Floral Initiation of Upland Cotton Gossypium hirsutum L. in Response to Temperatures

The effect of germination temperature, duration of high-intensitylight, and day temperature in modifying the influence of nighttemperature on the flowering process of the M-8 strain of Uplandcotton was examined. In general, night temperatures above 28°C caused the first floral branch to be formed at a higher node.The magnitude of the reaction was conditioned by the other environmentalfactors studied. Germination temperature had a slight but significanteffect on subsequent floral responses to night temperature.Plants given eight-hour periods of high-intensity light eachday were delayed more by high night temperature than those exposedto 14 or 24 hours of high light. At high day temperatures (28–32°C) the inhibiting influence of the high night temperature wasgreatly increased. High day temperatures delayed floral initiationif the night temperature was high (28–32°C) but causeda lowering of position of first floral branch when the nighttemperature was low (20–22°C). The enhancement offlowering by 32°C days and 22°C nights was expressednot only in the low node of first floral branch, but also inthe shorter time from planting to floral initiation.     

EXPERIMENTAL CONTROL OF FLOWERING IN LEMNA. II. SOME EFFECTS OF MEDIUM COMPOSITION,CHELATING AGENTS AND HIGH TEMPERATURES ON FLOWERING IN L. PERPUSILLA 6746

Hillman , William S. (Yale U., New Haven, Conn.) Experimental control of flowering in Lemna. II. Some effects of medium composition, chelating agents and high temperatures on flowering in L. perpusilla 6746. Amer. Jour. Bot. 46(7): 489–495. Illus. 1959.—-L. perpusilla 6746 flowers as a short-day plant on Hutner's medium (containing ethylenediaminetetraacetic acid [EDTA]) at constant temperatures from 25 to 30°C., but eventually flowers also in old cultures under 16 or 24 hr. of light. This old-culture flowering is more pronounced in dilute medium. Flowering is rapid under both long and short days at constant temperatures from 25 to 28°C. in media not containing EDTA; the addition of 10^-5 M EDTA or of similar or higher concentrations of numerous other chelating agents suppresses flowering under long days but not under short (8 hr. light). This effect does not depend on promotion or inhibition of vegetative growth. At 29 to 30°C., a short-day requirement is manifested even in media permitting flowering under long days at the lower temperatures. Temperatures above 31°C. completely inhibit flowering under all conditions. Brief periods of high temperature given to plants under short-day conditions inhibit flowering when given during the dark period but not during the light period. The implications of these observations for the further study of flowering are discussed.     

PHOTOCONTROL OF GROWTH AND FLOWERING OF CARYOPTERIS

Piringer , A. A., R. J. Downs , and H. A. Borthwick . (U. S. Dept. of Agriculture, ARS, Belts ville, Md.) Photocontrol of growth and flowering of Caryopteris . Amer. Jour. Bot. 50(1): 86–90. Illus. 1963.—Flower buds were initiated on plants of Caryopteris × clandonensis A. Simmonds (C. incana [Houtt.] Miq. × C. mongholica Bunge) on all photopcriods but developed to anthesis only when daily dark periods exceeded 8 hr. Anthesis occurred in not less than 22 days after the beginning of 11 or more short photoperiods. Treatments with short days could be interrupted by as many as 30 non-inductive long days without significant increase in the minimum number of short days required for anthesis. Anthesis, like floral initiation of many plants, was reversibly controlled by red and far-red radiation acting through phytochrome. The inductive effectiveness of long dark periods was nullified by 1 min of red light or about 1 hr of far red. It was modified by night temperature in the range 45–70.F and filament lengths of stamens were shorter at night temperatures of 60 than at 70 F.     

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.     

Physiological responses of Opuntia ficus-indica to growth temperature

The influences of various day/night air temperatures on net CO₂ uptake and nocturnal acid accumulation were determined for Opuntia ficus-indica, complementing previous studies on the water relations and responses to photosynthetically active radiation (PAR) for this widely cultivated cactus. As for other Crassulacean acid metabolism (CAM) plants, net nocturnal CO₂ uptake had a relatively low optimal temperature, ranging from 11°C for plants grown at day/night air temperatures of 10°C/0°C to 23°C at 45°C/35°C. Stomatal opening, which occurred essentially only at night and was measured by changes in water vapor conductance, progressively decreased as the measurement temperature was raised. The CO₂ residual conductance, which describes chlorenchyma properties, had a temperature optimum a few degrees higher than the optimum for net CO₂ uptake at all growth temperatures. Nocturnal CO₂ uptake and acid accumulation summed over the whole night were maximal for growth temperatures near 25°C/15°C, CO₂ uptake decreasing more rapidly than acid accumulation as the growth temperature was raised. At day/night air temperatures that led to substantial nocturnal acid accumulation (25°C/15°C.). 90% saturation of acid accumulation required a higher total daily PAR than at non-optimal growth temperatures (10°C/0°C and 35°C/25°C). Also, the optimal temperature of net CO₂ uptake shifted downward when the plants were under drought conditions at all three growth temperatures tested, possibly reflecting an increased fractional importance of respiration at the higher temperatures during drought. Thus, water status, ambient PAR, and growth temperatures must all be considered when predicting the temperature response of gas exchange for O. ficus-indica and presumably for other CAM plants.     

Effects of thermoperiods on the reproductive activity of females and on the maternal induction of larval diapause in the blowfly, <Emphasis Type="Italic">Calliphora vicina</Emphasis> R.-D. (Diptera,Calliphoridae)

The interaction of thermoperiod and photoperiod in their influence on the reproductive maturation of females and on the induction of the maternal effect determining larval diapause of the progeny of the blowfly, Calliphora vicina, was first investigated under laboratory conditions. Under the combination of a day length of 12 h with a thermoperiod (the alternation of 12 h long periods with temperatures of 10 and 20°C) the reproductive maturation of females was faster than at the corresponding mean constant temperature of 15°C. Under the “natural” thermoperiod, when the period with a temperature of 10°C coincided with “night-time” (the dark phase of the diurnal light-dark cycle) the maturation of females was slower than that under the “inverted” thermoperiod, when the period with a temperature of 10°C coincided with “day-time” (the light phase of the diurnal light-dark cycle). The proportion of diapausing individuals was maximal in the progeny of females kept at 20°C and decreased with the increase in temperature. Under thermoperiods (the alternations of 12 h long periods with temperatures of 20 and 26°C) the proportion of diapausing progeny was lower than that under the corresponding mean constant temperature of 23°C, but under the inverted thermoperiod with a high night temperature this effect was much stronger. In combination with the results of our previous studies, these data support the hypothesis that the effects of “night” and “day” temperatures are substantially different only when the thermal response interacts with a strong photoperiodic response.     

Influence of temperature and ethephon concentration on growth and composition of Cabernet Sauvignon grapevines

Potted Cabernet Sauvignon grapevines were acclimated to two different temperature regimes (25°C/15°C and 35°C/25°C day/night temperatures, respectively) until 100% bloom, when the vines were treated with either 0, 250, 500, or 750 ppm ethephon (Ethrel^®). Three days after ethephon application all vines were combined and held at 25°C/15°C in a phytotron room for 15 weeks. Growth was suppressed by a greater range of ethephon concentrations at the cool temperature, but effects were shorter-lived than at the high temperature. Generally, the 500 ppm treatment reduced vigor most effectively. The degree to which ethephon influences vine growth is mediated by temperature. Ethephon and temperature treatments caused significant differences in the concentrations of potassium, calcium, and magnesium in leaves.     

REVERSIBLE DEACTIVATION OF NEUROSPORA ASCOSPORES BY LOW TEMPERATURE

Sun, Clare Y., and Alfred S. Sussman. (U. Michigan, Ann Arbor.) Reversible deactivation of Neurospora ascospores by low temperature. Amer. Jour. Bot. 47(7): 589-593. Illus. 1960.—Heat-activated ascospores of Neurospora tetrasperma are reversibly deactivated after incubation at 4°C. for 36–48 hr. Two cycles of deactivation and reactivation are possible although the percentage germination decreases in the last cycle. By contrast, spores held at 20°C., or in glycerol at 4°C., will remain activated for much longer periods of time. If an incubation period at 20°C. greater than 30 min. is interposed before the activated spores are placed at 4°C., germination occurs despite the cold-treatment. Furfural-activated ascospores, when held at 4°C., are deactivated but can be reactivated only by heat, pointing up a difference between ascospores activated by these different means. Although a fraction of the stimulus afforded by heat-sensitization to chemical activators is preserved for 2 days at —20°C., it is dissipated completely after a short time at 4°C. These data are discussed on the basis of the suggestion that the reversible production of a substance initiates a series of steps which lead to germination. Thus, the temperature minimum of the forward reaction is greater than 4°C. whereas the back reaction proceeds at this temperature.     

Effects of temperature on virus-host relationships and on activity of the noninclusion virus of citrus red mites, Panonychus citri

The nonoccluded virus of citrus red mite retained full infectivity when exposed to 40.5°C for 24 hr within intact mite bodies but was inactivated at 46°C for 6 hr and 60°C for 1 hr. Exposures to 38°C for 28 days failed to destroy infectivity. Virus inoculated mites exposed to different temperature regimens had shortened periods of lethal infection at high temperatures and greatly lengthened periods of lethal infection at cool temperatures suggesting that failures in mite control by virus in the early spring and late fall may be due to previously unrecognized temperature relationships.     

Reduction in seed set upon exposure to high night temperature during flowering in maize

High temperature reduces crop production; however, little is known about the effects of high night temperature (HNT) on the development of male and female reproductive organs, pollination, kernel formation and grain yield in maize (Zea mays L.). Therefore, a temperature-controlled experiment was carried out using heat-sensitive maize hybrid and including three temperature treatments of 32/22°C (day/night; control), 32/26°C and 32/30°C during 14 consecutive days encompassing the flowering stage. When exposed to 30°C night temperature, grain yield and kernel number reduced by 23.8 and 25.1%, respectively, compared with the control. The decrease in grain yield was mainly because of the lower kernel number rather than change in kernel weight under HNT exposure around flowering. No significant differences in grain yield and kernel number were found between 22 and 26°C night temperatures. HNT had no significant effects on the onset of flowering time and anthesis-silking interval but significantly reduced time period of pollen shedding duration and pollen viability, and increased leaf night respiration. Different from high daytime temperature, HNT had no lasting effects on daytime leaf photosynthesis, biomass production and assimilate transportation. From the perspective of source–flow–sink relationship, the unchanged source and flow capacities during daytime are supposed to alleviate the adverse effects on sink strength caused by HNT compared with daytime heat stress. These new findings commendably filled the knowledge gaps concerning heat stress in maize.     

COMPARISON OF DAYLENGTH AND TEMPERATURE RESPONSES IN NICOTIANA AND ITS TAXONOMIC SECTIONS

Steinberg , Robert A. (U.S.D.A., Beltsville, Md.) Comparison of daylength and temperature responses in Nicotiana and its taxonomic sections. Amer. Jour. Bot. 46(4): 261–268. Illus. 1959.—Fifty-seven of the sixty species of Nicotiana were grown in the greenhouse under long- and short-day regimes. Supplemental tungsten light of about 30 ft.-c. (bench) was used to extend natural illumination to 16 hr. daily. Short-day controls received natural illumination for 9.5–12 hr. daily from about September to March. Two temperature levels were also employed—one with temperature held uniformly at about 25°C. and the other with a day temperature of about 20°C. and a night temperature of about 10°C. Daylength behavior of the species ranged from long-day to day-neutral to short-day. All species were brought into flower and all, except N. acaulis and N. ameghinoi, formed viable seed in at least 1 of the 4 environments. A modified classification of photoperiodic flowering responses based on rapidity and not ability to flower was adopted to permit quantitative comparison of species responses to both daylength and temperature. Very few species flowered equally rapidly (day-neutral) in both the 10- and 16-hr. day-lengths. Temperature level caused modifications in response from long-day to day-neutral and vice versa, and from short-day to day-neutral and vice versa. Data for N. glauca and some other species would indicate that a greater spread between temperature levels could possibly lead to opposite classifications at upper and lower temperatures. Excellent agreement was found between daylength responses of the species and the 14 taxonomic sections of Goodspeed for the genus Nicotiana. Only 2 of the sections (Paniculatae and Undulatae) were heterogeneous in that both included short- and long-day species in the same section. The native habitat of all short-day species was South America. Certain of the species gave a compensatory response to variations in light duration and low temperature similar to that given by sugarbeets and other biennials. This phenomenon may therefore be of general occurrence. Use of a quantitative expression for photoperiodic flowering responses is proposed to avoid ambiguity. It is the quotient of days from sowing to first blossom on short-days divided by that on long-days. The value 0.6₂₀°C. (9–12) would read short-day at 20°C. with 9–12 hr. daylengths. Close agreement was found in daylength flowering ratios in successive tests in the greenhouse. The ratios alter under cold treatment with species susceptible to low-temperature stimulation or inhibition of blossoming.     

ENVIRONMENTAL CONTROL OF INITIATION OF THE INFLORESCENCE,REPRODUCTIVE STRUCTURES,AND PROLIFERATIONS IN POA BULBOSA

Youngner , Victor B. (U. California, Los Angeles.) Environmental control of initiation of the inflorescence, reproductive structures, and proliferations in Poa bulbosa. Amer. Jour. Bot. 47(9): 753–757. Illus. 1960.—Environmental factors controlling inflorescence initiation and development of inflorescence proliferations were studied using controlled environments. Inflorescence initiation was promoted by long days (16 hr.) following vernalization of 1 wk. or more'at 10°C. Normal florets developed under high temperatures (21°–27°C.) and long days following initiation. Bulbils developed in place of normal florets at short days (8 hr.) and low temperatures (ca.20°C, or below). Long days and low temperatures or short days and high temperatures produced panicles containing both bulbils and normal florets. Two flower-promoting stimuli are proposed; one promoting inflorescence initiation and the second promoting floret differentiation. Both stimuli appear to be quantitative in their mode of action.     

EFFECTS OF DAY AND NIGHT TEMPERATURE AND TYPE OF CONTAINER ON THE GROWTH OF F1 HYBRID ANNUALS IN CONTROLLED ENVIRONMENTS

The effects of three day/night temperatures (18/12 C, 24/18 C, and 30/24 C), and two types of containers (clay and plastic) on the seedling growth of three F₁ hybrid annuals were determined after 14 days of controlled-environment treatment. A day/night temperature of 18/12 C was severely limiting to the early seedling growth of ‘Blue Blazer’ ageratum (Ageratum houstonianum Mill.), ‘Pink Cascade’ petunia (Petunia hybrida Vilm.), and ‘Double Eagle’ marigold (Tagetes erecta L.). Raising the day/night temperature to 24/18 C resulted in striking increases in growth in both clay and plastic pots. Fresh and dry weights of tops for all three species were increased four- to fivefold, and leaf areas of ageratum and petunia were increased more than fivefold irrespective of container. Height and node number were also increased significantly but to a much lesser degree. A day/night temperature of 30/24 C appeared to be optimum for early seedling growth of F₁ hybrid annuals the optimum being based on dry matter accumulation, stem elongation, node development, and leaf area production. Growth of lateral shoots was greatly stimulated at 30/24 C, especially in marigold. There were no appreciable differences in fresh or dry weight of tops or in leaf area between plants in clay and plastic containers at any of the three day/night temperatures.     

THE GROWTH OF A MIDWESTERN STRAIN OF SORGHUM HALEPENSE UNDER CONTROLLED CONDITIONS

Ingle , M., and B. J. Rogers . (Purdue U., Lafayette, Indiana.) The growth of a midwestern strain of Sorghum halepense under controlled conditions. Amer. Jour. Bot. 48(5): 392–396. Illus. 1961.—This grass, adapted to midwestern growing conditions, increased growth up to 32°C. under long-day conditions (16 hr.). In a 12-hr. photoperiod, growth increased up to 27°C; at 32°C. growth was reduced. Flowering usually occurred under photoperiods of 12 hr. or less, but this requirement was not absolute. Rhizome bud sprouting was found to be proportional to temperature. Regardless of the environmental conditions and variations tested, all buds on the rhizomes developed into aerial shoots and never into rhizomes.     

Temperature effects on endogenous indole-3-acetic acid levels in leaves and stamens of the normal and male sterile 'stamenless-2' mutant of tomato (Lycopersicon esculentum Mill.)

The indole-3-acetic acid (IAA) concentration in leaves and stamens of the normal and a temperature-sensitive male sterile ‘stamenless-2′ (sl-2/sl-2) mutant of tomato (Lycopersicon esculentum Mill.), grown under three temperature conditions, was measured by gas chromatography — mass spectrometry — selected ion monitoring (GC-MS-SIM) and by enzyme-linked immunosorbant assay (ELISA). At low (LTR, 18°C day/15°C night), intermediate (ITR, 23°C day/18°C night), and high temperatures (HTR, 28°C day/23°C night), the mutant leaves had approximately 10 to 20 times higher IAA concentrations, respectively, than the normal leaves under these temperature regimes. Similarly, the stamens of mutant flowers had approximately five and eight times higher IAA concentration at ITR and HTR, respectively, than the normal flowers. In the low temperature reverted mutant stamens, however, the level of IAA was similar to that in normal stamens. Also, with an increase in temperature, there was an increase in the level of IAA in the leaves and stamens of mutant plants. However, different temperatures had no appreciable effect on the IAA content of leaves and stamens of normal plants. It is suggested that the high IAA content in leaves and stamens of the stamenless-2 mutant is one of the factors associated with male sterility and carpellization of stamens in this mutant.     

Leaf senescence and grain filling affected by post-anthesis high temperatures in two different wheat cultivars

High temperature is a major factor affecting grain yield and plant senescence in wheat growing regions of central and east China. In this study, two different wheat cultivars, Yangmai 9 with low-grain protein concentration and Xuzhou 26 with high-grain protein concentration, were exposed to different temperature regimes in growth chambers during grain filling. Four day/night temperature regimes of 34°C/22°C, 32°C/24°C, 26°C/14°C, and 24°C/16°C were established to obtain two daily temperatures of 28 and 20°C, and two diurnal day/night temperature differences of 12 and 8°C. Concentration of a lipid peroxidation product malondialdehyde (MDA), activities of the antioxidants superoxide dismutase (SOD) and catalase (CAT), chlorophyll concentration (SPAD) in flag leaves and kernel weight were determined. Results show that activities of SOD and CAT in leaves increased markedly on 14 days after anthesis (DAA) for the high-temperature treatment (34°C/22°C) and then declined. As a result, MDA concentration in leaves increased significantly under high temperature (34°C/22°C and 32°C/24°C). Compared with optimum temperature treatment, high temperature reduced the concentration of soluble protein and SPAD values in flag leaves. Grain-filling rate increased slightly initially, but decreased significantly during late grain filling under high temperature. As a result, final grain weight was reduced markedly under high temperature. Decreases in the activities of SOD and CAT and increases in MDA concentration in leaves were more pronounced with a 12°C of day/night temperature difference when under high temperatures. Kernel weight was higher under 12°C of day/night temperature difference under optimum temperatures (24°C/16°C and 26°C/14°C). The responses to high-temperature regimes appeared to differ between the two wheat cultivars with different grain protein concentrations. It is concluded that a larger diurnal temperature difference hastened the senescence of flag leaves under high-temperature conditions, but retarded senescence under optimum temperature treatments of 26°C/14°C and 24°C/16°C.     

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.     

Growth and Development in the Young Tomato: III. THE EFFECT OF NIGHT AND DAY TEMPERATURES ON VEGETATIVE GROWTH

Tomato seedlings were grown in a 12-hour day at constant andalternating day and night temperatures ranging from 10°to 30° C. The pattern of results was similar at light intensitiesof 400 and 800 f.c. The maximum rate of dryweight accumulationoccurred at a constant temperature close to 25° C. The effectsof day and night temperatures on total dry weight showed a considerabledegree of independence. The optimum day temperature was 25°C irrespective of the night temperature; the optimum night temperatureincreased from 18° to 25° C over the whole range ofday temperature. On average, day temperature affected totaldry weight twice as much as night temperature. High night temperaturesto some extent compensated for low day temperatures. The optimumday and night temperatures for leaf growth were both 25°C. On average day temperature affected leaf growth one and ahalf times as much as night temperature. By 12-hourly sampling it was shown that the cotyledons and leavesgrow throughout both day and night and that high night temperatureaccelerates nocturnal growth (cotyledons by cell expansion,young leaves by cell multiplication). Plants having receivedonly one night at 25° C, as compared with 15° C, showa slightly greater assimilation during the following light period,apparently as a consequence of increased photosynthetic surface.The respiratory loss in dry weight during darkness was not significantlyaffected by temperature over the range 15–25° C.     

A new method to determine the energy saving night temperature for vegetative growth of Phalaenopsis

Knowledge of the energy saving night temperature (i.e. a relatively cool night temperature without affecting photosynthetic activity and physiology) and a better understanding of low night temperature effects on the photosynthetic physiology of Phalaenopsis would improve their production in terms of greenhouse temperature control and energy use. Therefore, Phalaenopsis‘Hercules’ was subjected to day temperatures of 27.5°C and night temperatures of 27.0°C, 24.2°C, 21.2°C, 18.3°C, 15.3°C or 12.3°C in a growth chamber. A new tool for the determination of the energy saving night temperature range was developed based on temperature response curves of leaf net CO₂ exchange, chlorophyll fluorescence, organic acid content and carbohydrate concentrations. The newly developed method was validated during a complete vegetative cultivation in a greenhouse environment with eight Phalaenopsis hybrids (i.e. ‘Boston’, ‘Bristol’, ‘Chalk Dust', ‘Fire Fly’, ‘Lennestadt’, ‘Liverpool’, ‘Precious’, ‘Vivaldi’) and day/night temperature set points of 28/28°C, 29/23°C and 29/17°C. Temperature response curves revealed an overall energy saving night temperature range for nocturnal CO₂ uptake, carbohydrate metabolism, organic acid accumulation and photosystem II (PSII) photochemistry of 17.1°C to 19.9°C for Phalaenopsis‘Hercules’. At the lower end of this energy saving night temperature range, a high malate‐to‐citrate ratio switched towards a low ratio and this transition seemed to alleviate effects of night chilling induced photoinhibition. At night temperatures of 24°C or higher, the degradation of starch, glucose and fructose indicated an increased respiratory CO₂ production. During the greenhouse validation experiment, the differences between the eight Phalaenopsis hybrids with regard to their response to the warm day/cool night temperature regimes were remarkably large. In general, the day/night temperature of 29/17°C led to a significantly lower biomass accumulation and less leaves which were in addition shorter, narrower and smaller in size as compared to the day/night temperature regimes of 28/28°C and 29/23°C. During week 25 of the cultivation period, plants matured and flower initiation steeply increased for all hybrids and in each day/night temperature regime. Before week 25, early spiking was only sufficiently suppressed in the 29/23°C and 29/17°C temperature regimes for three hybrids (‘Boston’, ‘Bristol’ and ‘Lennestadt’) but not in the other five hybrids. Although a considerable biochemical flexibility was demonstrated for Phalaenopsis‘Hercules’, inhibition of flowering after exposure to a combination of warm days and cool nights appeared to be largely hybrid dependent.