In vitro cultured plants frequently develop hyperhydric tissues. Hyperhydricity is a physiological disorder that causes stress and has a negative impact on plant growth and quality. The development of hyperhydricity was investigated in Arabidopsis thaliana seedlings. Hyperhydricity was induced by growing seedlings in liquid medium or on medium solidified with Gelrite. Hyperhydricity developed only occasionally on medium solidified with agar. The volumes of water and air in the apoplast of hyperhydric leaves were determined. Hyperhydric leaves contained 3-4 times more water in the apoplast than non-hyperhydric ones. The apoplastic air volume in non-hyperhydric leaves was 2-3 times larger than in hyperhydric leaves. Thus, in hyperhydric leaves, the increase in apoplastic water was accompanied by reduction in the apoplastic air volume. We propose that the apoplast of hyperhydric seedlings is waterlogged thereby impairing gas exchange by the symplast, which in turn can result in hypoxia and oxidative stress.
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