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.     

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.     

DIEL VARIATIONS IN OPTICAL PROPERTIES OF MICROMONAS PUSILLA (PRASINOPHYCEAE)1

Micromonas pusilla (Butcher) Manton et Parke, a marine prasinophyte, was used to investigate how cell growth and division affect optical properties of phytoplankton over the light:dark cycle. Measurements were made of cell size and concentration, attenuation and absorption coefficients, flow cytometric forward and side light scattering and chl fluorescence, and chl and carbon content. The refractive index was derived from observations and Mie scattering theory. Diel variations occurred, with cells increasing in size, light scattering, and carbon content during daytime photosynthesis and decreasing during nighttime division. Cells averaged 1.6 μm in diameter and exhibited phased division, with 1.3 divisions per day. Scattering changes resulted primarily from changes in cell size and not refractive index; absorption changes were consistent with a negligible package effect. Measurements over the diel cycle suggest that in M. pusilla carbon‐specific attenuation varies with cell size, and this relationship appears to extend to other phytoplankton species. Because M. pusilla is one of the smallest eukaryotic phytoplankton and belongs to a common marine genus, these results will be useful for interpreting in situ light scattering variation. The relationship between forward light scattering (FLS) and volume over the diel cycle for M. pusilla was similar to that determined for a variety of phytoplankton species over a large size range. We propose a method to estimate cellular carbon content directly from FLS, which will improve our estimates of the contribution of different phytoplankton groups to productivity and total carbon content in the oceans.     

Correlation between persistent forms of zeaxanthin-dependent energy dissipation and thylakoid protein phosphorylation

High light stress induced not only a sustained form of xanthophyll cycle-dependent energy dissipation but also sustained thylakoid protein phosphorylation. The effect of protein phosphatase inhibitors (fluoride and molybdate ions) on recovery from a 1-h exposure to a high PFD was examined in leaf discs of Parthenocissus quinquefolia (Virginia creeper). Inhibition of protein dephosphorylation induced zeaxanthin retention and sustained energy dissipation (NPQ) upon return to low PFD for recovery, but had no significant effects on pigment and Chl fluorescence characteristics under high light exposure. In addition, whole plants of Monstera deliciosa and spinach grown at low to moderate PFDs were transferred to high PFDs, and thylakoid protein phosphorylation pattern (assessed with anti-phosphothreonine antibody) as well as pigment and Chl fluorescence characteristics were examined over several days. A correlation was obtained between dark-sustained D1/D2 phosphorylation and dark-sustained zeaxanthin retention and maintenance of PS II in a state primed for energy dissipation in both species. The degree of these dark-sustained phenomena was more pronounced in M. deliciosa compared with spinach. Moreover, M. deliciosa but not spinach plants showed unusual phosphorylation patterns of Lhcb proteins with pronounced dark-sustained Lhcb phosphorylation even under low PFD growth conditions. Subsequent to the transfer to a high PFD, dark-sustained Lhcb protein phosphorylation was further enhanced. Thus, phosphorylation patterns of D1/D2 and Lhcb proteins differed from each other as well as among plant species. The results presented here suggest an association between dark-sustained D1/D2 phosphorylation and sustained retention of zeaxanthin and energy dissipation (NPQ) in light-stressed, and particularly photoinhibited, leaves. Functional implications of these observations are discussed.This revised version was published online in October 2005 with corrections to the Cover Date.     

CELL AND GROWTH CHARACTERISTICS OF TYPES A AND B OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) AS DETERMINED BY FLOW CYTOMETRY AND CHEMICAL ANALYSES1

Two morphotypes of Emiliania huxleyi (Lohmann 1902) Hay et al. 1967, types A and B, known to be unequally distributed in the oceans, were grown in dilution cultures at a range of photon flux densities (PFDs) (1.5–155 μmol photons·m^?2·s^?1) and two temperatures (10° and 15° C). Calcite carbon and organic carbon content of the cells as well as instantaneous growth rate, cell size, chlorophyll fluorescence, and light-scatter properties clearly depended on growth conditions and differed considerably for the two morphotypes. The ratio between calcite carbon and organic carbon production showed an optimum of 0.65 in E. huxleyi type A cells at PFD = 17.5. The ratio increased slightly with a temperature increase from 10° to 15°C but remained < 1.0 at both temperatures in light-limited cells. In contrast, calcite carbon production exceeded organic carbon production (ratio: 1.4–2.2) in phosphate-deprived cultures. Emiliania huxleyi type B generally showed a higher calcite carbon/organic carbon ratio than E. huxleyi type A, but the relation with PFD was similar. The content of calcite carbon and organic carbon as well as the instantaneous growth rate, cell size, chlorophyll fluorescence, and light-scatter properties showed large diel variations that were closely related to the division cycle. Our results show the importance of mapping the structure of any sampled cell population with respect to the phase in the cell division cycle, as this largely determines the outcome of not only “per cell” measurements but also short time (less than 24 h) flux measurements. For instance, dark production of calcite by E. huxleyi was negatively affected by cell division. Slowly growing (phosphate-stressed) cultures produced calcite in the light and in the dark. In contrast, rapidly growing cultures at 10°C produced calcite only in the light, whereas in the dark there was a significant loss of calcite due to dissolution.     

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.     

Differences in light energy utilisation and dissipation between dipterocarp rain forest tree seedlings

 The light environment within tropical rain forests varies considerably both spatially and temporally, and photon flux density (PFD) is considered to be an important factor determining the growth and survival of rain forest tree seedlings. In this paper we examine the ability of four ecologically contrasting dipterocarps (Dryobalanops lanceolata, Shorea leprosula, Hopea nervosa and Vatica oblongifolia) to utilise and dissipate light energy when grown in different light environments in lowland dipterocarp rain forest in the Danum Valley Conservation Area, Sabah, East Malaysia. Specifically we report (i) photosynthetic light response curves and associated fluorescence characteristics, including quantum yield (ΦPSII) and non-photochemical quenching (qN) and (ii) the extent to which photoinhibition occurs when plants grown in either high or low light are exposed to short bursts of high PFD. When grown in low light (artificial or forest shade) all four species had low light saturated rates of photosynthesis which were achieved at low PFDs. In addition, values of ΦPSII and qN were similar over a range of measurement PFDs. D. lanceolata and S. leprosula were also grown at high PFD and showed marked differences in their responses. S. leprosula demonstrated an ability to increase its rate of photosynthesis and there was a small increase in capacity to dissipate excess light energy non-photochemically at high PFDs. Partitioning of this qN into its fast, photo-protective (qE) and slow, photoinhibitory (qI) components indicated that there was an increase in qE quenching. In contrast, although D. lanceolata survived in the high light environment, greater rates of photosynthesis were not observed and the plants showed a greater capacity to dissipate energy non-photochemically. Partitioning of qN revealed that the majority of this increase was attributable to the slower relaxing phases. Received: 10 February 1996 / Accepted: 14 June 1996     

Daily carbon gain by Adenocaulon bicolor (Asteraceae), a redwood forest understory herb,in relation to its light environment

Summary The relationships between carbon gain and availability of sunfleck- and diffuse-light were determined for Adenocaulon bicolor by following the daily courses of assimilation and incident PFD on different days and locations in a redwood forest understory. Total PFD for the days sampled ranged from 1 to 4% of full sun values. Sunflecks accounted for 50 to 90% of the total PFD and were responsible for the majority of variation among days and locations. Each day had several clusters of sunfleck activity separated by relatively long intervals of diffuse light. Most sunflecks had maximum PFDs below the photosynthetic light-saturation point, and they had a median length and diffuse light interval separating them of 2 s. Daily carbon gain varied from 14 to 40 mmol m^-2d^-1 and was more strongly correlated with differences among days in total sunfleck PFD (r ²=0.81) than with variation in diffuse PFD (r ²=0.54). The assimilation that was attributable to sunflecks ranged from essentially zero on one day to 30 to 65% of the total on the other days. Carbon gain on most days was 70 to 80% of that predicted by a model based on the measured light dependences of assimilation. This model assumed an instantaneous response to changes in PFD, whereas incomplete photosynthetic induction probably limited the capacity to respond to sunflecks and therefore limited carbon gain on most days.     

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.     

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.     

Light adaptation/acclimation of photosynthesis and the regulation of ribulose-1,5-bisphosphate carboxylase activity in sun and shade plants

The consequences of light adaptation and acclimation of photosynthesis on photosynthetic nitrogen use efficiency (NUE), particularly as it relates to the efficiency of ribulose-1,5-bisphosphate carboxylase (Rubisco) use in photosynthetic CO₂ assimilation, was studied in the sun species Glycine max and the shade species Alocasia macrorrhiza. Both G. max and A. macrorrhiza were found to possess the capacity for light acclimation of CO₂ assimilation, but over distinctly different ranges of photon flux density (PFD). For each species, light acclimation of photosynthesis had little effect on the rate of photosynthesis per unit Rubisco protein or the light response of Rubisco carbamylation and CA 1P metabolism. In contrast, photosynthesis per unit Rubisco protein was significantly higher in G. max than in A. macrorrhiza, due in part to a lower total (fully carbamylated) molar activity (activity per unit enzyme) of A. macrorrhiza Rubisco than that of G. max. Comparison of the light response of Rubisco regulatory mechanisms between G. max and A. macrorrhiza indicated some degree of adaptation, such that carbamylation was higher and CA 1P levels lower at lower PFDs in the shade species than the sun species. However, this adjustment was not sufficient for Rubisco in low light grown A. macrorrhiza to be fully active at the growth PFD. Photosynthesis in A. macrorrhiza appeared to become RuBP regeneration-limited at lower PFDs than G. max, and this was probably the determinant of the light saturated rate of photosynthesis in the shade species. The low efficiency of Rubisco use in A. macrorrhiza was a major contributing factor to its five- to sixfold lower photosynthetic NUE than G. max. Shade species such as A. macrorrhiza appear to make far from maximal use of Rubisco protein N.     

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.     

Growth and photosynthetic rates of Phaeodactylum tricornutum Bohlin in a cyclical light field

Growth rates, cell chlorophyll a concentrations, and carbon uptake rates of Phaeodactylum tricornutum Bohlin grown in cyclical and constant light regimes at three mean irradiances (PFD) were compared. Light cycles with periods 5–38 min were used. Under high PFD the cyclical light regime resulted in both increased and decreased growth rates dependent on the signal frequency. Growth rates were universally depressed under mid and low PFD. Minimal influence of the light field was apparent on cell chlorophyll concentrations. The most marked effect of the fluctuating light was recorded on the carbon uptake rates with maximal influence demonstrated in those samples both grown and incubated in fluctuating light at high as opposed to mid and low PFDs. From the combination of light fields employed, the existence of both short and long term physiological controls of the carbon uptake rates were deduced, these being related to the time scales of carbon uptake and cell growth. A partial explanation to account for the observed increases in carbon uptake in terms of short period non-linear uptake kinetics was derived from two further experiments.     

Different strategies of photoacclimation by two strains of Emiliania huxleyi (Haptophyta)1

Photoacclimation involves the modification of components of the light and dark reactions to optimize photosynthesis following changes in available light. All of the energy required for photosynthesis comes from linear electron transport through PSII and PSI and is dependent upon the amount of light harvested by PSII relative to PSI (a*_PSII and a*_PSI). The amount of light harvested is determined by the effective absorption cross‐sections (σ_PSII, σ_PSI) and cellular contents of the PSII and PSI reaction center complexes (RCII, RCI). Here, we examine the effective absorption cross‐sections and reaction center contents for calcifying (B11) and noncalcifying (B92) strains of the globally important coccolithophorid Emiliania huxleyi (Lohmann) W. H. Hay et H. Mohler when grown under various photon flux densities (PFDs). The two strains displayed different “strategies” of acclimation. As growth PFD increased, B11 preferentially changed σ and the cellular content of chl a per cell over PSU “size” (the total cellular chl a content associated with the reaction center complexes); strain B92 preferentially changed PSU size over the cellular content of reaction complexes. Neither strategy was specifically consistent with the majority of previous studies from other microalgal species. For both strains, cellular light absorption for PSII and PSI was maintained close to unity across the range of growth PFDs since changes of σ_PSII and σ_PSI were reciprocated by those of RCIIs and RCIs per cell. Our results demonstrate a significant adaptive flexibility of E. huxleyi to photoacclimate. Finally, we calculated the amount of chl a associated with either photosystem to consider our interpretations of photoacclimation based on conventional determinations of PSU size.     

ALGAL PHOTOSYNTHETIC MEMBRANE COMPLEXES AND THE PHOTOSYNTHESIS-IRRADIANCE CURVE: A COMPARISON OF LIGHT-ADAPTATION RESPONSES IN CHLAMYDOMONAS REINHARDTII (CHLOROPHYTA)1

Chlamydomonas reinhardtii was grown at photon flux densities (PFDs) ranging from 47 to 400 μE.m^-2 s^-1. The total cellular content of chlorophyll (Chl) was twice as high in the low light (LL) versus high light (HL) grown cells. On an equal Chl basis, photosystem II (PSII) and cytochrome f (Cyt f) content was higher in HL cells, but photosystem I (PSI) concentration displayed little variation with the light intensity during cell growth. Consequently, there was a shift in the ratio of PSII / PSI and Cyt / PSI from near unity in LL cells to greater than two in HL cells. The functional Chl antenna size of PSII and PSI ranged from 460 and 170 Chl (a + b)in HL-grown cells to 620 and 370 Chl (a+ b)in LL-grown cells, respectively. The initial slope of the Chl-specific photosyn-thesis-irradiance (P-I) curve was similar in LL- and HL-grown cells, but the light saturated rate of photosynthesis was lower under LL. The response to low light was beneficial at the cellular level, since there was an enhancement of photosynthesis in LL. The PFD for the onset of light saturation, 1 was a factor of 2 lower in LL- relative to HL-grown photosythetic membranes. Since growth PFD varied by a factor of ten, photosynthesis shifted from being light-limited in the LL regime to light-saturated in the HL regime. The requirement for balanced absorption of light by the two photosystems constrains the PSII / PSI ratio to near unity when growth is light-limited, but such a constraint does not apply in HL conditions. Instead the concentration of individual electron transport complexes way be related to the pool size necessary for maximum rates of steady-state electron transport. Thus the stoichiometry of electron transport complexes changes in response to growth PFD and this change is correlated with the response flexlbility of algal photosynthesis in diverse light environments.     

NONEQUILIBRIUM RATES OF PHOTOSYNTHESIS AND RESPIRATION UNDER DYNAMIC LIGHT SUPPLY1

To identify processes that might account for differences in growth rates of rhodophytes under constant and dynamic light supply, we examined nonequilibrium gas exchange by measuring time courses of photoinduction, loss of photoinduction, and respiration rates immediately after the light–dark transition. Using the rhodophyte species Palmaria palmata (Huds.) Lamour and Lomentaria articulata (Huds.) Lyngb., we compared the effects of growth-saturating constant photon flux density (PFD) (95 μmol photons · m^?2· s^?1) to those of a dynamic light supply modeled on canopy movements in the intertidal zone (25 μmol photons · m^?2· s^?1 background PFD plus light flecks of 350 μmol photons · m^?2· s^?1, 0.1 Hz). The time required for P. palmata and L. articulata to be fully photoinduced was not affected by the dynamics of light supply. L. articulata required only 6 min of illumination with either fluctuating or constant light to be completely induced compared to 20 min for P. palmata. The latter species also lost photoinduction more rapidly than did L. articulata in the dark. There was no significant decline in photoinduction state for either species at the background PFD. The time courses of respiration after illumination with constant and fluctuating light were significantly different for P. palmata but not for L. articulata when the total photon dose was equal. In general, gas exchange of P. palmata appeared to be particularly sensitive to the temporal distribution of light supply whereas that of L. articulata was sensitive to the amplitude of variations, being photoinhibited at high PFD. These results are discussed in terms of the different mechanisms of inorganic carbon acquisition in the two species.     

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.     

Effect of photoperiod and light intensity on in vitro propagation of Alocasia amazonica

Plantlets of Alocasia amazonica regenerated under a photon flux density (PFD) of 15 or 30 μmol m⁻² s⁻¹ showed better growth and development than those grown under higher PFDs. While chlorophyll a and chlorophyll b decreased, the number of stomata increased with increasing PFD. Photoperiods also affected plantlet growth and stomatal development. Highest growth was observed for the short photoperiod (8/16 h) and for equinoctial (12/12 h) light and dark periods. Very few stomata developed in the leaves of plantlets grown under a short photoperiod (8/16 h) and the number of stomata increased with increasing light period. In conclusion, both light intensity and photoperiod independently affect growth of A. amazonica and development of stomata, depending on the intensity and duration of light treatment.     

Carbon allocation under light and nitrogen resource gradients in two model marine phytoplankton1

Marine phytoplankton have conserved elemental stoichiometry, but there can be significant deviations from this Redfield ratio. Moreover, phytoplankton allocate reduced carbon (C) to different biochemical pools based on nutritional status and light availability, adding complexity to this relationship. This allocation influences physiology, ecology, and biogeochemistry. Here, we present results on the physiological and biochemical properties of two evolutionarily distinct model marine phytoplankton, a diatom (cf. Staurosira sp. Ehrenberg) and a chlorophyte (Chlorella sp. M. Beijerinck) grown under light and nitrogen resource gradients to characterize how carbon is allocated under different energy and substrate conditions. We found that nitrogen (N)‐replete growth rate increased monotonically with light until it reached a threshold intensity (~200 μmol photons · m^?2 · s^?1). For Chlorella sp., the nitrogen quota (pg · μm^?3) was greatest below this threshold, beyond which it was reduced by the effect of N‐stress, while for Staurosira sp. there was no trend. Both species maintained constant maximum quantum yield of photosynthesis (mol C · mol photons^?1) over the range of light and N‐gradients studied (although each species used different photophysiological strategies). In both species, C:chl a (g · g^?1) increased as a function of light and N‐stress, while C:N (mol · mol^?1) and relative neutral lipid:C (rel. lipid · g^?1) were most strongly influenced by N‐stress above the threshold light intensity. These results demonstrated that the interaction of substrate (N‐availability) and energy gradients influenced C‐allocation, and that general patterns of biochemical responses may be conserved among phytoplankton; they provided a framework for predicting phytoplankton biochemical composition in ecological, biogeochemical, or biotechnological applications.     

Unimodal size scaling of phytoplankton growth and the size dependence of nutrient uptake and use

Phytoplankton size structure is key for the ecology and biogeochemistry of pelagic ecosystems, but the relationship between cell size and maximum growth rate (μ_max) is not yet well understood. We used cultures of 22 species of marine phytoplankton from five phyla, ranging from 0.1 to 10⁶ μm³ in cell volume (V_cell), to determine experimentally the size dependence of growth, metabolic rate, elemental stoichiometry and nutrient uptake. We show that both μ_max and carbon‐specific photosynthesis peak at intermediate cell sizes. Maximum nitrogen uptake rate (V_maxN) scales isometrically with V_cell, whereas nitrogen minimum quota scales as V_cell^0.84. Large cells thus possess high ability to take up nitrogen, relative to their requirements, and large storage capacity, but their growth is limited by the conversion of nutrients into biomass. Small species show similar volume‐specific V_maxN compared to their larger counterparts, but have higher nitrogen requirements. We suggest that the unimodal size scaling of phytoplankton growth arises from taxon‐independent, size‐related constraints in nutrient uptake, requirement and assimilation.