FREE PROLINE ACCUMULATION IN YOUNG SUGAR BEET PLANTS AND IN TISSUE CULTURE EXPLANTS UNDER WATER DEFICIENCY AS TOOLS FOR ASSESSMENT OF DROUGHT TOLERANCE
Proline is the most widely distributed metabolite that accumulates under various stress conditions, including the lack of water. To evaluate the suitability of proline accumulation triggered by drought stress to screen sugar beet genotypes for drought tolerance, we analysed accumulation of free proline in eleven genotypes classed in three levels of relative tolerance (low, medium, high), as assessed visually in field cultivation. Analysis was performed in two tests: 1) in greenhouse, where 90 days old plants were exposed to a short-term water deficiency and 2) in tissue culture where the lack of water was imposed by addition of 3 or 5% (w/v) polyethylene glycol (PEG, MW 6000). Both the in vitro test with increasing levels of PEG and the suspension of water supply in the greenhouse experiment showed large increases of free proline in tissues of sugar beet explants or leaves consequent to water restriction, as well as reduction in fresh weight, tissue water content and axillary bud formation. Stress effects varied considerably among genotypes classed at low, medium and high levels of field tolerance to drought stress, but were similar as class averages, except for proline in vitro, which was significantly higher for genotypes in the high tolerance group, and allowed separating them from those in the less tolerant groups. Proline response in the in vitro test correlated better than the response in greenhouse experiment with the field assessed drought tolerance of genotypes.