Start Of Short Days Research Articles

Elevated UV photon fluxes minimally affected cannabinoid concentration in a high-CBD cultivar.

Ultraviolet photons (UV) can damage critical biochemical processes. Plants synthesize photo-protective pigments that absorb UV to minimize damage. Cannabinoids absorb UV, so increased UV has the potential to increase cannabinoid synthesis. Studies in the 1980's provided some evidence for this hypothesis in low-cannabinoid cultivars, but recent studies did not find an increase in cannabinoid synthesis with increasing UV in high-cannabinoid cultivars. These studies used low UV photon fluxes, so we examined the effect of higher UV photon fluxes. We used fluorescent UV lights with 55% UV-B (280 to 314 nm) and 45% UV-A (315 to 399 nm). Treatments began three weeks after the start of short days and continued for five weeks until harvest. Established weighting factors were used to calculate the daily biologically effective UV photon flux (UV-PFDBE; 280 to 399 nm). Daily UV-PFDBE levels were 0, 0.02, 0.05, and 0.11 mol m-2 d-1 with a background daily light integral (DLI) of 30 mol m-2 d-1. This provided a ratio of daily UV-PFDBE to DLI of 41 to 218% of summer sunlight in the field. Cannabinoid concentration was 3 to 13% higher than the control in UV treated plants, but the effect was not statistically significant. Fv/Fm and flower yield were reduced only in the highest UV treatment. These data support recent literature and lead us to conclude that an elevated flux of UV photons is not an effective approach to increase cannabinoid concentration in high-cannabinoid cultivars.

Growth and development of poinsettia (Euphorbia pulcherrima) finished under reduced air temperature and bench-top root-zone heating

Greenhouse heating is an essential requirement for holiday poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch.) production, especially during the finish stage. Previous studies have shown that cultivars with early response attributes (short day initiation to finish in 6–8 weeks), moderate to high vigor, and naturally large bracts are most suitable for reduced temperature finishing (RTF) that leads to energy savings. We postulate that RTF in combination with root-zone heating (RZH) can further reduce greenhouse heating costs by allowing for even lower air temperature set points. The objective of this study was to quantify how RTF in combination with bench-top RZH influences growth, development, and quality of six red poinsettia cultivars. Rooted cuttings of poinsettia ‘Prestige Early Red’, ‘Prestige Red’, ‘Premium Red’, ‘Infinity Red’, ‘Viking Red’, and ‘Bravo Bright Red’ were transplanted and grown under a 16-h photoperiod consisting of natural days with daylength-extension lighting until 01 Oct. and a mean daily temperature (MDT) of 21.5°C [24/19°C (12h day/12h night)] until 15 Oct. On 15 Oct., the air temperature set point was reduced to a MDT of 15°C [19/13°C (8h day/16h night)], and plants were placed on a bench without RZH or with RZH set points of 21, 24, or 27°C; or plants were moved into a greenhouse without RZH and with a constant air temperature set point of 21°C (commercial control). Time to anthesis from the start of short days was reduced by 6, 4, and 7d, respectively, for ‘Prestige Early Red’, ‘Premium Red’, and ‘Viking Red’ when plants were finished on a RZH set point of 27°C compared to a RZH set point of 21°C. Time to marketability was generally longer for cultivars grown under RTF without RZH compared to all other treatments. Although plants grown under RTF with RZH matured at a similar rate to those in the commercial control, bract area index was reduced under RTF in most cases. For example, bract area of ‘Infinity Red’ was reduced by 27, 25, 18, or 19% when finished without RZH in ambient RTF or with a RZH set point of 21, 24, or 27°C, respectively, compared to the commercial control. Stem elongation responses varied by cultivar. Our data suggest that when RZH temperatures>24°C are utilized during poinsettia RTF, day/night air temperatures can be further reduced from the 20/13 and 21/17°C (12h/12h), recommended by previous research, to 19/13°C (8h/16h).

Development of Euphorbia pulcherrima under Reduced Finish Temperatures

The holiday poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch.) is the number two potted flowering crop sold in the United States with a reported wholesale value of $146 million in 2010. Profitability is increasingly threatened as the cost to heat greenhouses has increased by over 90% in the last 10 years. As energy costs continue to increase and poinsettia prices remain relatively constant, growers are seeking cultivars that can be finished under reduced temperatures. Our objectives were to quantify how reduced temperature finishing (RTF) 2 weeks after the start of short days influences height, bract area index, and time to anthesis of poinsettia. Eight red poinsettia cultivars were selected based on their early response attributes (initiate and finish within 6 to 8 weeks), moderate to high vigor, and naturally large bracts. Rooted cuttings were grown at day/night temperature set points (12 h/12 h) of 24/19 °C until 15 Oct. and under a 16-h photoperiod consisting of natural daylengths with day-extension lighting until 1 Oct. On 15 Oct., plants were transferred to day/night temperatures (12 h/12 h) of 20/14, 21/17, or 24/19 °C. Time to anthesis from the start of short days was 60 and 55 days at 24/19 °C and 76 and 68 days at a reduced finishing temperature of 20/14 °C for ‘Prestige Early Red’ and ‘Early Orion Red’, respectively. Final height was not significantly influenced by RTF in either cultivar. Our results indicate that RTF is a viable option that greenhouse growers can use to help reduce energy costs of carefully selected poinsettia cultivars.

Simulation of leaf area development based on dry matter partitioning and specific leaf area for cut chrysanthemum.

This work aims to predict time courses of leaf area index (LAI) based on dry matter partitioning into the leaves and on specific leaf area of newly formed leaf biomass (SLA(n)) for year-round cut chrysanthemum crops. In five glasshouse experiments, each consisting of several plant densities and planted throughout the year, periodic destructive measurements were conducted to develop empirical models for partitioning and for SLA(n). Dry matter partitioning into leaves, calculated as incremental leaf dry mass divided by incremental shoot dry mass between two destructive harvests, could be described accurately (R(2 )= 0.93) by a Gompertz function of relative time, R(t). R(t) is 0 at planting date, 1 at the start of short-days, and 2 at final harvest. SLA(n), calculated as the slope of a linear regression between periodic measurements of leaf dry mass (LDM) and LAI, showed a significant linear increase with the inverse of the daily incident photosynthetically active radiation (incident PAR, MJ m(-2 )d(-1)), averaged over the whole growing period, the average glasshouse temperature and plant density (R(2 )= 0.74). The models were validated by two independent experiments and with data from three commercial growers, each with four planting dates. Measured shoot dry mass increase, initial LAI and LDM, plant density, daily temperature and incident PAR were input into the model. Dynamics of LDM and LAI were predicted accurately by the model, although in the last part of the cultivation LAI was often overestimated. The slope of the linear regression of simulated against measured LDM varied between 0.95 and 1.09. For LAI this slope varied between 1.01 and 1.12. The models presented in this study are important for the development of a photosynthesis-driven crop growth model for cut chrysanthemum crops.

Influence of potential growth factors in predicting time to flowering in poinsettia ( Euphorbia pulcherrima)

A simple and robust model for the potential effect of several factors on the generative development of poinsettias is presented. Models for predicting time to flowering are developed following the `first passage time' modelling concept. A model selection procedure using cross-validation is employed. Data sets for pinched and non-pinched poinsettias include measurements of seven different potential factors. For pinched poinsettias prediction of time to flowering can be based on three of the factors: daily average temperature, daily artificial light intensity and plant density. Predictions can be made with adequate precision for practical use, with estimated mean prediction errors of 4.20 days when predicting from start of short day treatment and 3.64 when predicting from first visible cyathie. For non-pinched poinsettias, the number of long days prior to short day treatment is an additional relevant factor. Due to the unknown history of the cuttings the prediction of time to flowering is more difficult for non-pinched plants. Predicting from start of short days resulted in an estimated mean prediction error of 5.66 days, and using information about time of first visible cyathie did not improve the predictions.

Effect of anterior hypothalamic area lesions on photoperiod-induced shifts in reproductive activity of the ewe.

The areas of the brain involved in photoperiodic control of reproduction are not well defined. The objective of this study was to determine whether anterior hypothalamic area (AHA) lesions in the ewe affected the responses of the reproductive system to shifts in the length of the daily photoperiod and development of photorefractoriness to a constant short day photoperiod. Eleven intact ewes received bilateral radiofrequency lesions of the AHA (AHAX), and five received sham lesions (sham). The ewes then were placed in photochambers and exposed alternately to two approximately 90-day periods of long [16 h of light, 8 h of darkness (16L:8D)] and short (10L:14D) days and then to 10L:14D for an additional 165 days. Blood samples were collected twice weekly to monitor plasma profiles of progesterone, PRL, and total T4, and during the second 16L:8D photoperiod, hourly for one 24-h period to assess melatonin release. Lesions increased (P < 0.001) the interval between the start of long days and cessation of estrous cycles during both long day periods, but did not affect the interval between the start of short days and the onset of estrous cycles for either the first (P = 0.08) or second (P > 0.10) short day period. Consequently, the durations of both anestrous periods were shorter (P < 0.001) for AHAX than for sham ewes. AHA lesions did not affect (P > 0.10) diurnal patterns of melatonin release. No effects (P > 0.10) of lesions were evident on plasma patterns of PRL or total T4 for any short or long day photoperiod. Development of photorefractoriness to constant short days either did not occur or was markedly delayed in five of nine AHAX (P < 0.01) ewes, whereas the other four AHAX ewes became refractory at a time similar (P > 0.10) to that in sham ewes. Responses to inhibitory long day photoperiods and constant short days were highly (P < 0.05) correlated (r = 0.74) and appeared dependent upon the extent of the AHA lesion. These results suggest that AHA lesions disrupt neuronal pathways mediating the effects of shifts in photoperiod on reproductive activity and development of photorefractoriness to constant short days. Our results suggest that the effects of AHA lesions are confined to the termination of reproductive activity, and that different neural pathways participate in photostimulation vs. photosuppression or photorefractoriness.

DETERMINATION OF CAUSAL RELATIONSHIP BETWEEN PHOSPHORUS AND BRACT EDGE BURN OF POINSETTIAS

Increasing rates of triple superphosphate added to the root media have a positive correlation with bract edge burn on poinsettias (Whipker, Hammer,1992). We have studied the effects of application of increasing rates of phosphorus supplied from multiple sources. Analysis of P content and other elements were conducted on root media and foliar samples to determine source of toxicity.Euphorbia pulcherrrima Willd. cultivars Dark Red Hegg and Red Sails were potted on September 8, 1992 in a root medium of peat, perlite and soil (40:40:20, v/v) mixture amended with N, K, Ca and micronutrients. Prior to potting, the root media were also ammended with either .30, 2.37 or 4.75 kg P/m3 of superphosphate (0-18-0) or triple superphosphate (0-46-0). Two additional treatments of 75 mg P/liter and 150 mg P/liter supplied via 75% technical grade phosphoric acid in the irrigation system were also studied. Foliar samples were analyzed for N, P, K, Mg, Ca and flouride every two weeks after the start of short days. Root media samples were also collected and analyzed for pH, SS, NO3, NH4, P, K, Mg and Ca. Plant height, leaf dry weight and stem dry weight were measured on each sampling date as well as bract diameter and bract edge burn count at anthesis.Media P levels increased as phosphorus rate increased for all P sources. By the second sampling date, severe toxicity symptoms developed on foliage of the 4.75 kg P/m3 rate of superphosphate. This toxicity corresponded to high media P levels. Bract burn count at anthesis increased significantly at rates of 2.37 and 4.75 kg P/m3 of both superphosphate and triple superphosphate. The highest incidence of bract burn (21.875 bracts burned/plant) occured with application of 2.37 kg P/m3 of triple superphosphate.

FLOWERING AND GROWTH REGULATOR RESPONSE IN `FREEDOM' COMPARED TO OTHER POINSETTIA CULTIVARS

Euphorbia pulcherrima cvs. Freedom, SUPJIBI, and Celebrate 2 were sprayed with paclobutrazol or a tank mix of daminozide and chlormequat at week 40, 41, 42, 43, or 44. Application time had little effect on plant size. The tank mix had greater efficacy on `Freedom' but not on the other cultivars. Interactions for bract size indicated: 1) time of application had less effect on `Freedom', 2) there was little difference between the chemicals on `SUPJIBI', but the tank mix had greater efficacy on the other cultivars, and 3) the tank mix had greater efficacy than paclobutrazol during weeks 41, 42, and 43. `Freedom', `Celebrate 2', `SUPJIBI', and `V-14 Glory' were planted on 8 or 15 Aug. and placed under short days on 12, 19, or 26 Sept. `Freedom' reached anthesis between 30 Oct. and 6 Nov., about 5 days before `SUPJIBI' and `Celebrate 2' and 7-10 days ahead of `V-14 Glory'. `Freedom' planted in Aug. and given short days 14 days apart flowered only 7 days apart (40 to 47 days from start of short days), but when planted in Sept. flowering was in 54 days and each long day resulted in 1 day delay in flowering.

INFLUENCE OF INCREASED ROOT MEDIA PHOSPHORUS LEVELS IN POINSETTIA PRODUCTION

Acidification of the irrigation water with phosphoric acid is a common practice to avoid nutrient deficiencies/toxicities from alkaline root media. It has been suggested high phosphorus levels could cause phosphorus toxicity. Euphorbia pulcherrima Willd. cultivars Supjibi and Celebrate 2 cuttings were potted on June 6, 1991 in a root medium of peat, perlite and soil (40:40:20 by volume) amended with N, K, Ca and micro-nutrients, plus six phosphorus (0-40-0) rates of .89, 1.78, 3.55, 7.11, 10.67, and 14.22 kg/meter3. Foliar samples were analyzed for NH4, P, and K every two weeks after the start of short days. Root media samples were also collected and analyzed pH, SS and NO3, P, K and NH4. Bract diameter, bract edge burn, days to anthesis, and plant height were recorded at anthesis. Media P levels increased as the phosphorus rate increased, but a significant treatment*harvest interaction for media P was observed. There was decreased bract size and increased incidences of bract edge burn as phosphorus rate increased. Root media P levels did not affect the levels of other nutrient elements in the foliar samples. No visual symptoms of phosphorus toxicity was observed except for bract edge burn at anthesis.

Stem elongation and keeping quality in poinsettia ( Euphorbia pulcherrima Willd.) as affected by temperature and supplementary lighting

Euphorbia pulcherrima cultivars ‘Lilo’ and ‘Starlight’ were grown under five different day/night temperature (DT/NT) regimes from the start of short days (SD) to flowering for two different starting periods, 1 October (replicate A) and 12 October (replicate B). The average daily temperature (ADT) was kept at 18–20°C. Plants were given supplementary lighting with high pressure sodium lamps (HPS) for 10 h day −1 at three photosynthetic photon flux densities (PPFD): 12, 37 and 73 mol m −2 s −1. At the marketing stage the plants were moved to the interior holding room for testing the keeping quality. Both plant height and plant diameter were lower when the plants were grown at DT < NT (negative DIF) compared with plants grown at DT > NT (positive DIF) or with constant temperature (zero DIF). The response to DIF was stronger in replicate A than B, and considerably greater in ‘Starlight’ than in ‘Lilo’. Negative DIF-grown plants resulted in delayed visible cyathia and flowering compared with zero or positive DIF. Plants grown at constant temperature flowered earliest. Plants reached the marketing stage about 1 week earlier with 73 compared to 12 mol m −2 s −1 supplementary lighting. DT<NT (negative DIF) increased the abscission of cyathia in ‘Lilo’ during the first 2 weeks in an interior climate. Increasing PPFD reduced abscission of cyathia in both cultivars. Negative DIF during the growth period resulted in more bract necrosis than zero or positive DIF in ‘Lilo’. No bract necrosis was observed in ‘Starlight’. DIF had no influence on leaf abscission.

Chrysanthemum dry matter partitioning patterns along irradiance and temperature gradients

The influence of photosynthetic photon flux (PPF, 1.8−21.6 mol d−1 m−2) and day (DT) and night (NT) temperature (10–30 °C) on dry matter accumulation and partitioning was studied in Chrysanthemum morifolium Ramat. ’Bright Golden Anne’. Total plant dry matter varied from 3.6 to 17.2 g at flowering. Plants with the greatest dry matter were from treatments with high PPF levels and temperatures. Accumulation of dry matter in roots, stems, leaves and flowers examined on a normalized time and normalized dry matter basis showed similar trends independent of DT, NT and PPF during development. Accumulated dry matter in roots, stems and leaves increased to a maximum and then decreased as the flowers were developing. Maximum leaf, root and stem dry matter was reached at 81, 85 and 91%, respectively, of required time from start of short days (SD) to flower. Proportion root dry matter increased and proportion leaf dry matter decreased in the plants as PPF increased. Partitioning to roots decreased as the DT increased. The root/shoot dry matter ratio decreased as plants developed from start of SD to flowering at all studied combinations of PPF, DT and NT. A positive difference between DT and NT (DIF) resulted in a higher percentage stem dry matter compared to plants grown at a negative DIF. Partitioning to flowers was not strongly correlated with the levels of PPF, DT and NT.Key words: Chrysanthemum morifolium, Dendranthema grandiflora, dry matter accumulation and partitioning, temperature, irradiance

Quantification of Temperature Effects on Stem Elongation in Poinsettia

Elongation characteristics of each internode on a lateral shoot of poinsettia (Euphorbia pulcherrima Klotz) `Annette Hegg Dark Red' were determined from pinching through anthesis for plants grown with 36 day/night temperature (DT/NT) combinations between 16 and 30C. The Richards function was used to describe the elongation of each internode. The first internode developing on a lateral shoot was longer and matured faster than subsequent internodes. The length of the first internode was a function of the difference between day and night temperatures (DIF = DT - NT). Subsequent internodes elongated uniformly in the absence of flower initiation. In the absence of flower initiation, the length of an internode, after the first, was a function of DIF. Internodes were shorter as proximity to the inflorescence increased. Internode length after the start of short days was a function of DIF and the visible bud index where visible bud index = [(days from pinching to the day an internode began to elongate - days from pinching to the day of the start of flower initiation)/the number of days from pinching to visible bud]. A poinsettia lateral shoot elongation model was developed based on the derived functions for internode elongation. The model predicted lateral shoot length within one standard deviation of the mean for plants grown in a separate validation study with 16 combinations of DT/NT. The model allows the prediction of lateral shoot length on any day from pinching through anthesis based on temperature, the number of nodes on the lateral shoot, the time each internode on the lateral shoot began elongating, and the visible bud index at the start of elongation of each node.

Temperature Regime at Various Stages of Production Influences Growth and Flowering of Dendranthema × grandiflorum

For chrysanthemum [Dendranthema × grandiflorum (Ramat.) Kitamura], the hypothesis that a 12-hr 5C or 13C dark treatment could be used in conjunction with a 12-hr 27, 21, 17, or 13C light treatment for rapid flowering when applied during certain developmental stages was valid. Flowering of `Bright Golden Anne', planted on 23 Sept., was not delayed by 12-hr light/12-hr dark growth chamber treatments of 21/5C or 27/13C (day/night) if treated from planting (P) of the rooted cutting to the start of short days (SD), 3 weeks after start of SD to visible bud (VB) (SD + 3 to VB), or from VB to flower (F) when compared to the glasshouse control plants grown at 21/18C. Plants responded similarly if grown at 13/13C or 21/21C, but flowering was delayed compared to the 17/17C glass house control. Delays were absent, however, when 13/13C was used from P to SD, SD + 3 to VB, or when 17/13 or 21/13C was used from VB to F.

Development Rate during Four Phases of Chrysanthemum Growth as Determined by Preceding and Prevailing Temperatures

Abstract Time required to complete four developmental phases in chrysanthemum ‘Bright Golden Anne’ (Dendranthema grandiflora Tzvelev.) was determined under greenhouse conditions at constant temperature setpoints of 10, 15, 20, 25, 30, or 20 day/16C night. The four developmental phases were: I—start of short days to visible bud (2-mm-diameter terminal flower bud), II—visible bud to disbud (10-mm-diameter terminal flower bud), III—disbud to color (flower bud showing first color), and IV—color to flower. Plants either remained at the same temperature during all four phases or were moved to one other constant temperature regime after phase I, II, or III. Fastest development during all phases occurred at 20 day/16C night or constant 20C. Temperature of previous phases had less of an effect on future phases as plant development progressed. Low temperature (10C) during phase I delayed development in phase II, and high temperature (30C) during phase I and II delayed development during phase III. The length of each phase could be predicted based on the temperature preceding and during the phase. Optimum temperatures for fastest development during the four phases were calculated as 21.3, 20.3, 23.1, and 19.1C, respectively.

Path Analysis of Chrysanthemum Growth and Development

Abstract Plants of chrysanthemum (Dendranthema grandiflora Tzvelev.) were grown under one of 25 irradiance and temperature combinations from start of short days to flower. Four phases of development were defined as 1) the start of short days to the appearance of 4-mm terminal flower buds (phase I), 2) appearance of 4-mm terminal flower buds to removal of lateral flower buds when the terminal flower bud was 7 to 8 mm (phase II), 3) removal of lateral flower buds to flower buds showing first color (phase III), and 4) flower buds showing color to flowering (phase IV). Path analysis was used to study the influence of development time and relative dry weight gain during each of these four phases on development time and relative dry weight gain of subsequent phases. Relative dry matter accumulation during phases I, II, III, and IV significantly influenced cumulative relative dry weight gain, with phase I having the greatest influence. Increasing relative dry weight gain during phase I had a significant negative effect on relative dry weight gain in phase II. Time within each phase significantly affected total time to flower. Under the constant environmental conditions of this experiment, time in one phase did not influence the length of time in later phases.

High Temperature Effects on Growth and Floral Development of Chrysanthemum

Abstract Pinched plants of Chrysanthemum morifolium Ramat. ‘Orange Bowl’ and ‘Surf’ grown in a chamber maintained at 22° day/18°C night were transferred to 30° day/26° night at the beginning of week 1, 3, 5, or 7 after start of photoinduction period (15-hr nyctoperiod). Plants remained at high temperatures for 2, 4, 6, 8, or 10 weeks and then were returned to the 22°/18° chamber. Exposure to high temperatures during the first 4 weeks of short days increased the number of nodes, leaf area, stem length, and dry weight of leaves and stems. Rate of floret initiation and perianth differentiation decreased when exposed to high temperatures during the first 4 weeks of short days in ‘Orange Bowl’ but not in ‘Surf’. ‘Orange Bowl’ exposed to high temperatures for 10 weeks from the start of short days flowered 12 days later than plants grown at lower temperatures and formed bracteate buds. Flowering of ‘Orange Bowl’ grown at 22718° during the first 4 weeks of short days, then transferred to high temperatures, was not substantially delayed and flowers developed normally. Flowering was delayed 3 days when ‘Surf’ was exposed to high temperatures for 8 weeks from the start of short days. Exposure to high temperatures did not cause bracteate bud formation in ‘Surf’. With both cultivars, increasing the duration of high temperature exposure increased the time to flowering.

Variation in Cyathia Abscission of Poinsettia Cultivars in a Greenhouse and a Simulated Postharvest Environment

Abstract Differences in cyathia abscission of poinsettias (Euphorbia pulcherrima Willd.) ‘Annette Hegg Dark Red’ (Dark Red), ‘Annette Hegg Lady’ (Lady), ‘Annette Hegg Brilliant Diamond’ (Brilliant), ‘Gutbier V-14 Glory’ (V-14), and ‘Mikkel Triumph’ were evaluated chronologically based both on the number of days after the start of short days and on the number of days after anthesis. Seventy days after the start of short days, ‘V-14’ had the least abscission of the tested cultivars in the greenhouse or postharvest environment, while ‘Lady’ had the greatest abscission. In contrast, 7 days after anthesis, ‘V-14’ had the greatest abscission in the postharvest environment while ‘Brilliant’ and ‘Dark Red’ had the least abscission. The difference in ‘V-14’ ranking between evaluation method was due to ‘V-14’ reaching anthesis 7-10 days later than the other cultivars. Abscission was greater in the postharvest environment than in the greenhouse, probably due to the reduced photosynthetic photon flux (PPF) levels in the postharvest environment (5.1 mol·d−1·m−2 PPF in the greenhouse compared to 0.29 mol·d−1·m−2 PPF in the postharvest environment).

Flower formation inChrysanthemum morifolium: the influence of light level

SummaryGroups of chrysanthemum plants were transferred from a constant light level of 63 J cm-2 day-1 to higher levels at the start of short days, and at weekly intervals thereafter. The plants remained in their new light regime for one or two weeks before they were returned to the lower level. The results showed that flower initiation began earliest if transfer was made to a light level of 125 J cm-2 day-1 for two weeks at the start of short days. This benefit was retained throughout the subsequent period of growth at the lower light level and resulted in earlier flowering.The average level of visible radiation received in glasshouses in winter, even on the South Coast of England, is only 67 J cm-2 day-1 in the worst two months. The data suggest that if artificial light is used during this period to supplement the natural light and to raise the level of light energy received to at least 125 J cm-2 day-1 for the first two weeks of short days, earlier and more uniform flowering should . be obtained.Further...

Effect of gibberellin on the flowering of Christmas cactus. I

The present expriments were performed to study the flowering of Christmas cactus, a-short-day plant, following gibberellin application. 1. Gibberellin applied as an aqueous solution at the beginning of short-days caused delaying of the flowering. This delaying effect of gibberellin decreased when applied 7 or 10 days after the start of short-days. This result was also ascertained by the evidence that the flower buds stopped to increase in number due to the treatment of gibberellin. 2. When gibberellin had been applied before the start of short-days, the delaying effect on flowering decreased as the number of days prior to short-days was increased. The effect of gibberellin was demonstrated to remain in a plants at least for 20 days. 3. Gibberellin applied 20 or more days or even 14 days after the start of short-days caused the acceleration of the flowering. 4. With gibberellin treatment the plants generally produce peculiar flowers, the ovaries and the corolla tubes of which became longer and more slender. When gibberellin was applied 10 days after the start of short-days, it was observed that many unusual buds appeared, sometimes the ovaries of these buds were buried in leaf-like stems. 5. 5, 25 and 100 ppm of gibberellin had almost the same effect in delaying or acceler-ating the flowering. 6. With gibberellin treatment the number of flowers decreased, but increased when the plants were treated 30 days after the start of short--days. On the other hand, the plants. to which NAA was applied at an earlier time of short-days had more flowers than the non-treated control. 7. NAA applied 20 or less days after the start of short-days delayed the flowering by a few days independently of the time of application.