Evidence for a graft-transmissible substance which delays apical senescence in Pisum sativum L.

Apical senescence in an early flowering line of pea, G2, is greatly delayed by short days. This behavior is controlled by two dominant genes. Apical senescence of ungrafted, insensitive (I) lines is unaffected by photoperiod. When I-type scions with one of the two required genes were grafted onto G2, apical senescence of the I-type was delayed in short days, but not in long days. Flowering of the I-type was unaffected. The apex of the G2 stock was unaffected as well. Apical senescence of an I-type line lacking both photoperiod genes was not delayed when grafted on G2 in short days. It is concluded that G2 plants grown in short days produce a graft-transmissible factor which delays apical senescence of photoperiodically insensitive lines.     

The cell cycle in the shoot apex ofSilene during the first day of floral induction

Summary The length of the cell cycle was measured in the shoot apical meristem ofSilene coeli-rosa during the first day of an inductive photoperiod. The length of the cell cycle in the shoot apex of vegetative controls (those in short days) was about 18–20 hours. Exposure of plants to the long day resulted in an immediate shortening of the cell cycle to about 13 hours, roughly two thirds of that in short days. Measurements of the component phases of the cell cycle revealed that the shortened cycle in long days was the result of a decrease in the length of G 1 and perhaps S, whilst G 2 and M remained constant.     

The relationship between fruit growth and apical senescence in the G2 line of peas

In the G2 line of peas (Pisum sativum L.), senescence of the shoot apex (which precedes leaf senescence) only occurs in long days (LD) though flowering is independent of photoperiod. It has been suggested that the photoperiodic control of senescence in G2 is mediated through different rates of seed growth. In LD seed growth is more rapid than in short days (SD) and this places a greater nutrient drain on the plant. In addition, more flowers develop into fruits in LD than in SD: 32% of flower buds abort in SD while almost none abort in LD. Senescence is associated with early seed growth and does not occur in deflowered or deseeded plants. Seed development is completed in 30d in LD while it takes 40d in SD, though the seed weights are similar. The maximum rate of fresh-weight gain of all the growing seeds of eight fruits on a plant in SD (1,440 mg/d) does not reach the maximum rate of weight gain of a similar fruit complement in LD (1,720 mg/d). The appearance of senescence symptoms in the shoot apices of LD-grown G2 plants occurs, however, prior to the time of the greatest rate of seed-weight gain. In LD, four fruits with a combined maximum growth rate of 1,250 mg/d are sufficient to cause the appearance of senescence symptoms. This is a lower combined seed growth rate than in SD where senescence does not occur. The seeds in up to 12 fruits can be growing at any time in SD with a combined maximum seed-growth rate (1,660 mg/d), only slightly less than the maximum in LD, with no sign of senescence. It is concluded that the different rates of seed growth occasioned by different photoperiods bear no relation to senescence. However, photoperiod does alter the spatial relationship of the shoot apex and the filling fruits. In LD apical growth becomes slower as fruiting proceeds so that the distance between the filling fruits and the apex is decreased to only two nodes while in SD, because of the delayed fruit development compared to LD, the spatial separation between the fruits and the shoot apex is nine nodes. Even if the growth rate of the plant had remained constant in LD it is calculated that an equivalent fruit complement would still be located three nodes further from the apex in SD than in LD. This increased spatial separation of fruits and apex in SD compared to LD probably alters the source/sink distribution of photosynthate and leaf derived hormones so that larger amounts are available to the apex in SD than LD. Also any senescence factor exported from fruits is less likely to reach the apex in SD. In continuously deflorated plants of G2 the two uppermost expanded stipules enclose the apex in SD while in LD they open out. The effect is reversible. Thus photoperiod probably affects the apex and its growth, directly, i.e. independent of fruit development, and this is accentuated by the differing spatial relationships of the apex and fruits resulting from different fruit growth rates under the different photoperiodic conditions.Abbreviations LD long day(s) - SD short day(s)     

Photoperiodic and genetic control of carbon partitioning in peas and its relationship to apical senescence

Apical senescence but not flower initiation is delayed by short days (SD) compared to long days (LD) in pea plants (Pisum sativum L.) of genotype E Sn Hr. We recently reported that delay of senescence correlated with slower reproductive development, suggesting that fruits are weaker sinks for assimilates under delayed senescence conditions. Thus, we have examined assimilate partitioning in peas to determine if genotype and photoperiod regulate relative sink strength. Assimilate diversion by developing fruit has been implicated in senescence induction. A greater percentage of leaf-exported ¹⁴C was transported to fruits and a smaller percentage to the apical bud of G2 peas (genotype E Sn Hr) in LD than in SD. Relatively more of the ¹⁴C delivered to the apical bud of G2 peas was transported to flower buds than to young leaves in LD as compared to SD. There was no striking photoperiodic difference in carbon partitioning in genetic lines without the Sn Hr allele combination. The Sn Hr allele combination and photoperiod may regulate the relative strength of reproductive and vegetative sinks. Photoperiodic differences in sink strength early in reproduction suggest that these genes regulate sink strength by affecting the physiology of the whole plant. High vegetative sink strength in SD may maintain assimilate supply to the apical bud, delaying senescence.     

Genetic and Photoperiodic Control of the Relative Rates of Reproductive and Vegetative Development in Peas

The rates of leaf and flower production were determined in peas(Pisum sativum L.) of genotypes e sn hr (line 13), E Sn hr (line60), and E Sn Hr (line G2), to assess the role of the interactionof alleles Sn and Hr with photoperiod in development. The ratesat which flowers at successive nodes opened (AR) and leavesat successive nodes unfolded (PR) were constant. The AR wasfaster than the PR so that successive flowers opened at nodescloser to the apical bud. The rate at which this occurred wasindependent of photoperiod in line 13 but was slightly or markedlyslower in short days (SD) than long days (LD) in lines 60 andG2, respectively. The opening of flowers closer to the apicalbud of G2 peas in SD was so slow as to not be visually apparentduring the time of this study. The number of nodes between thefirst open flower and the apical bud was unaffected by photoperiodin line 13 but was greater in SD than LD in lines 60 and G2.The daylength effects are photoperiodic, since development ofG2 peas in LD with respect to the parameters measured was unaffectedby light intensity. It is concluded that photoperiod and theE Sn allele combination control the rate of reproductive developmentrelative to vegetative development in peas. The effects of ESn are magnified by the presence of the Hr allele. The constantrates of development measured are not consistent with declineof Sn allele expression with age. Delay of the rate of reproductivedevelopment relative to vegetative development correlated withdelay of apical senescence, suggesting that these processesare related. Pisum sativum, genotypes, photoperiod, flowering, reproductive development, vegetative development, senescence     

Export of organic materials from developing fruits of pea and its possible relation to apical senescence

In the G2 line of peas (Pisum sativum L.) senescence and death of the apical bud occurs only in long days (LD) in the presence of fruits. Removal of the fruits prevents apical senescence. One possible reason for the senescence-inducing effect of fruit is that the fruits produce a senescence-inducing factor which moves to the apical bud and is responsible for the effect. For this to be possible there must be a transport mechanism by which material may move from the pods to the apex. To examine the extent of fruit export, pods were labeled via photoassimilation of ¹⁴CO₂ beginning 12 days after anthesis. Under LD conditions, 1.14% of label fixed was transported from the pods with only 10.5% of this found in the apical bud and youngest leaves after 48 hours, the remainder being found principally in other developing fruits and mature leaves. During the onset of apical senescence, less total label was actually exported to the apical bud than at other times. In addition, more total export occurred from pods in short days than in LD, with the apical bud receiving a greater percentage than in LD. Thus the amount and distribution of export would not seem to support the idea of specific export of targeted senescence-promoting compounds. Girdling of the fruit peduncle did not change the characteristics of export suggesting movement via an apoplastic xylem pathway.     

Photoperiod-induced changes in gibberellin metabolism in relation to apical growth and senescence in genetic lines of peas (Pisum sativum L.)

In an early-flowering line of pea (G2) apical senescence occurs only in long days (LD), while growth in short days (SD) is indeterminate. In SD, G2 plants are known to produce a graft-transmissible substance which delays apical senescence in related lines that are photoperiod-insensitive with regard to apical senescence. Gibberellic acid (GA₃) applied to the apical bud of G2 plants in LD delayed apical senescence indefinitely, while N⁶-benzyladenine and -naphthaleneacetic acid were ineffective. Of the gibberellins native to pea, GA₉ had no effect whereas GA₂₀ had a moderate senescence-delaying effect. [³H]GA₉ metabolism in intact leaves of G2 plants was inhibited by LD and was restored by placing the plants back in SD. Leaves of photoperiod-insensitive lines (I-types) metabolized GA₉ readily regardless of photoperiod, but the metabolites differed qualitatively from those in G2 leaves. A polar GA₉ metabolite, GA_E, was found only in G2 plants in SD. The level of GA-like substances in methanol extracts from G2 plants dropped about 10-fold after the plants were moved from SD to LD; it was restored by transferring the plants back to SD. A polar zone of these GA-like materials co-chromatographed with GA_E. It is suggested that a polar gibberellin is synthesized by G2 plants in SD; this gibberellin promotes shoot growth and meristematic activity in the shoot apex, preventing senescence.Abbreviations GA gibberellin - GA₃ gibberellic acid - SD short days - LD long days     

An appraisal of the use of reciprocal transfer experiments: assessing the stages of photoperiod sensitivity in chrysanthemum cv. Snowdon (Chrysanthemum morifolium Ramat.)

Reciprocal transfer experiments can be used to describe the stages of photoperiod sensitivity in day-length-sensitive plants. However, there are inconsistencies in the literature concerning the techniques used and, more importantly, the assumptions made when analysing such data sets. This paper appraises the use of reciprocal transfer experiments, with chrysanthemum as a model (short day) plant.Experiments showed little evidence to suggest that axillary meristems were incapable of responding to a floral stimulus when released from apical dominance by pinching (even though the apex appeared vegetative). Five short days given after pinching resulted in sufficient induction to initiate an inflorescence, although seven short days were required to commit a plant to flower with a leaf number similar to plants grown in continuous short days. Floral initiation was then visible at the apex after nine short days. Once the inflorescence had been initiated, long days delayed the early stages of flower development.The results are discussed with reference to reciprocal transfer experiments in general, and specifically in relation to problems that arise when the length of a 'juvenile' phase is confounded with the number of inductive cycles for flower commitment.     

Apical Growth Cessation and Shoot Tip Abscission in Salix

Time course of apical shoot growth and shoot tip abortion in northern ecotypes (lat. 69°39′N, long. 18°37′E) of Salix pentandra and S. caprea have been investigated. In trees more than 15 years old growing under natural climatic conditions apical growth cessation and shoot tip abortion normally occurred in June-July when the day length still was 24 h. Application of GA₃, in spring to the apex effectively delayed growth cessation and shoot tip abortion. Application of kinetin was without effect. First-year seedlings of both species grew continuously at temperatue of 9 to 24°C in 24 h photoperiod. Short days induced apical growth cessation, but two to four (S. pentandra) or three to five (S. caprea) weeks of 12 h photoperiod were required to stop the elongation growth. The results indicated that the critical photoperiod for apical growth cessation in the used ecotype of S. pentandra was 16 to 18 h at 18°C. Short days had a minor effect only on the formation of apical leaf primordia in small seedlings. Development of axillary buds and radial growth were stimulated by short days when compared with long days. Small seedlings of both species (3 to 8 cm high at the start) formed terminal buds in short days, but in large seedlings (more than about 15 cm high) apical growth cessation was accompanied by shoot tip abortion. Abscisic acid applied to the apex or through a leaf did not induce growth cessation in S. pentandra seedlings grown in continuous light. The growth retardants CCC, B-9 and Phosphon D reduced growth rate under continuous light and induced shoot tip abortion in some plants. The effect of CCC was counteracted by GA₃. Apical growth cessation in short days was significantly delayed by a single GA₁ application.     

Quantitative Trait Loci for Thermal Time to Flowering and Photoperiod Responsiveness Discovered in Summer Annual-Type Brassica napus L

Time of flowering is a key adaptive trait in plants and is conditioned by the interaction of genes and environmental cues including length of photoperiod, ambient temperature and vernalisation. Here we investigated the photoperiod responsiveness of summer annual-types of Brassica napus (rapeseed, canola). A population of 131 doubled haploid lines derived from a cross between European and Australian parents was evaluated for days to flowering, thermal time to flowering (measured in degree-days) and the number of leaf nodes at flowering in a compact and efficient glasshouse-based experiment with replicated short and long day treatments. All three traits were under strong genetic control with heritability estimates ranging from 0.85–0.93. There was a very strong photoperiod effect with flowering in the population accelerated by 765 degree-days in the long day versus short day treatments. However, there was a strong genetic correlation of line effects (0.91) between the long and short day treatments and relatively low genotype x treatment interaction indicating that photoperiod had a similar effect across the population. Bivariate analysis of thermal time to flowering in short and long days revealed three main effect quantitative trait loci (QTLs) that accounted for 57.7% of the variation in the population and no significant interaction QTLs. These results provided insight into the contrasting adaptations of Australian and European varieties. Both parents responded to photoperiod and their alleles shifted the population to earlier flowering under long days. In addition, segregation of QTLs in the population caused wide transgressive segregation in thermal time to flowering. Potential candidate flowering time homologues located near QTLs were identified with the aid of the Brassica rapa reference genome sequence. We discuss how these results will help to guide the breeding of summer annual types of B. napus adapted to new and changing environments.     

Libido and scrotal circumference of rams as affected by season of the year and altered photoperiod

Two trials were conducted to evaluate the influence of season of the year and altered photoperiod on libido and scrotal circumference (SC). A 30-minute serving-capacity test was used to measure ram libido. the measures of libido were reaction time (RT), time from entry into the pen to first mount and/or service, total mounts (M), and total services (S). The serving-capacity test was conducted by placing a ram with four estrus-induced ewes and measuring RT and counting M and S. Prior to each serving-capacity test, SC was measured for each ram. Rams were tested every two weeks. In Trial I, eleven two-year-old Rambouillet rams from each of three selection lines -a high line (four rams; selected on the basis of high prolificacy), a low line (three rams; selected on the basis of low prolificacy) and a random bred control line (four rams) - were used in a one-year study. Rams were exposed to ambient conditions throughout the year. Rams were more active during the short days of fall and winter, i.e. normal breeding season, as evidenced by a greater number of total mounts and services plus a shorter reaction time. Selection line affected reproductive parameters measured, with the high line having more M and S and a shorter RT than the low line. However, SC was larger in the low line. In Trial II twelve rams were divided into two groups of six. The control group was exposed to ambient conditions from April 18 through July 24. The treated group was exposed to eight hours of light and 16 hours of darkness (8L:16D) from April 18 through July 24, simulating short days of fall and winter. Total services (S) in the 30-minute test interval were higher for rams subjected to the 8L:16D treatment (P<0.01; 2.7+/-0.2 vs 1.6+/-0.2 for 8L:16D and control, respectively). SC was 31.7+/-0.2 vs 30.2+/-0.2 for 8L:16D and control, respectively (P<0.01). Total mounts in 30 minutes were not affected by treatment (6.9+/-0.8 vs 5.7+/-0.8 for 8L:16D and control, respectively; P>0.10). No significant differences in any of the reproductive parameters between treatment groups were observed during the first 28 days. However, there were significant differences (P<0.05) observed between 8L:16D rams and control rams for SC during 42 to 84 days and S between days 42 to 70, respectively. Serving-capacity tests carried out about one month following the end of altered photoperiod trial showed no significant differences between treated and control rams, thus indicating that treatment had no carry-over effect.     

Effect of Photoperiod on Photosynthate Partitioning and Diurnal Rhythms in Sucrose Phosphate Synthase Activity in Leaves of Soybean (Glycine max L. [Merr.]) and Tobacco (Nicotiana tabacum L.)

Studies were conducted to identify the existence of diurnal rhythms in sucrose phosphate synthase (SPS) activity in leaves of three soybean (Glycine max L. [Merr.]) and two tobacco (Nicotiana tabacum L.) cultivars and the effect of photoperiod (15 versus 7 hours) on carbohydrate partitioning and the rhythm in enzyme activity. Acclimation of all the genotypes tested to a short day (7 hours) photoperiod resulted in increased rates of starch accumulation, whereas rates of translocation, foliar sucrose concentrations, and activities of SPS were decreased relative to plants acclimated to long days (15 hours). Under the long day photoperiod, two of the three soybean cultivars (`Ransom' and `Jupiter') and one of the two tobacco cultivars (`22NF') studied exhibited a significant diurnal rhythm in SPS activity. With the soybean cultivars, acclimation to short days reduced the activity of SPS (leaf fresh weight basis) and tended to dampen the amplitude of the rhythm. With the tobacco cultivars, photoperiod affected the shape of the SPS-activity rhythm. The mean values for SPS activity (calculated from observations made during the light period) were correlated positively with translocation rates and were correlated negatively with starch accumulation rates. Overall, the results support the postulate that SPS activity is closely associated with starch/sucrose levels in leaves, and that acclimation to changes in photoperiod may be associated with changes in the activity of SPS.     

Early photoperiod history and short-day responsiveness in Siberian hamsters

Siberian hamsters exhibit seasonal, photoperiod influenced cycles of reproductive activity, body size, pelage characteristics, and thermoregulatory behavior. Laboratory populations generally exhibit inter-individual variability in expression of photoperiod responsiveness, with a subset of individuals that fail to show the species typical responses to short photoperiod. This variability is partly explained by a genetic component, as it has been possible to increase the number of short-day nonresponders by artificial selection. Responsiveness to short photoperiod is also substantially influenced by photoperiod history in this species; hamsters that have been raised under long (16L) or very long (18L) day lengths are less likely to exhibit winter-type responses to short days as compared to hamsters raised under an intermediate (14L) day length. In the present experiment, we examined effects of age and early photoperiod history in a strain of Siberian hamsters that had been selected for short-day nonresponsiveness. Hamsters transferred into short photoperiod on the day of birth were uniform in exhibiting winter-type responses. However, hamsters raised until 25 days of age in either continuous illumination or in 16L exhibited variation in responsiveness when subsequently moved into short photoperiod. We conclude that virtually all hamsters of the short-day nonresponsive strain are born responsive to short days. Subsequent development of resistance to potential short day effects is dependent on age and/or photoperiod history.     

Postflowering photoperiod regulates vegetative growth and reproductive development of soybean

Soybean development is controlled by environmental factors, primarily photoperiod and temperature. To date, photoperiod effects on flowering have been well studied but the performances and mechanism of postflowering photoperiod responses have not been fully understood, especially for the photoperiod effects on vegetative growth after flowering. In the present study, the responses of vegetative growth and reproductive development in soybean to different postflowering photoperiod regimes were investigated in four separate experiments. Three varieties of different maturity groups (MG) including the early (Dongnong 36, MG 000), medium (Dandou 5, MG IV), and late (Zigongdongdou, MG IX) were exposed to two photoperiods, short (10, 12 h) and long (15, 16 or 18 h). The results showed that postflowering photoperiod not only regulated reproductive development but also affected vegetative growth. Even when flowers and pods were removed, short-day (SD) treatment promoted leaf senescence. The onset of leaf senescence among varieties tested appeared to be dependent on photoperiod sensitivity. Leaf senescence of the late-maturing variety of Zigongdongdou (sensitive to photoperiod) was delayed more significantly than that of the medium and early-maturing varieties (less sensitive to photoperiod). Long-day (LD) treatments delayed leaf senescence and seed maturation in the late-maturing variety of Zigongdongdou plants with only the SD-induced leaves produced before flowering. LD treatments imposed from the beginning bloom, beginning pod setting or beginning seed filling delayed leaf senescence and seed maturation of late-maturing soybean variety (Zigongdongdou). Results of night-break with red (R) and far-red (FR) light demonstrated that postflowering photoperiod responses of soybean were R/FR reversible reactions and the phytochromes seemed to be functional as receptors of photoperiod signals even after flowering. It was proposed that the regulation of photoperiod on development of soybean was effective from emergence through maturation, and the postflowering photoperiod signals were also mediated by phytochromes similar to those before flowering. The flowering reversion in late-MG soybean varieties under LD was a direct result of LD and was not due to secondary effect of abscission of pods and flowers. Soybean leaves not only received SD signals but also LD signals; furthermore, the LD effects reversed the SD effects and vice versa.     

Genetic and environmental influences on short-day responsiveness in Siberian hamsters (Phodopus sungorus)

Siberian hamsters are photoperiodic rodents that typically exhibit several physiological changes when exposed to a short-day photoperiod. However, development of the winter phenotype in short days is largely conditional on prior photoperiod history: Hamsters that have been reared in an exceptionally long day length (18 L) do not usually exhibit the winter phenotype after transfer to short days, whereas animals reared under "moderately" long days (16 L) are more variable in responsiveness to subsequent short-day exposure, with 20% to 30% generally failing to exhibit winter-type responses. Hamsters reared exclusively in an "intermediate" day length (14 L) are almost uniformly responsive to short photoperiod. In the present study, the authors examine the influence of photoperiod history on short-day responsiveness in a breeding line of hamsters that has been subjected to artificial selection for resistance to the effects of short days. The results demonstrate that photoperiod history is an important determinant of short-day responsiveness in both random-bred (UNS) hamsters and animals artificially selected and bred for nonresponsiveness to short photoperiod (PNR). The PNR hamsters have a reduced requirement for long-day exposure to evoke a state of unresponsiveness to short days. The results are discussed in relation to possible significance for the origin of population and species differences in photoperiod responsiveness.     

Hypothalamic Ventricular Ependymal Thyroid Hormone Deiodinases Are an Important Element of Circannual Timing in the Siberian Hamster (Phodopus sungorus)

Exposure to short days (SD) induces profound changes in the physiology and behaviour of Siberian hamsters, including gonadal regression and up to 30% loss in body weight. In a continuous SD environment after approximately 20 weeks, Siberian hamsters spontaneously revert to a long day (LD) phenotype, a phenomenon referred to as the photorefractory response. Previously we have identified a number of genes that are regulated by short photoperiod in the neuropil and ventricular ependymal (VE) cells of the hypothalamus, although their importance and contribution to photoperiod induced physiology is unclear. In this refractory model we hypothesised that the return to LD physiology involves reversal of SD expression levels of key hypothalamic genes to their LD values and thereby implicate genes required for LD physiology. Male Siberian hamsters were kept in either LD or SD for up to 39 weeks during which time SD hamster body weight decreased before increasing, after more than 20 weeks, back to LD values. Brain tissue was collected between 14 and 39 weeks for in situ hybridization to determine hypothalamic gene expression. In VE cells lining the third ventricle, expression of nestin, vimentin, Crbp1 and Gpr50 were down-regulated at 18 weeks in SD photoperiod, but expression was not restored to the LD level in photorefractory hamsters. Dio2, Mct8 and Tsh-r expression were altered by SD photoperiod and were fully restored, or even exceeded values found in LD hamsters in the refractory state. In hypothalamic nuclei, expression of Srif and Mc3r mRNAs was altered at 18 weeks in SD, but were similar to LD expression values in photorefractory hamsters. We conclude that in refractory hamsters not all VE cell functions are required to establish LD physiology. However, thyroid hormone signalling from ependymal cells and reversal of neuronal gene expression appear to be essential for the SD refractory response.     

Ethylene Synthesis and Floral Senescence following Compatible and Incompatible Pollinations in Petunia inflata

Ethylene production and floral senescence following compatible and incompatible pollinations were studied in a self-incompatible species, Petunia inflata. Both compatible and incompatible pollinations resulted in a burst of ethylene synthesis that peaked 3 hours after pollination. P. inflata pollen was found to carry large amounts of the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC). The amount of pollen-held ACC varied in different genetic backgrounds, and the magnitude of the peak correlated with the amount of ACC borne by the pollen. Aminooxyacetic acid (AOA), an inhibitor of ACC synthesis, had no inhibitory effect on this ethylene response, indicating that pollen-borne ACC was largely responsible for the early synthesis of ethylene. After compatible pollination, a second increase in ethylene synthesis began at 18 hours, and the first sign of senescence appeared at 36 hours. Upon treatment with AOA, the second phase of ethylene production was reduced by 95%, indicating that endogenous ACC synthesis was required for this phase of ethylene synthesis. AOA treatment also delayed senescence to 6 days after anthesis. After incompatible pollination, a second increase in ethylene production did not occur until 3 days, and the first sign of senescence occurred 12 hours later. Unpollinated flowers showed an increase in ethylene production 3 to 4 days after anthesis and displayed signs of senescence 1 day later. The significance of the early and late phases of pollination-induced ethylene synthesis is discussed.     

Induction of leaf senescence in Arabidopsis thaliana by long days through a light-dosage effect

Given the influence of photoperiod on reproductive development and whole-plant senescence in monocarpic plants, one would suspect that leaf senescence in these plants might be under photoperiodic control. In Arabidopsis thaliana , which is monocarpic and also a nonobligate long-day (LD) plant, LDs (16 h, 300 μmol m⁻² s⁻¹) caused leaves to die earlier than did short days (SDs, 10 h). Since leaf longevity was not paralleled by the reproductive development in the present study, the reproductive structures did not seem to be the primary controls of leaf senescence. The LD effect appeared to depend on the amount of light rather than on day length, for leaves given LDs at reduced light intensity (180 μmol m⁻² s⁻¹) lived longer than those in LDs with full light. In addition, the higher light intensity promoted chlorophyll loss and anthocyanin accumulation in LDs. Thus, senescence of these leaves seems to be governed by light dosage rather than photoperiod. Light may play a natural role in promoting the senescence of A. thaliana leaves.     

Studies on the Physiology of Flowering of Timothy (Phleutm pratense L.): I. Influence of Daylength and Temperature on Initiation and Differentiation of the Inflorescence

Spikelet initiation is advanced and the proportion of plantswhich attain the reproductive condition is increased in S. 48timothy by lengthening the photo-period from 14 to 24 hours.In shorter periods of light, reproduction is almost completelyinhibited, and in 8-hour short days plants remain vegetativeeven after 35 weeks. Spikelet initiation at the shoot apex occursafter exposure to 3–5 long days followed by short days.Initiation also occurs when extended daylength is replaced by‘light-breaks’ during long nights, or when a singleleaf is photo-induced while the remainder of the plant receivesshort days. High temperatures promote spikelet initiation incontinuous light; in photoperiods nearer the threshold for floweringthis response is reversed and a rise in temperature from 55°to 75° F. increasingly inhibits reproduction. Once initiationhas occurred, spike differentiation is hastened by increasesin temperature or photoperiod. Internode elongation begins atthe time of spikelet initiation, and is promoted by temperatureand photoperiod. Elongated vegetative shoots may be producedwhen spikelet initiation fails in threshold photoperiods orhigh temperatures.     

Short day lengths affect perinatal development of the male reproductive system in the Siberian hamster, Phodopus sungorus.

The Siberian hamster, Phodopus sungorus, breeds seasonally. In the laboratory, seasonal breeding can be controlled by photoperiod, which affects the duration of nightly melatonin secretion. Winterlike, short day lengths induce gonadal regression in adult animals, and pups born and maintained in short days undergo pubertal gonadal development later than animals born into long days. However, to date there have been no reports of gestational photoperiod affecting fetal development of reproductive systems. The spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus (BC) and levator ani (LA), compose a sexually dimorphic, androgen-sensitive neuromuscular system involved in male reproduction. The SNB neuromuscular system was studied in male Siberian hamsters maintained from conception in short-day (8 h light, 16 h dark; 8L:16D) versus long-day (16L:8D) conditions. On the day of birth, and at postnatal (PN) days 2 and 18, the BC/LA muscles of hamsters gestated and raised in the short photoperiod were significantly reduced relative to those of their long-day counterparts. Testes weights were not significantly different between groups until day 18. Thus, photoperiod exposure during gestation and after birth affects perinatal development of the SNB system in this species, and these effects can be seen as early as the day of birth. Because photoperiod did not significantly affect testes weights until PN18, these results suggest that either perinatal photoperiod affects fetal androgen production without affecting testes weight or it influences BC/LA development independently from androgen.