LIGHT DEPENDENCE OF GROWTH AND PHOTOSYNTHESIS IN PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

Phaeodactylum tricornutum Bohlin was maintained in exponential growth over a range of photon flux densities (PFD) from 7 to 230 μmol·m^?2s^?1. The chlorophyll a-specific light absorption coefficient, maximum quantum yield of photosynthesis, and C:N atom ratio were all independent of the PFD to which cells were acclimated. Carbon- and cell-specific, light-satuated, gross photosynthesis rates and dark respiration rates were largely independent of acclimation PFD. Decreases in the chlorophyll a-specific, gross photosynthesis rate and the carbon: chlorophyll ratio and increases of cell- or carbon-specific absorption coefficients were associated with an increase in cell chlorophyll a in cultures acclimated to low PFDs. The compensation PFD for growth was calculated to be 0.5 μmol·m^?2s^?1. The maintenance metabolic rate (2 × 10^?7s^?1), calculated on the basis of the compensation PFD, is an order of magnitude lower than the measured dark respiration rate(2.7 × 10^?6mol O₂·mol C^?1s^?1). Maintenance of high carbon-specific, light-saturated photosynthesis rates in cells acclimated to low PFDs may allow effective use of short exposures to high PFDs in a temporally variable light environment.     

THE INTERACTION BETWEEN INORGANIC CARBON ACQUISITION AND LIGHT SUPPLY IN PALMARIA PALMATA (RHODOPHYTA)1

The dependence of the carbon concentrating mechanism of Palmaria palmata (L.) Kuntze on the growth light level was examined 1) to determine whether or not there is a threshold photon flux density (PFD) at which the inorganic carbon uptake mechanism can operate and 2) to attempt to quantify the relative energetic costs of acclimation to the two different limiting factors, PFD and dissolved inorganic carbon (DIC) concentration. Plants were grown at six PFDs: 5, 25, 50, 75, 95, and 125 μmol photons. m^?2.s^?1. Growth rates increased with increasing PFD from 5 to 50 μmol photons. m^?2. s^?1 and were light-saturated at 75, 95, and 125 μmol photons. m^?2. s^?1 Values of δ¹³C increased continuously with increasing growth PFD and did not saturate over the range of light levels tested. Time-resolved fluorescence characteristics indicated a progressive photoacclimation below 50 μmol photons. m^?2. s^?1. Analysis of chlorophyll fluorescence induction showed three levels of light use efficirncy associated with growth at 5 or 25, 50, and >75 μmol photons. m^?2. s^?1. The light-haruesting efficiency was inversely proportional to the effectiveness of DIC acquisition in plants grown at the six PFDs. These data were interpreted to indicate that there is a physiological tradeoff between photosynthetic efficiency and bicarbonate use in this species.     

SIMAZINE-INDUCED INHIBITION IN PHOTOACCLIMATED POPULATIONS OF ANABAENA CIRCINALIS (CYANOPHYTA)1

The effects of the triazine herbicide, simazine, on photosynthetic oxygen evolution and growth rate in photoacclimated populations of Anabaena circinalis Rabenhorst were investigated. Chemostat populations were acclimated to photon flux densities (PFDs) of 50, 130, and 230 μmol·m^?2·s^?1 of photosynthetic active radiation (PAR), Decreases in chlorophyll a (Chl a). c-phycocyanin (CPC), and total carotenoid (TCar) contents and CPC: Chl a and CPC: TCar ratios of populations coincided with increasing PFD, Polynomial regression models that characterize inhibition of photosynthesis for populations acclimated to 50 and 130 μmol photons·m^?2·s^?1 PAR were distinct from the model for populations acclimated to 230 μmol photons·m^?2·s^?1 PAR. Simazine concentrations that, depressed oxygen evolution 50% compared to controls decreased with increasing PFD. Increases and decreases in both biomass and growth rate coincided with increasing PFD and simazine concentration, respectively. Simazine concentrations that depressed growth rate 50% compared to controls increased with decreasing PFD. The differences in photosynthetic and growth inhibition among photoacclimated populations indicate that sensitivity to photosystem II inhibitors is affected by alterations in pigment contents.     

ACCLIMATION OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) TO PHOTON FLUX DENSITY1

Growth rate, pigment composition, and noninvasive chl a fluorescence parameters were assessed for a noncalcifying strain of the prymnesiophyte Emiliania huxleyi Lohman grown at 50, 100, 200, and 800 μmol photons·m^?2·s^?1. Emiliania huxleyi grown at high photon flux density (PFD) was characterized by increased specific growth rates, 0.82 d^?1 for high PFD grown cells compared with 0.38 d^?1 for low PFD grown cells, and higher in vivo chl a specific attenuation coefficients that were most likely due to a decreased pigment package, consistent with the observed decrease in cellular photosynthetic pigment content. High PFD growth conditions also induced a 2.5‐fold increase in the pool of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin responsible for dissipation of excess energy. Dark‐adapted maximal photochemical efficiency (F_v/F_m) remained constant at around 0.58 for cells grown over the range of PFDs, and therefore the observed decline, from 0.57 to 0.33, in the PSII maximum efficiency in the light‐adapted state, (F_v′/F_m′), with increasing growth PFD was due to increased dissipation of excess energy, most likely via the xanthophyll cycle and not due to photoinhibition. The PSII operating efficiency (F_q′/F_m′) decreased from 0.48 to 0.21 with increasing growth PFD due to both saturation of photochemistry and an increase in nonphotochemical quenching. The changes in the physiological parameters with growth PFD enable E. huxleyi to maximize rates of photosynthesis under subsaturating conditions and protect the photosynthetic apparatus from excess energy while supporting higher saturating rates of photosynthesis under saturating PFDs.     

Chlorophyll fluorescence quenching and violaxanthin deepoxidation of FBPase antisense plants at low light intensities and low temperatures

Genetically modified potato (Solanum tuberosum L. cv. Desiree) and tobacco (Nicotiana tabacum cv. Samsun N.N.) plants were used to analyze the effects exerted by the chloroplastic (cp) fructose- 1,6-bisphosphatase (FBPase) on the regulation of light energy discrimination at the level of photosystem II. The cp-FBPase activity was progressively inhibited by an mRNA antisense to this FBPase. The chlorophyll fluorescence quenching parameters of these transgenic plants were compared to those of wild-type and transgenic plants that were acclimated to low temperatures. In particular various lines of the transgenic potato and tobacco plants were exposed to a temperature treatment of 10 and 20°C for 10 days. Light intensities were kept low to reduce photoinhibition so that we could analyze exclusively the effects of a modification in the carbon fixation cycle on the chlorophyll fluorescence quenching parameters. The photon flux densities (PFDs) employed at the level of the middle leaves of all plants were set to two different values of 10 μmol m^?2 s^?1 and 50 μmol m^?2 s^?1. Subsequent to this 10-day acclimation the chlorophyll-fluorescence parameters of all plants were measured. Photoinhibition as expressed by the F_y/F_m ratio was minor in plants subjected to a PFD of 10 μmol m^?2 s^?1. Higher photon fluence rates of 50 μmol m^?2 s^?1 at temperatures of 10°C gave rise to a significant reduction in the F_y/F_m ratios obtained from the transgenic plants which were characterized by a restriction in cp-FBPase capacity to 20% of normal activity. Furthermore, a progressive inhibition of the cp-FBPase activity induced an amplified nonphotochemical quenching of chlorophyll fluorescence with in the genetically manipulated species (except at 10°C and 50 μmol m^?2 s^?1). The increase in nonphotochemical quenching depended upon light and temperature. Photochemical quenching of light quanta within the antisense plants declined relative to that in the wild type. To further characterize the mechanisms producing higher levels of nonphotochemical fluorescence quenching. we analyzed several of the xanthophyll cycle pigments. The deepoxidation state of the xanthophyll cycle pigments in potato plants increased with attenuating FBPase activities under all conditions. For tobacco plants, this elevation of the deepoxidation state was only observed at a PFD of 50 μmol m^?2 s^?1.     

GROWTH AND PHOTOSYNTHESIS OF ULVA ROTUNDATA (CHLOROPHYTA) AS A FUNCTION OF TEMPERATURE AND SQUARE WAVE IRRADIANCE IN INDOOR CULTURE1

Temperature and photon flux density (PFD) vary independently in estuaries, e.g. high PFD may occur at any temperature, so it is necessary to consider synergistic effects of these factors on algal growth. Because natural PFD is highly variable and daylength changes confound seasonal temperature cycles, it is easier to interpret factorial experiments in controlled laboratory conditions. Clonal Ulva rotundata Blid. (Chlorophyta) has been studied extensively in outdoor culture. In this study it was maintained indoors under square wave photoperiods at five PFDs and three temperatures. Growth rate, photqsynthetic light response (P-I) curves, and photosystem II chlorophyll fluorescence properties were measured at the growth temperature following acclimation. Interactions between PFD and growth temperature were strongly indicated in all physiological parameters measured. Greatest PFD response occurred at the highest temperature, and the largest temperature response occurred at the highest PFD. Light-saturated photosynthesis (P_m) dark respiration (R_d), and light-limited quantum yield (Φ_m) were sufficient to describe acclimation status. The light-saturation parameter (I_k) was redundant and potentially misleading. Although U. rotundata exhibits a great amplitude of photoacclimation, it apparently has little capacity for temperature acclimation compared to the kelp, Laminaria saccharina, for which published data indicate similar photosynthetic rates over a broad range of growth temperatures. Diurnal variation of P_m and R_d at a growth PFD of ～ 1700 ± 200 μmol photons · m^?2· s^?1 was similar to the pattern observed previously in outdoor culture, suggesting endogenous control of these parameters. Quantum yield and the ratio of variable to maximum chlorophyll fluorescence (F_v/F_m), which were depressed in midday sunlight exceeding ～ 1500 μmol photons · m^?2· s^?1, were relatively invariant through the day in indoor culture, indicating that these parameters are controlled primarily by instantaneous PFD. Growth and fluorescence data are also presented for some other macroalgae for comparative purposes.     

PHOTOINHIBITION OF MECHANICALLY STIMULABLE BIOLUMINESCENCE IN THE AUTOTROPHIC DINOFLAGELLATE CERATIUM FUSUS (PYRROPHYTA)1

Ceratium fusus (Ehrenb.) Dujardin was exposed to light of different wavelengths and photon flux densities (PFDs) to examine their effects on mechanically stimulable bioluminescence (MSL). Photoinhibition of MSL was proportional to the logarithm of PFD. Exposure to I μmol photons·m^?2s^?1 of broadband blue light (ca. 400–500 nm) produced near-complete photoinhibition (≥90% reduction in MSL) with a threshold at ca. 0.01 μmol photons·m^?2·s^?1. The threshold of photoinhibition was ca. an order of magnitude greater for both broadband green (ca. 500–580 nm) and red light (ca. 660–700 nm). Exposure to narrow spectral bands (ca. 10 nm half bandwidth) from 400 and 700 nm at a PFD of 0.1 μmol photons·m^?2·s^?1 produced a maximal response of photoinhibition in the blue wavelengths (peak ca. 490 nm). A photoinhibition response (≥ 10%) in the green (ca. 500–540 nm) and red wavelengths (ca. 680 nm) occurred only at higher PFDs (1 and 10 μmol photons·m^?2·s^?1). The spectral response is similar to that reported for Gonyaulax polyedra Stein and Pyrocystis lunula Schütt and unlike that of Alexandrium tamarense (Lebour) Balech et Tangen. The dinoflagellate's own bioluminescence is two orders of magnitude too low to result in self-photoinhibition. The quantitative relationships developed in the laboratory predict photoinhibition of bioluminescence in populations of C. fusus in the North Atlantic Ocean.     

INFLUENCE OF IRRADIANCE ON THE FATTY ACID COMPOSITION OF PHYTOPLANKTON1

Eight species of marine phytoplankton commonly used in aquaculture were grown under a range of photon flux densities (PEDs) and analyzed for their fatty acid (FA) composition. Fatty and composition changed considerably at different PFDs although no consistent correlation between the relative proportion of a single FA and μ or chl a · cell^?1 was apparent. Within an individual species the percentage of certain fatty acids covaried with PFDs, growth rate and/or chl a · cell^?1. The light conditions which produced the greatest proportion of the essential fatty acids was species specific. Eicosapentaenoic acid. 20:5ω3 increased from 6.1% to 15.5% of the total fatty acids of Chaetoceros simplex Ostenfield grown at PFDs which decreased from 225 μE · m^?2· s^?1 to 6 μE · m^?2· s^?1, respectively. Most species had their greatest proportion of 20: 5ω3 at low levels of irradiance. Conversely, docosahexaenoic acid, 22:6ω3, decreased from 9.7% to 3.6% of the total fatty acids in Pavlova lutheri Droop as PFD decreased. The percentage of 22:6ω3 generally decreased with decreasing irradiances. In all diatoms the percentage of 16:0 was significantly correlated with PFD, and in three of five diatoms, with growth rate (μ). Results suggest that fatty acid composition is a highly dynamic component of cellular physiology, which responds significantly to variation in PFD.     

Relative enhancement of photosynthesis and growth at elevated CO2 is greater under sunflecks than uniform irradiance in a tropical rain forest tree seedling

The survivorship of dipterocarp seedlings in the deeply shaded understorey of South‐east Asian rain forests is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain. To investigate the effect of elevated CO₂ upon photosynthesis and growth under sunflecks, seedlings of Shorealeprosula were grown in controlled environment conditions at ambient or elevated CO₂. Equal total daily photon flux density (PFD) (～7·7 mol m^?2 d^?1) was supplied as either uniform irradiance (～170 µmol m^?2 s^?1) or shade/fleck sequences (～30 µmol m^?2 s^?1/～525 µmol m^?2 s^?1). Photosynthesis and growth were enhanced by elevated CO₂ treatments but lower under flecked irradiance treatments. Acclimation of photosynthetic capacity occurred in response to elevated CO₂ but not flecked irradiance. Importantly, the relative enhancement effects of elevated CO₂ were greater under sunflecks (growth 60%, carbon gain 89%) compared with uniform irradiance (growth 25%, carbon gain 59%). This was driven by two factors: (1) greater efficiency of dynamic photosynthesis (photosynthetic induction gain and loss, post‐irradiance gas exchange); and (2) photosynthetic enhancement being greatest at very low PFD. This allowed improved carbon gain during both clusters of lightflecks (73%) and intervening periods of deep shade (99%). The relatively greater enhancement of growth and photosynthesis at elevated CO₂ under sunflecks has important potential consequences for seedling regeneration processes and hence forest structure and composition.     

COMPARATIVE EFFECTS OF LIGHT ON PIGMENTS OF TWO STRAINS OF EMILIANIA HUXLEYI (HAPTOPHYTA)1

Calcifying and a noncalcifying strains of Emiliania huxleyi were cultured in nutrient replete turbidostats under a photon flux density (PFD) gradient from 50 to 600 μmol E·m^?2·s^?1. For both strains, growth was PFD‐saturated at 300 μmol E·m^?2·s^?1. The strains, although with clearly different physiological properties due to the presence or absence of calcification, showed the same trends and magnitude of change in their pigment compliment as a function of PFD. Light‐controlled pigment composition and the trends of change in pigment composition were identical in both strains. Fucoxanthin (Fuco) was the major carotenoid in the calcifying strain, while in the noncalcifying strain this role was assumed by 19′ hexanoyloxyfucoxanthin (19 Hex). The photoprotective pigments and 19 Hex, normalized to chl a, increased with increasing light, while chl a content per cell and chl c's and Fuco, normalized to chl a, decreased with increasing PFD. The sum of all carotenoids normalized to chl a was remarkably similar in all PFDs used. Collectively, our results suggest that 19 Hex was synthesized from Fuco with light as a modulating factor and that the total amount of carotenoids is strain‐specific and synthesized/catabolized in tandem with chl a to a genetically predefined level independent of PFD.     

Thermal acclimation and photoacclimation of photosynthesis in the brown alga Laminaria saccharina

Thermal acclimation and photoacclimation of photosynthesis were compared in Laminaria saccharina sporophytes grown at temperatures of 5 and 17 °C and irradiances of 15 and 150μmol photons m^?2 s^?1. When measured at a standard temperature (17°C), rates of light-saturated photosynthesis (P_max) were higher in 5 °C-grown algae (c. 3.0 μmol O₂ m^?2 s^?1) than in 17 °C-grown algae (c. 0.9 μmol O₂ m_-2 s_-1). Concentrations of Rubisco were also 3-fold higher (per unit protein) in 5 °C-grown algae than in algae grown at 17 °C. Light-limited photosynthesis responded similarly to high temperature and low light Photon yields (α) were higher in algae grown at high temperature (regardless of light), and at 5 °C in low light, than in algae grown at 5 °C in high light Differences in a were correlated with light absorption; both groups of 17 °C algae and 5 °C low-light algae absorbed c. 75% of incident light, whereas 5 °C high-light algae absorbed c. 55%. Increased absorption was correlated with increases in pigment content PSII reaction centre densities and the fucoxanthin-Chl ale protein complex (FCP). Changes in a were also attributed, in part, to changes in the maximum photon yield of photosynthesis (0_max). PSI reaction centre densities were unaffected by growth temperature, but the areal concentration of PSI in low-light-grown algae was twice that of high-light-grown algae (c. 160.0 versus 80.0 nmol m^?2). We suggest that complex metabolic regulation allows L, saccharina to optimize photosynthesis over the wide range of temperatures and light levels encountered in nature.     

Photosynthetic apparatus in primary leaves of barley seedlings grown under blue or red light of very low photon flux densities

Chlorophyll (Chl) a and Chl b contents, rate of CO₂ gas exchange, quenching coefficients of chlorophyll fluorescence, and endogenous phytohormones have been studied in primary leaves of barley seedlings cultivated under blue (BL) or red (RL) light. Photon flux densities (PFD) were between 0.3 and 12 mol m^-2 s^-1. Plants grown at PFD of 0.3 mol m^-2 s^-1 demonstrated in BL tenfold and in RL threefold decreased Chl content compared to plants grown at 12 mol m^-2 s^-1. Chl a/b ratio increased from 2.3–2.5 to 4.4–4.5 in BL, not in RL, following the decrease in PFD at plant cultivation from 12 to 0.3 mol m^-2 s^-1. Plants cultivated at weak BL demonstrated severalfold decreased rate of photosynthetic CO₂ uptake, whereas decrease in PFD of RL from 12 to 0.3 mol m^-2 s^-1 caused only 20% de cline in the rate of photosynthesis. Decrease in PFD during a plant cultivation reduced the maximum quantum yield of photosynthesis in BL, not in RL leaves. Light response curves of non-photochemical and photochemical quenching of chlorophyll fluorescence calculated on the basis of absorbed quanta were not affected by PFD of RL during plant cultivation. On the contrary, both non-photochemical quenching and accumulation of Q_A ^-, reduced primary acceptor of Photosystem II, occurred at lower amounts of absorbed quanta in leaves of BL plants grown at 0.3 than at 12 mol m^-2 s^-1. Two photoregulatory reactions were suggested to exert the light control of the development of photosynthetic apparatus in the range of low PFDs. The photoregulatory reaction saturating by very low PFDs of RL was supposed to be mediated by phytochrome. Phytochrome was proposed to enhance (as related to other pigment-protein complexes of thylakoids) the accu mulation of chlorophyll- b-binding light-harvesting complex of Photosystem II (LHC II). It acts independently of the pigment mediating the second photoregulatory reaction, as evidenced by the results of experiments with plant growth under mixed blue plus red light. The contents of cytokinins and indole-3-acetic acid in a leaf were not significantly affected by either light quality and PFD thus indicating those phytohormones not to be involved into photoregulatory processes.     

INORGANIC CARBON LIMITATION OF PHOTOSYNTHESIS IN ULVA ROTUNDATA (CHLOROPHYTA)1

A computerized oxygen electrode Astern was used to make rapid and accurate measurements of photosynthetic light and dissolved inorganic carbon (DIC) response cures with a macroalga. Ulva rotundata Blid. was grown in an outdoor, continuous flow system in seawater under sunlight or 9% of sunlight at Beaufort, North Carolina. The light compensation points in the shade- and sun-grown plants, measured in seawater, were at photon flux densities (PFDs) of 16 and 27 μmol. Photons·m^?2·s^?1, respectively but the quantum yield of O₂ evolution was not significantly different. Rates of photosynthesis in seawater per unit area of thallus under saturating light and rates of dark respiration were about 1.5-fold higher in sun- than in shade-grown plants. The concentration of DIC in seawater (approximately 2 mM) limited photosynthesis at absorbed PFDs above 60–70 μmol photons·m^?2·s^?1 Addition of 20 mM inorganic carbon had no effect on quantum yield but caused about a 1.5-fold increase in the light-saturated photosynthetic rate in both shade- and sun-grown Ulva. The effect of DIC supplementation was greatest in plants grown in October and least in plants grown in June. The light- and DIC-saturated rate of photosynthesis in seawater was similar to the maximum rate obtained by exposing Ulva to 10% CO₂, in the gas phase. The carbon isotope values (δ¹³C, reflecting the ¹³C/¹²C ratio compared to a standard) of Ulva grown in the same seawater supply were dependent on light and agitation. Samples from Beaufort Inlet were more negative (δ¹³C value, ?20.03‰) than those grown in bright light with agitation (δ¹³C value, ?17.78‰ outdoors; ?17.23‰ indoors), which may indicate DIC supply limited carbon uptake in seawater.     

INFLUENCE OF SALINITY AND TEMPERATURE ON GROWTH AND PHOTOSYNTHESIS IN THE EXTREMOPHILIC CHLOROPHYTE,NANNOCHLORIS SP.

Major, K. M. & Henley, W. J. Department of Botany, Oklahoma State University, Stillwater, OK 74078-3013 USA Preliminary data suggest Nannochloris sp., isolated from the Great Salt Plains National Wildlife Refuge, is a true extremophile. This alga is able to withstand salinities ranging from 0 to 150 ç and temperatures up to 45°C. To test the hypothesis that acclimation to high salinity confers tolerance to high temperature, experimental cultures were acclimated to salinities of 25 and 100 ç and/or temperatures of 23 and 38°C; irradiance (500 mol photons m^-2 s^-1) was saturating for both growth and photosynthesis. Cells acclimated to low salt and low temperature exhibited high photosynthetic performance in terms of both light-saturated photosynthesis (P_max; 45.0 fmol O₂ cell^-1 h^-1) and light-harvesting efficiency (0.103 fmol O₂ cell^-1 h^-1/mol photons m^-2 s^-1). However, high-salinity cells exhibited values for net P_max (18.1 fmol O₂ cell^-1 h^-1), (0.107 fmol O₂ cell^-1 h^-1/mol photons m^-2 s^-1) and growth rates (ca. 0.4 d^-1) that were equal to, or higher than, those of low-salinity cells when acclimated to high temperature. Both the amount of light required to achieve net photosynthesis (I_c) and that required to achieve light-saturated photosynthesis (I_k) were lower in high-salinity cells than those exhibited by low-salinity cells grown at high temperature; reductions in I_c and I_k were primarily due to increases in light-harvesting efficiency. We propose that an increase in growth temperature might release Nannochloris sp. from energy constraints associated with osmolyte production and low-temperature effects on enzyme activity. These data are consistent with effects of short-term temperature stress on Chl a fluorescence kinetics in this alga.     

Photon flux density and light quality induce changes in growth,stomatal development,photosynthesis and transpiration of <Emphasis Type="Italic">Withania Somnifera</Emphasis> (L.) Dunal. plantlets

The aim of the study was to establish whether the quantity and the quality of light affect growth and development of Withania somnifera plantlets. We have studied growth and histo-physiological parameters [stomatal characteristics, chloroplastic pigments concentrations, photosynthesis, and transpiration (E)] of W. somnifera plantlets regenerated under various light intensities, or monochromatic light or under a mixture of two colors of light in tissue culture conditions. Plantlets grown under a photon flux density (PFD) of 30 μmol m^-2 s^-1 showed greater growth and development than those raised under other PFDs. Chlorophylls and carotenoids, numbers of stomata, rate of photosynthesis (P_N) and transpiration (E), stomatal conductance (g_s), and water use efficiency (WUE) increased with increasing the PFD up to 60 μmol m^-2 s^-1. Light quality also affected plantlets growth and physiology. Highest growth was observed under fluorescent and in a mixture of blue and red light. Very few stomata were developed in any of the monochromatic light but under fluorescent or under a mixture of two colors stomatal numbers increased. Similarly, g_s, E, P_N, and WUE were also higher under fluorescent light and under a mixture of red and blue light. Regressional analysis showed a linear relationship between P_N (r ² = 70) and g_s and between E (r ² = 0.95) and g_s. In conclusion, both the quality and the quantity of light affect growth of plantlets, development of stomata and physiological responses differently depending on the intensity and the wavelength of light.     

Seasonal variation in energy fluxes and carbon dioxide exchange for a broad-leaved semi-arid savanna (Mopane woodland) in Southern Africa

We studied the seasonal variation in carbon dioxide, water vapour and energy fluxes in a broad‐leafed semi‐arid savanna in Southern Africa using the eddy covariance technique. The open woodland studied consisted of an overstorey dominated by Colophospermum mopane with a sparse understorey of grasses and herbs. Measurements presented here cover a 19‐month period from the end of the rainy season in March 1999 to the end of the dry season September 2000. During the wet season, sensible and latent heat fluxes showed a linear dependence on incoming solar radiation (I) with a Bowen ratio (β) typically just below unity. Although β was typically around 1 at low incoming solar radiation (150 W m^?2) during the dry season, it increased dramatically with I, typically being as high as 4 or 5 around solar noon. Thus, under these water‐limited conditions, almost all available energy was dissipated as sensible, rather than latent heat. Marked spikes of CO₂ release occurred at the onset of the rainfall season after isolated rainfall events and respiration dominated the balance well into the rainfall season. During this time, the ecosystem was a constant source of CO₂ with an average flux of 3–5 μmol m^?2 s^?1 to the atmosphere during both day and night. But later in the wet season, for example, in March 2000 under optimal soil moisture conditions, with maximum leaf canopy development (leaf area index 0.9–1.3), the peak ecosystem CO₂ influx was as much as 10 μmol m^?2 s^?1. The net ecosystem maximum photosynthesis at this time was estimated at 14 μmol m^?2 s^?1, with the woodland ecosystem a significant sink for CO₂. During the dry season, just before leaf fall in August, maximum day‐ and night‐time net ecosystem fluxes were typically ?3 μmol m^?2 s^?1 and 1–2 μmol m^?2 s^?1, respectively, with the ecosystem still being a marginal sink. Over the course of 12 months (March 1999–March 2000), the woodland was more or less carbon neutral, with a net uptake estimated at only about 1 mol C m^?2 yr^?1. The annual net photosynthesis (gross primary production) was estimated at 32.2 mol m^?2 yr^?1.     

High rates of net ecosystem carbon assimilation by Brachiara pasture in the Brazilian Cerrado

To investigate the consequences of land use on carbon and energy exchanges between the ecosystem and atmosphere, we measured CO₂ and water vapour fluxes over an introduced Brachiara brizantha pasture located in the Cerrado region of Central Brazil. Measurements using eddy covariance technique were carried out in field campaigns during the wet and dry seasons. Midday CO₂ net ecosystem exchange rates during the wet season were ?40 μmol m^?2 s^?1, which is more than twice the rate found in the dry season (?15 μmol m^?2 s^?1). This was observed despite similar magnitudes of irradiance, air and soil temperatures. During the wet season, inferred rates of canopy photosynthesis did not show any tendency to saturate at high solar radiation levels, with rates of around 50 μmol m^?2 s^?1 being observed at the maximum incoming photon flux densities of 2200 μmol m^?2 s^?1. This contrasted strongly to the dry period when light saturation occurred with 1500 μmol m^?2 s^?1 and with maximum canopy photosynthetic rates of only 20 μmol m^?2 s^?1. Both canopy photosynthetic rates and night‐time ecosystem CO₂ efflux rates were much greater than has been observed for cerrado native vegetation in both the wet and dry seasons. Indeed, observed CO₂ exchange rates were also much greater than has previously been reported for C₄ pastures in the tropics. The high rates in the wet season may have been attributable, at least in part, to the pasture not being grazed. Higher than expected net rates of carbon acquisition during the dry season may also have been attributable to some early rain events. Nevertheless, the present study demonstrates that well‐managed, productive tropical pastures can attain ecosystem gas exchange rates equivalent to fertilized C₄ crops growing in the temperate zone.     

Changes in photon flux can induce stomatal patchiness

Images of chlorophyll fluorescence were used to detect the occurrence of stomatal patchiness in leaves from eight species under variable photon flux conditions. Pronounced stomatal patchiness was induced within 5–10 min after PFD was changed from intermediate (～450 μmol quanta m^?2 s^?1) to low (～150 μmol quanta m^?2 s^?1) levels. This effect was completely reversible by returning PFD to intermediate levels. The pattern of heterogeneous fluorescence for each leaf was usually similar during repeated applications of medium and low PFD. In three species, stomatal patchiness could only be induced in slightly water-stressed plants. Leaves of more severely water-stressed Xanthium strumarium plants in low air humidity exhibited oscillations in fluorescence that corresponded with oscillatory changes in leaf diffusion conductance for water vapour. Stomatal patchiness was also induced by illuminating dark-adapted leaves with low PFD (below 200–300 μmol quanta m^?2 s^?1). Infiltration of leaves with distilled water showed that heterogeneous chlorophyll fluorescence was caused by changes in stomatal apertures.     

Dynamic responses of photosystem II and phycobilisomes to changing light in the cyanobacterium Synechococcus sp. PCC 7942

We have examined the molecular and photosynthetic responses of a planktonic cyanobacterium to shifts in light intensity over periods up to one generation (7 h). Synechococcus sp. PCC 7942 possesses two functionally distinct forms of the D1 protein, D1∶1 and D1∶2. Photosystem II (PSII) centers containing D1∶1 are less efficient and more susceptible to photoinhibition than are centers containing D 1∶2. Under 50 μmol photons· m^?2·s^?1, PSII centers contain D1∶1, but upon shifts to higher light (200 to 1000 μmol photons·m^?2·s^?1), D1∶1 is rapidly replaced by D 1∶2, with the rate of interchange dependent on the magnitude of the light shift. This interchange is readily reversed when cells are returned to 50 μmol photons·m^?2·s^?1. If, however, incubation under 200 μmol photons·m^?2·s^?1 is extended, D1∶1 content recovers and by 3 h after the light shift D1∶1 once again predominates. Oxygen evolution and chlorophyll (Chl) fluorescence measurements spanning the light shift and D1 interchanges showed an initial inhibition of photosynthesis at 200 μmol photons·m^?2·s^?1, which correlates with a proportional loss of total D1 protein and a cessation of growth. This was followed by recovery in photosynthesis and growth as the maximum level of D 1∶2 is reached after 2 h at 200 μmol photons·m^?2·s^?1. Thereafter, photosynthesis steadily declines with the loss of D1∶2 and the return of the less-efficient D1∶1. During the D1∶1/D1∶2 interchanges, no significant change occurs in the level of phycocyanin (PC) and Chl a, nor of the phycobilisome rod linkers. Nevertheless, the initial PC/Chl a ratio strongly influences the magnitude of photo inhibition and recovery during the light shifts. In Synechococcus sp. PCC 7942, the PC/Chl a ratio responds only slowly to light intensity or quality, while the rapid but transient interchange between D1∶1 and D 1∶2 modulates PSII activity to limit damage upon exposure to excess light.     

Nitrogen relations of the sorghum-Sfriga hermonthica hostparasite association: growth and photosynthesis

The extent to which the parasitic angiosperm Striga hermonthica reduces the growth of its sorghum host is dependent on the concentration of nitrogen (as NH₄NO₃ in 40% Long Ashton Solution) supplied to the plants. The biomass of 0.5,1 and 2 mol m^?3 N-grown infected plants was 22,30 and 66%, respectively, of uninfected plants after 140d growth. The biomass of 3 and 4 mol m^?3 N-grown infected plants differed little from uninfected plants. No grain was set in 0.5 and 1 mol m^?3 N-grown infected plants, grain yield reached 42 and 73% of controls in 2 and 3 mol m^?3 N-grown plants, and was unaffected in 4 mol m^?3 N-grown plants. Striga hermonthica also altered the allometry and architecture of the host, at all but the highest N concentration. Higher N concentration (3 and 4 mol m ^?3 N) reduced the growth of S. hermonthica. Foliar N concentrations in sorghum ranged from 11 mg g^?1 dwt. in 0.5 mol m^?3 N-grown plants, to 28 mg g^?1 dwt. in 4 mol m^?3 N-grown plants, and were not affected by S. hermonthica. Higher N concentrations were measured in S. hermonthica, and ranged from 18 to 45 mg g^?1 dwt. in 0.5 and 3 mol m^?3 N-grown plants, respectively. The relationship between photosynthesis (CO₂ flux) and N concentration differed between uninfected and infected sorghum. This was most apparent in 0.5 mol m^?3 N-grown plants, with rates of 16 and 11 μmol m^?2 s^?1 in uninfected and infected plants, respectively (at 1500–1800 μmol m^?2 s^?1 photosynthetic photon flux density). At higher N concentrations, this difference was smaller, with both sets of plants reaching 26 μmol m^?2 s^?1 at 4 mol m^?3 N. Varying the level of S. hermonthica infection showed that the effect of N on host photosynthesis cannot be explained by differences in the mass or number of parasites supported by the host. At low levels of infection in 1 mol m^?3 N-grown plants, the negative effect of the parasite was reversed, and photosynthesis in infected plants exceeded that in uninfected plants by 20%. Photosynthesis in S. hermonthica at 3 mol m^?3 N (8 μmol m^?2 s^?1) was double that in 0.5 mol m^?3 N-grown plants. Stable carbon isotope and gas exchange measurements data demonstrated that this higher level of autotrophic carbon fixation was accompanied by a lower dependency on hetero trophic carbon. The latter ranged from 27 to 6% in 0 5 mol m^?3 and 3 mol m^?3 N-grown plants, respectively.