Desensitization and recovery of phototropic responsiveness in Arabidopsis thaliana.

Phototropism is induced by blue light, which also induces desensitization, a partial or total loss of phototropic responsiveness. The fluence and fluence-rate dependence of desensitization and recovery from desensitization have been measured for etiolated and red light (669-nm) preirradiated Arabidopsis thaliana seedlings. The extent of desensitization increased as the fluence of the desensitizing 450-nm light was increased from 0.3 to 60 micromoles m-2 s-1. At equal fluences, blue light caused more desensitization when given at a fluence rate of 1.0 micromole m-2 s-1 than at 0.3 micromole m-2 s-1. In addition, seedlings irradiated with blue light at the higher fluence rate required a longer recovery time than seedlings irradiated at the lower fluence rate. A red light preirradiation, probably mediated via phytochrome, decreased the time required for recovery from desensitization. The minimum time for detectable recovery was about 65 s, and the maximum time observed was about 10 min. It is proposed that the descending arm of the fluence-response relationship for first positive phototropism is a consequence of desensitization, and that the time threshold for second positive phototropism establishes a period during which recovery from desensitization occurs.     

滇青冈幼苗的光合和生长对不同生长光强的适应

研究了3种光强(全日照的80%、20%和2%)下生长的滇青冈幼苗的光合-光强响应和生长状况。结果表明:3种光强生境滇青冈幼苗光饱和点和最大净光合速率没有显著差异,20%和2%生长光强下光补偿点显著低于80%生长光强。3种光强生境幼苗的光合饱和点在400~450μmol·m-2·s-1左右,光补偿点也很低(6~10μmol·m-2·s-1),表现出阴生植物的特征。但滇青冈幼苗较强的耐荫能力,是以牺牲生长为代价的。在2%的弱光生境中滇青冈幼苗比叶重显著降低,单位面积和单位干重叶绿素含量均显著增多,具有利用较强辐射的潜力,一旦林窗出现就可以利用增加的辐射提高光合速率,及时进入快速高生长。     

Light-stimulated cell expansion in bean (Phaseolus vulgaris L.) leaves

The quantity and quality of light required for light-stimulated cell expansion in leaves of Phaseolus vulgaris L. have been determined. Seedlings were grown in dim red light (RL; 4 micromoles photons m-2 s-1) until cell division in the primary leaves was completed, then excised discs were incubated in 10 mM sucrose plus 10 mM KCl in a variety of light treatments. The growth response of discs exposed to continuous white light (WL) for 16 h was saturated at 100 micromoles m-2 s-1, and did not show reciprocity. Extensive, but not continuous, illumination was needed for maximal growth. The wavelength dependence of disc expansion was determined from fluence-response curves obtained from 380 to 730 nm provided by the Okazaki Large Spectrograph. Blue (BL; 460 nm) and red light (RL; 660 nm) were most effective in promoting leaf cell growth, both in photosynthetically active and inhibited leaf discs. Far-red light (FR; 730 nm) reduced the effectiveness of RL, but not BL, indicating that phytochrome and a separate blue-light receptor mediate expansion of leaf cells.     

Photosystem II Excitation Pressure and Development of Resistance to Photoinhibition (I. Light-Harvesting Complex II Abundance and Zeaxanthin Content in Chlorella vulgaris)

The basis of the increased resistance to photoinhibition upon growth at low temperature was investigated. Photosystem II (PSII) excitation pressure was estimated in vivo as 1 - qp (photochemical quenching). We established that Chlorella vulgaris exposed to either 5[deg]C/150 [mu]mol m-2 s-1 or 27[deg]C/2200 [mu]mol m-2 s-1 experienced a high PSII excitation pressure of 0.70 to 0.75. In contrast, Chlorella exposed to either 27[deg]C/150 [mu]mol m-2 s-1 or 5[deg]C/20 [mu]mol m-2 s-1 experienced a low PSII excitation pressure of 0.10 to 0.20. Chlorella grown under either regime at high PSII excitation pressure exhibited: (a) 3-fold higher light-saturated rates of O2 evolution; (b) the complete conversion of PSII[alpha] centers to PSII[beta] centers; (c) a 3-fold lower epoxidation state of the xanthophyll cycle intermediates; (d) a 2.4-fold higher ratio of chlorophyll a/b; and (e) a lower abundance of light-harvesting polypeptides than Chlorella grown at either regime at low PSII excitation pressure. In addition, cells grown at 5[deg]C/150 [mu]mol m-2 s-1 exhibited resistance to photoinhibition comparable to that of cells grown at 27[deg]C/2200 [mu]mol m-2 s-1 and were 3- to 4-fold more resistant to photoinhibition than cells grown at either regime at low excitation pressure. We conclude that increased resistance to photoinhibition upon growth at low temperature reflects photosynthetic adjustment to high excitation pressure, which results in an increased capacity for nonradiative dissipation of excess light through zeaxanthin coupled with a lower probability of light absorption due to reduced chlorophyll per cell and decreased abundance of light-harvesting polypeptides.     

The effect of simulated microgravity on formation of the pigment apparatus in etiolated barley seedlings

The effect of horizontal clinorotation on the dynamics of the accumulation of the main photosynthetic pigments in the greening of 6-day-old etiolated barley seedlings has been studied. The content of protochlorophillide, the direct precursor of chlorophyll a, in clinorotated seedlings in the dark was 9–20% lower than in the control group. After exposure of barley seedlings to light for 12 h under clinorotation, chlorophyll accumulation lagged behind the control by 45% and reached the control value after 48–72 h. The total content of carotenoids increased many fold during greening; at the first stage the carotenoid level in clinorotated seedlings was less than in the control. The synthesis rates of δ-aminolevulinic acid and δ-aminolevulinate dehydratase activity in clinorotated seedlings were slower than in the control after 24 h of greening and after 72 h of greening reaching the control values. The activity of Mg-protoporphyrin IX chelatase catalyzing the incorporation of Mg ions in the structure of chlorophyll a, did not change when exposed to clinorotation. The results we obtained show inhibition of the initial stages of chlorophyll biosynthesis in the conditions of simulated microgravity. The light, to a certain extent, decreases the negative effect of microgravity on the formation of the photosynthetic apparatus in plants.     

Photosynthetic capacity and dry mass partitioning in dwarf and semi-dwarf wheat (Triticum aestivum L.).

Efficient use of space and high yields are critical for long-term food production aboard the International Space Station. The selection of a full dwarf wheat (less than 30 cm tall) with high photosynthetic and yield potential is a necessary prerequisite for growing wheat in the controlled, volume-limited environments available aboard long-term spaceflight missions. This study evaluated the photosynthetic capacity and carbon partitioning of a full-dwarf wheat cultivar, Super Dwarf, which is routinely used in spaceflight studies aboard U.S. space shuttle and NASA/Mir missions and made comparisons with other dwarf and semi-dwarf wheat cultivars utilized in other ground-based studies in plant space biology. Photosynthetic capacity of the flag leaf in two dwarf (Super Dwarf, BB-19), and three semi-dwarf (Veery-10, Yecora Rojo, IBWSN 199) wheat cultivars (Triticum aestivum L.) was assessed by measuring: net maximum photosynthetic rate, RuBP carboxylation efficiency, chlorophyll concentration and flag leaf area. Dry mass partitioning of carbohydrates to the leaves, sheaths, stems and ear was also assessed. Plants were grown under controlled environmental conditions in three replicate studies: slightly enriched CO2 (370 micromoles mol-1), high photosynthetic photon flux (1000 micromoles m-2 s-1; 58 mol m-2 d-1) for a 16 h photoperiod, 22/15 degrees C day/night temperatures, ample nutrients and water provided by one-half strength Hoagland's nutrient solution (Hoagland and Arnon, 1950). Photosynthetic capacity of the flag leaf was determined at anthesis using net CO2 exchange rate versus internal CO2 concentration curves measured under saturating light (2000 micromoles m-2 s-1) and CO2 (1000 micromoles mol-1). Dwarf wheat cultivars had greater photosynthetic capacities than the taller semi-dwarfs, they averaged 20% higher maximum net photosynthetic rates compared to the taller semi-dwarfs, but these higher rates occurred only at anthesis, had slightly greater carboxylation efficiencies and significantly increased chlorophyll concentrations per unit leaf area. The reduced-height wheat had significantly less dry mass fraction in the stem but greater dry mass partitioned to the ear than the taller semi-dwarfs (Yecora rojo, IBWSN-199). Studies with detached heads confirm that the head is a significant sink in the shorter wheat cultivars.     

Photoregulation of the Greening Process of Wheat Seedlings Grown in Red Light*

Wheat seedling grown with their shoot bottom exposed to red light (400 μmol m⁻² s⁻¹) either with constant illumination or light-dark cycles did not accumulate chlorophyll. This near-etiolation response was manifested by a critical threshold intensity of red light and did not need continuous illumination. The inhibition of the greening process resulted from reduced synthesis of glutamate-1-semialdehyde and consequent reduction in tetrapyrrole precursor 5-aminolevulinic acid. Red light perceived by the shoot bottom down regulated the protein and/or gene expression of enzymes involved in the biosynthesis of tetrapyrroles. The contents of endogenous cytokinins, i.e., isopentenyl-adenosine and dihydrozeatinriboside, were reduced in seedlings grown in red light having their shoot bottom exposed. Application of exogenous cytokinin and its analogue to roots of seedlings grown in red light reversed the down regulation of the greening process. The reversal of red-light-induced near-etiolation morphogenesis by far-red (200 μmol m⁻² s⁻¹) or blue (25 μmol m⁻² s⁻¹) light suggests that it could be a very high red-irradiance response of phytochrome, in the meristematic layers of the shoot bottom, that works in concert with blue light receptor(s). This work was supported by a competitive grant from the Department of Science and Technology, Govt. of India (DST/SP/SO/A-49/95) to BCT. Suchi Sood Varsha Gupta: Equal contributors     

光照强度对小麦苗期草酸氧化酶活性的影响

以小麦品种‘烟优361’(Triticum aestivum L.cv.Yanyou 361)萌发4 d幼苗为试验材料,分析了草酸氧化酶(OxO)在幼苗中的定位和表达,以及光照强度处理对小麦幼苗OxO活性的影响。实验结果显示,萌发后小麦幼苗的OxO分布在子叶与根的连接处和成熟的根中,其活性随光照强度的增加而下降;200μmol.m-2.s-1的强光显著抑制了OxO活性,该处理培养4 d幼苗的OxO活性仅为40μmol.m-2.s-1光照培养条件下的18.7%;强光还缩短OxO在苗期的表达时间,抑制了OxO的mRNA表达量。同时,光照强度还能影响小麦幼苗中H2O2的含量,200μmol.m-2.s-1处理幼苗的H2O2的含量显著下降,其培养4 d的幼苗H2O2含量仅为40μmol.m-2.s-1光照强度培养条件下的18.0%。研究发现,光照强度可通过调节OxO的活性和表达量来控制H2O2的产量,从而影响幼苗的生长发育。     

亚热带季风阔叶林不同林地几种植物光合作用的比较研究

比较了亚热带季风阔叶林不同林地几种植物的叶特性、叶含氮量、叶绿素含量、光合速率和气孔对CO_2的传导率,表明疏林的桃金娘(Rhodomyrtus tomentoso)和荷树(Schima superba)的叶厚和栅栏组织厚度较九节(Psychotria rubra)小,但单位面积干重较九节大,叶含氮量(N mg/g干重)较密林的罗伞(Ardisia quinquegona)低,疏林的桃金娘和荷树的光合作用饱和光强为600和550 μmol 光量子m~(-2),S~(-1),光补偿点分别为12和20 μmol,光量子m~(-2),s~(-1),两种植物的平均最大光合速率分别为11.9±0.4和7.5±1.8 μol CO_2,m~(-2),S~(-1),光合量子产率分别为0.056和0.048,而密林的罗伞光合速率变化对光强变化反应敏感,暗呼吸速率较低。光补偿点为5μmol,光量子m~(-2),S~(-1),显著低于疏林植物,疏林植物有着较高的Ci/Ca值和水分利用效率。亚热带季风阔叶林的不同林地的植物有着不同的光合作用特性,这可能与植物生境的光状况有关。     

不同光照强度和温度对金钗石斛生长的影响

为了系统地研究不同光照强度下温度对金钗石斛(Dendrobium nobile)生长的影响,在金钗石斛分蘖期,于80μmol·m-2·s-1、160μmol·m-2·s-1、320μmol·m-2·s-1、640μmol·m-2·s-1的不同光强下,各设置5个温度(15℃、20℃、25℃、30℃、35℃)梯度对石斛进行处理。结果表明：石斛的生长与代谢随温度由低到高,表现出弱—强—弱的变化规律;80μmol·m-2·S-1光强下,石斛生长以25～30℃较为适宜;160μmol·m-2·s-1光强下则以20～25℃为适宜温度范围;320μmol·m-2·s-1与640μmol·m-2·s-1的中、强光照下,25℃处理石斛的生长优势尤为明显;不同光强下,石斛鲜重的增长大多以25℃处理更快,繁殖力则以20℃与25℃处理较高,各光强下的MDA含量随温度升高而先降后升,且均以25℃最低;可溶性蛋白质、可溶性总糖及叶绿素含量则表现出随温度由低到高而先增后减的趋势,其含量最高点均出现在25℃左右;净光合速率和叶绿素含量随光强和温度的变化趋势基本一致;各种光强下的暗呼吸速率均随温度升高而增大。因此,在不同的光照条件下,石斛生长的适宜温度均在25℃左右。光温处理引起石斛生理生化过程明显的相应变化表现出：高温和弱光照条件有利于石斛的株高增长,但不利于产量和质量提高;石斛的生长与MDA含量呈显著负相关(r80＝-0.9082、r160＝-0.9816、r320＝-0.8075、r640＝-0.8586),与可溶性糖含量呈一定正相关(r80＝0.7673、r160＝0.8892、r320＝0.8179、r640＝0.9278),并且石斛的生长与可溶性蛋白质含量、叶绿素含量、光合速率之间的变化趋势基本一致。     

Photoactivation of Chlorophyll Synthesis and Cytochrome Oxidase Activity in Anaerobically Germinated Seedlings of Echinochloa crusgalli var. Oryzicola

When seeds of Echinochloa crusgalli var. oryzicola are germinated in dark anaerobic conditions (99.995% N₂), the seedlings do not have detectable protochlorophyll(ide). Two hours after exposure to light aerobic conditions, they begin to synthesize chlorophyll. The lag in greening is shorter in seedlings exposed to light for 24 hours before exposure to air. Seedlings maintained in light anaerobic conditions exhibit no lag in greening upon transfer to an aerobic environment. Preillumination of anaerobically grown seedlings does not result in any chlorophyll accumulation. Phytochrome is probably the receptor for photoactivation of chlorophyll synthesis, since activation is achieved by red light alone, but not by far red light or red plus far red light. The cytochrome oxidase activity in anaerobically germinated seedlings is 30% of the normal level found in aerobically grown seedlings. Preillumination was also found to activate the ability of anaerobically germinated seedlings to increase their cytochrome oxidase activity upon exposure to air.     

Effets de la fumitoxine A sur le tissu foliaire de Pisum sativum L.

Etiolated pea seedlings are treated with fumitoxin A, a mycotoxin produced by Aspergillus fumigatus Fres. The effects observed on the greening process relate to the production of chlorophyll and fatty acid biosynthesis. The effects vary in relation to the dose of toxin used. With high doses, fumitoxin A gives rise to a diminution of chlorophyll level and an inhibition of fatty acid biosynthesis. Lower doses cause a significant augmentation of chlorophyll level and at the same time, an acceleration of polyunsaturated fatty acid biosynthesis.     

矮嵩草光合作用与环境因素关系的比较研究

 以青海高原不同海拔地区生长的矮嵩草(Kobresia humilis)为材料,研究高山植物光合作用随海拔梯度的变化特征及对生长环境和低温胁迫的反应。随海拔升高矮嵩草叶绿素含量有降低的趋势,而叶绿素a／b值和类胡萝卜素含量则随海拔升高而增高。生长地区海拔越高矮嵩草光合速率、光补偿点、光饱和点越高;而光合表观量子产额则随海拔升高而降低。光呼吸强度有随海拔升高而降低的趋势。矮嵩草光合作用特性受生长环境因素的影响。低温胁迫导致矮嵩草光合速率、表观量子产额降低,低温下的光照加剧了光合作用抑制的程度。     

The Effect of Elevated [CO2] on Growth and Photosynthesis of Two Eucalyptus Species Exposed to High Temperatures and Water Deficits

Two species of eucalyptus (Eucalyptus macrorhyncha and Eucalyptus rossii) were grown for 8 weeks in either ambient (350 [mu]L L-1) or elevated (700 [mu]L L-1) CO2 concentrations, either well watered or without water additions, and subjected to a daily, 3-h high-temperature (45[deg]C, maximum) and high-light (1250 [mu]mol photons m-2 s-1, maximum) stress period. Water-stressed seedlings of E. macrorhyncha had higher leaf water potentials when grown in elevated [CO2]. Growth analysis indicated that increased [CO2] may allow eucalyptus species to perform better during conditions of low soil moisture. A down-regulation of photosynthetic capacity was observed for seedlings grown in elevated [CO2] when well watered but not when water stressed. Well-watered seedlings grown in elevated [CO2] had lower quantum efficiencies as measured by chlorophyll fluorescence (the ratio of variable to maximal chlorophyll fluorescence [Fv/Fm]) than seedlings grown in ambient [CO2] during the high-temperature stress period. However, no significant differences in Fv/Fm were observed between CO2 treatments when water was withheld. The reductions in dark-adapted Fv/Fm for plants grown in elevated [CO2] were not well correlated with increased xanthophyll cycle photoprotection. However, reductions in the Fv/Fm were correlated with increased levels of nonstructural carbohydrates. The reduction in quantum efficiencies for plants grown in elevated [CO2] is discussed in the context of feedback inhibition of electron transport associated with starch accumulation and variation in sink strength.     

苋菜的光合特性

宽菜Amaranthus cruentus cv.生长在调控的温室条件。在光强0至800μmol.m~(-2)S~(-1),光合速率(PN,μmol.CO_2m~(-2)、s~(-1))随光强(PFD,μmol、m~(-2)、s~(-1))增高而增大,其关系为PN=56.82 PFD×10~(-3)—2.13。光补偿点为60μmol.m~(-2)、s~(-1)。叶片在1400 μmol.m~(-2)、s~(-1)达到光合光饱和点。在叶温35℃,叶片/空气水蒸汽压陡度20 m Pa、Pa~(-1)和外界CO_2浓度340μ1、1~(-1),光饱和光合速率为51.63±4.90μ mol.CO_2、m~(-2)、S~(-1)。在光强0至600μmol.m~(-2)、s~(-1),气孔传道率随光强增高而增大。光强高于600μmol.m~(-2)、s~(-1),气孔传道率变化较小。细胞间CO_2浓度为120μ1.1~(-1)由于细胞间CO_2浓度在光合速率——CO_2关系曲线的转折点,可能表明光合作用不受气孔限制。结果表明,苋菜适于高光强环境生长,在干旱条件下具有高的光合速率。     

Water stress,CO2 and photoperiod influence hormone levels in wheat

'Super Dwarf' wheat (Triticum aestivum L.) plants have been grown from seed to maturity in the Mir space station where they were periodically exposed, because of microgravity and other constraints, to water deficit, waterlogging, high CO2 levels, and low light intensities. The plants produced many tillers, but none of them produced viable seed. Studies have been initiated to determine why the plants responded in these ways. In the present study, effects of the listed stresses on abscisic acid (ABA), indole-3-acetic acid (IAA) and isopentenyl adenosine ([9R]iP) levels in roots and leaves of plants grown under otherwise near optimal conditions on earth were measured. Hormones were extracted, purified by HPLC, and quantified by noncompetitive indirect ELISA. In response to water deficit, ABA levels increased in roots and leaves, IAA levels decreased in roots and leaves, and [9R]iP levels increased in leaves but decreased in roots. In response to waterlogging, ABA, IAA and [9R]iP levels briefly increased in roots and leaves and then decreased. When portions of the root system were exposed to waterlogging and/or water deficit, ABA levels in leaves increased while [9R]iP and IAA levels decreased. These responses were correlated with the percentage of the root system stressed. At a low photosynthetic photon flux (100 micromoles m-2 s-1), plants grown in continuous light had higher leaf ABA levels than plants grown using an 18 or 21 h photoperiod.     

Regulation of synthesis of the photosystem I reaction center

The in vivo biosynthesis of the P700 chlorophyll a-apoprotein was examined to determine whether this process is light regulated and to determine its relationship to chlorophyll accumulation during light- induced chloroplast development in barley (Hordeum vulgare L.). Rabbit antibodies to the 58,000-62,000-mol-wt apoprotein were used to measure relative synthesis rates by immunoprecipitation of in vivo labeled leaf proteins and to detect apoprotein accumulation on nitrocellulose protein blots. 5-d-old, dark-grown barley seedlings did not contain, or show net synthesis of, the 58,000-62,000-mol-wt polypeptide. When dark- grown barley seedlings were illuminated, net synthesis of the apoprotein was observed within the first 15 min of illumination and accumulated apoprotein was measurable after 1 h. After 4 h, P700 chlorophyll a-apoprotein biosynthesis accounted for up to 10% of the total cellular membrane protein synthesis. Changes in the rate of synthesis during chloroplast development suggest coordination between production of the 58,000-62,000-mol-wt polypeptide and the accumulation of chlorophyll. However, when plants were returned to darkness after a period of illumination (4 h) P700 chlorophyll a-apoprotein synthesis continued for a period of hours though at a reduced rate. Thus we found that neither illumination nor the rate of chlorophyll synthesis directly control the rate of apoprotein synthesis. The rapidity of the light-induced change in net synthesis of the apoprotein indicates that this response is tightly coupled to the primary events of light-induced chloroplast development. The data also demonstrate that de novo synthesis of the apoprotein is required for the onset of photosystem I activity in greening seedlings.     

Effects of CO2 and photosynthetic photon flux on yield, gas exchange and growth rate of Lactuca sativa L. 'Waldmann's Green.'

Enrichment of CO2 to 46 mmol m-3 (1000 mm3 dm-3) at a moderate photosynthetic photon flux (PPF) of 450 micromoles m-2 s-1 stimulated fresh and dry weight gain of lettuce leaves 39% to 75% relative to plants at 16 mmol m-3 CO2 (350 mm3 dm-3). Relative growth rate (RGR) was stimulated only during the first several days of exponential growth. Elevating CO2 above 46 mmol m-3 at moderate PPF had no further benefit. However, high PPF of 880-900 micromoles m-2 s-1 gave further, substantial increases in growth, RGR, net assimilation rate (NAR) and photosynthetic rate (Pn), but a decrease in leaf area ratio (LAR), at 46 or 69 mmol m-3 (1000 or 1500 mm3 dm-3) CO2, the differences being greater at the higher CO2 level. Enrichment of CO2 to a supraoptimal level of 92 mmol m-3 (2000 mm3 dm-3) at high PPF increased leaf area and LAR, decreased specific leaf weight, NAR and Pn and had no effect on leaf, stem and root dry weight or RGR relative to plants grown at 69 mmol m-3 CO2 after 8 d of treatment. The results of the study indicate that leaf lettuce growth is most responsive to a combination of high PPF and CO2 enrichment to 69 mmol m-3 for several days at the onset of exponential growth, after which optimizing resources might be conserved.     

The plasma membrane-associated NADH oxidase of spinach leaves responds to blue light

The plasma membrane-associated NADH oxidase (NOX) of spinach leaf disks is characterized by oscillations in activity with a regular period length of ca. 24 min. Within a single population of plants exposed to light at the same time, NOX activities of all plants function synchronously. Exposure of plants transferred from darkness to blue light (495 nm, 2 min, 50 micromoles m-2 s-1) resulted in a complex response pattern but with a new maximum in the rate of NOX activity 36 (24+12) min after illumination and then with maxima in the rate of NOX activity every 24 min thereafter. Transient maxima in NOX activity were observed as well after 9.3 + /- 1.4 and 20.7 +/- 2.1 min. The blue light response differed from the response to red (650 nm, 10 min, 50 micromoles m-2 s-1) or white light where activity maxima were initiated 12 min after the light exposure followed by maxima every 24 min thereafter. Green or yellow light was ineffective. The light response was independent of the time in the 24-min NOX cycle when the light was given. The net effects of blue and red light were ultimately the same with a new maximum in the rate of NOX activity at 12+24=36 min (and every 24 min thereafter), but the mechanisms appear to be distinct.