H. Lokstein et al., COMPARISON OF CHLOROPHYLL FLUORESCENCE QUENCHING IN LEAVES OF WILD-TYPE WITH A CHLOROPHYLL-B-LESS MUTANT OF BARLEY (HORDEUM-VULGARE L), Journal of photochemistry and photobiology.B, Biology, 19(3), 1993, pp. 217-225
A pulse amplitude-modulated fluorometric technique was employed to sep
arate photochemical (qP) and non-photochemical (qN) chlorophyll fluore
scence quenching in attached leaves of wild-type and a chlorophyll-b-l
ess mutant of barley (Hordeum vulgare L.). Whereas a significantly hig
her qN developed in the wild type in the intensity range at which the
photosynthetic light response became non-linear, there were virtually
no differences in qP until pronounced photoinhibition was apparent. On
monitoring the ''dark'' recovery of the maximum (F(M)) and dark level
(F0) fluorescence yield, three distinct kinetic phases were resolved,
which are ascribed to relaxation of high energy state quenching (qE),
state 1-state 2 transitions (qT) and quenching due to photoinhibition
(qI). The results provide evidence for heterogeneity of qE. The major
part of qE (related to a fast-relaxing phase of F(M) quenching) is st
rongly reduced in mutant leaves. A fast-relaxing phase of F0 quenching
, readily observed in wild-type leaves under high light, is absent in
the mutant under the same conditions. Hence, qE appears to be associat
ed with the photosystem II light-harvesting complex (LHC II). A medium
component of ''dark'' relaxation kinetics was observed in both mutant
and wild-type leaves. However, it appears attributable to qT only in
the latter at low light. Supported by the finding that there was no di
fference in xanthophyll pool size (on a chlorophyll a basis) between w
ild type and mutant, under high light conditions the medium phase may
reflect a slower-relaxing portion of qE, probably due to xanthophyll c
onversion. Under supersaturating light the very slowly relaxing qI com
ponent became dominating, affecting mutant leaves to a considerably gr
eater extent. The results stress the key role of LHC II not only as a
solar energy collector but also in protecting the photosynthetic appar
atus from adverse effects of excess excitation input.