PHOTOPERIOD IN XANTHIUM POPULATIONS FROM TEXAS AND MEXICO

Photoperiodic responses of Xanthium strumarium L. originating between 19° N in Mexico and 34° N in Texas varied among seedlings grown from seed under controlled conditions. The critical night lengths form a gradient from 9.5 hr in northern Texas to 10.75 hr in southern Texas and northeastern Mexico. Populations with critical night lengths of 9.5 and 9.75 hr showed a longer interval to flower bud formation under cooler temperatures (24 C day/15 C night) than under warmer temperatures (30 C day/24 C night). Three of four populations with a 10.75-hr critical night length showed a shorter interval under the cooler temperature regime. Although the Texas populations demonstrate a strong correlation of photoperiodic response with latitude, the Mexico populations show diverse photoperiodic timing from approximately the same latitude. The study emphasizes that a combination of critical night length and ripeness-to-flower (maturity) response forms the basis for reproductive adaptation in different climatic regimes in Texas and Mexico.     

THE EFFECT OF SHORT PERIODS OF HIGH TEMPERATURE DURING DAY AND NIGHT PERIODS ON PEA YIELDS

Karr , E. J. (Ohio State U., Columbus), A. J. Linck , and C. A. Swanson . The effect of short periods of high temperature during day and night periods on pea yields. Amer. Jour. Bot. 46(2) : 91-93. Illus. 1959.—The effect of high temperatures during periods of relatively short duration (3-4 days) at various stages following anthesis at the first bloom node was studied in relation to yield of peas at this node. Except for the periods of differential temperature treatments, the plants were maintained in a standard environment room (24°C., light, 12 hr.; 15°C., darkness, 12 hr.). Three different temperature regimes during the treatment periods were studied: high day temperature—standard night temperature (32°—15°C.) ; standard day temperature—high night temperature (24°—30°C.) ; and high day and night temperatures combined (32°—30°C.). The data reveal the existence of a relatively well-defined thermal-sensitive period, with maximal sensitivity to high day temperatures occurring at about 9-11 days from full bloom, and maximal sensitivity to high night temperatures occurring about 6-9 days from full bloom. High night temperatures proved more critical, resulting in a maximal reduction of 25% in yield, as opposed to about 8% for high day temperatures. The effect of high day and night temperatures combined tended to be roughly additive.     

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

The 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.     

Photoperiodic time measurement in the aphid Megoura viciae

Megoura produces parthenogenetic virginoparae in long day conditions, gamic oviparae in short days. The nature of this photoperiodic response has been analysed by rearing parent apterae in a wide range of circadian and non-circadian light cycles. By varying the light and dark components independently in a two-component cycle it has been established that the time measuring function is associated primarily with the dark period. There is no evidence that an endogenous circadian oscillation is implicated: thus (a) the ‘short day’ response is abolished by ‘night interruptions’ positioned in the early or late night. But this bimodal response pattern remains unchanged when the duration of the ‘main’ photoperiod is varied from ca. 6 hr to at least 25·5 hr. The stability of the maxima within the scotophase is inconsistent with the ‘coincidence’ models of photoperiodic timing that have been proposed. It is suggested that the essential timing process operates on the hour-glass principle, beginning anew with the onset of each period of darkness; (b) night interruption experiments employing very long (up to 72 hr) scanned dark periods yielded response maxima explicable in terms of the hour-glass hypothesis but did not reveal any circadian relationship between the maxima.The ‘dark reaction’ comprises a sequence of four stages, definable by the effects of light. Stage 1, extending from dark hr 0 to ca. 2·5, is fully photoreversible: at the next dark period the entire timing sequence is repeated up to the 9·5 hr critical night length. Towards the end of stage 1 reversibility is gradually lost and after a light interruption the reaction is resumed from a later time equivalent than dark hr 0; the subsequent critical night length is therefore reduced. The extent of the photoreversal is related to light duration. The period of maximum light insensitivity (stage 2) is attained at the end of the fourth hour. From ca. dark hr 5 to just short of the critical night length light exerts an increasingly promotive action which favours the production of virginoparae. This dark process is not photoreversible. Stage 4, which begins at hr 9·5, marks the end of the timing sequence. Light will not then annul the non-promotive action of the previous long night.Light has three effects which are determined by its duration and position within the cycle. The two terminal effects, mentioned above, are associated with the interception of dark stages 1 and 3 by either short (1 hr) or longer photoperiods. Light also prepares or primes the dark period timer. Thus the critical length is increased, and timing accuracy lost, if the preceding photoperiod is less than ca. 6 hr. Light during stage 4 has a priming action but no terminal function. Repeated cycles are ‘read’ in various ways, depending on the cycle structure. For example, if light intercepts stage 3, a two-component cycle is interpreted as the overlapping sequence light/dark/light. One and the same photoperiod then acts terminally in respect of the preceding dark period and as a primer for the next dark period.There is also a mechanism for summing the promotive effects produced by repeated interruption of dark stage 3. With complex (four-component) cycles both halves of the same cycle may contribute. ‘Product accumulation’ falls below threshold if the frequency of presentation of a given promotive cycle is too low. This occurs if there are very long, relatively non-promotive dark components. Such cycles are accepted as ‘continuous darkness’.     

Geographical variation and inheritance of the photoperiodic response controlling larval diapause in two distinct voltine ecotypes of the Asian cornborer Ostrinia furnacalis

The Asian corn borer Ostrinia furnacalis Guenée (Lepidoptera: Crambidae) enters facultative diapause as fully‐grown larvae in response to short day lengths during autumn. As a result of geographical variations in photoperiodic response, the moths from Nanchang (28.8°N, 115.9°E; NC strain) judge both LD 14 : 10 h and LD 15 : 9 h photocycles as long days and develop directly, whereas moths from Haerbin (44.9°N, 127.2°E; HB strain) judge the same photocycles as short days and enter diapause. Crosses between the two strains are used to evaluate the inheritance of diapause. The critical day lengths for diapause induction in the HB strain are significantly longer than those in the NC strain at all temperatures. The critical day length of F₁ progeny is intermediate between the two strains. However, the critical day length in all crosses is significantly longer with HB strain fathers or grandfathers than with NC strain fathers or grandfathers, indicating that the male parent has significantly more influence on the critical day length of subsequent progeny than the female. The results from all crosses under LD 14 : 10 h or LD 15 : 9 h photocycles at 25 °C show that the inheritance of diapause in O. furnacalis does not fit a purely additive hypothesis and that the capacity for diapause is transmitted genetically in the manner of incomplete dominance. The incidence of diapause for F₁ progeny under an LD 14 : 10 h photocycle is significantly higher than that under an LD 15 : 9 h photocycle, suggesting that the induction of diapause can be influenced by interactions between the F₁ genotype and photoperiod.     

Photoperiodic responses ofXanthium strumarium L. (Compositae) introduced and indigenous in eastern asia

Photoperiodic responses among populations ofXanthium strumarium L. in eastern Asia from Vladivostok, USSR, to Hong Kong and Taiwan include a wide range of adaptation. Among indigenous populations of thestrumarium morphological complex, responses range from near day neutrality (Vladivostok) to approximately a night length requirement of 9.5 hr (Hong Kong). Introduced populations in Japan include a night length requirement of 9.75 hr in theitalicum morphological complex and of 10.25–10.5 hr in thechinense morphological complex. The range of photoperiodic adaptation is nearly as wide as among populations of North America.     

Shifts in the semidian rhythm of flowering do determine night-break timing and critical night length in Pharbitis nil

There is a semidian (≈12 h) rhythm in the flowering response of the short-day plant Pharbitis nil Choisy following 90 min exposure to either far-red light/darkness or a temperature drop (27 °C to 12 °C) given at various times in constant conditions before an inductive dark period. This semidian rhythmic response to the temperature-drop pretreatments in the light is also evident through the inductive dark period without change of phase. Furthermore, those pretreatments which increase flowering also advance the time of maximum sensitivity to red light (R) interruptions of the dark period by up to 1.5 h and shorten the critical night length. Conversely, pretreatments which reduce flowering delay the time of maximum R inhibition by up to 1.5 h and increase the critical night length by the same amount. However the phase of a circadian rhythm of flowering response had no effect on either the time of maximum R inhibition or the critical night length. Thus, the semidian rhythm determines both the time of maximum R inhibition and the critical night length in Pharbitis. Received: 8 November 1997 / Accepted: 7 January 1998     

EXPERIMENTS ON THE PHOTOPERIODIC RESPONSE IN PINEAPPLE

Gowing , Donald P. (Pineapple Research Institute of Hawaii, Honolulu.) Experiments on the photoperiodic response in pineapple. Amer. Jour. Bot. 48(1): 16–21. 1961.—The initiation of flowering of ‘Smooth Cayenne’ pineapple plants is neither strictly a response to photoperiod (day lengths of 10 hr. 51 min.–13 hr. 24 min.) nor to a minimum temperature (minima from 50° to 72°F. in different areas) under natural Hawaiian conditions. Depending on the kind of planting material used and the time of planting, natural initiation of flowering may take place any month of the year. Slips planted in the fall generally initiate flowering in December of the following year. However, exposure of an 8-mo.-old slip-planting to a day length of 8 hours for 40 days starting Sept. 8 induced flowering irrespective of night temperatures from about 60 to 80°F. Interruption of the dark period by illumination at 30 ft.-c. from midnight to 1 a.m. suppressed the inductive effect. Lowering the night temperature to 60°F. was, of itself, non-inductive. Field-grown, 11-mo.-old plants treated in place responded similarly, in that 25 periods of 8-hr. day length starting Sept. 5 induced 60% of the plants to flower, and the night illumination suppressed the inductive effect as before. Daily application of 0.12 mg. of the major native pineapple auxin (indole-3-acetic acid) at the beginning of the dark period had no detectable effect on the short-day treatment, and similar application of an antiauxin (4-chlorophenoxyisobutyric acid) did not affect the suppression of flowering by the light-break. Supplemental illumination of field-grown 12-mo. plants to provide a photoperiod of more than 15 hr. daily from Nov. 4 to Jan. 30 did not suppress the natural initiation of flowering which occurred in early December (day length about 10 hr. 50 min.). ‘Smooth Cayenne’ pineapple is therefore a quantitative, but not an obligate, short-day plant.     

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

The 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 > 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.     

Seasonal adaptations of populations of the southwestern corn borer, Diatraea grandiosella, from tropical and temperate regions

Seasonal adaptations of populations of the southwestern corn borer, Diatraea grandiosella, obtained from south-central Mexico (19°N latitude) and southeast Missouri (37°N latitude) were compared. Day length and temperature were found to serve as environmental cues to programme the larval diapause of both populations, but different critical values were observed. The critical day length for diapause induction was about 13 hr light/day for Mexican larvae and about 15 hr light/day for Missouri larvae, and was relatively stable at 20 to 30°C. Mexican larvae displayed a less-intense diapause than did Missouri larvae. Some diapausing Mexican larvae maintained at 25 or 30°C pupated in about 15 days, regardless of the day length to which they were exposed. The rate of diapause development of Mexican larvae was high at day lengths between 14 hr and 16 hr, whereas that of Missouri larvae was accelerated at day lengths of 16 hr at 25 and 30°C. Diapause development of Mexican larvae was virtually unaffected by chilling at 10°C, whereas that of Missouri larvae continued at a low rate at 10°C. Selection of Mexican larvae for diapause showed that only four generations were needed to significantly increase the incidence of diapause.     

The photoperiodic control of wing development in the black bean aphid,Aphis fabae

Newly born presumptive gynoparae of Aphis fabae were transferred from their prenatal short-day rearing conditions (light-dark 12:12, 15°C) to a variety of postnatal photoperiodic regimes. Long days prevented wing formation and the majority of aphids developed into apterous or alate-apterous intermediate adults. Continued short days resulted, almost exclusively, in winged adults. The photoperiodic-response curve (T = 24) revealed a critical photoperiod of light-dark 13.5:10.5 and further investigations showed that morphogenesis depended more upon night- than day-length. Maximal apterization occurred with 8 or 9 h dark regardless of the length of the photophase but at photophases of 12 h or less the critical night length was reduced to 8.5 h. Night-interruption experiments revealed two peaks of photosensitivity when the scotophase was 12–14 h long but only a single peak was seen with longer dark periods. A series of experiments using early night interruptions followed by extended dark revealed apparent critical night lengths which decreased as the interruption was placed later in the scotophase. Resonance experiments involving 12 or 16 h photophases and extended scotophases at 15°C revealed long-day effects with scotophases shorter than critical and short-day effects with longer dark periods. However, similar regimes at 20°C produced three peaks of apterization 24 and 20 h apart indicating the possibility of a circadian element involved in the photoperiodic response. The results are compared with the photoperiodic responses of other insects.     

Inheritance of photoperiodic induction of larval diapause in the Asian corn borer Ostrinia furnacalis

The Asian corn borer Ostrinia furnacalis (Guenée) enters facultative diapause as fully‐developed larvae in response to short‐day conditions. As a consequence of geographical variation in photoperiodic response, moths from Nanchang (28°46′N, 115°50′E) enter diapause in response to short day‐lengths (D strain), even at the high temperatures whereas moths from Ledong (18°47′N, 108°89′E) exhibit almost no diapause under the same conditions (N strain). In the present study, crosses between the two strains are used to evaluate the inheritance of diapause under different photoperiods at temperatures of 22, 25 and 28 °C. The moths, both reciprocal crosses and backcrosses, show a clear long‐day response, similar to that of the D strain, suggesting that the photoperiodic response controlling diapause in this moth is heritable. However, the critical day‐length for induction of diapause is shorter in hybrids than in the D strain. The N strain also shows a short‐day photoperiodic response at the lower temperature of 22 °C, indicating that the N strain still has the capacity to enter a photoperiodically‐induced diapause, depending on the rearing temperature. The incidence of diapause in all crosses is highest with D strain fathers or grandfathers and lowest with N strain fathers or grandfathers, indicating that the male parent has significantly more influence on the incidence of diapause of subsequent progeny than the female. The results obtained from all crosses under LD 12 : 12 h or LD 13 : 11 h photocycles at 25 °C show that inheritance of diapause in O. furnacalis does not fit an additive hypothesis and that the capacity for diapause is transmitted genetically in the manner of incomplete dominance.     

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

Photoperiodic response curves were determined for two clones of the black bean aphid, Aphis fabae Scopoli, at three temperatures, 12.5, 15 and 17.5°C. Critical night lengths for the induction of winged females in an English clone (52° N) were 10.5, 11 and 11.5 h, respectively, and 10, 10.5 and 11 h in a Scottish clone (57° N). Critical night lengths for male induction were 10.5, 11 and 11 h at 12.5, 15 and 17.5°C in the English clone, and 10, 10.5 and 10.5 h, respectively, in the Scottish clone. High incidences of winged females and males were observed at all scotophases longer than the critical night length in both clones. In addition, in the English clone, the incidences of winged female and male producers in continuous darkness were 0% at 15 and 17.5°C, and 6% at 12.5°C. In the Scottish clone, however, continuous darkness resulted in high incidences of both winged female and male producers at 12.5 and 15°C, but 0% winged female producers and 6% male producers at 17.5°C. In scotophases shorter than the critical night length, including continuous light, no males or winged females were observed in either clone under the non‐crowded rearing conditions used. The results are discussed in terms of the ‘double circadian oscillator model’ for photoperiodic induction.     

The behaviour and coupling of the photoreceptor and hour-glass photoperiod timer at low temperature in the aphid Megoura viciae

In Megoura viciae photoperiodic time is measured during the scotophase by an hour-glass mechanism which is temperature compensated within the range 6–20°C. At lower temperatures the clock “loses time”, running at about half the normal rate at 0°C. The present experiments, based on the analysis of critical night lengths, showed that for short periods of scotophase chilling (4h or less) the same diminished rate is maintained irrespective of the position of the low temperature period within the scotophase. This occurs despite the known changes in spectral sensitivity of the photoreceptor. With longer periods of chilling (>6h) a normal critical response was not observed unless several hours of darkness intervened between the end of the chilling period and the temperature-modified critical night length. This is interpreted as a “recovery” process involving the photoreceptor but not the hour-glass. Additional data indicated that the sequence of changes in the photoreceptor, including the early resetting phase, is closely coupled to the progression of the timer even when photosensitivity is temporarily impaired by low temperature.     

Some effects of temperature on the hour glass photoperiod timer in the aphid Megoura viciae

“Resonance” experiments conducted at 3 temperatures do not support the view that the Megoura photoperiod (scotophase) timer has an oscillatory component. The main effects of temperature, which are consistent with an hour glass model, have been examined by exposing the aphids to temperatures ranging from −3 to 30°C for the first 4 h of the scotophase, subsequently returning them in each cycle to 15°C for measurement of the critical night length. During the temperature treatments the insects were removed from the plant to avoid indirect effects resulting from changes in the rate of development (this procedure causes a slight shift in the critical night length from 9.5 to 9.0 h). The timer was found to be almost perfectly compensated between 6 and 20°C, but at lower temperatures it runs slow, only one quarter the normal elapsed time being measured at −3°C. At higher temperatures (25 and 30°C) considerable irregularities appear, eventually resulting in the elimination of the critical night length. The results suggest that the timer may sometimes be stopped.Light perception was assessed by cooling the aphids in light. Photoreception was impaired in some individuals at temperatures as high as +6°C and became negligible at −3°C. The illuminated phase was then accepted as a dark interval of equal duration. Yet at this temperature the timer was still operating, albeit slowly. Their differing temperature thresholds point to some degree of separation in these two functions.High-temperature (25°C) experiments with continously feeding aphids showed that although scotophase heating was more effective, there was a distinct response when high temperatures were synchronized with the photophase. This was attributed to the operation of a “photoperiod counter” mechanism.     

THE PHOTOPERIODIC RESPONSE OF AMERICAN PROSOPIS AND ACACIA FROM A BROAD LATITUDINAL DISTRIBUTION

Prosopis and Acacia from North and South America were studied in the laboratory under constant and increasing night length under two temperature regimes. The Prosopis juliflora (Sw.) DC. complex showed strong populational differentiation; Acacia farnesiana (L.) Willd. showed minimal populational differentiation. In Prosopis a population from Oklahoma (36° N) showed the greatest inhibition of stem height under increasing night length. A population from Peru (6° S) showed least control of stem height by night length. In Acacia the response of the latitudinal series was relatively uniform within each of the 6 sets of light period-temperature conditions. In general the height increase for Acacia was two to three times greater under the warmer regime than under the cooler regime at a given night length. Since Acacia farnesiana occurs sympatrically with the Prosopis juliflora complex over a broad range of latitudes, it is of interest to find that the distributional mechanisms of these two leguminous trees are basically different.     

Cellular Lipid Fatty Acid Pattern Heterogeneity Between Reference and Recent Food Isolates of Listeria Monocytogenes as a Response to Cold Stress

Cells of four reference strains (Scott A, LO 28, CNL 895807 and ATCC 19115) and of five recent food isolates (A00M011, A00M018, A00M087, A00M092 and A00M123) of Listeria monocytogenes were grown until late exponential phase in Brain Heart Broth at two different temperatures (37 °C and 4 °C). Our results show that significant differences exist between the cellular lipid fatty acid profile of reference and recent food isolates. Like the reference strains, and in keeping with previous reports on the cellular lipid fatty acid profile of L. monocytogenes, the recent food isolates were characterised by the presence of ai15:0, i15:0 and ai17:0. In addition, the fatty acid ai13:0 was observed in all of the recent food isolates grown at 4 °C, whereas only two reference strains, Scott A and LO 28, showed ai13:0 in their cellular lipid fatty acid profile at 4 °C. When grown at 4 °C, the recent food isolates showed a mean aiC₁₅/aiC₁₇ ratio of 66, while reference strains were characterised by significantly lower ratios, ranging between 4.3 (ATCC 19115) and 28.9 (Scott A). These results showed that all of the recent food isolates, Scott A and LO28 strains use chain length and anteiso-branching (ai15:0) as their major response to cold temperature adaptation. However, the cold adaptation response of reference strains CNL 895807 and ATCC 19115 appears to be different.     

EXPERIMENTAL HYBRIDIZATION OF XANTHIUM STRUMARIUM (COMPOSITAE) FROM ASIA AND AMERICA. I. RESPONSES OF F1 HYBRIDS TO PHOTOPERIOD AND TEMPERATURE

Responses to photoperiod and temperature were compared for hybrids between Asiatic plants in the indigenous strumarium morphological complex and plants in five American complexes: oviforme, italicum, chinense, cavanillesii, and pennsylvancium. The F₁ hybrids between Hong Kong plants and various American plants showed intermediacy in photoperiodic response; however, the hybrid night requirement was more similar to that of the American parent. The Hong Kong plants are difficult to evaluate photoperiodically but showed a night length requirement of 9.25–9.50 hr. Day-neutral plants from India in the strumarium complex produced day-neutral F₁ hybrids in crosses with most American plants having night requirements less than 10 hr. The F₁ hybrids involving the day-neutral Indian plants and either Indian or Australian chinense plants showed a night requirement of 8.25–8.75 hr. The chinense parental plants had apparent critical nights of 10.25–10.50 hr. Crosses between the day-neutral Indian plants and Mexican plants with apparent critical nights of 10.75–11 hr produced F₁ hybrids requiring nights of 8.75–9.00 hr. The various hybrids tended to show the broader temperature tolerances of the American parents. The ripeness-to-flower (maturity) responses of seedlings tended to show the genetic controls of the parent with the faster developmental rate. The hybridization evidence suggests that photoperiodic responses are quantitatively controlled and inherited independently of morphology and ripeness-to-flower responses. The populations of India are highly diverse and probably reflect recombinations of photoperiodic and temperature responses between indigenous day-neutral plants and photoperiodic chinense plants introduced from North America.     

Thermosensitive genetic dwarfs of apple

The role of environment on the dwarfing (short internode) phenomenon of apple (Malus domestisca Borkh.) was investi gated and defined in controlled environmental chambers. Orchard-grown very dwarf, dwarf and semi-dwarf trees obtained by natural sibcrossing of spur-type cv. Golden Delicious and cv. Delicious, as well as standard cv. Golden Delicious, were propagated via in vitro techniques. Growth was rapid and none of the 4 types exhibited dwarf-like characteristics when grown at constant 27°C with 12, 14 or 16 h daylengths. Standard and very dwarf plants grew at nearly the same rate at constant 30°C, whereas growth nearly ceased on both types at constant 35°C after 7 days. Dwarf and very dwarf plants responded differently from standard and semi-dwarf plants when grown under alternating (ramped) night/day temperatures (15 or 20°C night ramped up to a daytime maximum over 8 h of 23, 28, 33 or 38°C, held for 2 h and then ramped down over 5 h to the night temperature). As the night/maximum day temperature differentia) increased from 0 to 23° under the ramping environments, growth of dwarf plants decreased markedly as compared to standard plants. When the same night/maximum day temperature differential occurred, the effect on decreasing shoot length was greater at the higher (20°C) night temperature. Increasing maximum day temperatures under the ramped environment also reduced leaf area plant^?1 but did not markedly affect leaf number, resulting in short internodes. When a period of constant temperature was followed by ramped temperatures or vice versa, the sequence of constant vs ramped environments made little difference in the final growth of the 4 plant types. The data point to high temperature as the major factor for causing dwarfing of the sensitive plant types. Increasing the differential between night and maximum day temperature resulted in short internode. dwarf plants with small leaves similar to orchard-grown dwarf trees.     

Effect of photoperiod and temperature on apical growth cessation in two ecotypes of Salix and Betula

Apical growth cessation as affected by photoperiod and temperature has been studied in seedlings of two latitudinal ecotypes of Salix and Betula. The critical photoperiod for apical growth cessation at constant temperatures of 15 and 21°C was about 22 h for a northern (69°C39′N) and about 15–16 h for a southern (59°C40′N) ecotype of Salix pentandra. Fluctuating day/night temperatures (21°C/9°C, 15°C/6°C) induced apical growth cessation in northern ecotypes even at 24–h photoperiod. Disagreements in critical photoperiods found in various studies are discussed.