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