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.     

Physiological responses to nitrogen and sulphur addition and raised temperature in <Emphasis Type="Italic">Sphagnum balticum</Emphasis>

Sphagnum, the main genus which forms boreal peat, is strongly affected by N and S deposition and raised temperature, but the physiological mechanisms behind the responses are largely unknown. We measured maximum photosynthetic rate (NP_max), maximum efficiency of photosystem II [variable fluorescence (F _v)/maximum fluorescence yield (F _m)] and concentrations of N, C, chlorophyll and carotenoids as responses to N and S addition and increased temperature in Sphagnum balticum (a widespread species in the northern peatlands) in a 12-year factorial experiment. NP_max did not differ between control (0.2 g N m⁻² year⁻¹) and high N (3.0 g N m⁻² year⁻¹), but was higher in the mid N treatment (1.5 g N m⁻² year⁻¹). N, C, carotenoids and chlorophyll concentration increased in shoot apices after N addition. F _v/F _m did not differ between N treatments. Increased temperature (+3.6°C) had a small negative effect on N concentration, but had no significant effect on NP_max or F _v/F _m. Addition of 2 g S m⁻² year⁻¹ showed a weak negative effect on NP_max and F _v/F _m. Our results suggest a unimodal response of NP_max to N addition and tissue N concentration in S. balticum, with an optimum N concentration for photosynthetic rate of ~13 mg N g⁻¹. In conclusion, high S deposition may reduce photosynthetic capacity in Sphagnum, but the negative effects may be relaxed under high N availability. We suggest that previously reported negative effects on Sphagnum productivity under high N deposition are not related to negative effects on the photosynthetic apparatus, but differences in optimum N concentration among Sphagnum species may affect their competitive ability under different N deposition regimes.     

Chlorophyll fluorescence and chlorophyll content in field-grown potato as affected by nitrogen supply, genotype, and plant age

Field experiments were conducted in Sicily (south Italy) to assess chlorophyll (Chl) fluorescence parameters in response of potato crop to nitrogen dose, to variation in genotype and in plant age, and to detect relationships between Chl content, fluorescence parameter F_v/F_m, and tuber yield. The experiment included five nitrogen doses (0, 10, 20, 30, and 40 g m⁻²) and four genotypes (Spunta, Sieglinde, Daytona, and Igea). Chl fluorescence parameters (initial fluorescence, F₀, maximum fluorescence, F_m, variable fluorescence, F_v, F_v/F_m, T_max (the time required to reach F_m), and Chl content were measured weekly between the appearance of the fifth and sixth leaves and the onset of plant senescence. A positive linear relationship was established between nitrogen supply and Chl content, F₀, and T_max. Nitrogen supply up to 10 g m⁻² also had a positive effect on F_m and F_v, but above this rate it reduced F_v/F_m. Spunta had the highest Chl content, F_m, F_v, and F_v/F_m, but the lowest F₀, whereas Sieglinde had the lowest Chl content, F_v, F_v/F_m, and T_max and the highest F₀. The cvs. Igea and Daytona exhibited intermediate Chl fluorescence parameters. Chl content and T_max decreased with increasing plant age, whereas F₀, F_m, and F_v increased until complete canopy development and thereafter declined until crop maturity. Tuber and plant dry matter yield were significantly correlated with Chl content, F₀, and T_max. Thus Chl fluorescence and content detect differences in the response of potato to N supply, can discriminate between genotypes, predict plant age, and yield performance under field conditions.     

Winter photoinhibition in needles of <Emphasis Type="Italic">Taxus baccata</Emphasis> seedlings acclimated to different light levels

Seasonal variability of maximum quantum yield of PSII photochemistry (F_v/F_m) was studied in needles of Taxus baccata seedlings acclimated to full light (HL, 100% solar irradiance), medium light (ML, 18% irradiance) or low light (LL, 5% irradiance). In HL plants, F_v/F_m was below 0.8 (i.e. state of photoinhibition) throughout the whole experimental period from November to May, with the greatest decline in January and February (when F_v/F_m value reached 0.37). In ML seedlings, significant declines of F_v/F_m occurred in January (with the lowest level at 0.666), whereas the decline in LL seedlings (down to 0.750) was not significant. Full recovery of F_v/F_m in HL seedlings was delayed until the end of May, in contrast to ML and LL seedlings. F_v/F_m was significantly correlated with daily mean (T _mean), maximal (T _max) and minimal (T _min) temperature and T _min was consistently the best predictor of F_v/F_m in HL and ML needles. Temperature averages obtained over 3 or 5 days prior to measurement were better predictors of F_v/F_m than 1- or 30-day averages. Thus our results indicate a strong light-dependent seasonal photoinhibition in needles of T. baccata as well as suggest a coupling of F_v/F_m to cumulative temperature from several preceding days. The dependence of sustained winter photoinhibition on light level to which the plants are acclimated was further demonstrated when plants from the three light environments were exposed to full daylight over single days in December, February and April and F_v/F_m was followed throughout the day to determine residual sensitivity of electron transport to ambient irradiance. In February, the treatment revealed a considerable midday increase in photoinhibition in ML plants, much less in HL (already downregulated) and none in LL plants. This suggested a greater capacity for photosynthetic utilization of electrons in LL plants and a readiness for rapid induction of photoinhibition in ML plants. Further differences between plants acclimated to contrasting light regimes were revealed during springtime de-acclimation, when short term regeneration dynamics of F_v/F_m and the relaxation of nonphotochemical quenching (NPQ) indicated a stronger persistent thermal mechanism for energy dissipation in HL plants. The ability of Taxus baccata to sustain winter photoinhibition from autumn until late spring can be beneficial for protection against an excessive light occurring together with frosts but may also restrict photosynthetic carbon gain by this shade-tolerant species when growing in well illuminated sites.     

Leaf Gas Exchange and Chlorophyll Fluorescence Parameters in Phaseolus Vulgaris as Affected by Nitrogen and Phosphorus Deficiency

The effects of N and P deficiency, isolated or in combination, on leaf gas exchange and fast chlorophyll (Chl) fluorescence emission were studied in common bean cv. Negrito. 10-d-old plants grown in aerated nutrient solution were supplied with high N (HN, 7.5 mol m⁻³) or low N (LN, 0.5 mol m⁻³), and also with high P (HP, 0.5 mol m⁻³) or low P (LP, 0.005 mol m⁻³). Regardless of the external P supply, in LN plants the initial fluorescence (F₀) increased 12 % in parallel to a quenching of about 14 % in maximum fluorescence (F_m). As a consequence, the variable to maximum fluorescence ratio (F_v/F_m) decreased by about 7 %, and the variable to initial fluorescence ratio (F_v/F₀) was lowered by 25 % in relation to control plants. In LP plants, F_v/F_m remained unchanged whilst F_v/F₀ decreased slightly as a result of 5 % decline in F_m. Under N deficiency, the net photosynthetic rate (P _N) halved at 6 d after imposition of treatment and so remained afterwards. As compared to LN plants, P _N declined in LP plants latter and to a less extent. From 12 d of P deprivation onwards. P _N fell down progressively to display rates similar to those of LN plants only at the end of the experiment. The greater P _N in LP plants was not reflected in larger biomass accumulation in relation to LN beans. In general, P and N limitation affected photosynthesis parameters and growth without showing any synergistic or additive effect between deficiency of both nutrients. This revised version was published online in June 2006 with corrections to the Cover Date.     

低温胁迫下丛枝菌根真菌对玉米光合特性的影响

利用盆栽试验,在15 ℃和5 ℃低温胁迫下研究了丛枝菌根（AM）真菌对玉米生长、叶绿素含量、叶绿素荧光和光合作用的影响.结果表明：低温胁迫抑制了AM真菌的侵染;接种AM真菌的玉米地上部和地下部干物质量、相对叶绿素含量高于不接种植株.与非菌根玉米相比,菌根玉米具有较高的最大荧光（F_m）、可变荧光（F_v）、最大光化学效率（F_v/F_m）和潜在光化学效率（F_v/F_o）及较低的初始荧光（F_o）,并且在5 ℃处理中差异显著.接种AM真菌使玉米叶片的净光合速率（P_n）和蒸腾速率（T_r）显著增强;低温胁迫下,菌根植株的气孔导度（G_s）显著高于非菌根植株;而胞间CO₂浓度（C_i）显著低于非菌根植株.表明AM真菌可通过提高叶绿素含量及改善叶片叶绿素荧光和光合作用来减轻低温胁迫对玉米植株造成的伤害,提高玉米耐受低温的能力,进而提高玉米的生物量,促进玉米生长.     

Influence of nitrogen supply and growth irradiance on photoinhibition and recovery in Heuchera americana (Saxifragaceae)

Prior work demonstrated that Heuchera americana, an evergreen herb inhabiting the deciduous forest understory in the southeastern United States, has a 3-4-fold greater photosynthetic capacity under the low-temperature, strong-light, open canopies of winter compared to the high-temperature, weak-light, closed canopies of summer. Moreover, despite the reductions in soil nitrogen, the chilling temperatures, and the increased quantum flux associated with winter, chronic photoinhibition was not observed in this species at this time of the year. We were interested in the photosynthetic acclimation and photoinhibition characteristics of this species when grown under contrasting light and nitrogen regimes. Newly expanded shade-acclimated leaves of forest-grown plants exposed to strong light varying in intensity and duration at 25°C showed a reduction in F_v/F_m (the ratio of variable to maximum room temperature chlorophyll fluorescence measured after dark adaptation), which was correlated with a decline in ø_a (the intrinsic quantum yield of CO₂-saturated O₂ evolution on an absorbed light basis). Plants grown in the glasshouse under contrasting light (high and low light; HL and LL, respectively) and nitrogen supply (high and low nitrogen; HN and LN, respectively) regimes showed that photosynthetic acclimation to HL was impaired in LN regimes. The HL-LN plants also had the lowest values of F_v/F_m and of ø on both incident and absorbed light bases and had 50% less chlorophyll (per unit area) compared to plants from other growth regimes. Controlled exposure to bright light at low temperatures (2-3°C) for 3 h resulted in a sharp decrease in F_v/F_m (and rise in F_o, the minimum fluorescence yield) in all plants. Shade-grown plants from both N regimes were highly susceptible to chronic photoinhibition, as indicated by a greater reduction in F_v/F_m and incomplete recovery after 18 h in weak light at 25°C. The HL-HN plants were the least susceptible to chronic photoinhibition, having the smallest decrease in F_v/F_m with near full recovery within 6 h. The decline in Fv/Fm in HL-LN plants was comparable to that of shade-acclimated plants, but recovered fully within 6 h. Low-N plants from both light regimes displayed greater increases in F_o which did not return to pretreatment levels after 18 h of recovery. These studies indicate that HL-LN plants were sensitive to chronic photoinhibition and, at the same time, had a high capacity for dynamic photoinhibition. Experimental garden studies showed that H. americana grown in an open field in summer were photoinhibited and did not fully recover overnight or during extended periods of weak light. These results are discussed in relation to the photosynthetic acclimation of H. americana under natural conditions.     

Wheat cultivars selected for high Fv/Fm under heat stress maintain high photosynthesis,total chlorophyll,stomatal conductance,transpiration and dry matter

The chlorophyll fluorescence parameter F_v/F_m reflects the maximum quantum efficiency of photosystem II (PSII) photochemistry and has been widely used for early stress detection in plants. Previously, we have used a three‐tiered approach of phenotyping by F_v/F_m to identify naturally existing genetic variation for tolerance to severe heat stress (3 days at 40°C in controlled conditions) in wheat (Triticum aestivum L.). Here we investigated the performance of the previously selected cultivars (high and low group based on F_v/F_m value) in terms of growth and photosynthetic traits under moderate heat stress (1 week at 36/30°C day/night temperature in greenhouse) closer to natural heat waves in North‐Western Europe. Dry matter accumulation after 7 days of heat stress was positively correlated to F_v/F_m. The high F_v/F_m group maintained significantly higher total chlorophyll and net photosynthetic rate (P_N) than the low group, accompanied by higher stomatal conductance (g_s), transpiration rate (E) and evaporative cooling of the leaf (ΔT). The difference in P_N between the groups was not caused by differences in PSII capacity or g_s as the variation in F_v/F_m and intracellular CO₂ (C_i) was non‐significant under the given heat stress. This study validated that our three‐tiered approach of phenotyping by F_v/F_m performed under increasing severity of heat was successful in identifying wheat cultivars differing in photosynthesis under moderate and agronomically more relevant heat stress. The identified cultivars may serve as a valuable resource for further studies to understand the physiological mechanisms underlying the genetic variability in heat sensitivity of photosynthesis.     

Influence of phytohormones on morphology and chlorophyll a fluorescence parameters in embryos of Fucus vesiculosus L. (Phaeophyceae)

While a variety of plant hormones from brown algae were described, there were few studies that examined the combined effects of these hormones on morphogenesis and photosynthetic physiology in developing fucoid embryos. We evaluated the effects of phytohormones to determine the extent, to which responses were similar to those of terrestrial plants. Kinetin, IAA, ABA, GA₃, and kinetin + IAA were added to seawater at a physiological concentration (1 mg/L), and embryos of Fucus vesiculosus L. were grown for 10 days. Photosynthetic activity of single embryos or embryo cells were characterized using the following fluorescence parameters: minimum fluorescence yield (F ₀), maximum quantum yield (F _v/F _m), relative maximum rate of electron transfer to photosystem II under saturation irradiances (rETR_max), photosynthetic efficiency under non-saturating irradiances (αETR) and saturation irradiance (E _k). In addition, embryo length and diameter and apical hair length and number were determined. Morphological changes associated with hormone treatments included an increase in the embryo length in the presence of IAA, an increase in the embryo diameter in the presence of IAA, kinetin, and kinetin + IAA, an increase in the maximum hair length and number in the presence of kinetin + IAA, and a decrease in the hair length and number in the presence of ABA. With respect to fluorescence parameters, significant effects of phytohormones included an increase in the F ₀ and F _v/F _m at kinetin treatment, a synergistic effect of kinetin + IAA on F _v/F _m, rETR_max, and αETR, a promotion of F _v/F _m by GA, and a decrease of the parameters by ABA. These results are consistent with the data on responses of land plants to the same hormones and suggest that brown algae have evolved regulatory mechanisms for morphogenesis and photosynthetic regulation similar to plants.     

The responses of photosynthetic gas exchange and chlorophyll a fluorescence to changes of irradiance and temperature in two species of Miscanthus

Photosynthetic CO₂ uptake and chlorophyll (Chl) a fluorescence of C₄ perennial grasses, Miscanthus floridulus (Labill) Warb and M. transmorrisonensis Hayata, from altitudes in central Taiwan of 390 and 2700 m, respectively, were studied at 10 and 25 °C to find if the species differ in their photosynthetic responses to a low temperature, and whether their photosystems 2 become more susceptible to the photoinhibition at low temperatures. For both species, the maximum photosynthetic rate (P_max) was reduced when the leaves were exposed to 10 °C. At irradiances higher than 400 µmol m^-2 s^-1, the values of F_v/F_m were reduced in both species at 10 °C but not at 25 °C, which indicated the photoinhibition at 10 °C. Reductions in P_max and the values of F_v/F_m at 10 °C were lesser in M. transmorrisonensis than in M. floridulus.     

USE OF PULSE‐AMPLITUDE‐MODULATED FLUORESCENCE TO ASSESS THE PHYSIOLOGICAL STATUS OF CLADOPHORA SP. ALONG A WATER QUALITY GRADIENT1

This study investigated the application of pulse‐amplitude‐modulated (PAM) fluorometry as a rapid assessment of benthic macroalgal physiological status. Maximum quantum efficiency (F_v/F_m), dark–light induction curves, and rapid fluorescence light‐response curves (RLC) were measured on the filamentous macroalgal Cladophora sp. from Lake Ontario on 5 d at 16 sites spanning a gradient of light and nutrient supply. For Cladophora sp. growing in situ, light limitation was assessed by comparing average daily irradiance with the light utilization efficiency parameter (α) derived from RLCs. In this study, there was a nonlinear relationship between F_v/F_m and the degree of P limitation in macroalgae. However, only light‐saturated Cladophora sp. showed a significant positive linear relationship between F_v/F_m and P nutrient status. The absence of this relationship among light‐limited algae indicates that their photosynthetic rate would be stimulated by increased water clarity, and not by increased P supply. PAM fluorescence measures were successfully able to identify light‐saturated macroalgae and, among these, assess the degree to which they were nutrient limited. These results enable us to test hypotheses arising from numeric models predicting the impact of changes in light penetration and nutrient supply on benthic primary production.     

Radiation Use Efficiency,Chlorophyll Fluorescence,and Reflectance Indices Associated with Ontogenic Changes in Water-Limited Chenopodium quinoa Leaves

Net photosynthetic rate, radiation use efficiency, chlorophyll (Chl) fluorescence, photochemical reflectance index (PRI), and leaf water potential were measured during a 25-d period of progressive water deficit in quinoa plants grown in a glasshouse in order to examine effects of water stress and ontogeny. All physiological parameters except F_v/F_m were sensitive to water stress. Ontogenic variations did not exist in F_v/F_m and leaf water potential, and were moderate to high in the other parameters. The complete recovery of photosynthetic parameters after re-irrigation was related with the stability in F_v/F_m. PRI showed significant correlation with predawn leaf water potential, F_m, and midday F_v/F_m. Thus PRI and Chl fluorescence may help in assessing physiological changes in quinoa plants across different developmental stages and water status.     

POTASSIUM ALLEVIATES THE INHIBITORY ACTION OF AMMONIUM UPON THE PHOTOSYNTHETIC RECOVERY OF NOSTOC FLAGELLIFORME (CYANOPHYCEAE) DURING REHYDRATION1

Effects of ammonium on the photosynthetic recovery of Nostoc flagelliforme Berk. et M. A. Curtis were assayed when being rehydrated in low‐K⁺ or high‐K⁺ medium. Its photosynthetic recovery was K⁺ limited after 3 years of dry storage. The potassium absorption of N. flagelliforme reached the maximum after 3 h rehydration in low‐K⁺ medium but at 5 min in high‐K⁺ medium. The K⁺ content of N. flagelliforme rehydrated in high‐K⁺ medium was much higher than that in low‐K⁺ medium. The maximal PSII quantum yield (F_v/F_m) value of N. flagelliforme decreased significantly when samples were rehydrated in low‐K⁺ medium treated with 5 mM NH₄Cl. However, the treatment of 20 mM NH₄Cl had little effect on its F_v/F_m value in high‐K⁺ medium. The relative F_v/F_m 24 h EC₅₀ (concentration at which 50% inhibition occurred) value of NH₄⁺ in high‐K⁺ medium (64.35 mM) was much higher than that in low‐K⁺ medium (22.17 mM). This finding indicated that high K⁺ could alleviate the inhibitory action of NH₄⁺ upon the photosynthetic recovery of N. flagelliforme during rehydration. In the presence of 10 mM tetraethylammonium chloride (TEACl), the relative F_v/F_m 24 h EC₅₀ value of NH₄⁺ was increased to 46.34 and 70.78 mM, respectively, in low‐K⁺ and high‐K⁺ media. This observation suggested that NH₄⁺ entered into N. flagelliforme cells via the K⁺ channel. Furthermore, NH₄⁺ could decrease K⁺ absorption in high‐K⁺ medium.     

The responses of photosynthetic gas exchange and chlorophyll a fluorescence to changes of irradiance and temperature in two species of Miscanthus

Photosynthetic CO₂ uptake and chlorophyll (Chl) a fluorescence of C₄ perennial grasses, Miscanthus floridulus (Labill) Warb and M. transmorrisonensis Hayata, from altitudes in central Taiwan of 390 and 2700 m, respectively, were studied at 10 and 25 °C to find if the species differ in their photosynthetic responses to a low temperature, and whether their photosystems 2 become more susceptible to the photoinhibition at low temperatures. For both species, the maximum photosynthetic rate (P_max) was reduced when the leaves were exposed to 10 °C. At irradiances higher than 400 μmol m^-2 s^-1, the values of F_v/F_m were reduced in both species at 10 °C but not at 25 °C, which indicated the photoinhibition at 10 °C. Reductions in P_max and the values of F_v/F_m at 10 °C were lesser in M. transmorrisonensis than in M. floridulus. This revised version was published online in June 2006 with corrections to the Cover Date.     

Silicon Alleviates Drought Stress of Rice Plants by Improving Plant Water Status,Photosynthesis and Mineral Nutrient Absorption

Drought is a major constraint for rice production in the rainfed lowlands in China. Silicon (Si) has been verified to play an important role in enhancing plant resistance to environmental stress. Two near-isogenic lines of rice (Oryza sativa L.), w-14 (drought susceptible) and w-20 (drought resistant), were selected to study the effects of exogenous Si application on the physiological traits and nutritional status of rice under drought stress. In wet conditions, Si supply had no effects on growth and physiological parameters of rice plants. Drought stress was found to reduce dry weight, root traits, water potential, photosynthetic parameters, basal quantum yield (F _v/F ₀), and maximum quantum efficiency of PSII photochemistry (F _v/F _m) in rice plants, while Si application significantly increased photosynthetic rate (Pr), transpiration rate (Tr), F _v/F ₀, and F _v/F _m of rice plants under drought stress. In addition, water stress increased K, Na, Ca, Mg, Fe content of rice plants, but Si treatment significantly reduced these nutrient level. These results suggested that silicon application was useful to increase drought resistance of rice through the enhancement of photochemical efficiency and adjustment of the mineral nutrient absorption in rice plants.     

Deschampsia antarctica Desv. primary photochemistry performs differently in plants grown in the field and laboratory

Primary photochemistry of photosystem II (F _v/F _m) of the Antarctic hair grass Deschampsia antarctica growing in the field (Robert Island, Maritime Antarctic) and in the laboratory was studied. Laboratory plants were grown at a photosynthetic photon flux density (PPFD) of 180 μmol m⁻² s⁻¹ and an optimal temperature (13 ± 1.5°C) for net photosynthesis. Subsequently, two groups of plants were exposed to low temperature (4 ± 1.5°C day/night) under two levels of PPFD (180 and 800 μmol m⁻² s⁻¹) and a control group was kept at 13 ± 1.5°C and PPFD of 800 μmol m⁻² s⁻¹. Chlorophyll fluorescence was measured during several days in field plants and weekly in the laboratory plants. Statistically significant differences were found in F _v/F _m (=0.75–0.83), F ₀ and F _m values of field plants over the measurement period between days with contrasting irradiances and temperature levels, suggesting that plants in the field show high photosynthetic efficiency. Laboratory plants under controlled conditions and exposed to low temperature under two light conditions showed significantly lower F _v/F _m and F _m. Moreover, they presented significantly less chlorophyll and carotenoid content than field plants. The differences in the performance of the photosynthetic apparatus between field- and laboratory-grown plants indicate that measurements performed in ex situ plants should be interpreted with caution.     

Induction of cyclic electron flow around photosystem 1 and state transition are correlated with salt tolerance in soybean

We investigated the role of cyclic electron flow around photosystem 1 (CEF1) and state transition (ST) in two soybean cultivars that differed in salt tolerance. The CEF1 and maximum photochemical efficiency (F_v/F_m) were determined under control and NaCl (50 mM) stress and the NaCl-induced light-harvesting complex 2 (LHC2) phosphorylation in vitro was analysed in light and dark. NaCl induced the increase of CEF1 more greatly in wild soybean Glycine cyrtoloba (cv. ACC547) than in cultivated soybean Glycine max (cv. Melrose). The F_v/F_m was reduced less in G. cyrtoloba than in G. max after 10-d NaCl stress. In G. cyrtoloba, the increase of CEF1 was associated with enhancement of LHC2 phosphorylation in thylakoid membrane under both dark and light. However, in G. max the NaCl treatment decreased the LHC2 phosphorylation. Treatment with photosynthetic electron flow inhibitors (DCMU, DBMIB) inhibited LHC2 phosphorylation more in G. max than in G. cyrtoloba. Thus the NaCl-induced up-regulation in CEF1 and ST might contribute to salt resistance of G. cyrtoloba.     

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

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

Leaf Photosynthesis of the Mangrove Avicennia Germinans as Affected by NaCl

In leaves of the mangrove species Avicennia germinans (L.) L. grown in salinities from 0 to 40 ‰, fluorescence, gas exchange, and δ¹³C analyses were done. Predawn values of F_v/F_m were about 0.75 in all the treatments suggesting that leaves did not suffer chronic photoinhibition. Conversely, midday F_v/F_m values decreased to about 0.55-0.60 which indicated strong down-regulation of photosynthesis in all treatments. Maximum photosynthetic rate (P _max) was 14.58 ± 0.22 µmol m^-2 s^-1 at 0 ‰ it decreased by 21 and 37 % in plants at salinities of 10 and 40 ‰, respectively. Stomatal conductance (g _s) was profoundly responsive in comparison to P _max which resulted in a high water use efficiency. This was further confirmed by δ¹³C values, which increased with salinity. From day 3, after salt was removed from the soil solution, P _max and g _s increased up to 13 and 30 %, respectively. However, the values were still considerably lower than those measured in plants grown without salt addition.     

Monitoring cashew seedlings during interactions with the fungus <Emphasis Type="Italic">Lasiodiplodia theobromae</Emphasis> using chlorophyll fluorescence imaging

The chlorophyll (Chl) fluorescence imaging technique was applied to cashew seedlings inoculated with the fungus Lasiodiplodia theobromae to assess any disturbances in the photosynthetic apparatus of the plants before the onset of visual symptoms. Two-month-old cashew plants were inoculated with mycelium of L. theobromae isolate Lt19 or Lt32. Dark-adapted and light-acclimated whole plants or previously labelled, single, mature leaf from each plant were evaluated weekly for Chl fluorescence parameters. From 21 to 28 days, inoculation with both isolates resulted in the significantly lower maximal photochemical quantum yield of PSII (F_v/F_m) than those for control samples, decreasing from values of 0.78 to 0.62. In contrast, the time response of the measured fluorescence transient curve from dark-acclimated plants increased in both whole plants and single mature leaves in inoculated plants compared with controls. The F_v/F_m images clearly exhibited photosynthetic perturbations 14 days after inoculation before any visual symptoms appeared. Additionally, decays in the effective quantum yield of PSII photochemistry and photochemical quenching coefficient were also observed over time. However, nonphotochemical quenching increased during the evaluation period. We conclude that F_v/F_m images are the effective way of detecting early metabolic perturbations in the photosynthetic apparatus of cashew seedlings caused by gummosis in both whole plants and single leaves and could be potentially employed in larger-scale screening systems.