Variations in light energy dissipation in <Emphasis Type="Italic">Woodfordia fruticosa</Emphasis> leaves during expansion

Young leaves of tropical trees frequently appear red in color, with the redness disappearing as the leaves mature. During leaf expansion, plants may employ photoprotective mechanisms to cope with high light intensities; however, the variations in anthocyanin contents, nonphotochemical quenching (NPQ), and photorespiration during leaf expansion are poorly understood. Here, we investigated pigment contents, gas exchange, and chlorophyll (Chl) fluorescence in Woodfordia fruticosa leaves during their expansion. Young red leaves had significantly lower Chl content than that of expanding or mature leaves, but they accumulated significantly higher anthocyanins and dissipated more excited light energy through NPQ. As the leaves matured, net photosynthetic rate, total electron flow through PSII, and electron flow for ribulose-1,5-bisphosphate oxygenation gradually increased. Our results provided evidence that photorespiration is of fundamental importance in regulating the photosynthetic electron flow and CO₂ assimilation during leaf expansion.     

Adaptation of the Photosynthetic Apparatus in Maize Leaves as a Result of Nitrogen Limitation : Relationships between Electron Transport and Carbon Assimilation

In maize (Zea mays L., cv Contessa), nitrogen (NO₃⁻) limitation resulted in a reduction in shoot growth and photosynthetic capacity and in an increase in the leaf zeaxanthin contents. Nitrogen deficiency had only a small effect on the quantum yield of CO₂ assimilation but a large effect on the light-saturated rate of photosynthesis. Linear relationships persisted between the quantum yield of CO₂ assimilation and that of photosystem II photochemistry in all circumstances. At high irradiances, large differences in photochemical quenching and nonphotochemical quenching of Chl a fluorescence as well as the ratio of variable to maximal fluorescence (Fv/Fm) were apparent between nitrogen-deficient plants and nitrogen-replete controls, whereas at low irradiances these parameters were comparable in all plants. Light intensity-dependent increases in nonphotochemical quenching were greatest in nitrogen-deficient plants as were the decreases in Fv/Fm ratio. In nitrogen-deficient plants, photochemical quenching decreased with increasing irradiance but remained higher than in controls at high irradiances. Thermal dissipative processes were enhanced as a result of nitrogen deficiency (nonphotochemical quenching was elevated and Fv/Fm was lowered) allowing PSII to remain relatively oxidised even when carbon metabolism was limited via nitrogen limitation.     

Photosynthetic performance and light response of two olive cultivars under different water and light regimes

The olive tree (Olea europaea L.) is commonly grown in the Mediterranean area, where it is adapted to resist periods characterized by severe drought and high irradiance levels. Photosynthetic efficiency (in terms of F_v/F_m and Φ_PSII), photochemical (q_P) and nonphotochemical quenching (NPQ) were determined in two-year-old olive plants (cultivars Coratina and Biancolilla) grown under two different light levels (exposed plants, EP, and shaded plants, SP) during a 21-day controlled water deficit. After reaching the maximum level of drought stress, plants were rewatered for 23 days. During the experimental period, measurements of gas exchange and chlorophyll (Chl) fluorescence were carried out to study the photosynthetic performance of olive plants. The synergical effect of drought stress and high irradiance levels caused a reduction of gas exchange and photosynthetic efficiency and these decreases were more marked in EP. EP showed a higher degree of photoinhibition, a higher NPQ and a lower q_P if compared to SP. Coratina was more sensitive to high light and drought stress but also showed a slower recovery during rewatering, whereas Biancolilla showed a less marked photosynthesis depression during drought and a considerable resilience during rewatering. The results confirm that photoinhibition due to high light intensity and water deficit can be an important factor that affects photosynthetic productivity in this species.     

CO2 response of cyclic electron flow around PSI (CEF-PSI) in tobacco leaves--relative electron fluxes through PSI and PSII determine the magnitude of non-photochemical quenching (NPQ) of Chl fluorescence

We hypothesized that cyclic electron flow around photosystem I (CEF-PSI) participates in the induction of non-photochemical quenching (NPQ) of chlorophyll (Chl) fluorescence when the rate of photosynthetic linear electron flow (LEF) is electron-acceptor limited. To test this hypothesis, the relationships among photosynthesis rate, electron fluxes through both PSI and PSII [Je(PSI) and Je(PSII)] and Chl fluorescence parameters were analyzed simultaneously in intact leaves of tobacco plants at several light intensities and partial pressures of ambient CO2 (Ca). At low light intensities, decreasing Ca lowered the photosynthesis rate, but Je(PSI) and Je(PSII) remained constant. Je(PSI) was larger than Je(PSII), indicating the existence of CEF-PSI. Increasing the light intensity enhanced photosynthesis and both Je(PSI) and Je (PSII). Je(PSI)/Je(PSII) also increased at high light and at high light and low Ca combined, showing a strong, positive relationship with NPQ of Chl fluorescence. These results indicated that CEF-PSI contributed to the dissipation of photon energy in excess of that consumed by photosynthesis by driving NPQ of Chl fluorescence. The main physiological function of CEF-PSI in photosynthesis of higher plants is discussed.     

Photosynthesis and protective mechanisms during ageing in transgenic tobacco leaves with over-expressed cytokinin oxidase/dehydrogenase and thus lowered cytokinin content

The content of cytokinins (CKs), the plant inhibitors of the final phase of plant development, senescence, is effectively controlled by irreversible degradation catalysed by cytokinin oxidase/dehydrogenase (CKX). In transgenic tobacco, denoted as AtCKX, with over-expressed CKX causing lowered CK content, we investigated changes in the time courses of chlorophyll (Chl) and xanthophyll (violaxanthin, antheraxanthin, zeaxanthin, neoxanthin, and lutein) contents. We also determined parameters of slow Chl fluorescence kinetics such as minimum Chl fluorescence yield in the darkadapted state F₀, maximum quantum yield of PS2 photochemistry (F_v/F_m), maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in photosystem 2 (PS2), F_v/F₀, non-photochemical quenching (NPQ), and effective quantum yield of photochemical energy conversion in PS2 (Φ₂). We used three different developmental leaf stages, old, mature, and young, and compared this with time courses of these characteristics in leaves with natural CK levels. The parameters F_v/F_m, F_v/F₀, and Φ₂ were unchanged during ageing in AtCKX plants in contrast to control ones where a significant decrease in old leaves was found. In control plants F₀ increased during ageing, but in the oldest leaf a considerable decrease was observed. This could indicate progressive damage to PS2 reaction centres and then detachment and rapid degradation of Chl. This is in agreement with time course of Chl content. NPQ decreased with age and was similar in both plant types. We observed a decline of xanthophyll contents in the oldest leaves in both plant types, but the contents were enhanced in AtCKX compared to control plants, especially of neoxanthin. The higher xanthophyll contents in the transgenic plants contribute to a better photoprotection and the fluorescence parameters indicated that photosynthetic apparatus was in better condition compared to control and it consequently postponed the onset of leaf senescence.     

Source-to-sink relationship between green leaves and green pseudobulbs of C<Subscript>3</Subscript> orchid in regulation of photosynthesis

In this paper, photosynthetic characteristics of green leaves (GL) and green pseudobulbs (GPSB) of C₃ orchid Oncidium Golden Wish were first studied. Light saturation for photosynthesis and maximum photosynthetic rates (P _max) were significantly higher in GL than in GPSB. The results of the optimal PSII quantum yield (F_v/F_m ratio), electron transport rate (ETR), the effective photochemical quantum yield (ΔF/F_m′) and nonphotochemical quenching (NPQ) of Chl fluorescence revealed that GPSB had lower light utilization than that of GL. Significantly higher photosynthetic pigments were found in GL than in GPSB. Alteration of source/sink ratio had no impact on all photosynthetic parameters for both GL and GPSB after a short term of 3 days or even a long term of 2 weeks of treatments although there were significant decreases in GL carbohydrate concentration of GL-darkened plants by the end of the day. However, decreases of all photosynthetic parameters of GL were observed in GL-darkened plants after 4 weeks of treatment compared to those of fully illuminated (FI) and GPSB-darkened plants. These results indicate that the level of carbohydrates in GL plays an important role in regulating their photosynthesis. Due to their lower photosynthetic capacities, GPSB function mainly as sinks. Darkening GPSB up to 2 weeks did not affect their own P _max and the P _max of GL and thus, did not result in significant decreases of total carbohydrate concentration of GPSB. As GPSB store a large amount of carbohydrates, it could also act as a source when the level of carbohydrates decreased. Thus, GL could depend on GPSB carbohydrates to regulate their photosynthesis when their source capacity was removed. However, 4 weeks after treatments, photosynthetic capacities of GL were significantly lower in GL- and GPSB-darkened plants than in FI plants, which could be due to the lower total soluble and insoluble sugar concentrations of both GL and GPSB in these plants.     

Changes in leaf photosynthesis of transgenic rice with silenced <Emphasis Type="Italic">OsBP-73</Emphasis> gene

In comparison with its wild type (WT), the transgenic (TG) rice with silenced OsBP-73 gene had significantly lower plant height, grain number per panicle, and leaf net photosynthetic rate (P _N). Also, the TG rice showed significantly lower chlorophyll (Chl), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), RuBPCO activase, and RuBP contents, photosystem 2 (PS2) photochemical efficiency (F_v/F_m and ΔF/F_m′), apparent quantum yield of carbon assimilation (Φ_c), carboxylation efficiency (CE), photosynthetic electron transport and photophosphorylation rates as well as sucrose phosphate synthase activity, but higher intercellular CO₂ concentration, sucrose, fructose, and glycerate 3-phosphate contents, and non-photochemical quenching of Chl fluorescence (NPQ). Thus the decreased P _N in the TG rice leaves is related to both RuBP carboxylation and RuBP regeneration limitations, and the latter is a predominant limitation to photosynthesis.     

Changes in photosynthesis, chlorophyll fluorescence, and antioxidant enzymes of mulberry (Morus ssp.) in response to salinity and high-temperature stress

Photosynthesis, chlorophyll (Chl) fluorescence, and antioxidant enzymes were measured in the mulberry (Morus spp.) cultivars Da 10, Hongguo 2, Anza 1, and Taiwan 72C002, which were subjected to salinity and high-temperature stress (STS; 0.1%, 0.3%, and 0.5% NaCl concentrations, 34.5°C–40.5°C/27.8°C–29.2°C day/night temperatures). Control plants were watered with 1 L of full-strength Hoagland’s nutrient solution with no added NaCl. Net photosynthetic rate (P _N), stomatal conductance (g _s), and effective quantum yield of photosystem II photochemistry (Φ_PSII) increased in Anza 1 and Taiwan 72C002 under 0.1% STS but decreased in Da 10 and Hongguo 2 compared with the control. However, all the above parameters, including Chl content, maximum quantum yield of photosystem II photochemistry (F_v/F_m), nonphotochemical quenching (NPQ), and maximum carboxylation velocity of Rubisco (V _cmax, decreased in Taiwan 72C002, Honggua 2, and Da 10 under 0.3% and 0.5% STS, suggesting that photoinhibition occurred under severe STS. Under STS, there were no significant changes in P _N, F_v/F_m, Φ_PSII, ascorbate peroxidase (APX) activity, superoxide dismutase (SOD) activity, catalase activity, superoxide anion radical (O ₂ ^? ) content, malondialdehyde (MDA) content, soluble sugar content (SSC), and leaf biomass in Anza 1 even at 0.5% STS, showing that Anza 1 displays high resistance to STS. In addition, peroxidase activity was significantly higher in Anza 1 than in the other mulberry cultivars. Significant adverse effects of severe salinity on photosynthesis and Chl fluorescence parameters were observed in Da 10. Additionally, SOD, peroxidase, and APX activities were lower in Da 10, whereas O ₂ ^? and MDA contents were higher in comparison with the other mulberry cultivars under 0.3% and 0.5% STS, suggesting that Da 10 had low resistance to STS. These results show that 0.1% STS had a positive effect on photosynthesis and Chl fluorescence parameters in Anza 1 and Taiwan 72C002, and higher peroxidase activity can to a certain extent explain the higher STS tolerance in Anza 1. Damages to DSM photosystems might be related to lower SOD, POD, and APX activities, which resulted in the accumulation of reactive oxygen species.     

Leaf gas exchange and chlorophyll a fluorescence in soybean leaves infected by Phakopsora pachyrhizi

Asian soybean rust (ASR), caused by Phakopsora pachyrhizi, is one of the most important foliar diseases affecting soybean production worldwide. This study aimed to investigate the photosynthetic performance (leaf gas exchange, chlorophyll (Chl) a fluorescence images and photosynthetic pigment pools) of soybean plants sprayed with Acibenzolar‐S‐Methyl (ASM) and the fungicide epoxiconazole + pyraclostrobin (Epo+Pyr) and further inoculated with P. pachyrhizi. The ASR symptoms progressed much faster on the leaves of plants from the control treatment (water spray) in comparison with the ASM and Epo+Pyr treatments. In general, the values for the leaf gas exchange parameters net carbon assimilation rate (A), stomatal conductance to water vapour (g_s), internal CO₂ concentration (C_i) and transpiration rate (E) increased for the infected plants sprayed with ASM or Epo+Pyr in comparison with plants from the control treatment. The values for the initial fluorescence (F_o), maximal fluorescence (F_m), maximal photosystem II quantum efficiency (F_v/F_m), effective photosystem II quantum yield (Y(II)) and quantum yield of regulated energy dissipation (Y(NPQ)) were consistently higher for the ASM and Epo+Pyr treatments in comparison with the control treatment at advanced stages of fungal infection. By contrast, the values for quantum yield of non‐regulated energy dissipation (Y(NO) were significantly lower for the ASM and Epo+Pyr treatments. The concentrations of total Chl a+b and carotenoids significantly increased for infected plants sprayed with ASM and Epo+Pyr in comparison with plants from the control treatment. The results of this study demonstrated that the spray of soybean plants with either ASM or Epo+Pyr contributed to reduce the negative effect of ASR on the photosynthesis of soybean plants.     

Effect of day/night temperature difference on chlorophyll content,photosynthesis and fluorescence parameters of tomato at fruit stage

In order to investigate the effect of day/night temperature difference (DIF) on photosynthetic characteristics of tomato plants (Solanum lycopersicum, cv. Jinguan 5) at fruit stage, an experiment was carried out in climate chambers. Five day/night temperature regimes (16/34, 19/31, 25/25, 31/19, and 34/16°C) with respective DIFs of -18, -12, 0, +12, and +18 were used and measured at mean daily temperature of 25°C. The results showed that chlorophyll (Chl) a, Chl b, net photosynthetic rate (P_N), stomatal conductance (g_s), maximum quantum yield of PSII photochemistry (F_v/F_m), effective quantum yield of PSII photochemistry (?_PSII), and photochemical quenching (qp) significantly increased under positive DIF, while they decreased with negative DIF. In contrast, the Chl a/b ratio and nonphotochemical quenching (NPQ) decreased under positive DIF, while increased with negative DIF. Chl a, Chl b, P_N, g_s, F_v/F_m, ?_PSII, and qp were larger under +12 DIF than those at +18 DIF, while Chl a/b and NPQ showed an opposite trend.     

Effects of light intensity on cyclic electron flow around PSI and its relationship to non-photochemical quenching of Chl fluorescence in tobacco leaves

We tested the hypothesis that plants grown under high light intensity (HL-plants) had a large activity of cyclic electron flow around PSI (CEF-PSI) compared with plants grown under low light (LL-plants). To evaluate the activity of CEF-PSI, the relationships between photosynthesis rate, quantum yields of both PSII and PSI, and Chl fluorescence parameters were analyzed simultaneously in intact leaves of tobacco plants which had been grown under different light intensities (150 and 1,100 micromol photons m(-2) s(-1), respectively) and with different amounts of nutrients supplied. HL-plants showed a larger value of non-photochemical quenching (NPQ) of Chl fluorescence at the limited activity of photosynthetic linear electron flow. Furthermore, HL-plants had a larger activity of CEF-PSI than LL-plants. These results suggested that HL-plants dissipated the excess photon energy through NPQ by enhancing the ability of CEF-PSI to induce acidification of the thylakoid lumen.     

Photosynthesis in different types of transgenic tobacco plants with elevated cytokinin content

Photosynthetic parameters were compared in three types of transgenic tobacco plants: ipt-transgenic plants with slightly elevated endogenous cytokinin (CK) content, Pssu-ipt-transgenic plants with markedly increased CK content, and zmp-transgenic plants with slightly elevated CK content accompanied by elevated auxin content. Slightly increased CK content promoted net photosynthetic rate (P_N) in both ipt- and zmp-transgenic plants, and chlorophyll (Chl) and carotenoid contents in zmp-transgenic plants. Morphology, growth characteristics, stomatal conductance, and parameters of Chl a fluorescence kinetics were similar in control and transgenic plants with slightly higher CK content. No significant effect of increased level of endogenous auxin (indole-3-acetic acid) on development of zmp-transgenic plants and measured parameters was found. Pssu-ipt-transgenic plants with highly increased CK content revealed suppressed root development, wilting of plants, and depression of P_N and stomatal conductance; however, Chl content was slightly increased and parameters of Chl a fluorescence kinetics did not indicate damage to photosynthetic apparatus. This revised version was published online in July 2006 with corrections to the Cover Date.     

Responses of photosynthetic parameters of <Emphasis Type="Italic">Mikania micrantha</Emphasis> and <Emphasis Type="Italic">Chromolaena odorata</Emphasis> to contrasting irradiance and soil moisture

Photosynthetic parameters were measured in two invasive weeds, Mikania micrantha and Chromolaena odorata, grown in soil under full, medium, and low irradiance and full, medium, and low water supply. Both species showed significantly higher net photosynthetic rate, quantum yield of PS 2 photochemistry and photochemical quenching coefficient under high than low irradiance. For M. micrantha, low irradiance caused decreased chlorophyll content (Chl), Chl a/b ratio and maximum photochemical efficiency of PS 2 (F_v/F_m), while drought decreased Chl content and F_v/F_m and increased nonphotochemical quenching (NPQ). However, these parameters were much less affected in C. odorata except that Chl content and NPQ slightly increased under drought and high irradiance. High irradiance increased xanthophyll pools in both species, especially M. micrantha under combination with drought.     

Closure of Stomata in Water-Stressed Pine Needles Results from the Decreased Water Potential of the Mesophyll Apoplast in the Substomatal Cavity

Pine (Pinus sylvestris L.) seedlings grown under controlled conditions were subjected to water deficit (external water potentials ranging from–0.15 to–1.5 MPa) by adding polyethylene glycol 6000 (PEG) to the nutrient solution. Following this treatment, the dry weights of plant shoots and roots, as well as the ratio of variable to maximum chlorophyll fluorescence (F_v/F_m), nonphotochemical quenching (NPQ) of chlorophyll excitations, photosynthetic CO₂/H₂O exchange, dark respiration of needles, and water potential of mesophyll apoplast in the substomatal cavity of pine needles, were measured. The imposed water deficit was followed by the inhibition of seedling growth, suppression of photosynthesis and transpiration, and by the decreased content of photosynthetic pigments. It is shown for the first time that the closure of stomata in the needles of water-stressed pine seedlings falls into the physiological reaction norm and is caused by the reduction of water potential in the mesophyll apoplast of the substomatal cavity.     

Effects of light quality on growth and development,photosynthetic characteristics and content of carbohydrates in tobacco (<Emphasis Type="Italic">Nicotiana tabacum</Emphasis> L.) plants

In this study, effects of yellow (Y), purple (P), red (R), blue (B), green (G), and white (W) light on growth and development of tobacco plants were evaluated. We showed that monochromatic light reduced the growth, net photosynthetic rate (P _N), stomatal conductance, intercellular CO₂, and transpiration rate of tobacco. Such a reduction in P _N occurred probably due to the stomatal limitation contrary to plants grown under W. Photochemical quenching coefficient (qP), maximal fluorescence of dark-adapted state, effective quantum yield of PSII photochemistry (Φ_PSII), and maximal quantum yield of PSII photochemistry (F_v/F_m) of plants decreased under all monochromatic illuminations. The decline in ΦPSII occurred mostly due to the reduction in qP. The increase in minimal fluorescence of dark-adapted state and the decrease in F_v/F_m indicated the damage or inactivation of the reaction center of PSII under monochromatic light. Plants under Y and G showed the maximal nonphotochemical quenching with minimum P _N compared with the W plants. Morphogenesis of plants was also affected by light quality. Under B light, plants exhibited smaller angles between stem and petiole, and the whole plants showed a compact type, while the angles increased under Y, P, R, and G and the plants were of an unconsolidated style. The total soluble sugar content increased significantly under B. The reducing sugar content increased under B but decreased significantly under R and G compared with W. In conclusion, different monochromatic light quality inhibited plants growth by reducing the activity of photosynthetic apparatus in plants. R and B light were more effective to drive photosynthesis and promote the plant growth, while Y and G light showed an suppression effect on plants growth. LEDs could be used as optimal light resources for plant cultivation in a greenhouse.     

Effect of Glomus mosseae on chlorophyll content, chlorophyll fluorescence parameters, and chloroplast ultrastructure of beach plum (Prunus maritima) under NaCl stress

The present study was undertaken to investigate the effect of Glomus mosseae on chlorophyll (Chl) content, Chl fluorescence parameters and chloroplast ultrastructure of beach plum seedlings under 2% NaCl stress. The results showed that compared to control, both Chl a and Chl b contents of NaCl + G. mosseae treatment were significantly lower during the salt stress, while Chl a/b ratio increased significantly. The increase of minimal fluorescence of darkadapted state (F₀), and the decrease of maximal fluorescence of dark-adapted state (F_m) and variable fluorescence (F_v) values were inhibited. The maximum quantum yield of PSII photochemistry (F_v/F_m), the maximum energy transformation potential of PSII photochemistry (F_v/F₀) and the effective quantum yield of PSII photochemistry (??_PSII) increased significantly, especially the latter two variables. The values of the photochemical quenching coefficient (q_P) and the nonphotochemical quenching (NPQ) were similar between G. mosseae inoculation and noninoculation. It could be concluded that G. mosseae inoculation could protect the photosystem II (PSII) of beach plum, enhance the efficiency of primary light energy conversion and improve the primitive response of photosynthesis under salinity stress. Meanwhile, G. mosseae inoculation was beneficial to maintain the integrity of thylakoid membrane and to protect the structure and function of chloroplast, which suggested that G. mosseae can alleviate the damage of NaCl stress to chloroplast.     

Imaging viral infection: studies on Nicotiana benthamiana plants infected with the pepper mild mottle tobamovirus

We have studied by kinetic Chl-fluorescence imaging (Chl-FI) Nicotiana benthamiana plants infected with the Italian strain of the pepper mild mottle tobamovirus (PMMoV-I). We have mapped leaf photosynthesis at different points of the fluorescence induction curve as well as at different post-infection times. Images of different fluorescence parameters were obtained to investigate which one could discriminate control from infected leaves in the absence of symptoms. The non-photochemical quenching (NPQ) of excess energy in photosystem II (PSII) seems to be the most adequate chlorophyll fluorescence parameter to assess the effect of tobamoviral infection on the chloroplast. Non-symptomatic mature leaves from inoculated plants displayed a very characteristic time-varying NPQ pattern. In addition, a correlation between NPQ amplification and virus localization by tissue-print was found, suggesting that an increase in the local NPQ values is associated with the areas invaded by the pathogen. Changes in chloroplast ultrastructure in non-symptomatic leaf areas showing different NPQ levels were also investigated. A gradient of ultrastructural modifications was observed among the different areas. The paper is dedicated to the memory of Prof. Dr López Gorgé (1935–2004)     

Analysis of Qualitative Contribution of Assimilatory and Non-Assimilatory De-Excitation Processes to Adaptation of Photosynthetic Apparatus of Barley Plants to High Irradiance

The adaptation of barley (Hordeum vulgare L. cv. Akcent) plants to low (LI, 50 µmol m^–2 s^–1) and high (HI, 1000 µmol m^–2 s^–1) growth irradiances was studied using the simultaneous measurements of the photosynthetic oxygen evolution and chlorophyll a (Chl a) fluorescence at room temperature. If measured under ambient CO₂ concentration, neither increase of the oxygen evolution rate (P) nor enhancement of non-radiative dissipation of the absorbed excitation energy within photosystem 2 (PS2) (determined as non-photochemical quenching of Chl a fluorescence, NPQ) were observed for HI plants compared with LI plants. Nevertheless, the HI plants exhibited a significantly higher proportion of Q_A in oxidised state (estimated from photochemical quenching of Chl a fluorescence, q_P), by 49–102 % at irradiances above 200 µmol m^–2 s^–1 and an about 1.5 fold increase of irradiance-saturated PS2 electron transport rate (ETR) as compared to LI plants. At high CO₂ concentration the degree of P stimulation was approximately three times higher for HI than for LI plants, and the irradiance-saturated P values at irradiances of 2 440 and 2 900 µmol m^–2 s^–1 were by 130 and 150 % higher for HI plants than for LI plants. We suggest that non-assimilatory electron transport dominates in the adaptation of the photosynthetic apparatus of barley grown at high irradiances under ambient CO₂ rather than an increased NPQ or an enhancement of irradiance-saturated photosynthesis.     

Photochemical and Thermal Phases of Chlorophyll a Fluorescence

The measurement of variable chlorophyll (Chl) a fluorescence is widely used as a convenient and versatile tool in photosynthesis research. In many applications empirical correlations and simplified models of Chl a fluorescence are used with success. Nevertheless, variable Chl a fluorescence provides only indirect and complex image of processes occurring within photosynthetic membranes and such simplifications have only limited validity. In this review we elucidate some controversial and still unresolved questions about the origin and interpretation of the variable Chl a fluorescence induction and the proper use of variable Chl a fluorescence for studies of photochemical events in photosystem 2 (PS2). Although the major part of variable Chl a fluorescence reflects the photochemical closure of the PS2 reaction centers (RCs) and can be considered as a function of the redox state of the primary acceptor Q_A, up to 50 % of the change in the Chl a fluorescence yield can be of secondary, nonphotochemical origin. We review the possible sources of the inherent heterogeneity in the origin of variable Chl a fluorescence. We also comment on the practical implications this bears for the use of variable Chl a fluorescence. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.