Effect of Short‐term Shading on the Cytology of Palisade Tissue in Mature Leaves of the Sunflower Helianthus Annuus

Mature sunflower leaves were exposed to partial shading (35 or 14% of normal sun) or darkness (0% of normal sun) for approximately 8 hr. During this period one‐half of each test leaf was shaded; the other half was used as a normal sun control. Palisade cell structure from both halves of each leaf was compared. Shading of leaves had little effect on organelle percent volume values (Vv) with exception of the starch compartment which decreased as shading increased. The surface to volume ratio (Sv) of the chloroplast thylakoids increased while the Sv of the mitochondrial membranes decreased as shading increased. Palisade cell volume did not change in shaded portions of the leaf, except in the fully shaded (dark) tissues where cell volume decreased. Changes in the actual volume of organelle compartments were strongly correlated with changes in cell volume. Thus a general osmotic response may account for some of the volume changes associated with differences in light intensity. Shading increased thylakoid surface areas 10–30% over the full sun controls. The ratio of stromal to granal thylakoid surface area remained constant in both the control and partially shaded samples. However, in darkened samples this ratio decreased as stromal membranes increased more than granal membranes. Changes observed in thylakoid surface areas associated with shading did not support thylakoid models which propose the interconversion of granal membranes to stromal membranes and vice versa.