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PubMed1998

Regulation of thermal dissipation of absorbed excitation energy and violaxanthin deepoxidation in the thylakoids of lactuca sativa. Photoprotective mechanism of a population of photosystem II centers

Non-photochemical quenching of chlorophyll a fluorescence is thought to be mainly associated with thermal dissipation of excitation energy taking place within the antenna and reaction center of PS II. In this report, non-photochemical fluorescence quenching was investigated in the fluorescence yields induced by a series of short and high-energy flashes after dark adaptation. The observation of period four fluorescence oscillations with increasing flash number indicates functioning O2 evolving centers. It was found that these PS II centers could not be identical to all the O2 evolving centers. Appreciable differences in antenna size and the number of centers were observed between the PS II centers contributing to fluorescence oscillations and the PS II centers that evolve the flash-induced steady-state O2 yield. Direct evidence for non-photochemical fluorescence quenching was provided by the numerical fitting of the fluorescence oscillations. This procedure revealed that a proportion of the centers exhibiting oscillating fluorescence yields, converted into quenching centers after each flash of a series (7% in February; 17% in June). The observed quenching could not be related to a dissipative process inside the reaction center. Instead, it was attributed to a change in the organization of some PS II centers in the membrane, possibly a conversion of PS II dimers into PS II monomers, resulting in a decreased absorption cross-section for these centers. Quenching resulting from energy de-excitation in the antenna was also observed. This was a light-initiated process, but the modification of the antenna occurred in the dark on a time scale of a few minutes. After this dark period and only on the first flash of a series, antenna quenching was revealed by a smaller absorption cross-section of the PS II centers involved in fluorescence oscillations. This process was reversed on the following flashes. The same period of darkness after illumination was necessary to allow maximum zeaxanthin formation to occur in the dark at a higher pH than the pH for optimum violaxanthin deepoxidation in the absence of preillumination. To explain this effect, comparable to that referred to as light activation for non-photochemical quenching (Ruban and Horton, Aust. J. Plant Physiol. 22 (1995) 221-230), we propose that upon preillumination (before darkness), the protons released in response to a net positive charge in these PS II centers, have access to proton binding groups acting in a cooperative way in LHC II. This accounts for the proton cooperativity as can be deduced from the pH dependence of the rate constant of violaxanthin deepoxidation (Hill coefficient n from 2 to 6). Copyright 1998 Elsevier Science B.V.

Journal Article↗
PubMed1999

Mapping morphological genes relative to molecular markers in lettuce (Lactuca sativa L.)

Two F2 populations were generated by crossing morphologically diverse genetic stocks in order to map 10 morphological traits relative to polymerase chain reaction-based molecular markers (RAPDs). Using one segregating population generated from crossing the experimental line, 'dwarf-2', with the butterhead cultivar, 'Saffier', the dwarf phenotype conditioned by the dwf2 locus was mapped using bulked segregation analysis to within 38 cM of the Adh3 locus. Using the second segregating population generated by crossing two experimental lines, 87-25-1M x 87-109M, nine traits [white seed (w), brown seed (br), salmon flower colour (sa), pale yellow flower colour (pa), virescent juvenile leaf colour (vi), plump involucre (pl), yellow seed (y), one of two complementary genes for anthocyanin expression (C or G) and anthocyanin spotting (Rs)] were linked to RAPD loci, but only six of them could be placed on an existing genetic map of lettuce generated by analysis of cv. 'Calmar' x cv. 'Kordaat'. A tenth trait, golden yellow (gy), remained unlinked. Approximately a third of the RAPD markers analysed segregated in both the 87-25-1M x 87-1090M and 'Calmar' x 'Kordaat' populations. In the genomic regions with multiple segregating loci in common, their relative orders and distances were mostly conserved. In one instance, linkage detected in the present study consolidated two separate groups on the earlier genetic map.

Journal Article↗