Morphological, structural, and ultrastructural key features of hyperhydricity in Beta vulgaris L. var. saccharifera

Hyperhydricity, as a phenomenon specific to in vitro plants, triggers a series of changes at the cell and tissue level, which modify the plants’ physiological processes. For a better understanding of this phenomenon, we found it useful to present here the fundamental research on establishing the steps of structural and ultrastructural degradation of cells and tissues, observed by light microscopy (LM) and transmission electron microscopy (TEM) analyses, in the leaves of Beta vulgaris var. saccharifera, at 30 days of secondary culture, and their correlation with viability in the third culture. Comparative with control (non-hyperhydric plants), three degradation steps of hyperhydricity were identified in plants regenerated on a culture medium with 2.5 mg·L-1 6-benzyladenine. The first step involved cell wall deformation, which lost its rigidity and became sinuous, causing the enlargement of the intercellular spaces. In the second step, these spaces formed gaps through their union, and the whole membrane system suffered: both chloroplasts and tonoplast were broken (the cytoplasm and vacuolar composition were mixed), the nuclear membrane presented undulations, just before the damage to the tonoplast, and the nucleus became pyknotic. In step 3, the cell showed the beginning of lysis, which leads to necrosis, the cell had nothing in common with a normal ultrastructure. For in vitro plants in this final step, there was no chance of surviving but in steps I and II, the viability was 55-75%. These features can be useful to producers to calculate the level of culture damage and start measures to prevent losses.