolism, in heterotrophic and autotrophic foliar fractions. Both green and white sections contained vitamin C and activity of L-galactose dehydrogenase. However, in the white parts mainly dehydroascorbate was found, whereas in the green parts the redox state of the ascorbate system varied with light or dark conditions. Characterisation of L-galactose dehydrogenase from illuminated green leaf sections showed increasing activity with increasingly alkaline pH-values and a temperature optimum of 25 degrees C. Since these properties were slightly different than those of L-galactose dehydrogenase activities obtained from albino or darkened green leaf sections, we suggest that the enzyme may be light-modulated.

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