Am. Gilmore et O. Bjorkman, TEMPERATURE-SENSITIVE COUPLING AND UNCOUPLING OF ATPASE-MEDIATED, NONRADIATIVE ENERGY-DISSIPATION - SIMILARITIES BETWEEN CHLOROPLASTS AND LEAVES, Planta, 197(4), 1995, pp. 646-654
The effects of temperature on the dark relaxation kinetics of nonradia
tive energy dissipation in photosystem II were compared in lettuce (La
ctuca sativa L.) chloroplasts and leaves of Aegialitis annulata R. Br.
After high levels of violaxanthin de-epoxidation in the light, Aegial
itis leaves showed a marked delay in the dark relaxation of nonradiati
ve dissipation, measured as nonphotochemical quenching (NPQ) of photos
ystem II chlorophyll a fluorescence. Aegialitis leaves also maintained
a moderately high adenylate energy charge at low temperatures during
and after high-light exposure, presumably because of their limited car
bon-fixation capacity. Similarly, dark-sustained NPQ could be induced
in lettuce chloroplasts after de-epoxidizing violaxanthin and light-ac
tivating the ATP synthase. The duration and extent of dark-sustained N
PQ were strongly enhanced by low temperatures in both chloroplasts and
leaves. Further, the NPQ sustained at low temperatures was rapidly re
versed upon warming. In lettuce chloroplasts, low temperatures sharply
decreased the ATP-hydrolysis rate while increasing the duration and e
xtent of the resultant trans-thylakoid proton gradient that elicits th
e NPQ. This was consistent with a higher degree of energy-coupling, pr
esumably due to reduced proton diffusion through the thylakoid membran
e at the lower temperatures. The chloroplast adenylate pool was in equ
ilibrium with the adenylate kinase and therefore both ATP and ADP cont
ributed to reverse coupling, The low-temperature-enhanced NPQ quenched
the yields of the dark level (F-o) and the maximal (F-m) fluorescence
proportionally in both chloroplasts and leaves. The extent of NPQ in
the dark was inversely related to the efficiency of photosystem II, an
d very similar linear relationships were obtained over a wide temperat
ure range in both chloroplasts and leaves. Likewise, the dark-sustaine
d absorbance changes, caused by violaxanthin de-epoxidation (A(508 nm)
) and energy-dependent light scattering (A(536) nm) were strikingly si
milar in chloroplasts and leaves. Therefore, we conclude that the dark
-sustained, low-temperature-stimulated NPQ in chloroplasts and leaves
is apparently directly dependent on lumen acidification and chloroplas
tic ATP hydrolysis. In leaves, the ATP required for sustained NPQ is e
vidently provided by oxidative phosphorylation in the mitochondria. Th
e functional significance of this quenching process and implications f
or measurements of photoprotection versus photodamage in leaves are di
scussed.