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 calcium deficiency on Coffea arabica. Nutrient changes and correlation of calcium levels with some photosynthetic parameters

Calcium deficiency was induced in hydroponically grown 1.5-years-old coffee plants with 12–14 pairs of leaves. Calcium was given in the form of Ca(NO₃)₂: 5, 2.5, 0.1, 0.01 and 0 mM. After 71 days of Ca-treatment root and shoot as well as total biomass were decreased by severe Ca-deficiency. However, a stronger decrease was observed for shoot growth as revealed by the increase in the root/shoot ratio. New leaves were affected showing decreases in the total leaf area and in Leaf Area Duration (LAD). After 91 days of deficiency, leaf protein concentration decreased (by about 45%) in the top leaves while nitrate reductase activity (NRA) and NO₃ content showed no significant changes. Total nitrogen and mineral concentrations (P, K, Ca, Mg and Na) were also determined in leaves and roots. With the decrease in calcium concentration in Ca-deficiency conditions, we observed concomitant increases in the concentrations of K⁺, Mg²⁺ and Na⁺ in leaves (maximal changes of 32% for K⁺, 96% for Mg²⁺ and 438% for Na⁺) and in roots (108% for K⁺, 86% for Mg²⁺ and 38% for Na⁺). Accordingly, the ratio between elements changed, including the ratio N/P, showing a non-equilibrium in the balance of nutrients. Significant correlations were obtained between Ca²⁺ concentration and some photosynthetic parameters. Ca-deficiency conditions would increase the loss of energy as expressed by the rise in a_E and decrease the photochemical efficiency, which confirms the importance of this element in the stabilization of chlorophyll and in the maintenance of good photochemical efficiency at PS II level.Abbreviations Chl Chlorophyll - Fv/Fm ratio of variable to maximal fluorescence - LAD leaf area duration - LHC II light harvesting complex of PS II - NRA nitrate reductase activity - PC photosynthetic capacity - PS II photosystem II - P₆₈₀ reaction center of PS II - q_N non-photochemical quenching - q_E high-energy dependent quenching - q_p photochemical quenching - SLA specific leaf area     

Low nitrogen stress regulates chlorophyll fluorescence in coordination with photosynthesis and Rubisco efficiency of rice

Nitrogen (N) is the basis of plant growth and development and, is considered as one of the priming agents to elevate a range of stresses. Plants use solar radiations through photosynthesis, which amasses the assimilatory components of crop yield to meet the global demand for food. Nitrogen is the main regulator in the allocation of photosynthetic apparatus which changes of the photosynthesis (P_n) and quantum yield (F_v/F_m) of the plant. In the present study, dynamics of the photosynthetic establishment, N-dependent relation with chlorophyll fluorescence attributes and Rubisco efficacy was evaluated in low-N tolerant (cv. CR Dhan 311) and low-N sensitive (cv. Rasi) rice cultivars under low-N and optimum-N conditions. There was a decrease in the stored leaf N under low-N condition, resulting in the decreased P_n and F_v/F_m efficiency of the plants through depletion in the activity and content of Rubisco. The P_n and F_v/F_m followed the parallel trend of leaf N content during low-N condition along with depletion of intercellular CO₂ concentration and overall conductance under low-N condition. Photosynthetic saturation curve cleared abrupt decrease of effective quantum yield in the low-N sensitive rice cultivar than the low-N tolerant rice. Also, the rapid light curve highlighted the unacclimated regulation of photochemical and non-photochemical quenching in the low-N condition. The low-N sensitive rice cultivar triumphed non-photochemical quenching, whereas the low-N tolerant rice cultivar rose gradually during the light curve. Our study suggested that the quantum yield is the key limitation for photosynthesis in low-N condition. Regulation of Rubisco, photochemical and non-photochemical quenching may help plants to grow under low-N level.

     

Simultaneous gas exchange and fluorescence measurements indicate differences in the response of sunflower,bean and maize to water stress

Gas exchange and fluorescence measurements of attached leaves of water stressed bean, sunflower and maize plants were carried out at two light intensities (250 mol quanta m^-2s^-1 and 850 mol quanta m^-2s^-1). Besides the restriction of transpiration and CO₂ uptake, the dissipation of excess light energy was clearly reflected in the light and dark reactions of photosynthesis under stress conditions. Bean and maize plants preferentially use non-photochemical quenching for light energy dissipation. In sunflower plants, excess light energy gave rise to photochemical quenching. Autoradiography of leaves after photosynthesis in ¹⁴CO₂ demonstrated the occurrence of leaf patchiness in sunflower and maize but not in bean. The contribution of CO₂ recycling within the leaves to energy dissipation was investigated by studies in 2.5% oxygen to suppress photorespiration. The participation of different energy dissipating mechanisms to quanta comsumption on agriculturally relevant species is discussed.Abbreviations F_o minimal fluorescence - F_m maximal fluorescence - F_p peak fluorescence - g leaf conductance - P_N net CO₂ uptake - q_N coefficient of non-photochemical quenching - q_P coefficient of photochemical quenching     

Response of photosynthesis to high light and drought for Arabidopsis thaliana grown under a UV-B enhanced light regime

Arabidopsis thaliana grown in a light regime that included ultraviolet-B (UV-B) radiation (6 kJ m⁻² d⁻¹) had similar light-saturated photosynthetic rates but up to 50% lower stomatal conductance rates, as compared to plants grown without UV-B radiation. Growth responses of Arabidopsis to UV-B radiation included lower leaf area (25%) and biomass (10%) and higher UV-B absorbing compounds (30%) and chlorophyll content (52%). Lower stomatal conductance rates for plants grown with UV-B radiation were, in part, due to lower stomatal density on the adaxial surface. Plants grown with UV-B radiation had more capacity to down regulate photochemical efficiency of photosystem II (PSII) as shown by up to 25% lower φ_PSII and 30% higher non-photochemical quenching of chlorophyll fluorescence under saturating light. These contributed to a smaller reduction in the maximum photochemical efficiency of PSII (F _v/F _m), greater dark-recovery of F _v/F _m, and higher light-saturated carbon assimilation and stomatal conductance and transpiration rates after a four-hour high light treatment for plants grown with UV-B radiation. Plants grown with UV-B were more tolerant to a 12 day drought treatment than plants grown without UV-B as indicated by two times higher photosynthetic rates and 12% higher relative water content. UV-B-grown plants also had three times higher proline content. Higher tolerance to drought stress for Arabidopsis plants grown under UV-B radiation may be attributed to both increased proline content and decreased stomatal conductance. Growth of Arabidopsis in a UV-B-enhanced light regime increased tolerance to high light exposure and drought stress.     

Limitations to photosynthesis inCoffea canephoraas a result of nitrogen and water availability

Plants of C. canephora grown in pots under low nitrogen (LN) or high nitrogen (HN) applications were submitted to either cyclic water stress or daily irrigation. Water deficit led to marked decreases in net carbon assimilation rate (A) and, to a lesser extent, in stomatal conductance (g_s), regardless of the N treatments. In well-watered plants, A appreciably increased in HN plants relative to LN plants without significant changes in g_s. As a whole, changes in internal CO₂ concentration predominantly reflected changes in A rather than in g_s. Under irrigated conditions, A, but not g_s, correlated with leaf N concentration in a curvilinear way. Photosynthetic nitrogen-use efficiency was considerably low, and decreased with increasing leaf N concentration. Limited N, but not water, slightly decreased the maximum photochemical efficiency of photosystem II (PSII). Under continuous irrigation, LN plants had a smaller quantum yield of electron transport (_PSII) through slight decreases of photochemical quenching (q_p) and capture efficiency of excitation energy by open PSII reaction centres, and increases in Stern-Volmer non-photochemical quenching. Under water-stressed conditions, changes in PSII photochemistry were apparent only in HN plants, with a 25 % decrease in _PSII, due mainly to variations in q_p. Biochemical constraints, rather than stomatal or photochemical limitations, provoked the decreases in A under limited supply of either N or water.     

Elevated CO2 and limited nitrogen nutrition can restrict excitation energy dissipation in photosystem II of Japanese white birch (Betula platyphylla var. japonica) leaves

Elevated atmospheric CO₂ concentration [CO₂] and different levels of nitrogen (N) nutrition can influence the amount of excess excitation energy in photosystem (PS) II and related photosynthetic properties. The interactive effect of two [CO₂] levels (ambient: 360 µM M⁻¹ and elevated: 720 µM M⁻¹) and two N levels (high: 700 mg N plant⁻¹ and low: 100 mg N plant⁻¹) on these properties was examined in seedlings of Japanese white birch (Betula platyphylla var. japonica) using simultaneous measurements of gas exchange and chlorophyll fluorescence. Photosynthetic acclimation to elevated [CO₂], as indicated by a decline in carboxylation efficiency (CE), was observed in plants grown at elevated [CO₂] especially under low N. Elevated [CO₂] resulted in a decrease in area-based leaf N content (N_area) irrespective of N treatment. The adverse effect of elevated [CO₂] and low N on CE may have been exacerbated by a greater accumulation of leaf sugar and starch contents in these plants leading to a lower electron transport rate (ETR). While these plants also showed higher non-photochemical quenching (Nq_P) that could offset the reduction in energy dissipation through ETR to some extent, they still have a higher risk of photoinhibition from excessive excitation energy in PSII as indicated by a decrease in photochemical quenching (q_P). However, chronic photoinhibition was not observed in plant grown at elevated [CO₂] and low N because they showed no difference in F_v/F_m (the maximum photochemical efficiency of PSII) from those grown at ambient [CO₂] and low N after an overnight dark adaptation. High levels of Nq_P in plants grown at elevated [CO₂] and low N reflect a near saturation of thermal energy dissipation. This impaired capacity of photoprotection would render these plants more vulnerable to photoinhibition in the event of additional environmental stresses such as drought, low or high temperature.     

长期施氮对旱地小麦灌浆期叶绿素荧光参数的影响

探讨了长期施氮对黄土旱塬冬小麦灌浆期叶绿紊荧光的影响．结果表明,PSⅡ反应中心的实际光化学效率(φPSⅡ)随叶片水分胁迫的加剧而降低,施氮可明显提高φPsⅡ．施氮量为0kg·hm^-2、90kg·hm^-2和180kg·hm^-2三个处理中午时叶片的φPSⅡ分别为0．197、0．279和0．283,与上午相比分别降低了57．7％、56．4％和40．2％;下午各处理的φPSⅡ均得以恢复,0kg·hm^-2和90kg·hm^-2处理分别恢复到上午的87．3％和81．5％,180kg·hm^-2处理则完全恢复,并稍高于上午．施氮可提高PSⅡ的最大光化学量子效率(Fv／Fm)、光化学猝灭系数(qP)和非光化学猝灭系数(qNP)等荧光参数,表明施氮一方面提高了光能转换效率和PSⅡ的潜在活性,另一方面增强了过剩光能的非光化学耗散,有利于保护光合机构免受环境胁迫的破坏．但90kg·hm^-2和180kg·hm^-2施氮处理之间对光合的促进作用差异不显著,表明过量施氮无助于光合性能的提高,作物的光合能力并不随施氮量而同比例的改善。     

Photosynthetic responses and proline content of mature and young leaves of sunflower plants under water deficit

In mature and young leaves of sunflower (Helianthus annuus L. cv. Catissol-01) plants grown in the greenhouse, photosynthetic rate, stomatal conductance, and transpiration rate declined during water stress independently of leaf age and recovered after 24-h rehydration. The intercellular CO₂ concentration, chlorophyll (Chl) content, and photochemical activity were not affected by water stress. However, non-photochemical quenching increased in mature stressed leaves. Rehydration recovered the levels of non-photochemical quenching and increased the F_v/F_m in young leaves. Drought did not alter the total Chl content. However, the accumulation of proline under drought was dependent on leaf age: higher content of proline was found in young leaves. After 24 h of rehydration the content of proline returned to the same contents as in control plants.     

Photoadaptation in the Green Alga Spongiochloris Sp. A Three-Fluorometer Study

The dark-adapted cells of the green alga Spongiochloris sp. were exposed to "white light" of 1000 μmol(photon) m⁻² s⁻¹ for 2 h and then dark adapted for 1.5 h. Changes of photochemical activities during photoadaptation were followed by measurement of chlorophyll (Chl) fluorescence kinetics, 77 K emission spectra, photosynthetic oxygen evolution, and pigment composition. We observed a build-up of slowly-relaxing non-photochemical quenching which led to a decrease of the F_v/F_m parameter and the connectivity. In contrast to the depression of F_v/F_m (35 %) and the rise of non-photochemical quenching (∼ 1.6), we observed an increase in effective absorption cross-section (20 %), Hill reaction (30 %), photosynthetic oxygen evolution (80 %), and electron transport rate estimated from the Chl fluorescence analysis (80 %). We showed an inconsistency in the presently used interpretation schemes, and ascribe the discrepancy between the increase of effective absorption cross-section and the photosynthetic activities on one side and the effective non-photochemical quenching on the other side to the build-up of a quenching mechanism which dissipates energy in closed reaction centres. Such a type of quenching changes the ratio between thermal dissipation and fluorescence without any effect on photochemical yield. In this case the F_v/F_m ratio cannot be used as a measure of the maximum photochemical yield of PS2. This revised version was published online in August 2006 with corrections to the Cover Date.     

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     

Acclimation of photosynthesis to nitrogen deficiency in Phaseolus vulgaris

Nitrogen deficiency diminishes consumption of photosynthates in anabolic metabolism. We studied adjustments of the photosynthetic machinery in nitrogen-deficient bean plants and found four phenomena. First, the number of chloroplasts per cell decreased. Chloroplasts of nitrogen starved leaves contained less pigments than those of control leaves, but the in vitro activities of light reactions did not change when measured on chlorophyll basis. Second, nitrogen deficiency induced cyclic electron transfer. The amounts of Rubisco and ferredoxin-NADP⁺ reductase decreased in nitrogen starved plants. Low activities of these enzymes are expected to lead to increase in reduction of oxygen by photosystem I. However, diaminobenzidine staining did not reveal hydrogen peroxide production in nitrogen starved plants. Measurements of far-red-light-induced redox changes of the primary donor of photosystem I suggested that instead of producing oxygen radicals, nitrogen starved plants develop a high activity of cyclic electron transport that competes with oxygen for electrons. Nitrogen starvation led to decrease in photochemical quenching and increase in non-photochemical quenching, indicating that cyclic electron transport reduces the plastoquinone pool and acidifies the lumen. A third effect is redistribution of excitation energy between the photosystems in favor of photosystem I. Thus, thylakoids of nitrogen starved plants appeared to be locked in state 2, which further protects photosystem II by decreasing its absorption cross-section. As a fourth response, the proportion of non-Q_B-reducing photosystem II reaction centers increased and the redox potential of the Q_B/Q_B⁻ pair decreased by 25 mV in a fraction of photosystem II centers of nitrogen starved plants.     

Chlorophyll fluorescence transients in a barley mutant lacking Photosystem I

We have compared the properties of a mutant of barley lacking Photosystem I (viridis-zb ⁶³ ) with the corresponding wild type using modulated fluorescence measurements. The mutant showed two unexpected characteristics. Firstly, there was a slow decline in the fluorescence signal in the light which was dependent on the presence of O₂ at concentrations similar to that in air; 2% O₂ in N₂ had no effect. The observed decline was mainly due to an increase in the non-photochemical quenching. Secondly, in the absence of O₂, saturating light pulses caused a pronounced transient decrease in the fluorescence signal; a similar effect could also be observed in wild type plants when neither CO₂ nor O₂ was present.Abbreviations PPFD- photosynthetic photon flux density - q_N- non-photochemical quenching of chlorophyll fluorescence - q_p- photochemical quenching of chlorophyll fluorescence     

24-epibrassionlide improves photosynthetic response of Rhododendron delavayi to drought

Rhododendron delavayi is an alpine evergreen ornamental plant with strong tolerance to drought stress. Brassinosteroids are promising agents for alleviating the negative effects of drought on plants, but the mechanism by which BRs induce plant resistance to drought is not well understood. The present study investigated the effects of exogenous spray of 24-epibrassionlide (EBR) at different concentrations (0~1 mg l⁻¹) on the physiological response of R. delavayi to drought caused by no watering for 10 days. With the increase in EBR concentration, net photosynthetic rate, stomatal conductance, transportation rate, light saturated photosynthetic rate, light compensation point, light saturation point, excitation energy capture efficiency of reaction center, actual photochemical efficiency of photosystem II (PSII), photochemical quenching and electron transport rate significantly increased, but there were no significant effects on photosynthetic pigment content. These results suggested that the EBR-induced improvement in CO₂ assimilation under drought was mainly related to stomatal and non-stomatal factors, and partially attributed to the increased photochemical efficiency of PSII. In addition, the leaf water potential increased with the increase in EBR concentration, while the malondialdehyde, superoxide dismutase, catalase, proline and soluble protein decreased. The results suggested EBR application partially alleviated the negative effect of drought on R. delavayi by improving water relations and decreasing lipid peroxidation and reactive oxygen species production. We concluded that exogenous application of EBR improved photosynthesis and alleviated the negative effects of drought-induced membrane peroxidation and severe oxidative stress.     

Role of calcium ion in protection against heat and high irradiance stress-induced oxidative damage to photosynthesis of wheat leaves

Cross stress of heat and high irradiance (HI) resulted in the accumulation of active oxygen species and photo-oxidative damage to photosynthetic apparatus of wheat leaves during grain development. Pre-treatment with calcium ion protected the photosynthetic system from oxidative damage by reducing O^-. ₂ production, inhibiting lipid peroxidation, and retarding electrolyte leakage from cell. Therefore, high F_v/F_m [maximal photochemical efficiency of photosystem 2 (PS2) while all PS2 reaction centres are open], F_m/F₀ (another expression for the maximal photochemical efficiency of PS2), Φ_PS2 (actual quantum yield of PS2 under actinic irradiation), q_P (photochemical quenching coefficient), and P _N (net photosynthetic rate) were maintained, and lower q_NP (non-photochemical quenching coefficient) of the leaves was kept under heat and HI stress. EGTA (a chelant of calcium ion) and LaCl₃ (a blocker of Ca²⁺ channel in cytoplasmic membrane) had the opposite effect. Thus Ca ion may help protect the photosynthetic system of wheat leaves from oxidative damage induced by the cross stress of heat and HI.     

Effects of rhizobia inoculation and nitrogen fertilization on photosynthetic physiology of soybean

Plant growth, contents of photosynthetic pigments, photosynthetic gas exchange, and chlorophyll (Chl) fluorescence in soybean [Glycine max (L.) Merr. cv. Heinong37] were investigated after it was inoculated with Sinorhizobium fredii USDA191 or treated with 5 mM (NH₄)₂SO₄ (N5) and 30 mM (NH₄)₂SO₄ (N30), respectively. In the plants following N5 fertilization, not only plant biomass, leaf area, and Chl content, but also net photosynthetic rate (P _N), stomatal conductance (g _s), carboxylation efficiency (CE), maximum photochemical efficiency (F_v/F_m) of photosystem 2 (PS2), and quantum yield of PS2 (Φ_PS2) were markedly improved as compared with the control plants. There were also positive effects on plant growth and plant photosynthesis after rhizobia inoculation, but the effects were much less than those of N5 fertilization. For N30 plants there were no significant positive effects on plant growth and photosynthetic capacity. Plant biomass, P _N, and g _s were similar to those of N-limited (control) plants. Φ_PS2 and photochemical quenching (q_P) were obviously declined while content of carotenoids and non-photochemical quenching (q_N) were significantly enhanced in N30 treated plants. This indicated that excess N supply may cause some negative effects on soybean plants.     

Photosynthesis and chlorophyll fluorescence response to low sink demand of tubers and roots in <Emphasis Type="Italic">Dahlia pinnata</Emphasis> source leaves

Photosynthetic rate (P_N) and chlorophyll (Chl) fluorescence induction of source leaves in response to a low sink demand created by girdling the branch (GB) between the root-tuber-system and the leaves were studied in Dahlia pinnata L. cv. Rigolet during the stage of rapid tuber growth in the greenhouse. GB resulted in significantly lower values of P_N, stomatal conductance (g_s), and transpiration rate (E), but in higher leaf temperature (T_l) compared with those of controls. With exception of maximum quantum yield of photosystem 2 (PS 2) photochemistry (F_v/F_m) and maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in PS 2 (F_v/F₀), no significant differences were observed in Chl fluorescence parameters between girdled and control leaves on days 1 and 2 after GB, indicating no apparent damage in the photosynthetic apparatus. However, longer girdling duration resulted in higher non-photochemical Chl fluorescence quenching (NPQ), but lower F_v/F₀, actual efficiency of energy conversion in PS 2 under steady-state conditions (Φ_PS2), and photochemical quenching coefficient (qP) in comparison with controls from 10:00 to 16:00 or 15:00 on days 4 and 5, respectively, indicating reversible injury in the photosynthetic apparatus.     

Changes in photosynthesis,fluorescence, and nitrogen metabolism of hawthorn (<Emphasis Type="Italic">Crataegus pinnatifida</Emphasis>) in response to exogenous glutamic acid

Photosynthesis, chlorophyll (Chl) a fluorescence, and nitrogen metabolism of hawthorn (Crataegus pinnatifida Bge.), subjected to exogenous L-glutamic acid (GLA) (200 mg l⁻¹, 400 mg l⁻¹, and 800 mg l⁻¹) that possibly affect secondary metabolic regulation, were measured. The results indicated that photosynthetic and fluorescence characteristics of hawthorn exhibited positive responses to the application of GLA. Different concentrations of GLA caused an increase in Chl content, net photosynthetic rate (P _N) and stomatal conductance (g _s) as well as transpiration rate (E), and improved the carboxylation efficiency (CE), apparent quantum yield (AQY) and maximum carboxylation velocity of Rubisco (V_cmax). Application of GLA could also enhance the maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in PSII (F_v/F₀), the maximal quantum yield of PSII (F_v/F_m), the probability that an absorbed photon will move an electron into the electron transport chain beyond Q_A (Φ_Eo) as well as the performance index on absorption basis (PI_ABS), but decreased the intercellular CO₂ concentration (C _i) and the minimal fluorescence (F₀). Application of GLA also induced an increase in nitrate reductase (NR; EC 1.6.6.1) and glutamine synthetase (GS; EC 6.3.1.2) activities, and increased the soluble protein content, leaf nitrogen (N) content and N accumulation in leaves as well as the plant biomass. However, the effects were different among different concentrations of GLA, and 800 mg l⁻¹ GLA was better. This finding suggested that application of GLA is recommended to improve the photosynthetic capacity by increasing the light energy conversion and CO₂ transfer as well as the photochemical efficiency of PSII, and enhanced the nitrogen metabolism and growth and development of plants.     

Salt stress affects water relations,photosynthesis, and oxidative defense mechanisms in Solanum melongena L.

Abstract

A greenhouse experiment was conducted to examine the salt-induced changes in some key physio-biochemical attributes in eggplant (cv. New Noble) plants. Eggplant plants were grown under varying levels (0, 50, 100, and 150 mM) of sodium chloride under greenhouse conditions supplied with natural light and other climatic conditions. Varying saline regimes in growth medium significantly reduced the shoot and root fresh and dry weights, shoot and root lengths, relative water content, chlorophyll a and b pigments, photosynthetic rate (A), water-use efficiency, stomatal conductance (g _s ), leaf and root K⁺, total phenolics, total soluble proteins, activity of superoxide dismutase (SOD), and leaf water and osmotic potentials in all eggplant plants. However, in contrast, saline regimes of the root growing medium did not affect transpiration rate (E), internal CO₂ concentration (C _i ), C _i /C _a ratio, photochemical quenching (qP), non-photochemical quenching, efficiency of photosystem-II (F _v /F _m ), leaf and root Ca²⁺ as well as ascorbic acid (AsA) contents in eggplant. A significant increase was observed in leaf turgor potential, free proline and glycinebetaine contents, leaf and root Na⁺ contents, malondialdehyde and hydrogen peroxide (H₂O₂) contents and activities of peroxidase (POD) and catalase (CAT) in eggplant plants under varying saline regimes. Overall, salt-induced growth reduction in eggplant plants was found to be associated with high accumulation of Na⁺ in both roots and shoots, which adversely affected photosynthetic capacity, chlorophyll pigments, K⁺ and Ca²⁺ contents, H₂O₂ and AsA levels and activities of SOD, POD, and CAT.     

Photoinactivation of photosystem II by cumulative exposure to short light pulses during the induction period of photosynthesis

Photoinactivation of Photosystem (PS) II in vivo was investigated by cumulative exposure of pea, rice and spinach leaves to light pulses of variable duration from 2 to 100 s, separated by dark intervals of 30 min. During each light pulse, photosynthetic induction occurred to an extent depending on the time of illumination, but steady-state photosynthesis had not been achieved. During photosynthetic induction, it is clearly demonstrated that reciprocity of irradiance and duration of illumination did not hold: hence the same cumulative photon exposure (mol m^–2) does not necessarily give the same extent of photoinactivation of PS II. This contrasts with the situation of steady-state photosynthesis where the photoinactivation of PS II exhibited reciprocity of irradiance and duration of illumination (Park et al. (1995) Planta 196: 401–411). We suggest that, for reciprocity to hold between irradiance and duration of illumination, there must be a balance between photochemical (qP) and non-photochemical (NPQ) quenching at all irradiances. The index of susceptibility to light stress, which represents an intrinsic ability of PS II to balance photochemical and non-photochemical quenching, is defined by the quotient (1-qP)/NPQ. Although constant in steady-state photosynthesis under a wide range of irradiance (Park et al. (1995). Plant Cell Physiol 36: 1163–1169), this index of susceptibility for spinach leaves declined extremely rapidly during photosynthetic induction at a given irradiance, and, at a given cumulative photon exposure, was dependent on irradiance. During photosynthetic induction, only limited photoprotective strategies are developed: while the transthylakoid pH gradient conferred some degree of photoprotection, neither D1 protein turnover nor the xanthophyll cycle was operative. Thus, PS II is more easily photoinactivated during photosynthetic induction, a phenomenon that may have relevance for understorey leaves experiencing infrequent, short sunflecks.Abbreviations D1 protein psbA gene product - DTT dithiothreitol - F_v, F_m, F_o variable, maximum, and initial (corresponding to open traps) chlorophyll fluorescence yield, respectively - NPQ non-photochemical quenching - PS Photosystem - Q_A primary quinone acceptor of PS II - qP photochemical quenching coefficient