The carotenoid zeaxanthin and ‘high-energy-state quenching’ of chlorophyll fluorescence

The possibility that zeaxanthin mediates the dissipation of an excess of excitation energy in the antenna chlorophyll of the photochemical apparatus has been tested through the use of an inhibitor of violaxanthin de-epoxidation, dithiothreitol (DTT), as well as through the comparison of two closely related organisms (green and blue-green algal lichens), one of which (blue-green algal lichen) naturally lacks the xanthophyll cycle. In spinach leaves, DTT inhibited a major component of the rapidly relaxing high-energy-state quenching' of chlorophyll fluorescence, which was associated with a quenching of the level of initial fluorescence (F₀) and exhibited a close correlation with the zeaxanthin content of leaves when fluorescence quenching was expressed as the rate constant for radiationless energy dissipation in the antenna chlorophyll. Green algal lichens, which possess the xanthophyll cycle, exhibited the same type of fluorescence quenching as that observed in leaves. Two groups of blue-green algal lichens were used for a comparison with these green algal lichens. A group of zeaxanthin-free blue-green algal lichens did not exhibit the type of chlorophyll fluorescence quenching indicative of energy dissipation in the pigment bed. In contrast, a group of blue-green algal lichens which had formed zeaxanthin slowly through reactions other than the xanthophyll cycle, did show a very similar response to that of leaves and green algal lichens. Fluorescence quenching indicative of radiationless energy dissipation in the antenna chlorophyll was the predominant component of high-energy-state quenching in spinach leaves under conditions allowing for high rates of steady-state photosynthesis. A second, but distinctly different type of high-energy-state quenching of chlorophyll fluorescence, which was not inhibited by DTT (i.e., it was zeaxanthin independent) and which is possibly associated with the photosystem II reaction center, occurred in addition to that associated with zeaxanthin in leaves under a range of conditions which were less favorable for linear photosynthetic electron flow. In intact chloroplasts isolated from (zeaxanthin-free) spinach leaves a combination of these two types of rapidly reversible fluorescence quenching occurred under all conditions examined.Abbreviations DTT dithiothreitol - F₀ (or F₀) yield of instantaneous fluorescence at open PS II reaction centers in the dark (or during actinic illumination) - F_M (or F_M) yield of maximum fluorescence induced by a saturation pulse of light in the dark (or during actinic illumination) - F_V (or F_V) yield of variable fluorescence induced by a saturating pulse of light in the dark (or during actinic illumination) - k _D rate constant for radiationless energy dissipation in the antenna chlorophyll - SV Stern-Volmer equation - PFD photon flux density - PS I photosystem I - PS II photosystem II - Q_A acceptor of photosystem II - q_N coefficient of nonphotochemical chlorophyll fluorescence quenching - q_P coefficient of photochemical chlorophyll fluorescence quenching     

‘Low-waves>’ in chlorophyll fluorescence kinetics indicate deprivation of bicarbonate*

A brief reversible lowering of chlorophyll fluorescence yield (so called low-waves) immediately after application of a saturating light pulse in parallel with a short-time enhancement of the P700 oxidation level was observed in the green alga Haematococcus pluvialis. The phenomenon occurred in the steady-state time region of fluorescence induction kinetics under mild acidic conditions, and was eliminated by bicarbonate. Shortly after expression of low-waves, the photosynthetic oxygen evolution rate decreased and the non-photochemical chlorophyll fluorescence quenching component increased. The enhancement of the non-photochemical chlorophyll fluorescence quenching component was nigericin-sensitive indicating its dependence on the transthylakoid proton gradient. On the other hand, the formation of low-waves was not removed by the uncoupler. Only when bicarbonate was applied additionally, the reversible short-term decrease in fluorescence yield following each saturating light flash was abolished. Dimethyl-4-nitroso-aniline as an artificial electron acceptor of Photosystem I did not limit the brief drops in fluorescence. However, formate as a competitive inhibitor of bicarbonate binding in Photosystem II induced low-wave formation. Therefore, our results suggest that low-waves in chlorophyll fluorescence kinetics indicate deprivation of bicarbonate in the reaction centre of Photosystem II. This revised version was published online in June 2006 with corrections to the Cover Date.     

Changes in lipid composition, water relations and gas exchange in leaves of two young ‘Chemlali’ and ‘Chetoui’ olive trees in response to water stress

The comparative responses of two young olive trees (Olea europaea L. ‘Chemlali’ and ‘Chetoui’) to drought stress were investigated during 1 month. Three-month-old own-rooted plants were subjected to two irrigation treatments: WW (well watered plants that were irrigated with fresh water to maintain a soil water content close to field capacity), and WS (water stressed plants by withholding water). Leaf water potential, gas exchange and leaf lipid composition were studied. ‘Chemlali’ was able to maintain higher leaf CO₂ assimilation rate and leaf stomatal conductance throughout the drought cycle compared to ‘Chetoui’. Water stress induced a larger decrease in the total lipid content in ‘Chetoui’ than in ‘Chemlali’. Interestingly, the constitution of different lipid classes was highly altered in ‘Chetoui’. Lipid changes in Chemlali, a drought tolerant cultivar, revealed more stability of its cellular membranes to drought stress as compared to the drought susceptible olive cultivar, Chétoui. Furthermore, in comparison to the controls, drought stressed plants showed an increase in the degree of unsaturation of leaf lipids in the two olive cultivars. Moreover, the results observed in Chemlali showed that besides changes in lipids composition this cultivar may have an efficient defence strategy which can be related on antioxidative production against oxidative stress.     

Zinc accumulation,photosynthetic gas exchange,and chlorophyll a fluorescence in Zn-stressed Miscanthus × giganteus plants

Photosynthetica - Accumulation and distribution of zinc within Miscanthus × giganteus plants grown on elevated Zn concentrations and their photosynthetic performance were investigated. High...     

Growth and inorganic composition of ‘Nova’ mandarin plants grafted on two commercial rootstocks in response to salinity and silicon

The effects of salinity and its combination with silicon (Si) were studied in ‘Nova’ mandarin plants grafted on Citrus aurantium L. or Swingle Citrumelo to determine: (1) which combination is more tolerant to salt stress and (2) the impact of Si in limiting the harmful effects of salinity. Six groups of plants were grown in a greenhouse for 120 days and irrigated with: (1) 50 % Hoagland’s solution (Control), (2) 50 % Hoagland’s solution plus 80 mM NaCl (NaCl), and (3) 50 % Hoagland’s solution plus 80 mM NaCl plus 0.5 mM Si (NaCl + Si). Grafted plants exhibited accumulation of Na and Cl in their tissues following exposure to salinity. The ability of S. Citrumelo to retain the toxic ions in the roots in corroboration with the observation that the dry weights (DWs) of S. Citrumelo tissues were not influenced by NaCl treatment indicates that this rootstock is more tolerant to salinity. Silicon supplementation into the saline medium promoted the accumulation of toxic ions, whereas, when compared to NaCl treatment, it increased the DW of S. Citrumelo roots. Mineral concentrations were significantly affected by rootstock, treatment, and their interaction with S. Citrumelo, which presented better nutrient status than Sour Orange; and Si which differed depending on citrus tissue. It appears that S. Citrumelo rootstock is the most tolerant for ‘Nova’ mandarin plants under salinity, whereas salt tolerance in grafted citrus plants is not improved by Si application, indicating that the beneficial role of Si depends on the cultivar or rootstock–scion combinations.     

Leaf characteristics and diurnal variation of chlorophyll fluorescence in leaves of the ‘bana’ vegetation of the amazon region

In six dominant species of the Amazonian ‘Bana’ vegetation, leaf blade characteristics, pigment composition, and chlorophyll (Chl) fluorescence parameters were measured in young and mature leaves under field conditions. Leaf δ¹³C was comparable in the six species, which suggested that both expanding and expanded leaves contained organic matter fixed under similar intercellular and ambient CO₂ concentration (C _i/C _a). High leaf C/N and negative δ¹⁵N values found in this habitat were consistent with the extreme soil N-deficiency. Analysis of Chl and carotenoids showed that expanding leaves had an incomplete development of photosynthetic antenna when compared to adult leaves. Dynamic inactivation of photosystem 2 (PS2) at midday was observed at both leaf ages as F_v/F_m decreased compared to predawn values. Adult leaves reached overnight F_v/F_m ratios typical of healthy leaves. Overnight recovery of F_v/F_m in expanding leaves was incomplete. F₀ remained unchanged from midday to predawn and F_v tended to increase from midday to predawn. The recovery from midday depression observed in adult leaves suggested an acclimatory down-regulation associated with photo-protection and non-damage of PS2.     

Effect of nickel concentration and soil pH on metal accumulation and growth in tropical agromining ‘metal crops’

Plant and Soil - Phosphorus (P) deficiency is a major constraint for rice production in the tropics. Field-specific P management is key for resource-limited farmers to increase yields with minimal...     

Integrin-β5 and zyxin mediate formation of ventral stress fibers in response to transforming growth factor β

Cell adhesion to the extracellular matrix is an essential element of various biological processes. TGF-β cytokines regulate the matrix components and cell–matrix adhesions. The present study investigates the molecular organization of TGF-β-induced matrix adhesions. The study demonstrates that in various mouse and human epithelial cells TGF-β induces cellular structures containing 2 matrix adhesions bridged by a stretch of actin fibers. These structures are similar to ventral stress fibers (VSFs). Suppression of integrin-β5 by RNA interference reduces VSFs in majority of cells (>75%), while overexpression of integrin-β5 fragments revealed a critical role of a distinct sequence in the cytoplasmic domain of integrin-β5 in the VSF structures. In addition, the integrity of actin fibers and Src kinase activity contribute to integrin-β5-mediated signaling and VSF formation. TGF-β-Smad signaling upregulates actin-regulatory proteins, such as caldesmon, zyxin, and zyxin-binding protein Csrp1 in mouse and human epithelial cells. Suppression of zyxin markedly inhibits formation of VSFs in response to TGF-β and integrin-β5. Zyxin is localized at actin fibers and matrix adhesions of VSFs and might bridge integrin-β5-mediated adhesions to actin fibers. These findings provide a platform for defining the molecular mechanism regulating the organization and activities of VSFs in response to TGF-β.     

Leaf–water relations and ion concentrations of the halophyte <Emphasis Type="Italic">Atriplex hortensis</Emphasis> in response to salinity and water stress

The impact of salinity and water stress was analyzed in the xero-halophyte Atriplex hortensis using two varieties: green orach (A. hortensis var. purpurea) and red orach (A. hortensis var. rubra). A. hortensis L. is a C₃ species well adapted to salt and drought conditions. To collect information on the physiological impact of different salt and water deficit levels on their water stress resistance, plants were exposed for 3 months to solution containing four levels of NaCl or to water stress regimes including four levels of field capacity. Osmotic potential at zero turgor Ψ_s⁰, osmotic potential at full turgor (Ψ_s¹⁰⁰), relative water content (RWC), ion concentration (Na⁺, K⁺, Ca²⁺, Mg²⁺, and Cl⁻), and malondialdehyde (MDA) were determined at the end of the treatment. The salinity and water stress induced a decrease in Ψ_s¹⁰⁰, Ψ_s⁰, and RWC in both varieties, recorded changes being higher in plants of red variety than those of green variety. Both varieties specifically accumulated Na⁺ in response to drought and salt stress, suggesting that this element could play a physiological role in the stress response of this xero-halophyte species. In contrast, the presence of NaCl and water stress induced a decrease in K⁺, Ca²⁺, and Mg²⁺ concentration in both varieties. Salinity clearly induced an increase in Cl^– concentration in all tissues, but water stress had no impact on this parameter. MDA concentration increased in response to water stress and exogenous NaCl. Based on these findings the more drought-tolerant red orach may be grown in water-limiting soils.     

Genotypic variability in physiological,biomass and yield response to drought stress?in pigeonpea

Three pigeonpea (Cajanus cajan L. Millsp.) genotypes- GT-1, AKP-1 and PRG-158 with varying crop duration, growth habit and flowering pattern were evaluated for variability in their response for drought stress. Drought stress was imposed at initiation of flowering and the observations on biomass and seed yield parameters were recorded at harvest. The magnitude of response of individual component to drought stress was found to be genotype specific. Drought stress significantly decreased photosynthetic rate (P_N), transpiration rate (Tr) and relative water content (RWC) in all the genotypes, however the magnitude of reduction differed with genotype. With drought stress, the reduction of P_N was highest in GT-1 while reduction in Tr was highest in PRG-158. The genotype AKP-1, accumulated significantly higher concentrations of osmotic solutes especially proline under water deficit stress, this facilitated it to maintain higher relative water content (RWC) and lower malondialdehyde (MDA) content as compared to other genotypes. Drought stress also impacted biomass production and their partitioning to vegetative and reproductive components at harvest. There was significant variability between the genotypes for seed yield under drought stress while it was non-significant under well-watered condition. Drought stress enhanced flower drop and decreased flower to pod conversion resulting in reduced pod number and seed number in PRG-158 and GT-1. The genotype AKP-1 recorded superior performance for seed yield under stress environment due to its ability in maintaining pod and seed number as well as improved test weight (100 seed weight). Under drought stress, significant positive association of seed yield with proline, seed number, pod number and test weight clearly indicating their role in drought tolerance.     

Physiological response to heat stress of tomato ‘Micro-Tom’ plants expressing high and low levels of mitochondrial sHSP23.6 protein

Tomato ‘Micro-Tom’ plants were transformed for high or low expression of the mitochondrial small “heat shock” protein (HSP) (MT-sHSP23.6) to evaluate their response to high temperature. The plants were raised for 59 days under a controlled temperature, photoperiod and photon flow density and then subjected to heat stress for 24 h at 37 °C, followed by a recovery period under normal conditions (21 ± 2 °C). The cycle was repeated. The chlorophyll a fluorescence intensity was measured, and the parameters of the JIP-test were calculated. The gas exchange was also evaluated. The JIP-test showed significantly different responses of the genotypes to heat stress. The parameters of photosystem I activity and the net assimilation of CO₂ increased during the first stress cycle in genotypes with a high expression of MT-sHSP23.6 and in non-transformed plants; however, the net assimilation of CO₂ decreased in genotypes with a low expression of MT-sHSP23.6. The data suggest that MT-sHSP23.6 participates in the heat tolerance mechanism, considering that the suppression of this protein resulted in greater physiological damage during heat stress.     

Rapid, transient, and proportional activation of σ(B) in response to osmotic stress in Listeria monocytogenes

The osmotic activation of sigma B (σ(B)) in Listeria monocytogenes was studied by monitoring expression of four known σ(B)-dependent genes, opuCA, lmo2230, lmo2085, and sigB. Activation was found to be rapid, transient, and proportional to the magnitude of the osmotic stress applied, features that underpin the adaptability of this pathogen.     

Glycerol stimulation of chlorophyll synthesis,embryogenesis, and carboxylation and sucrose metabolism enzymes in nucellar callus of ‘Hamlin’ sweet orange

Anthers of niger (Guizotia abyssinica. Cass) were inoculated onto five different media differing mainly in their inorganic and organic constituents and plant growth regulators to study their influence on callus induction (embryogenic/non-embryogenic) and plant regeneration. LS medium supplemented with 2 mg 1^-1 2,4-d, and 0.3 mg 1^-1 KN favoured the production of EC, whereas 2 mg 1^-1 BAP and 0.5 mg 1^-1 KN promoted the NEC from anthers. Different types of embryos were initiated upon transfer of EC to Chaleff's R-2 medium containing 2 mg 1^-1 NAA and 0.3 mg 1^-1 KN and/or 5 mg 1^-1 ABA. NEC when transferred onto the medium supplemented with 1 mg 1^-1 BAP and 0.1 mg 1^-1 NAA produced on an average 8–12 shoots/callus mass. Embryoids developed from the EC and shoots differentiated from NEC when cultured onto the Chaleff's R-2 and MS media respectively lacking growth regulators, they transformed into whole plantlets. The plantlets thus obtained were successfully hardened and grown to maturity for analysis of various plant characters.Abbreviations EC embryogenic callus - NEC non-embryogenic callus - 2,4-d 2,4-dichlorophenoxyacetic acid - NAA -naphthaleneacetic acid - ABA abscisic acid - BAP 6-benzylaminopurine - KN kinetin - MS Murashige and Skoog's medium - LS Linsmaier and Skoog's medium     

How do salinity and water stress affect transport of water,assimilates and ions to tomato fruits?

The study was conducted in order to determine whether water stress affects the accumulation of dry matter in tomato fruits similarly to salinity, and whether the increase in fruit dry matter content is solely a result of the decrease in water content. Although the rate of water transport to tomato fruits decreased throughout the entire season in saline water irrigated plants, accumulation rates of dry matter increased significantly. Phloem water transport contributed 80–85% of the total water transport in the control and water-stressed plants, and over 90% under salinity. The concentration of organic compounds in the phloem sap was increased by 40% by salinity. The rate of ions transported via the xylem was also significantly increased by salinity, but their contribution to fruit osmotic adjustment was less. The rate of fruit transpiration was also markedly reduced by salinity. Water stress also decreased the rate of water transport to the tomato fruit and increased the rate of dry matter accumulation, but much less than salinity. The similar changes, 10–15%, indicate that the rise in dry matter accumulation was a result of the decrease in water transport. Other parameters such as fruit transpiration rates, phloem and xylem sap concentration, relative transport via phloem and xylem, solutes contributing to osmotic adjustment of fruits and leaves, were only slightly affected by water stress. The smaller response of these parameters to water stress as compared to salinity could not be attributed to milder stress intensity, as leaf water potential was found to be more negative. Measuring fruit growth of girdled trusses, in which phloem flow was inactive, and comparing it with ungirdled trusses validated the mechanistic model. The relative transport of girdled as compared to ungirdled fruits resembled the calculated values of xylem transport.     

Branch bending affected floral bud development and nutrient accumulation in shoot terminals of ‘Fuji’ and ‘Gala’ apples

Branch bending has been practiced for decades in China to induce flower buds in ‘Fuji’ apple. However, the optimum bending angle is yet to be elucidated. The main objectives of this study were to compare the effect of branch bending angles (70°, 90° and 110°) on the flowering and nutrient accumulation of 1-year-old shoots of ‘Fuji’ and ‘Gala’ apples and to determine the optimum branch bending angle for each cultivar. In both cultivars, the production of spurs and terminal flower buds, and the total sugar concentration and the carbon-to-nitrogen (C/N) ratio in the shoot terminals increased, whereas the N concentration decreased with increasing bending angles. The nutrient concentration was significantly higher in spurs than in medium and long shoots. The distinction between the changing patterns of C and N concentrations in the bent shoots during the growing season in our study suggested the competition of these two nutrients caused by vegetative and reproductive growth at different growing times. In ‘Fuji’ apple, the proportion of flowering buds appeared to increase more rapidly with the increase of bending angle from 70° to 110° than that in ‘Gala’ apple, and particularly a higher proportion of spurs was observed on ‘Fuji’ branches bent at a larger angle. The increase in the total sugar concentration and the C/N ratio in the shoot terminals of the bent branches might be involved in inducing floral buds after bending. The optimum bending angle was about 90° for ‘Gala’ apple and 110° for ‘Fuji’ apple, respectively. Bending could help farmers to reduce the severity of biennial fruiting in ‘Fuji’ apple.     

Contrasting phosphorus (P) accumulation in response to soil P availability in ‘metal crops’ from P-impoverished soils

Plant and Soil - The current study investigated the potential influence of Enterobacter sp. FM-1 on plant growth and the accumulation of Cd and Pb in plants growing in highly contaminated soils....     

Clonal ability,height and growth form explain species’ response to habitat deterioration in Fennoscandian wooded meadows

Fennoscandian calcareous wooded meadows have high conservation value due to very high diversity which has been maintained by consistent mowing, but undergo species loss when this management is abandoned. We compared species richness and composition of regularly mown and abandoned wooded meadows in Estonia and established species groups with respect to their response to abandonment. These meadows were very species rich with a maximum of 43 species per 0.25 m². Species whose populations are maintained by mowing constituted >60 % of the floristic diversity of the mown wooded meadows. Abandonment suppressed species with a preference for dry infertile open habitats. Response to abandonment was related to clonality, height and growth form. The greatest negative response was associated with low graminoids, short-lived and non-clonal species, rosette and semi-rosette growth form. Weak competitors with low height and light seeds exhibited a slow decline; perennial life span and clonality enable a delay in local extinction of these species. Response groups had similar persistence at a national level, revealing that earlier assessment of species dynamics at larger spatial scales could underestimate the number of threatened species. Application of restoration measures could avoid local extinctions of many mowing-supported species, whereas some of the mowing-dependent species likely need reintroduction.     

The emission of floral scent from Lilium ‘siberia’ in response to light intensity and temperature

Floral scent is an important part of volatile compounds emitted from plants, and is influenced by many environmental factors. In this study, the floral scent emitted from Lilium ‘siberia’, a common breed of lily, was collected by dynamic headspace at different levels of light intensity (0, 100, 300, 600, 1,000, and 1,500 μmol m^?2 s^?1) and temperature (10, 20, 30, and 40 °C). Using the automated thermal desorption-gas chromatography/mass spectrometry (ATD-GC/MS) technique, the components and release amounts were subsequently identified to investigate the influence of light and temperature on the emission of floral scent. The results revealed that the numbers and release amounts of floral scent components were significantly influenced by light intensity and temperature, showing the similar pattern: first increasing and then decreasing. After light intensity treatment, the maximum numbers and release amounts mainly appeared at 600 and 1,000 μmol m^?2 s^?1. For temperature treatment, 30 °C resulted in the highest numbers and release amounts of the floral scent components. At different levels of light intensity and temperature, terpenoid compounds showed the highest numbers and release amounts among the component categories. α-Ocimene and linalool were the two terpenoid compounds with the highest release amounts, and accounted for the highest proportion. The results obtained provide evidence that both light intensity and temperature trigger the emission of floral scent. The particular response mechanisms must be investigated in future research.     

Growth, gas exchange, water-use efficiency, and carbon isotope composition of ‘Gale Gala’ apple trees grafted onto 9 wild Chinese rootstocks in response to drought stress

To determine the effects of rootstock choice on the scion response to drought stress, we compared the vegetative growth, biomass accumulation, gas exchange, and water-use efficiency (WUE) of ??Gale Gala?? apple (Malus domestica Borkh.) trees grafted onto nine wild Chinese Malus rootstocks. Compared with the well-watered control, drought treatment limited growth, as manifested by smaller increments in plant height (PH), trunk diameter (TD), total fresh biomass (TB), total dry biomass (TDB), total leaf area (LA), and relative growth rate (RGR). The extent of this effect differed among rootstocks. Stress conditions led to increases in the root/shoot ratio (RSR), leaf thickness (LT), water-holding capacity (WHC), carbon isotope composition (??¹³C), and WUE. Decreases were noted in stomatal density (SD), leaf relative water content (RWC), chlorophyll content (Chl), net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s), again varying by rootstock. Those that are generally considered more drought-tolerant, e.g., M. sieversii, M. prunifolia, and M. toringoides, had smaller declines in PH, TD, TB, TDB, LA, RGR, SD, RWC, Chl, P _N, E, and g _s and proportionally greater increases in RSR, LT, WHC, ??¹³C, and WUE compared with the droughtsensitive M. hupehensis and M. sieboldii. These results suggest that moisture stress has a significant dwarfing effect in the latter two species. Based on WUE calculations, trees on drought-tolerant rootstocks showed higher tolerance when stressed, whereas those on drought-sensitive rootstocks were less tolerant, as indicated by their lower WUE values.