Variation among Kalanchoe species in their flowering responses to photoperiod and short-day cycle number

Summary

Our objectives were to identify the critical daylength and number of short-day (SD) cycles necessary for flowering in Kalanchoe glaucescens, K. manginii, and K. uniflora. In Experiment I, plants were grown for 20 weeks under 9, 10, 11, 12, 13, 14, or 15 h photoperiods at 300 µmol m^–2 s^–1 for 8 h 55 min (9 h photoperiod), or 9 h and extended with dayextension (3 µmol m^–2 s^–1) lighting (10 – 15 h photoperiods). All species flowered when grown under photoperiods ranging from 9 – 12 h. The percentage of flowering plants decreased for all species as the photoperiod increased from 12 h to 14 h. No flowering occurred on plants grown under a 15 h photoperiod. Node numbers below the terminal inflorescence increased from 18 nodes to 28 nodes on K. glaucescens, from 12 nodes to 14 nodes on K. manginii, and from 12 nodes to 16 nodes on K. uniflora as the photoperiod increased from 12 h to 14 h, from 10 h to 12 h, and from

12 h to 13 h, respectively. Total flower numbers on K. uniflora decreased from 45 flowers to 13 flowers as the photoperiod increased from 9 h to 13 h. In Experiment II, plants were exposed to 0, 1, 2, 3, 4, 5, 6, 7, or 8 weeks of SD (8 h photoperiod) before being placed under night-interruption lighting (2 µmol m^–2 s^–1; between 22.00 – 0.200 h). One-hundred percent of K. glaucescens, K. manginii, and K. uniflora plants flowered when they received more than 1, 3, or 6 weeks of SD, respectively. The node number below the terminal inflorescence in each species was not affected by SD cycle-number. Total flower numbers per plant, and days to first open flower, were unaffected as the number of SD cycles exceeded the number required to induce flowering for all species.     

Optimizing the lighting regime for Alstroemeria with respect to photoperiod and fluence rates

The Alstroemeria cultivars Diamond, King Cardinal and Libelle were grown for 18 months under five lighting regimes with, and without, soil cooling. The aim was to optimize the daily investment of light energy from artificial sources with respect to photoperiod and photosynthetic fluence rates and to elucidate possible links between reactions to photoperiod and root-zone temperature. The more photons (photosynthetically active radiation, PAR) that were supplied to the plants per day (8, 11 and 13 mol m⁻²), the higher was the total production of flowering stems. The total yield from regimes with 13 mol m⁻² day⁻¹ was higher when the light was spread over 20 and 16 h compared to 12 h. In treatments with soil cooling, the plants flowered continuously under all combinations of photoperiods and photosynthetic fluence rates, and the summer and autumn recession in flower production that occurred for non-cooled ‘King Cardinal' and ‘Diamond' was the same under all lighting regimes. It is concluded that it might be more cost-effective to spread the daily investment of light over 20 rather than 16 or 12 h when the total energy budget and CO₂ costs are taken into consideration.     

Use of a cyclic high-pressure sodium lamp to inhibit flowering of chrysanthemum and velvet sage

Photoperiod is commonly controlled in the commercial production of ornamental crops to induce or prevent flowering. Flower induction in short-day (SD) plants can be prevented or delayed when the natural daylength is short by providing low-intensity lighting during the dark period. A stationary high-pressure sodium (HPS) lamp with an oscillating aluminum parabolic reflector (cyclic HPS) has been developed to provide intermittent lighting to greenhouse crops. We determined the efficacy of a cyclic HPS lamp at preventing flowering in SD plants garden chrysanthemum [Chrysanthemum × grandiflorum (Ramat.) Kitam.] ‘Bianca’, pot chrysanthemum ‘Auburn’, and velvet sage (Salvia leucantha L.) relative to traditional night interruption (NI) lighting strategies. Plants were grown in a glass-glazed greenhouse at a mean daily temperature of 19.5–20.7 °C with natural SD photoperiods. NI lighting was delivered during the middle of the night (2230–0230 h) from a 600 W cyclic HPS lamp mounted at one gable end of the greenhouse or from incandescent (INC) lamps that were illuminated for the entire 4 h (CONT INC) or for 6 min every 30 min for 4 h. Plants under cyclic HPS were grown at lateral distances of 1, 4, 7, 10, or 13 m from under the lamp. Control plants were grown under an uninterrupted 15 h skotoperiod. As the distance from the cyclic HPS lamp increased from 1 to 13 m, the maximum irradiance measured during the NI decreased from 25.4 to 0.3 μmol m⁻² s⁻¹ and time to visible inflorescence (VI) and the number of nodes at VI decreased. All species had a VI within 54 d, but ≤10% of plants flowered when grown at a lateral distance of 1 or 4 m from the cyclic HPS lamp or under CONT INC. Plants grown without NI had a VI 2 to 15 d earlier and flowered 7 to 24 d earlier than plants grown at 10 or 13 m from the cyclic HPS. All garden chrysanthemums flowered under cyclic INC, whereas velvet sage and pot chrysanthemum had 15% and 35% flowering, respectively. These results indicate that a cyclic HPS lamp can be used effectively to delay flower induction and prevent flowering in these species when NI is delivered at ≥2.4 μmol m⁻² s⁻¹.     

Night interruption promotes vegetative growth and flowering of Cymbidium

The effects of night interruption (NI) were examined on the vegetative growth and flowering of Cymbidium ‘Red Fire’ and ‘Yokihi’. Plants were grown under 9/15 h ambient light/dark (control), 9 h ambient light plus night interruption (22:00–02:00 h) with low light intensity at 3–7 μmol m⁻² s⁻¹ (LNI) and 9 h ambient light plus NI with high light intensity at 120 μmol m⁻² s⁻¹ (HNI) conditions. The number of leaves, leaf length, number of pseudobulbs and pseudobulb diameter increased in both LNI and HNI compared to controls for both cultivars. While none of the control plants flowered within 2 years, 100% of the ‘Yokihi’ and 80% of the ‘Red Fire’ plants grown under HNI condition flowered. In the LNI group, 60% of the plants flowered in both cultivars. Plants in the HNI group showed a decreased time to visible inflorescence and flowering than those in the LNI group. The number of inflorescences and florets were greater in the plants grown under HNI than those in the LNI group. The tallest plants at flowering were in the HNI group in both cultivars. NI with low light intensity can be used effectively to promote flower induction with increased growth rate during the juvenile stage in Cymbidium. To obtain high quality plants, however, NI with high light intensity strategies should be considered.     

Environmental factors affecting flowering of rice flower (Ozothamnus diosmifolius,Vent.)

Rice flower (Ozothamnus diosmifolius, Vent.), native to east Australia, is a spring flowering perennial shrub. It is a new cut flower plant, recently introduced into cultivation in Australia and in Israel. Its response to environmental conditions, which affect growth and flowering, are not yet known. The purpose of the present study was to evaluate the effects of growth temperature, photoperiod and total solar energy on flowering. Experiments were conducted with plants of the cv. Cook’s Snow White. Plants were grown in three cycles under controlled conditions in the phytotron, at four day/night temperature regimes: 17/9, 20/12, 23/15 and 26/18°C. Two photoperiods — short day (SD) of 10 h natural day light and long day (LD) of 10 h natural light plus 10 h incandescent light — were employed. High temperatures enhanced vegetative growth but blocked flowering under both LD and SD. Under medium–moderate temperatures plants were absolute LD plants and did not flower under SD conditions. Under lower temperatures plants flowered under both LD and SD, but SD delayed flowering. High total solar radiation under LD did not affect flowering time but greatly promoted the number of flowering stems.     

Influences of day and night temperatures on flowering of Fragaria x ananassa Duch., cvs. Korona and Elsanta,at different photoperiods

The effects of photoperiod (12, 13, 14, 15 or 16 h), day temperature (12, 15, 18, 24 or 27 °C) and night temperature (6, 9 or 12 °C) and their interactions on flower and inflorescence emergence were investigated by exposing 4 week old runner plants of strawberry cvs. Korona and Elsanta during a period of 3 weeks. A daily photoperiod of 12 or 13 h resulted in the highest number of plants with emerged flowers. A photoperiod of 14 h or more strongly reduced this number, while no flowers emerged at a photoperiod of 16 h. Plants exposed to photoperiods of 12 or 13 h flowered earlier and had longer flower trusses. A day temperature of 18 °C and/or a night temperature of 12 °C were optimal for plants to emerge flowers and resulted in the shortest time to flowering. A night temperature of 6 °C strongly reduced the number of plants that emerged flowers, especially when combined with lower day temperatures. Photoperiod and temperature had no effect on the number of inflorescences, all flowering plants produced on average one inflorescence. The number of flowers on the inflorescence increased with decreasing day temperature and when photoperiod was raised from 12 to 15 h. In general, ‘Korona’ was more sensitive to photoperiod and temperature as ‘Elsanta’, and had a lower optimal day temperature for flower emergence. Results of this experiment may be used to produce high quality plant material or to define optimal conditions when combining flower induction and fruit production.     

The Cause of Black Blotch Disease of the Raspberry

Summary

Actively growing, single-stemmed plants of three black currant cultivars, each with 15 – 16 nodes, were exposed to photoperiods of 10, 14, 15, 16, 17, 18, 20, and 24 h at 18°C for 8 weeks for determination of the critical photoperiods for growth cessation and floral initiation. In all three cultivars, growth cessation was induced by short day (SD) conditions, with a critical photoperiod of 16 h, and the response was advanced by decreasing the photoperiod. The critical photoperiod for 50% flowering was 16 h in the cultivars ‘Öjebyn’ and ‘Ben Tron’, and 17 h in ‘Kristin’. Unexpectedly, however, not all plants flowered after exposure to a 10 h photoperiod, and the number of flowers per plant increased several-fold as the photoperiod was increased from 10 h to 15 h in all cultivars. Apparently, this unexpected result was due to the fact that all plants had only 15 – 16 nodes at the start of the experiments, which is marginal for “ripeness to flower” in black currant. While growth cessation was almost immediate in a 10 h photoperiod, causing only a few additional leaves to be formed during the experiment, the slower response to longer photoperiods apparently enabled the plants to reach the critical size at an earlier stage of the treatment period. However, although plants with 15 – 16 nodes were only marginally responsive to SD induction, buds situated as far down the shoot as the fifth or sixth node were competent to flower. It is therefore suggested that the inability of small black currant plants to flower resides in limitations of the leaves to respond to SD and to produce a florigenic signal, while their buds are fully competent to respond to such a signal.     

Long-day control of flowering in everbearing strawberries

Summary

Photoperiod and temperature control of flowering in a number of perpetual-flowering or everbearing strawberry cultivars of widely varying pedigree has been studied in controlled environments. Flower bud initiation in the cultivars ‘Flamenco’, ‘Ridder’, ‘Rita’ and ‘Rondo’ was significantly advanced by long-day (LD) conditions at temperatures of 15°C and 21ºC; while, at 27ºC, flowering took place under LD conditions only. Some plants of the seed-propagated F₁-hybrid ‘Elan’, raised at 21°C, also flowered under short-day (SD) conditions at 27°C, but reverted to the vegetative state after a few weeks when maintained under these conditions. When vegetative plants growing in SD at 27°C were transferred to LD conditions at the same temperature, they consistently initiated flower buds and started flowering after about 4 weeks. At such a high temperature, flowering could thus be turned on and off by switching between SD and LD conditions. This applied to all the cultivars studied. Also the cultivar ‘Everest’, which was tested only at 21°C, produced similar results. Night interruption for 2 h was effective in bringing about the LD response. At 9°C, flowering was substantially delayed, especially in ‘Flamenco’ and, at this temperature, flowering was unaffected by photoperiod. Runner formation was generally promoted by high temperature and SD conditions, but the photoperiodic effect varied between experiments. We conclude that everbearing strawberry cultivars, in general, whether of the older European-type or the modern Californian-type originating from crosses with selections of Fragaria virginiana ssp. glauca, are qualitative (obligatory) LD plants at high temperature (27°C), and quantitative LD plants at intermediate temperatures. Only at temperatures below 10°C are these cultivars day-neutral.     

Response of pepper and eggplant to continuous light in relation to leaf chlorosis and activities of antioxidative enzymes

Activities of the oxygen radical-scavenging enzymes were determined in leaves of ‘Kyomidori’ pepper (Capsicum annuum L.) and ‘Senryo’ eggplant (Solanum melongena L.) seedlings under two photoperiod treatments; 12 h light/12 h dark (12/12 h) or with continuous light (24/0 h) for six days. Growing eggplant under continuous light resulted in leaf chlorosis after four days and a sharp decline in the chlorophyll content. In eggplants grown under 24/0 h photoperiod, the activities of antioxidative enzymes; superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) rapidly increased from day two. This was an indication of photooxidative damage. In contrast, leaf chlorosis did not occur in pepper plants grown under continuous light, while the chlorophyll content was even higher in the 24/0 h than in the 12/12 h photoperiod. Continuous light did not enhance the activities of SOD and POD in leaves of pepper. However, the CAT activity increased faster and was significantly higher in pepper than in eggplants grown under continuous illumination. Pepper plants did not appear to suffer from photooxidative damage and thus grew normally under continuous illumination.     

Optimal pollination environment of tetraploid ginger (Zingiber officinale Roscoe) evaluated by in vitro pollen germination and pollen tube growth in styles

Pollen germination percentages in vitro of a tetraploid ginger (Zingiber officinale Roscoe), ‘4×Sanshu’, tended to be highest at around 20°C. Pollen tube growth in the styles was greatly enhanced at 17°C, i.e., pollen tubes penetrated into the entire stylar length in 66.7% of the styles used. Pollen stored for at least 3 h under 40–80% relative humidity (RH) almost completely lost its germinability, whereas pollen incubated for up to 3 h under 100% RH retained a relatively higher germinability. Moreover, the pollen germinability that was lowered under 40–80% RH did not recover when the pollen was re-humidified for 2 h under 100% RH before inoculation onto a medium. When stigmas were pollinated with fresh pollen at 17°C under 100% RH, pollen tubes penetrated into the entire stylar length in 26.7% of the styles tested. Under 40–80% RH, however, pollen tubes stopped growing at the middle or the base of styles. From these results, I concluded that the optimal pollination environment of the tetraploid ginger was at 17–20°C under 100% RH.     

Interactions of photoperiod,temperature, duration of short-day treatment and plant age on flowering of Fragaria x ananassa Duch. cv. Korona

The effects of photoperiod (10, 12, 16, 20 or 24 h), day-temperature (12, 15, 18, 24 or 30 °C), the number of short days (14, 21 or 28 days), plant age (4, 8 or 12 weeks) and their interactions on flower and inflorescence emergence were investigated in strawberry cv. Korona. No flowers emerged in plants exposed to photoperiods of 16, 20 or 24 h or to a short-day treatment for 14 days. All plants exposed to short days at daily photoperiods of 10 or 12 h for 21 days or longer, emerged flowers at temperatures between 12 and 18 °C. A further increase in temperature led to a drastic decrease in the total number of flowers per plant. A short-day treatment (10 or 12 h photoperiod) of 28 days resulted in highest numbers of inflorescences and flowers per plant, while a short-day treatment of 21 days resulted in the highest numbers of flowers per inflorescence. Complete flower induction was observed in only 4-week-old runner plants. The number of inflorescences and the number of flowers per inflorescence increased with plant age. However, the start of flowering was delayed with increasing plant age.     

Responses of native Lupinus varius (L.) to culture conditions: effects of photoperiod and sowing time on growth and flowering characteristics

The effects of photoperiod and sowing time on growth and flowering characteristics of Lupinus varius were investigated during two growing periods to determine its responses to culture conditions as a potential native cut-flower crop. The seeds were sown in an unheated plastic greenhouse on 28 September, 28 October and 28 November under natural, 14- and 16-h day-length treatments. 14- and 16-h day-lengths were established by lengthening the natural day-lengths to 14 and 16 h with the use of night break photoperiodic lighting at 1.8–1.9 μmol m⁻² s⁻¹ in 400–700 nm. Photoperiodic lighting, in particular the 16-h day-length treatments, slightly (maximally 15 days) shortened days to flowering and increased plant height in all sowing times relative to natural photoperiods. There were no significant differences in stem and branch inflorescence diameters, in lengths of branch, in main and branch inflorescences in plants grown under natural photoperiod, and 16-h day-length treatments. The highest main inflorescence diameter, the number of branches per plant, and flower numbers on main and branch inflorescences were recorded in plants grown under natural photoperiods, whereas 14-h day-length treatments did not provide sufficient specimens to allow for the measurement of most of the characteristics studied. These findings were interpreted to indicate that L. varius behaves as a facultative long day plant. Additionally, there was a particular shortening of days to flower and growth, and flowering quality decreased linearly with delayed sowing dates under all photoperiodic treatments. The earliest and latest flowering dates were recorded for plants sown in September under 16-h day-length, and plants sown in November under natural photoperiods, respectively. Therefore, sowing in September under natural photoperiods or 16-h artificial day-length resulted in earlier flowering dates and a longer time from sowing to flowering and was consequently the best sowing time with respect to all of the characteristics considered in this study.     

Induction of bulbing of tulip shoots in vitro

Bulbing has been induced in adventitious shoots from floral stem explants of Tulipa ‘Merry Widow’. Incubation of 1-mm thick explants at 20°C on a medium containing 1 mg l⁻¹ NAA and BAP for 14–18 weeks, followed by 4°C for 8 weeks, produced a consistent bulbing response in shoots after transfer back to 20°C. Bulbing was further enhanced by transfer to 25°C and by raising the concentration of sucrose in the medium. Microscopy indicated the presence of developing meristematic centres in shoots 12 weeks of age. Shoots over 16 weeks of age containing meristematic centres gave a variable bulbing response with applied gibberellins and cold incubation, depending on whether gibberellins were supplied continuously in the medium or via a brief soak in sterile gibberellin solutions before sub-culturing. A GA₃ “soak” of 1.0 mg l⁻¹ was optimum for bulb production. The possible mechanisms of cold and gibberellin effects are discussed.     

Environmental control of growth and flowering of Rubus idaeus L. cv. Glen Ample

Environmental control of the annual growth cycle of ‘Glen Ample’ raspberry has been studied in order to facilitate crop manipulation for out-of-season production. Plants propagated from root buds were raised in long days (LD) at 21 °C and then exposed to different temperature and daylength conditions at varying ages. Shoot growth was monitored by weekly measurements and floral initiation by regular sampling and examination of axillary bud #5. Under natural summer daylight conditions at 60°N shoot growth was nearly doubled at 21 °C compared with 15 °C, while at 9 °C one half of the plants ceased growing and formed flower buds at midsummer. Developing shoots have a juvenile phase and could not be induced to flower before the 15-leaf stage. No significant reduction in induction requirements was found in larger plants. Plants exposed to natural light conditions from 10th August, had an immediate growth suppression at 9 and 12 °C with complete cessation after 4 weeks (by September 7). This coincided with the first appearance of floral primordia. At 15 °C both growth cessation and floral initiation occurred 2 weeks later (by September 21), while at 18 °C continuous growth with no floral initiation was maintained until early November when the photoperiod had fallen below 9 h. The critical photoperiod for growth cessation and floral initiation at 15 °C was 15 h. Plants exposed to 10-h photoperiods at 9 °C for 2–4 weeks had a transient growth suppression followed by resumed growth under subsequent high temperature and LD conditions, while exposure for 5 or 6 weeks resulted in complete growth cessation and dormancy induction. The critical induction period for floral initiation was 3 weeks although no transitional changes were visible in the bud before week 4. When exposed to inductive conditions for marginal periods of 3 or 4 weeks, an increasing proportion of the plants (20% and 67%, respectively), behaved as primocane flowering cultivars with recurrent growth and terminal flowering. It is concluded that growth cessation and floral initiation in raspberry are jointly controlled by low temperature and short day conditions and coincide in time as parallel outputs from the same internal induction mechanism.     

Flowering performance and yield of established and recent strawberry cultivars (Fragaria × ananassa) as affected by raising temperature and photoperiod

Plants of six strawberry cultivars were raised under controlled conditions and tested for flowering and yield potential. Short days (SD) at intermediate temperatures for 4 weeks in August induced profuse flowering in subsequent long days (LD) in all cultivars except the late-flowering ‘Malwina’. LD conditions induced flowering only in ‘Nobel’, which has an everbearing parent. ‘Nobel’ and ‘Saga’ exhibited broad temperature adaptation for SD floral induction, which was generally reduced or suppressed at 9 and 27°C. After autumn planting, all cultivars flowered most abundantly in plants raised in SD and intermediate temperatures. Flowering was earliest in ‘Nobel’ and ‘Rumba’. Plants that did not reach floral commitment after 4 weeks in SD continued and completed induction under subsequent natural SD conditions after planting in the field, demonstrating the capability of fractional induction. Berry yield varied in parallel with flowering in the field and was always higher in plants raised under SD conditions. The traditional cultivars ‘Florence’ and ‘Sonata’ out-yielded the more recent cultivars. Some cultivars lost more than two thirds of their initiated flowers during the winter with obvious consequences for their yields. With proper raising management, acceptable yields were obtained after autumn planting even in a cool Nordic climate.     

Manipulation of bolting and flowering in celery (Apium graveolens L. var. dulce). III. Effects of photoperiod and irradiance

SUMMARY

Photoperiods of 8 and 16 h during chilling at 5°C had no effect on bolting and macroscopic flower appearance in celery cv. New Dwarf White. Eight hour photoperiods during chilling however markedly increased the number of plants forming sessile flowers. Short photoperiods (8 h) after chilling decreased the proportion of young, but competent plants that bolted and flowered. Total darkness during chilling completely prevented any subsequent vernalization response either as bolting or as flowering. Reducing irradiance receipt by the plants during chilling from 85 to W m"2 (PAR) had no effect on their vernalization response. After chilling, a reduction in mean daily total irradiance in the glasshouse from 4.05 to 1.57 MJ m"2 d-1 had no effect on bolting and flowering. Confinement of competent plants to darkness for 4-8 d at 20°C just prior to chilling resulted in a highly significant delay (F>0.001) to bolting and reduced the number of plants flowering. Two days of darkness had no significant effect. The inhibitory effects of dark treatments prior to chilling was greater in plants chilled subsequently for six weeks than for nine weeks.     

Gibberellin substitution for the high night temperatures required for the long-day promotion of flowering in Gypsophila paniculata L.

Gibberellic acid (GA) promoted flowering of Gypsophila paniculata plants grown under long-day (LD) conditions at low night temperatures, which otherwise prevented the realization of the LD induction. GA did not induce flower formation of plants grown in short-days (SD), either under low winter temperatures or at the relatively high night temperatures of spring and summer, which allow flowering under LD. GA had only a marginal promotive influence under fully inductive conditions of LD and high temperatures. Thus, in gypsophila, GA substitutes for the high night temperature which is required as a background for the photoperiodic induction, but not for the long photoperiod itself.     

Plant growth regulators and flowering of Brunonia and Calandrinia sp.

Brunonia australis R. Br (Goodeniaceae) and Calandrinia (Portulacaceae), native to Australia, are potential new flowering potted plants. This research investigated the role of daylength and growth regulators, Gibberellic acid (GA₃) and paclobutrazol (Pac), to control vegetative growth, peduncle elongation and flowering of Brunonia and Calandrinia. Plants were grown under long days (16 h), short days (11 h) and 8 weeks under short day then transferred to long day (SDLDs). Plants in each daylength were treated with GA₃, Pac, and GA₃+ Pac. GA₃ was applied as 10 μL drop of 500 mg L⁻¹ concentration to the newest mature leaf. A single application of Pac was applied as a soil drench at 0.25 mg a.i. dose per plant. Both Brunonia and Calandrinia flowered earlier in long days but still flowered in short days, so both can be classified as facultative LD plants. Brunonia under SDLDs were more vigorous and attractive than plants under LDs while still being more compact than plants under SDs. In Brunonia, GA₃ promoted earlier flowering and increased the number of inflorescences under SDs. Pac at 0.25 mg a.i. per plant applied alone or in combination with GA₃ had extended flower development in Brunonia, and resulted in a reduced number of inflorescences per plant compared to the control plants. Vegetative growth of Calandrinia was similar under LDs, SDs and SDLDs, whereas GA₃ application increased plant size. Pac-treated Calandrinia looked compact and attractive, and Pac application did not affect time to flower and flower number.     

Influence of cutting position and temperature during rooting on adventitious root formation and axillary bud break of Stephanotis floribunda

Stock plants were grown in a glasshouse under standard growing conditions. Single-node leafbud cuttings were excised and numbered according to the position on the stock plant. Rooting took place at basal temperatures of 17,20 or 23°C and at different durations at 17 or 20°C followed by 23°C. The rooting period lasted 9 weeks.

The temperature of 17°C for 9 weeks completely suppressed root formation. A temperature of 20°C was decisive for root formation. The optimal rooting temperature was higher than 23°C. Temperature treatments of 17 or 20°C for 2–4 weeks only suppressed rooting slightly compared with the 23°C treatment. Cutting position on the stock plant affected the number of roots formed per cutting but not the rooting percentage. Best rooting was observed in cuttings from the middle part of the stock plant.

Axillary bud break was accelerated with increasing rooting temperature and decreasing duration of the lower temperatures. With increasing cutting position number (numbered from top to base), axillary bud break was considerably delayed.

Temperature treatments which delayed root formation also delayed axillary bud break. On the other hand, the cutting position on the stock plant, which had only a minor effect on the speed of root formation, had a pronounced effect on the speed of axillary bud break.     

Citrus hybrid seedlings reduce initial time to flower when grafted onto shiikuwasha rootstock

The long juvenile period of citrus seedlings before flowering is one of the major obstacles in citrus breeding. Under the citrus breeding program of National Institute of Fruit Tree Science in Japan, 299 citrus hybrid seedlings were grafted onto shiikuwasha (Citrus depressa HAYATA) and cultivated. Approximately one-third of the plants flowered two and a half years after grafting. The average length of the main stems of flowering plants was 308 cm, while that of non-flowering plants was 264 cm. Half of the plants having main stem length ≥300 cm formed flowers, while most of the plants having main stem length <210 cm formed no flower. These results indicate that plant vigor influences the flowering and that grafting effectively accelerated flowering. The proportion of flowering plants varied among cross combinations, ranging from 71.4% to 8.0% among the 10 cross combinations used.