Diurnal oscillatory movements of growing leaves of tobacco

The analysis of diurnal oscillatory movements of tobacco leaves was used in the diagnosis of viral infection of plants. The oscillatory helices circumscribed by a growing leaf of a healthy plant were regular, but some deviations, particularly in the transition points, were recorded. In order to make clear the cause of these irregularities of trajectory, the course of elongation of leaf petiole and blade in relation to localization and shift of zones of elongation during ontogenesis was analysed. The present analysis is similar to that described by the author's earlier experiments with pea roots. Oscillatory curves circumscribed by petiole, projected on a horizontal plane, were compared with curves circumscribed by the blade tip. The analysis of the leaves of different ages enabled us to study this process in dependence on growth rate. It was confirmed that oscillations are a result of elongation; the extent of oscillations is quantitatively dependent on the growth rate. An analysis of the zones of growth showed that in petiole the active meristems are localized near to its base while in the leaf lamina they move gradually during the ontogenesis from the apical to the basal part of the leaf blade. Active meristems of young rapidly growing leaves are localized approximately in the middle of the blade while those of old leaves were found in close proximity to the base of the lamina. The growth rate of petiole can be expressed in hundreds of mm per hour (4.8×10^?2 mm h^?1); half of this value was recorded for its apical part. The growth rate of leaf blade was found approximately ten times higher (3.2×10^?1 mm h^?1). The oscillatory movements of growing leaf consists of two integrate components: of oscillations originating in the base of the petiole and of oscillations of leaf blade the centrum of which is localized in the basal third of the blade. The arrangement of the experiments did not enable us to determine to what extent the phototropic response of leaf blade participates in leaf movements. The movements of leaves of an intact plant are evidently affected by rhythmic stem oscillations. Stem is an integral part of a system which participates in the transfer of information in plants.     

QUANTITATIVE MEASUREMENTS OF INTERACTIONS BETWEEN CROWN-GALL TUMORS AND THE PINTO BEAN HOST

Interactions between crown-gall tumors on the primary pinto bean leaf and the pinto bean seedling (Phaseolus vulgaris L. ‘Pinto‘) were estimated by quantitative measurements of tumor initiation and growth as affected by certain modifications of the host. Effects of the tumors on the host were estimated by measurements of host growth and correlation responses. The presence of crown-gall tumors was found to reduce the growth of the leaf in area but to nearly double the weight of the leaf 9 days after inoculation with Agrobacterium tumefaciens (Smith and Town.) Conn, strain B6. The presence of tumors on only one of the two primary leaves resulted in a decrease in the weight of the leaf without tumors, showing the tumors to be effective mobilization centers. Tumors also delayed the abscission of petiole explants and delayed the growth of the epicotyl bud, both reminiscent of auxin effects. The excision of the cotyledons, the epicotyl bud, or one of the pair of primary leaves at the time of inoculation increased the number of tumors initiated per leaf. Removing the epicotyl bud or one of the primary leaves, or placing a cytokinin on one of the leaves, altered leaf growth but failed to alter tumor growth, indicating that tumor growth is not affected by the changes responsible for the compensatory growth effects induced by these treatments. Tumor growth was shown to be generally correlated with leaf growth from day 2 through 8 after inoculation, suggesting that the factors normally limiting leaf growth in a determinate type leaf are also active in limiting tumor growth. The changes in the plant cell responsible for the enhanced rate of growth seen in crown-gall tumor cells, therefore, appear to occur in regulatory systems other than those normally limiting leaf growth. The regulatory systems that are affected may be identical with those activated in compensatory host growth effects.     

Elongation and circumnutation oscillations of hypocotyl of pine seedlings (Pinus silvestris L.)

The elongation of pine seedlings (Pinus silvestris L.) is associated with nutation movements. Trajectories of these growth oscillations were recorded by film technique in horizontal and vertical projection during a three day period of growth. On the basis of these data the parameters of elongation and nutation oscillations,i.e. rate, amplitudes and frequency of oscillations, were calculated and their changes during plant development compared. Oscillation trajectories are circular or elliptic spirals the amplitudes of which are increasing with the age of hypocotyl from 1.5 mm to 7.5 mm. The frequencies of nutations are decreasing during the growth from 0.5 to 0.2 rev. h^?1. On the other hand, the growth rate of hypocotyl increased from values near to 10^?3 mm h^?1 at the beginning of the experiment to 4×10^?1 mm h^?1 recorded at the end of the third day. The zone of nutation curvature was slightly transferred from the middle of the hypocotyl toward the apex and its location has not been identical with that of elongation. This indicates that the system controlling nutation oscillations need not be identical with that controlling direction of elongation. At a certain stage of development behaviour of the decapitated pine hypocotyl is analogical to that of the root without the centre of georeception. A possibility of analogy of the system controlling direction of hypocotyl growth and of the system proposed for geotropical control of root growth is discussed.     

Synchronization of oscillatory rhythms of stems and leaves

A previous report on the mechanism of diurnal oscillations is supplemented here by data concerning causes of irregularities in their oscillation trajectories. Cinematographic records showed that the final trajectory is significantly influenced by the circumnutation oscillations of the stem. Amplitudes of these oscillations which are small(ca 0.7 mm as compared with 5 X larger amplitudes of leaves when measured at the leaf tip) can be enlarged up to 10 times the value at the apical tip of the leaf. These nutations are responsible for side-deviations of space spirals of growing leaves in the direction of their shorter half axis. This explanation is based on the occurrence of side-deviations of leaf trajectories which correlate with the actual position of the nutation phase of the stem. Concurrent frequencies of the two oscillations (= diurnal rhythm) indicate that nyctinastic oscillations of the leaves are controlled by the same mechanism as is the circumnutation of the stem.     

Comparison of the effects of epibrassinolide and steroidal estrogens on adventitious root growth and early shoot development in mung bean cuttings

Concentrations of 24-epibrassinolide as low as 0.1 μ M consistently inhibited adventitious root formation and elongation in both hypocotyl and epicotyl cuttings from mung bean ( Phaseolus aureus L.). Similar, but less pronounced, inhibitory effects on root elongation were also observed with estrone sulphate and estradiol sulphate. With regards to root number, estrone sulphate enhanced this in both types of cutting, whereas estradiol sulphate was stimulatory in hypocotyl cuttings but inhibitory in epicotyl cuttings. Brassinolide caused a marked stimulation of epicotyl (but not hypocotyl) elongation and a swelling and splitting of the epicotyl in both types of cutting, whereas estrogens varied in their effect from inhibition of epicotyl growth to no effect. Root-applied brassinolide and estrogen sulphates brought about similar morphological abnormalities in shoots viz. epinasty and inrolling of primary leaves and delayed expansion of the first trifoliate leaf.     

The effect of lateral illumination on growth oscillations of pine seedling hypocotyls (Pinus silvestris L.)

Phototropic sensitivity of forest wood seedlings to lateral illumination was proposed as an early assay for distinquishing various ecotypes of woody plants of the same species. Statistical analysis showed that results were significantly influenced by heterogenity of experimental material caused by an interference of phototropic movements and natural oscillations of hypocotyl. Both movements of pine seedlings (Pinus silvestris L.) were registered by phase photography and their mechanism was analyzed. The apical part of growing hypocotyl illuminated from above oscillates in a space spiral with frequency 3.3 h at mean growth rate 0.66 mm h^?1. The mean size of spiral amplitudes is 2.9 mm. The oscillation rhythm is disturbed after the lateral illumination and a phase shift was observed as a result. A new oscillation rhythm with frequency 3.9 h and mean growth rate 0.69 mm h^?1 was stabilized after a period of time equal to one nutation turn. Oscillation amplitudes were increased to 4.3 mm. In addition to the radiation intensity the appearance of the phototropic response to light pulse is first of all effected by the actual position of the apical part of hypocotyl in the course of endogenous circumnutations. A uniform plant material for the early assay may be obtained by selection of seedlings which are at the beginning of lateral illumination in the same phase of nutation rhythm. Under such conditions the deviations of longitudinal axis of oscillating spirals characterize the actual phototropic curvature.     

A model for Ca2+ oscillations stimulated by the type 5 metabotropic glutamate receptor: an unusual mechanism based on repetitive, reversible phosphorylation of the receptor

In parallel with experimental investigations, the molecular mechanisms responsible for Ca²⁺ oscillations have been much investigated with computational models. In the vast majority of cell-types, these oscillations rely on the biphasic regulation of the inositol 1,4,5-trisphosphate (InsP₃) receptor by cytosolic Ca²⁺. However, when Ca²⁺ oscillations are initiated by agonist stimulation of the type 5 metabotropic glutamate (mGlu5) receptor, oscillatory behaviour is tightly controlled by repetitive cycles of receptor phosphorylation/dephosphorylation leading to the periodic activation/deactivation of the G protein-activated signalling cascade downstream of this G protein-coupled receptor. We present a minimal model for mGlu5 receptor-induced Ca²⁺ oscillations, taking into account receptor phosphorylation by a protein kinase C isoenzyme sensitive to diacylglycerol but not to Ca²⁺. Depending on the density of receptors and the level of stimulation, the model reproduces Ca²⁺ oscillations based on either a ‘dynamic uncoupling’ mechanism or InsP₃ receptor dynamics. When based on the former mechanism, Ca²⁺ oscillation frequency is insensitive to the level of stimulation, while the level of receptor expression is a determinant of oscillation frequency. When investigating the conditions for the occurrence of oscillations, the model predicts that dynamic uncoupling likely relies on a steep relationship between the activity of PKC and the amount of phosphorylated mGlu5 receptor. Finally, we use the model to simulate the adaptation of the signalling pathway during periods of prolonged stimulation associated with receptor desensitization/internalization. The model suggests that the existence of both oscillatory mechanisms could allow for a significant lengthening of the repetitive Ca²⁺ responses under these conditions.     

Dynamics of growth and the content of endogenous phytohormones during kidney bean scoto-and photomorphogenesis

The dynamics of growth and the contents of free and bound endogenous IAA, gibberellins (GA), cytokinins (zeatin and its riboside), and ABA in kidney bean plants (Phaseolus vulgaris L., cv. Belozernaya) grown in darkness or in the light was studied. Phytohormones were quantified in 5–15-day-old plants by the ELISA technique. Plant growth and phytohormone content were shown to depend on plant age and the conditions of illumination. During scotomorphogenesis, changes in the biomass and hypocotyl length were highly correlated with the content of GA, whereas during photomorphogeneses, these parameters were correlated with the content of zeatin. In darkness, epicotyl growth displayed a positive correlation with the content of GA, whereas in the light, the correlation was negative. Growth characteristics of the primary leaves were shown to correlate with IAA in darkness and with GA and zeatin in the light. At a low concentration of cytokinins in illuminated leaves, cell divisions occurred, whereas, at the higher cytokinin concentrations, cell expansion occurred. The highest content of GA was characteristic of leaves in the period of growth cessation. ABA accumulated during active leaf and root elongation and biomass increment in the light and during hypocotyl growth in darkness. After plant illumination, the ratio of auxins to cytokinins increased in bean roots and decreased in their epicotyls. Thus, light changed the developmental programs of bean plants, which was manifested in the changed rate and duration of growth of various organs (root, hypocotyl, epicotyl, and leaf). Some mechanisms of light action depended on the contents of IAA, ABA, GA, and cytokinins and the ratios between these phytohormones. Differences between scotonorphogenesis of mono-and dicotyledonous plants are discussed in relation to the levels of phytohormones in them.     

Interactions of photo- and geotropism with periodical oscillations of growing pea root (Pisum sativum L.)

Oscillation movements of primary roots of pea seedlings were investigated after low- and high-energy irradiation (10^?2 and 10⁶ W m^?2) which was applied continuously and in pulses at intervals of 3 and 30 min. Oscillation amplitudes of control roots grown in darkness were lower (1 mm) than those of irradiated ones recorded in our previous experiments. The elongation rate of roots grown under scattered and low-energy irradiation as well as their mean oscillation amplitude (2.6 mm) and frequency (8 h) correspond to the standard values recorded in previous experiments. The same effects on root growth and oscillations had scattered electronic flash irradiation. Side-irradiation did not influence the oscillation movements of growing roots. Their frequency and oscillation amplitudes were not changed. Their longitudal axis was, however, deflected 2.75° from the vertical one due to the negative phototropic response of the root. The growth oscillations may be, according to present results, interpreted as consequences of tropic growth controlling system. External factors, like irradiation, may influence oscillation parameters,i.e. their amplitudes and after application of side-irradiation also the direction of longitudinal axis of oscillation spirals. The resultant trajectory is composed from movements which resulted from geotropic and phototropic responses of the roots.     

表油菜素内酯对绿豆上胚轴内源IAA及其氧化酶的影响

用0.5ppm表油菜素内酯处理绿豆幼苗,显著促进上胚轴伸长生长,若切除真叶则可抑制表油菜素内酯诱导的效应。三碘苯甲酸(TIBA)也可抑制表油菜素内酯促进的伸长生长。外源IAA能部分恢复TIBA的抑制效应。经处理的上胚轴内源IAA含量明显高于对照。暗示表油菜素内酯可能通过对内源IAA的调节来促进绿豆上胚轴的伸长生长。 表油菜素内酯处理的绿豆上胚轴组织中,与生长素降解密切相关的IAA氧化酶以及过氧化物酶活性均明显低于对照。     

The response of growth and circadian rhythm to cycle length and photoperiod

Summary The growth of bean plants (Phaseolus vulgaris L., cv Red Kidney) is inhibited by cycle lengths of 36 and 48 hours. Maximal inhibition occurs when the length of the light period is equal to or shorter than 3/6 of the cycle length. The inhibition does not occur when the photofraction is 5/6 or longer. The rhythmic leaf movement in beans can be entrained to a 30-hour cycle with a photofraction of 3/6 or less. No entrainment occurs to 36-or 48-hour cycles, but such cycles with photofraction of 3/6 or less cause an irregular course of the rhythm. When the photofraction is 5/6 or greater, the leaf movement proceeds as in continuous light, independent of cycle length. In continuous light the rhythm persists for at least 12 days. The parallel response of growth and circadian rhythm to cycle length and photoperiod suggests that a circadian oscillation is involved in the growth process. It further indicates that the response of these phenomena to cycle length and photoperiod involves the same basic timing mechanism.With 4 Figures in the Text     

Membrane potentials and salt distribution in epidermal bladders and photosynthetic tissue of Mesembryanthemum crystallinum L.

Abstract Membrane potentials (pd's) of epidermal bladder cells and green leaf cells of Mesembryanthemum crystallinum L. are rather low (between ?10 and ?40 mV). During growth on 400 mM NaCl membrane potentials decrease further. As shown previously, plants grown on 400 mM NaCl show the diurnal changes of malate levels typical for plants having crassulacean acid metabolism. Therefore, in this study membrane potentials were measured at different times of the day, but no diurnal variations of pd were correlated with diurnal oscillations of malate levels. Resting potentials are similar in bladder cells and in green leaf cells and are similar in continuous light and darkness. Both bladder and leaf cells display light-triggered photosynthesis-dependent oscillations of the membrane potential although the bladder cells do not appear to be photosynthetically very active. This suggests electrical coupling between the bladder cells and the underlying green cells. Over a larger distance (2 mm) in the leaves, however, there is no direct evidence for electrical coupling. Cl^?, Na⁺, and K⁺ concentrations are similar in bladder cells and in the photosynthetically active tissue of leaves and stems. Bladder cells appear to contain high concentrations of free oxalate. The present findings corroborate earlier conclusions that the epidermal bladder cells of M. crystallinum function as peripheral water reservoirs providing protection from short term water stress.     

A comprehensive time‐course metabolite profiling of the model cyanobacterium Synechocystis sp. PCC 6803 under diurnal light:dark cycles

Cyanobacteria are a model photoautotroph and a chassis for the sustainable production of fuels and chemicals. Knowledge of photoautotrophic metabolism in the natural environment of day/night cycles is lacking, yet has implications for improved yield from plants, algae and cyanobacteria. Here, a thorough approach to characterizing diverse metabolites—including carbohydrates, lipids, amino acids, pigments, cofactors, nucleic acids and polysaccharides—in the model cyanobacterium Synechocystis sp. PCC 6803 (S. 6803) under sinusoidal diurnal light:dark cycles was developed and applied. A custom photobioreactor and multi‐platform mass spectrometry workflow enabled metabolite profiling every 30–120 min across a 24‐h diurnal sinusoidal LD (‘sinLD’) cycle peaking at 1600 μmol photons m^?2 sec^?1. We report widespread oscillations across the sinLD cycle with 90%, 94% and 40% of the identified polar/semi‐polar, non‐polar and polymeric metabolites displaying statistically significant oscillations, respectively. Microbial growth displayed distinct lag, biomass accumulation and cell division phases of growth. During the lag phase, amino acids and nucleic acids accumulated to high levels per cell followed by decreased levels during the biomass accumulation phase, presumably due to protein and DNA synthesis. Insoluble carbohydrates displayed sharp oscillations per cell at the day‐to‐night transition. Potential bottlenecks in central carbon metabolism are highlighted. Together, this report provides a comprehensive view of photosynthetic metabolite behavior with high temporal resolution, offering insight into the impact of growth synchronization to light cycles via circadian rhythms. Incorporation into computational modeling and metabolic engineering efforts promises to improve industrially relevant strain design.     

Oscillations in stomatal conductance of hybrid poplar leaves in the light and dark

Cycling of stomatal conductance in three hybrid poplar ( Populus sp.) cultivars was observed under a variety of conditions. Illumination of plants kept previously in the dark induced very large oscillations with a period of about 40 min and large oscillations with a shorter period (< 10 min) were superimposed on the longer cycles. During these oscillations, large changes in conductance could occur very rapidly (1.0 cm s⁻¹ in 3 min). Plants in constant light also displayed both long and short term cycles in conductance, but these were smaller in amplitude than those induced by sudden illumination. Stomatal oscillations were also observed in darkness and after darkening of previously illuminated plants. These oscillations had shorter (< 30 min) and less regular periods than those observed in the light. Such cycling in the dark is rare. Cycling of the two leaf surfaces was sometimes in synchrony in the light, and more so after a perturbation. Little synchrony between the two surfaces was observed in the dark. Stomatal movements of different leaves on a plant were usually relatively independent. Transient stomatal opening occurred following leaf excision in the light or dark, and often after sudden darkening of intact leaves. Also, stomata of intact leaves sometimes transiently closed following illumination.     

Growth and nutation parameters of primary root of pedunculated oak (Quercus robur L.)

The elongation of primary roots of pedunculated oak (Quercus robur L.), which is associated with nutation oscillations, was registered by a time lapse cinematography. The following characteristics were expressed graphically: relative growth rate [mm h-¹], frequency [rev. h-¹] and amplitudes [mm] of nutation oscillations, their direction and location of nutation curvatures. The growth-oscillation mechanism is interpreted as a part of the geocontrol system of the root and was evaluated on the basis of its physiological efficiency. Comparison of nutation periods showed that the control system of pedunculated oak is three times more and two times less efficient than that of pea and maize, respectively. Comparison of the oscillation amplitudes of growing roots gives similar results.     

Temporal and spatial variations in the oxygen-18 content of leaf water in different plant species

Temporal variations in the δ¹⁸ oxygen (δ¹⁸O) content of water transpired by leaves during a simulated diurnal cycle fluctuated around the δ¹⁸O content of the source water. Reconstructed variations in the δ¹⁸O values of leaf water differed markedly from those predicted by conventional models. Even when transpiring leaves were maintained under constant conditions for at least 3 h, strict isotopic steady-state conditions of leaf water (equality of the ¹⁸O/¹⁶O ratios in the input and transpired water) were rarely attained in a variety of plant species (Citrus reticu-lata, Citrus paradisi, Gossypium hirsutum, Helianthus annuns, Musa musaceae and Nicotinia tabacum). Isotopic analysis of water transpired by leaves indicated that leaves approach the isotopic steady state in two stages. The first stage takes 10 to 35 min (with a rate of change of about 3–3%h^?1), while in the second stage further approach to the isotopic steady state is asymptotic (with a rate of change of about 0–4% h^?1), and under conditions of low transpiration leaves can last for many hours. Substantial spatial isotopic heterogeneity was maintained even when leaves were at or near isotopic steady state. An underlying pattern in this isotopic heterogeneity is often discerned with increasing ¹⁸O/¹⁶O ratios from base to tip, and from the centre to the edges of the leaves. It is also shown that tissue water along these spatial isotopic gradients, as well as the average leaf water, can have ¹⁸O/¹⁶O ratios both lower and higher than those predicted by the conventional Craig and Gordon model. We concluded, first, that at any given time during the diurnal cycle of relative humidity the attainment of an isotopic steady state in leaf water cannot be assumed a priori and, secondly, that the isotopic enrichment pattern of leaf water reflects gradual enrichment along the water-flow pathway (e.g. as in a string of pools), rather than a single-step enrichment from source water, as is normally assumed.     

Relationship between Growth and Electric Oscillations in Bean Roots

Extracellular and intracellular electric potentials in bean roots are known to show electric oscillations along the longitudinal axis with a period of several minutes. The relationship between growth and the electric oscillations was studied using roots of adzuki (Phaseolus chrysanthos). We measured surface electric potentials with a multielectrode apparatus while simultaneously measuring elongation using a CCD camera and monitor. Roots having an electric oscillation grew faster than roots with no oscillation. Furthermore, elongation rate was higher in roots with higher oscillation frequency. Oscillation frequency had a strong dependence on temperature; i.e. Q₁₀ was estimated at 1.7. These results suggest a correlation between electric oscillation and elongation.     

Maintenance of the diurnal petiolar movement by exogenous indole-3-acetic acid under day-night cycles in Mimosa pudica

Effects of plant hormones on the diurnal movement of the petiole of Mimosa pudica L. were examined under the conditions of day-night cycles. Surgical removal of the leaf at the middle of the petiole led to gradual loss of the movement. Aqueous solutions of indole-3-acetic acid (IAA) applied as a pulse each day 10 μl at (10⁻⁶−10⁻⁵ M ) or continuously (10⁻⁸−10⁻⁶ M ) to the cut end of the petiole maintained a diurnal movement that has the same phase of oscillation as that shown in the intact leaf. Pulse treatments given at different times of the day had no effect on the phase of the oscillation. Higher concentrations of the acid gave larger amplitudes. Other hormones such as gibberellic acid, kinetin and [R,S]-abscisic acid or 1-aminocyclopropane-1-carboxylic acid did not show diurnal movement-maintaining activities of IAA. Gibberellic acid disturbed the phase of the diurnal movements and kept the petiole elevated at very high positions. A possible action mechanism of IAA is discussed.     

A digital sensor to measure real-time leaf movements and detect abiotic stress in plants

Plant and plant organ movements are the result of a complex integration of endogenous growth and developmental responses, partially controlled by the circadian clock, and external environmental cues. Monitoring of plant motion is typically done by image-based phenotyping techniques with the aid of computer vision algorithms. Here we present a method to measure leaf movements using a digital inertial measurement unit (IMU) sensor. The lightweight sensor is easily attachable to a leaf or plant organ and records angular traits in real-time for two dimensions (pitch and roll) with high resolution (measured sensor oscillations of 0.36 ± 0.53° for pitch and 0.50 ± 0.65° for roll). We were able to record simple movements such as petiole bending, as well as complex lamina motions, in several crops, ranging from tomato to banana. We also assessed growth responses in terms of lettuce rosette expansion and maize seedling stem movements. The IMU sensors are capable of detecting small changes of nutations (i.e. bending movements) in leaves of different ages and in different plant species. In addition, the sensor system can also monitor stress-induced leaf movements. We observed that unfavorable environmental conditions evoke certain leaf movements, such as drastic epinastic responses, as well as subtle fading of the amplitude of nutations. In summary, the presented digital sensor system enables continuous detection of a variety of leaf motions with high precision, and is a low-cost tool in the field of plant phenotyping, with potential applications in early stress detection.

An inertial measurement unit is capable of measuring dynamic and complex plant organ movements in real-time, and is suitable for early abiotic stress detection.     

Phosphorylation of the D1 photosystem II reaction center protein is controlled by an endogenous circadian rhythm

The light dependence of D1 phosphorylation is unique to higher plants, being constitutive in cyanobacteria and algae. In a photoautotrophic higher plant, Spirodela oligorrhiza, grown in greenhouse conditions under natural diurnal cycles of solar irradiation, the ratio of phosphorylated versus total D1 protein (D1-P index: [D1-P]/[D1] + [D1-P]) of photosystem II is shown to undergo reproducible diurnal oscillation. These oscillations were clearly out of phase with the period of maximum in light intensity. The timing of the D1-P index maximum was not affected by changes in temperature, the amount of D1 kinase activity present in the thylakoid membranes, the rate of D1 protein synthesis, or photoinhibition. However, when the dark period in a normal diurnal cycle was cut short artificially by transferring plants to continuous light conditions, the D1-P index timing shifted and reached a maximum within 4 to 5 h of light illumination. The resultant diurnal oscillation persisted for at least two cycles in continuous light, suggesting that the rhythm is endogenous (circadian) and is entrained by an external signal.