The growth of the wild-type and three salt tolerant mutants of barnyard

grass (Echinochloa crusgalli L.) under salt stress was investigated in relation

to oxidative stress and activities of the antioxidant enzymes superoxide

dismutase (SOD: EC 1.15.1.1), catalase (CAT: EC 1.11.1.6), phenol peroxidase

(POD: EC 1.11.1.7), glutathione reductase (GR: EC 1.8.1.7) and ascorbate

peroxidase (APX: EC 1.11.1.1). The three mutants (fows B17, B19 and B21)

grew significantly better than the wild-type under salt stress (200 mM NaCl)

but some salt sensitive individuals were still detectable in the populations of

the mutants though in smaller numbers compared with the wild-type. The salt

sensitive plants had slower growth rates, higher rates of lipid peroxidation and

higher levels of reactive oxygen species (ROS) in their leaves compared with

the more tolerant plants from the same genotype. These sensitivity responses

were maximized when the plants were grown under high light intensity

suggesting that the chloroplast could be a main source of ROS under salt

stress. However, the salt sensitivity did not correlate with reduced K+/Na+

ratios or enhanced Na+ uptake indicating that the sensitivity responses may

be mainly because of accumulation of ROS rather than ion toxicity. SOD

activities did not correlate to salt tolerance. Salt stress resulted in up to 10-fold

increase in CAT activity in the sensitive plants but lower activities were found

in the tolerant ones. In contrast, the activities of POD, APX and GR were

down regulated in the sensitive plants compared with the tolerant ones.

A correlation between plant growth, accumulation of ROS and differential

modulation of antioxidant enzymes is discussed. We conclude that loss of

activities of POD, APX and GR causes loss of fine regulation of ROS levels

and hence the plants experience oxidative stress although they have high

CAT activities.

Introduction

Salt stress