Biomass and Pigment Production in Three Isolates of Azolla I. Response to Water Stress

Two isolates of Azolla Lam. from Colombia, South America, andone from the USA (Florida) were selected for characterizationof their growth potentials in certain defined stress environments.These isolates were Amazonian (RAR) and Atlantic coastal (M-3)specimens of Azolla caroliniana Willd and a Floridian variety(LA) of Azolla filiculoides Lam. Osmotic stress (from polyethylene glycol-amended media) andosmotic/salt stress (from seawater-amended media) were the chosendeterminants in these trials. Their effects on growth responseswere monitored. Initial growth rates of LA and M-3 were stimulatedat 1 and 5 ppt seawater salts, relative to growth under controlconditions. RAR, however, was generally the most stable isolatein the experiments with seawater-induced ionic stress. LA wasmost stable in its biomass growth in experiments with polyethyleneglycol 6000, but chlorophyll accumulation data were again leastaffected in RAR. LA survived the longest of the three isolatesat a water potential of approximately —10 bars with eitherseawater (13 ppt salts) or polyethylene glycol (26 per cent).The latter amendment was usually more inhibitory to acetylenereduction rates in RAR and consistently more inhibitory in LA.RAR was tentatively judged by its reactions to water stressto be the superior isolate of the three. Azolla caroliniana Willd., Azolla filiculoides Lam., biomass and chlorophyll accumulation, acetylene reduction, osmotic/salt stress     

The Azolla, Anabaena azollae Relationship: II. Localization of Nitrogenase Activity as Assayed by Acetylene Reduction

Anaerobic (microaerophilic) acetylene reduction by Azolla caroliniana Willd. was dependent on light and saturated at approximately 450 foot candles. Maximum rates of acetylene reduction were 60 nmoles/mg chlorophyll minute. However, rates of 25 to 30 nmoles/mg chlorophyll minute were more common.     

Long-Term Chilling of Young Tomato Plants under Low Light. IV. Differential Responses of Chlorophyll Fluorescence Quenching Coefficients in Lycopersicon Species of Different Chilling Sensitivity

Temperature dependences of chlorophyll fluorescence quenchingcoefficients were studied in the cultivated tomato (Lycopersiconesculentum) and three lines of the chilling-tolerant L.peruvianumfrom different altitudes, i.e. LA 1373 (20 m a.s.l.), LA 2157(1,650 m a.s.l.) and LA 385 (2,400 m a.s.l.). At actinic lightintensity near light saturation of photosynthesis (370 µEm^–2 s^7minus;1), photochemical quenching (qP) increasedwith increasing temperature between 5 and 30°C. The temperature,at which qP reached the numerical value 0.5 [T (qP=0.5)] decreasedby 2.5–4.5°C after a chilling treatment of 14 daysat 10°C in L. peruvianum, indicating acclimation of thephotosynthetic dark reactions in this species. The final T (qP=0.5)attained after chilling could be arranged in the order L.esculentum>LA1373>LA 2157>LA 385. The fast relaxing non-photochemicalquenching (qN) component (qf, consisting mainly of energy-dependentquenching, qE) exhibited minima near the optimum temperaturefor photosynthesis. These minima shifted to lower temperaturesupon chilling in L. peruvianum. Photoinhibitory quenching (ql)was unaffected by chilling in the high altitude lines, but-increasedstrongly in LA 1373 and L. esculentum. Under low actinic light(40 µE m^–2 s^–1), temperature dependences ofqP and qN were nearly identical in L. esculentum and LA 385and revealed abrupt changes at approx. 8°C. It is concludedthat qP and ql, measured after defined chilling treatments,are valuable screening parameters for chilling tolerance inearly growth stages of Lycopersicon plants. (Received November 2, 1993; Accepted February 28, 1994)     

Growth Responses of Two Solanum Species to Contrasting Temperatures and Irradiance Levels: Relations to Photosynthesis, Dark Respiration and Chlorophyll Fluorescence

Potato production in the tropical lowlands during the rainyseason is constrained by high temperature and low irradiance.This study examined the effect of these two variables on drymatter production and allocation, using plant growth, leaf anatomy,gas exchange and chlorophyll fluorescence measurements. Plantsof two clones, Solanum goniocalyx cv. Garhuash Huayro (GH) andDTO-33, a heat tolerant clone of S. tuberosum x S. phureja,were grown in growth chambers at 33/25 °C or 20/10 °Cday/night temperature. At each temperature, plants were grownin either 12 h high irradiance (430–450 µmol m^–2s^–1 PAR) or 12 h low irradiance (250–280 µmolm^–2 s^–1) both with a 6–h photoperiod extensionof 6 µmol m^–2 s^–1. Plants were harvested after10 d (initial harvest) and after 20 d (final harvest). By theend of the study DTO-33 had produced more dry matter and hadtuberized, whereas GH had a greater leaf area ratio (LAR) andspecific leaf area (SLA). The highest relative growth rate (RGR)was at low temperature and low irradiance, possibly due to acombination of thin leaves with a large surface area. At thehigh temperature, low irradiance had the opposite effect, producingthe lowest net assimilation rate (NAR) and lowest RGR. Bothtuber number and weight were markedly reduced by high temperature.Low irradiance, in combination with high temperature, producedvirtually no tubers. Stomatal density, which was greater onGH than in DTO-33, was increased at high temperature. When measuredat 30 °C both clones, especially DTO-33, showed heat-adaptationin terms of ability to maintain a high rate of net photosynthesisat 30 °C. Plants grown at high irr-adiance and low temperaturehad the lowest net photosynthetic rate at 30 °C. Concurrentmeasurements of chlorophyll fluorescence indicated that onlythe initial (O) fluorescence parameter was affected. The dataconfirm the field observation that reduction in potato growthat high temperature can be aggravated by lower irradiance. Thisreduction is associated with a reduced leaf area and NAR. Growth analysis, heat adaptation, light     

Acclimation of Tomato Leaves to Changes in Light Intensity; Effects on the Function of the Thylakoid Membrane

When young tomato plants grown in high light (400 µmolquanta m^–2s^–1 PAR) were transferred to low light(100 µmol quanta m^–2s^–1 PAR), non-cyclic electrontransport capacity was decreased and the rate of dark re-oxidationof Q^–, the first quinone electron acceptor of photosystemII, was decreased within 1–2 d. In contrast, the amountof coupling factor CF₁, assayed by its ATPase activity, decreasedmore gradually over several days. The total chlorophyll contentper unit leaf area remained relatively constant, although thechlorophyll a/chlorophyll b ratio declined. When young tomato plants grown in low light were transferredto high light, the ATPase activity of isolated thylakoids increasedmarkedly within 1 d of transfer. This increase occurred morerapidly than changes in chlorophyll content per leaf area. Inaddition, in vivo chlorophyll fluorescence induction curvesindicate that forward electron transfer from Q^– occurredmore readily. The functional implications of these changes arediscussed. Key words: Tomato, leaves, light intensity, thylakoid membrane     

Growth and Nitrogen Fixation of Phaseolus vulgaris L. at Two Irradiances II. Nitrogen Fixation

Phaseolus vulgaris cv. Glamis plants grown at 7 and 28 W m^–228 W m^–2 in controlled environment cabinets showed copiousnodulation and high levels of acetylene reducing activity. Earlydifferences in nodulation were apparent before differences inphotosynthesis and were attributed to an effect of far-red lighton nodule development. Total plant nitrogen content was greater at 28 W m^–2 thanat 7 W ^–2 but nitrogen content as a percentage of d. wtwas greater at the lower irradiance level. Total acetylene reducing activity (nmol. min^–1 root^–1)was greater at 28 W m^–2 than at 7 W ^–2, but therewas no irradiance effect on specific activity (nmol. min^–1g d. wt of pink nodules^–1 or nmol. min^–1 pink nodule^–1). Transfer of 40-day-old plants from 7 W m^–2 to 28 W m^–2resulted in increased nodulesize(due toincreased size of infectedcells), accompanied by increased total, but not specific, acetylenereducing activity. Transfer of plants from 28 W m^–2 to 7 W m^–2 resultedin a fall of total acetylene reducing activity within 24 h,and senescence of large nodules. Specific acetylene reducingactivity was unaffected The results are interpreted as an effect of light on the productionof nitrogen fixing tissue, rather than on nitrogenase activity.     

Effects of Temperature on Growth and Nitrogen Fixation by Swards of Subterranean Clover

Subterranean clover (Trifolium subterraneum L.) cv. ‘Woogenellup’ swards were grown at 10, 15, 20 and 25 Cwith a 12 h photoperiod of 500 or 1000 µmol m^–2s^–1 [low and high photosynthetic photon flux density (PPFD)].Nitrogen-fixing swards received nutrient solution lacking combinednitrogen while control swards received a complete nutrient solution.Growth was measured by infra-red analysis of carbon dioxideexchange and by accumulation of dry matter. Swards were harvestedat intervals between 95 and 570 g d. wt m^–2 for estimationof nitrogenase activity by acetylene reduction and hydrogenevolution assays. Nitrogen fixation was also measured by increasein organic nitrogen. The growth rate was highest at 10 C at low PPFD, and at 10–15C at high PPFD. Nitrogen-fixing swards grew slower than thosereceiving combined nitrogen. Nitrogen fixation measured by increasein organic nitrogen responded similarly to the growth rate,as did acetylene reduction between 10 and 20 C. At 25 C therelationship between acetylene reduction and nitrogen fixationwas distrupted. The difference between the rates of acetylenereduction and hydrogen evolution, theoretically proportionalto nitrogen fixation, was not a reliable indicator of nitrogenfixation because hydrogen uptake developed. Trifolium subterraneum L, subterranean clover, growth, nitrogen fixation, temperature, acetylene reduction     

Changes in some Calvin Cycle Enzymes of the Tomato during Acclimation to Irradiance

The activities of three Calvin cycle enzymes, RuBPc (E.C. 4.1.1.39 [EC] ),3PGA phosphokinase (E.C. 2.7.2.3 [EC] ) and NADP-G3P dehydrogenase(E.C. 1.2.1.13 [EC] ), and the cytoplasmic enzyme PEPc (E.C. 4.1.1.31 [EC] )together with soluble protein and chlorophyll were measuredin extracts from young tomato leaves during acclimation to achange in irradiance. Leaf area and fresh weight were also measuredto show changes due to growth during treatments. Soluble proteinhad doubled on a unit leaf area basis 7 d after transfer from100 µmol quanta m^–2s^–1 PAR (low light) to400 µmol quanta m^–2s^–1 PAR (high light). Duringthis period the protein/chlorophyll ratio rose from 4•6to 10, RuBPc activity almost doubled and PEPc almost trebled.Following the reverse transfer from high to low light, solubleprotein decreased by 30% after 7 d and the protein/chlorophyllratio fell from 12 to 5•6. There was no change in RuBPcactivity 3 d after transfer from high to low light while PEPcactivity decreased by over 30%. There was no decrease in theactivity of 3PGA phosphokinase or NADP-G3P dehydrogenase 1 dafter transfer to low light, but decreases were apparent after3 d. The extracted kinase and dehydrogenase when fully activatedwere able to phosphorylate and reduce 3PGA at more than 2•5-foldits calculated rate of synthesis in the leaf. The data are discussedin relation to changes in the CO₂ exchange of the leaf. Key words: Photosynthetic acclimation, irradiance, tomato leaf, RuBP carboxylase     

Effects of Light on Transport by Azolla pinnata

Effects of light flux density (LFD) during growth and uptakeassay on induction of transport system and kinetics of transport were studied using the Azolla pinnata-Anabaena azollae association (Azolla). Theinduction and uptake kinetics of the transport system were determined using an automated system that measuredthe NO₃ concentration in the growth medium as a function oftime, using an on-line high performance liquid chromatograph(HPLC) with a UV-VIS detector. Full induction of the transport system required about 1.5 to 2.0 h and occurred without any apparent lag phase,regardless of the LFD provided. The level of induction of transport of Azolla grown at 600 µmol m^–2s^–1 LFD was higher than for that grown at 100 µmolm^–2 s^–1. Similarly, 600 µmol m^–1 s^–1LFD during the assay resulted in a higher level of inductionthan did 100 umol m^–2 s^–1. An increase in the LFDeither during the growth or the assay period increased the uptake rate; however, an increase in LFD duringthe latter period had greater effect. Azolla grown and assayedat 600 umol m^–2 s^–1 had the highest uptake rate. The uptake rate at 50 cm³ m^–3ambient CO₂ concentration was initially higher than at 305 cm³m^–3, but the uptake rate decreased rapidly with time andeventually dropped below that at 305 cm³ m^–3 CO₂. Thesedata suggest that the energy required for transport in Azolla may bypass the photosynthetic CO₂ fixationand carbon-cycling. Key words: carbon dioxide, concentration dependence, light flux density, uptake     

Effect of light regime upon growth rate and chemical composition of a clone of Skeletonema costatum from the Trondheimsfjord, Norway

Nutrient-saturated cultures of Skeletonema costatum were grownat 15C and 42 combinations of photon flux density (PFD) anddaylength. The growth rate increased with the daylength andPFD up to 460–630 µE m^–2 s^–1 (maximum2.5 doublings day At 2000 µE m^–2 s^–1 the growthrate was reduced by 45%. The chlorophyll (chl) content of thecells and the rate of production of carbon per unit chlorophylland ambient light increased for declining light regimes as didcellular nitrogen and carbon. The N/C ratio, cellular phosphorusand ratios between in vivo fluorescence, with and without DCMU,and chlorophyll varied negligibly. The ATP/C ratio was linearlyrelated to the growth rate. The results were described mathematically.The chl/C ratio was low both in strong light and in marginallylow light, corresponding to low cellular chlorophyll and highcellular carbon, respectively. The observed increase in cellularnitrogen and carbon at shade adaptation probably represent anincrease in the size of internal stores of organic nitrogenand may imply that Skeletonema cells become enriched with organicnitrogen when staying in nitrate-rich subsurface layers, e.g.in or below a nutricline. However, close to zero growth in marginallight the cells become greatly enriched with respect to everymeasured factor. Such cells may be physiologically resting stageswhich may ensure survival during dark periods and promote rapiddevelopment during the initial phase of blooms. Cultures andnatural blooms of Skeleronema in the Trondheimsfjord exhibitvery similar patterns of variation.     

The influence of irradiance on growth, photosynthesis and respiration of Gyrodinium cf. aureolum

The bloom-forming marine dinoflagellate Gyrodinium cf. aureolumwas grown in batch cultures over a range of irradiances (35–380µmolm^–2 s^–1 and growth, photosynthesis and respirationrates determined. Saturation of growth occurred at irradiancesof 100µmol m^–2 s^–1 Below this light level,decreases in growth rates and cell size, and a relative increasein carbon specific respiration rates, were observed. On theother hand, photosynthesis-irradiance relationships determinedfrom dissolved oxygen incubations showed that on a cellularand carbon basis, cultures grown at low irradiances had higherrates of light-limited and light-saturated photosynthesis, mainlyas a result of large increases in cell chlorophyll content.This adaptation strategy enables low-light-grown organisms toexploit available high irradiance through a relatively highphotosynthetic capacity. In cells grown at higher light levels(>100µmol m^–2 s^–1), excess photosynthatemay be diverted to storage rather than used for growth.     

Bisulphite-induced Inactivation of Growth and Chlorophyll Formation in Chlorella pyrenoidosa

Treatment of S-sufficient or S-deficient Chlorella pyrenoidosacells with NaHSO₃, during an 8-h period in the light, significantlydecreased their chlorophyll and dry matter contents when thecells were incubated in the presence or absence of SO₄²⁺. Incontrols lacking HSO₃^–, when the starting pH was 7.5,dry matter and chlorophyll contents increased slightly, whereasno significant changes in either occurred at a starting pH of3.0 when the cells and medium contained SO₄^–. In the dark,at both pH 3.0 and 7.5, dry matter and chlorophyll contentsdecreased slightly. Bisulphite treatment in the dark causedlittle decrease of either dry matter or chlorophyll when cellsand medium contained SO₄^2–. However, in its absence, drymatter decreased markedly, but there was little change of chlorophyllcontent in the dark. The interactions between HSO₃^– asa source of S and as an inhibitor of growth and chlorophyllformation are discussed in the context of the changes inducedby light and alternative sources of S. Overall, the harmfuleffects of HSO₃^– outweigh any role it has as a sourceof S, since its effects are ameliorated by SO₄^2–.     

Comparative Effects of Abscisic Acid and Methyl Jasmonate in Greening Cucumber Cotyledons and Its Application to a Bioassay for Abscisic Acid

In the cucumber cotyledon greening system, abscisic acid (ABA)is more potent inhibitor of growth and chlorophyll productionand/or destruction than methyl jasmonate (MJ). The inhibitoryeffect of ABA is apparent within 5 h of exposure to light whereasMJ is ineffective at all concentrations tested (10^–6 to10^–3 M). With longer exposure of 24 h to light and inthe presence of 40 mM KC1, the inhibition of growth and chlorophyllproduction by ABA is more pronounced whereas MJ does not inhibitgrowth and inhibits chlorophyll levels only at the higher concentrations.Both benzyladenine and KC1 stimulate chlorophyll productionand increase the fresh weights of the cucumber cotyledons andeither one of these compounds reverse the inhibitory effectsof ABA. Inhibition of chlorophyll production by ABA is valuableas a simple and rapid bioassay for abscisic acid. Under similarconditions cytokinins increase chlorophyll production and hencethe cucumber cotyledon greening system is ideal for detectingboth ABA which inhibits and cytokinins which stimulate chlorophyllproduction. (Received December 6, 1982; Accepted June 9, 1983)     

Growth and Preferences for Ammonium or Nitrate Uptake by Barley in Relation to Root Termperature

Barley plants (Hordewn vulgare L. cv. Atem) were grown fromseed for 28 d in flowing solution culture, during which timeroot temperature was lowered decrementally to 5?C. Plants werethen subjected to root temperatures of 3, 5, 7, 9, 11, 13, 17or 25 ?C, with common air temperature of 25/15 ?C (day/night).Changes in growth, plant total N, and NO₃^– levels, andnet uptake of NH₄⁺ and NO₃^– from a maintained concentrationof 10 mmol m^–3 NH₄NO₃ were measured over 14 d. Dry matterproduction increased 6-fold with increasing root temperaturebetween 3–25 ?C. The growth response was biphasic followingan increase in root temperature. Phase I, lasting about 5 d,was characterized by high root specific growth rates relativeto those of the shoot, particularly on a fresh weight basis.During Phase I the shoot dry weight specific growth rates wereinversely related to root temperature between 3–13 ?C.Phase 2, from 5–14 d, was characterized by the approachtowards, and/or attainment of, balanced exponential growth betweenshoots and roots. Concentrations of total N in plant dry matterincreased with root temperature between 3–25 ?C, moreso in the shoots than roots and most acutely in the youngestfully expanded leaf (2?l–6?9% N). When N contents wereexpressed on a tissue fresh weight basis the variation withtemperature lessened and the highest concentration in the shootwas at 11 ?C. Uptake of N increased with root temperature, andat all temperatures uptake of NH₄⁺, exceeded that of NO₃^–,irrespective of time. The proportions of total N uptake over14 d absorbed in the form of NH₄⁺ were (%): 86, 91, 75, 77,76, 73, 77, and 80, respectively, at 3, 5, 7, 9, Il, 13, 17,and 25 ?C. At all temperatures the preference for NH₄⁺ overNO₃^– uptake increased with time. An inverse relationshipbetween root temperature (3–11 ?C) and the uptake of NH₄⁺as a proportion of total N uptake was apparent during PhaseI. The possible mechanisms by which root temperature limitsgrowth and influences N uptake are discussed. Key words: Hordeum vulgare, root temperature, ammonium, nitrate, ion uptake, growth rate     

Reactions of birch leaves to changes in light during early ontogeny: comparison between in vivo and in vitro techniques to measure carbon uptake

Trends in several photosynthetic parameters and their responseto changed growth light were followed for 15 d in leaves ofyoung birch saplings using a rapid-response gas exchange measuringequipment. These in vivo measurements were compared to biochemicalassays that were made from the same leaves after the gas exchangestudies. The measurements were made on leaves that were selectedprior to the study and were at that time of similar age. Forthe first 7 d the photosynthetic parameters were followed fromthe growth conditions of moderate light (200 µmol m^–2s^–1; referred to as controls later in the text). On day7 some of the saplings were transferred to grow either underhigh (450 µmol m^–2 s^–1; referred to as highlight plants) or low (75 µmol m^–2 s^–1; referredto as low light plants) light and the capability of the preselectedleaves for acclimation was followed for 6 d. For comparison,at the end of the experiment the measurements were made on bothcontrols and on young leaves that had developed under high andlow light. Generally the in vivo measured rate of CO₂ uptake (gross photosynthesis)both at 310 ppm CO₂ and 2000 ppm CO₂ corresponded very wellto the biochemically determined CO₂ fixation capacity in vitroafter rapid extraction (measured as the initial and total activityof Rubisco, respectively). However, if the flux of CO₂ intothe chloroplasts was limited by the closure of the stomata,as was the case of the high light plants, then the in vitromeasured Rubisco activity was greater than the in vivo measuredCO₂ uptake. V_max, calculated from the mesophyll conductanceat 1% O₂, exceeded the initial activity of Rubisco (assayedat saturating RuBP and CO₂) constantly by 60%. The catalyticactivity of Rubisco in birch leaves was overall very low, evenwhen calculated from the total activity of Rubisco (K_cat 0.63–1.18 s^–1), when compared to herbaceous C₃ species. Signs of light acclimation were not observed in most of thephotosynthetic parameters and in chloroplast structure whenmature birch leaves were subjected to changes in growth lightfor 6 d. However, the change of the growth light either to highor low light caused day-to-day fluctuations in most of the measuredphotosynthetic parameters and in the case of the high lightplants signs of photoinhibition and photodestruction were alsoobserved (decrease in the amount of chlorophyll and increasein chlorophyll a/b ratio). As a result of these fluctuationsthese plants achieved a new and lower steady-state conditionbetween the light and dark reactions, as judged from the molarratio of RuBP to Rubisco binding site. Key words: Acclimation, photosynthesis, light, Rubisco, birch     

Aluminium and the Hydraulic Conductivity of Quercus rubra L. Root Systems

Two hydroponic experiments were conducted to determine the effectsof brief and prolonged AI³⁺ exposures on the hydraulic conductivity(Lp) of northern red oak (Quercus rubra L.) root systems. RootLp was determined using the pressure chamber method of Fiscus(1977). In the first experiment, 28- to 40-d-old seedlings weretreated for 4 d with complete nutrient solutions containingone of three Al concentrations (0.04, 1.85 or 3.71 mol m^–3)and either 0 or 50 mmol m^–3 P. Neither Lp nor daily transpirationwas affected by treatment. In Experiment II, seedlings were grown for 48–63 d incomplete solutions containing one of three Al concentrations(0, 0.75 or 2.00 mol m^–3) and either 10 or 250 mmol m^–3Ca. Lp and leaf area to root length ratio (LA/RL) were reducedwhen (AI³⁺/ Ca²⁺), the solution activity ratio, was 2.9 andhigher. Lp and LA/RL were also negatively correlated with Alconcentration and Al/Ca concentration ratio in the roots. Lpwas positively correlated with LA/RL in both experiments. Itis unclear whether Lp in the second experiment was reduced directlyby solution and root chemistry or whether Lp changed in responseto altered leaf/root balance. Key words: Al phytotoxicity, Al x Ca interaction, Quercus rubra, root hydraulic conductivity     

The Effect of Root Temperature on Growth and Uptake of Ammonium and Nitrate by Brassica napus L. in Flowing Solution Culture: I. GROWTH

Macduff, J. H., Hopper, M. J. and Wild, A. 1987. The effectof root temperature on growth and uptake of ammonium and nitrateby Brassica napus L. in flowing solution culture. I. Growth.—J.exp. Bot. 38: 42–52 Oilseed rape (Brassica napus L. cv. Bien venu) was grown for49 d in flowing nutrient solution at pH 6?0 with root temperaturedecrementally reduced from 20?C to 5?C; and then exposed todifferent root temperatures (3, 5, 7, 9, 11, 13,17 or 25?C)held constant for 14 d. The air temperature was 20/15?C day/nightand nitrogen was supplied automatically to maintain 10 mmolm^–3 NH₄NO₃ in solution. Total dry matter production wasexponential with time and similar at all root temperatures givinga specific growth rate of 0?0784 g g^–1 d^–1. Partitioningof dry matter was influenced by root temperature; shoot: rootratios increased during treatment at 17?C and 25?C but decreasedafter 5 d at 3?C and 5?C. The ratio of shoot specific growthrate: root specific growth rate increased with the ratio ofwater soluble carbohydrates (shoot: root). Concentrations ofwater soluble carbohydrates in shoot and root were inverselyrelated to root temperature; at 3, 5 and 7?C they increasedin stem + petioles throughout treatment, coinciding with a decreasein the weight of tissue water per unit dry matter. These resultssuggest that the accumulation of soluble carbohydrates at lowtemperature is the result of metabolic imbalance and of osmoticadjustment to water stress. Key words: Brassica napus, oilseed rape, root temperature, specific growth rate     

The Potential for Photoinhibition of Pinus sylvestris L. Seedlings Exposed to High Light and Low Soil Temperature

The effect of high light and root chilling on gas exchange,chlorophyll fluorescence, and bulk shoot water potential (_shoot)was examined for Pinus sylvestris seedlings. Transferring plantsfrom low light (200 µmol m^–2s^–1, PAR) anda soil temperature of 15 °C to high light (850 µmolm^–2 s^–1) and 1 °C caused >90% decrease innet photosynthesis and leaf conductance measured at 350 mm³dm^-3 CO₂, and a decrease in the ratio of variable to maximumfluorescence (F_v/F_m) from 0.83 to 0.63. The decrease in Fv/Fmwas, however, only marginally greater than when seedlings weretransferred from low to high light but kept at a soil temperatureof 15 °C. Thus, photoinhibition was a minor component ofthe substantial decrease observed for net photosynthesis at1 °C soil temperature. The decrease in net photosynthesisand _shoot at 1 °C was associated with an increase in calculatedintracellular CO₂ concentration, suggesting that non-stomatalfactors related to water stress were involved in inhibitingcarbon assimilation. Measurements at saturating external CO₂concentration, however, indicate that stomatal closure was thedominant factor limiting net photosynthesis at low soil temperature.This interpretation was confirmed with additional experimentsusing Pinus taeda and Picea engelmannii seedlings. Decreasesin gas-exchange variables at 5 °C soil temperature werenot associated with changes in _shoot Thus, hormonal factors,localized decreases in _needles or changes in xylem flux maymediate the response to moderate root chilling.     

The Effects of {delta}-Aminolevulinic Acid and Inhibitors of RNA and Protein Synthesis on Phytochrome Mediated Chlorophyll Accumulation in Sorghum bicolor

In 5-d-old etiolated seedlings of Sorghum bicolor, 12 h of darknessafter 5 min in red light eliminated a lag before the accumulationof chlorophylls in subsequent continuous white light. Increasingthe dark period to 24 h and 36 h, increased the rate of chlorophyllaccumulation in the later stages of greening. Exogenous -aminolevulinicacid neither completely removed the lag, nor increased the rateof chlorophyll accumulation. Cycloheximide (25 µg ml^–1)and 6-methyl purine (5.0 µg ml^–1), given continuouslyor only until the 12 h dark period following the red light irradiation,restored the lag and decreased the rate of chlorophyll accumulation.D-threo-chloramphenicol (400µg ml^–1) also decreasedthe rate of chlorophyll accumulation but did not restore thelag. Addition of these inhibitors even 12 h after red lightirradiation decreased the rate of chlorophyll accumulation.Rifampicin (Rifamycin SV, 400 µg ml^–1) did not havesuch effects. Key words: Chlorophylls, Phytochrome, -Aminolevulinic acid, Sorghum bicolor     

Effect of Light Quality on Chloroplast Replication in Spinach

Chloroplast growth and replication have been studied in spinachleaf dises cultured on sterile nutrient agar under lights ofdifferent spectral qualities. Over the intensity range 0.1-5mW cm^–2 both red (632 nm) and blue (488 nm) laser lightstimulate chloroplast division to the same extent as white light.By contrast low intensities (0.22–0.65 m Wcm^–2 ofboth white or green (525 nm) light are ineffective for chloroplastreplication but permit normal chlorophyll synthesis and greaterthan normal chloroplast growth. The large plastids of disesgrown in green light divide when exposed to high-intensity whitelight.