高山植物唐古特山莨菪和唐古特大黄对强太阳辐射光能的利用和耗散特性

以西宁地区人工栽培的唐古特山莨菪(Anisodus tanguticus)和唐古特大黄(Rheum tanguticum)为材料,比较研究了两典型高山植物对青藏高原强太阳辐射光能的利用和耗散特性。PSⅡ反应中心的最大光化学效率(F_v/F_m)、实际光化学量子效率(Φ_PSⅡ)和光合功能的相对限制(L_(PDF))的分析表明,强太阳辐射会导致光合作用的光抑制,但并不造成PSⅡ反应中心的不可逆破坏。猝灭分析表明,唐古特山莨菪的光化学猝灭系数(q_P)显著小于唐古特大黄,非光化学猝灭(NPQ)和(q_N)则相反(p<0.05),意味着唐古特山莨菪将PSⅡ反应中心吸收的过剩光能以热耗散等非光化学过程消耗的能力大于唐古特大黄,因而降低了用于光化学反应的份额。q_N的3组分中,q_Nf所占比例较大;尽管相对份额很小,中午强光下两高山植物的q_Nm都有增大趋势,表明它在过剩光能的非光化学耗散中也起重要作用。NPQ_S和q_Ns的日变化趋势很相似;同样,NPQ_F为NPQ的主要组分,且唐古特山莨菪的NPQ_F和q_Nf都显著大于唐古特大黄(p<0.05)。唐古特山莨菪PSⅡ天线色素吸收光能中分配于光化学反应的相对份额(P)始终低于唐古特大黄,而用于天线热能耗散的相对份额(D)则大于唐古特大黄,两者都具有极显著差异(p<0.01)。以上结果表明,唐古特山莨菪的Φ_PSⅡ较唐古特大黄小是因为PSⅡ天线色素吸收的光能中分配于光化学反应的相对份额或光化学猝灭的比例较小,而分配于天线热耗散的相对份额或非光化学过程的比例较大的缘故。唐古特山莨菪的NPQ和q_N较大,与NPQ_F和q_Nf以及NPQ_S和q_Ns都显著大于唐古特大黄有关(p<0.05)。     

How to correctly determine the different chlorophyll fluorescence parameters and the chlorophyll fluorescence decrease ratio R<Subscript>Fd</Subscript> of leaves with the PAM fluorometer

This contribution is a practical guide to the measurement of the different chlorophyll (Chl) fluorescence parameters and gives examples of their development under high-irradiance stress. From the Chl fluorescence induction kinetics upon irradiation of dark-adapted leaves, measured with the PAM fluorometer, various Chl fluorescence parameters, ratios, and quenching coefficients can be determined, which provide information on the functionality of the photosystem 2 (PS2) and the photosynthetic apparatus. These are the parameters F_v, F_m, F₀, F_m′, F_v′, NF, and ΔF, the Chl fluorescence ratios F_v/F_m, F_v/F₀, ΔF/F_m′, as well as the photochemical (q_P) and non-photochemical quenching coefficients (q_N, q_CN, and NPQ). q_N consists of three components (q_N = q_E + q_T + q_I), the contribution of which can be determined via Chl fluorescence relaxation kinetics measured in the dark period after the induction kinetics. The above Chl fluorescence parameters and ratios, many of which are measured in the dark-adapted state of leaves, primarily provide information on the functionality of PS2. In fully developed green and dark-green leaves these Chl fluorescence parameters, measured at the upper adaxial leaf side, only reflect the Chl fluorescence of a small portion of the leaf chloroplasts of the green palisade parenchyma cells at the upper outer leaf half. Thus, PAM fluorometer measurements have to be performed at both leaf sides to obtain information on all chloroplasts of the whole leaf. Combined high irradiance (HI) and heat stress, applied at the upper leaf side, strongly reduced the quantum yield of the photochemical energy conversion at the upper leaf half to nearly zero, whereas the Chl fluorescence signals measured at the lower leaf side were not or only little affected. During this HL-stress treatment, q_N, q_CN, and NPQ increased in both leaf sides, but to a much higher extent at the lower compared to the upper leaf side. q_N was the best indicator for non-photochemical quenching even during a stronger HL-stress, whereas q_CN and NPQ decreased with progressive stress even though non-photochemical quenching still continued. It is strongly recommended to determine, in addition to the classical fluorescence parameters, via the PAM fluorometer also the Chl fluorescence decrease ratio R_Fd (F_d/F_s), which, when measured at saturation irradiance is directly correlated to the net CO₂ assimilation rate (_{P N}) of leaves. This R_Fd-ratio can be determined from the Chl fluorescence induction kinetics measured with the PAM fluorometer using continuous saturating light (cSL) during 4–5 min. As the R_Fd-values are fast measurable indicators correlating with the photosynthetic activity of whole leaves, they should always be determined via the PAM fluorometer parallel to the other Chl fluorescence coefficients and ratios.     

Possible roles of two quinone molecules in direct and indirect proton pumps of bovine heart NADH-quinone oxidoreductase (complex I)

In many energy transducing systems which couple electron and proton transport, for example, bacterial photosynthetic reaction center, cytochrome bc₁-complex (complex III) and E. coli quinol oxidase (cytochrome bo₃ complex), two protein-associated quinone molecules are known to work together. T. Ohnishi and her collaborators reported that two distinct semiquinone species also play important roles in NADH-ubiquinone oxidoreductase (complex I). They were called SQ_Nf (fast relaxing semiquinone) and SQ_Ns (slow relaxing semiquinone). It was proposed that Q_Nf serves as a “direct” proton carrier in the semiquinone-gated proton pump (Ohnishi and Salerno, FEBS Letters 579 (2005) 4555), while Q_Ns works as a converter between one-electron and two-electron transport processes. This communication presents a revised hypothesis in which Q_Nf plays a role in a “direct” redox-driven proton pump, while Q_Ns triggers an “indirect” conformation-driven proton pump. Q_Nf and Q_Ns together serve as (1e^?/2e^?) converter, for the transfer of reducing equivalent to the Q-pool.     

Changes in Chlorophyll Fluorescence in Maize Plants with Imposed Rapid Dehydration at Different Leaf Ages

A comparison of the effects of a rapidly imposed water deficit with different leaf ages on chlorophyll a fluorescence and gas exchange was performed in maize (Zea mays L.) plants. The relationships between photosynthesis and leaf relative turgidity (RT) and ion leakage were further investigated. Leaf dehydration substantially decreased net photosynthetic rate (A) and stomatal conductance (G _s), particularly for older leaves. With dehydration time, F _v /F _m maintained a relatively stable level for youngest leaves but significantly decreased for the older leaves. The electron transport rate (ETR) sharply decreased with intensifying dehydration and remained at lower levels during continuous dehydration. The photochemical quenching of variable chlorophyll fluorescence (q _P) gradually decreased with dehydration intensity for the older leaves but increased for the youngest leaves, whereas dehydration did not affect the nonphotochemical chlorophyll fluorescence quenching (NPQ) for the youngest leaves but remarkably decreased it for the older leaves. The leaf RT was significantly and positively correlated with its F _v /F _m, ETR, and q _P, and the leaf ion leakage was significantly and negatively correlated with F _v /F _m and NPQ. Our results suggest that the photosynthetic systems of young and old leaves decline at different rates when exposed to rapid dehydration.     

Photosynthesis in Drought-Adapted Cassava

After 45 d of limited water supply, cassava (Manihot esculenta Crantz) exhibited pronounced reduction in shoot growth, high leaf fall, and decreased stomatal conductance. However, the water status of the remaining leaves was unaffected. This was combined with an amplified heliotropic response and drooping which minimises radiant energy interception at mid-day, suggesting that leaves are sensitive to high irradiance (I). In well-irrigated plants, CO₂-saturated oxygen evolution and net photosynthetic rate (P _N) in air were markedly higher (5-fold) in young (expanding) leaves than in mature leaves. Water limitation did not strongly modify CO₂-saturated oxygen evolution but it altered P _N in air for both types of leaves, although differently. The mature leaves of drought-adapted plants displayed residual rate of P _N and deteriorated photosystem 2 (PS2) photochemistry estimated from chlorophyll (Chl) a fluorescence measurements. In young leaves at moderate I, P _N was depressed by only 66 % in stressed plants. Moreover, the photochemical quenching of Chl a fluorescence and the quantum efficiency of PS2 photochemistry in young leaves were comparable in both control and stressed plants. In contrast at high I, P _N was almost null and marked decreases in the two fluorescence parameters were apparent. Hence the strong heliotropic response and drooping displayed by young leaves under water limitation is an important strategy for avoiding inactivation of P _N by high I and therefore for cassava tolerance to drought.     

Characteristics of Photosynthetic Apparatus in Mn-Starved Maize Leaves

The effects of Mn-deficiency on CO₂ assimilation and excitation energy distribution were studied using Mn-starved maize leaves. Mn-deficiency caused about 70 % loss in the photon-saturated net photosynthetic rate (P _N) compared to control leaves. The loss of P _N was associated with a strong decrease in the activity of oxygen evolution complex (OEC) and the linear electron transport driven by photosystem 2 (PS2) in Mn-deficienct leaves. The photochemical quenching of PS2 (q_P) and the maximum efficiency of PS2 photochemistry (F_v/F_m) decreased significantly in Mn-starved leaves under high irradiance, implicating that serious photoinhibition took place. However, the high-energy fluorescence quenching (q_E) decreased, which was associated with xanthophyll cycle. The results showed that the pool of de-epoxidation components of the xanthophyll cycle was lowered markedly owing to Mn deficiency. Linear electron transport driven by PS2 de-creased significantly and was approximately 70 % lower in Mn-deficient leaves than that in control, indicating less trans-thylakoid pH gradient was built in Mn deficient leaves. We suggest that the decrease of non-radiative dissipation depending on xanthophyll cycle in Mn-starved leaves is a result of the deficiency of trans-thylakoid pH gradient.     

Short-term responses of photosynthetic membrane lipids and photochemical efficiency in plants of <Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> and <Emphasis Type="Italic">Vigna unguiculata</Emphasis> submitted to high irradiance

Primary leaves of young plants of common bean (Phaseolus vulgaris cv. Carioca and Negro Huasteco) and cowpea (Vigna unguiculata Walp cv. Epace 10) were exposed to high irradiance (HI) of 2 000 μmol m⁻² s⁻¹ for 10, 20, and 30 min. The initial fluorescence (F₀) was nearly constant in response to HI in each genotype except for Carioca. A distinct reduction of maximum fluorescence (F_m) was clearly observed in stressed genotypes of beans after 20 min followed by a slight recovery for the longer stress times. In common bean, the maximum quantum yield (F_v/F_m) was reduced slowly from 10 to 30 min of HI. In cowpea, only a slight reduction of F_v/F_m was observed at 20 min followed by recovery to normal values at 30 min. HI resulted in changes in the photochemical (q_P) and non-photochemical (q_N) quenching in both species, but to a different extent. In cowpea plants, more efficiency in the use of the absorbed energy under photoinhibitory conditions was related to increase in q_P and decrease in q_N. In addition, lipid peroxidation changed significantly in common bean genotypes with an evident increase after 20 min of HI. Hence the photosynthetic apparatus of cowpea was more tolerant to HI than that of common bean and the integrity of cowpea cell membranes was apparently maintained under HI.     

Effects of different nitrogen forms on photosynthetic rate and the chlorophyll fluorescence induction kinetics of flue-cured tobacco

Net photosynthetic rate (P _N) of tobacco plants grown with NH₄-N as the only N source was the lowest all the times, while P _N grown only with NO₃-N was the greatest until 22^nd day, and P _N grown with both NO₃-N and NH₄-N (1 : 1) was the greatest. Maximal photochemical efficiency of photosystem 2 (PS2), F_v/F_m, and actual quantum yield of PS2 under actinic irradiation (Φ_PS2) in plants grown with only NH₄-N were greatest at early stage and then decreased and were smaller than those of other treatments. Photochemical quenching coefficient (q_P) and non-photochemical quenching coefficient (q_NP) in the NH₄-N plants were the greatest at all times. Hence excessive NH₄-N can decrease not only photochemical efficiency but also the efficiency of utilization of photon energy absorbed by pigments for photosynthesis. Therefore, excessive NH₄-N is a hindrance to photosynthesis of flue-cured tobacco. On the other hand, tobacco cultured with an appropriate mixture of NO₃-N with NH₄-N can sufficiently utilize photon energy and increase the efficiency of energy transformation.     

Photoinhibition of photosystem II in vivo is preceded by down-regulation through light-induced acidification of the lumen: Consequences for the mechanism of photoinhibition in vivo

The mechanism of photoinhibition of photosystem II (PSII) was studied in intact leaf discs of Spinacia oleracea L. and detached leaves of Vigna unguiculata L. The leaf material was exposed to different photon flux densities (PFDs) for 100 min, while non-photochemical (q_N) and photochemical quenching (q_p) of chlorophyll fluorescence were monitored. The ‘energy’ and redox state of PSII were manipulated quite independently of the PFD by application of different temperatures (5–20° C), [CO₂] and [O₂] at different PFDs. A linear or curvilinear relationship between q_p and photoinhibition of PSII was observed. When [CO₂] and [O₂] were both low (30 μl · l^?1 and 2%, respectively), PSII was less susceptible at a given q_p than at ambient or higher [CO₂] and photoinhibition became only substantial when q_p decreased below 0.3. When high levels of energy-dependent quenching (q_E) (between 0.6 and 0.8) were reached, a further increase of the PFD or a further decrease of the metabolic demand for ATP and NADPH led to a shift from q_E to photoinhibitory quenching (q_I). This shift indicated that photoinhibition was preceded by down-regulation through light-induced acidification of the lumen. We propose that photoinhibition took place in the centers down-regulated by q_E. The shift from q_E to q_I occurred concomitant with q_P decreasing to zero. The results clearly show that photoinhibition does not primarily depend on the photon density in the antenna, but that photoinhibition depends on the energy state of the membrane in combination with the redox balance of PSII. The results are discussed with regard to the mechanism of photoinhibition of PSII, considering, in particular, effects of light-induced acidification on the donor side of PSII. Interestingly, cold-acclimation of spinach leaves did not significantly affect the relationship between q_P, q_E and photoinhibition of PSII at low temperature.     

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.     

Comparison of photosynthetic activity of <Emphasis Type="Italic">Orychophragmus violaceus</Emphasis> and oil-seed rape

Orychophragmus violaceus, Brassica campestris cv. Chuanyou No.8, and Brassica juncea cv. Luzhousileng diurnal changes of net photosynthetic rate (P _N) and activities of carbonic anhydrase (CA) of leaves were studied. One uni-modal curve occurred at the diurnal changes of P _N in O. violaceus, but bimodal curves were found in B. campestris and B. juncea. Thus photosynthetic midday depression was not found in O. violaceus but in both Brassica species. Midday depression of P _N in O. violaceus was not related to high temperature or low humidity at midday but to the activity of CA.     

Chilling-Induced Photoinhibition in Two Oak Species: Are Evergreen Leaves Inherently Better Protected Than Deciduous Leaves?

We compared the sensitivity to cold stress, in terms of photosynthetic capacity and changes in chlorophyll fluorescence of photosystem 2 (PS2), of an evergreen and a deciduous oak species, which co-occur in the southeastern United States. We predicted that the evergreen species, Quercus virginiana, which must endure winter, is likely to have an inherently greater capacity for energy dissipation and to be less susceptible to chilling stress than the deciduous species, Quercus michauxii. Short-term cold stress in both species lead to greater than 50 % reduction in maximum photosynthetic rates, 60-70 % reduction in electron transport, and irreversible quenching of PS2 fluorescence. The kinetics of recovery in the dark after exposure to 1 h high irradiance (1000 μmol m^-2 s^-1) and chilling (5 °C) showed that the evergreen Q. virginiana exhibited more protective q_E and less irreversible quenching (q_I) than the deciduous Q. michauxii. The large q_E which we observed in Q. virginiana suggests that the capacity for photoprotection at low temperatures is not induced by a long-term acclimation to cold but preexists in evergreen leaves. This capacity may contribute to the ability of this species to maintain leaves during the winter. This revised version was published online in June 2006 with corrections to the Cover Date.     

EPR characterization of ubisemiquinones and iron-sulfur cluster N2, central components of the energy coupling in the NADH-ubiquinone oxidoreductase (complex I) in situ

The proton-translocating NADH-ubiquinone oxidoreductase (complex I) is the largest and least understood respiratory complex. The intrinsic redox components (FMN and iron–sulfur clusters) reside in the promontory part of the complex. Ubiquinone is the most possible key player in proton-pumping reactions in the membrane part. Here we report the presence of three distinct semiquinone species in complex I in situ, showing widely different spin relaxation profiles. As our first approach, the semiquinone forms were trapped during the steady state NADH-ubiquinone-1 (Q₁) reactions in the tightly coupled, activated bovine heart submitochondrial particles, and were named SQ_Nf (fast-relaxing component), SQ_Ns (slow-relaxing), and SQ_Nx (very slow relaxing). This indicates the presence of at least three different quinone-binding sites in complex I. In the current study, special attention was placed on the SQ_Nf, because of its high sensitivities to and to specific complex I inhibitors (rotenone and piericidin A) in a unique manner. Rotenone inhibits the forward electron transfer reaction more strongly than the reverse reaction, while piericidine A inhibits both reactions with a similar potency. Rotenone quenched the SQ_Nf signal at a much lower concentration than that required to quench the slower relaxing components (SQ_Ns and SQ_Nx). A close correlation was shown between the line shape alteration of the g = 2.05 signal of the cluster N2 and the quenching of the SQ_Nf signal, using two different experimental approaches: (1) changing the poise by the oligomycin titration which decreases proton leak across the SMP membrane; (2) inhibiting the reverse electron transfer with different concentrations of rotenone. These new experimental results further strengthen our earlier proposal that a direct spin-coupling occurs between SQ_Nf and cluster N2. We discuss the implications of these findings in connection with the energy coupling mechanism in complex I.     

Use of simultaneous analysis of gas-exchange and chlorophyll fluorescence quenching for analysing the effects of water stress on photosynthesis in apple leaves

Summary A convenient system for the rapid simultaneous measurement of both chlorophyll fluorescence quenching using a modulated light system, and of CO₂, and water vapour exchange by leaves is described. The system was used in a study of the effects of water deficits on the photosynthesis by apple leaves (Malus x domestica Borkh.). Apple leaves were found to have low values of steady-state variable fluorescence, and the existence of significant fluorescence with open traps (F_o) quenching necessitated the measurement and use of a corrected F_o in the calculation of quenching components. Long-term water stress had a marked effect on both gas-exchange and chlorophyll fluorescence quenching. Non-photochemical quenching (qn) in particular was increased in water-stressed leaves, and it was particularly sensitive to incident radiation in such leaves. In contrast, rapid dehydration only affected gas exchange. Relaxation of qn quenching in the dark was slow, taking approximately 10 min for a 50% recovery, in well-watered and in draughted plants, and whether or not the plants had been exposed to high light.     

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     

Gas Exchange,Membrane Permeability,and Ion Uptake in Two Species of Indian Jujube Differing in Salt Tolerance

Effect of NaCl (electrical conductivity of 0, 5, 10, 15, and 20 dS m^–1) on growth, gas exchange, and ion uptake in two Ziziphus species (Z. rotundifolia and Z. nummularia) differing in salt tolerance was studied. At 30 and 45 d after first leaf initiation, the dry mass of shoot and leaves, and rates of net photosynthesis (P _N) and transpiration (E) decreased significantly with increasing NaCl concentration whereas membrane injury and accumulation of proline increased. The sodium content was highest in the roots of Z. rotundifolia and in the leaves of Z. nummularia. Potassium content did not differ much in the roots but it was significantly higher in the leaves of Z. rotundifolia at 30 and 45 d of observations. Thus both these species were tolerant to salinity but at high salinity Z. rotundifolia performed better owing to its higher P _N and E, restricted translocation of sodium from root to leaves, and larger accumulation of potassium in the leaves.     

Mechanical Wounding Caused by Inoculation Influences the Photosynthetic Response of Nicotiana benthamiana Plants to Plum Pox Potyvirus

Plants of Nicotiana benthamiana (Gray) (60 d old) were mechanically inoculated by a spreading of the fourth and fifth leaves with inoculum with or without plum pox potyvirus (PPV). Changes in growth parameters and selected photosynthetic characteristics were followed in control and inoculated plants in the locally affected leaves (LA) during 11 d after inoculation (DAI), in systemically affected leaves immature at time of inoculation (SAI) during 14–25 DAI, and in systemically affected leaves developed after the inoculation (SAD) during 28–39 DAI. The pure mechanical damage caused by inoculation induced a decrease in the net photosynthetic rate (P _N) in LA and SAD leaves, and an increase in the steady-state value of the non-photochemical chlorophyll (Chl) fluorescence quenching q_N. The q_N increase appeared in certain time intervals in all measured leaves on plants, so it could be regarded as indication of a systemic reaction of plant to the local mechanical injury. The viral infection developed in LA leaves and spread to SAI and SAD leaves was documented by the ELISA-DASI method. The plant height and area of SAI and SAD leaves were lower in infected plants. The combined effect of mechanical damage and viral infection caused a decrease in P _N only in LA and SAD leaves. In SAD leaves, an increased relative height of the J step (V_J) in the O-J-I-P Chl fluorescence transient together with a lower B/A band ratio of thermoluminescence glow curves reflected a damage to the acceptor side of photosystem 2 (PS2) caused by the viral infection, and a faster kinetics of the induction of the photochemical quenching coefficient q_P of Chl fluorescence indicated a faster Q_A ^– re-oxidation in the remaining undamaged centres of PS2.     

'Photoinhibition' of Heliconia under natural tropical conditions: the importance of leaf orientation for light interception and leaf temperature

The influence of irradiance on photosynthesis under natural conditions was studied in aseasonal Singapore using three Heliconia taxa: H. rostrata, H. psittacorum × H. spathocircinata cv. Golden Torch and H. psittacorum cv. Tay. When grown under full sunlight, all three heliconias exhibited reduced phatosynthetic capacities and lowered chlorophyll content per leaf area as compared with those grown under intermediate and deep shade. A marked decrease in the chlorophyll fluorescence F_v/Fm ratio and an increase in photochemical quenching (1- q_p) and non-photochemical quenching (q_N) were observed in upper leaves of plants grown under full sunlight. Increases in q_N suggest that ‘photoinhibition’ (decreases in F_v/F_m) in Heliconia grown under natural tropical conditions are probably due to photoprotective energy dissipation processes. The quantum yield, the maximum photosynthetic rate, F_v/F_m and the chlorophyll content of upper leaves were lower than those of lower leaves on the same plants grown under full sunlight. Similarly, lower values were obtained for the tip (sun) portion than for the base (shaded) portion of the leaves. The changes in F_v/F_m and in the levels of (1 –q_p) in leaves grown under intermediate and deep shade were negligible in plants during the course of day. However, there was a steep decrease in F_v/F_m and an increase in the levels of (1 –q_p), along with an increase in incident light in the sun leaves. The lowest F_v/F_m and the highest level of (1 –q_p) indicated minimum PSII efficiency at midday in full sun. These results indicate that, in Heliconia, the top leaves (particularly leaf tips) experienced sustained decreases in PSII efficiency upon exposure to full sunlight. Although all three taxa exhibited sustained decreases in photosynthetic capacity in full sunlight, the sun leaves of ‘Tay’ showed higher photosynthetic capacity than those of the other two taxa. This could be due, at least in part, to the vertical leaf angle and smaller lamina area. When the upright leaves of ‘Tay’ were constrained to a horizontal angle, they exhibited lower PSII efficiency (F_vIF_m ratio), while horizontal leaves of ‘Rostrata’ and ‘Golden Torch’ inclined lo near-vertical angles showed increased efficiency. Thus, an increase in leaf angle helps to achieve a reduction in the sustained decrease in PSII efficiency by decreasing the levels of incident sunlight and subsequently the leaf temperature.     

Method for resolution and quantification of components of the non-photochemical quenching (<Emphasis Type="Italic">q</Emphasis><Subscript>N</Subscript>)

A new method of the chlorophyll (Chl) a fluorescence quenching analysis is described, which allows the calculation of values of (at least) three components of the non-photochemical quenching of the variable Chl a fluorescence (q _N) using a non-linear regression of a multi-exponential function within experimental data. Formulae for coefficients of the “energy”-dependent (ΔpH-dependent) quenching (q _E), the state-transition quenching (q _T) and the photo/inhibitory quenching (q _I) of Chl a fluorescence were found on the basis of three assumptions: (i) the dark relaxation kinetics of q _N, as well as of all its components, is of an exponential nature, (ii) the superposition principle is valid for individual Chl a fluorescence quenching processes and (iii) the same reference fluorescence level (namely the maximum variable Chl a fluorescence yield in the dark-adapted state, F _V) is used to define both q _N and its components. All definitions as well as the algorithms for analytical recognition of the q _N components are theoretically clarified and experimentally tested. The described theory results in a rather simple equation allowing to compute values for all q _N components (q _E, q _T, q _I) as well as the half-times of relaxation (τ_1/2) of corresponding quenching processes. It is demonstrated that under the above assumptions it holds: q _N = q _E + q _T + q _I. The theoretically derived equations are tested, and the results obtained are discussed for non-stressed and stressed photosynthetically active samples. Semi-empirical formulae for a fast estimation of values of the q _N components from experimental data are also given.     

Chlorophyll a Fluorescence Analysis in Response to Excitation Irradiance in Bean Plants (Phaseolus vulgaris L. and Vigna unguiculata L. Walp) Submitted to High Temperature Stress

Bean plants Phaseolus vulgaris L. (cv. Carioca and Negro Huasteco) and Vigna unguiculata L. Walp (cv. Epace-10) were grown in a growth chamber with a photosynthetic photon flux density of 200 mol m^–2 s^–1 at leaf level and air temperature of 25+1 °C. Fully expanded, first pair leaves of 12-d-old plants were submitted for 90 min to high temperature (25, 30, 35, 40, 45, and 48 °C). Chlorophyll a fluorescence parameters (ETR, q_P, q_N, and F₀) were investigated using a modulated fluorimeter at 25 °C during recovery considered here as 48 h after stress induction period. An accentuated decrease in q_P and an increase in q_N at 48 °C in Carioca and Negro Huasteco was not observed in Epace-10. In response to excitation irradiance a great potential for ETR was found in Negro Huasteco at 25 °C, also demonstrated by net photosynthetic rate. At 48 °C ETR was high for Epace-10 while it was equal to zero for Carioca and Negro Huasteco. Tolerance to high temperature observed in Epace-10 provided important information about the adaptative characteristics of Vigna cultivars to warm climates.