环境强光诱导玉簪叶片光抑制的机制

为进一步阐述光抑制的强光诱导和发生机制, 该文以喜阴植物玉簪(Hosta spp.)为材料研究其光抑制发生规律及其与环境光强的关系。结果表明, 全日照和遮阴条件下玉簪叶片发育分别形成适应强光和弱光的形态特征; 与遮阴处理相比, 强光下生长的玉簪光合速率和叶绿素含量较低, 但两种处理叶片最大光化学效率差异很小, 证明强光下植株可以正常生长且光合机构未发生严重的光抑制。将遮阴处生长的植株转移到全日照下, 光合速率和最大光化学效率急剧下降; 荧光诱导动力学曲线发生明显改变, 而且光系统II供体侧和受体侧荧光产量的变化幅度分别达到24.3%和34.2%, 表明玉簪由弱光转入强光后光系统II发生不可逆失活, 且受体侧受到的伤害较供体侧更严重。因此, 作者认为环境光强骤然提高并超过玉簪生长光强时很容易诱导其光合机构发生严重的光抑制。该研究对于理解植物适应光环境的策略以及喜阴植物的优质栽培有重要意义。     

Induction characteristics and response of photosynthetic quantum conversion to changes in irradiance in mulberry plants

A study was conducted, using rapid time course of chlorophyll (Chl) fluorescence parameters, and light-response curves of Chl fluorescence parameters, to determine the induction requirements and response of photosystem II (PSII) photochemistry and non-photochemical reactions after changes in irradiance in greenhouse mulberry plants. The induction of PSII photochemistry rapidly approached to steady state after leaves were treated from darkness to low irradiance (LI). When irradiance of leaves changed from darkness to high irradiance (HI), a biphasic induction was observed. A slight photoinhibition occurred in the leaves exposed to sunlight coming to the greenhouse, whereas a chronic photoinhibition occurred in the leaves fully exposed to sunlight outside the greenhouse. The chronic photoinhibition was demonstrated by sustained reduction of maximal quantum yield of PSII photochemistry (Fv/Fm). Moreover, the leaves of mulberry plants in greenhouse were sensitive to abrupt changes in irradiance and the sensitivity of leaves suffered in a short-term (1h) high light treatment was reduced, based on the changes in photosynthetic quantum conversion. These results demonstrated an inducible response of photosynthetic quantum conversion to changes in irradiance in mulberry.     

The irreversible photoinhibition of the photosystem II complex in leaves of Vicia faba under strong light

Exposure of the photosynthetic machinery to strong light causes the photoinhibition of the photosystem II complex. The recovery from the photoinhibition in vivo was characterized by monitoring the ratio of variable to maximum fluorescence (F_v/F_m) in detached leaves of broad bean (Vicia faba). The changes in the ratio were explained in terms of three components, namely, two saturating exponential components with half rise-times of about 15 and 120 min, respectively, and a non-recovery component. The non-recovery component increased gradually as the exposure to strong light was prolonged. Our results suggest that this irreversible component of the photoinhibition of the photosystem II complex was caused by severe stress due to strong light under which repair of the photosystem II complex was insufficient to allow full recovery. The irreversible photoinhibition is discussed in terms of both the physiology and ecology of plants.     

Inhibition of photosynthetic reactions under water stress: interaction with light level

When the shrub Nerium oleander L., growing under full natural daylight outdoors, was subjected to water stress, stomatal conductance declined, and so did non-stomatal components of photosynthesis, including the CO₂-saturated rate of CO₂ uptake by intact leaves and the activity of electron transport by chloroplasts isolated from stressed plants. This inactivation of photosynthetic activity was accompanied by changes in the fluorescence characteristics determined at 77 K (-196°C) for the upper leaf surface and from isolated chloroplasts. The maximum (F _M) and the variable (F _V) fluorescence yield at 692 nm were strongly quenched but there was little effect on the instantaneous (F _O) fluorescence. There was a concomitant quenching of the maximum and variable fluorescence at 734 nm. These results indicate an inactivation of the primary photochemistry associated with photosystem II. The lower, naturally shaded surfaces of the same leaves were much less affected than the upper surfaces and water-stress treatment of plants kept in deep shade had little or no effect on the fluorescence characteristics of either surface, or of chloroplasts isolated from the water-stressed leaves. The effects of subjecting N. oleander plants, growing in full daylight, to water stress are indistinguishable from those resulting when plants, grown under a lower light regime, are exposed to full daylight (photoinhibition). Both kinds of stress evidently cause an inactivation of the primary photochemistry associated with photosystem II. The results indicate that water stress predisposes the leaves to photoinhibition. Recovery from this inhibition, following restoration of favorable water relations, is very slow, indicating that photoinhibition is an important component of the damage to the photosynthetic system that takes place when plants are exposed to water stress in the field. The underlying causes of this water-stress-induced susceptibility to photoinhibition are unknown; stomatal closure or elevated leaf temperature cannot explain the increased susceptibility.Abbreviations and symbols Chl chlorophyll - PFD photon flux area density - PSI, PSII photosystem I, II - F _M, F _O, F _V maximum, instantaneous, variable fluorescence emission - leaf water potential C.I.W.-D.P.B. Publication No. 775     

Photosynthesis, photoprotection, and growth of shade-tolerant tropical tree seedlings under full sunlight

High solar radiation in the tropics is known to cause transient reduction in photosystem II (PSII) efficiency and CO(2) assimilation in sun-exposed leaves, but little is known how these responses affect the actual growth performance of tropical plants. The present study addresses this question. Seedlings of five woody neotropical forest species were cultivated under full sunlight and shaded conditions. In full sunlight, strong photoinhibition of PSII at midday was documented for the late-successional tree species Ormosia macrocalyx and Tetragastris panamensis and the understory/forest gap species, Piper reticulatum. In leaves of O. macrocalyx, PSII inhibition was accompanied by substantial midday depression of net CO(2) assimilation. Leaves of all species had increased pools of violaxanthin-cycle pigments. Other features of photoacclimation, such as increased Chl a/b ratio and contents of lutein, β-carotene and tocopherol varied. High light caused strong increase of tocopherol in leaves of T. panamensis and another late-successional species, Virola surinamensis. O. macrocalyx had low contents of tocopherol and UV-absorbing substances. Under full sunlight, biomass accumulation was not reduced in seedlings of T. panamensis, P. reticulatum, and V. surinamensis, but O. macrocalyx exhibited substantial growth inhibition. In the highly shade-tolerant understory species Psychotria marginata, full sunlight caused strongly reduced growth of most individuals. However, some plants showed relatively high growth rates under full sun approaching those of seedlings at 40?% ambient irradiance. It is concluded that shade-tolerant tropical tree seedlings can achieve efficient photoacclimation and high growth rates in full sunlight.     

Photoinhibition in tropical forest understorey species with short- and long-lived leaves

1. Shade-tolerant species that inhabit the understorey have a range of leaf lifetimes (from 1 to 8 years), which may indicate a variety of strategies for dealing with increases in light associated with tree-fall gaps. We hypothesized that species with long-lived leaves should be more tolerant of an increase in light levels than species with short-lived leaves.
2. In understorey plants of 12 shade-tolerant rain-forest species, photoinhibition, measured as a reduction in the chlorophyll fluorescence parameter F _v/ F _m when leaf discs were exposed to 1h at 1000μmol m^–2s^–1, was greater in species with short-lived leaves than species with long-lived leaves.
3. Less photoinhibition in species with long-lived leaves was not associated with higher levels of non-photochemical dissipation (NPQ) of absorbed light, but may be the result of a higher yield of photosystem II compared with short-lived leaves.
4. Thus, species with long-lived leaves are more tolerant of abrupt increases in light that occur when tree-fall gaps are formed than species with short-lived leaves.
5. Discs from leaves of all species growing in tree-fall gaps had higher levels of NPQ, yield of photosystem II and more rapid recovery from photoinhibition than leaves developed in the understorey; however, there were no differences among species with short- and long-lived leaves.     

叶片发育影响紫罗勒花青素的强光诱导和激发能分配

通过气体交换、叶绿素荧光、反射光谱和显微技术等研究了叶片发育与花青素强光诱导的关系及其对激发能分配的影响。结果表明,遮荫导致紫罗勒叶片变薄,花青素含量显著降低。当弱光下生长的植株转入强光后,转光前发育成熟的叶片花青素含量很低,而此后强光下发育成熟的叶片花青素含量高。转强光后,弱光下发育成熟的叶片光合速率低、光抑制严重,且天线耗散增强;强光下发育成熟的叶片净光合速率高,光抑制程度轻,天线耗散低。因此,我们认为叶片发育影响紫罗勒花青素合成的强光诱导,而转强光后花青素的诱导差异进一步改变了光合作用过程中的激发能分配。     

Photosynthetic acclimation of Tradescantia albiflora to growth irradiance: morphological, ultrastructural and growth responses

Tradescantia albiflora (Kunth), a trailing ground species naturally occurring in deep shade in rainforests, has an unusual photosynthetic acclimation profile for growth irradiance. Although capable of increasing its capacity for electron transport, photophosphorylation and carbon fixation when grown in full sunlight, Tradescantia has constant chlorophyll alb ratios, photosystem reaction centre stoichiometry and pigment-protein composition at all growth irradiances (Chow et al. 1991. Physiol. Plant. 81: 175–182). To gain an insight into the compensatory strategies which allow Tradescantia to grow in both high and low lights, plants were grown under shade cloth (100 to 1.4% relative growth irradiance) and leaf and chloroplast attributes were compared. While shade Tradescantia chloroplasts had three times more chlorophyll per chloroplast and twice the length of thylakoid membranes compared to plants grown in full sunlight, the ratios of appressed to nonappressed thylakoid membranes were constant. The average net surface charge density of destacked thylakoids was the same for plants grown at moderate and low-irradiance, consistent with their similar stacking profiles. Tradescantia plants grown in direct sunlight had 10-times more fresh and dry weight per plant compared to plants grown in shade, despite a lower photosynthetic capacity on a leaf area basis with partial photoinhibition. We conclude that having a light-harvesting apparatus permanently locked into the "shade-plant mode " does not necessarily prevent a plant from thriving in high light. Analyses of leaf growth at different irradiances provide a partial explanation of the manner in which Tradescantia compensates for very low photosynthetic capacity per unit leaf in sunlight.     

Physiological responses and photo-protective mechanisms of two Rhododendron plants to natural sunlight after long term shading

AimsResponses of plants to increased irradiance are governed by two strategies: an increase in the utilization of absorbed light and photo-protective mechanisms. Varied physiological responses to increased irradiance were observed in plant species with differing adaptabilities to light regimes. This research aims to explore the physiological responses and photo-protective mechanisms of two Rhododendron plants to changes in light regimes. MethodsChlorophyll fluorescence parameters and rapid light curves were measured for leaves of R. hybrida (a shade-tolerant species) and R. simsii (a light-loving and shade-tolerant species) following exposure to sunlight for five days after growing in the shade for one year.Important findings Natural sunlight decreased the efficiency of photochemical reaction by reducing the fraction of incident light in photochemical energy utilization and decreased thermal dissipation through regulating energy dissipation in photosystem II (PSII) in the leaves of R. hybrida. As a result, natural sunlight induced the accumulation of excess excitation energy in PSII, and caused photo-inhibition and even photodamage in the leaves of R. hybrida, which was suddenly transferred from long-term shading to sun exposures. The acclimation capacity to changes of growth light regimes was stronger in R. simsii than in R. hybrida, due to a higher capability for photochemical reaction, thermal dissipation and cyclic electron flows around photosystem I in the leaves of R. simsii. Rhododendron simsii could utilize a high fraction of incident light in photochemistry and regulate energy dissipation in PSII to protect the photosynthetic apparatus under both shading and natural sunlight condition. Therefore, high light intensity under natural sunlight did not cause photo-inhibition in R. simsii.     

两种杜鹃花属植物对长期遮阴后全光照环境的生理响应及其光保护机制

植物通过提高光能利用能力和光保护途径以响应环境光强的增加, 但不同植物对环境光强增加的生理响应存在差异, 从而导致植物对光环境的适应性不一致。为探讨植物对光环境变化的生理响应及其适应机制, 该文以遮阴条件下培养1年的2种杜鹃属(Rhododendron)植物比利时杜鹃(R. hybrida)和杜鹃(R. simsii)为材料, 对其由遮阴后转入全光照下培养5天时的叶片叶绿素荧光参数及其快速光曲线变化进行了比较研究, 以期从叶片吸收光能分配和光保护机制的角度探讨这2种植物对光环境变化的适应机制。结果表明: 全光照降低了喜阴植物比利时杜鹃叶片的光化学反应和热耗散能力, 且其吸收光能分配于光化学反应和调节性能量耗散部分的比例减少, 导致光系统II反应中心过量激发能积累, 造成了叶片光抑制甚至光破坏。杜鹃作为耐阴喜光植物对光环境变化具有较强的适应性, 具有较高的光化学反应、热耗散和环式电子传递能力等内在生理特性; 在遮阴和全光照两种光环境下均能维持较高的吸收光能在光化学反应和调节性能量耗散部分的分配比例, 从而保护了光合机构的正常运行, 是其全光照强光未造成叶片光抑制的原因。     

喜光榕树和耐荫榕树光适应机制的差异

100%和36%光强下生长的喜光的斜叶榕的光合能力高于耐荫的假斜叶榕,而热耗散能力与之相似,说明强光下斜叶榕主要通过光合作用利用光能和热耗散、假斜叶榕主要通过热耗散防御光破坏.100%光强下生长的两种榕树的日间光抑制程度相似,但叶表光强相同情况下各光强下生长的假斜叶榕的光抑制均比斜叶榕严重.100%光强下假斜叶榕叶片悬挂角大于斜叶榕,导致日间叶表光强低于斜叶榕,这可能是两种榕树日间光抑制程度相似的原因,表明叶片悬挂角的适应变化对假斜叶榕有重要的意义.     

Functional plasticity of photosynthetic apparatus and its resistance to photoinhibition in <Emphasis Type="Italic">Plantago media</Emphasis>

Morphological and functional characteristics of Plantago media L. leaves were compared for plants growing at different light regimes on limestone outcrops in Southern Timan (62°45′N, 55°49′E). The plants grown in open areas under exposure to full sunlight had small leaves with low pigment content and high specific leaf weight; these leaves exhibited high photosynthetic capacity and elevated water use efficiency at high irradiance. The maximum photochemical activity of photosystem II (F _v/F _m) in leaves of sun plants remained at the level of about 0.8 throughout the day. The photosynthetic apparatus of sun plants was resistant to excess photosynthetically active radiation, mostly due to non-photochemical quenching of chlorophyll fluorescence (qN). This quenching was promoted by elevated deepoxiation of violaxanthin cycle pigments. Accumulation of zeaxanthin, a photoprotective pigment in sun plant leaves was observed already in the morning hours. The plant leaves grown in the shade of dense herbage were significantly larger than the sun leaves, with pigment content 1.5–2.0 times greater than in sun leaves; these leaves had low qN values and did not need extensive deepoxidation of violaxanthin cycle pigments. The data reveal the morphophysiological plasticity of plantain plants in relation to lighting regime. Environmental conditions can facilitate the formation of the ecotype with photosynthetic apparatus resistant to photoinhibition. Owing to this adjustment, hoary plantain plants are capable of surviving in ecotopes with high insolation.     

Effects of shading on photosynthetic characteristics and chlorophyll fluorescence parameters in leaves of Hydrangea macrophylla

Aims The objectives were to investigate the effects of different light intensities on photosynthetic characteristics and chlorophyll fluorescence parameters, to clarify the physiological responses and photo-protective mechanisms of Hydrangea macrophylla to changes in light regimes in view of the distribution of energy absorbed and photosynthetic characteristics.Methods Three light regimes including natural and shade (shading rate 50% and 75% of natural light) were applied to plants for 60 days. After the treatment, the gas-exchange, chlorophyll a fluorescence and photosynthesis-light curves were measured by a portable leaf gas exchange system (LI-6400).Important findings The results showed that the weak light intensity treatment reduced dark respiration rate, light compensation point and light saturation point of plant, but increased apparent quantum yield, suggesting that plants had the physiological strategy to utilize the weakening light by reducing respiration. The net photosynthetic rate, intercellular CO₂ concentration, transpiration rate and water use efficiency of plants grown below 50% of natural light showed significant difference compared with natural and shading rate 75% of natural light. There were significant difference between natural and shade treatments in the maximal quantum efficiency of PSII (F_v/F_m), as indicated that it was significantly less at full light than that at 50% of natural light. Initial fluorescence intensity (F_o) of plants was higher at full light than that at 50% of natural light, suggesting that photoinhibition occurred in natural light. The non-photochemical quenching (NQP) decreased with the aggravation of shade stress, indicating that shading decreased the efficiency of photochemical reaction by reducing the fraction of incident light in photochemical energy utilization and decreased thermal dissipation through regulating energy distribution in photosystem II (PSII) in the leaves of Hydrangea macrophylla. In general, the 70% of incident light in photochemical energy utilization was distributed to thermal dissipation, 20% was distributed to non-regulated energy dissipation and 4% was distributed to effective photochemical reaction. In conclusion, responses of plants to increased irradiance are governed by strategy: to utilize a high fraction of incident light in photochemistry and regulate energy dissipation in PSII and weaken the accumulation of excess excitation energy in PSII to protect the photosynthetic apparatus in the leaves of H. macrophylla under saturated radiation.     

The long-term response to fluctuating light quality is an important and distinct light acclimation mechanism that supports survival of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> under low light conditions

The long-term response (LTR) of higher plants to varying light qualities increases the photosynthetic yield; however, the benefit of this improvement for physiology and survival of plants is largely unknown, and its functional relation to other light acclimation responses has never been investigated. To unravel positive effects of the LTR we acclimated Arabidopsis thaliana for several days to light sources, which preferentially excite photosystem I (PSI) or photosystem II (PSII). After acclimation, plants revealed characteristic differences in chlorophyll fluorescence, thylakoid membrane stacking, phosphorylation state of PSII subunits and photosynthetic yield of PSII and PSI. These LTR-induced changes in the structure, function and efficiency of the photosynthetic machinery are true effects by light quality acclimation, which could not be induced by light intensity variations in the low light range. In addition, high light stress experiments indicated that the LTR is not involved in photoinhibition; however, it lowers non-photochemical quenching (NPQ) by directing more absorbed light energy into photochemical work. NPQ in turn is not essential for the LTR, since npq mutants performed a normal acclimation. We quantified the beneficial potential of the LTR by comparing wild-type plants with the LTR-deficient mutant stn7. The mutant exhibited a decreased effective quantum yield and produced only half of seeds when grown under fluctuating light quality conditions. Thus, the LTR represents a distinct acclimation response in addition to other already known responses that clearly improves plant physiology under low light conditions resulting in a pronounced positive effect on plant fitness.     

Photosynthetic acclimation in eastern hemlock [Tsuga canadensis (L.) Carr.] seedlings following transfer of shade-grown seedlings to high light

 We studied photosynthetic acclimation of eastern hemlock [Tsuga canadensis (L.) Carr.] seedlings in the first month after sudden exposure of shade-grown seedlings to full sunlight. In a greenhouse experiment, seedlings were grown under full sun or 80% shade, and after 7 months, a sample of the shaded trees was transferred to full sun in the greenhouse. Photosynthetic responses of shaded, transferred, and sun trees were followed over the course of 26 days to track short to medium-term acclimation responses. A partial acclimation of photosynthesis at high light occurred in pre-existing (formed in the previous environment) and new foliage of transferred seedlings. This was associated with non-stomatal limitations to photosynthesis. Pre-existing foliage of transferred plants had a prolonged reduction in the ratio of variable to maximal fluorescence, and a limited capacity to adjust photochemical quenching or photosystem II quantum yield in the light to increasing light intensity compared to sun foliage, and apparently had some difficulty sustaining non-photochemical quenching. Seedling survival was only 58% among transferred seedlings, compared to 80% and 100% in the shade or sun groups, respectively. Photosystem II quantum yield in the light, and photochemical and non-photochemical quenching were similar between newly formed foliage of transferred and sun plants. These findings indicate that eastern hemlock depends strongly on the production of new foliage for photosynthetic adjustments to high light, and that development of photosynthetic competence may be a gradual process that occurs over successive foliar production cycles. Received: 12 May 1998 / Accepted: 27 July 1998     

Influence of light intensity during growth on photosynthesis and activity of several key photosynthetic enzymes in a C4 plant (Zea mays)

Maize ( Zea mays L. Hybrid Sweet Corn, Royal Crest), a C₄ plant, was grown under different light regimes, after which the rate of photosynthesis and activities of several photosynthetic enzymes (per unit leaf chlorophyll) were measured at different light intensities. Plants were grown outdoors under direct sunlight or 23% of direct sunlight, and in growth chambers at photosynthetic photon flux densities of about 20% and 8% of direct sunlight. The plants grown under direct sunlight had a higher light compensation point than plants grown under lower light. At a light intensity about 25% of direct sunlight, plants from all growth regimes had a similar rate of photosynthesis. Under saturating levels of light the plants grown under direct sunlight had a substantially higher rate of photosynthesis than plants grown under the lower light regimes. The higher photosynthetic capacity in the plants grown under direct sunlight was accompanied by an increased activity of several photosynthetic enzymes and in the amount of the soluble protein in the leaf. Among five photosynthetic enzymes examined, RuBP carboxylase (EC 4.1.1.39) and pyruvate, Pi dikinase (EC 2.7.9.1) were generally just sufficient to account for rates of photosynthesis under saturating light; thus, these may be rate limiting enzymes in C₄ photosynthesis. Pyruvate, Pi dikinase and NADP-malate dehydrogenase (EC 1.1.1.82) were the only enzymes examined which were light activated and increased in activity with increasing light intensity. In the low light grown plants the activity of pyruvate, Pi dikinase closely paralleled the photosynthetic rate measured under different light levels. With the plants grown under direct sunlight, as light intensity was increased the activation of pyruvate, Pi dikinase and NADP⁺-malate dehydrogenase proceeded more rapidly than photosynthesis.     

水淹对水芹叶片结构和光系统II光抑制的影响

通过探讨在水淹条件下水芹(Oenanthe javanica)叶片结构的变化以及出水对其光系统II功能和光抑制的影响, 阐明水芹光合机构在水淹条件下及出水后死亡的可能原因。结果表明: 水淹条件下新生沉水功能叶光系统II(PSII)最大光化学效率(Fv/Fm) 、电子传递活性与对照叶片差异很小, 但水淹使气生功能叶的Fv/Fm显著降低; 植株总生物量呈负增长趋势; 活体弱光条件下, 沉水叶出水后2小时叶片相对含水量(RWC)和Fv/Fm无显著变化; 中等光强和强光条件下其RWC和Fv/Fm迅速降低; 离体条件下, 5小时的中等光强对沉水叶的Fv/Fm影响不显著, 在随后的弱光下能恢复到出水时的初始状态; 强光能使沉水叶的Fv/Fm大幅降低, 且弱光下不能恢复到出水时的初始水平; 在解剖结构上, 水芹沉水叶的叶片总厚度、上下表皮厚度和气孔大小都显著低于气生叶, 而且沉水叶没有明显的栅栏组织分化, 但是沉水叶上表皮的气孔密度显著高于气生叶。研究结果表明, 水淹使水芹原气生叶PSII功能迅速衰退, 但对新生沉水叶片影响很小。水芹植株出水后, 沉水叶片结构变化使其在光下保水能力下降, 而强光导致了光合机构的光抑制和反应中心失活。田间条件下两者共同作用则加剧了对叶片光合机构的破坏, 进而致使其死亡。     

珍稀树种红花玉兰在华北地区的最适光环境

珍稀树种红花玉兰对其华南原产地的自然环境有良好的适应性, 但在华北地区却生长不良。通过对红花玉兰在华北地区一个生长季内对三种光照水平(100%、70%、40%全光照)的光合和生长响应分析, 结果表明:在70%全光照条件下, 红花玉兰幼苗的净光合速率、光饱和点、株高、基径、根生物量和茎生物量均达到最高水平。随着光照强度的减弱, 暗呼吸速率、光补偿点、比叶重量、叶片厚度和密度显著降低, 表观量子效率、最大荧光Fm、可变荧光Fv、Fm/Fo(Fo为初始荧光)、Fv/Fo、Fv/Fm、叶绿素含量、叶面积和叶柄角度均显著增大。说明70%全光照最适于一年生红花玉兰幼苗在华北地区的生长, 全光照和40%全光照条件下幼苗则因光量的过剩和不足而生长不良。因此建议将红花玉兰栽植在林缘或林窗地带, 可为这一珍稀濒危树种在华北地区的引种提供有利的适生光照环境。     

Photosystem II photochemistry and photosynthetic pigment composition in salt-adapted halophyte Artimisia anethifolia grown under outdoor conditions

The effects of high salinity (0-400 mmol/L NaCl) on photosystem II (PSII) photochemistry and photosynthetic pigment composition were investigated in the halophyte Artimisia anethifolia grown under outdoor conditions and exposed to full sunlight. High salinity resulted in an inhibition in plant growth and a significant accumulation of sodium and chloride in leaves. However, high salinity induced no effects on the actual PSII efficiency, the efficiency of excitation energy capture by open PSII reaction centres, photochemical quenching, and non-photochemical quenching at midday. High salinity also induced neither changes in the maximum efficiency of PSII photochemistry, the efficiency with which a trapped exciton can move an electron into the electron transport chain further than QA and the quantum yield of electron transport beyond QA, nor changes in absorption, trapping and electron transport fluxes per PSII reaction centre. No significant changes were observed in the levels of neoxanthin, lutein, beta-carotene, violaxanthin, antheraxanthin, and zeaxanthin expressed on a total chlorophyll basis in salt-adapted plants. Our results suggest that Artimisia anethifolia showed high resistance not only to high salinity, but also to photoinhibition even if it was treated with high salinity as high as 400 mmol/L NaCl and exposed to full sunlight. The results indicate that tolerance of PSII to high salinity and photoinhibition can be viewed as an important strategy for Artimisia anethifolia, a halophyte plant, to grow in very high saline soil.     

Effect of Nitrogen Supply on the Photosynthetic Performance of Leaves from Coffee Plants Exposed to Bright Light

Although Coffea arabica L. grows naturally in shaded habitats,it can be cultivated under high light intensity, but not withoutsevere photoinhibition mainly during the period of transferfrom the nursery into the field. The present work examines someof the changes in the photosynthetic performance induced byexposure to high light and the possibility of using enhancednitrogen levels to overcome photoinhibition. For that purpose,young plants of Coffea arabica L. (cv. Catuai) grown in a shadedgreenhouse were treated with 0, 1 and 2 mmol of nitrogen and4 weeks later exposed to full solar irradiation, outside. Visible damage due to exposure to full sunlight appeared within2 d in all plants, resulting in a reduced photosynthetic leafarea and drastic shedding of leaves in the unfertilized plants.These effects were considerably less in plants with the highestN dose. After 130 d of exposure, there was 100% mortality inplants receiving no extra nitrogen, compared with 30% in theplants treated with 2 mmol nitrogen. Photosynthesis rates, leafconductance and transpiration presented minimum values after4 d of light stress. Large changes in the photosynthetic capacity(measured at high CO₂ concentration and high light intensity),quantum efficiency and fluorescence yield (F_v/F_m) indicate thatnet photosynthesis rate in the air had been reduced by bothstomatal closure and by changes at the photochemical level.All indicators show that N-fertilized plants were less affectedby photoinhibition. Key words: Coffee plant, nitrogen, photoinhibition, photosynthesis