title: Development of photosystem-II activity during irradiance of etiolated Helianthus (Asteraceae) seedlings.
abstract: Light-induced development of photosystem (PS)-II activity was followed during irradiance of etiolated Helianthus annuus (sunflower) cotyledons using chlorophyll a fluorescence. Cotyledons from seedlings grown in continuous darkness for 6 d were exposed to 100 μmol photons·m(-2)·s(-1) for time periods of 1, 3, 6, and 12 h. Associated with increased time of irradiance exposure were significant: (1) increases in concentration of PS II, (2) increases in quantum efficiency of PS II, (3) decreases in the ratio of PS-II quinone(B) (Q(B))-nonreducing centers to total PS-II centers (PS-II Q(B)-nonreducing centers + PS-II Q(B)-reducing centers), and (4) decreases in the ratio of slow PS-II Q(B)-reducing centers to total PS-II Q(B)-reducing centers (fast PS-II Q(B)-reducing centers + slow PS-II Q(B)-reducing centers). The results support the hypotheses that development of PS II involves assembly of complexes which initially cannot reduce Q(B) and that heterogeneous aspects of PS-II pools during chloroplast maturation may represent different developmental states.
authors.affiliation: Department of Biology, Austin Peay State University, Clarksville, Tennessee 37044.
authors.forename: J G
authors.initials: JG
authors.lastname: Lebkuecher
authors.forename: K A
authors.initials: KA
authors.lastname: Haldeman
authors.forename: C E
authors.initials: CE
authors.lastname: Harris
authors.forename: S L
authors.initials: SL
authors.lastname: Holz
authors.forename: S A
authors.initials: SA
authors.lastname: Joudah
authors.forename: D A
authors.initials: DA
authors.lastname: Minton