Circadian regulation of leaf hydraulic conductance in sunflower (Helianthus annuus L. cv Margot)

The circadian regulation of leaf hydraulic conductance (K_leaf) was investigated in Helianthus annuus L. (sunflower). K_leaf was measured with an high pressure flow meter during the light and dark period from plants growing at a photoperiod of 12 h. K_leaf was 4.0 e−4 kg s⁻¹ m⁻² MPa⁻¹ during the light period (LL) and 30–40% less during the dark period (DL). When photoperiod was inverted and leaves were measured for K_leaf at their subjective light or dark periods, K_leaf adjusted to the new conditions requiring 48 h for increasing from dark to light values and 4 d for the opposite transition. Plants put in continuous dark showed K_leaf oscillating from light to dark values in phase with their subjective photoperiod indicating that K_leaf changes were induced by the circadian clock. Several cuts through the minor veins reduced leaf hydraulic resistance (R_leaf) of both LL and DL to the same value (1.0 e + 3 MPa m² s kg⁻¹) that equalled the vascular resistance (R_v). The contribution of the non-vascular leaf resistance (R_nv) to R_leaf was of 71.9% in DL and of 58.4% in LL. The dominant R_nv was shown to be reversibly modulated by mercurials, suggesting that aquaporins play a role in diurnal changes of K_leaf.     

The effect of photoperiod length on respiration in leaves of Saxifraga cernua L., an arctic herb

Abstract Rates of oxygen uptake were measured in leaves of Saxifraga cernua which had been exposed to an 18-h photoperiod. These rates were compared to those in plants which had been exposed to continuous light. Rates of total dark respiration and alternative pathway respiration measured at the end of the photoperiod gradually decreased over the initial 3 d of exposure to an 18-h photoperiod. Thereafter, respiratory rates were constant. Rates of total dark respiration and alternative pathway respiration decreased during the 6h dark period. Rates of normal and alternative pathway respiration are equally affected during the dark period. The respiratory rates had reached a new minimum level 3 d after the initiation of a dark period. These results suggest that respiration rates in arctic plants are high because of the long photoperiod in the arctic. The kinetics of photoperiod induced changes in respiration are slow enough to suggest the involvement of the biological clock in setting respiration rates. Indeed, total dark respiration and alternative pathway respiration show a definite circadian rhythm. Free-running experiments show that normal respiration changes much less (has a smaller amplitude of variation) than alternative pathway respiration and that alternative pathway respiration accounts for most of the rhythmicity of respiration.     

LEAF RESISTANCE TO WATER VAPOR TRANSFER IN SUCCULENT PLANTS: EFFECT OF THERMOPERIOD

Leaf resistance (R_L) of Kalanchoe blossfeldiana to water vapor transfer was determined with a resistance hygrometer. The diurnal leaf-resistance change followed a normal pattern (i.e., low in light and higher in dark) when plants were pretreated with cool thermoperiods or with thermoperiods having little diurnal temperature fluctuation. Large diurnal temperature fluctuations (30-18, 26-15 C) resulted in apparent nocturnal stomatal opening. Nocturnal stomatal opening was more apparent than real since leaf-resistance measurements indicated day stomatal closing rather than complete night opening. Low nocturnal leaf resistances ( < 10 sec/cm) were not measured in the dark; however, resistances tended to decrease toward the end of the dark period indicating some degree of nocturnal stomatal opening. Leaf resistances were generally higher than those reported for nonsucculent plants. The data suggested that gaseous diffusion (Q) into or out of the leaves of K. blossfeldiana would be adequately described by an equation of the form, Q = D Δ e R_L⁻¹. There was little or no indication that physiological long days (15 min of 660 mμ light in the middle of a 16-hr dark period), which prevented flowering and reduced organic acid accumulation, significantly affected leaf resistance. It was concluded that the photoperiod response effects of dark CO₂ fixation were probably not due to leaf-resistance changes and, therefore, not due to stomatal aperture changes.     

Seasonal changes in adrenal and gonadal activity in the quail, Perdicula asiatica: involvement of the pineal gland

The present study assessed annual adrenal gland activity in the Indian tropical Jungle bush quail, Perdicula asiatica. We also elucidated the role of the annual variations in gonadal steroids and melatonin in the regulation of its activity. Increasing day length (photoperiod), ambient temperature and rainfall are positively correlated with adrenal and gonadal functions, and inversely related to pineal gland activity. Pineal, adrenal and gonadal weights showed cyclical patterns relative to environmental factors, which were also correlated with plasma melatonin, corticosterone and gonadal steroids, respectively. In both sexes of P. asiatica, pineal gland weight and/or plasma melatonin levels were inversely related to adrenal lipids, (e.g. phospholipids, free and esterified cholesterol) and plasma corticosterone levels. Melatonin levels also showed an inverse relationship with plasma testosterone and estradiol levels. These studies indicate that changes in environmental factors promote annual variations in adrenal and gonadal activity probably by modulating the pineal gland. Melatonin receptors have been localized in the pars tuberalis, adrenal gland and gonads of birds, the pineal gland may, therefore, mediate environmental stimuli indirectly and directly to down regulate adrenal and gonadal activity, which run in parallel in this species.     

Description of the individual growth of Daphnia magna (Crustacea: Cladocera) through the von Bertalanffy growth equation. Effect of photoperiod and temperature

Daphnia magna is a cladoceran used as a model organism in aquatic ecology and ecotoxicology studies. Because growth is a critical parameter to study the effect of environmental conditions on the development of zooplankters, the somatic growth of D. magna was measured here and described by the von Bertalanffy growth equation (VBGE), a mathematical model widely used in fisheries management. For this purpose, the effect of two temperatures (20 and 25°C) and two photoperiod conditions (12:12 and 16:8, light:dark) was assayed. Experiments began with neonate females and were finished when parthenogenetic females reached the age of 41 days; they were fed the microalga Ankistrodesmus falcatus (400,000 cell ml⁻¹, 12 mg l⁻¹, dry weight). According to the VBGE, maximal length (L _max) was inversely correlated with the growth rate (K). The highest L _max (6.45 mm) was for the females grown at 20°C with the 12:12 photoperiod, whereas the maximum growth rate (K = 0.182 ± 0.010) was for individuals grown at 25°C with the 12:12 photoperiod. The number of clutches during the studied period was significantly higher for females grown at 25°C, 12:12. Temperature affected the growth rate and the maximum size in D. magna; interaction of temperature with photoperiod was also noteworthy. The VBGE was a nifty way to assess the effects of the tested environmental factors.     

Diurnal variations in the physiology of the goldfish,Carassius auratus

Abstract

Liver glycogen, liver lipid, liver triglycerides, plasma glucose, plasma total lipid, plasma cholesterol, plasma corticoids, hypothalamic serotonin and pituitary pro‐lactin levels were assayed at five times over a 24‐h period in Carassius auratus maintained under a specific photoperiod regime at various times throughout the year. Diurnal variations were observed in all parameters monitored. Daily variations of liver glycogen, plasma glucose, plasma lipid, plasma corticoids and hypothalamic serotonin were affected by time of feeding. Liver glycogen, plasma lipid and plasma corticoid levels were also affected by time of feeding. Diurnal variations of liver glycogen, plasma glucose and plasma lipid were influenced by light‐dark cycles. These data illustrate that feeding time, photoperiod and time of sacrifice are important considerations in the study of metabolic and hormonal parameters in fishes.     

The relationship between turgor pressure and titratable acidity in mesophyll cells of intact leaves of a Crassulacean-acid-metabolism plant,Kalanchoe daigremontiana Hamet et Perr

Day/night changes in turgor pressure (P) and titratable acidity content were investigated in the (Crassulacean-acid-metabolism (CAM) plant Kalanchoe daigremontiana. Measurements of P were made on individual mesophyll cells of intact attached leaves using the pressure-probe technique. Under conditions of high relative humidity, when transpiration rates were minimal, changes in P correlated well with changes in the level of titratable acidity. During the standard 12 h light/12 h dark cycle, maximum turgor pressure (0.15 MPa) occurred at the end of the dark period when the level of titratable acidity was highest (about 300 eq H⁺·g^-1 fresh weight). A close relationship between P and titratable acidity was also seen in leaves exposed to perturbations of the standard light/dark cycle. (The dark period was either prolonged, or else only CO₂-free air was supplied in this period). In plants deprived of irrigation for five weeks, diurnal changes in titratable acidity of the leaves were reduced (H=160 eq H⁺·g^-1 fresh weight) and P increased from essentially zero at the end of the light period to 0.02 MPa at the end of the dark period. Following more severe water stress (experiments were made on leaves which had been detached for five weeks), P was zero throughout day and night, yet small diurnal changes in titratable acidity were still measured. These findings are discussed in relation to a hypothesis by Lüttge et al. 1975 (Plant Physiol. 56,613-616) for the role of P in the regulation of acidification/de-acidification cycles of plants exhibiting CAM.Abbreviations CAM crassulacean acid metabolism - FW fresh weight - P turgor pressure     

CHANGES IN ENDOGENOUS CYTOKININ CONCENTRATIONS IN CHLORELLA (CHLOROPHYCEAE) IN RELATION TO LIGHT AND THE CELL CYCLE1

Endogenous cytokinins were quantified in synchronized Chlorella minutissima Fott et Novákova (MACC 361) and Chlorella sp. (MACC 458) grown in a 14:10 light:dark (L:D) photoperiod. In 24 h experiments, cell division occurred during the dark period, and cells increased in size during the light period. Cytokinin profiles were similar in both strains, consisting of five cis‐zeatin (cZ) and three N⁶‐(2‐isopentenyl)adenine (iP) derivatives. Cytokinin concentrations were low during the dark period and increased during the light period. In 48 h experiments using synchronized C. minutissima (MACC 361), half the cultures were maintained in continuous dark conditions for the second photoperiod. Cell division occurred during both dark periods, and cells increased in size during the light periods. Cultures kept in continuous dark did not increase in size following cell division. DNA analysis confirmed these results, with cultures grown in light having increased DNA concentrations prior to cell division, while cultures maintained in continuous dark had less DNA. Cytokinins (cZ and iP derivatives) were detected in all samples with concentrations increasing over the first 24 h. This increase was followed by a large increase, especially during the second light period where cytokinin concentrations increased 4‐fold. Cytokinin concentrations did not increase in cultures maintained in continuous dark conditions. In vivo deuterium‐labeling technology was used to measure cytokinin biosynthetic rates during the dark and light periods in C. minutissima with highest biosynthetic rates measured during the light period. These results show that there is a relationship between light, cell division, and cytokinins.     

Growth and physiology of Oscillatoria agardhii gomont cultivated in continuous culture with a light-dark cycle

The cyanobacterium Oscillatoria agardhii Gomont was cultivated with a diurnal light-dark cycle (photoperiod 16 h) in continuous culture. There were found to be large differences in specific synthesis rates of the different biopolymers. The specific rates of change of proteins and nucleic acids (except DNA) matched the dilution rate, both in the light and in the dark period. Carbohydrates were synthesized and stored at a very high rate during the photoperiod, and were metabolized for the provision of energy, and for biosynthesis of other biopolymers in the dark. Cell counts showed no evidence of phased synchrony, although this conclusion was contradicted by changes in DNA and pigments.     

Within-day Changes in the Polyribosome Content and in Synthesis of Proteins in Leaves of Capsicum annuum L

Capsicum annuum cv. California Wonder was grown in controlled environment with a 12-hour photoperiod. The polyribosome content of leaves varied from 60 to 72% of total ribosomes with the highest level occurring in the middle of the photoperiod and the lowest in the middle of the dark period. The variation was accounted for by changes in the content of large polyribosomes (hexamers and larger). There was no indication of an immediate effect on polyribosome content of light-on or light-off.

The synthesis of proteins at two times in the 24-hour cycle was compared using a dual isotope technique. Statistically significant results were obtained that suggested that protein(s) with molecular weights of 26,000 daltons were preferentially synthesized in the photoperiod compared to the dark period. No evidence was found for the differential synthesis of proteins within the photoperiod.

     

Effects of photoperiod on energy intake, thermogenesis and body mass in Eothenomys miletus in Hengduan Mountain region

The present study was designed to examine whether photoperiod alone was effective to induce seasonal changes in physiology in voles (Eothenomys.) from the Hengduan Mountain region. Eothenomys miletus were randomly assigned into either long photoperiod (LD; 16L: 8D) or short photoperiod (SD; 8L: 16D) for 4 weeks at constant temperature (25 °C). At the end of acclimation, SD voles showed lower body mass and body fat coupled with higher energy intake than LD voles. SD greatly enhanced the thermogenic capacity of E. miletus, as indicated by an elevated nonshivering thermogenesis (NST), mitochondrial protein in brown adipose tissue (BAT); basal metabolic rate (BMR) was also raised. Although no variations in serum leptin levels were found between SD and LD voles, serum leptin levels were positively correlated with body mass and body fat mass, and negatively correlated with energy intake and UCP1 content in BAT, respectively. To summarize, SD alone is effective in inducing higher thermogenic capacities and energy intake coupled with lower body mass and body fat mass in root voles. Leptin is potentially involved in the photoperiod induced body mass regulation and thermogenesis in E. miletus. Our study shows that SD alone is effective.     

Diurnal fluctuations of endogenous IAA content in aralia leaves

Diurnal variations in endogenous IAA levels inFatsia japonica leaves, maintaining constant other external factors such as temperature and relative humidity, were studied. Plants were cultivated in a growth chamber (20 °C, 75 % RH, 16 h photoperiod, 400 µmol m^-2 s^-1 PAR). IAA analyses were carried out by analytical IP-HPLC with on-line spectrofluorimetry. Rhythmic variation of endogenous IAA levels was found. At the onset of the light period the IAA concentration dropped very rapidly from 1070 pmol g^-1 (fr.m.) to 144 pmol g^-1 (fr.m.). This concentration was nearly constant throughout the entire light period. During the subsequent dark period the IAA levels increased again to about 1000 pmol g^-1 (fr.m.) at the end of the dark phase. These results were not confirmed in open field conditions where many other external factors probably influence the endogenous IAA content.     

Photoperiodic Control of Flowering in Dark-Grown Seedlings of Pharbitis nil Choisy : The Effect of Skeleton and Continuous Light Photoperiods

The control of night-break timing was studied in dark-grown seedlings of Pharbitis nil (Choisy cv. Violet) following a single continuous or skeleton photoperiod. There was a rhythmic response to a red (R) interruption of an inductive dark period, and the phasing of the rhythm was influenced by the preceding light treatment.

Following a continuous white light photoperiod of 6 hours or less, the points of maximum inhibition of flowering were constant in real time. Following a continuous photoperiod of more than 6 hours, maximum inhibition occurred at 9 and 32.5 hours after the end of the light period. The amplitude of the rhythm during the second circadian cycle was much reduced following prolonged photoperiods.

Following a skeleton photoperiod, the time of maximum sensitivity to a R interruption was always related to the second pulse of the skeleton, R₂, with the first point of maximum inhibition of flowering occurring after 12 to 18 hours and the second after 39 hours. Without a second R pulse, the time of maximum sensitivity to a R interruption was related to the initial R₁ pulse. A `light-off' or dusk signal was not mimicked by a R pulse ending a skeleton photoperiod; such a pulse only generated a `light-on' signal and initiated a new rhythm.

It is concluded that the timing of sensitivity to a R interruption of an inductive dark period in Pharbitis nil is controlled by a single circadian rhythm initiated by a light-on signal. After 6 hours in continuous white light, the phase of this rhythm is determined by the transition to darkness. Following an extended photoperiod, the timing characteristics were those of an hourglass; this seemed to be due to an effect on the coupling or expression of a single circadian timer during the second and subsequent cycles, rather than to the operation of a different timing mechanism.

In addition to the effects on timing, the photoperiod affected the magnitude of the flowering response.

     

How photoperiod influences body temperature selection in Lacerta viridis

Summary European green lizards, Lacerta viridis, show a distinct annual cycle in their day and nighttime selected body temperature (T _b) levels when monitored under natural photoperiod. The amplitude between daily photophase and scotophase temperatures varies throughout the year. Highest body temperatures with smallest day/night variation are selected from May through July. Throughout fall, the difference between day and nighttime selected T _b levels increases. Lizards inevitably enter a state of winter dormancy which terminates daily rhythmicity patterns. Under natural photoperiodic conditions, cessation of dormancy occurs spontaneously by mid-March, regardless whether high temperatures are available or not. Lacerta viridis respond to an artificial long photoperiod (16 h light, 8 h dark) at all times of the year with modifications in both diel patterns and levels of selected T _b to summer-like conditions. When, however, the natural photoperiod at different phases in the annual cycle is held constant for six to eight weeks, T _b selection of Lacerta viridis also remains stable at the level corresponding to the prevailing photoperiod. These results implicate that the photoperiod is a more prominent Zeitgeber for seasonal cueing of temperature selection than has been surmised in the past. Further, we suggest that the large variations recorded in daily T _b cycles do not imply that this lizard is an imprecise thermoregulator, but rather indicates an important integral process necessary for seasonal acclimatization.     

Apparent endogenous circadian rhythm inent-kaurene biosynthesis

Net synthesis of [¹⁴C]ent-kaurene from [¹⁴C]2-mevalonic acid was assayed in cell-free enzyme extracts prepared from Alaska pea (Pisum sativum L.) seedlings throughout 44 h of a regimen consisting of a 16-h day and an 8-h night. Activities generally followed an upward trend during the dark period and a downward trend during the photoperiod. Activity was also assayed in enzyme extracts prepared at intervals during a 12-h photoperiod and a following, continuous 36-h dark period after entrainment of plants to a regimen of 12-h days and 12-h nights.Ent-kaurene synthesis activity again followed an upward trend in enzyme extracts prepared during what would have been the entrainment dark period, and a downward trend during the entrainment photoperiod. The apparent endogenous rhythm ofent-kaurene biosynthesis may have implications for the regulation of gibberellin biosynthesis.     

Diurnal Fluctuations in the Activity of the Enzyme Nitrate Reductase in Lemna paucicostata

Diurnal fluctuations were observed in nitrate reducta.se activity in Lemna paucicostatu Hegelm, grown under 16-h photoperiod. The enzyme activity showed a single peak in the middle of the photo period, i.e. 8 h after the onset of light. The activity decreased during the latter half of the photoperiod and reached a minimum level in the middle of the dark period. The peak in enzyme activity persists in continuous light as well as continuous dark, at least for a couple of days, suggesting the existence of endogenous rhythmicity     

Larval diapause of Monochamus urussovi and photoperiodic effects on larval development in tree bolts

To determine the larval diapause and the effect of photoperiod on development in Monochamus urussovi (Coleoptera: Cerambycidae), larvae were reared on Abies sachalinensis and Picea jezoensis logs and bolts. Larvae stopped developing in the final instar at 25°C and 16L : 8D (16 h light and 8 h dark) whereas an exposure to 5°C in the dark (134 days) following acclimation at 12°C under natural daylength led to adult emergence. When larvae were reared under 8L : 16D or 16L : 8D at 25°C with an intervening period of chilling at 5°C in the dark (112 days), a photoperiod of 8L : 16D induced a shorter time required for adult emergence after being returned to 25°C, and smaller adult body size than 16L : 8D.     

Rhythms as photoperiodic timers in the control of flowring in Chenopodium rubrum L.

Summary In C. rubrum, the amount of flowering that is induced by a single dark period interrupting continuous light depends upon the duration of darkness. A rhythmic oscillation in sensitivity to the time that light terminates darkness regulates the level of flowering. The period length of this oscillation is close to 30 hours, peaks of the rhythm occurring at about 13, 43 and 73 h of darkness.Phasing of the rhythm by 6-, 12- and 18-h photoperiods was studied by exposing plants to a given photoperiod at different phases of the free-running oscillation in darkness. The shift in phase of the rhythm was then determined by varying the length of the dark period following the photoperiod; this dark period was terminated by continuous light.With a 6-h photoperiod the timing of both the light-on and light-off signals is shown to control rhythm phasing. However, when the photoperiod is increased to 12 or 18 h, only the light-off signal determines phasing of the rhythm. In prolonged periods of irradiation-12 to 62 h light—a durational response to light overrides any interaction between the timing of the light period and the position of the oscillation at which light is administered. Such prolonged periods of irradiation apparently suspend or otherwise interact with the rhythm so that, in a following dark period, it is reinitiated at a fixed phase relative to the time of the light-off signal to give a peak of the rhythm 13 h after the dusk signal.In daily photoperiodic cycles rhythm phasing by a 6-h photocycle was also estimated by progressively increasing the number of cycles given prior to a single dark period of varied duration.In confirmation of Bünning's (1936) hypothesis, calculated and observed phasing of the rhythm controlling flowering in c. rubrum accounts for the photoperiodic response of this species. Evidence is also discussed which indicates that the timing of disappearance of phytochrome P_fr may limit flowering over the early hours of darkness.