Ozone sensitivity and ethylenediurea protection in ash trees assessed by JIP chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence transient analysis

The effect of ethylenediurea (EDU) was tested using the chlorophyll (Chl) a fluorescence transient analysis, performed with JIP-test, to assess ambient ozone (O₃) effects on photosynthesis of adult trees under natural conditions. Twelve adult European ash (Fraxinus excelsior L.) trees, known to be sensitive or tolerant to O₃, determined by presence symptomatic (S) or absence asymptomatic (AS) trees of foliar symptoms in previous years, were treated either with distilled water containing 450 g m⁻³ EDU or with distilled water. Once a month across the growing season [the accumulated exposure over a threshold of 40 nmol(O₃) mol⁻¹ was 32.49 μmol mol⁻¹ h⁻¹], Chl a fluorescence transients were measured in vivo on dark-adapted leaves of 1-year-old labeled shoots, from the lower crown part. Twenty-five parameters were calculated. The maximum quantum yield of primary photochemistry (ϕ_Po or F_v/F_m) did not differentiate between S-and AS-trees, while increased Chl content and de-excitation rates suggested compensation of O₃ injury in S-trees. Seasonal reductions in absorbing fluxes and increase in heat and fluorescence dissipation processes was due to leaf ageing and drought, the latter suggesting water deficit influenced Chl a fluorescence stronger than ambient O₃ exposure. AS-trees showed elevated probability of connectivity among photosystem 2 units, a mechanism to stimulate energy dissipation and reduce photo-oxidative injury. EDU prevented the inactivation of reaction centers. This slight effect does not warrant EDU as a tool to assess O₃ effects on photosynthesis, while the JIP-test is suggested for a quantitative assessment in adult trees.     

Effect of gamma radiation on mutant induction of <Emphasis Type="Italic">Fagopyrum dibotrys</Emphasis> Hara

Agronomic traits, photosynthetic pigments, gas exchange, and chlorophyll (Chl) fluorescence parameters of red stem buckwheat (Fagopyrum dibotrys Hara) mutants induced by γ-radiation were compared with green control at seedling stage. Plant height, number of first-class branches, and rhizome biomass were inhibited significantly (p<0.01). Chl a, Chl b, and Chl a+b contents decreased with elevated dose of γ-rays, while increasing carotenoid content indicated that buckwheat was capable of adjusting to the radiation damage. Decrease in net photosynthetic rate was the result of both stomatal and non-stomatal limitations. Fluorescence parameters, such as F₀, F_m, F_v/F_m, F_v/F₀, Φ_PS2, electron transport rate, and photochemical quenching declined significantly (p<0.01) as compared with control due to photoinhibition, while non-photochemical quenching increased to enhance thermal dissipation. Lower parameters implied that leaf tissue was damaged significantly by high dose of γ-radiation and therefore leaf senescence was accelerated.     

Co<Subscript>2</Subscript> assimilation and chlorophyll fluorescence in green <Emphasis Type="Italic">versus</Emphasis> red <Emphasis Type="Italic">Berberis thunbergii</Emphasis> leaves measured with different quality irradiation

Photosynthesis, photorespiration, and chlorophyll (Chl) fluorescence in green and red Berberis thunbergii leaves were studied with two different measuring radiations, red (RR) and “white” (WR). The photosynthetic and photorespiration rates responded differently to the different radiation qualities, which indicate that the carboxylase and oxygenase activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) were affected. Differences in photosynthetic rate between the two color leaves were less under RR than under WR. However, this reduced difference in photosynthetic rate was not correlated with the stomatal response to the measuring radiation qualities. Compared with the WR, the RR reduced the differences in dark-adapted minimum and maximum fluorescence, steady-state fluorescence, light-adapted maximum fluorescence, and actual photochemical efficiency (Φ_PS2) of photosystem 2 (PS2), but enlarged the difference in non-photochemical quenching between the two color leaves. Differences in both maximum quantum yield of PS2 and ratio of Φ_PS2 to quantum yield of CO₂ fixation between the two color leaves were similar under the two measuring radiations. To exclude disturbance of radiation attenuation caused by anthocyanins, it is better to use RR to compare the photosynthesis and Chl fluorescence in green versus red leaves.     

Photosynthetic performance and acclimation of <Emphasis Type="Italic">Incarvillea delavayi</Emphasis> to water stress

The photosynthetic performance and related leaf traits of Incarvillea delavayi Bur. et Franch were studied at different water regimes to assess its capacity for photosynthetic acclimation to water stress. The initial response of I. delavayi to water stress was the closure of stomata, which resulted in down-regulation of photosynthesis. The stomatal limitation (S_L) represented the main component to photosynthetic limitations but non-stomatal limitation (NS_L) increased quickly with the increasing water stress, and had similar magnitude to SL under severe water stress (soil moisture 25–30 % of field capacity). Chlorophyll (Chl) a fluorescence parameters characterizing photosystem (PS) 2 photochemical efficiency (Φ_PS2), electron transport rate (J) and photochemical quenching (qP) decreased with the increasing water stress, indicating impaired photosynthetic apparatus. However, the water-stressed plants had a increased mesophyll CO₂ diffusional conductance, Chl a/b ratio, leaf nitrogen partitioning in RuBPCO and bioenergetics in later grown parts, indicating that I. delavay had a substantial physiological plasticity and showed a good tolerance to water stress.     

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

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

Elevated CO<Subscript>2</Subscript> reduces stomatal and metabolic limitations on photosynthesis caused by salinity in <Emphasis Type="Italic">Hordeum vulgare</Emphasis>

The future environment may be altered by high concentrations of salt in the soil and elevated [CO₂] in the atmosphere. These have opposite effects on photosynthesis. Generally, salt stress inhibits photosynthesis by stomatal and non-stomatal mechanisms; in contrast, elevated [CO₂] stimulates photosynthesis by increasing CO₂ availability in the Rubisco carboxylating site and by reducing photorespiration. However, few studies have focused on the interactive effects of these factors on photosynthesis. To elucidate this knowledge gap, we grew the barley plant, Hordeum vulgare (cv. Iranis), with and without salt stress at either ambient or elevated atmospheric [CO₂] (350 or 700 μmol mol⁻¹ CO₂, respectively). We measured growth, several photosynthetic and fluorescence parameters, and carbohydrate content. Under saline conditions, the photosynthetic rate decreased, mostly because of stomatal limitations. Increasing salinity progressively increased metabolic (photochemical and biochemical) limitation; this included an increase in non-photochemical quenching and a reduction in the PSII quantum yield. When salinity was combined with elevated CO₂, the rate of CO₂ diffusion to the carboxylating site increased, despite lower stomatal and internal conductance. The greater CO₂ availability increased the electron sink capacity, which alleviated the salt-induced metabolic limitations on the photosynthetic rate. Consequently, elevated CO₂ partially mitigated the saline effects on photosynthesis by maintaining favorable biochemistry and photochemistry in barley leaves.     

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

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

Effects of ozone exposure on growth and photosynthesis of the seedlings of <Emphasis Type="Italic">Liriodendron chinense</Emphasis> (Hemsl.) Sarg,a native tree species of subtropical China

Little is known about the response of trees to elevated ozone (O₃) in the subtropical region of China, where ambient O₃ concentrations are high enough to damage plants. In this study, pigment content, gas exchange and chlorophyll (Chl) a fluorescence in leaves of Liriodendron chinense (Hemsl.) Sarg seedlings, a deciduous broadleaf tree species native in subtropical regions, were investigated at 15, 40, and 58 days after O₃ fumigation (DAF) at a concentration of 150 mm³ m⁻³ (E-O₃). At the end of experiment, seedlings were harvested for biomass measurement. E-O₃ caused visible injuries on the mature leaves e.g. necrotic patches and accelerated early defoliation. Relative to the charcoal-filtered air (CF) treatment, E-O₃ significantly decreased shoot and root biomass, pigment content, light-saturated net photosynthesis (P _Nsat), stomatal conductance (g _s), maximum rate of carboxylation (V_cmax), photochemical quenching coefficient (q_p) and effective quantum yield of PSII photochemistry (Φ_PSII), and also caused a slight reduction in relative increase of basal diameter. Therefore, L. chinense can be assumed to be an O₃-sensitive tree species, which will be threatened by increasing ambient O₃ concentrations in China.     

Responses of leaf photosynthesis,pigments and chlorophyll fluorescence within canopy position in a boreal grass (<Emphasis Type="Italic">Phalaris arundinacea</Emphasis> L.) to elevated temperature and CO<Subscript>2</Subscript> under varying water regimes

The effects of elevated growth temperature (ambient + 3.5°C) and CO₂ (700 μmol mol⁻¹) on leaf photosynthesis, pigments and chlorophyll fluorescence of a boreal perennial grass (Phalaris arundinacea L.) under different water regimes (well watered to water shortage) were investigated. Layer-specific measurements were conducted on the top (younger leaf) and low (older leaf) canopy positions of the plants after anthesis. During the early development stages, elevated temperature enhanced the maximum rate of photosynthesis (P _max) of the top layer leaves and the aboveground biomass, which resulted in earlier senescence and lower photosynthesis and biomass at the later periods. At the stage of plant maturity, the content of chlorophyll (Chl), leaf nitrogen (N_L), and light response of effective photochemical efficiency (Φ_PSII) and electron transport rate (ETR) was significantly lower under elevated temperature than ambient temperature in leaves at both layers. CO₂ enrichment enhanced the photosynthesis but led to a decline of N_L and Chl content, as well as lower fluorescence parameters of Φ_PSII and ETR in leaves at both layers. In addition, the down-regulation by CO₂ elevation was significant at the low canopy position. Regardless of climate treatment, the water shortage had a strongly negative effect on the photosynthesis, biomass growth, and fluorescence parameters, particularly in the leaves from the low canopy position. Elevated temperature exacerbated the impact of water shortage, while CO₂ enrichment slightly alleviated the drought-induced adverse effects on P _max. We suggest that the light response of Φ_PSII and ETR, being more sensitive to leaf-age classes, reflect the photosynthetic responses to climatic treatments and drought stress better than the fluorescence parameters under dark adaptation.     

Drought effects on photosynthesis of the mangrove, Avicennia germinans, under contrasting salinities

 Drought effects on leaf photosynthesis of A. germinans growing under two contrasting salinities were studied in a Venezuelan fringe mangrove. During both wet and dry seasons, severe chronic-photoinhibition at predawn was not observed but strong down regulation occurred at midday during both seasons. Carbon assimilation rates (A, μmol CO₂ m⁻² s⁻¹) declined during the dry season from 11.9±1.8 to 7.0±1.5 and from 9.6±2.0 to 4.7±2.5 in plants from low and high salinity sites, respectively. Changes in carbon assimilation per unit of chlorophyll (A/Chl, mmol CO₂ mol⁻¹ Chl) were from 31.6±4.7 to 20.5±4.3 and from 21.9±4.7 to 15.2±8.2 in the low and high salinity plants, respectively. Therefore, neither changes in Chl nor seasonal differences in photoprotective down regulation could account fully for the decrease in leaf photosynthesis during drought. A reduction in CO₂ diffusion due to lowered stomatal conductance was not large enough to explain such a dramatic effect of drought on leaf photosynthesis. Stomatal response could be mitigated by the capability of A. germinans for osmotic adjustment under high salinity and/or drought. However, this intracellular salt accumulation may reduce carbon assimilation capacity further by decreasing the metabolism of leaf cells, increasing dark respiration and/or photorespiration. Received: 10 June 1998 / Accepted: 5 October 1998     

Photosynthetic characterization of Australian pitcher plant <Emphasis Type="Italic">Cephalotus follicularis</Emphasis>

Australian carnivorous pitcher plant Cephalotus follicularis Labill. produces two types of leaves. During the spring time, the plant produces a foliage type of noncarnivorous leaf called lamina. Later, the second type of leaf is produced — carnivorous pitcher. Using simultaneous measurements of gas exchange and chlorophyll (Chl) fluorescence photosynthetic efficiency of these two distinct forms of leaves were compared. In addition stomatal density, an important component of gas exchange, and Chl concentration were also determined. Pitcher trap had lower net photosynthetic rate (P _N) in comparison to noncarnivorous lamina, whereas the rate of respiration (R _D) was not significantly different. This was in accordance with lower stomatal density and Chl concentration in the pitcher trap. On the other hand maximum quantum yield of PSII (F_v/F_m) and effective quantum yield of photochemical energy conversion in PSII (Φ_PSII) was not significantly different. Nonphotochemical quenching (NPQ) was significantly higher in the lamina at higher irradiance. These data are in accordance with hypothesis that changing the leaf shape in carnivorous plants to make it a better trap generally makes it less efficient at photosynthesis. However, the pitcher of Cephalotus had much higher P _N than it was expected from the data set of the genus Nepenthes. Because it is not possible to optimize for contrasting function such as photosynthesis and carnivory, it is hypothesized that Cephalotus pitchers are less elaborated for carnivorous function than the pitchers of Nepenthes.     

Chlorophyll biosynthesis and chloroplast development in etiolated seedlings of <Emphasis Type="Italic">Ginkgo biloba</Emphasis> L.

Ginkgo biloba L. is a large tree native in China with evolutionary affinities to the conifers and cycads. However unlike conifers, the gymnosperm G. biloba is not able to synthesize chlorophyll (Chl) in the dark, in spite of the presence of genes encoding subunits of light-independent protochlorophyllide oxidoreductase (DPOR) in the plastid genome. The principal aims of the present study were to investigate the presence of DPOR protein subunits (ChlL, ChlN, ChlB) as well as the key regulatory step in Chl formation: aminolevulinic acid (ALA) synthesis and abundance of the key regulatory enzyme in its synthesis: glutamyl-tRNA reductase (GluTR). In addition, functional stage of photosynthetic apparatus and assembly of pigment-protein complexes were investigated. Dark-grown, illuminated and circadian-grown G. biloba seedlings were used in our experiments. Our results clearly showed that no protein subunits of DPOR were detected irrespective of light conditions, what is consistent with the absence of Chl and Chl-binding proteins (D1, LHCI, LHCIIb) in the dark. This correlates with low ALA-synthesizing capacity and low amount of GluTR. The concentration of protochlorophyllide (Pchlide) in the dark is low and non-photoactive form (Pchlide₆₃₃) was predominant. Plastids were developed as typical etioplasts with prollamelar body and few prothylakoid membranes. Continual illumination (24 h) only slightly stimulated ALA and Chl synthesis, although Pchlide content was reduced. Prollamelar bodies disappeared, but no grana were formed, what was consistent with the absence of D1, LHCI, LHCIIb proteins. Lightinduced development of photosynthetic apparatus is extremely slow, as indicated by Chl fluorescence and gas exchange measurements. Even after 72 h of continuous illumination, the values of maximum (F_v/F_m) and effective quantum yield (Φ_PSII) and rate of net photosynthesis (P _N) did not reach the values comparable with circadian-grown plants.     

Growth and photosynthetic responses of four landscape shrub species to elevated ozone

Attention should be paid to ozone (O₃) sensitivity of greening plant since ground-level O₃ concentrations are increasing especially in urban and suburban area. We studied the ecophysiological responses to elevated O₃ of four shrub species [Euonymus bungeanus Maxim. (EB), Photinia × fraseri (PF), Chionanthus retusus Lindl. & Paxt. (CR) and Cornus alba L. (CA)], which are often used for garden greening in China. Saplings of those species were exposed to high O₃ concentration (70 nmol mol⁻¹, 7 h d⁻¹ for 65 d) in open-top growth chambers. Responses to O₃ were assessed by gas exchanges, chlorophyll (Chl) fluorescence and dry mass. We found that elevated O₃ significantly decreased lightsaturated net photosynthetic rate (P _Nsat), transpiration rate (E) and stomatal conductance (g _s). The ratio of intercellular CO₂ to ambient CO₂ concentration (C _i/C _a) did not reduce under O₃ fumigation which suggested that the O₃-induced depressions of P _Nsat under O₃ fumigation were probably due to limitation of mesophyll processes rather than stomatal limitation. High O₃ exposure also significantly depressed the maximum efficiency of photosystem II (PSII) photochemistry in the dark-adapted state (F_v/F_m) which meant the O3-induced photoinhibition. Both root dry mass and root/shoot ratios were significantly decreased under ozone fumigation, but the total mass was unchanged. The responses of gas exchange such as P _Nsat in these four shrubs to O₃ exposure were species-specific. Highest loss of P _Nsat was observed in EB (−49.6%), while the CR had the lowest loss (−36.5%). Moreover, the O₃-exposed CR showed similar g _s as CF, reflecting that its O₃ flux might be unchanged under elevated O₃ environment. Ozone drastically decreased actual quantum yield of PSII (Φ_PSII) and electron transport rate (ETR) in EB while increased Φ_PSII and ETR in CR. Furthermore, the relative losses in P _Nsat positively correlated with the relative decreases in Φ_PSII and ETR which indicated that the impairment of photosynthesis was probably affected by the light reaction process. The light reaction of EB was impaired most seriously but that of CR was not damaged. All results indicated that EB was probably the most sensitive shrub species to O₃ while CR the most tolerant one. Therefore, CR might be an ideal choice for greening in ozone-polluted areas.     

Elevated CO<Subscript>2</Subscript> and temperature differentially affect photosynthesis and resource allocation in flag and penultimate leaves of wheat

Differences in acclimation to elevated growth CO₂ (700 μmol mol⁻¹, EC) and elevated temperature (ambient +4 °C, ET) in successive leaves of wheat were investigated in field chambers. At a common measurement CO₂, EC increased photosynthesis and the quantum yield of electron transport (Φ) early on in the growth of penultimate leaves, and later decreased them. In contrast, EC did not change photosynthesis, and increased Φ at later growth stages in the flag leaf. Contents of chlorophyll (Chl), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), and total soluble protein were initially higher and subsequently lower in penultimate than flag leaves. EC decreased RuBPCO protein content relative to soluble protein and Chl contents throughout the development of penultimate leaves. On the other hand, EC initially increased the RuBPCO:Chl and Chl a/b ratios, but later decreased them in flag leaves. In the flag leaves but not in the penultimate leaves, ET initially decreased initial and specific RuBPCO activities at ambient CO₂ (AC) and increased them at EC. Late in leaf growth, ET decreased Chl contents under AC in both kinds of leaves, and had no effect or a positive one under EC. Thus the differences between the two kinds of leaves were due to resource availability, and to EC-increased allocation of resources to photon harvesting in the penultimate leaves, but to increased allocation to carboxylation early on in growth, and to light harvesting subsequently, in the flag leaves.     

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.     

Singlet oxygen formation and chlorophyll a triplet excited state deactivation in the cytochrome b6f complex from Bryopsis?corticulans

We have attempted to investigate the correlation between the detergent-perturbed structural integrity of the Cyt b ₆ f complex from the marine green alga Bryopsis corticulans and its photo-protective properties, for which the nonionic detergents n-octyl-β-d-glucopyranoside (β-OG) and n-dodecyl-β-d-maltoside (β-DM), respectively, were used for the preparation of Cyt b ₆ f, and the singlet oxygen (¹O₂*) production as well as the triplet excited-state chlorophyll a (³Chl a*) formation and deactivation were examined by spectroscopic means. Near-infrared luminescence of ¹O₂ * (~1,270 nm) on photo-irradiation was detected for the β-OG preparation where the complex is mainly in oligomeric state, but not for the β-DM one in which the complex exists in dimeric form. Under anaerobic condition, photo-excitation of Chl a in the β-DM preparation generated ³Chl a* with a lower quantum yield of Φ_T ~ 0.02 and a longer lifetime of ~600 μs with respect to those as in the case of β-OG preparation, Φ_T ~ 0.12 and 200–300 μs. These results prove that the enzymatically active and intact Cyt b ₆ f complex on photo-excitation tends to produce little ³Chl a* or ¹O₂ *, which implies that the pigment–protein assembly of Cyt b ₆ f complex per se is crucial for photo-protection. F. Ma and X.-B. Chen contributed equally to this work.     

Changes in Photosynthetic Performance of Ceratonia Siliqua in Summer

In carob tree (Ceratonia siliqua) radiant energy saturated net photosynthetic rate (P _N) during summer was about 10 % of the spring values. This was accompanied by a reduction in stomatal conductance (g _s), which only partially explains the strong reduction in P _N. Photosynthetic capacity (P _max) and quantum yield (Φ), both measured under saturating CO₂, had the maximum in spring (about 34 μmol m⁻² s⁻¹ and 0.08 mol mol⁻¹, respectively) and both decreased in late summer to about 55 % of their spring values. Despite strong decreases in Φ, photoinhibition of photosystem 2 (PS2) was negligible or easily reversible in carob leaves subjected to summer drought, since F_v/F_m, measured in the morning, did not show appreciable changes. The recovery of affected parameters was very rapid after the first rains in late October. The chlorophyll (Chl) alb ratio in the end of the summer was 2.6, a value significantly lower than 3.6 obtained in the spring, suggesting that Chl a was preferentially reduced. This revised version was published online in August 2006 with corrections to the Cover Date.     

Chill-induced inhibition of photosynthesis was alleviated by 24-epibrassinolide pretreatment in cucumber during chilling and subsequent recovery

To investigate whether brassinosteroids (BRs) could be used to alleviate chill-induced inhibition of photosynthesis in cucumber (Cucumis sativus L) during chilling and subsequent recovery, the effects of exogenously applied 24-epibrassinolide (EBR) on gas exchange, chlorophyll fluorescence parameters, and antioxidant enzyme activity were studied. Cucumber plants were exposed to chilling under low light (12/8°C and 100 μmol m⁻² s⁻¹ PPFD) for 3 days and then recovered under normal temperature and high irradiance (28/18°C and 600 μmol m⁻² s⁻¹ PPFD) for 6 days. Chilling significantly decreased the net photosynthetic rate (P _N) and stomatal conductance (g _s), and increased rate of O₂ ^·− formation and H₂O₂ and malondialdehyde (MDA) content in cucumber leaves, but did not influence the optimal quantum yield of PSII (F_v/F_m). Chilling also decreased the effective quantum yield of PSII photochemistry (Φ_PSII) and photochemical quenching (q_P), but induced an increase in nonphotochemical quenching (NPQ), and the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX). High irradiance (600 μmol m⁻² s⁻¹) further aggravated the decrease in P _N, g _s, Φ_PSII and q_P, and enhanced the increase in reactive oxygen species (ROS) generation and accumulation in the first day of recovery after chilling. However, high irradiance induced a sharp decrease in F_v/F_m and NPQ, as well as the activities of SOD and APX on the first day of recovery. EBR pretreatment significantly alleviated chill-induced inhibition of photosynthesis during chilling stress and subsequent recovery period, which was mainly due to significant increases in g _s, Φ_PSII, q_P and NPQ. EBR pretreatment also reduced ROS generation and accumulation, and increased the activities of SOD and APX during chilling and subsequent recovery. Those results suggest that EBR pretreatment alleviates the chill reduction in photosynthesis and accelerated the recovery rate mainly by increasing of the stomatal conductance, the efficiency of utilization and dissipation of leaf absorbed light, and the activity of the ROS scavenging system during chilling and subsequent recovery period.     

Effects of elevated CO<Subscript>2</Subscript> and/or O<Subscript>3</Subscript> on hormone IAA in needles of Chinese pine

This study examined the effects of season-long exposure of Chinese pine (Pinus tabulaeformis) to elevated carbon dioxide (CO₂) and/or ozone (O₃) on indole-3-acetic acid (IAA) content, activities of IAA oxidase (IAAO) and peroxidase (POD) in needles. Trees grown in open-top chambers (OTC) were exposed to control (ambient O₃, 55 nmol mol⁻¹ + ambient CO₂, 350 μmol mol⁻¹, CK), elevated CO₂ (ambient O₃ + high CO₂, 700 μmol mol⁻¹, EC) and elevated O₃ (high O₃, 80 ± 8 nmol mol⁻¹ + ambient CO₂, EO) OTCs from 1 June to 30 September. Plants grown in elevated CO₂ OTC had a growth increase of axial shoot and needle length, compared to control, by 20% and 10% respectively, while the growth in elevated O₃ OTC was 43% and 7% less respectively, than control. An increase in IAA content and POD activity and decrease in IAAO activity were observed in trees exposed to elevated CO₂ concentration compared with control. Elevated O₃ decreased IAA content and had no significant effect on IAAO activity, but significantly increased POD activity. When trees pre-exposed to elevated CO₂ were transferred to elevated O₃ (EC–EO) or trees pre-exposed to elevated O₃ were transferred to elevated CO₂ (EO–EC), IAA content was lower while IAAO activity was higher than that transferred to CK (EC–CK or EO–CK), the change in IAA content was also related to IAAO activity. The results indicated that IAAO and POD activities in Chinese pine needles may be affected by the changes in the atmospheric environment, resulting in the change of IAA metabolism which in turn may cause changes in Chinese pine’s growth. An erratum to this article can be found at     

Why does photosynthesis decrease with needle age in Pinus pinaster?

Rates of photosynthesis vary with foliage age and typically decline from full-leaf expansion until senescence occurs. This age-related decline in photosynthesis is especially important in species that retain foliage for several years, yet it is not known whether the internal conductance to CO₂ movement (g _i) plays any role. More generally, g _i has been measured in only a few conifers and has never been measured in leaves or needles older than 1 year. The effect of ageing on g _i was investigated in Pinus pinaster, a species that retains needle for 4 or more years. Measurements were made in autumn when trees were not water limited and after leaf expansion was complete. Rates of net photosynthesis decreased with needle age, from 8 μmol m⁻² s⁻¹ in fully expanded current-year needles to 4.4 μmol m⁻² s⁻¹ in 3-year-old needles. The relative limitation due to internal conductance (0.24–0.35 out of 1) was in all cases larger than that due to stomatal conductance (0.13–0.19 out of 1). Internal conductance and stomatal conductance approximately scaled with rates of photosynthesis. Hence, there was no difference among year-classes in the relative limitations posed by internal and stomatal conductance or evidence that they cause the age-related decline in photosynthesis. There was little evidence that the age-related decline in photosynthesis was due to decreases in contents of N or Rubisco. The decrease in rates of photosynthesis from current-year to older needles was instead related to a twofold decrease in rates of photosynthesis per unit nitrogen and V _cmax/Rubisco (i.e., in vivo specific activity).