© 1989 Heron Publishing—Victoria, Canada
Seasonal variation in the photosynthetic capacity of a willow (Salix cv. Aquatica gigantea) canopy. 2 Comparison of the structure and function of chloroplasts at different levels in the canopy
E. M. Vapaavuori (1), A. Nurmi (2), A. H. Vuorinen (1) and T. Kangas (1)
1. The Finnish Forest Research Institute, Suonenjoki Research Station, SF-77600 Suonenjoki, Finland / 2. Department of General Botany, University of Helsinki, SF-00710 Helsinki, Finland / Received April 10, 1989
Summary
Development of leaf chloroplast ultrastructure at five levels in a willow (Salix cv. Aquatica gigantea) canopy was followed during one growing season in the field. Changes in chloroplast ultrastructure were compared with the
rate of CO2 uptake of the same leaves. The highest rates of CO2 uptake were recorded in young leaves exposed to full available sunlight. In these leaves, the area of the grana stacks was
less than 20% of the total chloroplast area and the degree of thylakoid stacking was less than 1.5. The chloroplasts of these
leaves contained large amounts of starch and small amounts of plastoglobuli. As the canopy grew and the leaves in the lower
parts of the canopy became shaded, the structure of the chloroplast thylakoids gradually changed. In leaves at the two lowest
levels of the canopy, the degree of stacking at the end of the growing season was close to 2 and correspondingly the rate
of CO2 uptake was
low. The areas of grana stacks and plastoglobuli in these chloroplasts increased and were about 30 and 10% of the chloroplast
area, respectively, by the end of the growing season. The increase in the degree of thylakoid stacking was caused by increased
biosynthesis of grana lamellae, which in general were thinner than the lamellae of young leaves. The length of the stroma
lamellae did not change with leaf age. Morphometric measurements showed that the structure of the chloroplasts in leaves 160
cm above ground was dynamic and responsive to environmental conditions so that photosynthetic capacity remained high for 7
weeks despite an increase in leaf shading.
