EFFECTS OF CANOPY POSITION AND IRRADIANCE ON THE LEAF PHYSIOLOGY AND MORPHOLOGY OF PENTACLETHRA MACROLOBA (MIMOSACEAE)

Pentaclethra macroloba (Willd.) Kuntze (Mimosaceae) is a dominant late-successional tree species in the Atlantic lowland forests of Costa Rica. Leaves of P. macroloba from three heights in the forest canopy were compared with leaves of seedlings grown in controlled environment chambers under four different irradiance levels. Changes in leaf characteristics along the canopy gradient paralleled changes resulting from the light gradient under controlled conditions. The effect of light or canopy position on light-saturated photosynthesis was small, with maximum photosynthesis increasing from 5 to 6.5 μmol m^−-2 s^−-1 from understory to canopy. Both chamber grown and field leaves showed large adjustments in photosynthetic efficiency at low light via reductions in dark respiration rates and increases in apparent quantum yields. Light saturation of all leaves occurred at or below 500 μmol m^−-2 s^−-1. Leaf thickness, specific leaf weight, and stomatal density increased to a greater extent than saturated photosynthesis with higher irradiance during growth or height in the canopy. As a result, there was a poor correspondence between leaf thickness and light-saturated photosynthesis on an area basis. It is concluded that Pentaclethra macroloba possesses the characteristics of a typical shade-tolerant species.     

Levels of Daily Light Doses Under Changed Day-Night Cycles Regulate Temporal Segregation of Photosynthesis and N2 Fixation in the Cyanobacterium Trichodesmium erythraeum IMS101

While the diazotrophic cyanobacterium Trichodesmium is known to display inverse diurnal performances of photosynthesis and N₂ fixation, such a phenomenon has not been well documented under different day-night (L-D) cycles and different levels of light dose exposed to the cells. Here, we show differences in growth, N₂ fixation and photosynthetic carbon fixation as well as photochemical performances of Trichodesmium IMS101 grown under 12L:12D, 8L:16D and 16L:8D L-D cycles at 70 μmol photons m^-2 s^-1 PAR (LL) and 350 μmol photons m^-2 s^-1 PAR (HL). The specific growth rate was the highest under LL and the lowest under HL under 16L:8D, and it increased under LL and decreased under HL with increased levels of daytime light doses exposed under the different light regimes, respectively. N₂ fixation and photosynthetic carbon fixation were affected differentially by changes in the day-night regimes, with the former increasing directly under LL with increased daytime light doses and decreased under HL over growth-saturating light levels. Temporal segregation of N₂ fixation from photosynthetic carbon fixation was evidenced under all day-night regimes, showing a time lag between the peak in N₂ fixation and dip in carbon fixation. Elongation of light period led to higher N₂ fixation rate under LL than under HL, while shortening the light exposure to 8 h delayed the N₂ fixation peaking time (at the end of light period) and extended it to night period. Photosynthetic carbon fixation rates and transfer of light photons were always higher under HL than LL, regardless of the day-night cycles. Conclusively, diel performance of N₂ fixation possesses functional plasticity, which was regulated by levels of light energy supplies either via changing light levels or length of light exposure.     

Does growth under elevated CO2 moderate photoacclimation in rice?

Acclimation of plant photosynthesis to light irradiance (photoacclimation) involves adjustments in levels of pigments and proteins and larger scale changes in leaf morphology. To investigate the impact of rising atmospheric CO₂ on crop physiology, we hypothesize that elevated CO₂ interacts with photoacclimation in rice (Oryza sativa). Rice was grown under high light (HL: 700 µmol m^?2 s^?1), low light (LL: 200 µmol m^?2 s^?1), ambient CO₂ (400 µl l^?1) and elevated CO₂ (1000 µl l^?1). Leaf six was measured throughout. Obscuring meristem tissue during development did not alter leaf thickness indicating that mature leaves are responsible for sensing light during photoacclimation. Elevated CO₂ raised growth chamber photosynthesis and increased tiller formation at both light levels, while it increased leaf length under LL but not under HL. Elevated CO₂ always resulted in increased leaf growth rate and tiller production. Changes in leaf thickness, leaf area, Rubisco content, stem and leaf starch, sucrose and fructose content were all dominated by irradiance and unaffected by CO₂. However, stomata responded differently; they were significantly smaller in LL grown plants compared to HL but this effect was significantly suppressed under elevated CO₂. Stomatal density was lower under LL, but this required elevated CO₂ and the magnitude was adaxial or abaxial surface‐dependent. We conclude that photoacclimation in rice involves a systemic signal. Furthermore, extra carbohydrate produced under elevated CO₂ is utilized in enhancing leaf and tiller growth and does not enhance or inhibit any feature of photoacclimation with the exception of stomatal morphology.     

The Response of Bean Plants to UV-B Radiation Under Different Irradiances of Background Visible Light

Plants of Phaseolus vulgaris L. (cv. Stella) were grown in controlledconditions under three different irradiances of visible lightwith or without UV-B (280–320nm) radiation. The biologicallyeffective UV-B radiation (UV-B_BE) was 6.17 kJ m^–2 d^–1,and simulated a c. 5% decrease in stratospheric ozone at 55.7?N,13.4?E. The photon flux densities of the photosyntheticallyactive radiation (PAR, 400–700 nm) were either 700 µmolm^–2–1 (HL), 500, µmol m^–2 s^–1(ML) or 230 µmol m^–2 s^–1 PAR (LL). Under highlight (HL) conditions plus UV-B radiation, bean plants appearedmost resistant to the enhanced levels of UV-B radiation, andresponded only by increasing leaf thickness by c. 18%. A smallincrease in UV screening pigments was also observed. Both thelower irradiances (ML and LL) increased the sensitivity of theplants to UV-B radiation. Changes in leaf structure were alsoobserved. Photosystem II was inhibited under ML and LL togetherwith UV-B radiation, as determined by Chi fluorescence inductionand calculation of the fluorescence half-rise times. Leaf reflectivitymeasurements showed that the amount of PAR able to penetrateleaves of UV-B treated plants was reduced, and that a possiblecorrelation may exist between the reduced PAR levels, loss ofChi and lowered photosynthetic activity, especially for LL +UV-Bgrown plants, where surface reflection from leaves was highest.Changes in leaf chlorophyll content were mostly confined toplants grown under LL + UV-B, where a decrease of c. 20% wasfound. With regard to protective pigments (the carotenoids andUV screening pigments) plants subjected to different visiblelight conditions responded differently. Among the growth parametersmeasured, there was a substantial decrease in leaf area, particularlyunder LL + UV-B (c. 47% relative to controls), where leaf dryweight was also reduced by c. 25%. Key words: Chlorophyll fluorescence induction, bean, flavonoids, Phaseolus vulgaris, reflectance, UV-B radiation     

Impact of irradiance on the C allocation in the coastal marine diatom Skeletonema marinoi Sarno and Zingone

Elemental stoichiometry and organic composition were investigated in an Adriatic strain of Skeletonema marinoi, cultured at 25 [low light (LL)] and 250 [high light (HL)]µmol photon m^?2 s^?1. Inorganic carbon acquisition, fixation and allocation, and silicic acid and orthophosphate uptake were also studied. The C : P ratio was below the Redfield ratio, especially at LL. In HL cells, N quota was halved, C quota was similar, silica quota was lower, growth rate and long‐term net primary productivity were almost doubled, relative to LL cells. The HL : LL cell quota ratios were 6 for lipid, 0.5 for protein and 0.4 for carbohydrate. Phosphoenolpyruvate carboxylase (PEPc) and glutamine synthetase (GS) activities were unaffected by the growth irradiance; phosphoenolpyruvate carboxykinase (PEPck) was 2.5‐fold more active in LL cells. This suggests that in S. marinoi, C₄ photosynthesis is unlikely, PEPc is anaplerotic and PEPck may be involved in the conversion of lipid C to carbohydrates, especially in LL cells. Because about 50% of the cost for the production of an HL cell is caused by lipid biosynthesis, we propose that the preferential allocation of C to lipid at HL takes advantage of the relatively high volume‐based energy content of lipids, in an organism that reduces its size at each vegetative cell division.     

Developmental phase-dependent photosynthetic responses to ultraviolet-B radiation: damage, defence, and adaptation of primary leaves of wheat seedlings

Alterations in photosynthetic capacity of primary leaves of wheat seedlings in response to ultraviolet-B (UV-B; 280–320 nm; 60 μmol m⁻² s⁻¹) exposure alone and in combination with photosynthetically active radiation (PAR; 400–800 nm; 200 μmol m⁻² s⁻¹) during different phases of leaf growth and development were assessed. UV-B exposure resulted in a phase-dependent differential loss in photosynthetic pigments, photochemical potential, photosystem 2 (PS2) quantum yield, and in vivo O₂ evolution. UV-B exposure induced maximum damage to the photosynthetic apparatus during senescence phase of development. The damages were partially alleviated when UV-B exposure was accompanied by PAR. UV-B induced an enhancement in accumulation of flavonoids during all phases of development while it caused a decline in anthocyanin content during senescence. The differential changes in these parameters demonstrated the adaptation ability of leaves to UV-B stress during all phases of development and the ability was modified in UV-B+ PAR exposed samples.     

Importance of AOX pathway in optimizing photosynthesis under high light stress: role of pyruvate and malate in activating AOX

The present study shows the importance of alternative oxidase (AOX) pathway in optimizing photosynthesis under high light (HL). The responses of photosynthesis and respiration were monitored as O₂ evolution and O₂ uptake in mesophyll protoplasts of pea pre‐incubated under different light intensities. Under HL (3000 µmol m^?2 s^?1), mesophyll protoplasts showed remarkable decrease in the rates of NaHCO₃‐dependent O₂ evolution (indicator of photosynthetic carbon assimilation), while decrease in the rates of respiratory O₂ uptake were marginal. While the capacity of AOX pathway increased significantly by two fold under HL, the capacity of cytochrome oxidase (COX) pathway decreased by >50% compared with capacities under darkness and normal light (NL). Further, the total cellular levels of pyruvate and malate, which are assimilatory products of active photosynthesis and stimulators of AOX activity, were increased remarkably parallel to the increase in AOX protein under HL. Upon restriction of AOX pathway using salicylhydroxamic acid (SHAM), the observed decrease in NaHCO₃‐dependent O₂ evolution or p‐benzoquinone (BQ)‐dependent O₂ evolution [indicator of photosystem II (PSII) activity] and the increase in total cellular levels of pyruvate and malate were further aggravated/promoted under HL. The significance of raised malate and pyruvate levels in activation of AOX protein/AOX pathway, which in turn play an important role in dissipating excess chloroplastic reducing equivalents and sustenance of photosynthetic carbon assimilation to balance the effects of HL stress on photosynthesis, was depicted as a model.     

Study on photosynthetic responses and chlorophyll fluorescence in Rhizophora mucronata seedlings under shade regimes

The photosynthetic performance of mangrove Rhizophora mucronata seedlings grown under seasonally full light (HL), 50 % shade (ML), and 80 % shade (LL) conditions was characterized by gas exchange, and chlorophyll fluorescence. The carboxylation efficiency significantly affected the seasonal change of the photosynthetic capacity. Temperature and light might have synergic effect on the carboxylation efficiency. The photosynthetic rate (PN) of R. mucronata seedlings under shade regimes, however, could not be attributed to variability in chlorophyll, C _i , ΦPSII, ETR or qP values but more to differences in carboxylation efficiency, g _max, and E _max. HL and ML plants had higher PN, g _s and E than the LL ones. Nevertheless, LL leaves exhibited low photoinhibition susceptibility. The high non-photochemical quenching in HL leaves may show that applied light intensity probably exceeded the photosynthetic capability. The findings indicate that ML treatments provided the best condition to obtain such carbon fixation capacity.     

Photosynthetic acclimation in relation to nitrogen allocation in cucumber leaves in response to changes in irradiance

Leaves deep in canopies can suddenly be exposed to increased irradiances following e.g. gap formation in forests or pruning in crops. Studies on the acclimation of photosynthesis to increased irradiance have mainly focused on the changes in photosynthetic capacity (A_max), although actual irradiance often remains below saturating level. We investigated the effect of changes in irradiance on the photosynthesis irradiance response and on nitrogen allocation in fully grown leaves of Cucumis sativus. Leaves that fully developed under low (50 µmol m^?2 s^?1) or moderate (200 µmol m^?2 s^?1) irradiance were subsequently exposed to, respectively, moderate (LM‐leaves) or low (ML‐leaves) irradiance or kept at constant irradiance level (LL‐ and MM‐leaves). Acclimation of photosynthesis occurred within 7 days with final A_max highest in MM‐leaves, lowest in LL‐leaves and intermediate in ML‐ and LM‐leaves, whereas full acclimation of thylakoid processes underlying photosystem II (PSII) efficiency and non‐photochemical quenching occurred in ML‐ and LM‐leaves. Dark respiration correlated with irradiance level, but not with A_max. Light‐limited quantum efficiency was similar in all leaves. The increase in photosynthesis at moderate irradiance in LM‐leaves was primarily driven by nitrogen import, and nitrogen remained allocated in a similar ratio to Rubisco and bioenergetics, while allocation to light harvesting relatively decreased. A contrary response of nitrogen was associated with the decrease in photosynthesis in ML‐leaves. Net assimilation of LM‐leaves under moderate irradiance remained lower than in MM‐leaves, revealing the importance of photosynthetic acclimation during the leaf developmental phase for crop productivity in scenarios with realistic, moderate fluctuations in irradiance that leaves can be exposed to.     

Comparison of responses of bean, pea and rape plants to UV-B radiation in darkness and in light

Effect of UV-B radiation on leaves of bean, pea and rape plants was studied. UV-B radiation (11.2 kJ·m⁻²) induced more distinct reduction of the primary photosynthesis activity when applied in darkness than the same UV-B dose, extended in time, and applied with photosynthetic active radiation (PAR). The pea plants were more susceptible to UV-B in darkness, but in the presence of PAR their tolerance was higher. The CO₂ fixation in the bean and rape plants, exposed to UV-B was decreased, but for the pea plants it remained unchanged. The UV-B irradiation caused an increase in the content of ultraviolet-absorbing pigments. Additionally, the bean plants grown at UV-B increased the thickness of leaves, described as SLW.     

Interactive effects of photon fluence rates and drought on CAM‐cycling in Delosperma tradescantioides (Mesembryanthemaceae)

The ability of photosynthesis and CAM to acclimate to low (220 µmol m^?2 s^?1; LL) and relatively high (550 µmol m^?2 s^?1; HL) photosynthetic photon flux densities (PPFD) was investigated in the CAM-cycling species Delosperma tradescantioides by means of CO₂ gas exchange and chlorophyll fluorescence analysis. Furthermore, the influence of short-term drought on malic acid accumulation and the activity of photosystem II (PSII) was studied to assess the possible interactions between drought and the prevailing PPFD in this species. HL plants showed features of sun versus shade acclimation relative to LL plants. Nocturnal malic acid accumulation (Δ-malate) and leaf water content also tended to be higher in HL plants. Irrespective of the PPFD during growth, the weak Δ-malate doubled within 3 days of drought. Despite largely restricted CO₂ uptake, photosynthetic activity as estimated from fluorescence analysis declined only ca 5%. After 7 days of drought, when plants showed CAM-idling and Δ-malate had decreased again, potential carbon assimilation was still ca 84% of that in well-watered plants and remained relatively constant throughout the day. Decarboxylation of malic acid accounted for ca 23% of potential assimilation assuming total oxidation of a maximum portion of this organic acid. Drought did not affect predawn maximum photochemical efficiency (F_v/F_m). Nonphotochemical quenching (qN) increased (24%) in response to desiccation and resulted in a more or less constant reduction state of PSII. This increase in qN resulted mainly from the change in its fast-relaxing component (qNF), while the slow component (qNS) was significant only at or above saturating PPFD in both HL and LL plants. The photon response characteristics of PSII, which differed between LL and HL plants, were unaffected by short-term drought. Photon harvesting and photon use were always adjusted to guarantee a low reduction state of PSII. Results suggest that in both LL and HL plants CAM-cycling may help to stabilize photosynthesis but to a large extent by other means than simply providing internally derived CO₂.     

Salt stress and fluctuating light have separate effects on photosynthetic acclimation,but interactively affect biomass

In nature, soil salinity and fluctuating light (FL) often occur concomitantly. However, it is unknown whether salt stress interacts with FL on leaf photosynthesis, architecture, biochemistry, pigmentation, mineral concentrations, as well as whole-plant biomass. To elucidate this, tomato (Solanum lycopersicum) seedlings were grown under constant light (C, 200 μmol m⁻² s⁻¹) or FL (5–650 μmol m⁻² s⁻¹), in combination with no (0 mM NaCl) or moderate (80 mM NaCl) salinity, for 14 days, at identical photoperiods and daily light integrals. FL and salt stress had separate effects on leaf anatomy, biochemistry and photosynthetic capacity: FL reduced leaf thickness as well as nitrogen, chlorophyll and carotenoid contents per unit leaf area, but rarely affected steady-state and dynamic photosynthetic properties along with abundance of key proteins in the electron transport chain. Salt stress, meanwhile, mainly disorganized chloroplast grana stacking, reduced stomatal density, size and aperture as well as photosynthetic capacity. Plant biomass was affected interactively by light regime and salt stress: FL reduced biomass in salt stressed plants by 17%, but it did not affect biomass of non-stressed plants. Our results stress the importance of considering FL when inferring effects of salt-stress on photosynthesis and productivity under fluctuating light intensities.     

Photosynthesis and transpiration of tomato and CO2 fluxes in a greenhouse under changing environmental conditions in winter

Responses of tomato leaves in a greenhouse to light and CO₂ were examined at the transient stage at the end of winter, when both photoperiod and irradiance gradually increase. Additionally, CO₂ fluxes were calculated for a greenhouse without supplementary lighting and without CO₂ enrichment based on CO₂ sinks (plant photosynthesis) and CO₂ sources (plant and substrate respiration). In January, tomato leaves in the greenhouse showed low photosynthesis with a maximum assimilation of 6–8 μmol CO₂ m⁻² s⁻¹, a quantum yield of 0.06 μmol CO₂ μmol⁻¹ photosynthetic active radiation (PAR) and a low light compensation point of 26 μmol PAR m⁻² s⁻¹, a combination which classifies them as shade leaves. In February, tomato leaves increased their light compensation point to 39 μmol PAR m⁻² s⁻¹ and quantum yield to 0.08, the former indicating the adaptation to increased irradiance and photoperiod. These tomato leaves increased their transpiration from 0.4 to 0.9 in January to ∼2 mmol H₂O m⁻² s⁻¹ in February. Both photosynthesis and transpiration were primarily limited by light but neither by stomatal conductivity nor by CO₂. In January, light response of photosynthesis, dark respiration and transpiration were negligibly affected by increasing CO₂ concentrations from 600 to 900 ppm CO₂ under low light conditions, indicating no benefit of CO₂ enrichment unless light intensity increased. In February, tomato leaves were photoinhibited at inherent greenhouse CO₂ concentrations on the first sunny day; this photoinhibition was further enhanced by an increased CO₂ concentration of 1000 ppm. CO₂ fluxes in the greenhouse appeared strongly dependent on solar radiation. After exceeding the light compensation point in the morning, greenhouse CO₂ concentrations decreased by 58 or by 110 ppm CO₂ h⁻¹ on a sunny day in January or February and by 23 ppm on overcast days in both months. Calculated per overall tomato canopy, plant photosynthesis contributed 42–50% to the morning CO₂ depletion in the greenhouse. Dark respiration of tomato leaves was ∼2 μmol CO₂ m⁻² s⁻¹ in January and ∼3 μmol CO₂ m⁻² s⁻¹ in February. This dark respiration resulted in rises of 15 and 17 ppm CO₂ h⁻¹ at night in the greenhouse compartment and was identified as primary source of CO₂. Respiration of the substrate used to grow the plants, which produced 7.3 ppm CO₂ h⁻¹, was identified as secondary source of CO₂. The combined plant and substrate respiration resulted in peaks of up to 900 ppm CO₂ in the greenhouse before dawn.     

Does Rubisco control the rate of photosynthesis and plant growth? An exercise in molecular ecophysiology

Experiments are described in which tobacco (Nicotiana tabacum L.) transformed with antisense rbcS to decrease expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) was used to evaluate the contribution of Rubisco to the control of photosynthetic rate, and the impact of a changed rate of photosynthesis on whole plant composition, allocation and growth. (1) The concept of flux control coefficients is introduced. It is discussed how, with adequate precautions, a set of wild-type and transgenic plants with varying expression of an enzyme can be used to obtain experimental values for its flux control coefficient. (2) The flux control coefficient of Rubisco for photosynthesis depends on the short-term conditions. It increases in high light, or low CO₂. (3) When plants are grown under constant irradiance, the flux control coefficient in the growth conditions is low (<0.2) at irradiances of up to 1000μmol quanta m⁻² s⁻¹. In a natural irradiance regime exceeding 1500μmol quanta m⁻² s⁻² over several hours the flux coefficient rose to 0.8–0.9. It is concluded that plants are able to adjust the balance between Rubisco and the remainder of the photosynthetic machinery, and thereby avoid a one-sided limitation of photosynthesis by Rubisco over a wide range of ambient growth irradiance regimes. (4) When the plants were grown on limiting inorganic nitrogen, Rubisco had a higher flux control coefficient (0.5). It is proposed that, in many growth conditions, part of the investment in Rubisco may be viewed as a nitrogen store, albeit bringing additional marginal advantages with respect to photosynthetic rate and water use efficiency. (5) A change in the rate of photosynthesis did not automatically translate into a change in growth rate. Several factors are identified which contribute to this buffering of growth against a changed photosynthetic rate. (6) There is an alteration in whole plant allocation, resulting in an increase in the leaf area ratio. The increase is mainly due to a higher leaf water content, and not to changes in shoot/root allocation. This increased investment in whole plant leaf area partly counteracts the decreased efficiency of photosynthesis at the biochemical level. (7) Plants with decreased Rubisco have a lower intrinsic water use efficiency and contain high levels of inorganic cations and anions. It is proposed that these are a consequence of the increased rate of transpiration, and that the resulting osmotic potential might be a contributory factor to the increased water content and expansion of the leaves. (8) Starch accumulation in source leaves is decreased when unit leaf photosynthesis is reduced, allowing a more efficient use of the fixed carbon. (9) Decreased availability of carbohydrates leads to a down-regulation of nitrate assimilation, acting via a decrease in nitrate reductase activity.     

Ex Vitro Phenotype Stability is Affected by In Vitro Cultivation

Plant phenotype stability during ex vitro growth, one of the main requirements of plant micropropagation, was tested on tobacco. Plants cultivated in vitro in the presence of 3 % sucrose under photon flux density (PFD) of 200 mol m^–2 s^–1 (3 % HL plants) showed the best growth and photosynthetic parameters in the course of 7-day acclimation. However, significant change in phenotype of these plants appeared under a decrease in PFD to 50 mol m^–2 s^–1 during further ex vitro growth (in the period of 7^th – 17^th day). Much higher internodia elongation was found in 3 % HL plants in comparison with plants grown in vitro on sucrose media under PFD of 50 mol m^–2 s^–1 (3 % LL) or without sucrose either under PFD of 50 mol m^–2 s^–1 or 200 mol m^–2 s^–1 (0 % LL, 0 % HL). It can be presumed that 3 % HL plants show permanent demand for high PFD. Neither ABA or chlorophyll contents nor de novo thylakoid membrane synthesis were related to the morphogenic effect of low PFD. Changeable contents of hexoses in leaves of 3 % HL and 3 % LL plants were in no direct correlation to the elongated growth.     

Effects of UV-B on flavonoids, ferulic acid, growth and photosynthesis in barley primary leaves

Barley (Hordeum vulgare L.) was grown with UV-B (280–320 nm) at levels simulating 25 nr 5% ozone depletion on the date of the summer solstice al 40°N latitude, with UV-A (320–400 nm), or with no supplemental irradiation. In plant growth chambers providing 300 μmol m^?2 s^?1 photosynthetically active radiation (PAR). UV-B-grown leaves elongated more slowly than controls but reached the same final length 1 day later. Leal specific fresh weight (mass leaf area^?1) was significantly increased by UV-B after the 7th day of growth. IV-B did not significantly affect leaf area, fresh weight, dry weight, total chlorophylls, total carotenoids or photosynthetic quantum efficiency. CO₂ assimilation was decreased by UV-B only at internal CO₂ levels above 250 μl l^?1. By the 8th day of growth, UV-B increased flavonoid (saponarin and lutonarin) accumulation in both the lower epidermis and the mesophyll: about 40% of the saponarin and 20% of the lutonarin were in the lower epidermis under all experimental conditions. Glasshouse conditions proved too variable for reproducible determination of growth and photosynthesis but were reliable for determining developmental changes in flavonoid (saponarin and lutonarin) accumulation and provided up to 800 μmol m^?2 s^?1 PAR. In the glasshouse UV-B-grown leaves had more flavonoids than controls al all stages from 5 to 30 days after planting: ca 509 more saponarin and 100% more lutonarin. Levels of soluble (vacuolar) ferulic acid esters were similar under all conditions on day 5. and on day 20 or later, but were significantly higher in UV-B-grown plants on days 10 and 15. UV-B decreased insoluble (cell-wall-bound) ferulic acid esters on a whole leaf basis but significantly increased this fraction in the lower epidermis. UV-A had no significant effects on growth, photosynthesis or ferulic acid, but it slightly increased flavonoid accumulation. The results are discussed in terms of secondary phenolics as a tissue-specific, developmentally regulated adaptive response to UV-B.     

The effects of enhanced ultraviolet-B and nitrogen supply on growth,photosynthesis and nutrient status of <Emphasis Type="Italic">Abies faxoniana</Emphasis> seedlings

Abies faxoniana is a key species in reforestation processes in the southeast of the Qinghai-Tibetan Plateau of China. The changes in growth, photosynthesis and nutrient status of A. faxoniana seedlings exposed to enhanced ultraviolet-B (UV-B), nitrogen supply and their combination were investigated. The experimental design included two levels of UV-B treatments (ambient UV-B, 11.02 KJ m⁻² day⁻¹; enhanced UV-B, 14.33 KJ m⁻² day⁻¹) and two nitrogen levels (0; 20 g N m⁻²). The results indicated that: (1) enhanced UV-B significantly caused a marked decline in growth parameters, net photosynthetic rate (Pn), photosynthetic pigments and F _v/F _m, (2) supplemental nitrogen supply increased the accumulation of total biomass, Pn, photosynthetic pigments and F _v/F _m under ambient UV-B, whereas supplemental nitrogen supply reduced Pn, and not affect biomass under enhanced UV-B, (3) enhanced UV-B or nitrogen supply changed the concentration of nutrient elements of various organs.     

Effects of Exogenous γ-Aminobutyric Acid (GABA) on Photosynthesis and Antioxidant System in Pepper (Capsicum annuum L.) Seedlings Under Low Light Stress

The effects of γ-aminobutyric acid (GABA) treatment on parameters of photosynthesis and antioxidant defense system were measured in pepper (Capsicum annuum L.) leaves under low-light (LL) stress. Seedlings exposed to LL stress showed increased chlorophyll content as well as decreased net photosynthetic rate (P _n), stomatal conductance (g _s), maximum quantum yield of PSII (F _v/F _m), actual PSII photochemical efficiency (Φ_PSII), electron transport rates and photochemical quenching coefficient (q _p). However, almost all the photosynthetic parameters above were enhanced markedly in seedlings treated with GABA under LL stress. Moreover, LL stress increased malondialdehyde (MDA) content, superoxide anion radical (O₂ ^·?) and hydrogen peroxide (H₂O₂) production. GABA-treated, LL-stressed seedlings exhibited lower MDA, O₂ ^·? and H₂O₂ production, and showed an activated antioxidant defense system, including increased activities of superoxide dismutase, catalase, ascorbate peroxidase, glutathione peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, ascorbate and glutathione. Moreover, seedlings subjected to LL stress showed increased endogenous GABA levels, and the level was further improved by application of exogenous GABA. These results suggest that GABA mitigates the LL-induced stress via regulating the antioxidant defense system and maintaining a high level of photochemical efficiency in pepper seedlings.

     

Protection of thylakoids against combined light and drought by a lumenal substance in the resurrection plant Haberlea rhodopensis

Background and Aims

Haberlea rhodopensis is a perennial, herbaceous, saxicolous, poikilohydric flowering plant that is able to survive desiccation to air-dried state under irradiance below 30 µmol m⁻² s⁻¹. However, desiccation at irradiance of 350 µmol m⁻² s⁻¹ induced irreversible changes in the photosynthetic apparatus, and mature leaves did not recover after rehydration. The aim here was to establish the causes and mechanisms of irreversible damage of the photosynthetic apparatus due to dehydration at high irradiance, and to elucidate the mechanisms determining recovery.

Methods

Changes in chloroplast structure, CO₂ assimilation, chlorophyll fluorescence parameters, fluorescence imaging and the polypeptide patterns during desiccation of Haberlea under medium (100 µmol m⁻² s⁻¹; ML) irradiance were compared with those under low (30 µmol m⁻² s⁻¹; LL) irradiance.

Key Results

Well-watered plants (control) at 100 µmol m⁻² s⁻¹ were not damaged. Plants desiccated at LL or ML had similar rates of water loss. Dehydration at ML decreased the quantum efficiency of photosystem II photochemistry, and particularly the CO₂ assimilation rate, more rapidly than at LL. Dehydration induced accumulation of stress proteins in leaves under both LL and ML. Photosynthetic activity and polypeptide composition were completely restored in LL plants after 1 week of rehydration, but changes persisted under ML conditions. Electron microscopy of structural changes in the chloroplast showed that the thylakoid lumen is filled with an electron-dense substance (dense luminal substance, DLS), while the thylakoid membranes are lightly stained. Upon dehydration and rehydration the DLS thinned and disappeared, the time course largely depending on the illumination: whereas DLS persisted during desiccation and started to disappear during late recovery under LL, it disappeared from the onset of dehydration and later was completely lost under ML.

Conclusions

Accumulation of DLS (possibly phenolics) in the thylakoid lumen is demonstrated and is proposed as a mechanism protecting the thylakoid membranes of H. rhodopensis during desiccation and recovery under LL. Disappearance of DLS during desiccation in ML could leave the thylakoid membranes without protection, allowing oxidative damage during dehydration and the initial rehydration, thus preventing recovery of photosynthesis.Key words: Haberlea rhodopensis, resurrection plant, electron microscopy, blue–green fluorescence, chlorophyll fluorescence