Days In Continuous Light Research Articles

Effects of continuous light deprivation on cucumber growth and antioxidant enzyme activities in solar greenhouse in winter

Cucumber Variety ‘Drite L108’ (Cucumis sativus L. Cv. Derit L108) was selected as the test material. In the solar greenhouse, different days (1, 3, 5, 7, 9 d) of light (PAR < 200 µmol·m-2·s-1) and normal light conditions were designed with shading nets to observe the growth indexes of cucumber plants and the changes of antioxidant enzyme activities in leaves. The results showed that: (1) continuous low light increased the SPAD (relative chlorophyll) value of cucumber leaves and decreased the net photosynthetic rate. The longer the continuous low light days are, the smaller the net photosynthetic rate of cucumber leaves and the worse the photosynthetic recovery ability would be. (2) The plant height, stem diameter and leaf area per plant were lower than CK, and the above indexes could not return to the normal level after 9 days of normal light recovery; the yield and marketability of cucumber fruit decreased under continuous low illumination. (3) The activities of SOD (superoxide dismutase) and POD (peroxidase) in cucumber leaves increased, the activities of CAT (catalase) first increased and then decreased, and the content of MDA (malondialdehyde) continued to increase. The longer the days of continuous light keep, the more seriously the cucumber leaves were damaged by membrane lipid peroxidation. After continuous light for more than 7 days, the metabolic function of cucumber leaves was difficult to recover to the normal level.

Diel patterns of microphytobenthic primary production in intertidal sediments: the role of photoperiod on the vertical migration circadian rhythm

Diel primary production patterns of intertidal microphytobenthos (MPB) have been attributed to short-term physiological changes in the photosynthetic apparatus or to diel changes in the photoautotrophic biomass in the sediment photic layer due to vertical migration. Diel changes in primary production and vertical migration are entrained by external factors like photoperiod and tides. However, the role of photoperiod and tides has not been experimentally separated to date. Here, we performed laboratory experiments with sediment cores kept in immersion, in the absence of tides, with photoperiod or under continuous light. Measurements of net production, made with O2 microsensors, and of spectral reflectance at the sediment surface showed that, in intertidal sediments, the photoperiod signal was the major driver of the diel patterns of net primary production and sediment oxygen availability through the vertical migration of the MPB photoautotrophic biomass. Vertical migration was controlled by an endogenous circadian rhythm entrained by photoperiod in the absence of tides. The pattern progressively disappeared after 3 days in continuous light but was immediately reset by photoperiod. Even though a potential contribution of a subjective in situ tidal signal cannot be completely discarded, Fourier and cross spectral analysis of temporal patterns indicated that the photosynthetic circadian rhythm was mainly characterized by light/dark migratory cycles.

Extremely robust photocurrent generation of titanium dioxide photoanodes bio-sensitized with recombinant microalgal light-harvesting proteins

Bio-dyes for light harvesting in dye-sensitized solar cells (DSSC) have the advantage of being environmentally-friendly, non-toxic alternatives, which can be produced in a sustainable fashion. Free photosynthetic pigments are unstable in the presence of light and oxygen, a situation which can hardly be avoided during the operation of DSSCs, especially in large-scale applications. We therefore investigated the recombinant light-harvesting protein LHCBM6, which naturally occurs in the photosynthetic apparatus of the green microalga Chlamydomonas reinhardtii as a bio-dye in DSSCs. Photocurrent densities of up to 0.87 and 0.94 mA·cm−2 were determined for the DSSCs and solar energy to electricity conversion efficiencies (η) reached about 0.3% (100 mW·cm−2; AM 1.5 G filter applied). Importantly, we observed an unprecedented stability of LHCII-based DSSCs within long DSSC operation times of at least 7 days in continuous light and show that operation times are restricted by electrolyte decomposition rather than reduced dye performance, as could be demonstrated by DSSC reactivation following re-supplementation with fresh electrolyte. To the best of our knowledge, this is the first study analysing bio-dye sensitized DSSCs over such long periods, which revealed that during illumination an activation of the DSSCs occurs.

Antagonistic Regulation of Flowering Time through Distinct Regulatory Subunits of Protein Phosphatase 2A

Protein phosphatase 2A (PP2A) consists of three types of subunits: a catalytic (C), a scaffolding (A), and a regulatory (B) subunit. In Arabidopsis thaliana and other organisms the regulatory B subunits are divided into at least three non-related groups, B55, B’ and B″. Flowering time in plants mutated in B55 or B' genes were investigated in this work. The PP2A-b55α and PP2A-b55β (knockout) lines showed earlier flowering than WT, whereas a PP2A-b’γ (knockdown) line showed late flowering. Average advancements of flowering in PP2A-b55 mutants were 3.4 days in continuous light, 6.6 days in 12 h days, and 8.2 days in 8 h days. Average delays in the PP2A-b’γ mutant line were 7.1 days in 16 h days and 4.7 days in 8 h days. Expression of marker genes of genetically distinct flowering pathways (CO, FLC, MYB33, SPL3), and the floral integrator (FT, SOC1) were tested in WT, pp2a mutants, and two known flowering time mutants elf6 and edm2. The results are compatible with B55 acting at and/or downstream of the floral integrator, in a non-identified pathway. B’ γ was involved in repression of FLC, the main flowering repressor gene. For B’γ the results are consistent with the subunit being a component in the major autonomous flowering pathway. In conclusion PP2A is both a positive and negative regulator of flowering time, depending on the type of regulatory subunit involved.

The ATP Binding Cassette Transporter AtMRP5 Modulates Anion and Calcium Channel Activities in Arabidopsis Guard Cells

Stomatal guard cells control CO(2) uptake and water loss between plants and the atmosphere. Stomatal closure in response to the drought stress hormone, abscisic acid (ABA), results from anion and K(+) release from guard cells. Previous studies have shown that cytosolic Ca(2+) elevation and ABA activate S-type anion channels in the plasma membrane of guard cells, leading to stomatal closure. However, membrane-bound regulators of abscisic acid signaling and guard cell anion channels remain unknown. Here we show that the ATP binding cassette (ABC) protein AtMRP5 is localized to the plasma membrane. Mutation in the AtMRP5 ABC protein impairs abscisic acid and cytosolic Ca(2+) activation of slow (S-type) anion channels in the plasma membrane of guard cells. Interestingly, atmrp5 insertion mutant guard cells also show impairment in abscisic acid activation of Ca(2+)-permeable channel currents in the plasma membrane of guard cells. These data provide evidence that the AtMRP5 ABC transporter is a central regulator of guard cell ion channel during abscisic acid and Ca(2+) signal transduction in guard cells.

Jasmonates Inhibit Flowering in Short-Day Plant Pharbitis nil

The role of jasmonates in the photoperiodic flower induction of short-day plant Pharbitis nil was investigated. The plants were grown in a special cycle: 72 h of darkness, 24 h of white light with lowered intensity, 24-h long inductive night, 14 days of continuous light. At 4 h of inductive night the cotyledons of non-induced plants contained about two times the amount of endogenous jasmonates (JA/JA-Me) compared to those induced. A 15-min long pulse of far red light (FR) applied at the end of a 24-h long white light phase inhibited flowering of P. nil. The concentration of jasmonates at 2 and 4 h of inductive night in the cotyledons of the plants treated with FR was similar. Red light (R) could reverse the effect of FR. R light applied after FR light decreased the content of jasmonates by about 50%. Methyl jasmonate (JA-Me) applied to cotyledons, shoot apices and cotyledon petioles of P. nil inhibited the formation of flower buds during the first half of a 24-h long inductive or 14-h long subinductive night. Application of JA-Me to the cotyledons was the most effective. None of the plants treated with JA-Me on the cotyledons in the middle of the inductive night formed terminal flower buds. The aspirin, ibuprofen and phenidone, jasmonates biosynthesis inhibitors partially reversed the effect of FR, stimulating the formation of axillary and terminal flower buds. Thus, the results obtained suggests that phytochrome system control both the photoperiodic flower induction and jasmonates metabolism. Jasmonates inhibit flowering in P. nil.

Esporulação de Mycosphaerella fijiensis em diferentes meios de cultura

Avaliou-se a esporulação de conídios de Mycosphaerella fijiensis (isolado LPM472) em sete meios de cultura (batata dextrose ágar, V8 ágar, V8 CaCO3 ágar, água de coco ágar, batata cenoura ágar, folha de banana ágar e micophil) sob quatro regimes de luminosidade (escuro contínuo, fotoperíodo de 12 h, luz contínua e seqüencial - dez dias no escuro e cinco dias sob luz contínua). O ensaio foi conduzido em delineamento inteiramente casualizado, com cinco repetições. A esporulação foi determinada em erlenmeyer de 125 ml, contendo 20 ml de seu respectivo meio de cultura e 0,5 ml de suspensão com 5 x 10(4) conídios de M. fijiensis/ml, mantidos a 25 ºC + 2 ºC, durante 15 dias. Não ocorreu esporulação em todos os meios de cultura sob regime de escuro contínuo, assim como no meio de folha de banana ágar, sob todos os regimes de luminosidade. No regime de fotoperíodo, ocorreu esporulação apenas nos meios V8 CaCO3 ágar e michophil, e no regime de luz contínua, apenas no micophil. No regime seqüencial, os meios de batata dextrose ágar e V8 CaCO3 ágar propiciaram as maiores produções de conídios.

Effect of 24 hours light on circadian rhythms of secretory enzymes and morphology of rat von Ebner’s glands

Von Ebner’s glands of the rat are minor salivary serous glands in the posterior portion of the tongue. They secrete two digestive enzymes, lingual lipase and amylase. In this investigation, circadian rhythm in feeding was established under a normal 12 h light/12 h dark cycle, with the rats eating primarily during the dark period. At lights on, the size of the acinar cells and the area of the inclusive secretory granules, and the amount of digestive enzyme activity (lingual lipase and amylase) remaining in the gland was significantly less than in the mid-afternoon, after very little daylight food consumption. However, after 7 days of continuous light the circadian rhythm was altered: the food consumption during the normal night-time hours (5 p.m. to 8 a.m.) went from 88% of total 24 h food consumption to 45%, and during normal daylight hours (8 a.m. to 5 p.m.) from 12% to 55%. These changes were correlated with histometric findings of a near reversal of the areas of acinar cells and secretory granules of a.m. and p.m. samples under continuous light. Lingual lipase activity in the glands went from 35% under 12 h light to 61% under continuous light in the a.m. and from 65% to 39% in the p.m. Amylase activity also showed nearly a reversal in activity remaining in the gland, from 36% at 12 h light to 58% at 24 h light in the a.m. and 64% to 41% for the p.m. samples. These results indicate that the von Ebner’s glands of the rat have a circadian rhythm of secretion and storage of secretory proteins that is subject to light entrainment similar to that seen in other exocrine glands such as the parotid and pancreas.

The Effect of Continuous Light on Refractive Error and the Ocular Components of the Rat

Phototoxic induced degeneration of the rat retina is a well-documented phenomenon resulting in losses of photoreceptors and their cell bodies, and an overall retinal thinning. This process may serve as a test of the hypothesis that the retinoscopic reflex originates from the inner limiting membrane of the retina. Retinal thinning should produce myopia in the absence of any other ocular component changes in a stable, mature eye if the inner limiting membrane model is correct. Phototoxic retinal degeneration was induced in 10 albino rats by exposure to 19 days of continuous light (1,800 cd m−2). Another 10 albino rats exposed to 12-hr on/12-hr off cycled light served as controls. Before and after the exposure to constant light, measures were made of refractive state by cycloplegic retinoscopy, corneal curvature and lens curvature by Purkinje image photography, and axial length of the globe by A-scan ultrasonography.Comparing pre- to post-exposure values, phototoxic degeneration resulted in a mean (±s.d.) myopic shift of −5.10±2.12 D (P<0.002). The corneal curvature also steepened significantly (0.17±0.11 mm, equivalent to −6.0 D;P<0.004), while the posterior curvature of the crystalline lens flattened by 0.21±0.22 equivalent mm (P<0.027), and the axial length shortened by 0.11±0.11 equivalent mm (all tests Wilcoxon signed-rank;P<0.025). Phototoxic rats underwent a mean retinal thinning of 49.6 μcompared to controls (Kruskal-Wallis test;P<0.0008). No refractive or ocular parameters changed significantly in the controls. Phototoxic degeneration in the rat has optical consequences beyond simple retinal thinning. The size of the eye and the curvature of refractive surfaces can be altered in a mature eye well after the completion of development. The multiple changes which occur prevent phototoxic retinal thinning from serving as a test of the inner limiting membrane model for retinoscopic reflections.

Habituation to Sound During Morning and Night Sessions in Pigeons ( Columba livia)

VALENTINUZZI, V. S. AND E. A. M. FERRARI. Habituation to sound during morning and night sessions in pigeons (Columba livia). PHYSIOL BEHAV 62(6) 1203–1209, 1997.—Habituation to acoustic stimulation was analyzed through night and morning sessions in pigeons submitted to a 12:12 h light-dark (LD) cycle. We demonstrate a lack of habituation in nocturnal sessions as compared to typical habituation curves in morning sessions. Temporal organization of the habituation process raises issues concerning the biological meaning of sound in adaptation to the environment and/or the possible interaction of well known endogenous physiological rhythms with learning. A second experiment was performed to establish whether the LD cycle was responsible for the night/day difference observed in the first experiment. In this experiment, the animals were subjected to two days of continuous light (LL) before the learning sessions. No difference in habituation according to time of day was observed in these conditions suggesting a direct effect of the LD cycle in the habituation to sound.

Circadian expression of the dnaK gene in the cyanobacterium Synechocystis sp. strain PCC 6803

The expression of the dnaK gene in the cyanobacterium Synechocystis sp. strain PCC 6803 was continuously monitored as bioluminescence by an automated monitoring system, using the bacterial luciferase genes (luxAB) of Vibrio harveyi as a reporter of promoter activity. A dnaK-reporting bioluminescent Synechocystis strain was constructed by fusing a promoterless segment of the luxAB gene set downstream of the promoter region of the Synechocystis dnaK gene and introduction of this gene fusion into a BglII site downstream of the ndhB gene in the Synechocystis chromosome. Bioluminescence from this strain was continuously monitored and oscillated with a period of about 22 h for at least 5 days in continuous light. The phase of the rhythm was reset by the timing of the 12-h dark period administered prior to the continuous light. The period of the rhythm was temperature compensated between 25 and 35 degrees C. Thus, the bioluminescence rhythm satisfied the three criteria of circadian rhythms. Furthermore, the abundance of dnaK mRNA also oscillated with a period of about 1 day for at least 2 days in continuous light conditions, indicating circadian control of dnaK gene expression in Synechocystis sp. strain PCC 6803.

Cytokinins in photoperiodic induction of flowering in Chenopodium species

Changes in cytokinin (zeatin – Z, zeatin riboside – ZR, isopentenyladenine – iP, isopentenyladenosine – iPA) levels were determined under light regimes inductive and non‐inductive for flowering in leaves, stems, roots and apical parts of short‐day Chenopodium rubrum and long‐day Chenopodium murale. In leaves. stems and roots of both plant species the level of cytokinins (in C. rubrum of Z and ZR, in C. murale of Z. ZR, iP and iPA) decreased by about 50% during the dark period and increased again during the subsequent light period, No significant changes in cytokinin levels were observed in continuous light. In apical parts of C. rubrum cytokinin level (Z, ZR, iP) was dramatically increased (by 400–500%) at the end of the dark period and decreased to about the original value during the following light period, while no changes were observed in continuous light. In apical parts of C. murale the level of cytokinins doubled during floral induction consisting of 10 days of continuous light. A red (R) break (15 min at the 6th h of darkness), which prevents flowering in C. rubrum, has no significant effect on cytokinin levels in leaves at the end of darkness. Cytokinin levels increased 1 h after R and decreased again rapidly. On the other hand, the increase of cytokinin level in the apical parts of C. rubrum was largely prevented by the R break. These effects of R on cytokinin levels were not reverted by far‐red (FR), while the effect on flowering was reverted. It may be concluded that there is no correlation between changes in cytokinin levels in leaves. Stems and roots and photoperiodic flower induction, as both species, representing different photoperiodic types, showed similar changes under the same light regime. The increase of cytokinin levels in apical parts of both photoperiodic species during floral induction suggests a role (increased cell division and branching) for cytokinins in apex evocation.

Initial characterization of a pea mutant with light-independent photomorphogenesis.

We have identified a mutant of pea cultivar Alaska that has many of the characteristics normally associated with light-grown seedlings even when grown in complete darkness. We have designated this mutant lip1, for light independent photomorphogenesis. Etiolated wild-type pea seedlings are white to slightly yellow in color and have a distinct morphology characterized by elongated epicotyls and buds containing unexpanded leaves with small, undifferentiated cells. In contrast, mutant seedlings grown under the same conditions are yellow in color and have short epicotyls and expanded leaves showing clear cellular differentiation. Transmission electron microscopy revealed partially developed, agranal plastids in the dark-grown mutant, unlike wild-type seedlings that contain etioplasts with prolamellar bodies. The mutant also exhibits a much shorter lag period for chlorophyll accumulation when etiolated seedlings are transferred from darkness to white light. The dark-grown mutant has 10-fold less spectrally detectable phytochrome, which can be attributed to a 10-fold reduction in the level of the PHYA polypeptide. Cab, Fed1, and RbcS transcripts are present in dark-grown mutant seedlings at levels comparable to those produced in light-grown material. The levels of these transcripts show a normal decrease when green plants grown for 15 days in a light/dark cycle are transferred to continuous darkness. However, transcript levels remain high during dark treatment of seedlings grown for 9 days in continuous light, indicating that the dark adaptation response in this mutant is developmentally plastic. The lip1 mutant has several features in common with the deetiolated Arabidopsis mutants det1, det2, and cop1. However, there are also several important differences, including varying effects on phytochrome levels, organ-specific gene expression, plastid development, and response to dark adaptation.

Does the tsetse parturition rhythm have a circadian basis?

Abstract. Under an LD 12:12h photoregime at constant temperature, parturition in Glossina morsitans centralis is a gated event occurring late in the afternoon. When flies are switched to continuous light the rhythm quickly dampens, but its persistence for at least two 24‐h cycles beyond the final scotophase suggests the rhythm has a circadian basis. A weak rhythm appears after 7 days of continuous light, perhaps in response to the daily disturbance caused by feeding. Return of the flies to LD 12:12h restores the rhythm after exposure to a single scotophase.

Effect of Oxygen and Temperature on the Induction of a Circadian Nitrogenase Activity Rhythm in Synechococcus RF-1

Summary Evidence is presented that the periodic cessation of photosynthetic CO 2 assimilation rather than O 2 evolution appears to induce an endogenous nitrogenase activity rhythm in Synechococcus RF-1. Apart from the inductive effect of L/D cycles, a 4 h cold pulse (0 °C) on four consecutive days in continuous light also induces the rhythm. At 0 °C the organism ceases to photosynthesize. The rhythms induced by means of L/D cycles at temperatures below 23 °C (15 °C-0 °C) exhibit similar phase shifts relative to the rhythms induced at 23 °C and higher temperatures. Synechococcus RF-1 does not exhibit nitrogenase activity in nitrogen-free medium at 5 °C or lower temperatures and appears to be unable to survive exposure to -5 °C for a few hours.

Changes in endogenous levels of three cytokinins during development of excised watermelon cotyledons

We have determined by an immunological method the endogenous levels of three cytokinins: dihydrozeatin riboside (DHZR), transzeatin riboside (tZR) and isope-ntenyladenosine (IPA) in watermelon (Citrullus vulgaris Schrad., cv. Fairfax) cotyledons that were either attached to the seedling or excised from the seed after imbibition and then grown on water. Both seedlings and cotyledons were grown either for 5 days in continuous light or for 3 days in the dark and 2 days in light. Our aim was to verify whether endogenous cytokinin levels are lower in excised than in attached cotyledons as could be expected since excised cotyledons are much more sensitive to exogenous cytokinin application. The levels of the three cytokinins were very low immediately after imbibition, but gradually increased during the following days. They were higher in excised cotyledons after 5 days of culture in the dark than in cotyledons of the same age that had developed on the seedling. Dihydrozeatin riboside was by far the most abundant of the three cytokinins in cotyledons as well as in the hypocotyl and the root. Irradiation reduced the level of DHZR, negating the concept that light promotes cotyledon development by increasing endogenous cytokinins. Transzeatin riboside when supplied exogenously, stimulated cotyledon development at a lower concentration than the other two cytokinins. Exogenous supply of ben-zyladenine (BA) induced a strong increase in endogenous tZR already after 24 h.

Synthesis and biological activity of allenic analogues of abscisic acid

Novel allenic analogues of abscisic acid (ABA) were synthesized and tested for ABA activity. The allene functional group was introduced by one of two methods: base catalysed isomerization of an enyne to an enallene conjugated to a ketone, or reduction of a 2-butyn-1,4-diol to an allenic alcohol. A 3:1 mixture of ( E)- and ( Z)-3-methyl-5-(4′,4′-ethylenedioxy-2′,6′,6′-trimethylcyclohex-2′-en-1′-ylidene)-2,4,5-pentatrienoic acids strongly inhibited growth of axenic duckweed ( Lemna gibba) causing a 50% reduction in frond number at 60 μg 1 −1 over 7 days in continuous light. Racemic ABA caused the same inhibition at 130 μ 1 −1. The mixture caused the production of turion-like structures, an effect known hitherto to be induced only by short photoperiods or by certain concentrations of ABA.

Ethylene production and metabolism of 1-aminocyclopropane-1-carboxylic acid in a long-day plant,Chenopodium murale L., as influenced by photoperiodic flower induction

Chenopodium murale plants, induced to flower by 5 days of continuous light, produced 43% more ethylene than vegetative plants kept under short days (16 h darkness, 8 h light). The 1-aminocyclopropane-1-carboxylic acid (ACC)-induced ethylene production, using saturating ACC concentration (10 mol·m−3) was also 55% higher in induced plants. Their ACC and N-malonyl-ACC (MACC) levels were also higher, the former increasing by 56% in both shoots and roots, the latter by 288% and 108% in shoots and roots, respectively. Administration of labeled [2,3-14C]ACC produced a very similar relative content of ACC and MACC in both treatments. The only process influenced by flower induction was ACC conversion to ethylene. Induced plants converted 66% more ACC than the vegetative ones. The effects of photoperiod on ethylene formation and metabolism in a long-day plant (LDP)C. murale and a short-day plant (SDP)C. rubrum are compared. Ethylene formation seems to be under photoperiodic control in both species, but its role in flower induction remains obscure.

Loss of short-day requirement for dormancy breaking of bulbils from gibberellic acid-treated plants in Sedum bulbiferum

Sedum bulbiferum forms bulbils at superterranean nodes under long-day conditions, and the detached bulbils sprout after exposure to short-days [1]. When gibberellic acid was applied to the mother plant at the start of the long-day induction period, the number of bulbils formed increased slightly and these bulbils sprouted on incubation in the dark but not under short days of continuous light. However, when gibberellic acid was applied directly to detached bulbils during incubation, the short-day requirement for sprouting was conserved. Gibberellic acid application to the mother plants enhanced sprouting ability of detached bulbils when incubated under illumination with blue, green or far-red light. However, presence of gibberellic acid during bulbil exposure to light did not induced marked enhancement in sprouting under blue, green of far-red light. Thus, gibberellic acid application to the mother plant modified light and photoperiodic requirements for the sprouting of detached bulbils of S. bulbiferum.

Nosema bombycis Replication in a Manduca sexta Cell Line

Nineteen B. glabrata (NIH-PR 1 strain) which had been individually exposed to 5-10 S. mansoni miracidia 58 days previously were placed in the apparatus. The average size of these snails was 15.2 mm + 1.65 mm. The snails were allowed to acclimatize to the apparatus for 3 days. Following this period, cercarial emergence from each snail was measured every 2 hr for 32 days. During this time, snails were kept on a 12 hr light/12 hr dark cycle except during days 8-22 and days 28-30, when the snails were kept in continuous light. During the experiment, the apparatus was kept in a walk-in environmental chamber. Temperature was kept constant at 25 C and light was supplied by 4 12-watt fluorescent bulbs. Snails were fed romaine lettuce ad libitum. The average number of cercariae emerging per snail per 2 hr interval is shown in Figure 2. Regardless of the lighting regimen, all snails had similar rhythms of cercarial emergence. During the 18 days of continuous light, 210 ? 144 cercariae emerged per snail per day, while during the 14 days with 12 hr of light and 12 hr of dark, 428 ? 283 cercariae emerged per snail per day. These values were significantly different in a Student's t-test. These results suggest that the diurnal emergence of S. mansoni cercariae is controlled by the light/dark cycle. Since the 24 hr period was abruptly abolished in continuous light, this aspect of the rhythm is probably not controlled by an endogenous oscillator. This agrees with the findings of other investigators (Nojima and Sato, 1982, loc. cit.). How the light/dark cycle controls cercarial emergence is not known. Duke (1952, J. Helminth. 26: 133-146) observed that the increased movements of S. mansoni cercariae lying beneath the surface epithelium of B. glabrata caused the emergence of the cercariae from the snails. Since irradiance changes cause increased movement of S. mansoni cercariae (Saladin, 1982, J. Parasit. 68: 120-124), changes in the amount of light passing through the thin surface epithelium of infected snails may stimulate cercarial emergence. This may account for our finding that over twice as many cercariae emerged during days with a light/dark change than during days with continuous light. The change from light to dark on day 23 may have stimulated the emergence of cercariae that had accumulated in the peripheral tissues of the snails during the previous days of continuous light. This is consistent with other reports (Nojima and Sato, 1982, loc. cit.; Luttermoser, 1955, J. Parasit. 41: 201-208) that the longer the period of darkness, the greater the number of cercariae emerging in the following light period.