Short Daylengths Research Articles

Flowering time control: gene network modelling and the link to quantitative genetics

Flowering is a key stage in plant development that initiates grain production and is vulnerable to stress. The genes controlling flowering time in the model plant Arabidopsis thaliana are reviewed. Interactions between these genes have been described previously by qualitative network diagrams. We mathematically relate environmentally dependent transcription, RNA processing, translation, and protein–protein interaction rates to resultant phenotypes. We have developed models (reported elsewhere) based on these concepts that simulate flowering times for novel A. thaliana genotype–environment combinations. Here we draw 12 contrasts between genetic network (GN) models of this type and quantitative genetics (QG), showing that both have equal contributions to make to an ideal theory. Physiological dominance and additivity are examined as emergent properties in the context of feed-forwards networks, an instance of which is the signal-integration portion of the A. thaliana flowering time network. Additivity is seen to be a complex, multi-gene property with contributions from mass balance in transcript production, the feed-forwards structure itself, and downstream promoter reaction thermodynamics. Higher level emergent properties are exemplified by critical short daylength (CSDL), which we relate to gene expression dynamics in rice (Oryza sativa). Next to be discussed are synergies between QG and GN relating to the quantitative trait locus (QTL) mapping of model coefficients. This suggests a new verification test useful in GN model development and in identifying needed updates to existing crop models. Finally, the utility of simple models is evinced by 80 years of QG theory and mathematical ecology.

Plant resistance to cold stress: Mechanisms and environmental signals triggering frost hardening and dehardening

This introductory overview shows that cold, in particular frost, stresses a plant in manifold ways and that the plant's response, being injurious or adaptive, must be considered a syndrome rather than a single reaction. In the course of the year perennial plants of the temperate climate zones undergo frost hardening in autumn and dehardening in spring. Using Scots pine (Pinus sylvestris L.) as a model plant the environmental signals inducing frost hardening and dehardening, respectively, were investigated. Over 2 years the changes in frost resistance of Scots pine needles were recorded together with the annual courses of day-length and ambient temperature. Both act as environmental signals for frost hardening and dehardening. Climate chamber experiments showed that short day-length as a signal triggering frost hardening could be replaced by irradiation with far red light, while red light inhibited hardening. The involvement of phytochrome as a signal receptor could be corroborated by respective night-break experiments. More rapid frost hardening than by short day or far red treatment was achieved by applying a short period (6 h) of mild frost which did not exceed the plant's cold resistance. Both types of signals were independently effective but the rates of frost hardening were not additive. The maximal rate of hardening was - 0.93 degrees C per day and frost tolerance of less than < - 72 degrees C was achieved. For dehardening, temperature was an even more effective signal than day-length.

Photoperiod-dependent modulation of cardiac excitation contraction coupling in the Siberian hamster

In mammals, changes in photoperiod regulate a diverse array of physiological and behavioral processes, an example of which in the Siberian hamster (Phodopus sungorus) is the expression of bouts of daily torpor following prolonged exposure to a short photoperiod. During torpor, body temperature drops dramatically; however, unlike in nonhibernating or nontorpid species, the myocardium retains the ability to contract and is resistant to the development of arrhythmias. In the present study, we sought to determine whether exposure to a short photoperiod results in alterations to cardiac excitation-contraction coupling, thus potentially enabling the heart to survive periods of low temperature during torpor. Experiments were performed on single ventricular myocytes freshly isolated from the hearts of Siberian hamsters that had been exposed to either 12 wk of short-day lengths (SD) or 12 wk of long-day lengths (LD). In SD-acclimated animals, the amplitude of the systolic Ca(2+) transient was increased (e.g., from 142 +/- 17 nmol/l in LD to 229 +/- 31 nmol/l in SD at 4 Hz; P < 0.001). The increased Ca(2+) transient amplitude in the SD-acclimated animals was not associated with any change in the shape or duration of the action potential. However, sarcoplasmic reticulum Ca(2+) content measured after current-clamp stimulation was increased in the SD-acclimated animals (at 4 Hz, 110 +/- 5 vs. 141 +/- 15 mumol/l, P < 0.05). We propose that short photoperiods reprogram the function of the cardiac sarcoplasmic reticulum, resulting in an increased Ca(2+) content, and that this may be a necessary precursor for maintenance of cardiac function during winter torpor.

The influence of photoperiod on the reproductive activity of female Honey possums, Tarsipes rostratus (Marsupialia: Tarsipedidae): assessed by faecal progestagens and oestradiol-17β

Six female Honey possums were kept on a 15L:9D light regime to simulate a long daylength over summer. After seven weeks, three females (Group 2) were changed to a shorter daylength of 10L:14D, while the other three females (Group 1) were maintained on the long daylength. Faecal oestradiol-17β and progestagen levels were measured during the experiment to detect any changes in reproductive rhythm, such as resumption of blastocyst development. Group 2 females were found to have very large and greatly expanded blastocysts with significantly higher levels of progestagens after the change to short photoperiod ( p < 0.05). In contrast, the Group 1 females had very small diapausing blastocysts and progestagen levels did not change throughout the study. Overall oestradiol levels also increased significantly in Group 2 females ( p < 0.05) but not in Group 1 females. These results demonstrate that a change from long to short days stimulates increased progestagen output (and oestradiol-17β to a lesser extent) that supports the growth and expansion of the blastocysts. Photoperiod, in particular its change to a shortening daylength, appears to be a stimulus for terminating diapause in the Honey possum during its first reproduction of the year. However, as subsequent breeding later in the year occurs when daylength is increasing, a similar role for photoperiod cannot be attributed and females may be entrained to other factors such as food resources.

Seasonal change in luteinizing hormone subunit mRNA in Japanese quail and effects of short daylength and low temperature

Changes in pituitary mRNA levels of LHβ-subunit (LHβ) and glycoprotein hormone α-subunit (common α) were investigated in male Japanese quail under natural and laboratory conditions to clarify the mechanisms of seasonal regulation of luteinizing hormone (LH) secretion. In Experiment 1, birds were kept in outdoor cages under natural conditions from August for 12 months. Both LHβ and common α mRNA levels decreased rapidly from August to September, and after a period of low levels from October through January, they began to increase in February and continued to increase until July. There were more pronounced seasonal changes in testicular weight and cloacal protrusion width with large decreases from August to September and increases from March to May. In Experiment 2, birds were kept on laboratory conditions and transferred from long to short daylengths at 20 or 9 °C and held for 14 days. Although common α mRNA levels, plasma LH concentrations, testicular weight, and cloacal protrusion area decreased on short days without low temperatures, levels of LHβ mRNA did not change. Short daylengths combined with low temperatures induced testicular regression and caused decrease in all the parameters measured. Low temperatures under long days did not induce any change in the parameters significantly. These results suggest that (1) synthesis as well as secretion of LH is regulated seasonally, (2) short daylength does not suppress LH synthesis completely unless combined with low ambient temperature, and (3) the effect of photoperiod on the endocrine system regulating LH secretion is predominant over the effect of ambient temperature but ambient temperature acts as an environmental cue to terminate reproductive activities at late summer to early autumn in Japanese quail.

Environmental Regulation of a 25 kDa Dehydrin in Relation to Rhododendron Cold Acclimation

The influence of photoperiod and temperature on the seasonal (fall to winter) cold acclimation and accumulation of a 25 kDa dehydrin in Rhododendron `Chionoides' was studied by exposing two groups of plants each in the greenhouse or outdoors to either a natural photoperiod (or short days) or an extended photoperiod (or long days) regime. Results suggest that the shortening daylength alone is sufficient to trigger both the first stage of cold acclimation and concomitant 25 kDa dehydrin induction. Exposure of the plants to natural photoperiod and temperatures induced the greatest cold hardiness and 25 kDa accumulation, while exposure to extended photoperiods (long days) and warmer temperatures (in the greenhouse) failed to induce any significant freezing tolerance in leaves. Whereas short days trigger the cold acclimation process initially, low inductive temperatures can eventually replace the photoperiod stimulus. Seasonal accumulation of 25 kDa dehydrin, on the other hand, appears to be predominantly effected by short photoperiods. Data indicated that the leaf water content of outdoor plants maintained under natural photoperiod was lower than that of plants grown under extended photoperiod. This was also true for the greenhouse plants at the first (September) and the last (January) sampling. It is hypothesized that early 25 kDa dehydrin accumulation may be due to short-day-induced cellular dehydration. Accumulation of two other dehydrins of 26 kDa and 32 kDa molecular masses does not appear to be associated with short day (SD)-induced first stage of cold acclimation. Results show that their accumulation may be regulated by low, subfreezing temperatures and may be associated with the second and/or third stage of cold acclimation of `Chionoides' rhododendron leaves.

Photoperiod and temperature differentially regulate the expression of two dehydrin genes during overwintering of birch (Betula pubescens Ehrh.).

The overwintering of trees in northern areas depends on processes regulated by photoperiod and temperature. To identify the physiological and genetic factors involved in this environmental control, three latitudinal ecotypes of pubescent birch (Betula pubescens Ehrh.) growing in a common garden experiment were used. Each ecotype responded to the shortening of the photoperiod according to its specific critical daylength, resulting in the induction of freezing tolerance and dehydration of buds first in the northern ecotype, followed by the central and southern ecotypes, respectively. By contrast, there was no clear difference in the timing of dormancy release, bud rehydration, and deacclimation in the spring, suggesting that these traits were controlled mainly by temperature. To elucidate the role of dehydrins (DHN) in the overwintering process, two DHN genomic clones were isolated from pubescent birch and expression of the corresponding genes, both in field and under controlled conditions, was characterized. BpuDhn1 was found to encode an Y(n)K(n)-type of basic DHN, while BpuDhn2 encoded an acidic, SK(n)-type of DHN. In field-grown trees the level of BpuDhn1 increased in buds during the autumn, while the level of BpuDhn2 was highest during the coldest winter months. Under controlled conditions BpuDhn1 increased in response to the combined effect of short daylength and low, non-freezing temperatures whereas the expression of BpuDhn2 was mainly controlled by low temperature while photoperiod had less effect on its expression. These results suggest that DHNs participate in the sensitive environmental regulation of the overwintering process in birch.

Photoperiod differentially modulates photic and nonphotic phase response curves of hamsters.

Circadian pacemakers respond to light pulses with phase adjustments that allow for daily synchronization to 24-h light-dark cycles. In Syrian hamsters, Mesocricetus auratus, light-induced phase shifts are larger after entrainment to short daylengths (e.g., 10 h light:14 h dark) vs. long daylengths (e.g., 14 h light:10 h dark). The present study assessed whether photoperiodic modulation of phase resetting magnitude extends to nonphotic perturbations of the circadian rhythm and, if so, whether the relationship parallels that of photic responses. Male Syrian hamsters, entrained for 31 days to either short or long daylengths, were transferred to novel wheel running cages for 2 h at times spanning the entire circadian cycle. Phase shifts induced by this stimulus varied with the circadian time of exposure, but the amplitude of the resulting phase response curve was not markedly influenced by photoperiod. Previously reported photoperiodic effects on photic phase resetting were verified under the current paradigm using 15-min light pulses. Photoperiodic modulation of phase resetting magnitude is input specific and may reflect alterations in the transmission of photic stimuli.

Photosynthetic and growth responses of three freshwater algae to phosphorus limitation and daylength

Summary1. Three common species of freshwater phytoplankton, the diatom Nitzschia sp., green alga Sphaerocystis schroeteri and cyanobacterium Phormidium luridum, were grown under contrasting daylengths [18 : 6 h light : dark cycles (LD) versus 6 : 18 h LD] and phosphorus (P) regimes (P‐sufficient versus 1 μm P). The rates of growth and photosynthesis, as well as growth efficiencies and pigment concentrations, were compared among treatments.2. The growth and photosynthetic parameters of the three species depended on both P status and daylength in a species‐specific way. The responses to P limitation depended on daylength and, conversely, the responses to daylength depended on P status.3. Growth rates and the maximum rates of photosynthesis (Pmax) of all species decreased under P limitation under both light regimes. However, the decrease of Pmax because of P limitation was greater under long daylength. The Pmax of the green alga S. schroeteri decreased the most (ca. sixfold) under P limitation compared with the other two species. The photosynthesis saturation parameter Ik also decreased under P limitation; the decline was significant in Nitzschia and Sphaerocystis. P‐limitation significantly increased photoinhibition (β) in Nitzschia and Sphaerocystis, but not in Phormidium. The excess photochemical capacity (the ratio of the maximum photosynthesis rate to the photosynthesis rate at the growth irradiance), characterising the ability to utilise fluctuating light, was significantly lower under P limitation.4. The growth efficiency (growth rate normalised to daylength) declined with increasing daylength in all species. Under short daylength the cyanobacterium Phormidium had the lowest growth efficiency of the three species.5. The cellular chlorophyll a concentration in both Nitzschia and Sphaerocystis was significantly higher under short daylength, but only under P‐sufficient conditions. In Nitzschia, under short daylength, P‐limitation significantly decreased cellular chlorophyll concentration. In contrast, P‐limitation increased cellular chlorophyll concentration in Sphaerocystis, but under long daylength only. The ratio of chlorophyll a to b in the green alga also declined under short daylength and under P‐limited conditions.

Photosynthetic photon flux density influences grass responses to extended photoperiod

While plant sensitivity to extended photoperiod has been well documented, virtually no attention has been given to the possibility that low-level photosynthetic photon flux density (PPFD) during the extended photoperiod has an influence on the expression of the photoperiodic response. This study was undertaken with four grasses (Pensacola bahiagrass, Paspalum notatum Flugge, Tifton 85 bermudagrass, Cynodon dactylon L., Florakirk bermudagrass, Florona stargrass, Cynodon nlemfuensis Vanderyst var. nlemfuensis) grown under field conditions to examine their photoperiodic responses under a PPFD gradient. Individual lamps were positioned above each experimental plot to impose a PPFD gradient (from 0.1 to 30 μmol m −2 s −1) during the extended photoperiod treatment. The experiment was undertaken during two seasons of short daylengths and plant responses were measured as mass accumulation and height. There was a linear increase in both measured variables with increasing PPFD up to a level where the plant responses were light saturated. Saturating PPFD was generally consistent between mass accumulation and height within a species, but there were substantial differences among grasses. Grasses with the greatest relative increase in mass accumulation under saturating PPFD for the extended photoperiod also were found to require the greatest PPFD for saturation. Overall, these results provided original observations on the quantitative response of plants to PPFD showing that there was a linear increase in response at very low PPFD and a saturating PPFD that varied among the grasses.

Acceleration of maturation of FSH and LH responses to photostimulation in prepubertal domestic hens by oestrogen.

Egg laying begins in domestic hens, reared on short daylengths, at about day 147 of age and is advanced by photostimulation after but not before about day 42 of age. The development of this response at day 42 may be facilitated by oestrogen. This hypothesis was investigated in prepubertal hens, reared on short daylengths, by comparing the effects of oestrogen treatment on pituitary and plasma FSH and LH responses to photostimulation (16 h light:8 h dark) for 1 week at days 34 and 54 of age. Oestradiol benzoate (0.5 mg kg(-1)) was injected i.m. on alternate days for 1 week before and after photostimulation. At day 34, pituitary LH content increased after photostimulation but plasma LH and FSH concentrations did not increase. At day 54, pituitary FSH content and plasma FSH and LH concentrations increased after photostimulation, whereas pituitary LH content did not increase. At days 34 and 54, oestrogen treatment decreased pituitary FSH and LH contents but did not block the stimulatory effect of photostimulation on pituitary FSH. At day 34 but not at day 54, photostimulation combined with oestrogen treatment increased plasma FSH and LH concentrations. Plasma LH but not plasma FSH concentration increased after GnRH-I injection at days 34 and 54. These observations are consistent with the hypothesis that, in prepubertal female chickens, maturation of the neuroendocrine mechanism mediating photoinduced FSH and LH release may be mediated by oestrogen. This effect of oestrogen on photoinduced LH release may be mediated by increased GnRH-I release or enhanced pituitary responsiveness to GnRH-I. It is proposed that neuroendocrine mechanisms controlling photoinduced FSH release may involve oestrogen-responsive interactions between pituitary paracrine factors, including activins and follistatin.

Acceleration of maturation of FSH and LH responses to photostimulation in prepubertal domestic hens by oestrogen

Egg laying begins in domestic hens, reared on short daylengths, at about day 147 of age and is advanced by photostimulation after but not before about day 42 of age. The development of this response at day 42 may be facilitated by oestrogen. This hypothesis was investigated in prepubertal hens, reared on short daylengths, by comparing the effects of oestrogen treatment on pituitary and plasma FSH and LH responses to photostimulation (16 h light:8 h dark) for 1 week at days 34 and 54 of age. Oestradiol benzoate (0.5 mg kg(-1)) was injected i.m. on alternate days for 1 week before and after photostimulation. At day 34, pituitary LH content increased after photostimulation but plasma LH and FSH concentrations did not increase. At day 54, pituitary FSH content and plasma FSH and LH concentrations increased after photostimulation, whereas pituitary LH content did not increase. At days 34 and 54, oestrogen treatment decreased pituitary FSH and LH contents but did not block the stimulatory effect of photostimulation on pituitary FSH. At day 34 but not at day 54, photostimulation combined with oestrogen treatment increased plasma FSH and LH concentrations. Plasma LH but not plasma FSH concentration increased after GnRH-I injection at days 34 and 54. These observations are consistent with the hypothesis that, in prepubertal female chickens, maturation of the neuroendocrine mechanism mediating photoinduced FSH and LH release may be mediated by oestrogen. This effect of oestrogen on photoinduced LH release may be mediated by increased GnRH-I release or enhanced pituitary responsiveness to GnRH-I. It is proposed that neuroendocrine mechanisms controlling photoinduced FSH release may involve oestrogen-responsive interactions between pituitary paracrine factors, including activins and follistatin.

Reciprocal effects in true potato seed breeding in short‐day length environments

AbstractReciprocal crosses may have significantly distinct performances regarding tuber characteristics in potato, suggesting the importance of cytoplasmic effects in this crop. The selection of parents for true potato seed breeding therefore needs to consider this potential effect when determining the direction of a cross. The aim of this research was to determine whether a broad‐based true potato seed breeding population, developed at the Centro Internacional de la Papa, could be affected by cytoplasmic effects in the short‐day length environments of the tropics. Two random sets of reciprocal biparental crosses were included in the first set of experiments, which were grown in two contrasting Peruvian locations. Only one out of 14 reciprocal crosses showed significantly distinct performance for tuber yield and tuber set. In the second set, only one of each of the 12 reciprocal crosses had distinct performance for vine earliness, days to flowering and flowering intensity, but four of the reciprocal crosses in the second set showed distinct pollen production. The results suggest that cytoplasmic effects in this breeding population are more important for reproductive characteristics such as pollen production than for tuber yield. This finding is not surprising because male sterility in potato results from the interaction between sensitive cytoplasm and dominant nuclear genes.

Enhanced expression of genes in the brains of larvae of Mamestra brassicae (Lepidoptera: Noctuidae) exposed to short daylength or fed Dopa

The cabbage armyworm, Mamestra brassicae, enters diapause in the early pupal stage. Pupal diapause is induced by rearing the larvae under short day lengths. We previously demonstrated that feeding Dopa during last larval instar induces pupal diapause even under long day lengths. In order to elucidate the mechanism by which pupal diapause is induced after experiencing short day lengths or fed Dopa under long day lengths, we analyzed gene expression in the brain of M. brassicae larvae under both of these conditions using a subtractive hybridization technique. After the secondary screen, 49 clones and 28 clones were identified as short day length or Dopa-feeding specific clones, respectively. All of these genes were sequenced and, using the base sequences of these clones, primers were synthesized. To confirm the genes enhanced specifically by these conditions, quantitative real-time PCR was carried out. This quantitative PCR analysis identified 15 and 1 clone whose expression was enhanced by the short day length conditions or Dopa-feeding, respectively. Among these clones, the gene with a high level of identity to receptor for activated protein kinase C (RACK) from Heliothis virescens is the most dramatically up-regulated under both conditions.

67 Patterns of reproduction and morphological variation in southern california populations of the lower intertidal kelp egregia menziesii (turner) areschoug

Egregia menziesii is a complex kelp with an adult morphology that varies considerably across its geographic range. Fertility in Egregia also varies geographically. We studied intertidal populations of Egregia at three southern California sites to determine spatial and temporal patterns of reproduction, recruit‐qment, and morphology. Growing axes of Egregia were collected monthly to quantify sporophyll production and lateral blade morphology. Band transects were surveyed quarterly (2001) and monthly (2002) to determine temporal patterns of sporophyte recruitment. Sporophylls were found throughout the year, but sporophyll densities showed a strong peak during winter periods of colder seawater temperatures and shorter daylengths. Sporophytes recruited almost exclusively from March to July, suggesting a gametophyte period of &lt;5 months. Lateral blade morphologies varied in a consistent pattern; thalli initially developed spatulate blades but then produced increasing numbers of filiform laterals as axes grew. Thalli in habitats populated by higher grazer densities had shorter axes, dominated by spatulate blades, while those growing under low grazing pressure exhibited longer fronds and more filiform laterals. Differences in lateral morphology had effects on thallus productivity and susceptibility to grazers. Filiform laterals had higher light‐saturated photosynthetic rates than spatulate laterals and a 1‐m long frond with filiform laterals was estimated to be 12% more productive than one with spatulate laterals. Kelp snails, but not urchins, consistently chose spatulate over filiform laterals, this suggests that Egregia thalli dominated by spatulate laterals are more susceptible to grazing molluscs, while lateral morphology has no effect on Egregia's susceptibility to urchins.

Seasonal control of penile development of Siberian hamsters (Phodopus sungorus) by daylength and testicular hormones

Seasonal variation in prepubertal penile growth has not previously been studied. The present study assessed the influence of daylength and androgens on penile development in the Siberian hamster (Phodopus sungorus). Adult penile masses were achieved at 18 and 8 weeks of age in hamsters maintained from birth under short (10 h light:14 h dark) versus long (14 h light:10 h dark) daylengths, respectively. Insulin-like growth factor I concentrations, previously implicated in penile growth, did not differ between hamsters maintained in short versus long daylengths. Gonadectomized juvenile males maintained in short and long daylengths and administered testosterone attained adult penile masses well in advance of untreated gonad-intact males maintained in short daylengths. Hamsters from both photoperiods, castrated as juveniles and first treated with testosterone in adulthood, also achieved adult penile masses. The photoinhibited gonad is insufficient to promote penile growth, and prepubertal gonadal secretions during short daylengths are not necessary for eventual penile development. Among young born near the end of the mating season, onset of neuroendocrine refractoriness to short daylengths at about 100 days of age and subsequent gonadal development induces growth in all reproductive tissues. Timing of puberty and increased androgen secretion controlled by daylength are the primary determinants of postnatal penile growth, which may also be affected by prenatal and early postnatal organizational actions of androgens.

Termination of neuroendocrine refractoriness to melatonin in Siberian hamsters (Phodopus sungorus).

Siberian hamsters maintained from birth in a short day length (DL), unlike their long-day counterparts, fail to undergo reproductive development by 5 weeks of age. Instead, reproductive maturation of short-day males is delayed for approximately 20 weeks, at which point neuroendocrine refractoriness to the inhibitory effects of short DLs develops, resulting in growth of the gonads. To terminate refractoriness and re-establish responsiveness to short photoperiods, 10-15 weeks of long-day exposure is required. We assessed whether continuous exposure to long days is necessary to terminate refractoriness or whether the first few weeks of long days initiate a process that culminates several months later in the breaking of refractoriness. Male hamsters refractory to short DLs were transferred to a long-day photoperiod, pinealectomized (PINx) after 0, 3, 6 or 15 weeks, and subsequently infused for 6 weeks with a short-day melatonin signal. This melatonin treatment induces gonadal regression in photosensitive but not in photorefractory hamsters. Six percent of males PINx at week 0 and 88% of those PINx at week 15 underwent gonadal atrophy by the end of the melatonin infusion treatment initiated on week 15. Among hamsters PINx on week 6, 17% versus 76% underwent testicular involution in response to melatonin infusions initiated on week 6 and week 15, respectively. This finding indicates that a fraction of the long days that hamsters experience during spring and summer are sufficient to trigger the processes that restore responsiveness to short DLs. Additional groups of pineal-intact photorefractory animals were given 3, 6 or 15 weeks of long-day exposure and then returned to a short DL for several months; only those treated for 15 weeks terminated refractoriness. The breaking of refractoriness, once triggered by long-day melatonin signals, proceeds to completion only in the absence of short-day melatonin signals.

Different maternal effects on diapause induction of tetranychid mites, Tetranychus urticae and T. kanzawai (Acari: Tetranychidae)

Prenatal maternal effects on diapause induction were examined in the tetranychid mites, Tetranychus urticae and T. kanzawai. In T. urticae, the incidence of diapause of offspring whose mothers had experienced short daylength (9L15D) was higher than that of the offspring whose mothers had experienced long daylength (16L8D) when the offspring developed under short daylength. However, this maternal effect was not conspicuous when the offspring developed under long daylength. The incidence of diapause in T. urticae offspring was higher when their mothers had experienced a high density during the juvenile period. By contrast, the photoperiod and density experienced by mothers did not affect the incidence of diapause in the offspring of T. kanzawai. However, the presence of senior female adults on the same leaf during the juvenile period enhanced the incidence of diapause in T. kanzawai. The difference in maternal effects observed between T. urticae and T. kanzawai is discussed with respect to their life-history characteristics.

Independence of circadian entrainment state and responses to melatonin in male Siberian hamsters

BackgroundSeasonal fluctuations in physiology and behavior depend on the duration of nocturnal melatonin secretion programmed by the circadian system. A melatonin signal of a given duration, however, can elicit different responses depending on whether an animal was previously exposed to longer or shorter photoperiod signals (i.e., its photoperiodic history). This report examined in male Siberian hamsters which of two aspects of photoperiod history – prior melatonin exposure or entrainment state of the circadian system – is critical for generating contingent responses to a common photoperiodic signal.ResultsIn Experiment #1, daily melatonin infusions of 5 or 10 h duration stimulated or inhibited gonadal growth, respectively, but had no effect on entrainment of the locomotor activity rhythm to long or short daylengths, thereby demonstrating that melatonin history and entrainment status could be experimentally dissociated. These manipulations were repeated in Experiment #2, and animals were subsequently exposed to a 12 week regimen of naturalistic melatonin signals shown in previous experiments to reveal photoperiodic history effects. Gonadal responses differed as a function of prior melatonin exposure but were unaffected by the circadian entrainment state. Experiment #3 demonstrated that a new photoperiodic history could be imparted during four weeks of exposure to long photoperiods. This effect, moreover, was blocked in animals treated concurrently with constant release melatonin capsules that obscured the endogenous melatonin signal: Following removal of the implants, the gonadal response depended not on the immediately antecedent circadian entrainment state, but on the more remote photoperiodic conditions prior to the melatonin implant.ConclusionsThe interpretation of photoperiodic signals as a function of prior conditions depends specifically on the history of melatonin exposure. The photoperiodic regulation of circadian entrainment state contributes minimally to the interpretation of melatonin signals.

Seasonal Alteration of the Cytosolic and Nuclear Ca2+ Concentrations in Overwintering Woody and Herbaceous Perennials in Relation to the Development of Dormancy and Cold Hardiness

Seasonal alteration of the cytosolic and nuclear Ca2+ concentrations of spruce (Picea engelmannii Parry) and brome grass (Bromus inermis Leyss) was investigated by the antimonate precipitation cytochemical technique. Electron microscopic (EM) observations revealed that electron-dense Ca2+ antimonate deposits, an indication of Ca2+ localization, were seen mainly in the vacuole, the cell wall and the intercellular space in samples of both species, collected on 14 July 1997. Few deposits were found in the cytosol and nuclei, showing a low resting level during summer months. On 8 Aug. 1997 following a decrease in daylength of 1 hour and 12 minutes, Ca2+ accumulation was initiated in spruce with increased cytosolic and nuclear Ca2+ deposits, but not in brome grass. On 8 Sept. 1997, Ca2+ accumulation occurred in the cytosol of brome grass. This followed a drop in ambient temperature to 12 °C. Cytosolic and nuclear Ca2+ deposits continued to increase in spruce. Controlled experiments confirmed that it was the low temperature, not shortening daylength, that triggered Ca2+ accumulation in brome grass. High cytosolic and nuclear Ca2+ concentrations lasted about three months in spruce from early August to early November. However, the high cytosolic and nuclear Ca2+ concentrations in brome grass lasted only about 20 days from early September to the end of the month. During winter and spring, both species had low resting cytosolic and nuclear Ca2+ concentrations. The relationship between the duration of the high cytosolic and nuclear Ca2+ concentrations and the status of the developed dormancy/cold hardiness is discussed in light of current findings.