Structural and functional characteristics of overwintering blackberry leave

The structural and functional changes in overwintering blackberry leaves (Rubus fruticosus L.) were investigated. The changes in frozen and thawed leaves were studied by light and electron microscopy. In addition to conventional chemical fixation, the frozen leaves were also examined by microwave-enhanced chemical fixations, and by freeze-substitution methods.The winter leaves were multivacuolated and contained chloroplasts with large grana. By light and electron microscopy it was shown that, in leaves frozen between -4 °C and-10 °C, some intercellular spaces lying near the stomata were much enlarged. In the cells of these leaves the cytoplasm was condensed and the organelles aggregated. These characteristics were registered in frozen tissue after all the fixation methods applied. After thawing, the normal structures of the leaf cells soon reappeared, as did the respiration and photosynthetic activity of the leaves. At the end of the winter period (beginning of March) the leaves still had the characteristics of winter leaves, although their hardening seemed to decline. Signs of senescence (large plastoglobulcs in the chloroplasts) were observed in the overwintered leaves in spring, some time before shedding