Photoinhibition of photosynthesis was studied in young and mature detached sun needles of cypress under high irradiance (HI) of about 1 900 μmol m-2 s-1. The degree of photoinhibition was determined by means of the ratio of variable to maximum chlorophyll (Chl) fluorescence (Fv/Fm) and electron transport measurements. Compared with the mature needles, the young needles, containing about half the amount of Chl a+b per unit area, exhibited a higher proportion of total carotenoids (Car) as xanthophyll cycle pigments and had an increased ratio of Car/Chl a+b. The potential efficiency of photosystem (PS) 2, Fv/Fm, markedly declined in HI-treated young needles without significant increase of F0 level. In contrast, the Fv/Fm ratio declined with significant increase of F0 level in mature needles. In isolated thylakoids, the rate of whole chain and PS2 activity markedly decreased in young HI-needles in comparison with mature needles. A smaller inhibition of PS1 activity was observed in both needles. In the subsequent dark incubation, fast recovery was found in both needle Types that reached maximum PS2 efficiencies similar to those observed in non-photoinhibited needles. The artificial exogenous electron donors DPC, NH2OH, and Mn2+ failed to restore the HI-induced loss of PS2 activity in mature needles, while DPC and NH2OH significantly restored it in young needles. Hence, HI-inactivation was on the donor side of PS2 in young needles and on the acceptor side of PS2 in mature needles. Quantification of the PS2 reaction centre proteins D1 and 33 kDa protein of water splitting complex following HI-exposure of needles showed pronounced differences between young and mature needles. The large loss of PS2 activity in HI-needles was due to the marked loss of D1 protein of the PS2 reaction centre in mature needles and of the 33 kDa protein in young needles. and N. La Porta ... [et al.]
Photosynthetic and respirátory electron transfers in cyanobacteria not only serve the bioenergetic needs of these prokaryotes during day and night time. The common use of the plastoquinone pool and the cytochrome (cyt) complex also establishes possibilities for sharing photosystem (PS) 2 plus dehydrogenases at the donor side and cyt c oxidase plus PSI at the acceptor side. Given metabolic conditions and radiant energy supply, the available choices may give rise to unusual combinations of connected electron transfer activities, for example PS2 and cyt c oxidase. In vivo measurements of energy storage in PSI cyclic photophosphorylation via photoacoustic spectroscopy, and of the P700 redox statě via absorbance changes at 820 nm detected with the pulse amplitudě modulation technique (PAM), as well as of PS2 fluorescence yield, all in the absence or presence of the cyt c oxidase inhibitor KCN were combined to demonstrate that the two pathways at the acceptor side communicate in vivo. This type of regulation serves proper poising of electron flow through and around PSI. The impaired cyt c oxidase activity (in this study achieved by addition of KCN) prevents a truly oxidized statě of P700 to be reached, which hampers electron passage from PS2. The relative overreduction of PS 1 in the KCN intoxicated samples reduces the electron flow directed to biosynthesis. The results illustrate the versatility of the P700 redox statě measurements at 820 nm as a means to study in vivo electron fluxes in cyanobacteria.
Chlorophyll fluorescence parameters of Quercus pubescens Willd. as response to heat shock (HS) by immersing leaves for 5 and 15 min in water of temperatures between 38 and 59 °C were examined. Fluorescence was measured after different periods of recovery (15, 30, 90, 210, and 1 440 min at 24/26 °C night/day temperature and 100 % humidity). The effective quantum yield of photosystem 2 (Y) in control and HS-treated leaves was always measured after previous 15 min irradiation. Under a 5 min HS, Y did not change after using temperatures below 44 °C, was rapidly restored after HS of moderate temperatures (44-48 °C), and progressively decreased and recovered eventually to the initial value after HS of high temperatures (48-52 °C). Y did not recover after HS with temperatures higher than 52 °C. Increase in the duration of HS from 5 to 15 min lead to change of the initial Y at each HS temperature, but the recovery processes were similar to those characteristic after 5 min incubation. The processes of recovery may depend mainly on the specificity of injuries caused by different heat shock temperatures. Thus Q. pubescens is able to preserve and recover the functional potential of its photosynthetic apparatus in response to HS up to 52 °C. and A. Dascaliuc, T. Ralea, P. Cuza.
Insect-infested (II) acorns germinated 3 d earlier than the healthy (H) ones. However, germination ratio of II-acorns was strongly decreased compared with H-acorns and there were great differences in activities of amylase. We found an apparently lower net photosynthetic rate and total chlorophyll contents of the first true leaf of II-acorns than of the H-ones. Maximal photochemical efficiency of photosystem 2 (PS2, Fv/Fm) decreased in seedlings germinated from II-acorns than from the H-ones. Infestation of insects, especially for weevil (Curculio spp.) had significantly negative effects on length of taproots, height of plants, dry mass (DM) of roots and the first fully expanded true leaf. Leaf area and total N content of the first true leaf declined due to limitation of resource reserves in cotyledons. Damage of cotyledons caused by weevil accounted much for poor development of seedlings germinated from II-acorns. A mutual relationship between seedling establishment and seed-infesting insects may exist due to high predation on H-acorns by small rodents. and X. F. Yi, Z. B. Zhang
Influence of manganese (Mn) toxicity on photosynthesis in ricebean (Vigna umbellata) was studied by the measurement of gas exchange characteristics and chlorophyll fluorescence parameters. The net photosynthetic rate (PN), transpiration rate (E), and stomatal conductance (gs) were reduced with increasing Mn concentration in nutrient solution. The reduction in gs and E was more pronounced at 6 d of Mn treatment. However, PN declined at 2 d of Mn treatment implying that the reduction in photosynthesis was not due to the direct effect of Mn on stomatal regulation. Mn did not affect the maximum efficiency of photosystem 2 (PS2) photochemistry (Fv/Fm). A reduction in photochemical quenching (qP) and excitation capture efficiency of open PS2 (Fv'/Fm') with a concomitant increase in qN was observed. This implies that reduced demand for ATP and NADPH due to the reduction in photosynthesis causes a down-regulation of PS2 photochemistry and thus a high pH gradient (increase in qN) and limited electron transport (decreased qP). and Desiraju Subrahmanyam, V. S. Rathore.
The combined effects of water stress (WS) and low irradiance (LI) on growth, photosynthesis, osmotic adjustment, and lipid peroxidation were studied in dragon spruce (Picea asperata Mast.) seedlings grown under two water treatments (well watered, 100 % of field capacity, and water stressed, 30 % of field capacity) and two irradiances (HI, 100 % of full sunlight and LI, 15 % of full sunlight). WS reduced growth, chlorophyll (Chl) a and b contents, net photosynthetic rate, transpiration rate, stomatal conductance, and effective quantum yield of photosystem 2 (Y) but increased free proline and malondialdehyde contents. LI increased Chl contents and decreased Y, photochemical quenching (qP), and non-photochemical quenching (qN) under both water treatments. Hence the seedlings in the understory were more sensitive to drought than to LI. and Y. Yang ... [et al.].
Two types of photosystem 2-light-harvesting complex 2 (PS2-LHC2) supercomplexes with similar pigment and protein composition were isolated directly from thylakoid membranes by sucrose density gradient centrifugation. Electron microscopy and single particle image analysis revealed the first Type as single unpaired PS2-LHC2 supercomplexes, whereas the second Type was characterized as pairs of two PS2-LHC2 supercomplexes attached together by their stromal sides. Unstacking of thylakoid membranes resulted in a spontaneous disintegration of the paired supercomplexes into single unpaired particles. A model of the organisation of the pigment-protein complexes in grana region is proposed. and L. Bumba, M. Hušák, F. Vácha.
Watermelon (Citrillus lanatus) plants were grown for two consecutive years in open-top chambers with three different ozone concentrations (O3-free air, O3 ambient, and air with additional O3; CFA, NFA, and NFA+O3) and three nitrogen fertilizer concentrations [0, 14.0, and 29.6 g N per pot; N0, N1, and N2). There was an interaction between ozone and N fertilizer for the major parameters studied. O3 and N2 treatments led to a significant decrease in maximum efficiency of photosystem 2 (PS2) photochemistry (Fv/Fm), and induced a significant decrease in the actual quantum yield of PS2 (ΦPS2), due mainly to the increased closure of PS2 reaction centres (qP) and to an increase in the non-photochemical quenching (NPQ). On the other hand, these plants exhibited an increased susceptibility to photoinhibition, which could be associated with an increased fraction of reduced QA. An increase in lipid peroxidation indicated that damage was occurring at the membrane levels. High N concentration enhanced the detrimental effects of ozone on the fluorescence parameter induction and lipid peroxidation. All these negative alterations led to a decreased yield. and A. Calatayud, F. Pomares, E. Barreno.
Seeds of Suaeda salsa were cultured in dark for 3 d and betacyanin accumulation in seedlings was promoted significantly. Then the seedlings with accumulated betacyanin (C+B) were transferred to 14/10 h light/dark and used for chilling treatment 15 d later. Photosystem 2 (PS2) photochemistry, D1 protein content, and xanthophyll cycle during the chilling-induced photoinhibition (exposed to 5 °C at a moderate photon flux density of 500 µmol m-2 s-1 for 3 h) and the subsequent restoration were compared between the C+B seedlings and the control (C) ones. The maximal efficiency of PS2 photochemistry (Fv/Fm), the efficiency of excitation energy capture by open PS2 centres (Fv'/Fm'), and the yield of PS2 electron transport (ΦPS2) of the C+B and C leaves both decreased during photoinhibition. However, smaller decreases in Fv/Fm, Fv'/Fm', and ΦPS2 were observed in the C+B leaves than in C ones. At the same time, the deepoxidation state of xanthophyll cycle, indicated by (A+Z)/(V+A+Z) ratio, increased rapidly but the D1 protein content decreased considerably during the photoinhibition. The increase in rate of (A+Z)/(V+A+Z) was higher but the D1 protein turnover was slower in C+B than C leaves. After photoinhibition treatment, the plants were transferred to a dim irradiation (10 µmol m-2 s-1) at 25 °C for restoration. During restoration, the chlorophyll (Chl) fluorescence parameters, D1 protein content, and xanthophyll cycle components relaxed gradually, but the rate and level of restoration in the C+B leaves was greater than those in the C leaves. The addition of betacyanins to the thylakoid solution in vitro resulted in similar changes of Fv/Fm, D1 protein content, and (A+Z)/(V+A+Z) ratio during the chilling process. Therefore, betacyanin accumulation in S. salsa seedlings may result in higher resistance to photoinhibition, larger slowing down of D1 protein turnover, and enhancement of non-radiative energy dissipation associated with xanthophyll cycle, as well as in greater restoration after photoinhibition than in the control when subjected to chilling at moderate irradiance. and C.-Q. Wang, T. Liu
Prosopis juliflora is a hardy plant tolerant to drought, salinity, extremes of soil pH, and heavy metal stress. We isolated and characterized a photosystem 2 (PS2) gene PsbR (Pj PsbR) and its promoter. Northern analysis for Pj PsbR in P. juliflora leaves under 25 % polyethylene glycol stress showed steady decrease in transcript level at 12, 24, and 48 h after stress application. Under 90 mM H2O2 stress, transcript level dropped drastically at 12 h, but increased again compared to the control at 24 h. A 1.7 kb fragment upstream the 5' UTR of this gene (putative promoter) was isolated and analyzed in silico. Several putative cis-acting DNA elements were identified in this sequence. and G. Suja, A. Parida.