The Effects of Light Intensity at Different Stages in Flower Initiation and Development of Chrysanthemum morifolium

Rooted chrysanthemum cuttings were raised in growth cabinetsat a ‘standard’, constant light level of 125 J cm^–2day^–1 and were induced to flower by giving daily cyclesof 8 h light followed by 16 h of uninterrupted darkness. Sampleswere transferred at weekly intervals to either a higher lightlevel of 375 J cm^–2 day^–1 or a lower light levelof 31 J cm^–2 day^–1 where they remained for 2 weeksbefore they were returned. It was shown that receptacle formationnormally began in the second week of inductive short-days (short-days8–14) and floret initiation in the third week (short-days15–21) when plants were grown throughout in the standardenvironment Receptacle formation was delayed and significantlymore leaves were formed below the flower if short-days 8–14were spent in low light. Transfer to high light for this periodhad no apparent effect Transfer to low light at the onset offloret formation retarded their further development and reducedthe total number of florets formed Transfer to high light atthis stage slightly increased the number of florets formed butdid not appear to affect their development. The data for stageof flower development after 11 weeks of growth reflected theseeffects of light level on the early development of the apex The total and flower dry weights of plants transferred to lowlight for 2 weeks were generally smaller than those of plantswhich had remained throughout in the standard condition Highlight only produced lasting increases in weight when the transferswere made during the later stages of growth. Most of the additionaldry matter formed at this time was accounted for in increasedflower weight The correlation between flower development andflower weight ratio previously found under constant environmentsalso applied to transferred plants except when the light levelwas changed for the last 2 weeks of growth; relatively highvalues of flower weight ratio were then obtained by transferto high light and relatively low values by transfer to low 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     

Effect of Light Intensity on Adaptation of Dunaliella to Very High Salt Concentrations

Light intensity was found to have a strong effect on the adaptationto high salt concentrations of a green microalga, Dunaliellaparva, normally grown at low and medium salt concentrations.At high light intensities (200 µmol m^–2 s^–1)the cell glycerol content increased in parallel with an increasein external salt concentration; protein synthesis and cell divisioncontinued with no period of arrest. At low light intensitiesno glycerol synthesis occurred as the external salt concentrationwas raised; protein synthesis and cell division were equallyarrested. The importance of high light intensity during theinitial phase of increase of salt concentration was demonstratedand was found to be a requirement for protein synthesis andcell division. In experiments designed to discover the effectof light intensity on cells growing in media in which the saltconcentration was kept constant, it was confirmed that the lightintensity needed for growth increases as the salt concentrationof the medium is increased. Key words: Dunaliella, salt concentration, light intensity, growth rates     

A Comparison of the Effects of Light Intensity and Duration on Chrysanthemum morifolium cv. Bright Golden Anne in Controlled Environments: I. Growth Analysis

Rooted cuttings were grown at a constant temperature of 15.6°C in two light intensities (22 and 33 J m–² sec–¹)in either 8- or 12-h days. The combination of the lower lightand the longer day gave the same daily light total as the reversecombination. Although departures from reciprocity were verysmall, total dry weight, specific leaf area, leaf area ratio,leaf weight ratio, and specific water content of stems weresignificantly greater for the lower light-longer day treatment.Only for specific water content of roots was the value for thelower light-shorter day treatment significantly smaller. Unitleaf rate was a linear function of light total, the two treatmentswith the same daily total being almost identical. In comparisonwith previous experiments, the plants had smaller leaf areasrelative to their rate of dry-matter increment. This is attributedto the lower day temperature. Flowering was earliest in thehigher light-shorter day combination, but surprisingly in thelonger day the higher intensity did not result in earlier floweringthan the lower intensity. The relationship between flower weightratio and flower development was the same as for other experimentsat different temperatures.     

Effect of Light Intensity, Carbon Dioxide Concentration, and Leaf Temperature on Gas Exchange of Spray Carnation Plants

The rates of CO₂ assimilation by potted spray carnation plants(cv. Cerise Royalette) were determined over a wide range oflight intensities (45–450 W m^–2 PAR), CO₂ concentrations(200–3100 vpm), and leaf temperatures (5–35 °C).Assimilation rates varied with these factors in a way similarto the response of single leaves of other temperate crops, althoughthe absolute values were lower. The optimal temperature forCO₂ assimilation was between 5 and 10 °C at 45 W m^–2PAR but it increased progressively with increasing light intensityand CO₂ concentration up to 27 °C at 450 W m^–2 PARand 3100 vpm CO₂ as expressed by the equation TOpt = –6.47-h 2.336 In G + 0.031951 where C is CO₂ concentration in vpmand I is photo-synthetically active radiation in W m^–2.CO₂ enrichment also increased stomatal resistance, especiallyat high light intensities. The influence of these results on optimalization of temperaturesand CO₂ concentrations for carnation crops subjected to dailylight variation, and the discrepancy between optimal temperaturesfor growth and net photosynthesis, are discussed briefly     

The Influence of Light Intensity on the Activation and Operation of the Hydroxyl Efflux System of Chara corallina

The interrelationships between light intensity and the activationof OH^– bands was investigated. The lag period prior toOH^– efflux activation was longer than the photosyntheticinduction period. It was found that this lag period dependedupon the light regime employed as well as the photosyntheticcapacity of the cell. The response of the cell to low light intensities revealed thatall OH^– bands were not of equal status. Below a criticallight intensity the cell did not develop any bands even afterprolonged illumination. An hypothesis is presented to accountfor these results, interms of total cell OH^– band activationand the regulation of the HCO₃^– and OH^– transportsystems. It is proposed that the electrical properties of theChara corallina plasmalemma, observed at high pH values, canbe explained on the basis of the hypothesis presented in thispaper     

The Effects of Light Intensity and External Potassium Level on Root/Shoot Ratio and Rates of Potassium Uptake in Perennial Ryegrass (Lolium perenne L.)

Loliun perenne L. (cv.S. 23) was grown on vermiculite in winterin a heated greenhouse for 8 weeks under factorial combinationsof two potassium regimes (nominally 6 parts/10⁶ and 156 parts/10⁶in Hewitt's solution) and three densities of artificially supplementedvisible radiation flux (36.1, 7.3, and 2.2 W m^–2). Growthand potassium uptake were studied through the calculation ofvarious growth functions from fitted curves. There was little effect of potassium treatment but the experimentalmaterial responded markedly to light. Leaf-area ratio in thethree treatments showed extreme plasticity in increasing from2–3 x 10^–2 through 6 x 10^–2 to 8–9 x10^–2 m² g^–1 as light intensity decreased. Correspondingdecreases in unit leaf rate, however, caused over-all reductionsin relative growth rate. Specific absorption rates for potassium (A_K, dry-weight basis)were strongly reduced at the lower light intensities but alsodisplayed complex ontogenetic drifts. Values of the allometricconstant, k (the ratio of root and shoot relative growth rates),decreased from c. 0.7 at 36.1 W m^–2 through c. 0.3 at7.3 W m^–2 to a value not significantly different fromzero (P < 0.05) at 2.2 W m^–2. In material grown under the two higher light intensities a constantinverse relationship was found between the mass ratio of rootand shoot and the corresponding activity ratio. The resultsconform to this model: Mass ratio = –0.001+45.0 (1/activityratio) where activity ratio is expressed as specific absorptionrate for potassium (in µg g root^–1 h^–1)/unitshoot rate (rate of increase of whole-plant dry weight per unitshoot dry weight, in mg g shoot^–1 h^–1). The implicationsof this relationship are discussed.     

The Effects of Light Intensity and Carbon Dioxide Concentration on the Growth of Chrysanthemum morifolium cv. Bright Golden Anne

Rooted cuttings were grown in controlled-environment cabinetsat daily visible light totals of 31, 63, 125, and 250 J cm^–28-h day^–1 and carbon dioxide concentrations of 325 and600 ppm. The experiment was repeated on another occasion withthe inclusion of a further carbon dioxide level of 900 ppm.A 5-h tungsten night break was used in the first week to delayflower initiation The plants in the various treatment combinationswere sampled by frequent small harvests for leaf area and freshand dry weights of leaf, stem, root, and flower, and also forvarious morphological features. Other growth measures were obtainedby manipulation of the primary data, including the fitting ofprogress curves. Plants were respaced at intervals to minimizemutual shading. There was an increase in total dry-matter production with increasinglight and carbon dioxide, with a small positive interactionbetween them. Plants in one experiment had a somewhat higherunit leaf rate and a lower leaf-area ratio, the latter beingdue to a slightly smaller leaf-weight ratio. The effects ofadditional carbon dioxide were largely accounted for by increasedphotosynthesis. Although there were substantial differencesin specific leaf area between treatment combinations withineach experiment, the leaf-weight ratio was little altered inthe period of vegetative growth. The inverse relationship betweenspecific leaf area and unit leaf rate showed a very similartrend for all combinations of light and carbon dioxide concentration.Leaf area was a linear function of absolute leaf water contentfor all treatment combinations within an experiment, but therewas a small significant difference between occasions. Flower development was extremely delayed in the lowest lightlevel and substantially delayed at the next higher level. Thenumber of leaves below the flower decreased with increasinglight level Flower weight increased with increasing light above63 J cm^–2 8-h day^–1 and with increasing carbon dioxideconcentration, there being a positive interaction between them The initial weight and leaf area of cuttings differed for thetwo experiments, and although the results on the two occasionswere in the same direction, their magnitudes were different.Some of the discrepancy was eliminated by expressing the variousgrowth measures as functions of plant dry weight, but therewas evidently a difference in the potential for growth of thetwo batches of cuttings. The plants which were initially smallerhad a higher average unit leaf rate which, due to a higher leafwater content, was not offset by a lower leaf area ratio.     

A Comparative Study of Germination Responses to High Irradiance Light

Seeds of 19 native British herbaceous species (14 grasses andfive forbs) were exposed to white light at three photon fluencerates: high (19–2 mol m^–2 d^–1), medium (9·6mol m^–1 d^–1) and low (2·3 mol m^–2 d^–2) These photon doses have been found by previous workers to inhibitgermination in several species. High and low photon doses wereapplied only as continuous light, but the medium dose was appliedas both continuous light and as a 12 h light/12 h dark photoperiod.All four treatments, plus a dark control, were carried out at15 °C; the high and low doses were also applied with a dailyalternation of 10/20 °C. The majority of species (15) fell into one of two groups. Inseven species germination was relatively high and consistentacross all treatments, including darkness; in the other eightspp germination was inhibited only in darkness. Mostly thesedata confirmed published results for the same species In contrast in Agrostis capillaris and A. stolonifera germinationwas high only at alternating temperatures, irrespective of photondose, but was also slightly promoted by a constant temperaturecombined with light/dark alternations. Only in Bromus sterilisand B. ereclus was germination inhibited by light, in B. erectusat all photon doses and in B. sterilis only at the highest photondose These results suggest that inhibition of germination by highirradiance light is not widespread among native British species Aira caryophyllea, Arrenatherum elatius, Festuca ovina, F. rubra, Hordeum murinum, Milhim effusum, Silene dioica, Achillea millefolium, Brachypodium syhaticum, Digitalis purpurea, Holcus lanatus, Leucanlhemum vulgare, Phleum pratense, Poa trivialis, Taraxacum officinale, Agrostis capillaris, A. stolonifera, Bromus erectus, B. sterilis, seed, dormancy, germination, light, High irradiance reaction, alternating temperatures, photoperiod     

Effect of Light Intensity on Growth of Natural Populations of Dactylis glomerata L.

The growth of two natural populations of cocksfoot from contrastingclimatic regions, Norway and Portugal, was studied in two photoperiodsat three temperatures with three levels of light energy (48,144, and 240 W m^–2 in the wavelength interval 400–700nm). There was a consistent increase in relative growth-rate(RGR) in response to increased light energy up to 144 W m^–2,but above this energy level there was either no change, or,in some treatments, a decline. Net assimilation rate (NAR) increased,whilst leaf area ratio decreased from the lowest to the highestenergy level in most treatments. The decrease of LAR with increasedlight energy could be attributed to a decrease of both leafweight ratio (LWR) and specific leaf area (SLA), a greater proportionof dry matter being distributed to plant parts other than leaf.This effect occurred although there was a positive relationshipbetween light energy and relative leaf growth-rate (RLGR). Populationdifferences in these growth attributes were most marked in thetreatments with low-temperature and short-day conditions. Theefficiency of energy conversion of visible radiation declinedfrom 3–4 per cent at the lowest energy level to 1–2per cent at the highest energy level.     

Influence of light intensity on diel variations in rates of growth, respiration and organic release of a marine diatom: comparison of diurnally constant and fluctuating light

Effects of variations in light intensity on diel patterns ofgrowth, respiration and organic release of Skeletonema costatum(Grev.) Cleve in a cyclostat were evaluated. Light intensitywas either constant during the tight period at levels from 1500to 15 µEm^–2 s^–1 or fluctuated throughout thelight period from 500 to 10 µEm^–2 s^–1 at ratesof either 1 or 12 cycles day^–1. Periodicity in cell divisionwas observed only at light intensities of 130 Em^–2 s^–1and was decreased under diurnally varying tight. Under all lightconditions carbon and pigment growth were maximal during thelight period but well coupled throughout the 24 h period. Carbonassimilation during the dark period varied from 19 to 34% oftotal daily production and was a linear function of growth rate.Respiratory activity during the light period increased relativeto total daily respiration as growth rate increased. The increasein night-time carbon assimilation with growth rate interactedwith night-time respiration through the refixation of respiredcarbon, thus, influencing the pattern of respiratory loss ofcarbon. Rates of organic release (E^c) were maximal during thelight period and did variations consistently increased withtight intensity. Fluctuating light increased E^c relative toconstant light. Net growth efficiency was maximal at 130 µEm^–2s^–1 when cell division periodicity was greatest. Underother light conditions relatively higher rates of cell divisionoccorred at night and cell division periodicity was reducedas well as net growth efficiency. Cellular chemical fractionationindicated that under high or variable light conditions fixedcarbon was stored during the tight period for subsequent synthesisof protein and pigments, and division at night. Such an uncouplingof photosynthesis and other growth parameters resulted in greatermetabolic costs to the cell. ¹Present address: Marine Biology, Lamont Doherty GeologicalObservatory, Palisades, NY 10964, USA     

Some Effects of Light Intensity, Daylength and Temperature on Growth of Fruiting and Non-fruiting Watermelon (Citrullus lanatus)

Plants of watermelon [Citrullus lanatus(Thunb.) Matsum. &Nakai, cv. Early Yates] were grown for up to 3 months aftergermination in controlled environment cabinets, and variousaspects of vegetative growth and fruit development were measured.Effects of light intensity were studied by comparing growthat 8, 16 and 32 klx at constant temperature and daylength (25^°C, 14 h). Effects of daylength were studied by comparing8, 14 and 24 h at constant light intensity and temperature (32klx, 25 ^°C), and effects of tem perature were studied bycomparing 20^°, 25^°, 30^°, 35^° and 40 ^°C atconstant light intensity and day- length (32 klx, 14 h). Withincreasing light intensity and daylength lateral growth waspromoted whereas main shoots were less affected. Increase intemperature above 25 ^°C resulted in longer main shoots andprolific lateral growth, due both to more and to longer laterals.Environmental differences had little effect on internode lengthbut did affect the size of basal leaves. However, an increasein total leaf area at higher temperatures or with Continuouslight was mainly due to more leaves rather than larger leaves.The presence of developing fruit greatly reduced vegetativegrowth of plants. Main shoot length, lateral growth, numberof leaves, and even size of individual leaves, were all reduced.This reduction did not apply to d. wt of whole plants. Fruitingplants were very efficient, on a leaf area basis, in accumulatingd. wt. At 25 ^°C at the two higher light intensities with14 h days the presence of one developing fruit was inhibitoryto the setting of any subsequent fruit. With short days or lowlight, more fruits were set but they were small. With continuouslight or high temperature more than one fruit could developand they were large.     

Further Effects of Light Intensity, Carbon Dioxide Concentration, and Day Temperature on the Growth of Chrysanthemum morifolium cv. Bright Golden Anne in Controlled Environments

In earlier work the effects of light intensity over the range31 to 250 J cm^–2 day^–1 and carbon dioxide concentrationfrom 325 to 900 ppm with 8-h days at 18.3 °C and 16-h nightsat 15.6 °C were described. The present paper is concernedwith three further experiments with light levels up to 375 Jcm^–2 day^–1 (which corresponds to the daily totalin a glasshouse in southern England in early May or August andthe intensity is approximately that of mid-winter sunshine),carbon dioxide concentration up to 1500 ppm, and day temperaturesof 18.3 to 29.4 °C. Final plant weight was increased by light over the range 125–375J cm^–2 day^–1 and by carbon dioxide over the range325–900 ppm, with positive interaction between them; thisinteraction was increased by raising the temperature to 23.9°C and somewhat more at 29.4 °C day temperature. Leaf-arearatio and specific leaf area were reduced by increasing eitherlight or carbon dioxide but there was little effect of temperature.Leaf-weight ratios were uniform within experiments but therewere small consistent differences between one experiment andthe other two which also affected leaf-area ratios. Mean unit leaf rate was scarcely affected by day temperatureover the range investigated. There were the usual increasesdue to increased light and carbon dioxide concentration anda consistent difference in absolute value between one experimentand the other two. These differences in mean unit leaf rateare illustrated in detail in the ontogenetic trend of unit leafrate and plant size. Lower unit leaf rates were to a considerableextent compensated for by increased leaf-area ratios in theusual way. Despite the substantial differences in day temperature the specificwater contents (g water g dry weight^–1) differed little,showing in the majority of cases higher values in the highertemperature for otherwise similar treatment combinations. Flower development was somewhat delayed at 23.9 °C day temperature,and substantially so at 29.4 °C. Lateral branch length wasincreased at 23.9 °C and excessively so at 29.4 °C.This reveals quite clearly that a temperature optimum for vegetativegrowth may not be the optimum for flowering performance norproduce a desirable plant shape. Despite the marked effects of temperature on rate of flowerdevelopment, the relationship between flower development andthe ratio of flower to total weight was the same for all treatmentcombinations in all three experiments and coincident with thatreported earlier. Gasometric determinations indicated that respiratory loss bythe whole plant was a smaller proportion of net photosyntheticgain at a temperature of 29.4 °C than at 18.3 °C andwas likewise a smaller proportion at 1500 ppm carbon dioxidethan at 325 ppm. If photorespiration of leaves is assumed tobe as great as their dark respiration, the respiratory lossesare in the range of 31–50 per cent of the gross gain.Greater rates of photorespiration would increase the proportionaterespiratory loss.     

Effects of Blue Light Pretreatments on Internode Extension Growth in Mustard Seedlings after the Transition to Darkness: Analysis of the Interaction with Phytochrome

The effects of blue light (B) pretreatments on internode extensiongrowth and their possible interaction with phytochrome mediatedresponses were examined in Sinapis alba seedlings grown for11 d under 280 µmol m^–2 s^–1 of continuousblue-deficient light from low pressure sodium lamps (SOX). SupplementaryB (16 µmol m^–2 s^–1) caused no detectable inhibitionof the first internode growth rate under continuous SOX, butgrowth rate was inhibited after transfer to darkness. This effect,and the growth promotion caused by far-red bend-of-day' lightpulses were additive. The addition of B at 16 µmol m^–2s^–1 during 11 d, or only during the first 9 or 10 d orthe latest 0.75, 1 or 2 d of the SOX pretreatment caused approximatelythe same extent of inhibition after the transition to darkness.A single hour of supplementary B before darkness caused morethan 50% of the maximum inhibition. However, 24 h of lower fluencerates of B (4 or 7 µmol m^–2 s^–1) were ineffective.Covering the internode during the supplementary B period didnot prevent the response to B after the transition to darkness.Far-red light given simultaneously with B (instead of the SOXbackground) reduced the inhibitory effect of B. Above a given threshold fluence rate, B perceived mainly inthe leaves inhibits extension growth in subsequent darkness,provided that high phytochrome photo-equilibria are presentduring the irradiation with B. Once triggered, this effect doesnot interact significantly with the ‘end-of-day’phytochrome effect. Key words: Blue light, extension growth, phytochrome     

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.     

Control of Plasma Membrane Cl- Fluxes in Chara corallina by External Cl- and Light

The efflux of Cl^– at the plasma membrane of Chara wasstudied in relation to two treatments known to affect the flux:that of removal of external Cl^– and of light. It is shownthat although removal of external Cl^– results in a rapidreduction in Cl^– efflux (consistent with a direct effectof external Cl^– on the transport system) the magnitudeof this reduction in the dark is greater than the measured darkinflux. Therefore, in the dark at least, it is proposed that1:1 exchange diffusion cannot account for the trans-stimulationof efflux by external Cl^–. Light induces an inhibition of efflux and a concomitant stimulationof Cl^– influx at 20 °C, but at 10 °C the responsesto light of the two fluxes can be separated temporally. It istherefore suggested that the fluxes are not reciprocally dependenton the same factor which mediates in the light response. Furtherconsiderations show that it is unlikely that the cytoplasmicCl^– concentration mediates in the light response of eitherflux, but that changes in cytoplasmic pH may do so.     

The Regulation of Root Growth in Cress Seedlings by Light and Gravity

Horizontal growth of seedling roots of Lepidium sativum L. cv.Curled is sensitive to white light with a photon flux densityas low as 7.5 µEinsteins m^–2 S^–1. Inhibitionof growth is positively correlated with light intensity. Rootsgrowing in the vertical plane are much less sensitive to light.The light inhibition of horizontal root growth can be overcomeby the application of an axial force. The effects of light andaxial force are reversible. Although geotropic responsivenessis not dependent on exposure to light, there is evidence thatlight induces a heightened geosensitivity. The results are discussedin relation to the counter-current theory of the regulationof root growth.     

Effect of Nitrate Concentration on the Root: Shoot Ratio in Dactylis glomerata L. and on the Kinetics of Growth in the Vegetative Phase

The vegetative growth of Dactylis glomerata L. in sand was studiedunder controlled light, temperature, and nutritional conditions.Plants were daily supplied with three nutrient solutions ofdifferent nitrate concentrations (10^–2, 10^–3 and2 x 10^–4 mol I^–1). For each concentration, growthobeyed an exponential law between the fourth and seventh weeksafter sowing. The time constant of the exponential was the samefor the shoot as for the root, and showed no significant variationwith nitrate concentration. The kinetic results and the strong dependence of the root: shootratio on nitrate concentration are discussed on the basis ofThornley's model. Hypothesizing that the molecular mechanismsof nitrate absorption are independent of the nitrate concentrationof the nutrient solution, we derived a relationship betweenthe root: shoot ratio and nitrate concentration. This relationshipwas found to be compatible with the experimental results. Dactylis glomerata L., vegetative phase, kinetics of growth, root: shoot equilibrium, nitrate absorption     

The Importance of Light Intensity for Pollen Tube Growth and Embryo Survival in Wheat x Maize Crosses

The success of Triticum aestivumxZea mays crosses, used to producewheat doubled haploids, is influenced by light intensity. Toexamine the basis for this response, pollen tube growth, embryosurvival and indicators of photosynthetic rate were measuredin two wheat cultivars (‘Karamu’ and ‘Kotuku’)crossed with maize at two irradiance levels (250 or 750 µmolm^-2s^-1, PAR). Pollen tube growth was significantly affectedby light intensity in ‘Karamu’ plants but not in‘Kotuku’ plants, despite both cultivars being pollinatedby the same maize source. The percentage of pollen tubes reachingthe cavity between the ovarian wall and integuments, or in themicropyle of ‘Karamu’ plants at high light intensity(65%) was nearly three-times greater than that at low lightintensity (22%). Thus, either low light intensity can affectthe maternal wheat plant in a way that inhibits pollen tubegrowth and/or high light intensity may promote pollen tube growthin ‘Karamu’ plants. Significant differences in ratesof electron transport in plants grown at the two light intensitiesindicated that the rate of photosynthesis may also have an effecton pollen tube growth. These results have importance for improvingthe efficiency of wheat x maize crosses and other wide cerealcrosses. Copyright 2001 Annals of Botany Company Intergeneric hybridization, light intensity, pollen tube growth, embryo survival, Triticum aestivum, wheat,Zea mays , maize     

Growth and light absorption of some planktonic cyanobacteria, diatoms and Chlorophyceae under simulated natural light fluctuations

Specific growth rates of Limnozhrix redekei, Planktothrix agardhii(cyanobacteria), Synedraacus, Stephanodiscus minutulus (diatoms),Scenedesmus acuminatus and Scenedesmus armatus (Chlorophyceae)were compared under different time structures of illumination,but the same daily light exposure, at 20C. Fluctuating irradiancesimulating a uniform rapid transport of the algal cells acrossthe aquatic light field on a cloudless day with Z^eu/Z^mix=1 wascompared with constant irradiance throughout the same photoperiodof 12 h length as well as a photoperiod of 6 h length. Fluctuatinglight (30 min for a cycle) resulted in a decrease in specificgrowth rates as compared with constant irradiance at the samephotoperiod length. This decrease amounts to 15–20% fordiatoms, 20–25% for Chlorophyceae and 35–40% forcyanobacteria, respectively. The decrease is somewhat lowerif the fluctuations simulating mixing are slower (60 min fora cycle). The specific growth rate is also decreased by a shorterphotoperiod, but this effect is more species specific. Regardingthe in vivo absorption spectra, fluctuating light or a shorterphotoperiod has little or no effect on the Chlorophyceae anddiatoms studied, whereas cyanobacteria show an increase in lightabsorption by chlorophyll a and phycobilins.