The Effect of Neutral Shade on the Structure of Mature, Sun-Acclimated Palisade Cells of Helianthus annuus L. II. 3-Day Exposure

Abstract

Portions of mature leaves of Helianthus annuus var. Mammoth, field grown under full sunlight (FS), were shaded for 3 days at 40%, 25%, or 0% FS. Nonshaded leaf areas served as FS controls. The palisade cell structure of shaded and control leaf areas was analzyed stereologically. There were no detectable shade effects on the volume density values of the major organelle compartments except for a decrease in the starch compartment. The surface density values of chloroplast thylakoid membranes were significantly greater in shaded than in nonshaded chloroplasts at all levels of shading. The proportion of granal thylakoid surface area was greater than that for stromal thylakoids in shaded compared to nonshaded chloroplasts. There were no discernible shade effects on mean palisade cell volume. The total chlorophyll (chl)/mm3 of leaf and chl a/b ratios were lower in the shaded leaves than in the controls except for the full-shade sample (0%), which had higher chl a/b ratios than the control. Thylakoid packing ratios for palisade cells (mm2 thylakoid/mm2 leaf surface) were greater in the shade than in sun areas for 25% and 0% FS. Considering the overall response of thylakoid surface areas and changes in leaf anatomy, there was a greater light-gathering potential in the shaded half of the leaf. Although some shade response was observed in 40% FS (688 μmol s-1 m-2), the most pronounced responses occurred at 25% FS (347 μmol s-1 m-2). Thus, the threshold light level for shade response in sunflowers is near the latter value. These studies also show that cells within the same leaf respond independently to local irradiance levels.