COMPARISON OF CHLOROPHYLL FLUORESCENCE QUENCHING IN LEAVES OF WILD-TYPE WITH A CHLOROPHYLL-B-LESS MUTANT OF BARLEY (HORDEUM-VULGARE L)

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.