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.     

Influence of Manganese Toxicity on Photosynthesis in Ricebean (Vigna Umbellata) Seedlings

Influence of manganese (Mn) toxicity on photosynthesis in ricebean (Vigna umbellata) was studied by the measurement of gas exchange characteristics and chlorophyll fluorescence parameters. The net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s) were reduced with increasing Mn concentration in nutrient solution. The reduction in g _s and E was more pronounced at 6 d of Mn treatment. However, P _N declined at 2 d of Mn treatment implying that the reduction in photosynthesis was not due to the direct effect of Mn on stomatal regulation. Mn did not affect the maximum efficiency of photosystem 2 (PS2) photochemistry (F_v/F_m). A reduction in photochemical quenching (q_P) and excitation capture efficiency of open PS2 (F_v′/F_m′) with a concomitant increase in q_N was observed. This implies that reduced demand for ATP and NADPH due to the reduction in photosynthesis causes a down-regulation of PS2 photochemistry and thus a high pH gradient (increase in q_N) and limited electron transport (decreased q_P). This revised version was published online in August 2006 with corrections to the Cover Date.     

High photosynthetic efficiency of a rice (Oryza sativa L.) xantha mutant

Comparative analysis revealed that a xantha rice mutant (cv. Huangyu B) had higher ratios of chlorophyll (Chl) a/b and carotenoids/Chl, and higher photosynthetic efficiency than its wild type parent (cv. II32 B). Unexpectedly, the mutant had higher net photosynthetic rate (P _N) than II32 B. This might have resulted from its lower non-photochemical quenching (q_N) but higher maximal photochemical efficiency (F_V/F_M), higher excitation energy capture efficiency of photosystem 2 (PS2) reaction centres (F_V′/F_M′), higher photochemical quenching (q_P), higher effective PS2 quantum yield (Φ_PS2), and higher non-cyclic electron transport rate (ETR). This is the first report of a chlorophyll mutant that has higher photosynthetic efficiency and main Chl fluorescence parameters than its wild type. This mutant could become a unique material both for the basic research on photosynthesis and for the development of high yielding rice cultivars.     

Changes in Chlorophyll Fluorescence During the Course of Photoperiod and in Response to Drought in Casuarina equisetifolia Forst. and Forst.

The effects of drought and the diurnal changes in photosynthetic electron transport were studied in non-nodulated plants of Casuarina equisetifolia. The induction of fluorescence showed a slightly higher I step in water-stressed than control plants, and the time from the start of irradiation to the P step of induction was significantly shortened by drought. The quantum efficiency of photosystem 2 (PS2) in the dark-adapted state (F_v/F_m) was generally not affected by drought, whereas it decreased during the central hours of the day. The decrease in quantum yield of PS2 electron transport (₂) in water-stressed plants was associated with decreases in the photochemical efficiency of open (oxidised) PS2 centres (F_v'/F_m') and increases in non-photochemical quenching (q_N) rather than with increased closure of PS2 centres (lowered photochemical quenching, q_P). In contrast, the changes in quantum yield of electron transport during the day were related to changes in q_P rather than in F_v'/F_m'. When chlorophyll fluorescence was measured at the same irradiance during the day, a greater q_N was observed at the end of the drying cycle than after watering, and early and late in the photoperiod than in the central hours of the day. The greater q_N at the beginning and end of the day did not prevent an increase in energy not used photochemically nor dissipated non-photochemically. Drought did not affect this excess of photon energy.     

Partitioning of photosynthetic electron flow between CO<Subscript>2</Subscript> assimilation and O<Subscript>2</Subscript> reduction in sunflower plants under water deficit

In sunflower (Helianthus annuus L.) grown under controlled conditions and subjected to drought by withholding watering, net photosynthetic rate (P _N) and stomatal conductance (g _s) of attached leaves decreased as leaf water potential (Ψ_w) declined from −0.3 to −2.9 MPa. Although g _s decreased over the whole range of Ψ_w, nearly constant values in the intercellular CO₂ concentrations (C _i) were observed as Ψ_w decreased to −1.8 MPa, but C _i increased as Ψ_w decreased further. Relative quantum yield, photochemical quenching, and the apparent quantum yield of photosynthesis decreased with water deficit, whereas non-photochemical quenching (q_NP) increased progressively. A highly significant negative relationship between q_NP and ATP content was observed. Water deficit did not alter the pyridine nucleotide concentration but decreased ATP content suggesting metabolic impairment. At a photon flux density of 550 μmol m⁻² s⁻¹, the allocation of electrons from photosystem (PS) 2 to O₂ reduction was increased by 51 %, while the allocation to CO₂ assimilation was diminished by 32 %, as Ψ_w declined from −0.3 to −2.9 MPa. A significant linear relationship between mean P _N and the rate of total linear electron transport was observed in well watered plants, the correlation becoming curvilinear when water deficit increased. The maximum quantum yield of PS2 was not affected by water deficit, whereas q_P declined only at very severe stress and the excess photon energy was dissipated by increasing q_NP indicating that a greater proportion of the energy was thermally dissipated. This accounted for the apparent down-regulation of PS2 and supported the protective role of q_NP against photoinhibition in sunflower.     

Low concentrations of NaHSO3 increase photosynthesis, biomass, and attenuate photoinhibition in Satsuma mandarin (Citrus unshiu Marc.) plants

Spraying low concentrated (0.5–5.0 mM) solutions of NaHSO₃ on Satsuma mandarin (Citrus unshiu Marc.) leaves resulted in enhancement (maximal about 15 % at 1 mM NaHSO₃) of net photosynthetic rate (P _N) for 6 d. The potential photochemical efficiency of photosystem 2 (PS2, F_v/F_m) and the quantum yield of PS2 electron transport (Φ_PS2) were increased under strong photon flux density (PFD). The slow phase of millisecond delayed light emission (ms-DLE) was increased, showing that the transmembrane proton motive force related to photophosphorylation was enhanced. We also observed that low concentrations of NaHSO₃ promoted the production of ATP in irradiated leaves. We suggest that the increase in P _N in Satsuma mandarin leaves caused by low concentrations of NaHSO₃ solution may have been due to the stimulation of photophosphorylation and, hence, the increase in photochemical efficiency through speeding-up of PS2 electron transport. Photoinhibition of photosynthesis in leaves was modified by NaHSO₃ treatment under high PFD. Hence the increase in leaf dry mass seems to be associated with the mitigation of photoinhibition caused by strong PFD.     

Chlorophyll Fluorescence Parameters: The Definitions,Photosynthetic Meaning,and Mutual Relationships

Chlorophyll fluorescence parameters (Chl FPs) derived from the slow (long-term) induction kinetics of modulated Chl a fluorescence are reviewed and analysed with respect to their application in photosynthesis research. Only four mutually independent Chl FPs, calculated from values of five essential Chl fluorescence (ChlF) yields, are distinguished as the basic ones. These are: the maximum quantum yield of PS2 photochemistry (_{P O}), the photochemical quenching of variable ChlF (q_P), the non-photochemical quenching of variable ChlF (q_N), and the relative change of minimum ChlF (q_O). _{P O} refers to the dark-adapted state of a thylakoid membrane, q_P, q_N and q_O characterise the light-adapted state. It is demonstrated that all other Chl FPs can be determined using this quartet of parameters. Moreover, three FPs related to the non-radiative energy dissipation within thylakoid membranes are evaluated, namely: the non-photochemical ChlF quenching (NPQ), the complete non-photochemical quenching of ChlF (q_CN), and the effective quantum yield of non-photochemical processes in PS2 (_N). New FPs, the total quenching of variable ChlF (q_TV) and the absolute quenching of ChlF (q_A) which allow to quantify co-action of the photochemical and non-photochemical processes during a light period are defined and analysed. The interpretation of Chl FPs and recommendations for their application in the photosynthesis research are also given. Some alternative FPs used in the laboratory practice have only an approximate character and can lead to incorrect conclusions if applied to stressed plants. They are reviewed and compared with the standard ones. All formulae and conclusions discussed herein are verified using experimental values obtained on young seedlings of the Norway spruce (Picea abies [L.] Karst.).     

Photosynthetic responses of radish (<Emphasis Type="Italic">Raphanus sativus</Emphasis> var. <Emphasis Type="Italic">longipinnatus</Emphasis>) plants to infection by turnip mosaic virus

Plant growth, chlorophyll (Chl) content, photosynthetic gas exchange, ribulose-1,5-bisphosphate carboxylase (RuBPCO) enzyme activity, and Chl fluorescence in radish (Raphanus sativus var. longipinnatus) plants were examined after turnip mosaic virus (TuMV) infection. Plant fresh mass, dry mass, Chl content, net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), and RuBPCO activity were significantly lower in infected plants after 5 weeks of virus infection as compared to healthy plants. The 5-week virus infection did not induce significant differences in intercellular CO₂ concentration (C _i, photochemical efficiency of photosystem 2, PS2 (F_v/F_m), excitation capture efficiency of open PS2 reaction centres (F_v'/F_m'), effective quantum efficiency of photosystem 2 (ΔF/F_m'), and photochemical quenching (q_P), but non-photochemical quenching (q_N) and alternative electron sink (AES) were significantly enhanced. Thus the decreased plant biomass of TuMV-infected plants might be associated with the decreased photosynthetic activity mainly due to reduced RuBPCO activity.     

Expression of the large isoform of ribulose-1,5-bisphosphate carboxylase/oxygenase activase gene driven by rbcS promoter in Oryza sativa enhances the photosynthetic capacity

In order to investigate the effect of large isoform of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activase (RuBPCO-A) on photosynthesis, cDNA of the enzyme (rca) was transferred to rice cultivars (Oryza sativa f. japonica cv. Nipponbare) under the control of RuBPCO small subunit gene promoter (rbcS) via Agrobacterium tumefaciens-mediated transformation. Transgenic rice plants were identified by polymerase chain reaction (PCR) and Southern and Western blot analyses. Net photosynthetic rate (P _N) values of the T₁ transgenic lines 34 (T34) and 40 (T40) were 45.26 and 46.32 % higher than that of the control plants, respectively. At the same time, their carboxylation efficiency and RuBPCO initial activity, quantum yield of electron transport in photosystem 2 (Φ_PS2), and steady state photochemical fluorescence quenching (q_P) increased. In addition, heading time of the transgenic rice was advanced. Thus increasing the amount of large isoform of RuBPCO-A in the transgenic rice might have a stimulatory effect on both photosynthesis and plant growth.     

Photosynthesis and non-photochemical excitation quenching components of chlorophyll excitation in maize and field bean during chilling at different photon flux density

The influence of chilling (8 °C, 5 d) at two photon flux densities [PFD, L = 200 and H = 500 μmol(photon) m⁻² s⁻¹] on the gas exchange and chlorophyll fluorescence was investigated in chilling-tolerant and chilling-sensitive maize hybrids (Zea mays L., K383×K130, K185×K217) and one cultivar of field bean (Vicia faba L. minor, cv. Nadwiślański). The net photosynthetic rate (P _N) for the both studied plant species was inhibited at 8 °C. P _N of both maize hybrids additionally decreased during chilling. Changes in the quantum efficiency of PS2 electron transport (Φ_PS2) as a response to chilling and PFD were similar to P _N. Measurements of Φ_PS2/Φ_CO2 ratio showed that in field bean seedlings strong alternative photochemical sinks of energy did not appear during chilling. However, the high increment in Φ_PS2/Φ_CO2 for maize hybrids can indicate reactions associated with chill damage generation. At 8 °C the non-photochemical quenching (NPQ) increased in all plants with chilling duration and PFD. The appearance of protective (q_I,p) and damage (q_I,d) components of q_I and a decrease in q_E (energy dependent quenching) took place. NPQ components of field bean and maize hybrids differed from each other. The amount of protective NPQ (q_E + q_I,p) components as part of total NPQ was higher in field bean than in maize hybrids at both PFD. On 5^th day of chilling, the sum of q_E and q_I,p was 26.7 % of NPQ in tolerant maize hybrids and 17.6 % of NPQ in the sensitive one (averages for both PFD). The increased PFD inhibited the ability of all plants to perform protective dissipation of absorbed energy. The understanding of the genotypic variation of NPQ components in maize may have implications for the future selection of plants with a high chilling tolerance.     

Photosystem 2 activity of <Emphasis Type="Italic">Citrus volkameriana</Emphasis> (L.) leaves as affected by Mn nutrition and irradiance

Citrus volkameriana (L.) plants were grown for 43 d in nutrient solutions containing 0, 2, 14, 98, or 686 μM Mn (Mn₀, Mn₂, Mn₁₄, Mn₉₈, and Mn₆₈₆, respectively). To adequately investigate the combined effects of Mn nutrition and irradiance on photosystem 2 (PS2) activity, irradiance response curves for electron transport rate (ETR), nonphotochemical quenching (q_N), photochemical quenching (q_P), and real photochemical efficiency of PS2 (Φ_PS2) were recorded under 10 different irradiances (66, 96, 136, 226, 336, 536, 811, 1 211, 1 911, and 3 111 μmol m⁻² s⁻¹, I₆₆ to I₃₁₁₁, respectively) generated with the PAM-2000 fluorometer. Leaf chlorophyll content was significantly lower under Mn excess (Mn₆₈₆) compared to Mn₀; its highest values were recorded in the treatments Mn₂-Mn₉₈. However, ETR and Φ_PS2 values were significantly lower under Mn₀ compared to the other Mn treatments, when plants were exposed to irradiances ≥96 μmol m⁻² s⁻¹. Furthermore, Mn₀ plants had significantly higher values of q_N and lower values of q_P at irradiances ≤226 and ≥336 μmol m⁻² s⁻¹, respectively, than those grown under Mn₂-Mn₆₈₆. Irrespective of Mn treatment, the values of Φ_PS2 and q_N decreased, while those of q_P increased progressively by increasing irradiance from I₁₃₆ to I₃₁₁₁. Finally, Mn₂-Mn₉₈ plants were less sensitive to photoinhibition of photosynthesis (≥811 μmol m⁻² s⁻¹) than the Mn₆₈₆ (≥536 μmol m⁻² s⁻¹) and Mn₀ (≥336 μmol m⁻² s⁻¹) ones.     

Chlorophyll fluorescence in micropropagated <Emphasis Type="Italic">Rhododendron ponticum</Emphasis> subsp. <Emphasis Type="Italic">baeticum</Emphasis> plants in response to different irradiances

The aim of this study was to investigate acclimation of micropropagated plants of Rhododendron ponticum subsp. baeticum to different irradiances and recovery after exposure to high irradiance. Plants grown under high (HL) or intermediate (IL) irradiances displayed higher values of maximum electron transport rate (ETR_max) and light saturation coefficient (E_k) than plants grown under low irradiance (LL). The capacity of tolerance to photoinhibition (as assessed by the response of photochemical quenching, q_p) varied as follows: HL > IL > LL. Thermal energy dissipation (q_N) was also affected by growth irradiance, with higher saturating values being observed in HL plants. Light-response curves suggested a gradual replacement of q_p by q_N with increasing irradiance. Following exposure to irradiance higher than 1500 μmol m⁻² s⁻¹, a prolonged reduction of the maximal photochemical efficiency of PS 2 (F_v/F_m) was observed in LL plants, indicating the occurrence of chronic photoinhibition. In contrary, the decrease in F_v/F_m was quickly reverted in HL plants, pointing to a reversible photoinhibition.     

Diurnal Changes of Gas Exchange,Chlorophyll Fluorescence,and Stomatal Aperture of Hybrid Poplar Clones Subjected to Midday Light Stress

Diurnal changes in net photosynthetic rate (P _N), chlorophyll (Chl) fluorescence, and stomatal aperture of several hybrid poplar clones subjected to midday light stress were measured in July and August of 1996. Midday depression of P _N, photosystem 2 (PS2) efficiency, stomatal conductance (g _s), and stomatal aperture was observed in all clones, though at differing rates among them. Non-uniform stomatal closure occurred at noon and at other times, requiring a modification of intercellular CO₂ concentration (C ₁). A linear relationship was found between g _s and stomatal aperture. More than half of the photons absorbed by PS2 centre dissipated thermally when subjected to light stress at noon. There was a linear relationship between the rate of PS2 photochemical electron transport (PxPFD) and P _N. There was a consensus for two fluorescence indicators (1 – q_P/q_N and (F_m' – F)/F_m') in assessment of susceptibility of photoinhibition in the clones. According to P _N, Chl fluorescence, and stomatal aperture, we conclude that midday depression of photosynthesis can be attributed to both stomatal and non-stomatal limitations.     

Suppression of nitrification and nitrous oxide emission by the tropical grass Brachiaria humidicola

Nitrification by soil nitrifiers may result in substantial losses of applied nitrogen through NO₃ ^– leaching and N₂O emission. The biological inhibition of nitrification by crop plants or pasture species is not well known. This study was conducted to evaluate the ability of three pasture species, Brachiaria humidicola, B. decumbens and Melinis minutiflora to inhibit nitrification. Plants were grown in a growth chamber for sixty days, fertilized with (NH₄)₂SO₄. After harvesting, the soil was incubated with (NH₄)₂SO₄ for 24 days. Ammonium oxidizing bacteria (AOB), NH₄-N levels, and N₂O emission were monitored at 4 d intervals. Among the species studied, B. humidicola inhibited nitrification and maintained NH₄-N in soil to a much greater extent than the other two species. This nitrification inhibition lasted for 12 days after initiation of soil incubation study (i.e. from 60 DAS when the plants were harvested). The AOB populations and N₂O emission from the soil were significantly lower in the soils where B. humidicola has been grown compared to the other two species. Root exudates and soil extracts of B. humidicola suppressed AOB populations, whereas those of B. decumbens and M. minutiflora did not. The results are in consistence with the hypothesis that B. humidicola suppressed nitrification and N₂O emissions through an inhibitory effect on the AOB population.     

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     

Effect of Cd on growth, photosynthetic gas exchange, and chlorophyll fluorescence of wild and Cd-sensitive mutant rice

Growth, photosynthetic gas exchange, and chlorophyll fluorescence characteristics were investigated in wild type (WT) and Cd-sensitive mutant rice (Oryza sativa L.) plants using 50 μM Cd treatment for 12 d followed by a 3-d recovery. Under Cd stress, net dry mass and pigment contents were significantly lower in the mutant plants than in the WT. The mutant had lower net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s) than WT rice, however, it had higher intercellular CO₂ concentration (C _i), indicating that non-stomatal factors accounted for the inhibition of P _N. Maximal photochemical efficiency of photosystem 2 (F_v/F_m), effective quantum yield of PS2 (Φ_PS2), and photochemical quenching (q_P) decreased much in the mutant under Cd stress. Cd content in roots and leaves of the mutant was significantly higher than those in the WT. Hence Cd toxicity was associated with the marked increases in Cd contents of plant tissue. After the recovery for 3 d, the WT rice had higher capacity to recover from Cd injury than the mutant.     

Effects of Ozone Fumigation on Photosynthesis and Membrane Permeability in Leaves of Spring Barley, Meadow Fescue, and Winter Rape

Seedlings of spring barley, meadow fescue, and winter rape were fumigated with 180 μg kg⁻¹ of ozone for 12 d, and effect of O₃ on photosynthesis and cell membrane permeability of fumigated plants was determined. Electrolyte leakage and chlorophyll fluorescence were measured after 6, 9, and 12 d of fumigation, while net photosynthetic rate (P _N) and stomatal conductance (g _s) were measured 9 d after the start of ozone exposure. O₃ treatment did not change membrane permeability in fescue and barley leaves, while in rape a significant decrease in ion leakage was noted within the whole experiment. O₃ did not change the photochemical efficiency of photosystem 2 (PS2), i.e., F_v/F_m, and the initial fluorescence (F₀). The values of half-rise time (t_1/2) from F₀ to maximal fluorescence (F_m) decreased in fescue and barley after 6 and 9 d of fumigation. P _N decreased significantly in ozonated plants, in the three species. The greatest decrease in P _N was observed in ozonated barley plants (17 % of the control). The ozone-induced decrease in P _N was due to the closure of stomata. Rape was more resistant to ozone than fescue or barley. Apparently, the rape plants show a large adaptation to ozone and prevent loss of membrane integrity leading to ion leakage. This revised version was published online in August 2006 with corrections to the Cover Date.     

Response of Photosynthetic Apparatus of Spring Barley (Hordeum vulgare L.) to Combined Effect of Elevated CO2 Concentration and Different Growth Irradiance

The response of barley (Hordeum vulgare L. cv. Akcent) to various photosynthetic photon flux densities (PPFDs) and elevated [CO₂] [700 μmol (CO₂) mol⁻¹; EC] was studied by gas exchange, chlorophyll (Chl) a fluorescence, and pigment analysis. In comparison with barley grown under ambient [CO₂] [350 μmol (CO₂) mol⁻¹; AC] the EC acclimation resulted in a decrease in photosynthetic capacity, reduced stomatal conductance, and decreased total Chl content. The extent of acclimation depression of photosynthesis, the most pronounced for the plants grown at 730 μmol m⁻² s⁻¹ (PPFD₇₃₀), may be related to the degree of sink-limitation. The increased non-radiative dissipation of absorbed photon energy for all EC plants corresponded to the higher de-epoxidation state of xanthophylls only for PPFD₇₃₀ barley. Further, a pronounced decrease in photosystem 2 (PS2) photochemical efficiency (given as F_V/F_M) for EC plants grown at 730 and 1 200 μmol m⁻² s⁻¹ in comparison with AC barley was related to the reduced epoxidation of antheraxanthin and zeaxanthin back to violaxanthin in darkness. Thus the EC conditions sensitise the photosynthetic apparatus of high-irradiance acclimated barley plants (particularly PPFD₇₃₀) to the photoinactivation of PS2. This revised version was published online in August 2006 with corrections to the Cover Date.     

Chlorophyll fluorescence as a tool for evaluation of drought stress in strawberry

The effect of water deficit on chlorophyll fluorescence, sugar content, and growth parameters of strawberry (Fragaria×ananassa Duch. cv. Elsanta) was studied. Drought stress caused significant reductions in leaf water potential, fresh and dry masses, leaf area, and leaf number. A gradual reduction of photochemical quenching (q_P) and quantum efficiency (Φ_PS2) was observed under drought stress while non-photochemical quenching (q_N) increased. Maximum efficiency of photosystem 2 (F_v/F_m) was not affected by drought stress.     

Effect of daily photon receipt on the susceptibility of dwarf bean (Phaseolus vulgaris L.) leaves to photoinhibition of photosynthesis

Bean (Phaseolus vulgaris L.) plants were grown at two light periods of 8 and 13 h with a similar photon flux density (PFD) giving a daily photon receipt (DPR) of 17.9 and 38.2 mol · m–2, respectively. Shoot growth and leaf area development were followed at regular intervals and diurnal whole-plant photosynthesis measured. Single mature trifoliate leaves were exposed to photoinhibitory treatments at PFDs of 800 and 1400 mol · m–2 · s–1 and at temperatures of 12 and 20°C. Chlorophyll fluorescence and photon yields were measured at regular intervals throughout each treatment. Plants grown in 13 h had significantly greater leaf areas than those grown in 8 h. There were no differences in maximum rates of photosynthesis, photon yields and only minor but significant differences in F_v/F_m for plants in the two treatments, showing photosynthetic characteristics were dependent on PFD but not DPR. A significant decline in photosynthesis and F_v/F_m occurred over the 13-h but little change in photosynthesis for plants in the 8 h, indicating some feedback inhibition of photosynthesis was occurring. Plants grown in 8 h were consistently more susceptible to photoinhibition of photosynthesis at all treatments than 13-h plants. Nevertheless, photoinhibition was exacerbated by increases in PFD, and by decreases in temperature for leaves from both treatments. However, for plants from the 8-h day, exposing leaves to 12°C and 1400 mol · m–2 · s–1 caused photo-oxidation and severe bleaching but no visible damage on leaves from 13-h-grown plants. Closure of the photosystem II reaction-centre pool was partially correlated with increasing extents of photoinhibition but the relationship was similar for plants from both treatments. There remains no clear explanation for their wide differences in susceptibility to photoinhibition.Abbreviations and Symbols DPR daily photon receipt - F₀ and F_m initial and maximal fluorescence - F_v/F_m fluorescence ratio in dark-treated leaves - F/F_m intrinsic efficiency of PSII during illumination - PFD photon flux density - _i photon yield (incident basis) - _psi quantum yield of PSII electron transport - P_max maximum rate of photosynthesis - q_N non-photochemical quenching coefficient - q_P photochemical quenching coefficient Many thanks to my colleague William Laing who spent a considerable effort in developing the programme to run the photosynthesis apparatus. I am also indebted to one reviewer with whom I corresponded to resolve some issues in the paper. This project was funded by the New Zealand Foundation for Research, Science and Technology.