Dissipation of Excitation Energy During Development of Photosystem 2 Photochemistry in Helianthus annuus

Etiolated sunflower cotyledons developed in complete darkness and lacking photosystem (PS) 2 were exposed to continuous 200 µmol(photon) m^–2 s^–1 white light for 1, 3, 6, 12, and 18 h prior to evaluations of excitation-energy dissipation using modulated chlorophyll a fluorescence. Photochemical potential of PS2, measured as the dark-adapted quantum efficiency of PS2 (F_V(M)/F_M), and thermal dissipation from the antenna pigment-protein complex, measured as the Stern-Volmer non-photochemical quenching coefficient (NPQ), increased to 12 h of irradiation. Following 12 h of irradiation, thermal dissipation from the antennae pigment-protein complex decreased while the efficiency of excitation capture by PS2 centers (F_V/F_M) and light-adapted quantum efficiency of PS2 (_PS2) continued to increase to 18 h of irradiation. The fraction of the oxidized state of Q_A, measured by the photochemical quenching coefficient (q_P), remained near optimal and was not changed significantly by irradiation time. Hence during the development of maximum photochemical potential of PS2 in sunflower etioplasts, which initially lacked PS2, enhanced thermal dissipation helps limit excitation energy reaching PS2 centers. Changes of the magnitude of thermal dissipation help maintain an optimum fraction of the oxidized state of Q_A during the development of PS2 photochemistry.     

Foliar δ13C within a temperate deciduous forest: spatial,temporal, and species sources of variation

Summary Foliar ¹³C-abundance (¹³C) was analyzed in the dominant trees of a temperate deciduous forest in east Tennessee (Walker Branch Watershed) to investigate the variation in foliar ¹³C as a function of time (within-year and between years), space (canopy height, watershed topography and habitat) and species (deciduous and coniferous taxa). Various hypotheses were tested by analyzing (i) samples collected from the field during the growing season and (ii) foliar tissues maintained in an archived collection. The ¹³C-value for leaves from the tops of trees was 2 to 3%. more positive than for leaves sampled at lower heights in the canopy. Quercus prinus leaves sampled just prior to autumn leaf fall had significantly more negative ¹³C-values than those sampled during midsummer. On the more xeric ridges, needles of Pinus spp. had more positive ¹³C-values than leaves from deciduous species. Foliar ¹³C-values differed significantly as a function of topography. Deciduous leaves from xeric sites (ridges and slopes) had more positive ¹³C-values than those from mesic (riparian and cove) environments. On the more xeric sites, foliar ¹³C was significantly more positive in 1988 (a dry year) relative to that in 1989 (a year with above-normal precipitation). In contrast, leaf ¹³C in trees from mesic valley bottoms did not differ significantly among years with disparate precipitation. Patterns in foliar ¹³C indicated a higher ratio of net CO₂ assimilation to transpiration (A/E) for trees in more xeric versus mesic habitats, and for trees in xeric habitats during years of drought versus years of normal precipitation. However, A/E (units of mmol CO₂ fixed/mol H₂O transpired) calculated on the basis of ¹³C-values for leaves from the more xeric sites was higher in a wet year (6.6±1.2) versus a dry year (3.4±0.4). This difference was attributed to higher transpiration (and therefore lower A/E) in the year with lower relative humidity and higher average daily temperature. The calculated A/E values for the forest in 1988–89, based on ¹³C, were within ±55% of estimates made over a 17 day period at this site in 1984 using micrometeorological methods.     

Indirect effects of insect herbivory on leaf gas exchange in soybean

Herbivory can affect plant carbon gain directly by removing photosynthetic leaf tissue and indirectly by inducing the production of costly defensive compounds or disrupting the movement of water and nutrients. The indirect effects of herbivory on carbon and water fluxes of soybean leaves were investigated using gas exchange, chlorophyll fluorescence and thermal imaging. Herbivory by Popillia japonica and Helicoverpa zea (Boddie) caused a 20–90% increase in transpiration from soybean leaflets without affecting carbon assimilation rates or photosynthetic efficiency (Φ_PSII). Mechanical damage to interveinal tissue increased transpiration up to 150%. The spatial pattern of leaf temperature indicated that water loss occurred from injuries to the cuticle as well as from cut edges. A fluorescent tracer (sulforhodamine G) indicated that water evaporated from the apoplast approximately 100 µm away from the cut edges of damaged leaves. The rate of water loss from damaged leaves remained significantly higher than from control leaves for 6 d, during which time they lost 45% more water than control leaves (0.72 mol H₂O per cm of damaged perimeter). Profligate water loss through the perimeter of damaged tissue indicates that herbivory may exacerbate water stress of soybeans under field conditions.     

Inhibition of chlorophyll biosynthesis by α′,α′-dipyridyl during greening of groundnut leaves

The site of inhibition of chlorophyll biosynthesis by α′,α′-dipyridyl was found to be at the level of conversion of chlorophyllide (672 nm) to chlorophyll (678 nm) during greening of groundnut leaves. This inhibition was partially reversed by certain divalent cations.     

Acclimation to irradiance in seedlings of three tropical rain forest Garcinia species after simulated gap formation

We investigated the acclimation of seedlings of three tropical rain forest sub-canopy Garcinia species (G. xanthochymus, G. cowa, and G. bracteata) after transfer from 4.5 (LI) to 40 % (HI) sunlight and 12.5 (MI) sunlight to HI (LH₁ and LH₂ denoting transfer from LI to HI and MI to HI transfer, respectively). The changes of chlorophyll (Chl) fluorescence, net photosynthetic rate (P _N), dark respiration rate (R _D), Chl content per unit area (Chl_area), leaf mass per unit area (LMA), and seedling mortality were monitored over two months after transfer. These parameters together with leaf anatomy of transferred and control seedlings (kept in LI, MI, and HI) were also examined after two months. No seedlings died during the two months. F_v/F_m, P _N, and Chl_area of the transferred seedlings decreased in the first 3 to 12 d. LH₁ leaves showed larger reduction in F_v/F_m (>23 % vs. <16 %) and slower recovery of F_v/F_m than LH₂ leaves. P _N started to recover after about one week of I transfer and approached higher values in all G. cowa seedlings and G. xanthochymus LH₁ seedlings than those before the transfer. However, P _N of G. bracteata seedlings approached the values before transfer. The final P _N values in leaves of transferred G. xanthochymus and G. cowa seedlings approached that of leaves kept in HI, while the final P _N values of transferred leaves of G. bracteata were significantly lower than that of leaves grown under HI (p<0.05). R _D of G. xanthochymus LH₁ seedlings and all G. cowa seedlings increased and approached the value of the seedlings in HI. The final Chl_area of both G. xanthochymus and G. cowa approached the values before transfer, but that of G. bracteata did not recover to the level before transfer. The final Chl_area of all transferred seedlings was not significantly different from that of seedlings in HI except that G. cowa LH₁ seedlings had higher Chl_area than that in HI. LMA decreased within 2 d and then increased continuously until about 30 d and approached the value under HI. Spongy/palisade mesophyll ratio decreased after transfer because of the increase in palisade thickness. Leaf thickness did not change, so LMA increase of transferred seedlings was mainly due to the increase of leaf density. Thus the mature leaves under LI and MI of G. xanthochymus and G. cowa are able to acclimate to HI by leaf physiological and anatomical adjustment, while G. bracteata had limited ability to acclimate to HI.     

Extinction coefficients of cytochromes b559 and c550 of Thermosynechococcus elongatus and Cyt b559/PS II stoichiometry of higher plants

“Reduced minus oxidized” difference extinction coefficients Δ? in the α-bands of Cyt b559 and Cyt c550 were determined by using functionally and structurally well-characterized PS II core complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus. Values of 25.1 ± 1.0 mM⁻¹ cm⁻¹ and 27.0 ± 1.0 mM⁻¹ cm⁻¹ were obtained for Cyt b559 and Cyt c550, respectively. Anaerobic redox titrations covering the wide range from −250 up to +450 mV revealed that the heme groups of both Cyt b559 and Cyt c550 exhibit homogenous redox properties in the sample preparation used, with E_m values at pH 6.5 of 244 ± 11 mV and −94 ± 21 mV, respectively. No HP form of Cyt b559 could be detected. Experiments performed on PS II membrane fragments of higher plants where the content of the high potential form of Cyt b559 was varied by special treatments (pH, heat) have shown that the α-band extinction of Cyt b559 does not depend on the redox form of the heme group. Based on the results of this study the Cyt b559/PSII stoichiometry is inferred to be 1:1 not only in thermophilic cyanobacteria as known from the crystal structure but also in PSII of plants. Possible interrelationships between the structure of the Q_B site and the microenvironment of the heme group of Cyt b559 are discussed.     

The light-harvesting and protective functions of carotenoids in photosynthetic membranes

Siefermann-Harms, D. 1987. The light-harvesting and protective functions of carotenoids in photosynthetic membranes.     

Study of the non-photochemical dark rise in chlorophyll fluorescence in pre-illuminated leaves of various C3 and C4 plants submitted to partial anaerobiosis

Changes in chlorophyll fluorescence yield were studied during a dark period in pre-illuminated leaves of various C3 and C4 plants. The oxygen content in the gaseous atmosphere was either normal (21 kPa) or low (1.5 or 0.36 kPa). C3 and C4 plants of the NAD malic enzyme subgroup showed an initial rise in fluorescence at the onset of the dark period with an amplitude depending on the O₂ level in the gas. In C4 plants belonging to the other two subgroups, the slow rise was absent or of very low size. At high [O₂], the fluorescence level decreased in some minutes to the initial F₀ level (determined in dark-adapted leaves). Conversely at low [O₂], the fluorescence yield remained higher than F₀ in all the C4 plants studied, whereas it decreased slowly to the F₀ level in the different C3 plants. At low [O₂], the fluorescence level decreased rapidly to F₀ when introducing for 30 s, a high O₂ level or when giving a 15-s far-red illumination. At the end of these treatments, the fluorescence level re-increased. These results demonstrate the presence at low [O₂] of highly fluorescent ‘closed' photosystem II centres containing Q^-_A in equilibrium with reduced plastoquinone molecules of the chloroplastic pool. Reoxidation of the plastoquinone pool would be dependent on the functioning of an oxidase probably dependent on a chlororespiration process fully active at O₂ levels higher than 2 kPa. The source of reducing equivalents for the plastoquinone pool is discussed.