Short-term changes in free radical scavengers and chloroplast pigments in Pinus canariensis needles as affected by mild drought stress

Potted Pinus canariensis seedlings were subjected to mild drought by withholding irrigation for one week. This treatment induced a reduction in maximum stomatal conductance (50 mmol m−2 s−1) compared to irrigated controls (130 mmol m−2 s−1). Needle water potentials of non-irrigated trees were maintained at control level (-0.44 MPa). Such a mild drought is a potential oxidative stressor due to the production of active oxygen species (AOS) in illuminated chloroplasts which lack CO2 due to stomatal closure. Photoprotective pigments (e.g. the xanthophyll cycle) may avoid this situation through light energy dissipation, and antioxidants such as ascorbic acid, tocopherols, and glutathione, may detoxify AOS. Concentrations of ascorbate, glutathione, chlorophyll, and the xanthophyll cycle carotenoids were minimal in the evening (under low light) compared to light-saturated conditions. α-Carotene was highest in the evening. These short-term changes were not affected by drought. The xanthophyll cycle pool tended to be more de-epoxidized in strongly illuminated needles of non-irrigated trees at the beginning of the experiment, but this effect was transient. The glutathione pool was more oxidized in needles of non-irrigated trees (up to 20 percnt; of total vs. 10 percnt; at control) after the xanthophyll changes took place, whereas the redox state of ascorbate remained stable.