© 1989 Heron Publishing—Victoria, Canada
Seasonal variation in the photosynthetic capacity of a willow (Salix cv. Aquatica gigantea) canopy. 1 Changes in the activity
and amount of ribulose 1,5-bisphosphate carboxylase-oxygenase and the content of nitrogen and chlorophyll at different levels
in the canopy
E. M. Vapaavuori and A. H. Vuorinen
The Finnish Forest Research Institute, Suonenjoki Research Station, SF-77600 Suonenjoki, Finland / Received April 10, 1989
Summary
The role of ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) in the regulation of photosynthesis in the field was
assessed by following changes in the amount and activity of Rubisco and in the amounts of chlorophyll and total nitrogen in
a willow (Salix cv. Aquatica gigantea) canopy during two
growing seasons. An additional aim of this study was to determine how nitrogen is partitioned in the canopy and whether Rubisco
and chlorophyll serve as sources of nitrogen.
During both growing seasons, the total activity and the amount of Rubisco were high in young, sun-exposed leaves and decreased
as the leaves aged and became shaded. In 1985, the specific activity was low (on average 0.8 μmol mm–1 mg–1 Rubisco protein) compared with the values obtained in 1986 (on average 1.8 μmol min–1 mg–1 Rubisco protein). These differences in the specific activity of Rubisco between the two years might be associated with changes
in partitioning of nitrogen. During the cool summer of 1985, about 20% of the total nitrogen was partitioned to Rubisco compared
with about 13% during the warm summer of 1986. Rubisco comprised a high proportion (about 60%) of the total soluble protein
throughout the 1985 growing season, whereas during the 1986 growing season, the proportion of Rubisco in the total soluble
protein fraction was lower and decreased markedly as the leaves became shaded. Chlorophyll did not serve as a source of nitrogen
in the canopy. On the contrary, the proportion of total nitrogen in chlorophyll increased with time, which indicates acclimation
of the thylakoid membranes to decreasing irradiance.
The degree of activation of Rubisco in samples collected in 1986 was high in young, sun-exposed leaves, which suggests that
in these leaves, Rubisco may limit the rate of CO2 uptake. In the shaded leaves of the lower canopy, the degree of activation of Rubisco was low, which indicates that in these
leaves, photosynthesis is limited by factors other than Rubisco.
