The Susceptibility of Cucumber and Sweet Pepper to Chilling Under Low Irradiance is Related to Energy Dissipation and Water-Water Cycle

The photoprotection of energy dissipation and water-water cycle were investigated by comparing chilling sensitivity of photosystems 2 (PS2) and 1 (PS1) in two chilling-sensitive plants, cucumber and sweet pepper, upon exposure to 4 °C under low irradiance (100 μmol m⁻² s⁻¹) for 6 h. During chilling stress, the maximum photochemical efficiency of PS2 (F_v/F_m) decreased only slightly in both plants, but the oxidisable P700 decreased markedly, which indicated that PS1 was more sensitive to chilling treatment under low irradiance than PS2. Sweet pepper leaves had lower F_v/F_m, higher non-photochemical quenching (NPQ), and higher oxidisable P700 during chilling stress. Activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX) in cucumber leaves was higher, but APX activity decreased apparently compared to that at room temperature. The productions of active oxygen species (H₂O₂, O₂ ⁻) increased in both plants, faster in cucumber leaves than in sweet pepper leaves. In sweet pepper leaves, a stronger de-epoxidation of the xanthophyll cycle pigments, a higher NPQ could act as a major protective mechanism to reduce the formation of active oxygen species during stress. Thus sensitivity of both plants to chilling under low irradiance was dominated by the protective mechanisms between PS1 and PS2, especially the energy dissipation and the water-water cycle. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature and irradiance impacts on the growth,pigmentation and photosystem II quantum yields of <Emphasis Type="Italic">Haematococcus pluvialis</Emphasis> (Chlorophyceae)

The microalga Haematococcus pluvialis Flotow has been the subject of a number of studies concerned with maximizing astaxanthin production for use in animal feeds and for human consumption. Several of these studies have specifically attempted to ascertain the optimal temperature and irradiance combination for growth of H. pluvialis, but there has been a great deal of disagreement between laboratories. “Ideal” levels of temperature and irradiance have been reported to range from 14 to 28°C and 30 to 200 μmol photons m⁻² s⁻¹. The objective of the present study was to simultaneously explore temperature and irradiance effects for a single strain of H. pluvialis (UTEX 2505) across an experimental region that encompassed the reported “optimal” combinations of these factors for multiple strains. To this end, a two-dimensional experimental design based on response surface methodology (RSM) was created. Maximum growth rates for UTEX 2505 were achieved at 27°C and 260 μmol photons m⁻² s⁻¹, while maximum quantum yield for stable charge separation at PSII (F_v/F_m) was achieved at 27°C and 80 μmol photons m⁻² s⁻¹. Maximum pigment concentrations correlated closely with maximum F_v/F_m. Numeric optimization of growth rate and F_v/F_m produced an optimal combination of 27°C and 250 μmol photons m⁻² s⁻¹. Polynomial models of the various response surfaces were validated with multiple points and were found to be very useful for predicting several H. pluvialis UTEX 2505 responses across the entire two-dimensional experimental design space.     

Characterization of acclimation of Hordeum vulgare to high irradiation based on different responses of photosynthetic activity and pigment composition

The ability of spring barley (Hordeum vulgare cv. Akcent) to adjust the composition and function of the photosynthetic apparatus to growth irradiances of 25–1200 μmol m⁻² s⁻¹ was studied by gas exchange and chlorophyll a fluorescence measurements and high-performance liquid chromatography. The increased growth irradiance stimulated light- and CO₂-saturated rates of CO₂ assimilation expressed on a leaf area basis up to 730 μmol m⁻² s⁻¹ (HL730), whereas at an irradiance of 1200 μmol m⁻² s⁻¹ (EHL1200) both rates decreased significantly. Further, the acclimation to EHL1200 was associated with an extremely high chlorophyll a/b ratio (3.97), a more than doubled xanthophyll cycle pool (VAZ) and a six-fold higher de-epoxidation state of the xanthophyll cycle pigments as compared to barley grown under 25 μmol m⁻² s⁻¹ (LL25). EHL1200 plants also exhibited a long-term inhibition of Photosystem II (PS II) photochemical efficiency (F _v/F _m). Photosynthetic capacity, chlorophyll a/b and VAZ revealed a linear trend of dependence on PS II excitation pressure in a certain range of growth irradiances (100–730 μmol m⁻² s⁻¹). The deviation from linearity of these relationships for EHL1200 barley is discussed. In addition, the role of increased VAZ and/or accumulation of zeaxanthin and antheraxanthin in acclimation of barley to high irradiance is studied with respect to regulation of non-radiative dissipation and/or photochemical efficiency within PS II. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photosynthetic acclimation to photon irradiance and its relation to chlorophyll fluorescence and carbon assimilation in the halotolerant green alga Dunaliella viridis

This work describes the long-term acclimation of the halotolerant microalga Dunaliella viridis to different photon irradiance, ranging from darkness to 1500 μmol m⁻² s⁻¹. In order to assess the effects of long-term photoinhibition, changes in oxygen production rate, pigment composition, xanthophyll cycle and in vivo chlorophyll fluorescence using the saturating pulse method were measured. Growth rate was maximal at intermediate irradiance (250 and 700 μmol m⁻² s⁻¹). The increase in growth irradiance from 700 to 1500 μmol m⁻² s⁻¹ did not lead to further significant changes in pigment composition or EPS, indicating saturation in the pigment response to high light. Changes in Photosystem II optimum quantum yield (F_v/F_m) evidenced photoinhibition at 700 and especially at 1500 μmol m⁻² s⁻¹. The relation between photosynthetic electron flow rate and photosyntetic O₂ evolution was linear for cultures in darkness shifting to curvilinear as growth irradiance increased, suggesting the interference of the energy dissipation processes in oxygen evolution. Carbon assimilation efficiencies were studied in relation to changes in growth rate, internal carbon and nitrogen composition, and organic carbon released to the external medium. All illuminated cultures showed a high capability to maintain a C:N ratio between 6 and 7. The percentage of organic carbon released to the external medium increased to its maximum under high irradiance (1500 μmol m⁻² s⁻¹). These results suggest that the release of organic carbon could act as a secondary dissipation process when the xanthophyll cycle is saturated. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photosynthetic Response of Carrots to Varying Irradiances

Response to irradiance of leaf net photosynthetic rates (P _N) of four carrot cultivars: Cascade, Caro Choice (CC), Oranza, and Red Core Chantenay (RCC) were examined in a controlled environment. Gas exchange measurements were conducted at photosynthetic active radiation (PAR) from 100 to 1 000 μmol m⁻² s⁻¹ at 20 °C and 350 μmol (CO₂) mol⁻¹(air). The values of P _N were fitted to a rectangular hyperbolic nonlinear regression model. P _N for all cultivars increased similarly with increasing PAR but Cascade and Oranza generally had higher P _N than CC. None of the cultivars reached saturation at 1 000 μmol m⁻² s⁻¹. The predicted P _N at saturation (P _Nmax) for Cascade, CC, Oranza, and RCC were 19.78, 16.40, 19.79, and 18.11 μmol (CO₂) m⁻² s⁻¹, respectively. The compensation irradiance (I _c) occurred at 54 μmol m⁻² s⁻¹ for Cascade, 36 μmol m⁻² s⁻¹ for CC, 45 μmol m⁻² s⁻¹ for Oranza, and 25 μmol m⁻² s⁻¹ for RCC. The quantum yield among the cultivars ranged between 0.057–0.033 mol(CO₂) mol⁻¹(PAR) and did not differ. Dark respiration varied from 2.66 μmol m⁻² s⁻¹ for Cascade to 0.85 μmol m⁻² s⁻¹ for RCC. As P _N increased with PAR, intercellular CO₂ decreased in a non-linear manner. Increasing PAR increased stomatal conductance and transpiration rate to a peak between 600 and 800 μmol m⁻² s⁻¹ followed by a steep decline resulting in sharp increases in water use efficiency. This revised version was published online in August 2006 with corrections to the Cover Date.     

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

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

Characteristics of sun- and shade-adapted populations of an endangered plant <Emphasis Type="Italic">Primulina tabacum</Emphasis> Hance

Primulina tabacum Hance is an endangered perennial herb distributed in calcium-rich and nitrogen-limited soil of the karst limestone areas in southern China. The morphological, ultrastructural, and physiological traits were determined for P. tabacum populations growing in three different environment conditions: twilight zone of a cave (site TZ, extremely low light intensity), at a cave entrance (site EZ, low light intensity), and in an open area (site OA, high light intensity). At site OA, P. tabacum plants were exposed to high light (635 μmol m⁻² s⁻¹ of mean daily photosynthetically active radiation) with drought stress, and expressed traits to minimize light capture and water loss. Compared to plants at sites EZ and TZ, those at site OA had thicker leaves with higher densities of stomata and pubescence, higher palisade/spongy ratio, higher light-saturated rate of net photosynthetic rate (P _max), higher biomass, higher non-photochemical quenching coefficient (NPQ), and higher light saturation point (LSP) but fewer grana per chloroplast and less thylakoid stacking per granum. In contrast, P. tabacum growing at the cave vicinities: EZ (mean daily irradiance 59 μmol m⁻² s⁻¹) and TZ (mean daily irradiance 11 μmol m⁻² s⁻¹) showed typical shade-adapted characteristics for optimum light capture. The presence of sun- and shade-adapted characteristics indicates that P. tabacum has different strategies to cope with different environments but whether these strategies reflect genetic selection or phenological plasticity is yet to be determined. Such variability in physiological and morphological traits is important for the survival of P. tabacum in heterogeneous light conditions.     

Photosynthetic parameters of young Brazil nut (<Emphasis Type="Italic">Bertholletia excelsa</Emphasis> H. B.) plants subjected to fertilization in a degraded area in Central Amazonia

In an experimental site for reforestation of degraded area, three-year-old plants of Bertholletia excelsa Humb. & Bonpl. were subjected to different fertilization treatments: T0 = unfertilized control, T1 = green fertilization (branches and leaves) and T2 = chemical fertilization. Higher net photosynthetic rates (P _N) were observed in T1 [13.2±1.0 μmol(CO₂) m⁻² s⁻¹] compared to T2 [8.0±1.8 μmol(CO₂) m⁻² s⁻¹] and T0 [4.8±1.3 μmol(CO₂) m⁻² s⁻¹]. Stomatal conductance (g _s), transpiration rate (E) and water use efficiency (WUE) of individuals of T1 and T2 did not differ significantly, however, they were by 88, 55 and 63%, respectively, higher in T1 than in the control. The mean values of variable fluorescence (F_v), performance index (P.I.) and total chlorophyll [Chl (a+b)] were higher in T1. Our results indicate that green fertilization improves photosynthetic structure and function in plants of B. excelsa in young phase.     

Photosynthetic and leaf anatomical characteristics of Castanea sativa: a comparison between in vitro and nursery plants

The anatomic and functional leaf characteristics related to photosynthetic performance of Castanea sativa growing in vitro and in nursery were compared. The irradiance saturated photosynthesis in in vitro grown plantlets was significantly lower compared to nursery plants (65 vs. 722 μmol m⁻² s⁻¹). The maximum photosynthetic rate (P_Nmax) was 4.0 and 10.0 μmol(CO₂) m⁻² s⁻¹ in in vitro microshoots and nursery plant leaves, respectively. Carboxylation efficiency (C_E) and electron transport rate (ETR) were three-folds higher in nursery plants than in microshoots. The nonphotochemical quenching (NPQ) was saturated at 80 μmol m⁻² s⁻¹ in microshoots suggesting limited photoprotection by thermal dissipation. The microshoots had wide open, spherical stomata and higher stomatal density than nursery plants and they had almost no epicuticular wax. Consequently, the microshoots had high stomatal conductance and high transpiration rate. These anatomic and functional leaf characteristics are likely major causes of the low survival rates of plantlets after ex vitro transfer.     

Effects of water stress on photosynthetic activity,dry mass partitioning and some associated metabolic changes in four provenances of neem (<Emphasis Type="Italic">Azadirachta indica</Emphasis> A. Juss)

Chlorophyll (Chl) content, dry mass, relative water content (RWC), leaf mass per area (LMA), proline (Pro) content, malondialdehyde (MDA) content, superoxide dismutase (SOD) and peroxidase (POD) activity, P _N-PAR response curves and gas exchange were studied to determine the effects of water stress on photosynthetic activity, dry mass partitioning and metabolic changes in four provenances of neem (Azadirachta indica A. Juss). The results indicated that provenance differences existed in the adaptation response to water stress that included changes to growth strategies coupled with ecophysiological and metabolic adjustments. As water stress increased, stomatal conductance (g _s), net photosynthetic rate (P _N), transpiration rate (E), and leaf RWC decreased while LMA increased in all provenances. Dry mass was reduced in droughted plants and the percentage increased in dry mass allocated to roots, and enzyme activities of SOD and POD were highest in neem originating from Kalyani (KA) provenance and lowest in neem originating from New Dehli (ND) provenance. In contrast, water stress increased MDA content least in KA and most in ND. Furthermore, neem originating from ND also had the greatest decrease in Chl a/b ratio while the ratio was least affected in neem originating from KA. These findings suggest neem originating from KA may have more drought resistance than neem originating from ND. The data from P _N-PAR response curves are less clear. While these curves showed that drought stress increased compensation irradiance (I _c) and dark respiration (R _D) and decreased saturation irradiance (I _s) and maximum net photosynthetic rate (P _max), the extent of decline in P _max was provenance dependent. P _max under non-waterlimiting conditions was higher in neem originating from Jodhpur (MA) (about 14 μmol m⁻² s⁻¹) than in the other three provenances (all about 10 μmol m⁻² s⁻¹), but mild water stress had minimal effect on P _max of these three provenances whereas P _max of MA provenance declined to 10 μmol m⁻² s⁻¹, i.e. a similar value. However, under severe water stress P _max of MA and KA provenances had declined to 40% of non-stressed values (about 6 and 4 μmol m⁻² s⁻¹, respectively) whereas the decline in P _max of neem originating from Kulapachta (KU) and ND provenances was about 50% of nonstressed values (about 5 μmol m⁻² s⁻¹). These data suggest the P _N responses of KU and ND provenances are most tolerant, and KA and MA least tolerant to increasing water stress, but also suggest MA provenance could be the most desired under both non-water-limiting and water-limiting conditions due to highest P _max in all conditions.     

Effect of temperature and irradiance on the uptake of ammonium and nitrate by <Emphasis Type="Italic">Laurencia brongniartii</Emphasis> (Rhodophyta,Ceramiales)

The effects of temperature (20, 24 and 28 °C) and irradiance (15 and 40 μmol photon m⁻² s⁻¹) on the nitrate and ammonium uptake rates of the subtropical red alga, Laurencia brongniartii, were investigated to prepare for tank cultivation. Nitrate uptake followed saturation kinetics and was faster at higher irradiances and temperatures. In contrast, ammonium uptake was linear over the experimental range and was not affected by an increase in temperature. A parameter, β, was calculated to compare substrate uptake rates of nitrate along the linear portion of the uptake curve with that of ammonium. For nitrate, β was lower at low irradiance and higher at high irradiance (β = 0.007 ± 0.003 and 0.030 ± 0.002 [μmol N L⁻¹ (μmol N g_ww⁻¹ d⁻)⁻¹], respectively). However, β was 0.023 ± 0.002 and 0.034 ± 0.002 [μmol N L⁻¹ (μmol N g_ww⁻¹ d⁻¹)⁻¹] for ammonium, suggesting a preference for ammonium over nitrate.     

Physiological differences in the growth of <Emphasis Type="Italic">Sargassum horneri</Emphasis> between the germling and adult stages

The effects of temperature, irradiance, and daylength on Sargassum horneri growth were examined at the germling and adult stages to discern their physiological differences. Temperature–irradiance (10, 15, 20, 25, 30°C × 20, 40, 80 μmol photons m⁻²s⁻¹) and daylength (8, 12, 16, 24 h) experiments were carried out. The germlings and blades of S. horneri grew over a wide range of temperatures (10–25°C), irradiances (20–80 μmol photons m⁻²s⁻¹), and daylengths (8–24 h). At the optimal growth conditions, the relative growth rates (RGR) of the germlings were 21% day⁻¹ (25°C, 20 μmol photons m⁻²s⁻¹) and 13% day⁻¹ (8 h daylength). In contrast, the RGRs of the blade weights were 4% day⁻¹ (15°C, 20 μmol photons m⁻²s⁻¹) and 5% day⁻¹ (12 h daylength). Negative growth rates were found at 20 μmol photons m⁻²s⁻¹ of 20°C and 25°C treatments after 12 days. This phenomenon coincides with the necrosis of S. horneri blades in field populations. In conclusion, we found physiological differences between S. horneri germlings and adults with respect to daylength and temperature optima. The growth of S. horneri germlings could be enhanced at 25°C, 20 μmol photons m⁻²s⁻¹, and 8 h daylength for construction of Sargassum beds and restoration of barren areas.     

Eco-physiological responses of nitrogen-fixing cyanobacteria to light

The eco-physiological responses of three nitrogen-fixing cyanobacteria (N-fixing cyanobacteria), Aphanizomenon gracile, Anabaena minderi, and Ana. torques-reginae, to light were assessed under nutrient saturation. The N-fixing cyanobacteria were isolated into monocultures from a natural bloom in a shallow colored lake and their growth irradiance parameters and pigment composition were assessed. The different ecological traits related to light use (μ_max, α, I _k) suggest that these N-fixing cyanobacteria are well adapted to low light conditions at sufficient nutrients, yet interspecific differences were observed. Aphanizomenon gracile and Anabaena minderi had high relative growth rates at low irradiances (ca. 70% of those in high light), low half saturation constant for light-limited growth (I _k < 9.09 μmol photon m⁻² s⁻¹) and high efficiency (α < 0.11 day⁻¹ μmol photon⁻¹ m² s). Conversely, Ana. torques-reginae showed poorer light competitiveness: low relative growth rates at low irradiances (ca. 40% of those in high light), low α (0.009 day⁻¹ μmol photon⁻¹ m² s) and higher I _k (35.5 μmol photon m⁻² s⁻¹). Final densities in Aphanizomenon gracile and Anabaena minderi reached bloom densities at irradiances above 30 μmol photon m⁻² s⁻¹ with different hierarchy depending on irradiance, whereas Ana. torques-reginae never achieved bloom densities. All species had very low densities at irradiances ≤17 μmol photon m⁻² s⁻¹, thus no N-fixing blooms would be expected at these irradiances. Also, under prolonged darkness and at lowest irradiance (0 and 3 μmol photon m⁻² s⁻¹) akinetes were degraded, suggesting that in ecosystems with permanently dark sediments, the prevalence of N-fixing cyanobacteria should not be favored. All species displayed peaks of phycocyanin, but no phycoeritrin, probably due to the prevailing red light in the ecosystem from which they were isolated.     

Relations between electron transport rates determined by pulse amplitude modulated chlorophyll fluorescence and oxygen evolution in macroalgae under different light conditions

The relationship between O₂-based gross photosynthesis (GP) and in vivo chlorophyll fluorescence of Photosystem II-based electron transport rate (ETR) as well as the relationship between effective quantum yield of fluorescence (Φ_PSII) and quantum yield of oxygen evolution (Φ_{O_2}) were examined in the green algae Ulva rotundata and Ulva olivascens and the red alga Porphyra leucosticta collected from the field and incubated for 3 days at 100 μmol m⁻² s⁻¹ in nutrient enriched seawater. Maximal GP was twice as high in Ulva species than that measured in P. leucosticta. In all species ETR was saturated at much higher irradiance than GP. The initial slope of ETR versus absorbed irradiance was higher than that of GP versus absorbed irradiance. Only under absorbed irradiances below saturation or at values of GP <2 μmol O₂ m⁻² s⁻¹ a linear relationship was observed. In the linear phase, calculated O₂ evolved /ETR molar ratios were closed to the theoretical value of 0.25 in Ulva species. In P. leucosticta, the estimated GP was associated to the estimated ETR only at high irradiances. ETR was determined under white light, red light emitting by diodes and solar radiation. In Ulva species the maximal ETR was reached under red light and solar radiation whereas in P. leucosticta the maximal ETR was reached under white light and minimal under red light. These results are in agreement with the known action spectra for photosynthesis in these species. In the case of P. leucosticta, GP and ETR were additionally determined under saturating irradiance in algae pre-incubated for one week under white light at different irradiances and at white light (100 μmol m⁻² s⁻¹) enriched with far-red light. GP and growth rate increased at a growth irradiance of 500 μmol m⁻² s⁻¹ becoming photoinhibited at higher irradiances, while ETR increased when algae were exposed to the highest growth irradiance applied (2000 μmol m⁻² s⁻¹). The calculated O₂ evolved /ETR molar ratios were close to the theoretical value of 0.25 when algae were pre-incubated under 500–1000 μmol m⁻² s⁻¹. The enrichment by FR light provoked a decrease in both GP and ETR and an increase of nonphotochemical quenching although the irradiance of PAR was maintained at a constant level. In addition to C assimilation, other electron sinks, such as nitrogen assimilation, affected the GP–ETR relationship. The slopes of GP versus ETR or Φ_PSII versus Φ_{O_2} were lower in the algae with the highest N assimilation capacity, estimated as nitrate reductase activity and internal nitrogen contents, i.e., Ulva rotundata and Porphyra leucosticta, than that observed in U. olivascens. The possible mechanisms to explain this discrepancy between GP and ETR are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Partial quantification of pigments extracted from the zooxanthellate octocoral <Emphasis Type="Italic">Sinularia flexibilis</Emphasis> at varying irradiances

Chlorophyll-a (chl-a) and carotenoid pigments of the zooxanthellate octocoral Sinularia flexibilis were analyzed using high performance liquid chromatography following exposure to three light intensities for over 30 days. From the coral fragments located at different light intensities, a total carotenoid of >41 μg g⁻¹ dry weight, including peridinin, xanthophylls (likely diadinoxanthin + diatoxanthin), and chl-a as the most abundant pigments, with minor contents of astaxantin and β-carotene were detected. The whole content of chl-a weighed 5 μg g⁻¹ dry weight in all coral colonies. Chl-a and carotenoids contributed 11.2% and 88.2%, respectively, to all pigments detected, and together accounted for 99.4% of the total pigments present. The highest contents of carotenoids and chl-a was observed in the coral grafts placed in an irradiance of 100 μmol quanta m⁻² s⁻¹; they showed lower ratios of total carotenoids: chl-a compared to those exposed to 400 μmol quanta m⁻² s⁻¹ after >30 days of incubation. The ratios of peridinin and xanthophylls with respect to chl-a from the colonies at 400 μmol quanta m⁻² s⁻¹ were approximately double those observed at irradiances of 100 and 200 μmol quanta m⁻² s⁻¹. Partial quantification of pigments in this study showed that the carotenoids of S. flexibilis showed a decrease at irradiances above 100 μmol quanta m⁻² s⁻¹, with the exception of an increase in β-carotene at 200 μmol quanta m⁻² s⁻¹.     

Life-cycle of <Emphasis Type="Italic">Scinaia interrupta</Emphasis> (Nemaliales,Rhodophyta)

The life-cycle of Scinaia interrupta (A.P. de Candolle) M. J. Wynne was investigated in vitro using four irradiance regimes: 4, 8, 12 and 16 μmol photons m⁻² s⁻¹. A triphasic heteromorphic life-cycle was observed. Carpospores released by cystocarps of gametophytes collected in the field developed into filamentous tetrasporophytes, which produced tetrahedral tetrasporangia. Tetrasporangial development was accelerated under higher irradiance levels. Tetraspores germinated into filamentous protonemal gametophytes, initially identical to the tetrasporophyte. Filamentous gametophytes developed apical utricles and gave rise directly to the fleshy gametophyte. Further development of the fleshy gametophyte was not observed at the lowest irradiance regime (4 μmol photons m⁻² s⁻¹). The present study reports for the first time the influence of the irradiance regime on the initial tetrasporangial development and in the development of the fleshy gametophyte, and reinforces the importance of light intensity on Scinaia life-cycle. Production of apical utricles by the filamentous gametophyte is newly reported for the genus.     

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.     

Photosynthesis of a temperate fallow C<Subscript>3</Subscript> herbaceous ecosystem: measurements and model simulations at the leaf and canopy levels

The objectives of the study were to characterize photosynthesis of temperate fallow C₃herbaceous species and examine the performance of a simple photosynthesis model (based on the Farquhar’s equations) to simulate carbon fluxes at the leaf and canopy levels. The maximum rate of carboxylation at 25°C (V _m0) was estimated for sunlit leaves using in situ gas exchange data under saturating irradiance. Throughout the seasons, leaf measurements indicate that values of V _m0 were similar for the four major species of the fallow. The rate declined from March (100 μmol m⁻² s⁻¹) to July (50 μmol m⁻² s⁻¹) and remained almost constant until November. The maximum quantum yield estimated for Potentilla reptans L. (dominant species) was 0.082 mol(CO₂) mol⁻¹(photon absorbed), similar to values already published for C₃ species. Leaf area index (LAI) increased from winter (less than 0.2 m² m⁻²) to spring (up to 4 m² m⁻²). Rates of canopy photosynthesis (measured with a canopy chamber) strongly depended on LAI and temperature, in addition to irradiance. They reached a maximum of 25 μmol m⁻² s⁻¹ and were intermediate between those published for C₄ grassland or cultivated species, and on woody species. At leaf level, simulations gave realistic predictions. At canopy level, the model had the ability to reproduce the effects of environmental and seasonal conditions. However, simulations underestimated the photosynthetic activity of the fallow canopy.     

Irradiance prior to and during desiccation improves the tolerance to excess irradiance in the desiccated state of the old forest lichen Lobaria pulmonaria

Hydrated thalli of the lichen Lobaria pulmonaria were either preconditioned to dim irradiance (DI, 5 μmol m⁻² s⁻¹) or medium irradiance (MI, 200 μmol m⁻² s⁻¹) for 6 h. After this 6 h period, the thalli were allowed to desiccate under the two respective irradiances. Thereafter, these dry lichens were exposed to high irradiance (HI, 1 000 μmol m⁻² s⁻¹) for 60 h. After this HI treatment, the maximal photochemical quantum yield (F_V/F_M) and the de-epoxidation state of xanthophyll cycle pigments (DEPS) were highest in thalli preconditioned to MI. Hence irradiance in the last hydrated period before sampling is significant for the physiological state of lichens. A standardized irradiance pre-treatment before start of experiments is recommended.     

Small scale vertical gradients of Arctic ice algal photophysiological properties

Photosynthetic parameters of phytoplankton and sea ice algae from landfast sea ice of the Chukchi Sea off Point Barrow, Alaska, were assessed in spring 2005 and winter through spring 2006 using Pulse Amplitude Modulated (PAM) fluorometry including estimates of maximum quantum efficiency (F _v/F _m), maximum relative electron transport rate (rETR_max), photosynthetic efficiency (α), and the photoadaptive index (E _k). The use of centrifuged brine samples allowed to document vertical gradients in ice algal acclimation with 5 cm vertical resolution for the first time. Bottom ice algae (0–5 cm from ice–water interface) expressed low F _v/F _m (0.331–0.426) and low α (0.098–0.130 (μmol photons m⁻²s⁻¹)⁻¹) in December. F _v/F _m and α increased in March and May (0.468–0.588 and 0.141–0.438 (μmol photons m⁻²s⁻¹)⁻¹, respectively) indicating increased photosynthetic activity. In addition, increases in rETR_max (3.3–16.4 a.u.) and E _k (20–88 μmol photons m⁻² s⁻¹) from December to May illustrates a higher potential for primary productivity as communities become better acclimated to under-ice light conditions. In conclusion, photosynthetic performance by ice algae (as assessed by PAM fluorometry) was tightly linked to sea ice salinity, temperature, and inorganic nutrient concentrations (mainly nitrogen).