© 2005 Heron Publishing—Victoria, Canada
Vertical, horizontal and azimuthal variations in leaf photosynthetic characteristics within a Fagus crenata crown in relation to light acclimation
Atsuhiro Iio (1), Hisakazu Fukasawa (1), Yachiho Nose (1), Shuri Kato (2) and Yoshitaka Kakubari (1, 3)
1. Faculty of Agriculture, University of Shizuoka, Ohya 836, Shizuoka 422-8529, Japan / 2. Faculty of Applied Biological Sciences, University of Gifu, Yanagido 1-1,Gifu 501-1193, Japan / 3. Corresponding author (afykaku@agr.shizuoka.ac.jp) / Received June 14, 2004; accepted November 5, 2004; published online March 1, 2005
Summary
An understanding of spatial variations in gas exchange parameters in relation to the light environment is crucial for modeling
canopy photosynthesis. We measured vertical, horizontal and azimuthal (north and south) variations in photosynthetic capacity
(i.e., the maximum rate of carboxylation: Vcmax), nitrogen content (N), leaf mass per area (LMA) and chlorophyll content (Chl) in relation to relative photosynthetic photon
flux (rPPF) within a Fagus crenata Blume crown. The horizontal gradient of rPPF was similar in magnitude to the vertical gradient of rPPF from the upper to
the lower crown. The rPPF in the north quadrant of the crown was slightly lower than in the south quadrant. Nitrogen content
per area (Narea), LMA and Vcmax were strictly proportional to rPPF, irrespective of the vertical direction, horizontal direction and crown azimuth, whereas
nitrogen content per dry mass, Chl per area and photosynthetic capacity per dry mass (Vm) were fairly constant.
Statistical analyses separating vertical trends from horizontal and azimuthal trends indicated that, although horizontal and
vertical light acclimation of leaf properties were similar, there were two significant azimuthal variations: (1) Vcmax was lower in north-facing leaves than in south-facing leaves for a given Narea, indicating low photosynthetic nitrogen-use efficiency (PNUE) of north-facing leaves; and (2) Vcmax was lower in north-facing leaves than in south-facing leaves for a given LMA, indicating low Vm of the north-facing leaves. With respect to the low PNUE of the north-facing leaves, there were no significant azimuthal
variations in leaf CO2 conductance from the stomata to the carboxylation site. Biochemical analysis indicated that azimuthal variations in nitrogen
allocation to ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and in nitrogen allocation between carboxylation (Rubisco
and other Calvin cycle enzymes) and light harvesting machinery (Chl pigment-protein complexes) were not the main contributor
to the difference in PNUE between north- and south-facing leaves. Lower specific activity of Rubisco may be responsible for
the low PNUE of the north-facing leaves. Anatomical analysis indicated that not only high leaf density, which is compatible
with a greater fraction of non-photosynthetic tissue, but also thick photosynthetic tissue contributed to the low Vm in the north-facing leaves. These azimuthal variations may need to be considered when modeling canopy photosynthesis based
on the Narea–Vcmax or LMA–Vcmax relationship.
Keywords:
leaf mass per area, maximum rate of carboxylation, nitrogen content, relative photosynthetic photon flux, three-dimensional
variation.
