Organic solutes protect drought-tolerant Populus x euramericana against reactive oxygen species

The mechanisms by which drought-induced oxidative stress is tolerated in poplar were appreciated by the ability of leaf extracts to scavenge reactive oxygen species (ROS) generated in vitro by a xanthine oxidase-hypoxanthine system. Their effects were detected by the protection towards the denaturation of malate dehydrogenase (MDH) of crude and dialysed extracts of a drought-sensitive (Populus x euramericana Luisa Avanzo) and a drought-tolerant (I! x euramericana, Dorskamp) poplar exposed to control and osmotic stress (- 0.336 MPa). At the beginning of the incubation period in presence of ROS, MDH from crude extract of control Dorskamp was protected against the denaturation; boiled crude extracts of control Dorskamp also protected MDH from the dialysed extract of control Luisa Avanzo. In control Dorskamp, cysteine, proline, polyamines and ascorbate added in vitro were efficient ROS scavengers. Compared with the control condition, the rate of MDH denaturation in presence of ROS decreased in a dialysed extract of stressed Luisa Avanzo but increased in that of stressed Dorskamp. Under stress conditions, proline in vitro enhanced the rate of MDH denaturation by ROS in Luisa Avanzo, bur: competed as ascorbate with ROS in Dorskamp. In Dorskamp, the 12 h-exposure to osmotic stress resulted in a decrease in catalase and gutathione reductase activities, in an enhancement of superoxide dismutase, ascorbate peroxidase and contents of some putative leaf antioxidants (putrescine and glutathione), but did not affect the contents of spermine, spermidine and ascorbate. The consequences of extract/solute effects are discussed for each clone as regards the drought response.