Effects of Different Photoperiods and Chilling Treatments on Growth and Reproduction of Cardamine flexuosa With. and Its Ecological Implications

Abstract Phenotypic variability of Cardamine flexuosa (Cruciferae) was examined in response to different lengths of exposure to low temperature (5°C) at the juvenile stage, and to two photoperiod regimes (8 and 16 hrs day-lengths) in the subsequent growth period. The results indicated that this species had a facultative chilling or long-day requirement for flowering. The long-day and chilling treatments both caused an earlier onset of stem internode elongation. Longer chilling treatments reduced the number of nodes and increased internode length under both long and short photoperiod regimes. Prolonged chilling treatments followed by long-day photoperiod remarkably induced more numerous basal branches and inflorescences, as a result producing more siliques. Remarkable rosette leaves Were formed at the base of the main stem when partial chilling and the short-day treatment suppressed stem internode elongation. The responses to chilling and long-day treatment closely resembled the synchronized flowering of this species in spring in the field. Delayed flowering under the short-day treatment resembled size- or age-dependent flowering in late summer to autumn in the field populations.     

The Effects of Photoperiod, Glucose and Gibberellic Acid on Growth In Vitro and Flowering of Chenopodium Murale

In vitro culture of long-day plant Chenopodium murale L was established. The effects of photoperiod, glucose and gibberellic acid (GA₃) on flowering and growth in vitro were investigated. Oscillatory changes of photoperiodic sensitivity were noticeable with regard to plant age. The plants induced at the phase of the 1^st and the 3^rd pair of leaves flowered to higher degree than those induced at the phase of 2^nd pair. Plants induced at the phase of the 1^st pair of leaves flowered to 17 % on 5 % glucose-containing medium and the addition of 5 mg dm^-3 GA₃ resulted in maximum flowering (43 %). Neither glucose nor GA₃ were able to compensate for photoperiodic requirements for flowering. Hypocotyl growth was decreased and the 1^st internode elongation and development of leaves were increased due to inductive photoperiodic conditions, as compared to non-inductive ones.     

Growth Chamber Illumination and Photomorphogenetic Efficacy I. Physiological action of infrared radiation beyond 750 nm

Plants cultivated in growth chambers under such artificial illumination as fluorescent tubes show certain characteristic differences from plants grown in nature or under incandescent lamps. The plants grown under fluorescent light are shorter and more darkly pigmented; they also show different daylength requirements for certain photoperiodic responses. It was assumed that these effects were caused by the relative deficiency of far red irradiation (λ= 700–750 nm) in the emission from the fluorescent lamps. This assumption was tested on such light sensitive phenomena as stem elongation, flowering, the formation of resting organs, chlorophyll synthesis, and production yield in several plant genera. In all cases the plants were grown under long days (16 h). Six common types of growth chamber illumination were tested for their efficacy for long-day induction. The spectral energy distribution of the lamps was measured. The emission from the various lamp types showed only small differences within the region of visible light (λ: 400–750 nm), but much larger diversity in the ratio of visible to infrared radiation (λ: 750–1000 nm). A strong correlation exists between the relative emission of infrared radiation and the photomorphogenetic efficacy of the lamps. Under fluorescent light the long-day effect was weak or missing, but increased with increasing infrared emission. High infrared emission was also favourable for high production yield.     

Photocontrol of stem elongation in light-grown plants of Fuchsia hybrida

Stems of the caulescent long-day plant, Fuchsia hybrida cv Lord Byron, showed 2 types of response to light. In one, internode length was increased by far-red irradiation given at the end of an 8 h photoperiod: the response was no greater with prolonged exposure and was less when the start of far-red was delayed. The effect of far-red was reversible by a subsequent exposure to red light. Internode length was inversely proportional to the P_fr/P ratio established before entry to darkness and there was no evidence for loss of P_fr during a 16 h dark period. The inhibitory effect of P_fr acted at a relatively late stage of internode growth. With the development of successive internodes a second response appeared in which stems lengthened following prolonged daily exposures to red or far-red light, or mixtures of the two, or to brief breaks with red or white light. In these later internodes, a short exposure to far-red near the middle of the night was not reversible by red because red alone promoted elongation at this time. Internode length increased with increase in the daily duration of light and, when light was given throughout an otherwise dark period of 16 h, with increase in illuminance to a saturation value of 200 lx from tungsten lamps. Elongation increased as a linear function of decrease in photostationary state of phytochrome down to P_fr/P0.3; however, internodes were shorter in far-red light than in 25% red/red+far-red. It was concluded that stem length is a net response to two modes of phytochrome action. An inductive effect of P_fr inhibits a late stage in internode expansion, and a phytochrome reaction which operates only in light (and may involve pigment cycling) promotes an early stage of internode development. Stem elongation is thus a function both of the daily duration of light and its red/red+far-red content. The outgrowth of axillary buds was controlled by the first type of phytochrome action only.Abbreviations and symbols FR far red light - R red light - P phytochrome - P_fr phytochrome in the far-red light absorbing form - SD 8 h short days - LDP long-day plant - SDP short-day plant     

Red light enhancement of the phototropic response of etiolated pea stems

In the subapical third internode of 7-day-old etiolated pea seedlings, the magnitude of phototropic curvature in response to continuous unilateral blue illumination is increased when seedlings are pre-exposed to brief red light. The effect of red light on blue light-induced phototropism becomes manifest maximally 4 or more hours after red illumination, and closely parallels the promotive action of red light on the elongation of the subapical cells. Ethylene inhibits phototropic curvature by an inhibitory action on cell elongation without affecting the lateral transport of auxin. Pretreatment of seedlings with gibberellic acid causes increased phototropic curvature, but experiments using ¹⁴C-gibberellic acid indicate that gibberellic acid itself is not laterally transported under phototropic stimuli. Neither red light nor gibberellic acid treatment has any promotive effect on blue light-induced lateral transport of ³H-indoleacetic acid. Under conditions where phototropic curvature is increased by red light treatment, low concentrations of indoleacetic acid applied in lanolin paste to the apical cut end of the seedling cause an increased elongation response in subapical tissue. This could explain increased phototropic curvature caused by red light treatment.     

Phytotron experiments in Pisum

Summary The flowering behavior of 59 Pisum mutants and 228 recombinants was studied in the phytotron in four different photoperiods (continuous light, long-day 18/6 h, short-day 12/12 h, extreme short-day 6/18 h). There was no or little difference in the response of the genotypes to long-day and permanent light, whereas great differences were observed between long- and short-day 12/12 h and between the two short-day trials. About half the genotypes tested were unable to survive or to flower in extreme short-day. Some recombinants, however, had an almost normal development under these unfavorable conditions. Gene fis controls the photoperiodic reaction of the plants: they are unable to flower in short-day. Gene fds negatively influences gene efr for earliness: it causes a strong delay of flowering of efr recombinants in long-day and suppresses the formation of functionable flowers in short-day. Most of the genotypes tested showed a specific reaction to the four photoperiods different from that of the mother variety and the other genotypes. The practical aim of our phytotron experiments is the preselection of Pisum genotypes which might be suited for cultivation in countries with short-day climate.This paper is dedicated to Professor Karl-Ernst Wohlfarth-Bottermann on his 65th birthday     

Is the Root Effect on Flowering of Chenopodium rubrum Mediated by Cytokinins?

Root removal enhances flowering in the short day plant Chenopodium rubrum. The extent of this effect depends on the de-rooting time with respect to photoperiodic induction. The largest promotive effect is observed when de-rooting coincides with the start of the inductive treatment or, to a lesser extent, when performed before it. De-rooting 24 h after induction has no effect on flowering. The flower-inducing action of de-rooting 24 h before the start of induction is increased by benzylaminopurine (BAP), whether applied simultaneously with de-rooting or 24 h later. At the beginning of darkness, BAP inhibits flowering slightly when applied simultaneously with de-rooting but inhibits it strongly when applied 24 h later. Flowering in plants de-rooted 24 h after induction is inhibited strongly by BAP. Root removal at the beginning of inductive darkness does not change the level of endogenous cytokinins in induced shoot explants, but under continuous light the level of cytokinins in shoot explants decreases during the same period compared with the level in the shoots of intact plants. BAP does not affect the level of endogenous cytokinins in light but causes an apparent increase in induced segments. Thus, two phases of the de-rooting effect and cytokinin treatment may be distinguished: one in which flowering is enhanced by both treatments and which is linked directly to photoperiodic flower induction, and the other in which both treatments are inhibitory to flowering and which is related to morphogenetic events following induction. The time courses of the effectiveness of de-rooting and BAP treatment differ slightly, suggesting that the effect of de-rooting cannot be attributed solely to cytokinin deprivation. Received February 27, 1998; accepted March 3, 1998     

Photocontrol of petiole elongation in light-grown strawberry plants

Summary The mode of phytochrome control of elongation growth was studied in fully-green strawberry (Fragaria x Ananassa Duch.) plants. Petiole growth showed two distinct types of response to light. In one, the end-of-day response, petioles were lengthened by low-intensity far-red irradiation for 1 h immediately following the 8 h photoperiod. The response was little or no greater with prolonged exposure and less when the start of far-red was delayed. It was already evident in the first leaf to emerge after treatment began. With the development of successive leaves a second, photoperiodic, type of response appeared, in which petioles lengthened following only prolonged exposure to red, far-red, mixtures of the two, or tungsten lighting, all at low levels of intensity. As with the inhibition of flowering in previous experiments, irradiation with red light during the second half of the otherwise long dark period gave the greatest response.Abbreviations and Symbols FR far-red light - HIR high irradiance response - R red light - P_r phytochrome in the red light absorbing form - P_fr phytochrome in the far-red light absorbing form - SDP short-day plant - LDP long-day plant - PAR photosynthetically active radiation     

Photoperiodism and enzyme rhythms: Kinetic characteristics of the photoperiodic induction of Crassulacean acid metabolism

Summary The effect of photoperiod on Crassulacean acid metabolism (CAM) in Kalanchoe blossfeldiana Poellniz, cv. Tom Thumb, has characteristics similar to its effect on flowering in this plant (although these two phenomena are not causally related). The photoperiodic control of CAM is based on (a) dependance on phytochrome, (b) an endogenous circadian rhythm of sensitivity to photoperiodic signals, (c) a balance between specific positive (increase in enzyme capacity) and negative (inhibitory substances) effects of the photoperiod. Variations in malate content, capacity of phosphoenolpyruvate (PEP) carboxylase, and capacity of CAM inhibitors in young leaves were measured under photoperiodic conditions noninductive for CAM and after transfer into photoperiodic conditions inductive for CAM. Essential characteristics of the photoperiodic induction of CAM are: 1) lag time for malate accumulation; 2) after-effect of the inductive photoperiod on the malate accumulation, on the increase in PEP carboxylase capacity, and on the decrease in the level of long-day produced inhibitors; final levels of malate, enzyme capacity and inhibitor are proportional to the number of inductive day-night cycles; 3) cireadian rhythm in PEP carboxylase capacity with a fixed phase under noninductive photoperiods and a continuously shifting phase under inductive photoperiods, after complex advancing and delaying transients. Kinetic similarities indicate that photoperiodic control of different physiological functions, namely, CAM and flowering, may be achieved through similar mechanisms. Preliminary results with species of Bryophyllum and Sedum support this hypothesis. Phase relationships suggest different degrees of coupling between endogenous enzymic rhythm and photoperiod, depending on whether the plants are under long days or short days.     

An assessment of morphogenetic fluctuation during reproductive phase change in Arabidopsis

Background and Aims

Reproductive phase change in Arabidopsis thaliana is characterized by two transitions in phytomer identity, the differentiation of the first elongate internode (bolting transition) and of the first flower (floral transition). An evaluation of the dynamics of these transitions was sought by examining the precision of the corresponding phytomer identity changes.

Methods

The length of the first elongate internode and the frequency of chimeric inflorescence structures, e.g. paraclades not subtended by a leaf (no-leaf/paraclades) and flowers subtended by a bract (bract/flowers), were measured in the Wassilewskija (Ws) accession and 47 early flowering mutants under a wide range of photoperiods. The impact of photoperiodic perturbations applied to Ws plants at different times of development was also evaluated.

Key Results

In Ws, both types of characters were remarkably constant across photoperiods in spite of a high degree of interindividual variability. Bract/flowers were not normally produced in Ws, but they were observed in conditions that suggest enhanced light signalling, e.g. in response to continuous light perturbations and in mutants with reduced hypocotyl elongation. In contrast, no-leaf/paraclades were normally present in approx. 20 % of Ws plants, and their frequency was increased in conditions that suggest reduced light signalling, e.g. in mutants with altered specification of long-day responses. The length of the first elongate internode was unrelated to the rate of stem elongation and to the regulation of reproductive phase change.

Conclusions

Bract/flowers and no-leaf/paraclades corresponded to opposite effects on the floral transition that reflected different dynamics of progression to flowering. In contrast, the length of the first elongate internode was only indirectly related to the regulation of reproductive phase change and was mainly dependent on global morphogenetic constraints. This paper proposes that morphogenetic variability could be used to identify critical phases of development and characterize the canalization of developmental patterns.     

Are two photoreceptors involved in the flowering of a long-day plant?

The effect of daylength extension with narrow spectral bands on the flowering of a long-day plant, Brassica campestris L. cv. Ceres, was investigated to obtain clues to the identity of the photoreceptor involved. Extension of a 9 h photoperiod with 5 h of light pulses at various wavelengths resulted in maximal flowering occurring after irradiation at 710 nm, less at 730 nm, and none at 550, 660 and 750 nm. Flowering at 710 and 730 nm was negated by simultaneous exposures at 550 nm, but not at 660 nm. A short preirradiation at 660 nm enabled a following irradiation at 750 nm to induce flowering. This latter induction was prevented by 550 nm irradiation.
Short flashes of light at 710 nm induced flowering that was negated by a following flash at 550 nm but not at 660 nm. The negation by 550 nm radiation was prevented by subsequent flashes at 710 nm, indicating photoreversibility. A flash at 660 nm enabled subsequent light flashes at 750 nm to initiate flowering that was reversed by a following 550 nm flash.
From the results showing the necessity of red and far-red lights, it is proposed that flowering in this long-day plant is due to two photoreceptors - one is phytochrome and the other an unknown pigment with far-red, green photoreversible properties. By using fluence response data, it is deduced that the unidentified photoreceptor has weak absorption bands in the far-red, but has a strong absorption band in the green. Flowering is induced when effects of red light absorbed by phytochrome interact with effects of far-red light absorbed by the unidentified photoreceptor.     

Bulblet Formation in Achimenes longiflora

The bulblet formation in Achimenes longiflora D.C. cv. Major was investigated under controlled conditions in growth chambers. The bulblet formation in Achimenes longiflora cuttings was found to be inhibited by long-day treatment such as continuous illumination. The degree of inhibition depended, however, on the quality of the light. High intensity far red and especially infrared emission was needed to effectively suppress the bulbing. As opposed to this, red light was found to be stimulatory. When red light (660 nm) was substituted for 20% of the white light, the bulbing was promoted over 400%. Gibberellic acid was totally inhibitory in all cases. The cytokinins benzyladenine and 6-γ,γ-dimethylallylaminopurine were strongly promotive and even inductive under inhibitory light conditions. The growth inhibitor N-dimethylamino succinamic acid also stimulated bulblet formation. α-Naphthyl-acetic acid was strongly inhibitory, an effect which could not be overcome by benzyladenine.     

Photoperiodism and photocontrol of stem elongation in two photomorphogenic mutants of Pisum sativum L.

The photomorphogenic mutation lv in the garden pea (Pisum sativum L.), which appears to reduce the response to light-stable phytochrome, has been isolated on a tall, late photoperiodic genetic background and its effects further characterised. Plants possessing lv have a reduced flowering response to photoperiod relative to wild-type plants, indicating that light-stable phytochrome may have a flower-inhibitory role in the flowering response of long-day plants to photoperiod. In general, lv plants are longer and have reduced leaf development relative to Lv plants. These differences are maximised under continuous light from fluorescent lamps (containing negligible far-red (FR) light), and decrease with addition of FR to the incident light. Enrichment of white light from fluorescent lamps with FR promotes stem elongation in the wild type but causes a reduction in elongation in the lv mutant. This “negative” shade-avoidance response appears to be the consequence of a strong inhibitory effect of light rich in FR, revealed in lv plants in the absence of a normal response to red (R) light. These results indicate that the wild-type response to the R: FR ratio may be comprised of two distinct photoresponses, one in which FR supplementation promotes elongation by reducing the inhibitory effect of R, and the other in which light rich in FR actively inhibits elongation. This hypothesis is discussed in relation to functional differentiation of phytochrome types in the light-grown plant. Gene lw has been reported previously to reduce internode length and the response to gibberellin A₁, and to delay flowering. The present study shows that the lw mutation confers an increased response to photoperiod. In all these responses the lw phenotype is superficially “opposite” to the lv phenotype. The possibility that the mutation might primarily affect light perception was therefore considered. The degree of dwarfing of lw plants was found to depend upon light quality and quantity. Dwarfing is more extreme in plants grown under continuous R light than in those grown in continuous FR or blue light or in darkness. Studies of the fluence-rate response show that the lw mutation imparts a lower fluence requirement for inhibition of elongation by white light from fluorescent lamps. Dark-grown lw plants are more strongly inhibited by a R pulse than are wild-type plants but, as in the wild type, this inhibition remains reversible by FR. Light-grown lw plants show an exaggerated elongation response to end-of-day FR light. Taken together, these findings indicate that the lw mutant may be hypersensitive to phytochrome action.     

Biliverdin reductase-induced phytochrome chromophore deficiency in transgenic tobacco

Targeted expression of mammalian biliverdin IXalpha reductase (BVR), an enzyme that metabolically inactivates linear tetrapyrrole precursors of the phytochrome chromophore, was used to examine the physiological functions of phytochromes in the qualitative short-day tobacco (Nicotiana tabacum cv Maryland Mammoth) plant. Comparative phenotypic and photobiological analyses of plastid- and cytosol-targeted BVR lines showed that multiple phytochrome-regulated processes, such as hypocotyl and internode elongation, anthocyanin synthesis, and photoperiodic regulation of flowering, were altered in all lines examined. The phytochrome-mediated processes of carotenoid and chlorophyll accumulation were strongly impaired in plastid-targeted lines, but were relatively unaffected in cytosol-targeted lines. Under certain growth conditions, plastid-targeted BVR expression was found to nearly abolish the qualitative inhibition of flowering by long-day photoperiods. The distinct phenotypes of the plastid-targeted BVR lines implicate a regulatory role for bilins in plastid development or, alternatively, reflect the consequence of altered tetrapyrrole metabolism in plastids due to bilin depletion.     

Wavelength Dependency of Light-Induced Effects on Photoperiodic Clock in the Migratory Blackheaded Bunting (Emberiza melanocephala)

The effects of light wavelength on photoperiodic clock were determined in the migratory male blackheaded bunting (Emberiza melanocephala). We constructed an action spectrum for photoperiodic induction (body fattening, gain in body mass, and gonadal recrudescence) by exposing birds for 4.5 weeks to 13 h light per day (L:D = 13:11 h) of white (control), blue (450 nm), or red (640 nm) color at irradiances ranging from 0.028 to 1.4 W m^?2. The threshold light irradiance for photoinduction was about 10-fold higher for blue, compared to red and white light. Phase-dependent effects of light wavelength on the photoperiodic clock were further examined in the next two sets of skeleton photoperiods (SKPs). In the first set of SKPs, birds were exposed for four weeks to asymmetrical light periods (L:D:L:D = 6:6:1:11 h) at 0.25 ± 0.01 W m^?2; two light periods applied were of the same (450 nm: blue:blue, B:B; 640 nm, red:red, R:R) or different (blue:red, B:R or red:blue, R:B) wavelengths, or of white:white (W:W, controls). Photoperiodic induction occurred under R:R and B:R, but not under B:B and R:B light conditions; the W:W condition induced an intermediate response. The second set of SKPs used symmetrical light periods (L:D:L:D = 1:11:1:11 h), and measured effects also on the activity rhythm. Birds were first exposed to one of the four SKPs (R:R, B:B, R:B, or B:R) for three weeks, subsequently were released into dim constant light (LL_dim; ?0.01 W m^?2, the night light used in an L:D cycle) for two weeks, and then were returned to respective SKPs for another three weeks. Activity was greater in the R:R compared to B:B, and in B:R compared to R:B light condition. Zugunruhe (intense nighttime activity, indicating migratory restlessness in a caged situation) developed under the R:R and B:R, but not the B:B and R:B, light condition. Under LL_dim, all birds free-ran with a period >24 h, the Zugunruhe had a circadian period longer than the daytime activity, and the re-entrainment to SKPs was influenced by the position of light periods relative to circadian phase of the activity rhythm. Photoperiodic induction at the end of 8 weeks was found in the R:R and B:R, but not in B:B, light conditions; in the R:B condition only one bird had initiated testes. Taken together, these results suggest that in the blackheaded bunting, the circadian photoperiodic clock is differentially responsive to light wavelengths; this responsiveness is phase-dependent, and the development of Zugunruhe reflects a true circadian function. Wavelength-dependent response of the photoperiodic clock could be part of an adaptive strategy in evolution of the seasonality in reproduction and migration among photoperiodic species under wild conditions.     

Light conditions of photoperiodic induction of flowering inChenopodium murale L. ecotype 197 - Early flowering long-day plant

A method of cultivation and effectiveness of different light sources and light regimes in photoperiodic induction of flowering in non-rosette long-day plantChenopodium murale L. ecotype 197 are described. Under the described conditions of cultivation 5 days, of continuous light produced by incandescent bulbs (TESLA 74 3x40 W, red 4.9 μWcm-²nn-¹, far-red 7.4 μWcn-2nm-¹, blue 0.25 μW cm-²nn-¹) induced flowering in the majority of plants.     

Growth control by phytochrome of de-etiolated bean hypocotyls mediated by chlorophyll fluorescence

The elongation of hypocotyls excised from de-etiolated seedlings of beans (Phaseolus vulgaris L. cv. British Wax) is inhibited by light, blue and red irradiations being equally effective. Conditions which decrease chlorophyll fluorescence, such as CO₂-free air, abolish the inhibitory effect of blue irradiation and enhance the inhibition by red light. Conversely, conditions which increase chlorophyll fluorescence, such as a N₂ atmosphere or irradiation through a chlorophyll filter, abolish the inhibitory effect of red light and enhance the inhibition by blue irradiation. The inhibitory effect of blue light is reversible by red irradiation under increased fluorescence as well as by far red. We propose that the chlorophyll fluorescence excited by blue and red irradiations in λ_F > 660 nm and λ_F > 720 nm, respectively, is responsible for the inhibitory effect of blue light and the reduction of the inhibitory effect of non fluorescing red light. Both red and blue wavelengths seem, therefore, to control hypocotyl elongation through phytochrome.     

Stem elongation and floral initiation on in vitro chicory root explants: influence of photoperiod

Chicory root explants (Cichorium intybus L.) were cultured in vitro under different photoperiods. In complete darkness, strong stem elongation, but no flowering induction was observed. We suggest that this stem elongation could be homologous to the pit growth in chicory heads in vivo. Under a photoperiod of 12 h (LI=±40 E m^–2 s^–1), only vegetative growth was observed. Photoperiods of 16 h or more light a day induced the in vitro explants to develop stems bearing flower buds. When the in vitro cultures were kept in the dark for different durations starting from the first day of culture and afterwards transferred to long-day conditions, 4 days dark were sufficient to cause a decrease in flowering induction. We suggest that during the dark culture, a flowering inhibitory process was started.     

Phytochromes A1 and B1 have distinct functions in the photoperiodic control of flowering in the obligate long-day plant Nicotiana sylvestris

The obligate long-day plant Nicotiana sylvestris with a nominal critical day length of 12 h was used to dissect the roles of two major phytochromes (phyA1 and phyB1) in the photoperiodic control of flowering using transgenic plants under-expressing PHYA1 (SUA2), over-expressing PHYB1 (SOB36), or cosuppressing the PHYB1 gene (SCB35). When tungsten filament lamps were used to extend an 8 h main photoperiod, SCB35 and SOB36 flowered earlier and later, respectively, than wild-type plants, while flowering was greatly delayed in SUA2. These results are consistent with those obtained with other long-day plants in that phyB has a negative role in the control of flowering, while phyA is required for sensing day-length extensions. However, evidence was obtained for a positive role for PHYB1 in the control of flowering. Firstly, transgenic plants under-expressing both PHYA1 and PHYB1 exhibited extreme insensitivity to day-length extensions. Secondly, flowering in SCB35 was completely repressed under 8 h extensions with far-red-deficient light from fluorescent lamps. This indicates that the dual requirement for both far-red and red for maximum floral induction is mediated by an interaction between phyA1 and phyB1. In addition, a diurnal periodicity to the sensitivity of both negative and positive light signals was observed. This is consistent with existing models in which photoperiodic time measurement is not based on the actual measurement of the duration of either the light or dark period, but rather the coincidence of endogenous rhythms of sensitivity - both positive and negative - and the presence of light cues.