Activation of the ascorbate-glutathione cycle is an early response of micropropagated Vitis vinifera L. explants transferred to ex vitro

Micropropagated grapevine ( Vitis vinifera L. var. Touriga Nacional) was transferred from heterotrophic growth conditions and PPFD of 50 mmol m(-2) s(-1) to ex vitro under PPFD fourfold higher, which causes photooxidative stress. We have analyzed the first 7 d of growth, focusing on the protective role of the ascorbateglutathione cycle and relating its activity to the functioning of photosynthetic electron transport. Immediately after transfer, reversible photoinhibition was observed. Both the photochemical efficiency of PSII and PSII excitation capture efficiency were transiently affected. Concomitantly, a transient rise in H2O2 content was observed on day 1, followed by increased activity of the ascorbate- glutathione cycle as determined by changes to ascorbate and glutathione levels and redox states, enzyme activities, and expression of associated genes. The upregulation of ascorbate peroxidase (APX1 and APX3), dehydroascorbate reductase ( DHAR), monodehydroascorbate reductase (MDHAR), and glutathione reductase (GOR2) was observed from day 1, followed by catalase ( CAT) on day 2. Accompanying changes to ascorbate- glutathione cycle activity, the expression of superoxide dismutase isoforms increased. A slight H2O2 peak, not accompanied by photoinhibition, was observed on day 6. A decrease in the ascorbate and glutathione pools also pointed to the development of new leaves, with the concurring translocation of metabolites.