differential regulation of photorespiratory gene expression by moderate and severe salt and drought stress in relation to oxidative stress.

The tolerant C3 plant Pancratium maritimum L. was used to investigate the contribution of photorespiration

to the oxidative load under salt and drought stress. 7-weeks old plants were salt-stressed by 150 or

300mMNaCl or drought-stressed by withholding water for 11 or 21 days. The growth, photosynthesis (A)

and transpiration rates (E) were reduced by all stress treatments proportionally to the severity of stress.

The rate of photorespiration was remarkably higher under moderate stresses than under severe stresses

as revealed by large increase in the photorespiratory indicators Gly/Ser ratio and glyoxylate content

and as substantiated by higher expression levels of photorespiratory enzymes. Nonetheless, indicators

of oxidative stress namely, malondialdehyde (MDA) and hydrogen peroxide (H2O2) contents showed

greater increase under severe stresses suggesting that the increase in the oxidative load under severe

stresses is independent of photorespiration rate. The lower contents of MDA and H2O2 under moderate

stresses (compared to these under severe stresses) in spite of the strong upregulation of photorespiration

indicate efficient detoxification of photorespiration-generated H2O2 as shown by efficient upregulation

of catalase (CAT) and peroxidase (POD). It is thus concluded that photorespiration may not be major

contributor to the oxidative load under salt and drought stress