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We measured needle pigment content and photosynthetic rates of 1-year-old western larch (Larix occidentalis Nutt.) during autumn foliar senescence. Chlorophyll (Chl) and carotenoid (xanthophyll + b-carotene) contents of needles declined 11 and 17%, respectively, before CO(2) assimilation rate began to decline. Chlorophyll a/b ratio, Chl/carotenoid ratio, photochemical efficiency (F(v)/F(m)), and photochemical quenching did not begin to decline until late in senescence. Internal CO(2)/ambient CO(2) did not change during needle yellowing. In seedlings in warmed soil (average 3 degrees C above natural conditions), the decline in needle chlorophyll content was delayed by 10 days and the decline in CO(2) assimilation rate was delayed by 5 days, compared with seedlings in soil at ambient temperature. In seedlings exposed to an extended 16-h photoperiod, the decline in needle chlorophyll content was delayed by 32 days, and the decline in CO(2) assimilation rate was delayed by 21 days, compared with seedlings exposed to natural day lengths. In addition to delaying the onset of needle senescence, the treatments affected the sequence of events during senescence. Differences among treatment groups provide evidence that the onset of pigment loss and photosynthetic decline and the sequence of events during needle senescence are affected by soil temperature and day length.

The detergent octyl--gucopyranoside (30 mh r in 2 mHsnolar Tris-maleate, pH 7.0) preferentiafly extracts complexes containing protein and chiorophylbs a plus b (CP) from spinach, leaving a residue hbly enriched in CP I (P700-chlorophyll a protein). Use of the detergent results in a relatively gentle extraction since little free chlorophyll is formed and since sodium dodecyl sulfate-gel electrophoresis (on 10% acrylamide) of the extract also reveals the presence of two minor chlorophyll a complexes (apparent molecular weight, 47,000 and 43,000) instead of the usual single complex. The major complex preserved is CP 64, a chlorophyll a/b complex (apparent molecular wegh 64,000) which is an oligomer of another chlorophyl a/b complex, CP 27, the Nght-harvesting complex (apparent molecular weight, 27,000). Dissociation of each complex reveals two polypeptides (molecular weight, 32,000 and 28,000) and limited proteolysis confirms that those of CP 64 have the same structure as those of CP 27.An additional chlorophyl a/b complex (apparent molecular weight, 29,000) is clearly separable from CP 27, and differs from it and CP 64 in having a higher chlorophyll a/b ratio and a single polypeptide (molecular weight, 29,000) which differs structurally from those of the other complexes.Considerable information about chloroplast photosynthetic membranes has accumulated from photochemical experiments and electron microscope studies. The need to integrate material from these two approaches into a model of the molecular architecture of the membrane has led to studies involving membrane fractionation using a wide variety of detergents (anionic, nonionic, and zwitterionic). Electrophoresis of the extracts on SDS gels generally reveals two major CP1 complexes, called CP I (the P700-Chl a protein of photosystem I) and CP II (the light-harvesting Chl a/b complex) (25). There is often a third major band which appears to be an oligomer of CP 11 (2,3,9,13,15,22). In addition, a number of minor complexes have been reported (2,10,12,17), comprising only a small fraction of the total Chl. It has become clear that, for good preservation of complexes during electrophoresis, it is essential to use low detergent to Chl ratios and to perform all operations at 4 C. Using careful procedures, up to 10 CP complexes have been resolved on gels (12). However, not all these complexes have been characterized and it is not always possible to compare minor complexes in different systems.The relationship of CP II to its putative oligomer and to the light-harvesting complex isolated by nonelectrophoretic methods 'Abbreviation: CP: chlorophyll-protein (complex).(6, 23) is still far from clear. Several workers have suggested that the oligomer is more representative of the native light-harvesting complex because more of the oligomer is obtained when gentle conditions are used (2,3,9,12,15,22). The story is complicated by a lack ofagreement about the number ofpolypeptides involved, whether one (9, 22, 23), two (1,3,4,19,27), or more than two (6,12). This pap...

Pb 2+ and Zn 2+ inhibition of photosystem I1 (PSII) activity was reported to be mediated via displacement of native inorganic cofactors (CI-, Ca 2+ and Mn 2+) from the oxygen evolving complex, OEC [Rashid and Popovic (1990) FEBS Lett. 271, 181-184; Rashid et al. (1991) Photosynth. Res. 30, 123-130]. Since the binding sites of these cofactors are protected by a shield of three extrinsic polypeptides (17, 23 and 33 kDa), we investigated whether these metal ions affect the extrinsic polypeptide shield of OEC. By immunoblotting with antibodies recognizing the 23 and 33 kDa polypeptides, we showed that both the metal ions significantly dissociated the 23 kDa (+17 kDa) polypeptide, and partially dissociated the 33 kDa. Ca 2+, one of the important inorganic cofactors of oxygen evolution, strongly prevented the dissociating action of Pb 2+ but did not prevent the action of Zn 2+. The probable molecular mechanism of action of Pb 2+ and Zn 2+ on PSII OEC is discussed.