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.     

Species-specific acclimation to strong shade modifies susceptibility of conifers to photoinhibition

Photoinhibitory processes in the photosynthetic apparatus of the seedlings of Abies alba (Mill.), Picea abies (Karst.), and Pinus mugo (Turra) growing under strong shade (5 % of full solar irradiance) or full irradiance conditions were investigated in winter and spring using chlorophyll a fluorescence techniques. The extent of photoinhibition in needles as indicated by a decrease in maximum quantum yield of PS II photochemistry (F_v/F_m) depended on species, air temperature and acclimation to the light environment. Unexpectedly, shade-tolerant Abies alba was less affected by low-temperature photoinhibition compared to the other species. F_v/F_m recovered with increasing air temperature. During winter, the seedlings of Picea abies growing in shade showed higher F_v/F_m than those from full light. Non-photochemical quenching of fluorescence (NPQ) measured at the same levels of actinic light was higher in needles acclimated to full light except for Abies alba in February. Photosynthetic performance in term of ETR (apparent electron transfer rate) was also higher in full light-acclimated needles. In April, at ambient temperature, recovery of PS II efficiency from the stress induced by illumination with saturating light was faster in the needles of Picea abies than in those of Abies alba. The shade-acclimated needles of Abies alba and Picea abies showed greater down-regulation of PS II induced by high light stress.     

Diurnal changes in chlorophyll fluorescence and light utilization in Colocasia esculenta leaves grown in marshy waterlogged area

Diurnal cycle of chlorophyll fluorescence parameters was done in Colocasia esculenta L. (swamp taro) grown in marshy land under sun or under shade. The sun leaves maintained higher electron transport rate (ETR) and steady state to initial fluorescence ratio (F_s/F₀) than shade leaves. In spite of lower ETR, higher photochemical quenching (PQ), and effective quantum yield of photosystem 2 (Φ_PS2) was evident in shade plants compared to plants exposed to higher irradiance. ETR increased linearly with increase in irradiance more under low irradiance (r ² = 0.84) compared to higher irradiance (r ² = 0.62). The maximum quantum yield of PS 2 (F_v/F_m) did not differ much in sun and shade leaves with the exception of midday when excess of light energy absorbed by plants under sun was thermally dissipated. Hence swamp taro plants adopted different strategies to utilize radiation under different irradiances. At higher irradiance, there was faster decline in proportion of open PS 2 centers (PQ) and excess light energy was dissipated through non-photochemical quenching (NPQ). Under shade, absorbed energy was effectively utilized resulting in higher Φ_PS2.     

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.     

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.     

Period four oscillations in chlorophyll a fluorescence

The discovery of period four oscillations of the fluorescence yield under flashing light demonstrated that not only the redox state of the Photosystem II (PS II) electron acceptor Q_A, but also the oxygen evolving cycle (described by the S states) modulates the fluorescence yield of chlorophyll (Chl). The positive charges accumulated on the donor side of PS II act on the fluorescence yield (measured in the Q_A ⁻ state during a strong flash) through the concentration of the quencher P₆₈₀ ⁺, the oxidized form of PS II reaction center Chl a. However, the period four oscillations of the fluorescence yield detected 1 s after a strong flash (in the P₆₈₀Q_A state) have not yet been fully explained. This revised version was published online in June 2006 with corrections to the Cover Date.     

光强转换对不同生长环境下桑树叶片光化学效率的影响

以桑树品种‘蒙古桑’为试验材料,利用叶绿素荧光技术研究了光强转换对生长在不同光强下的桑树叶片实际光化学效率(ΦPSⅡ)、电子传递速率(ETR)和非光化学淬灭(NPQ)的影响,分析了非光化学淬灭(NPQ)3个组分的变化.结果表明:当光强从黑暗或弱光转换到自然光条件下,自然光桑树叶片的光量子转化效率高于弱光叶片,ΦPSⅡ、ETR诱导平衡较快,NPQ诱导呈先升后降趋势.自然光叶片在强光下状态转换淬灭组分(qT)占NPQ的18%,而弱光叶片qT仅占NPQ的7%.与弱光桑树叶片相比,自然光桑树叶片可以通过较高的光量子转化效率和较强的调节激发能在PSⅠ和PSⅡ之间的分配能力来适应光强的变化.     

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.     

Very high light resistant mutants of Chlamydomonas reinhardtii: Responses of Photosystem II,nonphotochemical quenching and xanthophyll pigments to light and CO2

We have isolated very high light resistant nuclear mutants (VHL ^R) in Chlamydomonas reinhardtii, that grow in 1500–2000 mol photons m^–2 s^–1 (VHL) lethal to wildtype. Four nonallelic mutants have been characterized in terms of Photosystem II (PS II) function, nonphotochemical quenching (NPQ) and xanthophyll pigments in relation to acclimation and survival under light stress. In one class of VHL ^R mutants isolated from wild type (S4 and S9), VHL resistance was accompanied by slower PS II electron transfer, reduced connectivity between PS II centers and decreased PS II efficiency. These lesions in PS II function were already present in the herbicide resistant D1 mutant A251L (L ^*) from which another class of VHL ^R mutants (L4 and L30) were isolated, confirming that optimal PS II function was not critical for survival in very high light. Survival of all four VHL ^R mutants was independent of CO₂ availability, whereas photoprotective processes were not. The de-epoxidation state (DPS) of the xanthophyll cycle pigments in high light (HL, 600 mol photons m^–2 s^–1) was strongly depressed when all genotypes were grown in 5% CO₂. In S4 and S9 grown in air under HL and VHL, high DPS was well correlated with high NPQ. However when the same genotypes were grown in 5% CO₂, high DPS did not result in high NPQ, probably because high photosynthetic rates decreased thylakoid pH. Although high NPQ lowered the reduction state of PS II in air compared to 5% CO₂ at HL in wildtype, S4 and S9, this did not occur during growth of S4 and S9 in VHL. L ^* and VHL ^R mutants L4 and L30, also showed high DPS with low NPQ when grown air or 5% CO₂, possibly because they were unable to maintain sufficiently high pH due to constitutively impaired PS II electron transport. Although dissipation of excess photon energy through NPQ may contribute to VHL resistance, there is little evidence that the different genes conferring the VHL ^R phenotype affect this form of photoprotection. Rather, the decline of chlorophyll per biomass in all VHL ^R mutants grown under VHL suggests these genes may be involved in regulating antenna components and photosystem stoichiometries.This revised version was published online in October 2005 with corrections to the Cover Date.     

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 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.     

Acclimation to light in seedlings of <Emphasis Type="Italic">Quercus petraea</Emphasis> (Mattuschka) Liebl. and <Emphasis Type="Italic">Quercus pyrenaica</Emphasis> Willd. planted along a forest-edge gradient

Photosynthetic acclimation of two co-occurring deciduous oaks (Quercus petraea and Quercus pyrenaica) to a natural light gradient was studied during one growing season. In the spring of 2003, 90 seedlings per species were planted along a transect resulting from a dense Pinus sylvestris stand, an adjacent thinned area and a 10-m-wide firebreak (16.5–60.9% Global Site Factor (GSF)). In two dates of the following summer, we measured leaf gas exchange, carboxylation efficiency (CE), chlorophyll and nitrogen content, light–response curves of chlorophyll a fluorescence parameters, and leaf mass per area (LMA). Summer was mild, as evidenced by leaf predawn water potential (Ψ_pd), which reduced the interactive effect of water stress on the response of seedlings to light. Q. pyrenaica had higher LMA, CE, stomatal conductance (g _{s max}) and photosynthesis per unit area than Q. petraea at all growth irradiances. , LMA, g _{s max} and electron transport rate (ETR) all increased with light availability (GSF) in a similar fashion in both species. Light had also a clear effect on the organization of Photosystem II (PS II), as deduced by chlorophyll a fluorescence curves. Chlorophyll concentration (Chl_m) decreased with increasing light availability in Q. pyrenaica but it did not in Q. petraea. Seedlings of Q. petraea acclimated to higher irradiances showed a greater non-photochemical quenching (NPQ) than those of Q. pyrenaica. This suggests a higher susceptibility to high light in Q. petraea, which would be consistent with a better adaptation to shade, inferred from the lower LMA or the lower rate of photosynthesis.     

Chlorophyll a Fluorescence Response of Norway Spruce Needles to the Long-Term Effect of Elevated CO2 in Relation to Their Position Within the Canopy

The long-term impact of elevated CO₂ concentration on photosynthetic activity of sun-exposed (E) versus shaded (S) foliage was investigated in a Picea abies stand (age 12 years) after three years of cultivation in adjustable-lamella-domes (ALD). One ALD is supplied with either ambient air [ca. 350 µmol(CO₂) mol^–1; AC-variant) and the second with elevated CO₂ concentration [ambient plus 350 µmol(CO₂) mol^–1; EC-variant). The pronounced vertical profile of the photosynthetically active radiation (PAR) led to the typical differentiation of the photosynthetic apparatus between the S- and E-needles in the AC-variant estimated from the irradiance-responses of various parameters of the room temperature chlorophyll (Chl) a fluorescence parameters. Namely, electron transport rate (ETR), photochemical efficiency of photosystem 2, PS2 (_PS2), irradiance-saturated values of non-photochemical quenching of minimum (SV₀) and maximum (NPQ) fluorescence levels, and photochemical fluorescence quenching (q_p) at higher irradiances were all significantly higher for E-needles as compared with the S-ones. The prolonged exposure to EC did not cause any stimulation of ETR for the E-needles but a strongly positive effect of EC on ETR was observed for the S-needles resulting in more than doubled ETR capacity in comparison with S-needles from the AC-variant. For the E-needles in EC-variant a slightly steeper reduction of the _PS2 and q_p occurred with the increasing irradiance as compared to the E-needles of AC-variant. On the contrary, the S-needles in EC variant revealed a significantly greater capacity to maintain a high _PS2 at irradiances lower than 200 µmol m^–2 s^–1 and to prevent the over-reduction of the PS2 reaction centres. Moreover, compared to the AC-variant the relation between SV₀ and NPQ exhibited a strong decrease (up to 72 %) of the SV₀-NPQ slope for the E-needles and an increase (up to 76 %) of this value for the S-needles. Hence the E- and S-foliage responded differently to the long-term impact of EC. Moreover, this exposure was responsible for the smoothing of the PAR utilisation vertical gradient in PS2 photochemical and non-photochemical reactions within the canopy.     

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.     

Diurnal variation of gas exchange, chlorophyll fluorescence, and xanthophyll cycle components of maize hybrids released in different years

Diurnal variation of gas exchange, chlorophyll (Chl) fluorescence, and xanthophyll cycle components of three maize (Zea mays L.) hybrids released in different years, i.e. Baimaya (1950s), Zhongdan2 (1970s), and Nongda108 (1990s), were compared. On cloudless days, the newer hybrids always had higher net photosynthetic rate (P _N), especially at noon, than the older ones. At noon, all the hybrids decreased their maximal yield of photosystem 2 (PS2) photochemistry (F_v/F_m) and actual quantum yield of PS2 (Φ_PS2), the newer ones always showing higher values. Generally, the newer hybrids displayed higher photochemical quenching of Chl (q_P) and lower non-photochemical quenching (NPQ). The interhybrid differences in P _N may be owing to their differential photochemical efficiency. A midday depression in P _N occurred in all hybrids, which might be caused by serious photoinhibition or by decreased stomatal conductance. However, midday depression in P _N was more obvious in the older hybrids, especially when leaves were senescent. The higher de-epoxidation state of the xanthophylls was noted in older hybrids, which was confirmed by their larger NPQ. The newer maize hybrids did not need a strong de-epoxidation state since they had a better photosynthetic quantum conversion rate and a lower NPQ.     

Acclimation of the diatom Stephanodiscus neoastraea and the cyanobacterium Planktothrix agardhii to simulated natural light fluctuations

Functional and structural characteristics of the photosynthetic apparatus were studied in the diatom Stephanodiscus neoastraea and the cyanobacterium Planktothrix agardhii which were grown semi-continuously under constant irradiance or under simulated natural light fluctuations. The light fluctuations consisted of 24 oscillations of exponentially increasing and decreasing irradiance over a 12-h light period. Maximum irradiance was 1100 μmol photons m⁻² s⁻¹ with the ratio of maximum to minimum intensities being 100, simulating Langmuir circulations with a ratio of euphotic to mixing depth of 1. S. neoastraea acclimated to the light fluctuations by doubling the number and halving the size of photosynthetic units (PS II) while the amount of chlorophylls and carotenoids remained unchanged. The chlorophyll-specific maximum photosynthetic rate was enhanced while the slope of the photosynthesis versus irradiance curves was not influenced by the light fluctuations. Acclimation of P. agardhii was mainly characterized by an increase in chlorophyll content. Both photosystems showed only little changes in number and size. Maximum photosynthetic rate, saturating irradiance and initial slope of the photosynthesis versus irradiance curves did not vary. Although both high and low light were contained in the fluctuating light, an analogy to low or high light acclimation was not found for the diatom nor for the cyanobacterium acclimated to light fluctuations. We suggest that the acclimation to fluctuating light is a response type outside the known scheme of low and high light acclimation. This revised version was published online in June 2006 with corrections to the Cover Date.     

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

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

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.     

Diurnal cycle of chlorophyll fluorescence in <Emphasis Type="Italic">Phalaenopsis</Emphasis>

Chlorophyll (Chl) fluorescence of warm day/cool night temperature exposed Phalaenopsis plants was measured hourly during 48 h to study the simultaneous temperature and irradiance response of the photosynthetic physiology. The daily pattern of fluorescence kinetics showed abrupt changes of photochemical quenching (q_P), non-photochemical quenching (NPQ) and quantum yield of photosystem II electron transport (Φ_PSII) upon transition from day to night and vice versa. During the day, the course of Φ_PSII and NPQ was related to the air temperature pattern, while maximum quantum efficiency of PSII photochemistry (F_v/F_m) revealed a rather light dependent response. Information on these daily dynamics in fluorescence kinetics is important with respect to meaningful data collection and interpretation.     

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.