Single-cell C(4) photosynthesis: efficiency and acclimation of Bienertia sinuspersici to growth under low light.

Traditionally, it was believed that C(4) photosynthesis required two types of chlorenchyma cells to concentrate CO(2) within the leaf. However, several species have been identified that perform C(4) photosynthesis using dimorphic chloroplasts within an individual cell. The goal of this research was to determine how growth under limited light affects leaf structure, biochemistry and efficiency of the single-cell CO(2) -concentrating mechanism in Bienertia sinuspersici. Measurements of rates of CO(2) assimilation and CO(2) isotope exchange in response to light intensity and O(2) were used to determine the efficiency of the CO(2) -concentrating mechanism in plants grown under moderate and low light. In addition, enzyme assays, chlorophyll content and light microscopy of leaves were used to characterize acclimation to light-limited growth conditions. There was acclimation to growth under low light with a decrease in capacity for photosynthesis when exposed to high light. This was associated with a decreased investment in biochemistry for carbon assimilation with only subtle changes in leaf structure and anatomy. The capture and assimilation of CO(2) delivered by the C(4) cycle was lower in low-light-grown plants. Low-light-grown plants were able to acclimate to maintain structural and functional features for the performance of efficient single-cell C(4) photosynthesis.

The efficiency of the CO2-concentrating mechanism during single-cell C4 photosynthesis.

The photosynthetic efficiency of the CO(2)-concentrating mechanism in two forms of single-cell C(4) photosynthesis in the family Chenopodiaceae was characterized. The Bienertioid-type single-cell C(4) uses peripheral and central cytoplasmic compartments (Bienertia sinuspersici), while the Borszczowioid single-cell C(4) uses distal and proximal compartments of the cell (Suaeda aralocaspica). C(4) photosynthesis within a single-cell raises questions about the efficiency of this type of CO(2) -concentrating mechanism compared with the Kranz-type. We used measurements of leaf CO(2) isotope exchange (Δ(13) C) to compare the efficiency of the single-cell and Kranz-type forms of C(4) photosynthesis under various temperature and light conditions. Comparisons were made between the single-cell C(4) and a sister Kranz form, S. eltonica[NAD malic enzyme (NAD ME) type], and with Flaveria bidentis[NADP malic enzyme (NADP-ME) type with Kranz Atriplicoid anatomy]. There were similar levels of Δ(13) C discrimination and CO(2) leakiness (Φ) in the single-cell species compared with the Kranz-type. Increasing leaf temperature (25 to 30 °C) and light intensity caused a decrease in Δ(13) C and Φ across all C(4) types. Notably, B. sinuspersici had higher Δ(13) C and Φ than S. aralocaspica under lower light. These results demonstrate that rates of photosynthesis and efficiency of the CO(2) -concentrating mechanisms in single-cell C(4) plants are similar to those in Kranz-type.

Germination of dimorphic seeds of the desert annual halophyte Suaeda aralocaspica (Chenopodiaceae), a C4 plant without Kranz anatomy.

BACKGROUND AND AIMS: Suaeda aralocaspica is a C4 summer annual halophyte without Kranz anatomy that is restricted to the deserts of central Asia. It produces two distinct types of seeds that differ in colour, shape and size. The primary aims of the present study were to compare the dormancy and germination characteristics of dimorphic seeds of S. aralocaspica and to develop a conceptual model of their dynamics. METHODS: Temperatures simulating those in the natural habitat of S. aralocaspica were used to test for primary dormancy and germination behaviour of fresh brown and black seeds. The effects of cold stratification, gibberellic acid, seed coat scarification, seed coat removal and dry storage on dormancy breaking were tested in black seeds. Germination percentage and recovery responses of brown seeds, non-treated black seeds and 8-week cold-stratified black seeds to salt stress were tested. KEY RESULTS: Brown seeds were non-dormant, whereas black seeds had non-deep Type 2 physiological dormancy (PD). Germination percentage and rate of germination of brown seeds and of variously pretreated black seeds were significantly higher than those of non-pretreated black seeds. Exposure of seeds to various salinities had significant effects on germination, germination recovery and induction into secondary dormancy. A conceptual model is presented that ties these results together and puts them into an ecological context. CONCLUSIONS: The two seed morphs of S. aralocaspica exhibit distinct differences in dormancy and germination characteristics. Suaeda aralocaspica is the first cold desert halophyte for which non-deep Type 2 PD has been documented.

Effect of ultraviolet-B radiation on salt marsh vegetation: Trends of the genus Salicornia along the Americas.

The effects of natural UV-B radiation on growth, photosynthetic and photoprotective pigment composition of different Salicornia species were analyzed in salt marshes at three different sites along the Americas (Puerto Rico, southern Brazil and Patagonia, Argentina). Plants were exposed to different levels of UV-B radiation for 1-2 years in situ as well as in outdoor garden UV-B exclusion experiments. Different UV-B levels were obtained by covering plants with UV-B opaque (blocked 93-100% of ambient UV-B) and UV-B attenuating (near-ambient) filters (reduced 20-25% of UV-B). Unfiltered plants were exposed to natural irradiance. UV-B filters had significant effects on temperature and photosynthetic pigments (due to changes in PAR; 400-700 nm). The growth of Salicornia species was inhibited after 35 to 88 days of exposure to mean UV-B radiation dosages between 3.6 and 4.1 kJ m(-2) day(-1). The highest number of branches on the main shoot (S. bigelovii and S. gaudichaudiana) and longest total length of the branches (S. gaudichaudiana) were observed in the UV-B opaque treatment. Salicornia species responded to increasing levels of UV-B radiation by increasing the amount of UV-B absorbing pigments up to 330%. Chromatographic analyses of seedlings and adult S. bigelovii plants found seven different UV-B absorbing flavonoids that are likely to serve as UV-B filtering pigments. No evidence of differential sensitivity or resilience to UV-B radiation was found between Salicornia species from low-mid latitudes and a previously published study of a high-latitude population.

Identification of betacyanin and effects of environmental factors on its accumulation in halophyte Suaeda salsa.

Effects of temperature, light, salinity and developmental phases on the accumulation of red pigments in the C(3) halophyte Suaeda salsa were studied, and the physical and chemical characteristics of the red pigments were also analyzed. The results indicate that: these red pigments are insoluble in organic solvents but free in water, the pigments are red-violet and stable under acidic condition while yellow and unstable under alkaline condition, and they absorb the highest value at wavelength near 538 nm, light suppresses their accumulation and enhances their decomposition. These results suggest that these red pigments are betacyanins. Darkness, low temperatures and high salinity enhance betacyanin accumulation in Suaeda salsa, and darkness in the germination phase is one of the most important environmental factors for the betacyanin accumulation.

Influence of high concentrations of mineral salts on production process and NaCl accumulation by Salicornia europaea plants as a constituent of the LSS phototroph link.

Use of halophytes (salt-tolerant vegetation), in a particular vegetable Salicornia europaea plants which are capable of utilizing NaCl in rather high concentrations, is one of possible means of NaCl incorporation into mass exchange of bioregenerative life support systems. In preliminary experiments it was shown that S. europaea plants, basically, could grow on urine pretreated with physicochemical processing and urease-enzyme decomposing of urea with the subsequent ammonia distillation. But at the same time inhibition of the growth process of the plants was observed. The purpose of the given work was to find out the influence of excessive quantities of some mineral elements contained in products of physicochemical processing of urine on the production process and NaCl accumulation by S. europaea plants. As the content of mineral salts in the human liquid wastes (urine) changed within certain limits, two variants of experimental solutions were examined. In the first variant, the concentration of mineral salts was equivalent to the minimum salt content in the urine and was: K - 1.5 g/l, P - 0.5 g/l, S - 0.5 g/l, Mg - 0.07 g/l, Ca - 0.2 g/l. In the second experimental variant, the content of mineral salts corresponded to the maximum salt content in urine and was the following: K - 3.0 g/l, P - 0.7 g/l, S - 1.2 g/l, Mg - 0.2 g/l, Ca - 0.97 g/l. As the control, the Tokarev nutrient solution containing nitrogen in the form of a urea, and the Knop nutrient solution with nitrogen in the nitrate form were used. N quantity in all four variants made up 177 mg/l. Air temperature was 24 degrees C, illumination was continuous. Light intensity was 690 micromoles/m2s of photosynthetically active radiation. NaCl concentration in solutions was 1%. Our researches showed that the dry aboveground biomass of an average plant of the first variant practically did not differ from the control and totaled 11 g. In the second variant, S. europaea productivity decreased and the dry aboveground biomass of an average plant totaled 8 g. The increase of K quantity in the experimental solutions resulted in an elevated content of the element in the plants. The increase of K uptake in the second experimental variant was accompanied by a 30-50% decrease of Na content in comparison with the other variants. Comparative Na content in the other variants was practically identical. N, Mg and P content in the control and experimental variants was also practically identical. The increase of S quantity in the second experimental variant also increased S uptake by the plants. But Ca quantity, accumulated in aboveground plants biomass in the experimental variants was lower than in the control. NaCl uptake by plants, depending on the concentration of mineral salts in the experimental solutions, ranged from 8 g (maximum salt content) up to 15 g (minimum salt content) on a plant growth area that totaled 0.032 m2. Thus, high concentrations of mineral salts simulating the content of mineral salts contained in urine did not result in a significant decrease of S. europaea productivity. The present work also considers the influence of higher light intensity concentrations on productivity and NaCl accumulation by S. europaea plants grown on experimental solutions with high salt content.

The effect of plant growth regulators, nitric oxide, nitrate, nitrite and light on the germination of dimorphic seeds of Suaeda salsa under saline conditions.

Suaeda salsa, a leaf succulent shrub in the family Chenopodiaceae, is one of the most important halophytes in China. Suaeda salsa produces dimorphic seeds (soft brown seeds and hard black seeds). Seeds of S. salsa were collected from the coastal salt flats near Huanghua City, China. Experiments were conducted to determine the salinity-alleviating effect of plant growth regulators, nitric oxide, nitrate, nitrite and light on the germination of dimorphic seeds of S. salsa. Brown seeds had a higher germination rate than black seeds in all experiments. Black seeds were more sensitive to salt in the absence of light in comparison to brown seeds. Brown seeds absorbed water more quickly in comparison to black seeds and were found to be more tolerant of salt stress. Our results showed that 1-aminocyclopropane-1-carboxylate (ACC, the immediate precursor of ethylene), nitrite, GA(4) and BA improved seed germination in the presence of salt. However, nitrate, GA(1), GA(3) failed to alleviate salt stress. ABA inhibited seed germination and seedling growth. Possible mechanisms involved in the alleviation of salt stress in S. salsa seeds and the ecological adaptation of the seeds to the environment are discussed.

Light-dependent development of single cell C4 photosynthesis in cotyledons of Borszczowia aralocaspica (Chenopodiaceae) during transformation from a storage to a photosynthetic organ.

BACKGROUND AND AIMS: Previous work has shown that Borszczowia aralocaspica (Chenopodiaceae) accomplishes C4 photosynthesis in a unique, polarized single-cell system in leaves. Mature cotyledons have the same structure as leaves, with chlorenchyma cells having biochemical polarization of dimorphic chloroplasts and C4 functions at opposite ends of the cell. KEY RESULTS: Development of the single-celled C4 syndrome in cotyledons was characterized. In mature seeds, all cell layers are already present in the cotyledons, which contain mostly lipids and little starch. The incipient chlorenchyma cells have a few plastids towards the centre of the cell. Eight days after germination and growth in the dark, small plastids are evenly distributed around the periphery of the expanding cells. Immunolocalization studies show slight labelling of Rubisco in plastids in seeds, including chlorenchyma, hypodermal and water storage, but not epidermal, cells. After imbibition and 8 d of growth in the dark labelling for Rubisco progressively increased, being most prominent in chlorenchyma cells. There was no immunolabelling for the plastid C4 enzyme pyruvate, Pi dikinase under these conditions. Cotyledons developing in light show formation of chlorenchyma tissue, induction of the cytosolic enzyme phosphoenolpyruvate carboxylase and development of dimorphic chloroplasts at opposite ends of the cells. Proximal chloroplasts have well-developed grana, store starch and contain Rubisco; those located distally have reduced grana, lack starch and contain pyruvate, Pi dikinase. CONCLUSIONS: The results show cotyledons developing in the dark have a single structural plastid type which expresses Rubisco, while light induces formation of dimorphic chloroplasts from the single plastid pool, synthesis of C4 enzymes, and biochemical and structural polarization leading to the single-cell C4 syndrome.

The effect of natural UV-B radiation on a perennial Salicornia salt-marsh in Bahía San Sebastián, Tierra del Fuego, Argentina: a 3-year field study.

The Antarctic ozone hole and a general depletion of the stratospheric ozone layer cause increased levels of ultraviolet-B solar radiation (UV-B) over Tierra del Fuego, the southernmost tip of South America. For three consecutive growing seasons (1997-2000), we studied the biological impacts (morphology, physiology, demography and phenology) of natural UV-B radiation on a perennial Salicornia ambigua Michx. community in San Sebastian Bay (53 degrees S and 68 degrees W), Tierra del Fuego, Argentina. This is the first UV-B screening experiment on a subantarctic halophytic community. The shortwave UV-B spectrum (280 to 320 nm) was excluded by covering plots with UV-B blocking film (Mylar). These plots were compared to controls covered with UV-B transparent (Aclar) plastic screens, and unscreened plots. Shoot length in Salicornia was not affected by UV-B. Exposure to natural UV-B reduced biomass and density (by 17% and 38%, respectively). Concentration of UV-shielding pigments and cuticle thickness were both significantly higher (25-48% and 21-40%, respectively) in plants receiving ambient UV-B. The increase in cuticle thickness persisted throughout the growing season, whereas pigment concentration was higher at the beginning of the growing season. Also, the number of dead shoots was higher in plants exposed to UV-B. At the end of the growing season (March) shoot mortality was higher in plants exposed to ambient UV-B, and post-flowering senescence was 30 days earlier. Slight changes in the relative composition of Salicornia to Puccinellia were seen. The reduction observed in Salicornia shoot density under ambient UV-B was cumulative over time; 23% in the first growing-season, rising to 38% by the third growing-season. A similar incremental increase in pigment absorption at 305 nm was seen; 25% in the first and 48% in the third growing season.

Salinity treatment shows no effects on photosystem II photochemistry, but increases the resistance of photosystem II to heat stress in halophyte Suaeda salsa.

Photosynthetic gas exchange, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, and the polyphasic fluorescence transients were used to evaluate PSII photochemistry in the halophyte Suaeda salsa exposed to a combination of high salinity (100-400 mM NaCl) and heat stress (35-47.5 degrees C, air temperature). CO(2) assimilation rate increased slightly with increasing salt concentration up to 300 mM NaCl and showed no decrease even at 400 mM NaCl. Salinity treatment showed neither effects on the maximal efficiency of PSII photochemistry (F(v)/F(m)), the rapid fluorescence induction kinetics, and the polyphasic fluorescence transients in dark-adapted leaves, nor effects on the efficiency of excitation energy capture by open PSII reaction centres (F(v)'/F(m)') and the actual PSII effciency (Phi(PSII)), photochemical quenching (q(P)), and non-photochemical quenching (q(N)) in light-adapted leaves. The results indicate that high salinity had no effects on PSII photochemistry either in a dark-adapted state or in a light-adapted state. With increasing temperature, CO(2) assimilation rate decreased significantly and no net CO(2) assimilation was observed at 47.5 degrees C. Salinity treatment had no effect on the response of CO(2) assimilation to high temperature when temperature was below 40 degrees C. At 45 degrees C, CO(2) assimilation rate in control plants decreased to zero, but the salt-adapted plants still maintained some CO(2) assimilation capacity. On the other hand, the responses of PSII photochemistry to heat stress was modified by salinity treatment. When temperature was above 35 degrees C, the declines in F(v)/F(m), Phi(PSII), F(v)'/F(m)', and q(P) were smaller in salt-adapted leaves compared to control leaves. This increased thermostability was independent of the degree of salinity, since no significant changes in the above-described fluorescence parameters were observed among the plants treated with different concentrations of NaCl. During heat stress, a very clear K step as a specific indicator of damage to the O(2)-evolving complex in the polyphasic fluorescence transients appeared in control plants, but did not get pronounced in salt-adapted plants. In addition, a greater increase in the ratio (F(i)-F(o))/(F(p)-F(o)) which is an expression of the proportion of the Q(B)-non-reducing PSII centres was observed in control plants rather than in salt-adapted plants. The results suggest that the increased thermostability of PSII seems to be associated with the increased resistance of the O(2)-evolving complex and the reaction centres of PSII to high temperature.

A three-state model for energy trapping and chlorophyll fluorescence in photosystem II incorporating radical pair recombination.

The multiphasic fluorescence induction kinetics upon a high intensity light pulse have been measured and analyzed at a time resolution of 10 micros in intact leaves of Peperomia metallica and Chenopodium album and in chloroplasts isolated from the latter. Current theories and models on the relation between chlorophyll fluorescence yield and primary photochemistry in photosystem II (PSII) are inadequate to describe changes in the initial phase of fluorescence induction and in the dark fluorescence level F(0) caused by pre-energization of the system with single turnover excitation(s). A novel model is presented, which gives a quantitative relation between the efficiencies of primary photochemistry, energy trapping, and radical pair recombination in PSII. The model takes into account that at least two turnovers are required for stationary closure of a reaction center. An open reaction center is transferred with high efficiency into its semiclosed (-open) state. This state is characterized by Q(A) and P680 in the fully reduced state and a lifetime equal to the inverse of the rate constant of Q(A)(-) oxidation (approx. 250 micros). The fluorescence yield of the system with 100% of the centers in the semiclosed state is 50% of the maximal yield with all centers in the closed state at fluorescence level F(m). A situation with approximately 100% of the centers in the semiclosed state is reached after a single turnover excitation in the presence of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU). The lifetime of this state under these conditions is approximately 10 s. Closure of a semiclosed (-open) center occurs with low efficiency in a second turnover. The low(er) efficiency is caused by the rate of P(+) reduction by the secondary donor Y(Z) being competitive with the rate of radical pair recombination in second and following turnovers. The single-turnover-induced alterations in the initial kinetics of the fluorescence concomitantly with a 15-25% increase in F(o) can be simulated with the present so called three-state model of energy trapping. The experimental data suggest evidence for an electrostatic effect of local charges in the vicinity of the reaction center affecting the rate of radical pair recombination in the reaction center.