V. Spunda et al., RESPONSES OF PHOTOSYSTEM 2 PHOTOCHEMISTRY AND PIGMENT COMPOSITION IN NEEDLES OF NORWAY SPRUCE SAPLINGS TO INCREASED RADIATION LEVEL, Photosynthetica, 28(3), 1993, pp. 401-413
To reveal the dynamics of short-term photosynthetic acclimation to inc
reased irradiance, the light response of photochemical (q(P)), non-pho
tochemical (q(N)) and F0 (q0) quenchings of chlorophyll (Chl) fluoresc
ence and Chl and carotenoids composition in Norway spruce needles were
monitored within three days after transfer of saplings from low diffu
se irradiance (maximum photosynthetic photon fluence density PPFD 50 m
umol m-2 s-1) to direct sun radiation (maximum PPFD 2000 mumol m-2 s-1
). Irradiance responses of fluorescence quenching coefficients reveale
d the occurrence of substantial changes in partitioning of excitation
energy between photochemical reactions and radiationless dissipation w
ithin two days. The saturating irradiance for q(N) and the capacity of
non-radiative dissipation processes was shifted from about 450 mumol
m-2 S-1 to 1620 mumol M-2 s-1. Whereas immediately after exposure to f
ull sunlight Q(A) was completely reduced at 1620 mumol m-2 s-1, two da
ys later 40% Of Q(A) was still present in oxidized form at this irradi
ance. A fast pigment photobleaching at noon prevented the over-reducti
on Of Q(A) and thus it was one of the possible short-term acclimation
processes. No severe photoinhibition of photosystem 2 (PS 2) photochem
istry occurred within the period of investigation as can be judged fro
m the high F(v)/F(m) value.