A Quantitative Study of Daily Variation in the Cellular Ultrastructure of Palisade Chlorenchyma from Sunflower Leaves

Stereology was used to describe cytological changes which occur in palisade cells of fully expanded leaves as part of their normal daily activity. These changes were evaluated by describing the relationship between organelle volume and cell volume as ratio values (i.e. percentage volumes, Vv; surface-to-volume, Sv). These ratios describe an average cell in terms of its volume commitment to each organelle compartment. Cells were also described in terms of actual volume (μm3) or surface area (μm2) of membrane present in an average cell. ANOVA-LSD and Mann-Whitney statistics indicate significant changes occur in the ratio values of the vacuole, chloroplast, oil, starch and microbody compartments over the 24 h period. This indicates a re-allocation of cell space to these compartments during this period. The Sv ratio of internal membranes of the chloroplast and mitochondria showed no significant change over 24 h indicating that there is a constant relationship between volume and membrane surface area in these organelles. Significant changes occurred in average cell volume over 24 h with maximum volume during the dark period. Since changes in cell volume affected the actual volume expression of all of the organelle compartments there were diurnal variation in the actual size of these compartments, including the internal membranes of chloroplasts and mitochondria which more than doubled in surface area.