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.     

Diurnal changes in chlorophylla fluorescence and carotenoid composition inOpuntia ficus-indica,a CAM plant,and in three C3 species in Portugal during summer

Summary Diurnal changes in chlorophylla fluorescence were determined in four species, differing in life form, in Portugal during the summer of 1989. These includedOpuntia ficus-indica, a CAM plant, andHelianthus annuus, Ficus carica andArbutus unedo, three C₃ species. Steady state fluorescence yield,F _S, and maximum fluorescence yield,F _M′, were determined at different times of the day. Using the model of Genty et al. (1989), the photon use efficiency of photosystem II electron transport,φ _e, was calculated from (F _M′−F _S)/F _M′. Diurnal changes in relative rate of non-cyclic electron transport through photosystem II,J _e, were derived by multiplyingφ _e by the incident photon flux density (PFD). WhenJ _e, determined for each species for various points in time throughout the day, was plotted against corresponding values of PFD, the light response curves obtained showed thatJ _e was linearly dependent on PFD in low light and approached saturation in high light. The highest values ofJ _e were observed inHelianthus annuus, followed byOpuntia ficus-indica, Ficus carica andArbutus unedo. The proportion of the xanthophyll zeaxanthin to total carotenoids, determined around noon, was inversely related to maximum rates ofJ _e.     

The efficiency of electron transfer from QA − to the donor side of Photosystem II decreases during induction of photosynthesis: Evidences from chlorophyll fluorescence and photoacoustic techniques

The amplitudes ratio of the fast and slow phases (A_fast/A_slow) in the kinetics of the dark relaxation of variable chlorophyll fluorescence (F_V) was studied after various periods of illumination of dark-adapted primary barley leaves. Simultaneously, photosynthetic activity was monitored using the photoacoustic technique and the photochemical and non-photochemical fluorescence quenching parameters. The ratio A_fast/A_slow changed with the preceding illumination time in a two-step manner. During the first stage of photosynthetic induction (0–20 s of illumination), characterized by a drop in O₂-dependent photoacoustic signal following an initial spike and by a relatively stable small value of photochemical F_V quenching, the ratio A_fast/A_slow remained practically unaltered. During the second stage (20–60 s of illumination), when both the rate of O₂ evolution and the photochemical F_V quenching were found to be sharply developed, a marked increase in the above ratio was also observed. A linear correlation was found between the value of the photochemical quenching and the ratio A_fast/A_slow during the second phase of photosynthetic induction. It is concluded that the slow phase appearing in the kinetics of F_V dark relaxation is not due to the existence of Photosystem II reaction centres lacking the ability to reduce P700⁺ with high rates, but is instead related to the limitation of electron release from Photosystem I during the initial stage of the induction period of photosynthesis. This limitation keeps the intersystem electron carriers in the reduced state and thus increases the probability of back electron transfer from Q_A ^– to the donor side of Photosystem II.Abbreviations A_fast/A_slow the ratio of magnitudes between the fast and slow phases of dark relaxation of variable fluorescence - F_O initial level of chlorophyll fluorescence - F_V variable chlorophyll fluorescence (F-F_O) - (F_V)_S the yield of variable chlorophyll fluorescence under saturating pulse in illuminated leaves - (F_V)_M the yield of variable chlorophyll fluorescence under saturating pulse in dark-adapted leaves - PA photoacoustic - PSI Photosystem I - PS II Photosystem II - qN non-photochemical quenching - qQ photochemical quenching     

Effect of 24-Epibrassinolide on Chlorophyll Fluorescence and Photosynthetic CO<Subscript>2</Subscript> Assimilation in <Emphasis Type="Italic">Vicia faba</Emphasis> Plants Treated with the Photosynthesis-Inhibiting Herbicide Terbutryn

Simultaneous measurements of chlorophyll (Chl) fluorescence and CO₂ assimilation (A) in Vicia faba leaves were taken during the first weeks of growth to evaluate the protective effect of 24-epibrassinolide (EBR) against damage caused by the application of the herbicide terbutryn (Terb) at pre-emergence. V. faba seeds were incubated for 24 h in EBR solutions (2 × 10⁻⁶ or 2 × 10⁻⁵ mM) and immediately sown. Terb was applied at recommended doses (1.47 or 1.96 kg ha⁻¹) at pre-emergence. The highest dose of Terb strongly decreased CO₂ assimilation, the maximum quantum yield of PSII photochemistry in the dark-adapted state (F _V/F _M), the nonphotochemical quenching (NPQ), and the effective quantum yield (ΔF/F′_M) during the first 3–4 weeks after plant emergence. Moreover, Terb increased the basal quantum yield of nonphotochemical processes (F ₀/F _M)_, the degree of reaction center closure (1 − q _p), and the fraction of light absorbed in PSII antennae that was dissipated via thermal energy dissipation in the antennae (1 − F′_V/F′_M). The herbicide also significantly reduced plant growth at the end of the experiment as well as plant length, dry weight, and number of leaves. The application of EBR to V. faba seeds before sowing strongly diminished the effect of Terb on fluorescence parameters and CO₂ assimilation, which recovered 13 days after plant emergence and showed values similar to those of control plants. The protective effect of EBR on CO₂ assimilation was detected at a photosynthetic photon flux density (PFD) of 650 μmol m⁻² s⁻¹ and the effect on ΔF/F′_M and photosynthetic electron transport (J) was detected under actinic lightings up to 1750 μmol m⁻² s⁻¹. The highest dose of EBR also counteracted the decrease in plant growth caused by Terb, and plants registered the same growth values as controls.     

Changes of Donor and Acceptor Side in Photosystem 2 Complex Induced by Iron Deficiency in Attached Soybean and Maize Leaves

Photosynthesis in iron-deficient soybean and maize leaves decreased drastically. The quantum yield of photosystem 2 (PS2) electron transport (Φ_PS2), the efficiency of excitation energy capture by open PS2 reaction centres (F_v′/F_m′), and photochemical quenching coefficient (q_P) under high irradiance were lowered significantly by iron deficiency, but non-photochemical quenching (NPQ) increased markedly. The analysis of the polyphasic rise of fluorescence transient showed that iron depletion induced a pronounced K step both in soybean and maize leaves. The maximal quantum yield of PS2 photochemistry (Φ_po) decreased only slightly, however, the efficiency with which a trapped exciton can move an electron into the electron transport chain further than Q_A (Ψ₀) and the quantum yield of electron transport beyond Q_A (Ψ_Eo) in iron deficient leaves decreased more significantly compared with that in control. Thus not only the donor side but also the acceptor of PS2 was probably damaged in iron deficient soybean and maize leaves. This revised version was published online in August 2006 with corrections to the Cover Date.     

Within-canopy variation in the rate of development of photosynthetic capacity is proportional to integrated quantum flux density in temperate deciduous trees

The present study was undertaken to test for the hypothesis that the rate of development in the capacity for photosynthetic electron transport per unit area (J_max;A), and maximum carboxylase activity of Rubisco (V_cmax;A) is proportional to average integrated daily quantum flux density (Q_int) in a mixed deciduous forest dominated by the shade‐intolerant species Populus tremula L., and the shade‐tolerant species Tilia cordata Mill. We distinguished between the age‐dependent changes in net assimilation rates due to modifications in leaf dry mass per unit area (M_A), foliar nitrogen content per unit dry mass (N_M), and fractional partitioning of foliar nitrogen in the proteins of photosynthetic electron transport (F_B), Rubisco (F_R) and in light‐harvesting chlorophyll‐protein complexes (V_cmax;A ∝ M_AN_MF_R; J_max;A ∝ M_AN_MF_B). In both species, increases in J_max;A and V_cmax;A during leaf development were primarily determined by nitrogen allocation to growing leaves, increases in leaf nitrogen partitioning in photosynthetic machinery, and increases in M_A. Canopy differences in the rate of development of leaf photosynthetic capacity were mainly controlled by the rate of change in M_A. There was only small within‐canopy variation in the initial rate of biomass accumulation per unit Q_int (slope of M_A versus leaf age relationship per unit Q_int), suggesting that canopy differences in the rate of development of J_max;A and V_cmax;A are directly proportional to Q_int. Nevertheless, M_A, nitrogen, J_max;A and V_cmax;A of mature leaves were not proportional to Q_int because of a finite M_A in leaves immediately after bud‐burst (light‐independent component of M_A). M_A, leaf chlorophyll contents and chlorophyll : N ratio of mature leaves were best correlated with the integrated average quantum flux density during leaf development, suggesting that foliar photosynthetic apparatus, once developed, is not affected by day‐to‐day fluctuations in Q_int. However, for the upper canopy leaves of P. tremula and for the entire canopy of T. cordata, there was a continuous decline in N contents per unit dry mass in mature non‐senescent leaves on the order of 15–20% for a change of leaf age from 40 to 120 d, possibly manifesting nitrogen reallocation to bud formation. The decline in N contents led to similar decreases in leaf photosynthetic capacity and foliar chlorophyll contents. These data demonstrate that light‐dependent variation in the rate of developmental changes in M_A determines canopy differences in photosynthetic capacity, whereas foliar photosynthetic apparatus is essentially constant in fully developed leaves.     

Photorespiration and photoprotection of grapevine (<Emphasis Type="Italic">Vitis vinifera</Emphasis> L. cv. Cabernet Sauvignon) under water stress

In order to investigate the photoprotective function of photorespiration in grapevine under water stress, potted grapevines (Vitis vinifera L. cv. Cabernet Sauvignon) were randomly divided into three uniform groups for well-watered [watered every morning to keep the relative water content (RWC) of soil over 70 %], water-stress adapted (drought-adapted at 30 % relative soil water content for 30 days), and water stress without adaptation treatment (water-stressed to 30 % relative soil water content for 3 days). Net assimilation rate (A _N), stomatal conductance (g _s), substomatal CO₂ concentration (C _i), transpiration rate (E), actual photochemical efficiency of PSII (Φ_PSII), and maximum photochemical efficiency of PSII (F_v/F_m) were recorded by combining measurements of gas exchange and chlorophyll fluorescence. Gross photorespiration (P_r), photosynthetic electron partitioning (J_C/J_T), photochemical quenching coefficient (qP), and non-photochemical quenching (NPQ) were also calculated. The ratio of net assimilation rate to transpiration rate (A _N/E) was used as an indicator of water use efficiency (WUE). A _N, apparent P_r, Φ_PSII, F_v/F_m, qp, and g _s decreased, NPQ increased, and gross P_r sustained at a high level under water stress. This suggests that both photorespiration and energy dissipation play important roles in protecting photosynthetic apparatus against photoinhibition. C _i in water-stressed plants without adaptation treatment increased, which indicates the leaves suffered a non-stomatal limitation, while the water-stress adaped plants only suffered a stomatal limitation indicated by low C _i.     

Up-regulation of cyclic electron flow and down-regulation of linear electron flow in antisense-<Emphasis Type="Italic">rca</Emphasis> mutant rice

To investigate how excess excitation energy is dissipated in a ribulose-1,5-bisphospate carboxylase/oxygenase activase antisense transgenic rice with net photosynthetic rate (P _N) half of that of wild type parent, we measured the response curve of P _N to intercellular CO₂ concentration (C _i), electron transport rate (ETR), quantum yield of open photosystem 2 (PS2) reaction centres under irradiation (F_v′/F_m′), efficiency of total PS2 centres (Φ_PS2), photochemical (q_P) and non-photochemical quenching (NPQ), post-irradiation transient increase in chlorophyll (Chl) fluorescence (PITICF), and P700⁺ re-reduction. Carboxylation efficiency dependence on C _i, ETR at saturation irradiance, and F_v′/F_m′, Φ_PS2, and q_P under the irradiation were significantly lower in the mutant. However, NPQ, energy-dependent quenching (q_E), PITICF, and P700⁺ re-reduction were significantly higher in the mutant. Hence the mutant down-regulates linear ETR and stimulates cyclic electron flow around PS1, which may generate the ΔpH to support NPQ and q_E for dissipation of excess excitation energy.     

Cyclic electron flow around Photosystem II in vivo

The oxygen flash yield (Y_O2) and photochemical yield of PS II (_{PS II}) were simultaneously detected in intact Chlorella cells on a bare platinum oxygen rate electrode. The two yields were measured as a function of background irradiance in the steady-state and following a transition from light to darkness. During steady-state illumination at moderate irradiance levels, Y_O2 and _{PS II} followed each other, suggesting a close coupling between the oxidation of water and Q_A reduction (Falkowski et al. (1988) Biochim. Biophys. Acta 933: 432–443). Following a light-to-dark transition, however, the relationship between Q_A reduction and the fraction of PS II reaction centers capable of evolving O₂ became temporarily uncoupled. _{PS II} recovered to the preillumination levels within 5–10 s, while the Y_O2 required up to 60 s to recover under aerobic conditions. The recovery of Y_O2 was independent of the redox state of Q_A, but was accompanied by a 30% increase in the functional absorption cross-section of PS II (_{PS II}). The hysteresis between Y_O2 and the reduction of Q_A during the light-to-dark transition was dependent upon the reduction level of the plastoquinone pool and does not appear to be due to a direct radiative charge back-reaction, but rather is a consequence of a transient cyclic electron flow around PS II. The cycle is engaged in vivo only when the plastoquinone pool is reduced. Hence, the plastoquinone pool can act as a clutch that disconnects the oxygen evolution from photochemical charge separation in PS II.Abbreviations ADRY acceleration of the deactivation reactions of the water-splitting enzyme (agents) - Chl chlorophyll - cyt cytochrome - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_O minimum fluorescence yield in the dark-adapted state - F_I minimum fluorescence yield under ambient irradiance or during transition from the light-adapted state - F_M maximum fluorescence yield in the dark-adapted state - F_M maximum fluorescence yield under ambient irradiance or during transition from light-adapted state - F_V, F_V variable fluorescence (F_V=F_M–F_O ; F_V=F_M–F_I) - FRR fast repetition rate (fluorometer) - _{PS II} quantum yield of Q_A reduction (_{PS II}=(F_M – F_O)/F_M or _{PS II})=(F_M= – F_I=)/F_M=) - LHCII Chl a/b light harvesting complexes of Photosystem II - OEC oxygen evolving complex of PS II - P680 reaction center chlorophyll of PS II - PQ plastoquinone - POH₂ plastoquinol - PS I Photosystem I - PS II Photosystem II - RC II reaction centers of Photosystem II - PS II the effective absorption cross-section of PHotosystem II - TL thermoluminescence - Y_O2 oxygen flash yield The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Photoinhibition and xanthophyll cycle activity in bayberry (Myrica rubra) leaves induced by high irradiance

The effect of high irradiance (HI, photosynthetically active photon flux density of 1 300 μmol m⁻² s⁻¹) on net photosynthetic rate (P _N), chlorophyll fluorescence parameters, and xanthophyll cycle components were studied in fruit tree bayberry leaves. HI induced the photoinhibition and inactivation of photosystem 2 (PS2) reaction centres (RCs), which was characterized by decreased P _N, maximum yield of fluorescence after dark adaptation (F_m), photochemical efficiency of PS2 (F_v/F_m) and quantum yield of PS2 (Φ_PS2), and increased reduction state of Q_A (1-q_P) and non-photochemical quenching (NPQ). Initial fluorescence (F₀) showed a decrease after the first 2 h, and subsequently increased from the third hour exposure to HI. Furthermore, a greater increase in the ratio (F_i-F₀)/(F_p-F₀) which is an expression of the proportion of the Q_B non-reducing PS2 centres, whereas a remarked decrease in the slope of F_i to F_p which represents the rate of Q_A reduction was observed in leaves after HI exposure. Additionally, HI caused an increase in the pool size of the xanthophyll cycle pigments and sustained elevated contents of zeaxanthin (Z), antheraxanthin (A), and de-epoxidation state (DES) at the end of the irradiation period. During HI, decreased F_m, F_v/F_m, Φ_PS2, NPQ, slope of F_i to F_p, V+A+Z, and DES, and increased F₀, 1-q_P, ratio (F_i-F₀)/(F_p-F₀), and V were observed in dithiothreitol (DTT)-fed leaves compared to control ones under the same conditions. Hence photoinhibition caused by HI in bayberry was probably attributed to inactivation of PS2 RCs, and photoprotection from photodamage were mainly related to the xanthophyll cycle-dependent heat dissipation in excess photons.     

Photosynthesis of lichen symbiotic alga Trebouxia erici as affected by irradiance and osmotic stress

The relation between oxygen evolution rate (OER) and quantum yield of photochemical reactions in photosystem 2 (Φ_PS2) was examined in lichen symbiotic alga Trebouxia erici Ahmadjian (strain UTEX 911) exposed to different irradiances and osmotic stress (2 M sucrose for 60 h). Linear relationship was found between OER and Φ_PS2 in control cell suspension within irradiance range of 0 – 500 μmol m⁻² s⁻¹. Under osmotic stress, OER and Φ_PS2 were significantly reduced. Relation between OER and Φ_PS2 was curvilinear due to strong osmotically-induced inhibition of OER at high irradiance. The highest used irradiance (500 μmol m⁻² s⁻¹) was photoinhibitory for osmotically-stressed T. erici because non-photochemical quenching (NPQ) increased substantially. Energy-dependent quenching represented major part of NPQ increase. Osmotic stress led also to the reduction of capacity of photochemical processes in PS 2 (F_V/F_M) and increase in F₀/F_M. These changes indicated negative effects of osmoticum on structure and function of photosynthetic apparatus.     

Photosystem 2 photochemistry and pigment composition of a yellow mutant of rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) under different irradiances

A yellow leaf colouration mutant (named ycm) generated from rice T-DNA insertion lines was identified with less grana lamellae and low thylakoid membrane protein contents. At weak irradiance [50 μmol(photon) m⁻² s⁻¹], chlorophyll (Chl) contents of ycm were ≈20 % of those of WT and Chl a/b ratios were 3-fold that of wild type (WT). The leaf of ycm showed lower values in the actual photosystem 2 (PS2) efficiency (Φ_PS2), photochemical quenching (q_P), and the efficiency of excitation capture by open PS2 centres 1 (F_v′/F_m′) than those of WT, except no difference in the maximal efficiency of PS2 photochemistry (F_v/F_m). With progress in irradiance [100 and 200 μmol(photon) m⁻² s⁻¹], there was a change in the photosynthetic pigment stoichiometry. In ycm, the increase of total Chl contents and the decrease in Chl a/b ratio were observed. Φ_PS2, q_P, and F_v′/F_m′ of ycm increased gradually along with the increase of irradiance but still much less than in WT. The increase of xanthophyll ratio [(Z+A)/(V+A+Z)] associated with non-photochemical quenching (q_N) was found in ycm which suggested that ycm dissipated excess energy through the turnover of xanthophylls. No significant differences in pigment composition were observed in WT under various irradiances, except Chl a/b ratio that gradually decreased. Hence the ycm mutant developed much more tardily than WT, which was caused by low photon energy utilization independent of irradiance.     

Involvement of betacyanin in chilling-induced photoinhibition in leaves of <Emphasis Type="Italic">Suaeda salsa</Emphasis>

Seeds of Suaeda salsa were cultured in dark for 3 d and betacyanin accumulation in seedlings was promoted significantly. Then the seedlings with accumulated betacyanin (C+B) were transferred to 14/10 h light/dark and used for chilling treatment 15 d later. Photosystem 2 (PS2) photochemistry, D1 protein content, and xanthophyll cycle during the chilling-induced photoinhibition (exposed to 5 °C at a moderate photon flux density of 500 μmol m⁻² s⁻¹ for 3 h) and the subsequent restoration were compared between the C+B seedlings and the control (C) ones. The maximal efficiency of PS2 photochemistry (F_v/F_m), the efficiency of excitation energy capture by open PS2 centres (F_v′/F_m′), and the yield of PS2 electron transport (Φ_PS2) of the C+B and C leaves both decreased during photoinhibition. However, smaller decreases in F_v/F_m, F_v′/F_m′, and Φ_PS2 were observed in the C+B leaves than in C ones. At the same time, the deepoxidation state of xanthophyll cycle, indicated by (A+Z)/(V+A+Z) ratio, increased rapidly but the D1 protein content decreased considerably during the photoinhibition. The increase in rate of (A+Z)/(V+A+Z) was higher but the D1 protein turnover was slower in C+B than C leaves. After photoinhibition treatment, the plants were transferred to a dim irradiation (10 μmol m⁻² s⁻¹) at 25 °C for restoration. During restoration, the chlorophyll (Chl) fluorescence parameters, D1 protein content, and xanthophyll cycle components relaxed gradually, but the rate and level of restoration in the C+B leaves was greater than those in the C leaves. The addition of betacyanins to the thylakoid solution in vitro resulted in similar changes of F_v/F_m, D1 protein content, and (A+Z)/(V+A+Z) ratio during the chilling process. Therefore, betacyanin accumulation in S. salsa seedlings may result in higher resistance to photoinhibition, larger slowing down of D1 protein turnover, and enhancement of non-radiative energy dissipation associated with xanthophyll cycle, as well as in greater restoration after photoinhibition than in the control when subjected to chilling at moderate irradiance.     

Effect of light intensity on partitioning of photosynthetic electron transport to photorespiration in four subtropical forest plants

Photosynthetic rate (P_n) and the partitioning of noncyclic photosynthetic electron transport to photorespiration (J_O) in seedlings of four subtropical woody plants growing at three light intensities were studied in the summer time by measurements of chlorophyll fluorescence and CO₂ exchange. ExceptSchima superba, an upper canopy tree species, the tree speciesCastanopsis fissa and two understory shrubsPsychotria rubra, Ardisia quinquegona had the highestP _n at 36% of sunlight intensity. The total photosynthetic electron transport rate (J_F) and the ratio ofJ _O/J_F were elevated in leaves under full sunlight.J _O/J_F ratio reached 0.5–0.6 and coincided with the increasing of oxygenation rate of Rubisco (V_O), the activity of glycolate oxidase and photorespiration rate at full sunlight. It is suggested that an increasing partitioning proportion of photosynthetic electron transport to photorespiration might be one of the protective regulation mechanisms in forest plant under strong summer light and high temperature conditions.     

Effects of different irradiances on the photosynthetic process during ex-vitro acclimation of Anoectochilus plantlets

Six months old in vitro-grown Anoectochilus formosanus plantlets were transferred to ex-vitro acclimation under low irradiance, LI [60 μmol(photon) m⁻² s⁻¹], intermediate irradiance, II [180 μmol(photon) m⁻² s⁻¹], and high irradiance, HI [300 μmol(photon) m⁻² s⁻¹] for 30 d. Imposition of II led to a significant increase of chlorophyll (Chl) b content, rates of net photosynthesis (P _N) and transpiration (E), stomatal conductance (g _s), electron transfer rate (ETR), quantum yield of electron transport from water through photosystem 2 (Φ_PS2), and activity of ribulose-1,5-bisphosphate carboxylase/ oxygenase (RuBPCO, EC 4.1.1.39). This indicates that Anoectochilus was better acclimated at II compared to LI treatment. On the other hand, HI acclimation led to a significant reduction of Chl a and b, P _N, E, g _s, photochemical quenching, dark-adapted quantum efficiency of open PS2 centres (F_v/F_m), probability of an absorbed photon reaching an open PS2 reaction centre (F_v′/F_m′), ETR, Φ_PS2, and energy efficiency of CO₂ fixation (Φ_CO2/Φ_PS2). This indicates that HI treatment considerably exceeded the photo-protective capacity and Anoectochilus suffered HI induced damage to the photosynthetic apparatus. Imposition of HI significantly increased the contents of antheraxanthin and zeaxanthin (ZEA), non-photochemical quenching, and conversion of violaxanthin to ZEA. Thus Anoectochilus modifies its system to dissipate excess excitation energy and to protect the photosynthetic machinery.     

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.     

Effects of the interaction between ozone and carbon dioxide on gas exchange,photosystem II and antioxidants in rice leaves

To understand the interactive effects of O₃ and CO₂ on rice leaves; gas exchange, chlorophyll (Chl) fluorescence, ascorbic acid and glutathione were examined under acute (5 h), combined exposures of O₃ (0, 0.1, or 0.3 cm³ m⁻³, expressed as O₀, O_0.1, or O_0.3, respectively), and CO₂ (400 or 800 cm³ m⁻³, expressed as C⁴⁰⁰ or C⁸⁰⁰, respectively) in natural-light gas-exposure chambers. The net photosynthetic rate (P _N), maximum (F_v/F_m) and operating (F_q′/F_m′) quantum efficiencies of photosystem II (PSII) in young (8^th) leaves decreased during O₃ exposure. However, these were ameliorated by C⁸⁰⁰ and fully recovered within 3 d in clean air (O⁰ + C⁴⁰⁰) except for the O^0.3 + C⁴⁰⁰ plants. The maximum PSII efficiency at 1,500 μmol m⁻² s⁻¹ PPFD (F_v′/F_m′) for the O^0.3 + C₄₀₀ plants decreased for all measurement times, likely because leaves with severely inhibited P _N also had a severely damaged PSII. The P _N of the flag (16^th) leaves at heading decreased under O₃ exposure, but the decline was smaller and the recovery was faster than that of the 8^th leaves. The F_q′/F_m′ of the flag leaves in the O^0.3 + C⁴⁰⁰ and O^0.3 + C⁸⁰⁰ plants decreased just after gas exposure, but the F_v/F_m was not affected. These effects indicate that elevated CO₂ interactively ameliorated the inhibition of photosynthesis induced by O₃ exposure. However, changes in antioxidant levels did not explain the above interaction.     

Excess irradiance causes early symptoms of senescence during leaf expansion in photoautotrophically <Emphasis Type="Italic">in vitro</Emphasis> grown tobacco plants

Photosynthetic parameters, growth, and pigment contents were determined during expansion of the fourth leaf of in vitro photoautotrophically cultured Nicotiana tabacum L. plants at three irradiances [photosynthetically active radiation (400–700 nm): low, LI 60 μmol m⁻² s⁻¹; middle, MI 180 μmol m⁻² s⁻¹; and high, HI 270 μmol m⁻² s⁻¹]. During leaf expansion, several symptoms usually accompanying leaf senescence appeared very early in HI and then in MI plants. Symptoms of senescence in developing leaves were: decreasing chlorophyll (Chl) a+b content and Chl a/b ratio, decreasing both maximum (F_V/F_M) and actual (Φ_PS2) photochemical efficiency of photosystem 2, and increasing non-photochemical quenching. Nevertheless, net photosynthetic oxygen evolution rate (P _N) did not decrease consistently with decrease in Chl content, but exhibited a typical ontogenetic course with gradual increase. P _N reached its maximum before full leaf expansion and then tended to decline. Thus excess irradiance during in vitro cultivation did not cause early start of leaf senescence, but impaired photosynthetic performance and Chl content in leaves and changed their typical ontogenetic course.     

Photosynthesis,chlorophyll fluorescence,and antioxidant enzyme responses of invasive weed <Emphasis Type="Italic">Mikania micrantha</Emphasis> to <Emphasis Type="Italic">Bemisia tabaci</Emphasis> infestation

In a glasshouse, Bemisia tabaci infestation largely reduced response of photosynthesis to irradiance and CO₂ concentration of Mikania micrantha compared with the non-infested control (C) ones. The maximum irradiance-saturated photosynthetic rate (P _max) and saturation irradiance (SI) of the infested M. micrantha were only 21.3 % and 6.5 % of the C-plants, respectively. B. tabaci infestation led to the reduction of contents of chlorophyll and carotenoids in M. micrantha, which was accompanied with the decrease of actual photosystem 2 (PS2) efficiency (Φ_PS2), efficiency of excitation energy capture by open PS2 reaction centres (F_v′/F_m′), electron transport rate (ETR), and photochemical quenching (q_P). Moreover, superoxide dismutase and catalase activities significantly decreased while proline and glutathione contents significantly increased in infested M. micrantha. Hence B. tabaci infestation not only induced direct damage of photosynthetic apparatus but also altered the antioxidant enzymes activities in M. micrantha, which might as consequences accelerate senescence of this weed.     

Photosystem I-dependent cyclic electron transport is important in controlling Photosystem II activity in leaves under conditions of water stress

Leaves of the C₃ plant Brassica oleracea were illuminated with red and/or far-red light of different photon flux densities, with or without additional short pulses of high intensity red light, in air or in an atmosphere containing reduced levels of CO₂ and/or oxygen. In the absence of CO₂, far-red light increased light scattering, an indicator of the transthylakoid proton gradient, more than red light, although the red and far-red beams were balanced so as to excite Photosystem II to a comparable extent. On red background light, far-red supported a transthylakoid electrical field as indicated by the electrochromic P₅₁₅ signal. Reducing the oxygen content of the gas phase increased far-red induced light scattering and caused a secondary decrease in the small light scattering signal induced by red light. CO₂ inhibited the light-induced scattering responses irrespective of the mode of excitation. Short pulses of high intensity red light given to a background to red and/or far-red light induced appreciable additional light scattering after the flashes only, when CO₂ levels were decreased to or below the CO₂ compensation point, and when far-red background light was present. While pulse-induced light scattering increased, non-photochemical fluorescence quenching increased and F₀ fluorescence decreased indicating increased radiationless dissipation of excitation energy even when the quinone acceptor Q_A in the reaction center of Photosystem II was largely oxidized. The observations indicate that in the presence of proper redox poising of the chloroplast electron transport chain cyclic electron transport supports a transthylakoid proton gradient which is capable of controlling Photosystem II activity. The data are discussed in relation to protection of the photosynthetic apparatus against photoinactivation.Abbreviations F, F_M, F'_M, F"_M, F₀, F'₀ chlorophyll fluorescence levels - _exc quantum efficiency of excitation energy capture by open Photosystem II - _{PS II} quantum efficiency of electron flow through Photosystem II - P₅₁₅ field indicating rapid absorbance change peaking at 522 nm - P₇₀₀ primary donor of Photosystem I - Q_A primary quinone acceptor in Photosystem II - Q_N non-photochemical fluorescence quenching - Q_q photochemical quenching of chlorophyll fluorescence