Photoperiodic Rhythm Research Articles

Circadian Clock Contributes to Modulate Salinity Stress-Responsive Antioxidative Mechanisms and Chloroplast Proteome in Spinacia oleracea

Extreme abiotic stresses such as drought, salinity, and temperature reduce crop productivity significantly and pose a serious threat to the area of land used for agriculture. Therefore, there is a pressing need to create crops that can thrive in these circumstances. It has been noted that plants can maintain defense mechanisms during any environmental changes and anticipate diurnal patterns correct to a circadian-based clock. Therefore, the main aim of this study was to investigate the role of circadian core oscillators in response to salinity stress in an important vegetable crop, spinach, and obtain evidence to better understand salinity stress adaptation for crop productivity. Therefore, the current study was carried out to examine the circadian clock-based (morning–evening loop) salinity stress defense mechanism in spinach (Spinacia oleracea), a leafy vegetable crop with significant economic importance and health benefits. In the presence of dawn and dusk, the circadian clock-based defense mechanism was observed using the genotypes “Delhi Green” and “Malav Jyoti.” A photoperiodic rhythm consists of 4-h intervals for 12 h (morning–evening loop) in spinach was demonstrated under the salinity stress treatments (20 mM and 50 mM). The clock-controlled a large fraction of growth parameters such as plant height, biomass, and root-shoot ratio under salinity stress. Conversely, salinity stress resulted in upregulation of antioxidative parameters such as superoxide dismutase, ascorbate peroxidase, catalase, and other stress markers such as thiobarbituric acid reactive substances, proline content, and localizations of H2O2 and O2−1 but was altered and maintained at a certain photoperiodic time interval of the circadian clock. In distinction to results observed from antioxidative measurements performed with an early and late circadian duration of salt-treated plants, 10 am and 2 pm were revealed to be the rhythmic times for controlling salinity stress. Likewise, comprehensive measurements of the photosynthetic system under salinity stress at specific photoperiodic circadian time intervals, including net-photosynthetic rate, transpiration, stomatal conductance, PSII quantum yield, and stomata structure, were made at 10 am and 2 pm. The salinity stress response was down-streamed and the clock also regulated chloroplastic protein expression. Thus, according to our findings, photoperiodic circadian rhythms, particularly the morning–evening loop, enhanced plant survival rates by modulating cellular antioxidant mechanisms and chloroplastic proteins that further helped to reduce the effects of salinity stress.

Rice homolog of Sin3-associated polypeptide 30, OsSFL1, mediates histone deacetylation to regulate flowering time during short days.

Rice homolog of Sin3-associated polypeptide 30, OsSFL1, mediates histone deacetylation to regulate flowering time during short days.

Shortened Seasonal Photoperiodic Cycles Accelerate Aging of the Diurnal and Circadian Locomotor Activity Rhythms in a Primate

The gray mouse lemur (Microcebus murinus), a prosimian primate, exhibits seasonal rhythms strictly controlled by photoperiodic variations. Previous studies indicated that longevity can be altered by long-term acceleration of seasonal rhythms, providing a model for assessing various aspects of aging. To assess the effect of aging and accelerated aging on the circadian system of this primate, we compared the circadian rhythm of the locomotor activity in adult mouse lemurs (2-4.5 years, n = 9), aged mouse lemurs (5-9 years, n = 10), and adult mouse lemurs that had been exposed from birth to a shortened seasonal photoperiodic cycle (2-4.5 years, n = 7). Compared to adult animals, aged mouse lemurs showed a significant increase in intradaily variability and an advanced activity onset. Aging was characterized by a decrease in amplitude, with both a decrease in nocturnal activity and an increase in daytime activity. When maintained in constant dim red light, aged animals exhibited a shortening of the free-running period (22.8 +/- 0.1 h) compared to adult animals (23.5 +/- 0.1 h). A 3- to 5-year exposure to an accelerated seasonal photoperiodic rhythm ("annual" duration of 5 months) in accelerated mouse lemurs produced disturbances of the locomotor activity rhythm that resembled those of aged mouse lemurs, whether animals were studied in entrained or in free-running conditions. The present study demonstrated a weakened and fragmented locomotor activity rhythm during normal aging in this primate. Increasing the number of expressed seasonal cycles accelerated aging of parameters related to circadian rhythmicity in adult animals.

Daily light regulates seasonal responses in the migratory male redheaded bunting (Emberiza bruniceps)

This study analyzed the role of day length in regulation of seasonal body fattening and testicular growth in a latitudinal Palaearctic-Indian migrant, the redheaded bunting (Emberiza bruniceps). When exposed to increasing photoperiods (hours of light: hours of darkness; 11.5L:12.5D, 12L:12D, 12.5L:11.5D, 13L:11D, 14L:10D, and 18L:6D) for 9-12 weeks, buntings responded in a photoperiod-dependent manner and underwent growth and regression cycle under photoperiods of > or =12 hr per day. Also, the response to a long photoperiod of birds that were held under natural photoperiods at 27 degrees N for 2 years was similar to those who arrived the same year from their breeding grounds ( approximately 40 degrees N), suggesting that the experience of higher amplitude day-night (light-dark, LD) cycles during migratory and breeding seasons were not critical for the subsequent response (initiation-termination-reinitiation) cycle. Another experiment examined entrainment of the circadian photoperiodic rhythm in buntings by subjecting them to T=24+/-2 hr LD-cycles with 8 hr photophase and to T=22 and 24 hr with 11 hr photophase. The results showed a reduction in critical day length under T=22 hr LD-cycle. In the last experiment, we constructed an action spectrum for photoperiodic induction by exposing birds for 4.5 weeks to 13L:11D of white (control), blue (450 nm), or red (640 nm) light at irradiances ranging from 0.028 to 1.4 W m(-2). The threshold light irradiance for photoinduction was about 10-fold higher for blue light, than for red and white lights. These results conclude that the daily light of the environment regulates the endogenous program that times seasonal responses in body fattening and testicular cycles of the redheaded bunting.

Pineal gland (melatonin) affects the parturition time, but not luteal function and fetal growth, in pregnant rats.

The purpose of the present study was to investigate the role of pineal gland (melatonin) on parturition time, luteal function, and fetal growth in pregnant rats. Cycling rats were subjected to pinealectomy or sham operation under ether anesthesia; and pinealectomized rats immediately underwent implantation of a melatonin capsule (PINX + Mel group) or a vehicle-containing capsule (PINX group), and sham operated rats also underwent implantation of a vehicle-containing capsule (control group). All rats were maintained under the same photoperiod conditions (14 L:10 D) and were induced pregnancy. Blood samples were obtained on days 7, 12, 15, 17, 19, and 21 of pregnancy to measure serum progesterone concentrations, and parturition times were recorded on days 22 and 23. In the next experiment, pregnant PINX rats received subcutaneous injection of melatonin (10 microg/body) at 08:00 h (PINX + 8 h group) or at 20:00 h (PINX + 20 h group) from day 15 to the end of pregnancy, and parturition times were recorded. Parturition times of rats in the PINX group, the PINX + Mel group or the PINX + 8 h group, but not the PINX + 20 h group, were significantly different compared with those in the control group. Pinealectomy or melatonin implantation did not affect serum progesterone concentrations during pregnancy or the number and weight of fetuses or corpora lutea. The present results indicate that pineal gland (melatonin rhythm) synchronizing with photoperiodic rhythm is likely to be an important determinant of parturition time, but it does not affect progesterone production or fetal growth in pregnant rats.

Photoperiodic rhythm of wool growth and its contribution to seasonal wool production by the Merino, Polwarth, and their reciprocal crosses in southern Australia

Twelve wethers of each of the Merino and Polwarth breeds and their reciprocal crosses were fed at1 of 2 levels in individual pens for 14 months. Periodic clean wool weight, yield, fibre diameter, and the coefficient of variation (CV%) for fibre diameter were measured from midside patches harvested every6 weeks.There was a significant effect of time of wool harvest on clean wool weight per cm2, yield, fibre diameter(all P<0.01), and its CV% (P<0.05). A trough in wool production occurred about August-September and a peak in January. The amplitude of the photoperiodic rhythm of wool growth (difference between maximum and minimum wool growth per cm2 per 42 days as a percentage of the average) was 40%. Yield also displayed a substantial photoperiodic rhythm, but that of fibre diameter was less pronounced. Mean staple strength was 32.3 N/ktex, the mean break occurring at a distance of 60% of the staple length from the tip; there was no effect of genotype or level of feeding. From comparison with published Tasmanian field data, the seasonal wool growth pattern of sheep at pasture appears similar to the photoperiodic rhythm but occurs about a month earlier. It was concluded that the photoperiodic rhythm of wool production of Tasmanian Merino-based breeds should not be a barrier to manipulation of wool quality (staple strength and fibre diameter) by nutritional management.

Photic, Thermic, and Sibling Influences on the Hatching Rhythm of the Southern Green Stink Bug, Nezara viridula (L.)

The hatching of Nezara viridula was found to be a function of a photoperiodically entrained rhythm. Periodicity of hatching found in a constant photoperiod was shifted by 12 h from that found in a 14:10 (L:D) photoperiod and the former was probably a reflection of periodicity of oviposition. Both the onset and distribution of the photoperiodic hatching rhythm were dependent on the temperature of incubation. Although emergence was temperature dependent, it was not thermoperiodically entrained. The synchronization of hatching within an egg mass was the result of the interaction of emerging nymphs stimulating hatching in adjacent eggs and was not due to independent timing of eclosion of each egg. Thus, in addition to timing eclosion at the onset of light, the photoperiodic rhythm may also have functioned to synchronize development so that all eggs in an egg mass were simultaneously capable of hatching when stimulated by the emergence of siblings.

The perception of dusk signals in photoperiodic time‐measurement

A critical duration of darkness must be exceeded for the photoperiodic induction of flowering in short‐day plants. This requires detection of the light/dark transition at dusk and the coupling of this information to a time‐measuring system.Lowering the Pfr/Ptot, ratio photochemically at the end of the day did not accelerate the onset of dark timing in Pharbitis nil Choisy cv. Violet. Time‐measurement was initiated when, with no change in spectral quality, the irradiance fell below a threshold value. Thus, if the light/dark transition at dusk is sensed by a reduction in Pfr, this reduction can be achieved as rapidly through thermal reactions as through photochemical ones. When given at hourly intervals during a 6‐h extension of a 24‐h main light period in white light, pulses of red light were as effective as continuous red light in delaying the onset of timing; pulses every 2 or 3 h were less effective. The effectiveness of intermittent red light indicates that phytochrome is the photoreceptor and the requirement for frequent exposures suggests that Pfr is lost rapidly in the dark. However, the red light pulses could not be reversed by far‐red light, which argues against this hypothesis. An alternative explanation is that the perception of light as being continuous occurs only when “new” Pfr is regenerated sufficiently frequently.The nature of the coupling of the dusk signal to the time‐measuring system is discussed and it is suggested that the effect of each red light pulse is to delay the phase of the photoperiodic rhythm by 1–3 h.

Light Rquirements for Photoperiodic Sensitivity in Cotyledons of Dark‐grown Pharbitis nil

AbstractThe light requirements for induction of flowering by a long dark period were investigated in dark‐grown seedlings of Pharbitis nil Chois, cv. Violet. The cotyledons bcame photoperiodically sensitive to a 24 h dark period by two 1 min red irradiations (6.3 μmol m−2 S−1) separated by a 24 h dark period. The reversibility of the effect of brief red irradiations, and the effectiveness of low energies of red irradiation suggest the involvement of phytochrome in the induction of photoperiodic sensitivity. Partial de‐etiolation occurred after these brief periods of red irradiation but the seedlings were not capable of net CO2 uptakeeven 7 h after the start of the main light period that followed the critical dark period. A changing response to the duration of the priod of darkness given between the two short red irradiations showed the the correct phasing of an endogenous photoperiodic rhythm is needed for the attainment of photoperiodic snsitivity.

Mosquito Nectar Feeding in Wisconsin in Relation to Twilight and Microclimate1

Twenty-three mosquito species were collected feeding on floral nectar at 3 study sites. Each of the 9 most abundant species exhibited specific peak feeding periods with respect to twilight. Twilight times were expressed in crep-units. Aedes stimulans peak feeding occurred around 0.5 creps or mid-civil twilight. Aedes canadensis, Aedes communis, Aedes sticticus, Aedes vexans, Coquillettidia perturbans , and Culex restuans exhibited peak feeding activity around 1.0 to 1.5 creps or early nautical twilight, while Anopheles earlei and Anopheles walkeri fed later in nautical twilight. These 2 anophelines are known to be more active late at night, especially in search of a blood meal. Aedes stimulans fed on nectar for some time before sunset; this species is known for its aggressive daytime feeding activities, suggesting that nectar-feeding and blood-feeding activities with respect to photoperiod may be similar. Males and females of some species, such as C. perturbans , exhibited different peak feeding times, while for other species, such as A. vexans, A. canadensis, C. restuans , and A. earlei , both sexes fed at almost identical times, the correlation coefficient (r) of feeding being generally greater than 0.900. Microclimatic factors greatly influenced feeding, reducing it when they existed as extremes. Wind above 10 km/hr acted as an inhibitor to feeding, as did temperature below 10.0/C and relative humidity below 48%. The data suggest that the time of peak feeding activity is controlled by a circadian or photoperiodic rhythm, but that the level of illumination affects the initiation, cessation, and duration of feeding on a given night.

DISCUSSION

It was pointed out that Petter-Rousseaux (1972) was able to obtain two periods of reproduction per year in a small Loris, Microcebus murinus, by photoperiodic manipulations, namely the use of a photoperiodic rhythm of two periods in one year. Secretory activity of LH, prolactin and TSH from the pituitary cells coincided very closely with the phases of sexual activity. The types of pituitary cells were identified by an immunofluorescence technique which uses antibodies against specific pituitary hormones. Weaning during different periods of lactation probably accounts for the variation in the interval between weaning and the reestablishment of the cycle. The influence of weaning on cyclicity is greater in rhesus monkeys than in baboons, and when weaning occurs shortly after the first month of birth, cyclicity is resumed within a month.

Induction of Growth and Division Synchrony in Plasmodium vinckei chabaudi by Photoperiodic Rhythm

Growth and division of P. vinckei chabaudi, like that of P. berghei, appear to be synchronized by the photoperiodic rhythm. Also like P. berghei, the timing of cell division of P. vinckei chabaudi in the laboratory is always related to solar time. A photoperiodic rhythm of appropriate characteristic has been necessary in our laboratory to establish the synchrony. However, the photoperiodic rhythm has not appeared to control the timing of the cell division. The appearance of synchrony in P. vinckei chabaudi is more dramatic than it is in P. berghei. Cell growth and division is a subject of great interest to many investigators. The understanding of some of the unknown factors surrounding this phenomenon could in fact have far-reaching implications. In studying synchronous growth and division in plasmodia through the host-parasite relationship, variables such as light cycle (Taliaferro, 1925; Ramakrishnan and Prakash, 1951; Arold et al., 1969), diet (Boyd, 1933), and temperature (Kovic and Zeuthen, 1967; Hawking, 1970) have been considered. Most results indicate that various species of plasmodia have prominent patterns of growth and division which are usually at some multiple of 24 hr. Thus, malaria parasites are known with a generation time of 24, 48, and 72 hr. Very little has actually been published on the growth pattern of the asexual forms of Plasmodium vinckei chabaudi; perhaps this is partially due to its rather recent isolation by Dr. Irene Landau (1965a). Through a personal communication to us (March, 1970), Dr. Frank Hawking referred to P. chabaudi as "an excellent parasite for cycle studies," and his data suggested 24 hr as the generation time of this new malaria parasite. Depending upon experimental conditions, the behavior of P. v. chabaudi in mice has been described as following two basically different patterns of infection. The first is that Received for publication 29 June 1971. * Supported by U. S. Army Contract No. DA-49193-MD-2545 from the U. S. Army Research and Development Command, Office of the Surgeon General. This paper is contribution No. 963 from the Army Research Program on Malaria. of a relatively low-level infection lasting several weeks and from which the animal recovers (Stauber and Ott, 1967; Voller and Bidwell, 1968; Cox, 1970). The second pattern is that of an ever-increasing parasitemia, leading to subsequent death of the host within a week after injection (Landau, 1965b; Stauber and Ott, 1967). In this laboratory we have almost always observed the latter pattern. Previous studies in this laboratory have shown that certain photoperiodic rhythms can induce growth and division synchrony in P. berghei (Arnold et al., 1969). Thus, light quality, intensity, and duration have been de cribed as important factors in mediating an "augmented synchrony." It is our purpose in the present study to compare P. v. chabaudi to P. berghei, by observing how this newly isolated rodent malaria parasite responds to our defined experimental environment which is already known to induce a synchronous growth and division in P. berghei. MATERIALS AND METHODS

Synchronization of growth and division of rat-adapted Plasmodium vinckei chabaudi by photoperiodic rhythm

Synchronization of growth and division of rat-adapted Plasmodium vinckei chabaudi by photoperiodic rhythm

Role of the Endocrine System in Controlling Growth and Division Synchrony of Plasmodium berghei in Mice

The malaria parasite of mice may be induced by certain photoperiodic rhythms to show an augmented growth and division synchrony. This process is mediated by the testes and pineal but not by other endocrine organs. Certain steroids will compensate for the missing testes. Four hydroxy-17alpha-methyl testosterone and 4-chlorotestosterone produced an effect nearly equal to that seen in the intact animal. Estrogen, progesterone, and testosterone used alone and in combination were not able to replace the testes. In addition to the partial control of growth and division synchrony, the testes also influence the vascular capture and release of mature parasites along the walls of the blood vessels. Hypophysectomy had no effect on growth and division synchrony or on parasite sequestration even after partial gonadal atrophy had occurred. Neither cyclic peaks in concentration of replacement androgens nor the photoperiodic rhythm are the time clue to which the growth and division synchrony of the parasite is entrained. The nature of the time clue is still unknown. Malaria infections have a striking dependence on the solar day. This is apparent from the fact that the generation time of most species of parasite is an integral multiple of 24 hr. In addition, many species of malaria are so closely tied to a diurnal rhythm that all parasites of the species are found in growth and division synchrony in the same host individual. A species of rodent malaria, Plasmodium berghei, offers certain insights into the mechanism of parasite entrainment to a host rhythm. In this system of mouse-with-parasite, the entrainment only occurs when the host mouse is exposed to sunlight or a nearly equivalent light for at least 16 hr a day. Parasite division occurs between 12 midnight and 6 AM during the time of entrainment (Arnold et al., 1969a). The timing of cell division is not by the timing of the light rhythm. The on-off cycle of the light can be moved in any fashion around the clock as long as the duration of light is at least 16 hr. The parasites still divide from 12 midnight to 6 AM solar time. In some respects, P. berghei is a convenient indicator of the host rhythm. The parasite has a 24-hr generation time. There are no problems with multiple broods of parasites. There are convenient morphologic indicators of the Received for publication 1 April 1969. * This study supported by U. S. Army Contract No. DA-49-193-MD-2545 from the U. S. Army Research and Development Command, Office of the Surgeon General. This paper is contribution No. 561 from the Army Research Program on Malaria. age of the parasite. On the other hand, there are problems with sequestration (the vascular capture and release of mature parasites). Vascular sequestration is another curious feature of the host-parasite relationship. In many species of malaria, including P. berghei, the older and dividing forms of the parasite, together with the host red cell, stick to the blood vessel surfaces, until such time as cell division is complete (Garnham, 1966). On completion of cell division, the young forms usually circulate freely in the blood until they reach a degree of maturity appropriate for sequestration. The cycle is then repeated. In preliminary work from this laboratory, we have found that the pineal body is part of the mediating system for both growth and division synchrony and for vascular sequestration (Arnold et al., 1969b). The effect of the pineal body on growth and division synchrony can be replaced in part by vitamin K, vitamin E, and ubiquinone (Arnold et al., 1969c). Although the presence of an intact pineal or its chemical alternates, vitamin K, vitamin E, or ubiquinone, is needed for growth and division synchrony of P. berghei, none of these factors appears to act as the master clock. The diurnal growth rhythm of the parasites remains on the same time schedule regardless of the time of administration of the ubiquinone or of the phase relationship to the photoperiodic rhythm. In addition, the photop2riodic rhythm must be maintained if chemical replacement is to work in the pinealectomized animal. For this reason, a continued search for other medi-

Chemical Agents Effective in Mediating Control of Growth and Division Synchrony of Plasmodium berghei in Pinealectomized Mice

Pinealectomy abolishes the growth and division synchrony of Plasmodium berghei which is normally augmented by the photoperiodic rhythm. The augmented synchrony typical of the intact animal may be restored by ubiquinone, members of the vitamin K series, or partly by alpha-tocopherol. This restoration of growth and division synchrony by the 1,4-quinones or the 6-chromanols requires the continued exposure of the pinealectomized animal to the appropriate photoperiodic rhythm. Diets deficient in vitamin K or vitamin E will have the same effect on growth and division synchrony as does pinealectomy. The vascular release of mature schizonts, which is also under the control of the pineal, is only partially restored by the 1,4-quinones, or 6-chromanols. Pinealectomy can be shown to abolish the effects of photoperiodism in augmenting growth and division synchrony of Plasmodium berghei in the mouse (Arnold et al., 1969b). In addition, pinealectomy changes the pattern of mature schizont sequestration of the same infection in mice. The mechanism by which the pineal is activated is not yet known, nor is the mechanism known by which the pineal carries out its part in regulating growth and division synchrony. Similar uncertainties apply to the means by which the pineal regulates vascular capture and release of mature schizonts. It seems likely that the pineal is the junction point for a neural pathway and a chemical pathway. In studying the nature of the pineal as a chemical transducer, we have explored the role of several known chemical agents

The Role of the Pineal in Mediating Photoperiodic Control of Growth and Division Synchrony and Capillary Sequestration of Plasmodium berghei in Mice

Control over growth and division synchrony as well as vascular capture and release of late growth forms is shown to be mediated by the pineal body. Vascular trapping of these late growth forms seems to be most intense in the liver, spleen, and bone marrow. Since we observed these older parasites adhering to the vascular wall, vascular trapping is probably by a mechanism other than phagocytosis. Certain types of photoperiodic rhythm have been shown to influence the growth of Plasmodium berghei, a malarial parasite of rodents. Two effects of the photoperiodic rhythm have so far been observed. The first is the regulation of growth and division synchrony, and the second appears to be the tendency of segmenting parasites to sequester or to be sequestered in certain parts of the vascular system (Arnold, Lalli, and Martin, 1969). Growth and division of malarial parasites of many species have been entrained to 1-, 2-, and 3-day patterns, primarily by light rhythms. Each species of host and parasite has special characteristics. Infection of mice by P. berghei is a particularly interesting example of synchrony because the generation time of the parasite is 24 hr, and thus fits a diurnal pattern (Arnold et al., loc. cit.). One of the more interesting aspects of the problem of synchrony in P. berghei is that light rhythms require specific periods of light and dark exposure for effective augmentation of growth and division synchrony. As we have previously shown only certain intensities and spectra of light produce this synchronizing effect on parasite growth. In addition to the process of growth and division synchrony, the photoperiodic rhythm Received for publication 19 July 1968. * Supported by U. S. Army Contract No. DA-49193-MD-2545 from the U. S. Army Research and Development Command Office of the Surgeon General. This paper is contribution No. 405 from the Army Research Program on Malaria. appears to modify the sequestration of segmenting parasites in the vascular system. This phenomenon is frequently observed with several species of malarial parasites in addition to P. berghei in mice. A considerable amount of morbidity and mortality of the host, for instance, is produced by capillary blockade caused by the sequestration of mature schizonts of P. falciparum in man (Rigdon and Fletcher, 1945; Edington, 1967). For the most part, vascular sequestration of the parasite involves its older dividing asexual forms (Alger, 1963). This process of sequestration has been called margination by some authors because these older parasites appear to stick to the walls of the capillaries. It seems likely that once an erythrocytic form of the parasite is phagocytized, it is destroyed. Therefore, the capture and release of living parasites by the vascular endothelium would require a mechanism other than that of phagocytosis since these parasites are not destroyed. The capture and release of parasites by the vascular system is accentuated by the photoperiodic mechanisms. For example, we showed in a previous study that the segmenting forms in a synchronous infection were largely absent from the circulating blood at 3 AM when the light intensity was under 100 foot candles (ft-c). Normally, at 3 AM we would expect a maximum number of mature schizonts. When the light intensity was raised to 1,000 ft-c, sequestration seemed to be complete again. In this present study, we hope to

Augmentation of Growth and Division Synchrony and of Vascular Sequestration of Plasmodium berghei by the Photoperiodic Rhythm

Growth and division pattern of Plasmodium berghei in mice is either absent or hard to detect under most circumstances. An augmented pattern of growth and division synchrony occurs under the influence of certain light rhythms. These light rhythms require specific time patterns, specific light quality, and specific light intensities. Generation times of the parasite can be calculated from syn- chronous growth patterns and correspond to the length of the solar day. The final merozoite number does not seem to be modified by the entrainment mechanism. Diet exerts a permissive effect on the augmentation of growth and division synchrony by light, but neither light nor diet seem to play a role in the operation of the master clock. Sequestration of mature schizonts in blood vessels is also depend- ent on photoperiodism. One light rhythm will produce synchronous growth and division with most of the cell division occurring in the peripheral blood, while other light rhythms will induce growth and division synchrony with most of the cell division occuring in cells sequestered in the blood vessels. In addition, vascular capture and release of late growth forms appear also to be under the control of the photoperiodic rhythm.