THE CHANGING CONTRIBUTION OF LHC I TO PHOTOSYSTEM-I ACTIVITY DURING CHLOROPLAST BIOGENESIS IN WHEAT

During chloroplast biogenesis in 5-day-old wheat leaves, the apparent quantum yield of Photosystem I (PS I) electron transport activity of isolated thylakoids increases. From measurements of apparent PS I quantum yield using 440 and 470 nm radiation, it is demonstrated that this increase is attributable to an increase in the contribution of a chlorophyll-b-containing component of the PS I antenna. Immunoblot analyses of constituent polypeptides of the core complex of PS I (core complex I) and the light-harvesting chlorophyll a/b-protein complexes associated with PS I (LHC I) and PS II (LHC II) indicated that during chloroplast development the appearance and accumulation of LHC I in the thylakoids lag behind those of LHC II and core complex I. This finding was supported by changes observed in the 77 K fluorescence emission spectra of isolated thylakoids; the 740 nm emisson peak, associated with LHC I, increased markedly relative to the 686 nm peak during development. The effects of Mg2+ depletion on the apparent PS I quantum yield indicated that light captured by LHC II did not make an increasing contribution to PS I photochemical activity during development. It is concluded that the increase in the light capture ability to PS I in wheat thylakoids during chloroplast biogenesis is attributable to the delay in accumulation of LHC I relative to core complex I.