Low Light-grown Plants Research Articles

Contrasting Responses to Stress Displayed by Tobacco Overexpressing an Algal Plastid Terminal Oxidase in the Chloroplast.

The plastid terminal oxidase (PTOX) – an interfacial diiron carboxylate protein found in the thylakoid membranes of chloroplasts – oxidizes plastoquinol and reduces molecular oxygen to water. It is believed to play a physiologically important role in the response of some plant species to light and salt (NaCl) stress by diverting excess electrons to oxygen thereby protecting photosystem II (PSII) from photodamage. PTOX is therefore a candidate for engineering stress tolerance in crop plants. Previously, we used chloroplast transformation technology to over express PTOX1 from the green alga Chlamydomonas reinhardtii in tobacco (generating line Nt-PTOX-OE). Contrary to expectation, growth of Nt-PTOX-OE plants was more sensitive to light stress. Here we have examined in detail the effects of PTOX1 on photosynthesis in Nt-PTOX-OE tobacco plants grown at two different light intensities. Under ‘low light’ (50 μmol photons m–2 s–1) conditions, Nt-PTOX-OE and WT plants showed similar photosynthetic activities. In contrast, under ‘high light’ (125 μmol photons m–2 s–1) conditions, Nt-PTOX-OE showed less PSII activity than WT while photosystem I (PSI) activity was unaffected. Nt-PTOX-OE grown under high light also failed to increase the chlorophyll a/b ratio and the maximum rate of CO2 assimilation compared to low-light grown plants, suggesting a defect in acclimation. In contrast, Nt-PTOX-OE plants showed much better germination, root length, and shoot biomass accumulation than WT when exposed to high levels of NaCl and showed better recovery and less chlorophyll bleaching after NaCl stress when grown hydroponically. Overall, our results strengthen the link between PTOX and the resistance of plants to salt stress.

Acclimation of Swedish and Italian ecotypes of Arabidopsis thaliana to light intensity.

This study addressed whether ecotypes of Arabidopsis thaliana from Sweden and Italy exhibited differences in foliar acclimation to high versus low growth light intensity, and compared CO2 uptake under growth conditions with light- and CO2-saturated intrinsic photosynthetic capacity and leaf morphological and vascular features. Differential responses between ecotypes occurred mainly at the scale of leaf architecture, with thicker leaves with higher intrinsic photosynthetic capacities and chlorophyll contents per leaf area, but no difference in photosynthetic capacity on a chlorophyll basis, in high light-grown leaves of the Swedish versus the Italian ecotype. Greater intrinsic photosynthetic capacity per leaf area in the Swedish ecotype was accompanied by a greater capacity of vascular infrastructure for sugar and water transport, but this was not associated with greater CO2 uptake rates under growth conditions. The Swedish ecotype with its thick leaves is thus constructed for high intrinsic photosynthetic and vascular flux capacity even under growth chamber conditions that may not permit full utilization of this potential. Conversely, the Swedish ecotype was less tolerant of low growth light intensity than the Italian ecotype, with smaller rosette areas and lesser aboveground biomass accumulation in low light-grown plants. Foliar vein density and stomatal density were both enhanced by high growth light intensity with no significant difference between ecotypes, and the ratio of water to sugar conduits was also similar between the two ecotypes during light acclimation. These findings add to the understanding of the foliar vasculature's role in plant photosynthetic acclimation and adaptation.

Photoprotective capacity of non-photochemical quenching in plants acclimated to different light intensities.

Arabidopsis plants grown at low light were exposed to a gradually increasing actinic light routine. This method allows for the discerning of the photoprotective component of NPQ, pNPQ and photoinhibition. They exhibited lower values of Photosystem II (PSII) yield in comparison to high-light grown plants, and higher calculated dark fluorescence level (F'o calc.) than the measured one (F'o act.). As a result, in low-light grown plants, the values of qP measured in the dark appeared higher than 1. Normally, F'o act. and F'o calc. match well at moderate light intensities but F'o act. becomes higher at increasing intensities due to reaction centre (RCII) damage; this indicates the onset of photoinhibition. To explain the unusual increase of qP in the dark in low-light grown plants, we have undertaken an analysis of PSII antenna size using biochemical and spectroscopic approaches. Sucrose gradient separation of thylakoid membrane complexes and fast fluorescence induction experiments illustrated that the relative PSII cross section does not increase appreciably with the rise in PSII antenna size in the low-light grown plants. This suggests that part of the increased LHCII antenna is less efficiently coupled to the RCII. A model based upon the existence of an uncoupled population LHCII is proposed to explain the discrepancies in calculated and measured values of F'o.

Light Intensity Alters the Extent of Arsenic Toxicity in Helianthus annuus L. Seedlings

The present study is aimed at assessing the extent of arsenic (As) toxicity under three different light intensities-optimum (400μmole photon m(-2)s(-1)), sub-optimum (225μmole photon m(-2)s(-1)), and low (75μmole photon m(-2)s(-1))-exposed to Helianthus annuus L. var. DRSF-113 seedlings by examining various physiological and biochemical parameters. Irrespective of the light intensities under which H. annuus L. seedlings were grown, there was an As dose (low, i.e., 6mgkg(-1) soil, As1; and high, i.e., 12mgkg(-1) soil, As2)-dependent decrease in all the growth parameters, viz., fresh mass, shoot length, and root length. Optimum light-grown seedlings exhibited better growth performance than the sub-optimum and low light-grown seedlings; however, low light-grown plants had maximum root and shoot lengths. Accumulation of As in the plant tissues depended upon its concentration used, proximity of the plant tissue, and intensity of the light. Greater intensity of light allowed greater assimilation of photosynthates accompanied by more uptake of nutrients along with As from the medium. The levels of chlorophyll a, b, and carotenoids declined with increasing concentrations of As. Seedlings acquired maximum Chl a and b under optimum light which were more compatible to face As1 and As2 doses of As, also evident from the overall status of enzymatic (SOD, POD, CAT, and GST) and non-enzymatic antioxidant (Pro).

Identification of hydroxy-plastochromanol in Arabidopsis leaves.

In the present study we have identified hydroxy-plastochromanol in plants for the first time. This compound was found both in low light and high light-grown Arabidopsis plants, however, under high light stress its level was considerably increased. Hydroxy-plastochromanol accumulated also during ageing of leaves of low light-grown plants, similarly as in the case of other prenyllipids. Our results indicate that hydroxy-plastochromanol found in leaves is probably formed as a result of plastochromanol oxidation by singlet oxygen generated in photosystem II during photosynthesis. These data also support the hypothesis that plastochromanol is an efficient antioxidant in vivo, similarly as tocopherols and plastoquinol.

Trends in leaf photosynthesis in historical rice varieties developed in the Philippines since 1966

Crop improvement in terms of yield is rarely linked to leaf photosynthesis. However, in certain crop plants such as rice, it is predicted that an increase in photosynthetic rate will be required to support future grain yield potential. In order to understand the relationships between yield improvement and leaf photosynthesis, controlled environment conditions were used to grow 10 varieties which were released from the International Rice Research Institute (IRRI) between 1966 and 1995 and one newly developed line. Two growth light intensities were used: high light (1500 micromol m(-2) s(-1)) and low light (300 micromol m(-2) s(-1)). Gas exchange, leaf protein, chlorophyll, and leaf morphology were measured in the ninth leaf on the main stem. A high level of variation was observed among high light-grown plants for light-saturated photosynthetic rate per unit leaf area (P(max)), stomatal conductance (g), content of ribulose bisphosphate carboxylase-oxygenase (Rubisco), and total leaf protein content. Notably, between 1966 and 1980 there was a decline in P(max), g, leaf protein, chlorophyll, and Rubisco content. Values recovered in those varieties released after 1980. This striking trend coincides with a previous published observation that grain yield in IRRI varieties released prior to 1980 correlated with harvest index whereas that for those released after 1980 correlated with biomass. P(max) showed significant correlations with both g and Rubisco content. Large differences were observed between high light- and low light-grown plants (photoacclimation). The photoacclimation 'range' for P(max) correlated with P(max) in high light-grown plants. It is concluded that (i) leaf photosynthesis may be systematically affected by breeding strategy; (ii) P(max) is a useful target for yield improvements where yield is limited by biomass production rather than partitioning; and (iii) the capacity for photoacclimation is related to high P(max) values.

Dependence of reaction center-type energy-dependent quenching on photosystem II antenna size

The effects of photosystem II antenna size on reaction center-type energy-dependent quenching (qE) were examined in rice plants grown under two different light intensities using both wild type and qE-less (OsPsbS knockout) mutant plants. Reaction center-type qE was detected by measuring non-photochemical quenching at 50 μmol photons m − 2 s − 1 white light intensity. We observed that in low light-grown rice plants, reaction center-type qE was higher than in high light-grown plants, and the amount of reaction center-type qE did not depend on zeaxanthin accumulation. This was confirmed in Arabidopsis npq1–2 mutant plants that lack zeaxanthin due to a mutation in the violaxanthin de-epoxidase enzyme. Although the electron transport rate measured at a light intensity of 50 μmol photons m − 2 s − 1 was the same in high light- and low light-grown wild type and mutant plants lacking PsbS protein, the generation of energy-dependent quenching was completely impaired only in mutant plants. Analyses of the pigment content, Lhcb proteins and D1 protein of PSII showed that the antenna size was larger in low light-grown plants, and this correlated with the amount of reaction center-type qE. Our results mark the first time that the reaction center-type qE has been shown to depend on photosystem II antenna size and, although it depends on the existence of PsbS protein, the extent of reaction center-type qE does not correlate with the transcript levels of PsbS protein. The presence of reaction center-type energy-dependent quenching, in addition to antenna-type quenching, in higher plants for dissipation of excess light energy demonstrates the complexity and flexibility of the photosynthetic apparatus of higher plants to respond to different environmental conditions.

The PsbQ Protein Is Required in Arabidopsis for Photosystem II Assembly/Stability and Photoautotrophy under Low Light Conditions

RNA interference was used to simultaneously suppress the expression of the two genes that encode the PsbQ proteins of Photosystem II (PS II) in Arabidopsis thaliana, psbQ-1 (At4g21280) and psbQ-2 (At4g05180). Two independent PsbQ-deficient plant lines were examined. These plant lines produced little detectable PsbQ protein. Under normal growth light conditions, the wild type and mutant plants were visually indistinguishable. Additionally, analysis of steady state oxygen evolution rates and chlorophyll fluorescence characteristics indicated little alteration of photosynthetic capacity in the mutant plants. No loss of other PS II proteins was evident. Interestingly, flash oxygen yield analysis performed on thylakoid membranes isolated from the mutant and wild type plants indicated that the oxygen-evolving complex was quite unstable in the mutants. Furthermore, the lifetime of the S2 state of the oxygen-evolving complex appeared to be increased in these plants. Incubation of the wild type and mutant plants under low light growth conditions led to a significantly stronger observed phenotype in the mutants. The mutant plants progressively yellowed (after 2 weeks) and eventually died (after 3-4 weeks). The wild type plants exhibited only slight yellowing after 4 weeks under low light conditions. The mutant plants exhibited a large loss of a number of PS II components, including CP47 and the D2 protein, under low light conditions. Additionally, significant alterations of their fluorescence characteristics were observed, including an increased FO and decreased FV, yielding a large loss in PS II quantum efficiency (FV/FM). Analysis of QA- decay kinetics in the absence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea indicated a defect in electron transfer from QA- to QB, whereas experiments performed in the presence of this herbicide indicated that the recombination rate between QA- and the S2 state was strongly retarded. These results indicate that the loss of the PsbQ protein induces significant changes in Photosystem II function, particularly in low light-grown plants, and that the PsbQ protein is required for photoautotrophic growth under low light conditions.

PsbR, a Missing Link in the Assembly of the Oxygen-evolving Complex of Plant Photosystem II

The oxygen-evolving complex of eukaryotic photosystem II (PSII) consists of three extrinsic nuclear-encoded subunits, PsbO (33 kDa), PsbP (23 kDa), and PsbQ (17 kDa). Additionally, the 10-kDa PsbR protein has been found in plant PSII and anticipated to play a role in water oxidation, yet the physiological significance of PsbR has remained obscure. Using the Arabidopsis psbR mutant, we showed that the light-saturated rate of oxygen evolution is strongly reduced in the absence of PsbR, particularly in low light-grown plants. Lack of PsbR also induced a reduction in the content of both the PsbP and the PsbQ proteins, and a near depletion of these proteins was observed under steady state low light conditions. This regulation occurred post-transcriptionally and likely involves a proteolytic degradation of the PsbP and PsbQ proteins in the absence of an assembly partner, proposed to be the PsbR protein. Stable assembly of PsbR in the PSII core complex was, in turn, shown to require a chloroplast-encoded intrinsic low molecular mass PSII subunit PsbJ. Our results provided evidence that PsbR is an important link in the PSII core complex for stable assembly of the oxygen-evolving complex protein PsbP, whereas the effects on the assembly of PsbQ are probably indirect. The physiological role of the PsbR, PsbP, and PsbQ proteins is discussed in light of their peculiar expression in response to growth light conditions.

The acclimative response of the main light-harvesting chlorophyll a/ b–protein complex of photosystem II (LHCII) to elevated irradiances at the level of trimeric subunits

The changes in structural organization of the major light-harvesting chlorophyll a/ b–protein complex of photosystem II (LHC II) at the level of trimeric subcomplexes were studied in spinach plants grown under low light conditions (50 μmol quanta m −2 s −1) and then acclimated to elevated irradiances. By monitoring photochemical quenching of fluorescence yield ( q P), photosystem II (PS II) functional status was assessed in leaves of plants acclimated to a range of elevated irradiances. Three separate acclimative irradiances were selected for the experiments, reflecting: limiting light conditions (150 μmol quanta m −2 s −1), near to the inflexion point on the irradiance curve conditions (300 μmol quanta m −2 s −1) and an excessive light, causing a moderate stress in the form of down regulation of PS II (450 μmol quanta m −2 s −1). An immunoblot analysis showed that there was a clear decline in an abundance on chlorophyll basis of Lhcb1-3 apoproteins as an acclimative irradiance increased from 50 to 450 μmol quanta m −2 s −1, with Lhcb1 decreasing to a lesser extent than Lhcb2 and Lhcb3 (only at excessive irradiance). When analyzed by non-denaturing isoelectric focusing BBY membrane fragments (PSII α-enriched, stacked thylakoid membranes) isolated from low light-grown plants were resolved into nine fractions, seven of which (labelled 3–9) were established by us previously [Jackowski and Pielucha, J. Photochem. Photobiol. B: Biol. 64 (2001) 45] to be LHC II subcomplexes representing mixed populations of closely similar trimers, comprising permutations of Lhcb1 and Lhcb2 (subcomplexes 3–7) or Lhcb1-3 (subcomplexes 8 and 9). A heterogeneity with regard to accumulation behaviour of LHC II subcomplexes in response to elevated irradiances was revealed. The subcomplexes 5 and 6 were accumulating at similar level, regardless of the light irradiance experienced. Another group consisting of the subcomplexes 3 and 4 (the most basic ones) showed a progressive increase in relative abundance with increasing an irradiance intensity whereas the subcomplexes 7–9 (the most acidic ones) exhibited a progressive decline in their relative abundance during an acclimation of spinach plants to elevated irradiances thus they may collectively represent an elevated irradiance-responsive subunit of LHCII.

Responses of Photosynthesis and Biomass Allocation of an Understorey Herb, Pteridophyllum racemosum, to Gradual Increases in Irradiance

In wave-regenerating Abies forests in central Japan, light availability on the forest floor of a mature-tree stand increases gradually over 4 to 5 years during the senescence of trees. The understorey evergreen perennialPteridophyllum racemosum Sieb. et Zucc. (Papaveraceae) is commonly found in such forests. To determine the acclimation characteristics of this species to gradual changes in light availability, leaf photosynthetic traits and biomass allocation patterns were determined along a transect running from fully shaded understorey to a canopy gap. Along the transect, the photon flux density on the forest floor relative to full sun (RPFD) varied from 2.3–36.7%. The light-saturated net photosynthetic rate of field-grown plants increased as the light environment increased from 2.3–14.1% RPFD. However, as light conditions increased from moderate (14.1%) to high (36.7%), the light-saturated net photosynthetic rate decreased to a level comparable to that exhibited by low-light grown plants. On the other hand, root mass per unit leaf area (RMA) increased with increasing light conditions. A higher RMA contributes to a higher water uptake capacity per unit leaf area which in turn supports higher rates of photosynthesis and transpiration in high light. We conclude that in wave-regenerating forests P. racemosum can survive in high-light environments by avoiding a severe water deficit and heat load by increasing RMA.

Structural and functional heterogeneity in the major light-harvesting complexes of higher plants

The major light-harvesting complex (LHC IIb) of higher plants plays a crucial role in capturing light energy for photosynthesis and in regulating the flow of energy within the photosynthetic apparatus. Multiple isoforms of the protein bind chlorophyll and xanthophyll chromophores, but it is commonly believed that the pigment-binding properties of different LHC IIb complexes are conserved within and between species. We have investigated the structure and function of different LHC IIb complexes isolated from Arabidopsis thaliana grown under different light conditions. LHC IIb isolated from low light-grown plants shows increased amounts of the Lhcb2 gene product, increased binding of chlorophyll a, and altered energy transfer characteristics. We suggest that Lhcb2 specifically binds at least one additional chlorophyll a compared to the Lhcb1 gene product, and that differences in the functioning of LHC IIb from high and low light-grown plants are a direct consequence of the change in polypeptide composition. We show that changes in LHC IIb composition are accompanied by changes in photosynthetic function in vivo and discuss the possible functional significance of LHC IIb heterogeneity.

Ascorbate Metabolism in Potato Leaves Supplied with Exogenous Ascorbate

Photosynthesis and leaf ascorbate were measured in potato (Solanum tuberosum L.) plants grown in low light and then transferred to high light. Total foliar ascorbate content in low light-grown plants was 4.72+/-0.42 micromol mg(-1)chl. Over 80% of the ascorbate pool was found in the reduced form irrespective of position on the stem. No statistically-significant light-dependent effects were observed. Leaf discs supplied with [14C]-ascorbate in the dark showed significant ascorbate uptake such that after a 16h incubation over half of the total ascorbate pool in the discs was labelled [14C]-ascorbate. No ascorbate efflux from the leaves occurred during the period of [14C]-ascorbate uptake. The total amount of ascorbate did not increase, however, implying modified ascorbate turnover. The turnover of the [14C]-ascorbate in the leaves occurred at similar rates in both light and darkness. Little degradation of labelled ascorbate was observed, suggesting that uptake of exogenous ascorbate leads to inhibition of de novo ascorbate biosynthesis in potato leaf discs.

Chlorophyll fluorescence quenching, zeaxanthin formation and light scattering in intact leaves of triazine-resistant and triazine-susceptible Chenopodium album plants

The triazine-resistant biotype of Chenopodium album has an impaired activity of photosystem II. In vivo, this leads to a higher sensitivity to photoinhibition, especially when the plants have been grown at a high irradiance. The activities of several protecting energy dissipation pathways were compared in triazine-resistant and triazine-susceptible biotypes which were grown at low and high irradiances. It was found that the resistant plants have more light-induced zeaxanthin formation and a larger change in light scattering than does the susceptible biotype. The differences were more pronounced when the plants were grown at a high irradiance. Stern-Volmer plots indicated that high light-grown plants show a linear relationship between non-photochemical quenching q N and zeaxanthin content, and also between q N and change in light scattering. In low light-grown plants, the zeaxanthin content and the change in light scattering are not linearly related to q N and there is a strong increase in the value of q N already at low levels of zeaxanthin content and change in light scattering. The latter effect is smaller in the resistant biotype. Photorespiration is found to be more important as an energy dissipation pathway in resistant plants compared with susceptible plants. It is concluded that the increased sensitivity to photoinhibition of resistant plants is not caused by a lower activity of the photoprotective pathways, including qN, zeaxanthin formation and photorespiration. Instead, it is suggested that the shade-type characteristics of the chloroplasts of the resistant plants are responsible for the greater sensitivity to photoinhibition.

Flexibility of Crassulacean Acid Metabolism in Kalanchoe pinnata (Lam.) Pers. I. Response to Irradiance and Supply of Nitrogen and Water

Summary Plants of Kalanchoe pinnata (Lam.) Pers. were grown in the glass chamber of a phytotron in high-light (16-23 mol photons m -2 day -1 ) and in the shade (0.8-2.1 mol photons m -2 day -1 ) respectively, without or with nitrogen nutrition (12 mol m -3 NO 3 - in irrigation solution) and well watered or droughted for 18-22 days (high-light) and 26 to 32 days (shade) respectively, prior to measurements and analyses. Part of the high-light plants were transferred to the shade and part of the shade plants to high-light before measurements. Gas-exchange (H 2 O-vapour, CO 2 ) was measured and contents of total leaf nitrogen, chlorophyll a and b and carotenoids were analysed. Both high-light and low-light grown plants showed typical CAM gas exchange. When high-light plants were transferred to the shade under all conditions diurnal net CO 2 -balance was negative. Shade plants transferred to high-light greatly increased their daily net CO 2 -uptake, particularly by extended CO 2 -fixation at high rates in the light period, if supplied with H 2 O and N. Thus, shade plants were very successful in high-light provided N and H 2 O were not limiting. Shade adaptation of shade-plants as compared to high-light plants was indicated by higher absolute chlorophyll and carotenoid contents and lower chlorophyll a/b as well as lower CO 2 -uptake to chlorophyll (a + b) ratios. The results show, that the factors irradiance, nitrogen-nutrition and water-availability interact in eliciting flexible responses of the photosynthetic mechanism in the constitutive CAM-plant K. pinnata . Highlight plants were not shade tolerant; low-light plants were shade tolerant but not shade demanding.

Microstegium vimineum, a shade adapted C 4 grass

Microstegium vimineum, a C 4 plant of the family Poaceae, is shown to be extremely adaptable to growing under shade conditions. The capacity for dry matter production of the species was similar from 18% to 100% full sunlight. Even at 5% full sunlight substantial growth occurred (17% of that at full sunlight). In comparison, growth of the C 4 species Digitaria sanguinalis and Sporobolus airoides declined rapidly below full sunlight with no growth occurring at 5% full sunlight. A number of measurements on leaves of M. vimineum grown under 5% of full sunlight versus full sunlight reflects the ability of this species to adapt to low light. Under low light there was a 2-fold increase in leaf area per leaf with an approx. 2-fold decrease in leaf thickness. The soluble protein/ chlorophyll ratio increased about 3.5-fold in the low light-grown plants due to an increase in chlorophyll/leaf area and a decrease in soluble protein/leaf area. Under both light regimes the plants had Kranz anatomy and high levels of phosphoenolpyruvate (PEP) carboxylase which are known features of C 4 plants. The results indicate that there are no inherent limitations in the C 4 mechanism or in the efficiency of energy utilization for carbon assimilation which prevents C 4 plants from adapting to low light conditions.