Critical Night Length Research Articles

Identification and mapping of major-effect flowering time loci Autoflower1 and Early1 in Cannabis sativa L.

Flowering time is an important trait for all major market classes of hemp (Cannabis sativa), affecting yields and quality of grain, fiber, and cannabinoids. C. sativa is usually considered a short-day plant, flowering once night length reaches a critical threshold. Variations in flowering time within and across cultivars in outdoor grown populations have been previously identified, likely corresponding to genetic differences in this critical night length. Further, some C. sativa are photoperiod insensitive, colloquially referred to as “autoflowering.” This trait has anecdotally been described as a simple recessive trait with major impacts on phenology and yield. In this work, the locus responsible for the “autoflower” trait (Autoflower1), as well as a major-effect flowering time locus, Early1, were mapped using bulked segregant analysis. Breeder-friendly high-throughput molecular marker assays were subsequently developed for both loci. Also detailed are the flowering responses of diverse cultivars grown in continuous light and the result of crossing two photoperiod insensitive cultivars of differing pedigree.

Nanda-Hamner Curves Show Huge Latitudinal Variation but No Circadian Components in Drosophila Montana Photoperiodism.

Insect species with a wide distribution offer a great opportunity to trace latitudinal variation in the photoperiodic regulation of traits important in reproduction and stress tolerances. We measured this variation in the photoperiodic time-measuring system underlying reproductive diapause in Drosophila montana, using a Nanda-Hamner (NH) protocol. None of the study strains showed diel rhythmicity in female diapause proportions under a constant day length (12 h) and varying night lengths in photoperiods ranging from 16 to 84 h at 16°C. In the northernmost strains (above 55°N), nearly all females entered diapause under all photoperiods and about half of them even in continuous darkness, while the females of the southern strains showed high diapause proportions only in the circadian 24 h photoperiod. Significant correlation between the strains’ mean diapause proportions in ≥ 24 h photoperiods and critical day length (CDL; half of the females enter diapause) suggests at least partial causal connection between the traits. Interestingly, females of the northern strains entered diapause even in ≤ 24 h photoperiods, where the night length was shorter than their critical night length (24 h - CDL), but where the females experienced a higher number of Light:Dark cycles than in 24 h photoperiods. NH experiments, performed on the control and selection lines in our previous selection experiment, and completed here, gave similar results and confirmed that selection for shorter, southern-type CDL decreases female diapausing rate in non-circadian photoperiods. Overall, our study shows that D. montana females measure night length quantitatively, that the photoperiodic counter may play a prominent but slightly different role in extra short and extra long photoperiods and that northern strains show high stability against perturbations in the photoperiod length and in the presence of LD cycles. These features are best explained by the quantitative versions of the damped external coincidence model.

Constitutive expression of CsGI alters critical night length for flowering by changing the photo-sensitive phase of anti-florigen induction in chrysanthemum

Chrysanthemum is a typical short day (SD) flowering plant that requires a longer night period than a critical minimum duration to successfully flower. We identified FLOWERING LOCUS T-LIKE 3 (FTL3) and ANTI-FLORIGENIC FT/TFL1 FAMILY PROTEIN (AFT) as a florigen and antiflorigen, respectively, in a wild diploid chrysanthemum (Chrysanthemum seticuspe). Expression of the genes that produce these proteins, CsFTL3 and CsAFT, is induced in the leaves under SD or a noninductive photoperiod, respectively, and the balance between them determines the progression of floral transition and anthesis. However, how CsFTL3 and CsAFT are regulated to define the critical night length for flowering in chrysanthemum is unclear. In this study, we focused on the circadian clock-related gene GIGANTEA (GI) of C. seticuspe (CsGI) and generated transgenic C. seticuspe plants overexpressing CsGI (CsGI-OX). Under a strongly inductive SD (8 L/16D) photoperiod, floral transition occurred at almost the same time in both wild-type and CsGI-OX plants. However, under a moderately inductive (12 L/12D) photoperiod, the floral transition in CsGI-OX plants was strongly suppressed, suggesting that the critical night length for flowering was lengthened for CsGI-OX plants. Under the 12 L/12D photoperiod, CsAFT was upregulated in CsGI-OX plants. Giving a night break (NB) 10 h after dusk was the most effective time to inhibit flowering in wild-type plants, while the most effective time for NB was extended to dawn (12 and 14 h after dusk) in CsGI-OX plants. In wild-type plants, a red-light pulse delivered 8 or 10 h after dusk induced maximal CsAFT expression, but the length of the time period over which CsAFT could be induced by red light was extended until subjective dawn in CsGI-OX plants. Therefore, CsGI-OX plants required a longer dark period to maintain lower levels of CsAFT, and their critical night length for flowering was thus lengthened. These results suggested that CsGI has an important role in the control of photoperiodic flowering through shaping the gate for CsAFT induction by light in chrysanthemum.

Enhancement of Flowering by End-of-Day Far Red Light Irradiation in Chrysanthemum

We investigated the effects of end-of-day FR light irradiation (EOD-FR) on flowering in chrysanthemum. EOD-FR treatment in the low-temperature period promoted flowering in the autumn-flowering chrysanthemum cultivar ‘Jimba’. EOD-FR treatment with season flowering cultivation in autumn-flowering chrysanthemum promoted flowering in 14 out of 20 cultivars. EOD-FR treatment promoted flowering in short-day cultivation from the time of planting in 11 out of 20 cultivars of autumn-flowering chrysanthemum. EOD-FR treatment of summer-autumn-flowering chrysanthemum in the June-flowering cropping type promoted flowering in 19 out of 30 cultivars. EOD-FR treatment weakly promoted flowering near the critical night length in the autumn-flowering chrysanthemum cultivar ‘Jimba’; however, EOD-FR showed no effect on the duration of Dusk-NBmax of the chrysanthemum cultivar ‘Jimba’. The results of this study show that EOD-FR promotes flowering as well as internode elongation in chrysanthemum depending on the growing conditions.

Divergence in larval diapause induction between the rice and water-oat populations of the striped stem borer, Chilo suppressalis (Walker) (Lepidoptera: Crambidae).

Differences in diapause traits can result in the seasonal reproductive isolation of host plant-associated insect populations and thereby facilitate the population divergence. The striped stem borer, Chilo suppressalis, has two host plant-associated populations: rice population and water-oat population. Several studies have found evidence that seasonal reproductive isolation between these populations is at least partially due to interpopulation differences in diapause. However, there still lack unambiguous evidence comparing characteristics of diapause induction for both populations. We compared the photoperiodic response and the age of peak photoperiod sensitivity of these populations and used RNA-Seq to compare the molecular response of diapause induction between populations. The photoperiodic response of the two populations differed at 25°C; the critical night length of larvae from the rice population was 11h and 20min, whereas no obvious critical night length was in those from the water-oat population. In rice population, larvae were most sensitive to photoperiod at 9-12days of age, whereas in water-oat population, larvae were the most sensitive to photoperiod at 9-10days of age. The RNA-Seq results indicated that there were several differences in the molecular response of diapause induction and small overlap in differentially expressed genes (DEGs) between populations. Furthermore, GO analysis indicated that both rice and water-oat population's DEGs were significantly enriched in heme and iron binding. Besides, water-oat population's DEGs were significantly enriched in metabolizing nutrients but rice population's DEGs do not. Thus, our results described differences in diapause induction between rice and water-oat populations of C. suppressalis which could affect the timing of diapause and thereby contribute to the seasonal reproductive isolation of these host plant-associated populations. In conclusion, this work suggests that difference in diapause induction could promote the population divergence in insects associated with different host plants.

Drosophila ezoana uses an hour-glass or highly damped circadian clock for measuring night length and inducing diapause.

Insects inhabiting the temperate zones measure seasonal changes in day or night length to enter the overwintering diapause. Diapause induction occurs after the duration of the night exceeds a critical night length (CNL). Our understanding of the time measurement mechanisms is continuously evolving subsequent to Bünning's proposal that circadian systems play the clock role in photoperiodic time measurement (Bünning, 1936). Initially, the photoperiodic clocks were considered to be either based on circadian oscillators or on simple hour‐glasses, depending on ‘positive’ or ‘negative’ responses in Nanda–Hamner and Bünsow experiments (Nanda & Hammer, 1958; Bünsow, 1960). However, there are also species whose responses can be regarded as neither ‘positive’, nor as ‘negative’, such as the Northern Drosophila species Drosophila ezoana, which is investigated in the present study. In addition, modelling efforts show that the ‘positive’ and ‘negative’ Nanda–Hamner responses can also be provoked by circadian oscillators that are damped to different degrees: animals with highly sustained circadian clocks will respond ‘positive’ and those with heavily damped circadian clocks will respond ‘negative’. In the present study, an experimental assay is proposed that characterizes the photoperiodic oscillators by determining the effects of non‐24‐h light/dark cycles (T‐cycles) on critical night length. It is predicted that there is (i) a change in the critical night length as a function of T‐cycle period in sustained‐oscillator‐based clocks and (ii) a fixed night‐length measurement (i.e. no change in critical night length) in damped‐oscillator‐based clocks. Drosophila ezoana flies show a critical night length of approximately 7 h irrespective of T‐cycle period, suggesting a damped‐oscillator‐based photoperiodic clock. The conclusion is strengthened by activity recordings revealing that the activity rhythm of D. ezoana flies also dampens in constant darkness.

Timing growth and development of Campanula by daily light integral and supplemental light level in a cost-efficient light control system

Two campanula species Campanula portenschlagiana (‘Blue Get Mee’) and Campanula cochlearifolia (‘Blue Wonder’) were grown in a cost-efficient light control system and the effect of supplemental light level and daily light integral (DLI) on growth and development was quantified. The alternative light control system (DynaLight desktop) automatically defines the most cost-efficient use of supplemental light based on predefined setpoints for daily photosynthesis integral (DPI), forecasted solar irradiance and the market price on electricity. It saves energy in high-cost periods of electricity, but creates irregular light periods which may disturb circadian rhythms and thereby affect plant growth and flower development. Plants were grown in four treatments controlled by DynaLight desktop with two setpoints for DPI (300 and 600mmolm−2leafday−1) and two levels of supplemental lighting (48 and 84μmolm−2s−1). We found that differences in supplemental light levels, daily light hours or DLI had no effect on leaf area expansion and stem elongation, but there was a linear relation between dry matter accumulation and cumulative light integral (CLI) in both species, and a linear relation between the number of flowers and buds and CLI in ‘Blue Get Mee’. The results demonstrate that DLI was the main limiting factor for prediction of growth and development when two campanula species were grown in a cost-efficient light control system where the number of daily light hours was often below the critical day length of 14h. However the light hours was always distributed in two or more light periods during the day, which thereby prevented dark periods longer than the critical night length of 9h for campanulas.

暗期干扰对棉铃虫两个不同地理种群滞育抑制作用的比较

至今,所测试昆虫的光周期反应均表明,光周期反应对暗期干扰高度敏感,短暂的光脉冲都可在不同程度上逆转长夜效应,抑制滞育的发生。在研究了棉铃虫<em>Helicoverpa armigera</em>泰安种群(36.15°N, 116.59°E)和喀佐种群(41.34°N, 120.27°E)光周期反应的基础上,在滞育诱导的短光周期下(L12:D12 和L9:D15),分别测试了暗期不同时段1h光脉冲对这两个不同地理种群滞育抑制的影响。25和22℃下的光周期反应显示了泰安种群在长暗期11-14 h 的滞育率均显著低于喀佐种群;泰安种群的临界暗长分别为11.7 h和11.5 h,喀佐种群分别为10.5 h和10.3 h,泰安种群均比喀佐种群长1.2 h。在所测试的暗期干扰实验中,除了极少数光脉冲干扰点外,泰安种群蛹滞育率显著低于喀佐种群,但两者的滞育反应曲线基本相似。在短光周期L9:D15下,泰安种群和喀佐种群均显示了光脉冲落入暗期的第9-11小时最有效地抑制了滞育的发生。在短光周期L12∶D12下,泰安种群和喀佐种群在25℃时均显示了光脉冲落入暗期的第3-4小时和第10小时导致了最低的滞育发生;但在22℃时,喀佐种群只在暗期的第3-4小时显示了最高的滞育抑制。这些结果揭示了偏南的泰安种群对暗期干扰的敏感性强于偏北的喀佐种群,但这两个地理种群的最高光敏感位点基本相同。;Photoperiodic time measurement in insects can be affected by interrupting the dark phase of the light dark cycle with short supplementary light pulses. All the photoperiodic responses tested are highly sensitive to night interruption, and the long night effect is reversed by a light break. Most past studies revealed two points of apparent light sensitivity (the so-called A and B peaks), but the response pattern varied considerably among different species. Some species had only one peak (or trough), either A or B. Even the same species may have different patterns of response depending on the experimental conditions. However, it is still not clear whether differences exist in the effect of night-interruption on diapause-averting among different geographic populations. The cotton bollworm, <em>Helicoverpa armigera </em>(Hübner) is one of the most serious crop pests in Asia, Australia, Africa and Europe. The species is distributed across much of China and attacks many host plants. The widely distributed<em> H. armigera</em> encounters a great diversity in climatic conditions in different places and shows different critical day lengths or night lengths (the day length that elicits 50% diapause response) among different geographic populations. In the present study, the photoperiodic response and effects of night-interruption on diapause-averting, in which the scotophases of L9:D15 (Light 9 h:Dark 15 h) and L12∶D12 were interrupted by 1 h light pulse, were investigated at 22℃ and 25℃ in Taian (TA, 36.15°N, 116.59°E) and Kazuo (KZ, 41.34°N, 120.27°E) populations of the cotton bollworm<em> H. armigera</em>. The photoperiodic response curves showed that the incidences of diapause in the scotophases of 11-14 h were significantly lower in the Taian population than in the Kazuo population. The critical night length in the Taian population at 25℃ and 22℃ was 11 h 42 min and 11 h 30 min, respectively, and 1h 20 min longer than that in Kazuo population (10 h 30 min and 10 h 18min). In all night-interruption experiments, the incidences of diapause in almost all positions of light pulse in the Taian population were significantly lower than that in the Kazuo population. However, the diapause response curves were similar in the two populations. Under L9: D15, the incidence of diapause was inhibited most effectively in both the Taian population and the Kazuo populations when the light pulses were placed at 9-11 h in the darkness. Under L12:D12 the incidence of diapause was lowest at 25℃ in both Taian population and Kazuo population when the light pulses were placed at 3-4 h and 10 h in the darkness. However, the Kazuo population only showed a peak of diapaus-averting occurring in 3-4 h in the darkness at 22℃. <br> Our results reveal that effect of night-interruption on diapause-averting is different among different geographic population. The southern Taian population is more sensitive to night-interruption than the northern Kazuo populations, but the most highly photosensitive position is similar for both populations.<br> A light pulse plays two possible roles in photoperiodic time measurement. First it ‘initiates’ or ‘resets’ the clock; this has been called the ‘entraining agent’ or ‘priming agent’. Second it ‘measures’ a critical duration of day or night; this has been called the ‘inductive agent’ or ‘terminating agent’. The present study in <em>H. armigera</em> reveals that a light pulse can reset the process measuring night length, and thus influences the incidence of diapause.

Diapause induction and photoperiodic clock in Chilo suppressalis (Walker) (Lepidoptera: Crambidae)

AbstractThe mature larvae of the rice stem borer, Chilo suppressalis Walker (Lepidoptera: Crambidae) enters facultative diapause in response to short‐day conditions in the autumn (August–September). Diapause induction and photoperiodic clock mechanism were investigated in C. suppressalis larvae reared on an artificial diet in the present study. The critical night length for diapause induction was about 9 h 53 min to 10 h 39 min at 22 to 28°C. The third‐instar larvae were found to be relatively sensitive to diapause induction. Photoperiodic response under non‐24‐h light–dark cycles showed that scotophase length played an essential role in the induction of larval diapause in C. suppressalis, and consecutive exposure to long‐night cycles was necessary for a high diapause incidence. In the Nanda–Hamner experiment, diapause incidence peaked at scotophase of 12 h and dropped rapidly at scotophases &gt; 24 h. In the Bünsow experiment, diapause incidence was clearly suppressed, especially at the light pulse located 8 h in the scotophase. Both the Nanda–Hamner and Bünsow experiments showed no rhythmic fluctuations with a period of about 24 h; thus the photoperiodic clock in C. suppressalis is a non‐oscillatory hourglass timer or a rapidly damping circadian oscillator.

Identification and analysis of the germin-like gene family in soybean

BackgroundGermin and germin-like proteins constitute a ubiquitous family of plant proteins. A role of some family members in defense against pathogen attack had been proposed based on gene regulation studies and transgenic approaches. Soybean (G. max L. Merr.) germin genes had not been characterized at the molecular and functional levels.ResultsIn the present study, twenty-one germin gene members in soybean cultivar 'Maple Arrow' (partial resistance to Sclerotinia stem rot of soybean) were identified by in silico identification and RACE method (GmGER 1 to GmGER 21). A genome-wide analyses of these germin-like protein genes using a bioinformatics approach showed that the genes located on chromosomes 8, 1, 15, 20, 16, 19, 7, 3 and 10, on which more disease-resistant genes were located on. Sequence comparison revealed that the genes encoded three germin-like domains. The phylogenetic relationships and functional diversity of the germin gene family of soybean were analyzed among diverse genera. The expression of the GmGER genes treated with exogenous IAA suggested that GmGER genes might be regulated by auxin. Transgenic tobacco that expressed the GmGER 9 gene exhibited high tolerance to the salt stress. In addition, the GmGER mRNA increased transiently at darkness and peaked at a time that corresponded approximately to the critical night length. The mRNA did not accumulate significantly under the constant light condition, and did not change greatly under the SD and LD treatments.ConclusionsThis study provides a complex overview of the GmGER genes in soybean. Phylogenetic analysis suggested that the germin and germin-like genes of the plant species that had been founded might be evolved by independent gene duplication events. The experiment indicated that germin genes exhibited diverse expression patterns during soybean development. The different time courses of the mRNAs accumulation of GmGER genes in soybean leaves appeared to have a regular photoperiodic reaction in darkness. Also the GmGER genes were proved to response to abiotic stress (such as auxin and salt), suggesting that these paralogous genes were likely involved in complex biological processes in soybean.

Diapause induction, maintenance and termination in the rice stem borer Chilo suppressalis (Walker)

The rice stem borer, Chilo suppressalis, enters facultative diapause as fully grown larvae in response to short-day conditions during the autumn. Our results showed that the critical night length for diapause induction in C. suppressalis was between 10 h 22 min and 10 h 45 min at 22, 25 and 28 °C, 11 h 18 min at 31 °C, and between 10 h 5 min and 10 h 20 min under field conditions (average temperature ranged from 27.2 to 30.7 °C). The diapause incidence declined in ultra-long nights (18–22 h scotophases) and DD, and increased in ultra-short nights (2–6 h scotophases) and LL. Moreover, we found that the third instar was the stage most sensitive to the photoperiod, and night length played an essential role in the initiation of diapause. Night-interruption experiments with a 1-h light pulse at LD 12:12 (light 12:dark 12) exhibited two troughs of diapause inhibition, with one occurring in early scotophase and the other in late scotophase. Field observations for six years showed that most larvae entered winter diapause in August in response to declining day lengths, despite the high temperatures prevailing during August. By periodically transferring the field-collected overwintering larvae to different photoperiods and temperatures, the results showed that photoperiod had a significant influence on diapause development during the early phase of diapause, while high temperature significantly accelerated the termination of larval diapause.

Bud break and spring frost hardiness in Picea abies seedlings in response to photoperiod and temperature treatments

Spring frost may result in detrimental damage in newly planted Picea abies (L.) Karst. seedlings if their growth rhythm is not sufficiently adapted to the climatic conditions on the forest site. The aims of this study were to evaluate how bud break and spring frost hardiness were influenced by short-day (SD) treatments with different timing and different temperature regimes during bud formation. Following winter storage, frost hardiness was tested after 1, 3 and 5 weeks in forcing conditions. The SD treatment advanced bud break compared with the control seedlings. In comparison, the effects of timing and the different temperatures on bud break were small. The SD treatment improved frost hardiness in first-year needles during dehardening. The early SD treatment resulted in improved frost hardiness in first-year needles and greater root collar diameter compared with later SD treatment. To avoid a second bud flush, it is important that a critical night length is attained when the SD treatment terminates. Low temperatures following the SD treatment resulted in increased hardiness of the needles and decreased hardiness of the stems. The contrasting effect of temperature in different plant tissues demonstrates the importance of examining different tissues following freezing tests.

Circadian rhythms and the evolution of photoperiodic timing in insects

Abstract.This review discusses possible evolutionary trends in insect photoperiodism, mainly from a chronobiological perspective. A crucial step was the forging of a link between the hormones regulating diapause and the systems of biological rhythms, circadian or circannual, which have independently evolved in eukaryotes to synchronize physiology and behaviour to the daily cycles of light and darkness. In many of these responses a central feature is that the circadian system resets to a constant phase at the beginning of the subjective night, and then ‘measures’ the duration of the next scotophase. In ‘external coincidence’, one version of such a clock, light now has a dual role. First, it serves to entrain the circadian system to the stream of pulses making up the light/dark cycle and, second, it regulates the nondiapause/diapause switch in development by illuminating/not illuminating a specific light sensitive phase falling at the end of the critical night length. Important work by A. D. Lees on the aphid Megoura viciae using so‐called ‘night interruption experiments' demonstrates that pulses falling early in the night lead to long‐day effects that are reversible by a subsequent dark period longer than the critical night length and also show maximal sensitivity in the blue–green range of the spectrum. Pulses falling in the latter half of the night, however, produce long‐day effects that are irreversible by a subsequent long‐night and show a spectral sensitivity extending into the red. With movement to higher latitudes, insects develop genetic clines in various parameters, including critical night length, the number of long‐night cycles needed for diapause induction, the strength of the response, and the ‘depth’ or intensity of the diapause thus induced. Evidence for these and other types of photoperiodic response suggests that they provided strong selective advantages for insect survival.

Photoperiodism of diapause induction in Thyrassia penangae (Lepidoptera: Zygaenidae)

Thyrassia penangae enters winter diapause as a prepupa in a cocoon. Photoperiodism of diapause induction was systematically investigated in this moth. The photoperiodic response curves under 24-h light–dark cycles showed that this insect is a typical long-day species. The critical daylength was 13 h 30 min at 25 °C, 13 h at 30 °C and 12 h 20 min at 28 °C. Transferring experiments from a short day (LD 12:12) to a long day (LD 15:9) or vice versa indicated that photoperiodic sensitivity mainly occurs during the larval period. In experiments using non-24-h light–dark cycles, when the length of photophase exceeded the critical daylength (13.5 h), was diapause inhibited effectively, even when the length of scotophase exceeded the critical nightlength (10.5 h). Only when a long scotophase was combined with a short photophase, diapause was induced effectively. This result suggests that daylength measurement is more important than nightlength measurement in T. penangae. Night interruption experiments under 24-h light–dark cycles exhibited two points of apparent light sensitivity, but the photosensitive position was highly influenced by temperature and the length of scotophase. Nanda–Hamner experiments failed to reveal the involvement of a circadian system in this photoperiodic time measurement. All light–dark cycles from LD 12:12 to LD 12:72 resulted in a short day response, and all cycles from LD 14:4 to LD 14:72 resulted in a long day response, suggesting that photoperiodic time measurement in this moth is performed by a day-interval timer or an hourglass-like clock.

On critical night lengths and temperature compensation in the photoperiodic response of two geographical clones of the black bean aphid, Aphis fabae

On critical night lengths and temperature compensation in the photoperiodic response of two geographical clones of the black bean aphid, Aphis fabae

Effects of early long-night treatment on diameter and height growth, second flush and frost tolerance in two-year-old Picea abies container seedlings

The object of this study was to obtain Norway spruce seedlings with buds set, ready for summer planting from 1 July. An early long-night treatment prevented flushing of the newly formed terminal buds and ceased height growth, but slightly reduced hardiness in buds and needles. Nevertheless, a sufficient hardiness level in the autumn was acquired at a Norwegian nursery at 59°46′ N, with plants of the local provenance given a long-night treatment (14 h) for 13 days from 25 June. Similar treatment at a nursery at 64°30′ N did not give the same result; all treatments led to a second flush with resumed growth of the local provenance. A trial with seed lots from several provenances was therefore performed at this nursery, and a significant correlation was found between the critical night length of the seed lot and their ability to produce non-flushing buds; the longer the critical night length of the seed lot, the fewer non-flushing buds. Responses at the northern nursery are probably due to the lack of a dark period after termination of the treatment, and too short a treatment period to attain bud dormancy. An early and successful long-night treatment will also produce shorter seedlings with a larger root collar diameter.

The role of the main photophase on dark‐time measurement used for diapause determination in the Indian meal moth, Plodia interpunctella

Abstract The Indian meal moth Plodia interpunctella Hubner (Lepidoptera: Pyralidae) measures night length and enters diapause as a last‐instar larva. To examine the role of photophase on dark‐time measurement, the main LD 7 : 17 h photoperiod is disrupted by various lengths of darkness at 25 °C. When the light phase is not disrupted, the incidence of diapause is 76%. As the dark pulse disrupting a 7‐h photophase becomes longer, the incidence of diapause decreases. To detect the dynamic kinetics of the time‐measuring process, the main scotophase of 17 h is scanned by a 2‐h light pulse. When the dark pulse in a 7‐h photophase is fixed at 1 h after dawn and its duration is varied systematically from 1 to 3 h, or when the end of the dark pulse is fixed at 1 h before dusk, diapause is prevented completely by a 2‐h light pulse inserted in the middle of 17‐h darkness. These results are compared with those of a single night interruption of a 17‐h scotophase with a 2‐h light pulse but with an intact 7‐h photophase. The disruption of a 7‐h photophase by a dark pulse shifts the descending and ascending slopes of the response curve to some extent toward dawn and dusk, respectively, indicating that the dark pulse tends to shorten the critical length of dark time for diapause induction. When the main photophase (7 h) is interrupted by a 1‐h dark pulse at 3–4 h after dawn, the 2‐h scanning light pulse in the main scotophase (17 h) appears to act effectively as a dusk signal in the early scotophase. However, those in the mid‐ and late scotophase do not define the critical night length from dusk as sharply as for the critical night length from a 2‐h light pulse to dawn. The results indicate the importance of photophase in the dark‐time measurement.

Critical night length for bud set and its variation in two photoperiodic ecotypes of Betula pendula

We studied the variation in critical night length for bud set in two photoperiodic ecotypes (two latitudinally distant stands) of silver birch (Betula pendula Roth) in three phytotron experiments. Seeds from 21 open-pollinated mother trees in a southern (Tuusula, 60 degrees N) and a northern (Kittilä, 67 degrees N) Finnish stand were germinated and grown for 4 weeks in a 24-h photoperiod in a greenhouse and then moved to different night length treatments at 18 degrees C for 4 to 6 weeks. Night lengths from 5 to 8.5 h were used for southern origin seedlings and from 1 to 4.5 h for northern origin seedlings. At the end of the treatments, apical bud set was observed and the percentage of seedlings with bud set calculated for each treatment and tree progeny. The critical night lengths (CNL) for 50% bud set were determined separately for seedlings from each mother tree by regression analysis. In both ecotypes, the mean percentage of seedlings with bud set was lowest for the shortest night lengths and increased rapidly as night lengths increased. Mean CNL with its 95% confidence interval for the southern and northern ecotypes was 6.3 +/- 0.2 and 3.1 +/- 0.3 h, respectively. The CNL of the two ecotypes differed significantly in three experiments. Within-ecotype variance of the CNL was significantly higher in the northern ecotype (0.484) than in the southern ecotype (0.150). Significant differences in CNL were detected between individual mother trees of the southern ecotype, but not between mother trees of the northern ecotype. The ranking of individual mother trees, based on CNL, differed in the three experiments.

Photoperiodic response of diapause induction in the pine caterpillar, Dendrolimus punctatus

AbstractThe pine caterpillar, Dendrolimus punctatus (Walker) (Lepidoptera: Lasiocampidae), is a multivoltine pest of pine trees in China, overwintering as larvae. Winter diapause was induced by short day length. The critical night length was about 10 h 40 min at 25, 28, and 31 °C in the field, showing a temperature‐compensated diapause induction. Transfer experiments from a short night (L16:D8) to a long night (L12:D12) or vice versa at different times after hatching showed that sensitivity to day length was restricted to the first 14 days; the required day number for a 50% response at 25 °C was about 3.5 days for short nights but 7.5 days for long nights, indicating that short nights are photoperiodically more effective. When four successive short nights (L16:D8) were used to interrupt the long‐night regime (L12:D12) at different development stages and vice versa, the results showed that the highest sensitivity to photoperiod occurred on the 4th−8th day, corresponding to the second larval instar. Experiments of alternating short‐night (L16:D8) and long‐night (L12:D12) cycles during the larval period showed that the information of short nights as well as long nights could be accumulated. By rearing the larvae under conditions other than 24‐h light–dark cycles, we clearly showed that the dark period (scotophase) played a major role in the determination of diapause. The Nanda‐Hamner and Bünsow experiments failed to reveal rhythmic fluctuations with a period of about 24 h in the occurrence of diapause. Therefore, the photoperiodic clock in D. punctatus is an hourglass timer or a damped circadian oscillator.

Effects of red, far‐red and blue light in maintaining growth in latitudinal populations of Norway spruce (Picea abies)

Seedlings of trees with a free growth pattern cease growth when night-lengths become shorter than a critical value, and this critical night-length (CNL) decreases with increasing latitude of origin. In northern populations, the light quality also appears to play an important role and a clinal variation in requirement for far-red (FR) light has been documented. In this study we dissected the light quality requirements for maintaining growth in different latitudinal populations of Norway spruce (Picea abies (L.) H. Karst.) using light emitting diodes for red (R), FR and blue (B) light, as 12 h day extension to provide 24 h photoperiod. At equal spectral photon flux, FR light was more effective than R light in maintaining growth, and the requirement of both R and FR increased with northern latitude of origin. One-to-one mixtures of R and FR light were more effective in maintaining growth than either FR or R light alone, indicating a possible interaction between R and FR light maintaining growth. Using the blue light as day extension could not prevent growth cessation in any of the populations, but delayed the bud set slightly in all populations. Our results suggest that phytochrome(s) are the primary photoreceptors in high irradiance responses maintaining growth in Norway spruce seedlings.