Photon Fluence Rate Research Articles

Nitrate and phosphate removal by Spirulina platensis.

The cyanobacterium Spirulina platensis was used to verify the possibility of employing microalgal biomass to reduce the contents of nitrate and phosphate in wastewaters. Batch tests were carried out in 0.5 dm3 Erlenmeyer flasks under conditions of light limitation (40 micromol quanta m(-2) s(-1)) at a starting biomass level of 0.50 g/dm3 and varying temperature in the range 23-40 degrees C. In this way, the best temperature for the growth of this microalga (30 degrees C) was determined and the related thermodynamic parameters were estimated. All removed nitrate was used for biomass growth (biotic removal), whereas phosphate appeared to be removed mainly by chemical precipitation (abiotic removal). The best results in terms of specific and volumetric growth rates (mu=0.044 day(-1), Qx=33.2 mg dm(-3) day(-1)) as well as volumetric rate and final yield of nitrogen removal (Q(N-NO3-)=3.26 mg dm(-3) day(-1), Y(N-NO3-)=0.739) were obtained at 30 degrees C, whereas phosphorus was more effectively removed at a lower temperature. In order to simulate full-scale studies, batch tests of nitrate and phosphate removal were also performed in 5.0 dm3 vessels (mini-ponds) at the optimum temperature (30 degrees C) but increasing the photon fluence rate to 80 micromol quanta m(-2) s(-1) and varying the initial biomass concentration from 0.25 to 0.86 g/dm3. These additional tests demonstrated that an increase in the inoculum level up to 0.75 g/dm3 enhanced both NO3- and PO4(3-) removal, confirming a strict dependence of these processes on biomass activity. In addition, the larger surface area of the ponds and the higher light intensity improved removal yields and kinetics compared to the flasks, particularly concerning phosphorus removal (mu=0.032-0.050 day(-1), Qx=34.7-42.4 mg dm(-3) day(-1), Q(N-NO3-)=3.24-4.06 mg dm(-3) day(-1), Y(N-NO3-)=0.750-0.879, Q(P-PO4(3-))=0.312-0.623 mg dm(-3) day(-1), and Y(P-PO4(3-))=0.224-0.440).

Use of light and inorganic carbon acquisition by two morphotypes of Zostera noltii Hornem

The affinity for dissolved inorganic carbon (DIC) and the mechanisms to use HCO 3 − as a source of DIC for photosynthesis were investigated in two morphotypes of Zostera noltii Hornem. Both morphotypes were collected at Ria Formosa lagoon (Southern Portugal) at two different levels in the intertidal. Affinity for DIC at saturating photon fluence rate (PFR), estimated as photosynthetic conductance for CO 2 ( g p(CO 2) ), was reduced by 75% in the Z. noltii plants adapted to shade conditions (lower intertidal) in comparison to the sun morphotype (45×10 −6 and 182×10 −6 m s −1, respectively), indicating that the plants acclimated to sun conditions (higher intertidal) had a higher capacity to use HCO 3 − as DIC source for photosynthesis. Since external carbonic anhydrase activity was negligible and a large inhibitory effect was produced by Tris buffer addition, this HCO 3 − use was attributed to the operation of H + ATPases creating low pH zones in periplasmic space. The photosynthetic CO 2-flux supported for this mechanism was calculated to be 53 μmol O 2 m −2 s −1 in sun morphotype, about 80% out of maximum photosynthesis rate. In order to determine the possible photosynthetic energy cost of the HCO 3 − use, the effect of decreasing light on photosynthetic rates and g p(CO 2) was estimated. Photosynthetic conductance decreased in both morphotypes at non-saturating PFR. This dependence of g p(CO 2) on PFR indicated the existence of a positive interactive effect between DIC and PFR which was more pronounced in the shade morphotype since the ascending slope of O 2 evolution vs. PFR curves at limiting PFRs was reduced from 7.2 to 2.3 mmol O 2 mol photon −1 at 4 and 0.5 mol m −3 of DIC, respectively.

Spectroscopic and Photochemical Characterization of a Deep Ocean Proteorhodopsin

A second group of proteorhodopsin-encoding genes (blue-absorbing proteorhodopsin, BPR) differing by 20-30% in predicted primary structure from the first-discovered green-absorbing (GPR) group has been detected in picoplankton from Hawaiian deep sea water. Here we compare BPR and GPR absorption spectra, photochemical reactions, and proton transport activity. The photochemical reaction cycle of Hawaiian deep ocean BPR in cells is 10-fold slower than that of GPR with very low accumulation of a deprotonated Schiff base intermediate in cells and exhibits mechanistic differences, some of which are due to its glutamine residue rather than leucine at position 105. In contrast to GPR and other characterized microbial rhodopsins, spectral titrations of BPR indicate that a second titratable group, in addition to the retinylidene Schiff base counterion Asp-97, modulates the absorption spectrum near neutral pH. Mutant analysis confirms that Asp-97 and Glu-108 are proton acceptor and proton donor, respectively, in retinylidene Schiff base proton transfer reactions during the BPR photocycle as previously shown for GPR, but BPR contains an alternative acceptor evident in its D97N mutant, possibly the same as the second titratable group modulating the absorption spectrum. BPR, similar to GPR, carries out outward light-driven proton transport in Escherichia coli vesicles but with a reduced translocation rate attributable to its slower photocycle. In energized E. coli cells at physiological pH, the net effect of BPR photocycling is to generate proton currents dominated by a triggered proton influx, rather than efflux as observed with GPR-containing cells. Reversal of the proton current with the K+-ionophore valinomycin supports that the influx is because of voltage-gated channels in the E. coli cell membrane. These observations demonstrate diversity in photochemistry and mechanism among proteorhodopsins. Calculations of photon fluence rates at different ocean depths show that the difference in photocycle rates between GPR and BPR as well as their different absorption maxima may be explained as an adaptation to the different light intensities available in their respective marine environments. Finally, the results raise the possibility of regulatory (i.e. sensory) rather than energy harvesting functions of some members of the proteorhodopsin family.

Concerted response of xanthophyll-cycle pigments in a marine diatom, Chaetoceros gracilis, to shifts in light condition

SUMMARY The interconversion rate of diadinoxanthin (DD) cycle under high irradiance and subsequent darkness was analyzed using the cultivated centric marine diatom, Chaetoceros gracilis Schutt. A prompt de-epoxidation from diadinoxanthin to diatoxanthin (DT) occurred immediately after the onset of higher irradiance. The fist-order rate constant, k, for this de-epoxidation was 0.1–0.2 min-1 irrespective of the irradiance. The difference in photon fluence rate lead to the difference of the final amount of DT, leaving the rate constant at almost the same value. After turning off the light, epoxidation from DT to DD occurred. The first-order constant of epoxidation was much slower than that of de-epoxidation: 0.005 – 0.009 min-1. Independent of this epoxidation process, de novo synthesis of DD-cycle pigments was also observed under the subsequent darkness. Based on these findings, a common nature of the DD-cycle as a protection mechanism for photo-systems was demonstrated for C. gracilis.

Contents of Volume

The affinity for dissolved inorganic carbon (DIC) and the mechanisms to use HCO3− as a source of DIC for photosynthesis were investigated in two morphotypes of Zostera noltii Hornem. Both morphotypes were collected at Ria Formosa lagoon (Southern Portugal) at two different levels in the intertidal. Affinity for DIC at saturating photon fluence rate (PFR), estimated as photosynthetic conductance for CO2 (gp(CO2)), was reduced by 75% in the Z. noltii plants adapted to shade conditions (lower intertidal) in comparison to the sun morphotype (45×10−6 and 182×10−6 m s−1, respectively), indicating that the plants acclimated to sun conditions (higher intertidal) had a higher capacity to use HCO3− as DIC source for photosynthesis. Since external carbonic anhydrase activity was negligible and a large inhibitory effect was produced by Tris buffer addition, this HCO3− use was attributed to the operation of H+ ATPases creating low pH zones in periplasmic space. The photosynthetic CO2-flux supported for this mechanism was calculated to be 53 μmol O2 m−2 s−1 in sun morphotype, about 80% out of maximum photosynthesis rate. In order to determine the possible photosynthetic energy cost of the HCO3− use, the effect of decreasing light on photosynthetic rates and gp(CO2) was estimated. Photosynthetic conductance decreased in both morphotypes at non-saturating PFR. This dependence of gp(CO2) on PFR indicated the existence of a positive interactive effect between DIC and PFR which was more pronounced in the shade morphotype since the ascending slope of O2 evolution vs. PFR curves at limiting PFRs was reduced from 7.2 to 2.3 mmol O2 mol photon−1 at 4 and 0.5 mol m−3 of DIC, respectively.

Divergent light-, ascorbate-, and oxidative stress-dependent regulation of expression of the peroxiredoxin gene family in Arabidopsis.

Peroxiredoxins (prxs) are peroxidases with broad substrate specificity. The seven prx genes expressed in Arabidopsis shoots were analyzed for their expressional response to changing photon fluence rates, oxidative stress, and ascorbate application. The results reveal a highly variable and gene-specific response to reducing and oxidizing conditions. The steady-state transcript amounts of the chloroplast-targeted prxs, namely the two-cysteine (2-Cys) prxs, prx Q and prx II E, decreased upon application of ascorbate. prx Q also responded to peroxides and diamide treatment. prx II B was induced by tertiary butylhydroperoxide, but rather unaffected by ascorbate. The strongest responses were observed for prx II C, which was induced with all treatments. The two Arabidopsis 2-Cys Prxs and four Prx II proteins were expressed heterologously in Escherichia coli. In an in vitro test system, they all showed peroxidase activity, but could be distinguished by their ability to accept dithiothreitol and thioredoxin as electron donor in the regeneration reaction. The midpoint redox potentials (E(m)') of Prx II B, Prx II C, and Prx II E were around -290 mV and, thus, less negative than E(m)' of Prx II F, 2-Cys Prx A, and 2-Cys Prx B (-307 to -322 mV). The data characterize expression and function of the mitochondrial Prx II F and the chloroplast Prx II E for the first time, to our knowledge. Antibodies directed against 2-Cys Prx and Prx II C showed a slight up-regulation of Prx II protein in strong light and of 2-Cys Prx upon transfer both to high and low light. The results are discussed in context with the subcellular localization of the Prx gene products.

Observations on the effect of salinity and photon fluence rate on the induction of sporulation and rhizoid formation in the green alga Enteromorpha prolifera (Muller) J. Agardh (Chlorophyta, Ulvales)

Although most members of the genus Enteromorpha are important edible green algae, some species are also potentially economically valuable crops. Samples of E. prolifera were obtained from the Yoshino River estuary, Tokushima, Japan and cultured in laboratory conditions at 10°C, a salinity of 20 psu, under white light with 12 h light: 12 h dark cycle and at a photon fluence rate (PFR) of 40 μmol/m2 per s for 30 days. In the present study, the effect of salinity and PFR on the induction of reproductive cells and rhizoid formation were investigated. Synchronous formation of swarmers by thalli was induced in excised disks of 1.2 mm diameter after 2-5 days incubation. The optimum salinity for maturation of reproductive cells was between 5.0 and 52.0 psu, and between 13.2 and 45.3 psu for swarmer release, although the lower limit for swarmer release was 5.0 psu. Maturation of reproductive cells and swarmer release required a PFR higher than 16 μmol/m2 per s. The minimum PFR for swarmer release was 8 μmol/m2 per s. Many rhizoids were formed between 1.6 and 52.0 psu and photon fluence rates between 8 and 320 μmol/m2 per s. Rhizoids were formed in a polarized manner.

Stomatal uptake and cuticular adsorption contribute to dry deposition of NH3 and NO2 to needles of adult spruce (Picea abies) trees.

• In the present study NH 3 and NO 2 exchange between the atmosphere and needles of adult spruce ( Picea abies ) trees at a field site ('Höglwald') exposed to high loads of atmospheric nitrogen was assessed. • Twigs were fumigated with different NH 3 (C NH3 ) or NO 2 (C NO2 ) concentrations using the dynamic chamber technique. Beside fluxes of NH 3 (J NH3 ) and NO 2 (J NO2 ), transpiration (J H2O ), leaf conductance for water vapour (g H2O ), photosynthetic activity (J co2 ), photosynthetic photon fluence rate (PPFR), air temperature (T) and relative air humidity (RH) were determined. • Both fluxes, J NH3 and J NO2 , depended linearly on C NH3 and C NO2 in concentration ranges representative for the field site and g H2O as a measure of stomatal aperture. For both trace gases compensation points could be determined amounting to 2.5nmolmol -1 for NH 3 and to 1.7nmolmol -1 for NO 2 . • The fluxes of NH 3 and NO 2 could not be explained exclusively by exchange through the stomata. In both NH 3 and NO 2 fumigation experiments additional deposition onto the needle surface was observed and increased with increasing C NH3 and C NO2 . 15 N[NH 3 ] fumigation experiments with adult spruce trees confirmed the results of gas exchange measurements and revealed that NH 3 -N deposited to spruce needles is subjected to long distance transport within the plant, supplying the plant with additional nitrogen from the atmosphere.

Effects of photoacclimation on plasma membrane ferricyanide reductase from the rhodophyta Gracilaria tenuistipitata

The red macroalga Gracilaria tenuistipitata has a plasma membrane redox system easily assayed as a ferricyanide reductase (EC 1.6.99.13) activity. This activity is associated with acidification of the medium and is modulated by light conditions. G. tenuistipitata thalli adapt to light conditions by changes affecting their morphology, physiology and biochemistry. Thalli adapted to high photon fluence rates exhibited lower ferricyanide reductase activity than those adapted to darkness or low irradiance (40 μmol photon m −2 s −1). When thalli were changed from low to high irradiance (500 μmol photons m −2 s −1), there was a continuous decrease in ferricyanide reductase activity with time. When these thalli were changed to darkness, they recovered to control values of ferricyanide reductase activity after a 24 h additional incubation. Adaption of thalli to different qualities of light also produced significant changes in their ferricyanide reductase activity: under the same low irradiance (40 μmol photons m −2 s −1), thalli exposed for two or more weeks to blue or red light exhibited higher ferricyanide reductase activity levels than those exposed to white light; after four weeks of culture, thalli under blue light had higher ferricyanide reductase activity levels than those under red light. It is concluded that the differences in ferricyanide reductase activity observed under different light conditions are the result of acclimation and that this activity could play a relevant role in G. tenuistipitata bioenergetics.

Effect of photon fluence rates of white and blue-green light on growth efficiency and Pigment Content Of Three Diatom Species In Batch Cultures

This study provides information on the combined effect of light quality and photon fluence rates (light intensity) on growth efficiency and pigment content of the coastal plankton diatoms Chaetoceros sp., Skeletonema costatum and Thalassiosira pseudonana. The relative growth efficiencies of the three species were significantly higher at 210 µE m-2s-1, under mixed bluegreen light for Chaetoceros sp. and S. costatum, and with white light for T. pseudonana. The chlorophyll a contents of the three species grown under blue-green light, alone or mixed with white light, were significantly higher (P < 0.001) than those with white light and, in all cases, contents were lower at high photon fluence rates, while carotenoids showed inverse trends. The relative growth efficiencies of the three species were significantly different (P < 0.001). There were also changes in the relative pigment concentrations associated with the growth phase, probably due to autoshading.

Limited acclimation of photosynthesis to blue light in the seaweed Gracilaria tenuistipitata.

Changes in photosynthetic capacity of the seaweed Gracilaria tenuistipitata Zhang et Xia acclimated to monochromatic blue light were studied. For this purpose, affinity for external inorganic carbon, light use efficiency, carbonic anhydrase (CA; EC 4.2.1.1) activity and content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) were determined in thalli acclimated to 45 &mgr;mol m-2 s-1 of blue light. Thalli cultured in white light of the same photon fluence rate were used as a control. Lower maximal photosynthetic rates (i.e. at light and carbon saturation) were obtained in the thalli cultured in blue light. Apparently, this lower photosynthetic capacity was not due to differences in affinity and/or capacity for use of external dissolved inorganic carbon (DIC) since (1) CA activity did not change significantly and (2) similar values of photosynthetic conductance for DIC at alkaline pH were obtained (0.95 x 10-6 m s-1). In addition, the pool size of Rubisco was not modified by the blue light treatment since there were no significant differences in Rubisco content between white (12.14% of soluble proteins) and blue light (12.13% of soluble proteins) treatments. In contrast, Fv/Fm was increased by 11% and photosynthetic efficiency for oxygen production was reduced by 50% in blue light. This absence of correlation between quantum yields for maximum stable charge separation of photosystem II and oxygen evolution suggests that blue light promote changes in rates of photosynthetic electron flow.

The influence of drought, shade and availability of mineral nutrients on exudate phenolics of Dittrichia viscosa

Summary Exudate phenolics are thought to defend leaf surfaces against microorganisms and interfere with neighboring plants if leached to the soil. We investigated the effects of resource availability on growth and accumulation of epicuticular water soluble phenolics in potted seedlings of Dittrichia viscosa grown in the field under two levels of nutrients, water and photon fluence rates. As expected, growth was enhanced under additional nutrients and suppressed under water stress. Under shade, aboveground biomass accumulation was not affected, yet the total leaf area was considerably increased at the expense of dry mass invested on area basis. Glandular hair density was not affected by any treatment. The concentration of leaf exudate phenolics was increased under water stress and decreased under additional nutrients and shade, in accordance to the prevailing hypotheses predicting environmental effects oncarbon-based defensive secondary metabolites. Yet, when we considered total epicuticular phenolic production on a plant basis (i.e. the amount leachable to the soil, which better indicates its allelopathic potential), the changes in total leaf area compensated (shade) or over-compensated(nutrients, water stress) the changes in concentration. Thus, total leachable epicuticular phenolics were higher under additional nutrients and lower under water stress. We conclude that environmental factors and the corresponding physiological constrains in the production of exudate phenolics may act in opposite directions when surface protection and interference with neighbors is considered.

The donor side of photosystem II is impaired in a Cd 2+-tolerant mutant strain of the unicellular green alga Chlamydomonas reinhardtii

Summary Growth of a cadmium-tolerant mutant strain of the unicellular green alga Chlamydomonas reinhardtii was found to be impaired under photoautrotrophic, but not under mixotrophic conditions. As compared to wild-type cells, oxygen evolution by the photoautotrophically grown mutant was considerably decreased and higher photon fluence rates were required both for light compensation of oxygen consumption/production and maximal oxygen evolution. The capability for oxygen production was decreased in Chlamydomonas reinhardtii cells when grown in the presence of acetate without aeration. Wild-type cells grown under these conditions showed a rather low but significant oxygen evolution immediately after transfer to photoautotrophic conditions. This residual oxygen production was completely suppressed in the presence of acetate, obviously due to acetate inhibition of the water-splitting complex. In the case of our cadmium-tolerant mutant strain, however, residual oxygen production was measured even in the presence of acetate. After removal of acetate, oxygen evolution by the cadmium-tolerant mutant strain was increased to higher rates than measured for wild-type cells, but considerably higher photon fluence rates were required both for light compensation of oxygen consumption/production and maximal oxygen evolution. The conclusion that the donor side of photosystem II is affected in our cadmium-tolerant mutant strain was further corroborated by a stronger decrease of the fluorescence level caused by hydroxylamine.

The influence of selenium on photosynthesis and "light-enhanced dark respiration" (LEDR) in the flagellate Euglena gracilis after exposure to ultraviolet radiation

Photodamage of photosynthesis arises from oxidative damage. One of the protective mechanisms is to convert excessive absorbed energy into thermal radiation. Another mechanism could be to strengthen the antioxidative capacity of plants and algae. Selenium is important in antioxidation in humans and may play a role in antioxidative mechanisms in plants. The aim of this investigation was to study the role of selenium in "light-enhanced dark respiration" (LEDR) and photosynthesis in the flagellate Euglena gracilis, after exposure to ultraviolet radiation (UV-radiation). Selenium was added into the growth medium at different concentrations of selenite (10-7, 10-8, 10-9 and 10-10 M, Na2SeO3· 5H2O). E. gracilis were given six different light pulses with a photon fluence rate of 59, 163, 600, 1180, 2080 and 3340 μmol m-2 s-1 and periods of darkness between the light pulses. Photosynthetic saturation occurred at irradiances higher than 600 μmol m-2 s-1 and at the highest irradiance the photosynthetic rate decreased due to photoinhibition. Without any exposure to UV-radiation (UV-A, 320-400 nm, of 1.02 W m-2 plus UV-B, 280-320 nm, of 0.73 W m-2) LEDR increased with increasing photon fluence rate. After 40 min exposure to UV-radiation, photosynthetic rate and LEDR as functions of photon fluence rate were reduced. Neither in control (no UV-radiation) or when measured immediately after exposure to UV-radiation selenium had no stimulating effects on photosynthesis and LEDR. However, after UV-treatment and 24 h of recovery the presence of selenium led to an increase in photosynthesis and LEDR at higher irradiances. The results indicates that selenium might play a role in the repair mechanisms in E. gracilis after UV treatments.

Effect of temperature and photon fluence rate on gametophytes and young sporophytes of Laminaria ochroleuca Pylaie

We analysed the effects of temperature and photon fluence rate on meiospore germination, growth and fertility of gametophytes, and growth of young sporophytes of Laminaria ochroleuca. Maximum percentages of germination (91–98%) were obtained at 15°C and 18°C, independent of photon fluence rate. Optimal development of female gametophyte and maximum fecundity and reproductive success of gametophytes occurred at 15°C and 18°C and at 20 and 40 µmol m–2 s–1. Maximum relative growth rate of young sporophytes after 2 weeks of culture was achieved under the same conditions. L. ochroleuca gametophytes cannot reproduce and growth of its sporophytes is not competitive at temperatures close to 10°C.

Photoregulation of growth and branching of plum shoots: Physiological action of two photosystems

Plum shoot proliferation was investigated in terms of two distinct processes: axillary bud differentiation and axillary shoot development. Results showed that light quality influenced bud differentiation and interacted with apical dominance in determining shoot outgrowth, resulting in a differentiated structure of shoot clusters and type of branching. Results suggested that blue light, acting through its photoreceptor, increased the number of axillary buds differentiated from apical meristem, but did not remove the apical dominance. Red light removed apical dominance, while reducing the formation of axillary buds; both events appeared to be dependent on the putative amount of phytochrome active form, and independent of light photon fluence rate. On the contrary, blue light action appeared to be dependent on photon fluence rate. In addition, apparent blue-red interactions related to photomorphogenic events fit an antagonistic model for branching regulated by light via cryptochrome and phytochrome photoreceptors. Our results show that the dynamics of shoot cluster development is the product of two events: the formation of new axillary buds and their release from apical dominance.

O-Phenylenediamine-induced DNA damage and mutagenicity in tobacco seedlings is light-dependent

Of the three isomers of the aromatic amine phenylenediamine (PDA), only o-PDA, but not m- and p-PDA, induced DNA damage (as measured by the Comet assay), and somatic mutations in the leaves of the chlorophyll-deficient tester strain Nicotiana tabacum var. xanthi. With increasing light intensity (0, 30, 80 or 140 μmol m −2 s −1 photosynthetic photon fluence rate) during a 72 h mutagenic treatment of tobacco seedlings, o-PDA-induced DNA damage and the yield of somatic mutations were significantly increased. The peroxidase inhibitor diethyldithiocarbamate (DEDTC) repressed o-PDA-induced DNA damage. The effect of light is caused by the light-dependent increase of peroxidase activity and the accumulation of hydrogen peroxide, which participate in the metabolic activation of the promutagen o-PDA to mutagenic product(s). In contrast, DNA damage induced by the direct-acting alkylating mutagen ethyl methanesulphonate was the same whether treatment was in the light or in the dark, and was not repressed by the peroxidase inhibitor DEDTC.

Branching responses of a plagiotropic clonal herb to localised incidence of light simulating that reflected from vegetation

Plants sense the presence of neighbouring vegetation through phytochrome photoreceptors perceiving a lowered red to far-red ratio (R:FR) of light reflected from such vegetation. We hypothesised that it would be advantageous for the grassland clonal herb, Trifolium repens, to have an inhibitory branching response to perception by leaves of light reflected from neighbouring vegetation (i.e. light with lowered R:FR ratio) but have no response to interception of such light by the plagiotropic stem. We tested whether photoreception of reflected light by plagiotropic stems resulted in a different branching response to photoreception by leaves and whether leaf ontogeny influenced the response. To simulate light reflected from vegetation, FR light-emitting-diodes were used to supplement controlled environment room light so that the R:FR ratio, but not the photosynthetic photon fluence rate, of light incident at the stem or leaf of a phytomer of T. repens was lowered from 1.20 to 0.25. The plagiotropic stems were unresponsive to light simulating that reflected from vegetation. This response differs from that of stems of orthotropic species, indicating that plagiotropic stems have evolved an organ-specific photobiology. Treatment of the mature leaf with light of lowered FR ratio reduced phytomer production only of the branch in the axil of the treated leaf. Similar treatment of the immature leaf retarded, in addition, branching at basal phytomers on the same side of the primary stem axis. Thus the response to light simulating that reflected from neighbouring vegetation depended upon whether the light was incident at the stem or the leaf and on the stage of leaf development. We argue that such responses improve the performance and fitness of T. repens within grassland habitats by allowing axillary buds on plagiotropic stems to branch freely when stems are in receipt of light reflected from vegetation while leaves are in full light.

Effects of light quality on somatic embryogenesis in Araujia sericifera.

The effects of photoperiod, light quality and end-of-day (EOD) phytochrome photoconversion on somatic embryogenesis (SE) of Araujia sericifera petals have been studied. Petals from immature flowers were cultured under 8- and 16-h photoperiods using Gro-lux fluorescent lamps. The photon fluence rate was 90-100 µmol m-2 s-1 and the red (R):far-red (FR) ratio was 98. R, FR, R followed by FR (R-FR) and FR followed by R (FR-R) light treatments were applied for 3 weeks at the end of the photoperiods. In a set of experiments, DL-alpha-difluoromethylarginine (DFMA) or methylglyoxal bis(guanylhydrazone) (MGBG), both inhibitors of polyamine biosynthesis, were added to the culture medium in order to study the involvement of polyamine metabolism. The level of SE was the same in long (LD) and short (SD) days. Thus, the light effect was accomplished after 8 h. All EOD treatments that decreased the Pfr level inhibited SE when applied after SD, but not after LD. The FR-R treatment after LD caused an additional stimulatory effect on SE, even in the presence of polyamine inhibitors. DFMA inhibited SE in both SD and LD, but MGBG did not modify SE in either SD or LD. The R, FR and R-FR treatments did not alter the level of SE when applied after LD in the presence of DFMA or MGBG. However, these treatments decreased SE after SD when the medium contained polyamine inhibitors. Our results suggest that Gro-lux lamps, which produce an extremely high R:FR ratio, promote SE in A. sericifera and a timing response to phytochrome photoconversion during photoperiodic induction. Thus, our data corroborate the involvement of phytochromes and polyamines in SE in A. sericifera, which responded as a light-dominant long-day plant.

Carbon allocation and sugar status in individual cells of barley leaves affects expression of Sucrose: Fructan 6‐Fructosyltransferase gene

SummaryHeterogeneity of function in plant tissues requires precise regulation of metabolic systems controlling resource allocation at the level of single cells. Differential distribution of metabolites between epidermis, mesophyll and parenchymatous bundle sheath (PBS) cells has already been demonstrated in barley (Hordeum vulgare L.) source leaves and partitioning of sugars in barley leaves has been proven to be highly cell‐type specific. Carbohydrate metabolism of barley leaves induced to accumulate sucrose and fructans was investigated further at the individual cell level, using single cell sampling and analysis techniques. A novel approach using reverse transcription followed by polymerase chain reaction (RT‐PCR) was used to analyse expression of the enzyme Sucrose: Fructan 6‐Fructosyltransferase (6‐SFT) in individual cells of barley leaves. Accumulation of sucrose and the synthesis of fructans were induced by reducing export of sugars from leaves (root cooling), by increasing photosynthesis (high photon fluence rates), or by supplying sugars exogenously. In all cases there was a positive qualitative correlation between the presence of 6‐SFT transcripts and accumulation of fructan and sucrose in both mesophyll and PBS cells.