12.
Using attached and detached leaves of
Acer palmatum Thunb. and
Rhaphiolepsis umbellata Makino, pulse-modulated chlorophyll fluorescence and CO
2 exchange were measured. Quantum yield of photosynthesis was determined from the fluorescence parameter
(Fm′−Fs)/Fm′, where (
Fm′−Fs) was defined as the difference between steady state chlorophyll fluorescence (
Fs) and maximum fluorescence (
Fm′) elicited by a saturating light pulse. The rate of electron transport through photosystem II (total electron flow) was calculated
from the product of quantum yield and
A (PFD), where
A is the rate of absorbed photons as given by leaf absorptance, and PFD is the photon flux density at the leaf surface. The
rate of electron transport dependant on CO
2 uptake (assimilative electron flow) was calculated from the gross photosynthetic rate in a leaf. The difference between the
rates of total and assimilative electron transport was denoted as the rate of non-assimilative electron transport which depends
on photorespiration and oxygen reduction. Available data provided quantitative information on the rate of non-assimilative
electron flow in intact leaves. When leaf photosynthesis of
A. palmatum was measured under sunlight, the rates of total and assimilative electron transport were determined to be approximately 900
and 150 μmol equiv. e/mg Chl·h, respectively. The difference (750 μmol equiv. e/mg Chl·h) was attributed to the activity of
non-assimilative electron flow. The ratio of total to assimilative electron flow was found to increase gradually with rising
in irradiance. The results suggest that non-assimilative electron flow occurred at much higher rate than assimilative electron
flow at high irradiance. Implications of the results are briefly discussed in relation to photosynthesis limitation in tree
leaves.
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