A relationship between irradiation, carbohydrates and rooting in cuttings of Pisum sativum

Rooting ability was studied for cuttings derived from pea plants ( Pisum sativum , L. cv. Alaska) grown in controlled environment rooms. When the cuttings were rooted at 70 μmol m⁻² s⁻, ¹ (photosynthetic photon flux density) or more, a stock plant irradiance at 100 μmol m⁻² s⁻¹ decreased rooting ability in cuttings compared to 5 μmol m⁻², s⁻¹, However, cuttings rooted at 160 μmol m⁻² s⁻¹ formed more roots compared to 5 (μmol m⁻² s⁻¹. Although a high irradiance increased the number of roots formed, it could not overcome a decreased potential for root formation in stock plants grown at high irradiance. Light compensation point and dark respiration of cuttings decreased by 70% during the rooting period, and the final levels were strongly influenced by the irradiance to the cuttings. Respiratory O₂ uptake decreased in the apex and the base of the cutting from day 2 onwards, whereas a constant level was found in the leaves. Only the content of extractable fructose, glucose, sucrose and starch varied during the early part of the rooting period. We conclude that the observed changes in the cuttings are initiated by excision of the root system, and are not involved in the initiation of adventitious roots.     

Singlet oxygen inhibits the repair of photosystem II by suppressing the translation elongation of the D1 protein in Synechocystis sp. PCC 6803

Singlet oxygen, generated during photosynthesis, is a strong oxidant that can, potentially, damage various molecules of biological importance. We investigated the effects in vivo of singlet oxygen on the photodamage to photosystem II (PSII) in the cyanobacterium Synechocystis sp. PCC 6803. Increases in intracellular concentrations of singlet oxygen, caused by the presence of photosensitizers, such as rose bengal and ethyl eosin, stimulated the apparent photodamage to PSII. However, actual photodamage to PSII, as assessed in the presence of chloramphenicol, was unaffected by the production of singlet oxygen. These observations suggest that singlet oxygen produced by added photosensitizers acts by inhibiting the repair of photodamaged PSII. Labeling of proteins in vivo revealed that singlet oxygen inhibited the synthesis of proteins de novo and, in particular, the synthesis of the D1 protein. Northern blotting analysis indicated that the accumulation of psbA mRNAs, which encode the D1 protein, was unaffected by the production of singlet oxygen. Subcellular localization of polysomes with bound psbA mRNAs suggested that the primary target of singlet oxygen might be the elongation step of translation.     

Effect of light and gibberellic acid on photosynthesis during leaf senescence of alstroemeria cut flowers.

The functioning of the photosynthetic apparatus during leaf senescence was investigated in alstroemeria cut flowers by a combination of gas-exchange measurements and analysis of in vivo chlorophyll fluorescence. Chlorophyll loss in leaves of alstroemeria cut flowers is delayed by light and by a treatment of the cut flowers with gibberellic acid (GA₃). The maximal photosynthesis of the leaves was approximately 6 μmol CO₂ m⁻² s⁻¹ at I 350 μmol m⁻² s⁻¹ (PAR) which is relatively low for intact C₃ leaves. Qualitatively the gas-exchange rates followed the decline in chlorophyll content for the various treatments, i.e. light and GA₃-treatment delayed the decline in photosynthetic rates. However, when chlorophyll loss could not yet be observed in the leaves, photosynthetic rates were already strongly decreased. In vivo fluorescence measurements revealed that the decrease in CO₂ uptake is (partly) due to a decreased electron flow through photosystem II. Furthermore, analysis of the fluorescence data showed a high nonphotochemical quenching under all experimental conditions, indicating that the consumption of reducing power in the Calvin cycle is very low. The chlorophyll, remaining after 9 days incubation of leaves with GA₃ in the dark should be considered as a 'cosmetic' pigment without any function in the supply of assimilates to the flowers.     

Influence of irradiance on in vitro growth and proliferation of Vaccinium corymbosum (highbush blueberry) and subsequent rooting in vivo

The effects of light on in vitro proliferation and subsequent in vivo rooting and acclimatisation of Vaccinium corymbosum were investigated. The shoots were exposed in vitro to different irradiances (total radiation ranging from 55 to 240 μmol m⁻² s⁻¹) for 7 to 60 days. In vitro growth and proliferation and the possible consequences on in vivo rooting were observed.
As compared to the control treatment (55 μmol m⁻² s⁻¹), higher irradiances improved proliferation and rooting ratios only with short applications (7 days). Short but high (210 μmol m⁻² s⁻¹) exposures applied at the end of the proliferation phase increased in vivo growth and rooting of the shoots. The shoots treated with strong light for longer times (14 and 28 days) showed both inhibition of growth and red colour of leaves and sprouts, and were less vigorous when transferred in vivo.     

Effect of ultraviolet radiation on accumulation and leakage of 86Rb+ in guard cells of Vicia faba

The effects of UV-C (254 nm), UV-A (365 nm) and broad-band UV (280–380 nm) on guard cells of Vicia faba L. cv. Long Pod were investigated in the presence of white light (450 μmol m⁻² s⁻¹). UV-C (7 μmol m⁻² s⁻¹) was found to cause leakage of ⁸⁶Rb⁺ from guard cells, while UV-A (0.3 μmol m⁻² s⁻¹) stimulated increased uptake in these cells. A relatively small stimulatory effect was observed by broad-band UV (3 μmol m⁻² s⁻¹) during the first 30 min of irradiation with an apparent equilibration of influx and efflux thereafter. Leakage of ⁸⁶Rb⁺ from guard cells continued despite the removal of UV-C and an increase in the amount of white light from 450 to 1500 μmol m⁻² s⁻¹, suggesting that membranes were irreversibly damaged. Irradiation of guard cells with UV-C for 30, 45 and 90 min indicated that these cells began to be affected already by 30 min UV-C irradiation.     

Inhibition of hypocotyl elongation by ultraviolet-B radiation in de-etiolating tomato seedlings. I. The photoreceptor

Broad-band UV-B radiation inhibited hypocotyl elongation in etiolated tomato ( Lycopersicon esculentum Mill. cv. Alisa Craig) seedlings. This inhibition could be elicited by < 3 μmol m⁻² s⁻¹ of UV-B radiation provided against a background of white light (> 620 μmol m⁻² s⁻¹ between 320 and 800 nm), and was similar in wild-type and phytochrome-1-deficient aurea mutant seedlings. These observations suggest that the effect of UV-B radiation is not mediated by phytochrome. An activity spectrum obtained by delivering 1 μmol m⁻² s⁻¹ of monochromatic UV radiation against a while light background (63 μmol m⁻² s⁻¹ showed maximum effectiveness around 300 nm, which suggests that DNA or aromatic residues in proteins are not the chromophores mediating UV-B induced inhibition of elongation. Chemicals that affect the normal (photo)chemistry of flavins and possibly pterins (KI, NaN, and phenylacetic acid) largely abolished the inhibitor) effect of broad-hand UV-B radiation when applied to the root zone before irradiation. KI was effective at concentrations < 10⁻⁴ M , which have been shown in vitro to be effective in quenching the triplet excited stales of flavins but not fluorescence from pterine or singlet states of flavins. Elimination of blue light or reduction of UV-A, two sources of flavin excitation, promoted hypocotyl elongation, but did not affect the inhibition of elongation evened by UV-B. Kl applied after UV-B irradiation had no effect on the inhibition response. Taken together these findings suggest that the chromophore of the photoreceptor system invoked in UV-B perception by tomato seedlings during de-etiolation may be a flavin.     

Photosynthesis and transpiration by young Larix kaempferi trees: C3 responses to light and temperature

The effects of photon flux density and temperature on net photosynthesis and transpiration rates of mature and immature leaves of three-year-old Japanese larch Larix kaempferi (Lamb.) Sarg. trees were determined with an infrared, differential open gas analysis system. Net photosynthetic response to increasing photon flux densities was similar for different foliage positions and stage of maturity. Light compensation was between 25 and 50 μmol m⁻² s⁻¹. Rates of photosynthesis increased rapidly at photon flux densities above the compensation level and became saturated between 800 and 1000 μmol m⁻² s⁻¹. Transpiration rates at constant temperature likewise increased with increasing photon flux density, and leveled off between 800 and 1000 μmol m⁻² s⁻¹. Photosynthetic response to temperature was determined in saturating light and was similar for all foliage positions; it increased steadily from low temperatures to an optimum range betweeen 15 and 21°C and then decreased rapidly above 21°C. Transpiration rate, however, increased continuously with rising temperature up to the experimental maximum. CO₂ compensation concentrations for mature foliage varied between 58 and 59 μl l⁻¹; however, foliage borne at the apex of the terminal leader compensated at 75 μl l⁻¹. None of these data support the claim that Japanese larch possesses C₄ photosynthetic characteristics.     

Regulation of the photosynthetic capacity of primary bean leaves by the red:far-red ratio and photosynthetic photon flux density of incident light

The main objective of the present work was to examine the effects of the red:far-red ratio (R:FR) prevailing during leaf development on the photosynthetic capacity of mature leaves. Plants of Phaseolus vulgaris L. cv. Balin de Albenga were grown from time of emergence in a controlled environment room, 25 ± 3°C, 12-h photoperiod, with different light treatments:a) high photosynthetic photon flux density (PPFD) = 800 μmol m⁻¹ s⁻¹+ high R:FR= 1.3;b) low PPFD= 300 μmol m⁻² s⁻¹+ high R:FR= 1.3; c) high PPFD=800 μmol m⁻² s⁻¹+ low R:FR= 0.7; d) low PPFD= 300 μmol m⁻²s⁻¹+ low R:FR=0.7. With an R:FR ratio of 1.3, a decrease in irradiance during leaf growth reduced photosynthesis when measured at moderate to high PPFD; but when measured at low PPFD, leaves expanded under low irradiance actually had photosynthesis rates higher than those of leaves grown in high irradiance. A low R:FR ratio during development reduced the photosynthetic capacity of the leaves. In leaves expanded under R:FR = 0.7 and high irradiance photosynthesis was reduced by 42 to 89%, depending on the PPFD at which measurements were made, whereas for leaves developed at R:FR = 0.7 and low irradiance photosynthesis decreased by 21 to 24%, compared to leaves under R:FR = 1.3 and similar irradiance. The reduced photosynthetic capacity under R:FR = 0.7 and high irradiance. In natural environments, leaves may experience low R:FR conditions temporarily during their development, and this may affect their future photosynthetic capacity in full sunlight.     

Influence of solar ultraviolet radiation on photosynthesis and motility of marine phytoplankton

Abstract: The effects of solar ultraviolet radiation (UV) on carbon uptake, oxygen evolution and motility of marine phytoplankton were investigated in coastal waters at Kristineberg Marine Research Station on the west coast of Sweden (58° 30'N, 11° 30'E). The mean irradiances at noon above the water surface during the investigation period were: photosynthetic active radiation (PAR, 400–700 nm) 1670 μmol m⁻² s⁻¹; ultraviolet-A radiation (UV-A, 320–400 nm) 35.9 W m⁻² and ultraviolet-B radiation (UV-B, 280–320 nm) 1.7 W m⁻². UV-B radiation was much more attenuated with depth in the water column than were PAR and UV-A radiation. UV-B radiation could not be detected at depths greater than 100–150 cm. Inhibition of carbon uptake by UV-A and UV-B in natural phytoplankton populations was greatest at 50 cm depth and the effects of UV-B were greater than those of UV-A. At depths greater than 50 cm there was almost no effect of ultraviolet radiation on carbon uptake. PAR, UV-A and UV-B decreased oxygen evolution by the dinoflagellate Prorocentrum minimum . Inhibition of oxygen evolution was greater after 4 h than 2 h but it was not possible to distinguish the negative effects of the different light regimes. The motility of P. minimum was not affected by PAR, UV-A and UV-B. The importance of exposure of phytoplankton to different light regimes before being exposed to natural solar radiation is discussed.     

Changes in pigment levels, Rubisco and respiratory enzyme activity of Ficus benjamina during acclimation to low irradiance

The objective of the present study was to determine the influence of reduced irradiance on the activities of ribulose bisphosphate carboxylase-oxygenase (Rubisco) and respiratory enzymes. Rooted cuttings of the tropical epiphyte. Ficus benjamina L., were grown in a shaded environment that excluded approximately 50% of the natural photosynthetically active irradiance (890 μmol m⁻² s⁻¹) for 4 months. Established plants were transferred and grown for 10 months under a range of irradiance levels with daily average maxima varying from a full-sun environment to 20% full sun (100%−1735; 50%−890; 40%−695; and 20%−303 μmol m⁻²s⁻¹). Chlorophyll, carotenoid and soluble protein content increased in Ficus leaves as irradiance level decreased, while Rubisco increased on a fresh weight basis but decreased on a protein basis. Glycolytic enzymes, enolase and pyruvate kinase, showed higher activities in full-sun plants on a protein and fresh weight basis. However, the activity of two mitochondrial enzymes, aconitase and malate dehydrogenase, was not different under the various irradiance levels. When transferred to a very low irradiance environment (18 μmol m⁻² s⁻¹), mature leaves exhibited increased chlorophyll and carotenoid levels regardless of previous irradiance treatment. Exposure to very low irradiance resulted in a large increase in enolase and pyruvate kinase activities. Only plants grown under full sun conditions showed a decline in Rubisco activity following growth at very low irradiance. Together, these studies demonstrate the ability of mature leaves of Ficus to biochemically adjust photosynthetic and respiratory components over a wide range of irradiance.     

CIRCADIAN GROWTH IN PORPHYRA UMBILICALIS (RHODOPHYTA): SPECTRAL SENSITIVITY OF THE CIRCADIAN SYSTEM

The circadian rhythm in growth of the red macroalga Porphyra umbilicalis (Linnaeus) J. Agardh was investigated under different spectral light conditions in laboratory-grown thalli. A free-running rhythm was observed in constant green or red light at irradiances of 2.5 to 20 μmol photons·m⁻²·s⁻¹, whereas arhythmicity occurred in constant blue light at 6–20 μmol photons·m⁻²·s⁻¹. The circadian oscillator controlling growth rhythmicity in Porphyra uses most of the visible sunlight spectrum and possibly multiple photoreceptors with a high sensitivity for blue light and a lower sensitivity for red light. This was inferred from three experimental results: (1) The free-running period, τ, of the growth rhythm decreased with increasing irradiance, from approximately 25 h at 2.5 μmol photons·m⁻²·s⁻¹ to 22 h at 20 μmol photons·m⁻²·s⁻¹ in red or green light, (2) Dark pulses of 3 h duration, interrupting otherwise continuous green or red light, caused advances during the subjective day and delays during the subjective night; the circadian oscillator in Porphyra can discriminate darkness from green or red light, and (3) Low-irradiance blue light pulses (2.5 μmol photons·m⁻²·s⁻¹) shifted the growth rhythm in red light of higher irradiance (e.g. 10 μmol photons·m⁻²·s⁻¹), and a strong, high amplitude, type 0 phase response curve was obtained that is usually observed with light pulses shifting a circadian rhythm in otherwise continuous darkness.     

Light effects on in vitro adventitious root formation in axillary shoots of mature Prunus serotina

Light effects on in vitro adventitious root formation in axillary shoots of a 95-year-old black cherry ( Prunus serotina Ehrh.) were examined using microcuttings derived from cultured vegetative buds. Three studies were performed: 1) complete darkness and 4 levels of continuous white light irradiance were tested at 70, 278, 555 and 833 μmol m⁻² s⁻¹; 2) white, red, yellow and blue light were tested to assess the importance of spectral quality; and 3) the effect of blue light at intensities of 7,15, 22 and 30 μmol m⁻² s⁻¹ was also studied, Measurements included rooting percentage, total number of roots per shoot, and shoot and root dry weight. There was a strong negative effect of white light intensity upon root formation. Blue light between 15 and 22 μmol m⁻²: s⁻¹ significantly retarded root formation and completely inhibited it at 36 μmol m⁻² s⁻¹. Shoots treated with yellow light exhibited the highest rooting percentage, mean number of roots per shoot, and root dry weight.     

Photoinhibition and the xanthophyll cycle are not enhanced in the salt-acclimated halophyte Artimisia anethifolia

The effects of high salinity (up to 400 m M NaCl) on photosystem II (PSII) photochemistry, photoinhibition and the xanthophyll cycle were investigated in the halophyte Artimisia anethifolia grown under outdoor conditions. In order to examine the changes in PSII photochemistry, photoinhibition, thermal dissipation associated with the xanthophyll cycle in salt-acclimated plants, the experiments were conducted at midday on a clear day (maximal irradiance 1500 μmol m⁻¹ s⁻¹) and on a cloudy day (maximal irradiance 700 μmol m⁻¹ s⁻¹), respectively. With increasing salt concentration, the accumulation of sodium and chloride in leaves increased considerably while the relative growth rate and CO₂ assimilation rate decreased significantly. Salinity induced no effects on PSII photochemistry, thermal energy dissipation, and the contents of the xanthophyll cycle pigments either on a clear day or on a cloudy day. However, when compared with those on a cloudy day, PSII photochemistry decreased and thermal energy dissipation increased significantly in both control and salt-acclimated plants on a clear day. The levels of zeaxanthin and antheraxanthin at the expense of violaxanthin were higher on a clear day than on a cloudy day. The results suggest that photoinhibition and the xanthophyll cycle were not induced by high salinity but by high light only in A. anethifolia plants. The results also suggest that A. anethifolia showed high resistance not only to high salinity, but also to photoinhibition even when it was treated with high salinity and exposed to full sunlight.     

Mechanism of copper-enhanced photoinhibition in thylakoid membranes

The effect of copper on photoinhibition of photosystem II (PSII) in vitro was studied in bean ( Phaseolus vulgaris L. cv. Dufrix) and pumpkin ( Cucurbita pepo L.) thylakoids. The thylakoids were illuminated at 200–2 000 μmol photons m⁻² s⁻¹ in the presence of 70–1 830 added Cu²⁺ ions per PSII. Three lines of evidence show that the irreversible damage of PSII caused by illumination of thylakoids in the presence of Cu²⁺ was mainly due to donor-side photoinhibition resulting from inhibition of the PSII donor side by Cu²⁺. First, addition of an artificial electron donor partially restored PSII activity of thylakoids that had been illuminated in the presence of Cu²⁺. Second, already moderate light was enough to cause rapid inhibition of PSII, and the inhibition could be saturated by light. Third, the extrinsic polypeptides of the oxygen-evolving complex were found to become oxidized by the combined effect of Cu²⁺ and light. The presence of oxygen was not necessary for the copper-induced enhancement of photoinhibition of PSII. When the illumination was prolonged, copper caused a gradual collapse of the thylakoid structure by increasing degradation of thylakoid proteins.     

Induction of leaf senescence in Arabidopsis thaliana by long days through a light-dosage effect

Given the influence of photoperiod on reproductive development and whole-plant senescence in monocarpic plants, one would suspect that leaf senescence in these plants might be under photoperiodic control. In Arabidopsis thaliana , which is monocarpic and also a nonobligate long-day (LD) plant, LDs (16 h, 300 μmol m⁻² s⁻¹) caused leaves to die earlier than did short days (SDs, 10 h). Since leaf longevity was not paralleled by the reproductive development in the present study, the reproductive structures did not seem to be the primary controls of leaf senescence. The LD effect appeared to depend on the amount of light rather than on day length, for leaves given LDs at reduced light intensity (180 μmol m⁻² s⁻¹) lived longer than those in LDs with full light. In addition, the higher light intensity promoted chlorophyll loss and anthocyanin accumulation in LDs. Thus, senescence of these leaves seems to be governed by light dosage rather than photoperiod. Light may play a natural role in promoting the senescence of A. thaliana leaves.     

Photophobic stop-response in a dinoflagellate: Modulation by preirradiation

Gyrodinium dorsum Kofoid responds photophobically to flashes of blue light. The photophobic response consists of a cessation of movement (stop-response). Without background light and after a flash fluence above 10 J m⁻², 75–85% of the cells show a stop-response, while only 50% of the cells show this response at 5 J m⁻². With a flash fluence of 5 J m⁻², background light of different wavelengths either increases (614 nm. 5.5–18.2 μmol m⁻² s⁻¹) or decreases (700 nm, 18.4–36.0 μmol m⁻² s⁻¹) the stop-response. Two hypotheses for the mechanism of the modulation by background light of the photophobic response are discussed: an effect of light on the balance of the photosynthetic system (PS I/PS II) or an effect on a phytochrome-like pigment (P_r/P_fr). This study supports the idea that a phytochrome-like pigment works in combination with a blue light-absorbing pigment. It was also found that cells of Gyrodinium dorsum cultured in red light (39.8 μmol m⁻²) had a higher absorption in the red region of the absorption spectra than those cultured in white light (92.7 μmol m⁻²).     

PHYSIOLOGICAL RESPONSES TO TEMPERATURE AND IRRADIANCE IN SPIROGYRA (ZYGNEMATALES, CHAROPHYCEAE)1

Spirogyra Link (1820) is an anabranched filamentous green alga that forms free-floating mats in shallow waters. It occurs widely in static waters such as ponds and ditches, sheltered littoral areas of lakes, and stow-flowing streams. Field observations of its seasonal distribution suggest that the 70-μm-wide filament form of Spirogyra should have a cool temperature and high irradiance optimum for net photosynthesis. Measurements of net photosynthesis and respiration were marie at 58 combinations of tight and temperature in a controlled environment facility. Optimum conditions were 25°C and 1500 μmol photons m⁻² s⁻¹, at which net photosynthesis averaged 75.7 mg O₂ gdm⁻¹ h⁻¹. Net photosynthesis was positive at temperatures from 5° to 35°C at most irradiances except at combinations of extremely low irradiances and high temperatures (7 and 23 μmol photons m⁻² s⁻¹ at 30°C and 7, 23, and 35 μmol photons m⁻² s⁻¹ at 35°C). Respiration rates increased with both temperature and prior irradiance. Light-enhanced respiration rates were significantly greater than dark respiration rates following irradiances of 750 μmol photons m⁻² s⁻¹ or greater. Polynomials were fitted to the data to generate response surfaces; such response surfaces can be used to represent net photosynthesis and respiration in ecological models. The data indicate that the alga can tolerate the cool water and high irradiances characteristic of early spring but cannot maintain positive net photosynthesis under conditions of high temperature and low light (e.g. when exposed to self-shading ).     

PHOTOSYNTHETIC RESPONSE OF STREAM PERIPHYTON TO FLUCTUATING LIGHT REGIMES

  An experiment was conducted on intact algal assemblages of stream periphyton to test their response to fluctuating and constant light regimes having the same mean intensity. The light regimes (in μmol·m⁻²·s⁻¹) were constant light at 100, light fluctuating between 50 and 150 with a period of 5 min, and light fluctuating between 10 and 460 with periods of either 4:1 or 8:2 min. Compared to the rates measured under 100 in μmol·m⁻²·s⁻¹ constant light conditions, fluctuations ranging between 50 and 150 in μmol·m⁻²·s⁻¹ with a 5-min period produced a 23% greater rate of photosynthesis. Conversely, fluctuations between 10 and 460 in μmol·m⁻²·s⁻¹ led to a 59%–74% decrease in photosynthetic activity. Detailed examination of periphytic algal responses to fluctuating light revealed that higher light intensities produced steeper photosynthesis/time slopes, but it was the combined interaction with lower light intensity that ultimately determined overall photosynthetic rate for a given light regime. This study offers compelling evidence that variable light regimes have important consequences for algal photosynthesis in natural streams.     

Irradiance levels affect growth parameters and carotenoid pigments in kale and spinach grown in a controlled environment

Carotenoids play critical roles in both light harvesting and energy dissipation for the protection of photosynthetic structures. However, limited research is available on the impact of irradiance on the production of secondary plant compounds, such as carotenoid pigments. Kale ( Brassica oleracea L.) and spinach ( Spinacia oleracea L.) are two leafy vegetables high in lutein and β-carotene carotenoids. The objectives of this study were to determine the effects of different irradiance levels on tissue biomass, elemental nutrient concentrations, and lutein β-carotene and chlorophyll (chl) pigment accumulation in the leaves of kale and spinach. 'Winterbor' kale and 'Melody' spinach were grown in nutrient solution culture in growth chambers at average irradiance levels of 125, 200, 335, 460, and 620 μmol m⁻² s⁻¹. Highest tissue lutein β-carotene and chls occurred at 335 μmol m⁻² s⁻¹ for kale, and 200 μmol m⁻² s⁻¹ for spinach. The accumulations of lutein and β-carotene were significantly different among irradiance levels for kale, but were not significantly different for spinach. However, lutein and β-carotene accumulation was significant for spinach when computed on a dry mass basis. Identifying effects of irradiance on carotenoid accumulation in kale and spinach is important information for growers producing these crops for dry capsule supplements and fresh markets.     

Laboratory models of diel vertical migration in the dinoflagellate Ceratium hirundinella

SUMMARY. Diel vertical migrations of a dinoflagellate, Ceratium hirundinella , were induced in a laboratory tube (1.63 × 0.15 m) under a light-dark cycle. The timing of vertical migrations differed between cultures in the exponential and stationary phases of growth; the latter showed a greater coincidence with the light regime.
Migration of cells into the surface layers occurred at low values of surface irradiance (<550 μeinsteins m⁻² s⁻¹). At irradiances more closely approaching summer sunshine (> 1300 μE m⁻² s⁻¹) there was a marked avoidance of surface waters, and population maxima were found at depths associated with a relative irradiance level of 10% or c. 150 μE m⁻² s⁻¹). Thermal stratification restricted downward movement of cells into the cooler layers. The combination of high surface irradiance and thermal stratification resulted in large, stable, sub-surface maxima of Ceratium , similar to those observed in natural waters under comparable environmental conditions.