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Roberto Hauagge and James N. Cummins

Apple seedlings have a shallow dormancy, as has been observed in many other species. The length of bud dormancy in high-chilling-requirement seedlings does not reflect their genetic constitution well if dormancy is induced before they are 200 days old. Seedling populations sprayed with paclobutrazol and/or ethephon displayed bud dormancy periods resembling those of older populations of similar genetic constitution. Terminal bud formation and dormancy could not be induced by continuously exposing apple seedlings to low temperature (8 ± 1C) and short photoperiod, even after extended periods. Stomate operation may not be completely functional under these conditions. Terminal bud formation was induced by holding apple seedlings above 20C. Additional exposure to low temperature (8 ± 1C) induced leaf fall. These findings suggest the existence of an active regulatory factor that induces terminal bud formation and dormancy and is either turned on or synthesized above 15 to 17C. Chemical names used: β- [(4-chlorophenyl)methyl]- α -(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol(paclobutrazol);(2-chloroethyl)phosphoric acid (ethephon).

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Tony K. Wolf and M. Kay Warren

Examination of `Riesling' grape (Vitis vinifera L.) in Virginia suggested that a high incidence of bud necrosis (BN) in some vineyards was associated with canopy shade and rapid shoot growth. BN appeared to originate as an abortion and dehydration of the primary, and occasionally secondary, buds of the developing dormant bud. BN frequency was lowest among the basal four nodes of a given shoot or cane, and increased in frequency through node 20. Experiments were conducted in 1991 and 1992 to evaluate the specific involvement of shoot growth rate and canopy shade on `Riesling' BN. Shoot growth rate (SGR), measured in a 17-day period around bloom, had a significant, positive relationship with BN in one of two vineyards. BN was positively associated with cane diameter and average internode length. Applying the growth retardant paclobutrazol significantly reduced SGR and BN incidence up to 80% among nodes 6 to 15 in two separate vineyards. Artificial shade (64% or 92% reduction in photosynthetic photon flux), suspended over vine canopies in the 3-week period before véraison, did not affect BN. Shoots of canopies that had been thinned before bloom to 10 shoots/m of canopy expressed slightly lower BN levels than shoots sampled from canopies that had been thinned to 20 shoots per meter. `Riesling' BN appeared more influenced by shoot vigor than shade under Virginia growing conditions. Chemical name β-[(4-chlorophenyl)methyl]-α-(1,1-dimethyl-ethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).

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David A. Gilbertz

Spray applications of uniconazole (UC) or paclobutrazol (PB) were applied 0, 2, or 4 weeks after pinching Dendranthema grandiflora (Tzvelev). `Bright Golden Anne' cuttings planted 4 per 15 cm pot. Cuttings were controlled to 3 shoots each, averaging 5.4 and 14.9 cm at 2 and 4 weeks, respectively.

Final height was affected interactively by week of application and chemical treatment. Treatment at pinch caused less stem elongation than later treatments, probably due to persistence of PB and UC activity until flowering. At week 4, 67% of stem elongation had already occurred and, therefore, less retardation was possible. Of the 4 triazole treatments, PB at 30 mg 1-1 (20 ml per pot) applied at pinch produced heights similar to daminozide 5000 mg 1-1 applied at 2 weeks. PB at 60 mg 1-1 gave similar height control as UC 15 mg 1-1. UC 30 mg 1-1 treated plants were shortest regardless of treatment timing, averaging 16.9 cm applied week 2.

Other growth data was pooled for week of application and for chemical treatment since there was no interaction. Flowering was delayd 2 days and flower dry weight was reduced up to 26% by treatment at pinch compared to later treatments. Flower diameter was only minimally affected by treatments.

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Jeff B. Million, James E. Barrett, and Terril A. Nell

Drench applications of paclobutrazol (PBZ) are becoming increasingly popular as a means for controlling height in potted plants, and research is being conducted to quantify the distribution of PBZ following applications. In one trial, 120 ml of 0 or 1 mg 1-1 PBZ were applied to 15-cm pots filled with either Vergro Klay Mix (no bark) or Metro Mix 500 (bark). A bioassay using broccoli (Brassica oleracea L. Italica) seedlings was used to quantify PBZ in leachates and media following treatment drenches. Leachate PBZ concentrations were lower for Vergro than for Metro Mix 500; however, leachates for both media were <0.1 mg·liter–1. Concentrations of PBZ in media decreased with depth and were four to 10 times higher in the uppermost 2.5 cm than in lower horizons. For the uppermost 2.5 cm of media, higher PBZ concentrations were recovered in Metro Mix 500 than in Vergro. A follow-up study will compare surface vs. subsurface application methods on the movement of PBZ into pots.

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A. Abu El-Kashab, A.F. El-Sammak, A.A. Elaidy, M.l. Salama, and M. Rieger

We studied the effect of a 200-mg·liter–1 foliar application of paclobutrazol (PBZ) on growth and physiological responses of Prunus persica `Nemaguard' (salt-sensitive) and Olea europea `Manzanillo' (salt-tolerant) to salt stress. One-year-old trees were grown in 3 sand: 3 field soil: 4 pine bark media in 20-cm pots in a greenhouse and were irrigated with nutrient solutions adjusted with 0, 9, 18, or 36 mmol NaCl for peach and 0, 36, 72, 108 mmol NaCI for olive. Dry weight, photosynthesis, and leaf conductance decreased with increasing salinity for both species. However, leaf expansion rate was unaffected by NaCl. PBZ reduced dry weight for peach only, but PBZ increased photosynthesis and reduced leaf expansion rate for both species. Relative water content was decreased by salt but increased by PBZ. PBZ reduced the foliar Na and Cl content in peach but not olive. Olive had less Na in leaves than peach at 36 mmol NaCI, accumulated less C in leaves in all salt treatments, and had higher foliar Na without symptom expression. PBZ may reduce salt stress in sensitive species like peach by reducing foliar Na and Cl accumulation but has less influence on the salinity response of the more salt-tolerant olive.

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Juan P. Brigard, Richard L. Harkess, and Brian S. Baldwin

Tomato seedling hypocotyls elongate rapidly after germination resulting in weak seedlings. The effects of 0, 250, 500, 750, or 1000 mg paclobutrazol (PB)/L seed soak and soaking times from 1 to 12 hours on tomato (Solanum lycopersicum L.) seed germination, seedling growth, and plant growth were tested. Adequate height control was obtained with 250 mg PB/L while soaking time did not affect seedling growth. In a second experiment, PB was tested at 0, 50, 100, 150, 200, or 250 mg PB/L soaking the seed for 1 hour. A concentration of PB at 100 mg·L–1 provided optimum control of hypocotyl elongation with minimal residual effect on subsequent plant growth. In a third experiment, seed soaked at the different PB concentrations were germinated and grown under light intensities of 0.09, 50, 70, or 120 μmol·m–2·s–1. Seedlings grown under 0.09 μmol·m–2·s–1 were not affected by PB treatment and did not develop an epicotyl. PB seed soak treatment gave greater growth suppression under 50 μmol·m-2·s-1 than under the two higher light levels. Soaking tomato seeds in 100 mg PB/L for 1 hour prevented early hypocotyl stretch of tomato seedlings with no long term effects on plant growth. This treatment effectively prevented excessive hypocotyl elongation when seeds were germinated under low PAR while not over controlling elongation under high PAR conditions.

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Michael R. Evans, Harold F. Wilkins, and Wesley P. Hackett

Exogenous foliar spray applications of gibberellic acid (GA3) applied at 7- or 14-day intervals providing 50 or 125 μg per plant inhibited long-day (LD) floral initiation in poinsettia [Euphorbia pulcherrima (Willd. ex. Klotzsch)]. Periodic application of GA3 resulted in an additional number of nodes being produced by the plant before floral initiation equivalent to the number of nodes over which GA3 was applied. Further, GA, application eliminated the nodal position dependence of the long-day node number (LDNN) of axillary meristems observed in control plants. It was concluded that GA3 application inhibited the inclusion of nodes into the LDNN count and thus inhibited ontogenetic aging of the meristem. Exogenous application of GA, also inhibited LD floral initiation, while application of GA4 had no effect. Application of GA7 delayed LD floral initiation, but plants did initiate cyathia by the termination of the experiment. All gibberellins increased the average internode lengths similarly. The gibberllin-biosynthesis inhibitors chlormequat and paclobutrazol had no effect on LD floral initiation when applied as single or multiple foliar sprays or as soil drenches, although heights and internode lengths were reduced by application of the inhibitors. The LDNN of plants grown at 31C was significantly higher than of plants grown at 16, 21, or 26C. All plants eventually initiated cyathia regardless of temperature. When plants were grown under a range of day/night temperatures, an increase in the LDNN occurred only when plants were grown at 31C during the day. Chemical names used: 2-chloroethyl-trimethyl-ammonium chloride (chlormequat); (+/-)-(R*,R*)-β -(4-chlorophenyl)methyl-α -(1,1-dimethylethyl)-1-H-1,2,4-triazole-1-ethanol (paclobutrazol).

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A. Hagiladi and A.A. Watad

Potted Cordyline terminalis L. `Prins Albert', a foliage plant, was treated with foliar sprays or growth medium drenches of paclobutrazol for plant growth control. Paclobutrazol effectively reduced shoot length measured 4 months following application, the drench being more effective than the spray. Application of paclobutrazol at 200 ppm by either method gave a desirable compact and marketable product. Drench applications at 1000 ppm promoted side-shoot formation. Leaf morphology was altered from an elongated to a more oval form as the paclobutrazol concentration increased, but leaf count was not affected by paclobutrazol, except for the highest drench concentration, which reduced leaf count by 10%. Chemical name used: β– [(4-chlorophenyl)methyl] –α– (1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).

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Rodney Serres and Brent McCown

The capability to uniformlyinduce flowering in cranberry (Vaccinium macrocarpon Ait. `Stevens') in < 1 year from microculture was investigated to accelerate cranberry breeding and to study woody plant reproductive biotechnology. Flower buds were induced on newly micropropagated cranberry plants during the first growing season. A treatment of 2.5 mg of paclobutrazol applied as a soil drench per 2- to 3-month-old potted plant in midsummer, when the plants were grown in coldframes under natural daylength and air temperatures, resulted in 70% of the plants flowering. Plants not treated with paclobutrazol did not flower. Reduced but significant flower bud set was observed on plants treated with paclobutrazol but grown in the greenhouse under natural daylength. Flowering was stimulated by cold treatment coupled with gibberellin sprays and/or repotting to nonpaclobutrazol-treated medium. Chemical name used: β -[(4-chlorophenyl)methyl]-ct-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).

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Jeff B. Million, James E. Barrett, Terril A. Nell, and David G. Clark

A broccoli (Brassica oleracea var. botrytis L.) seedling bioassay was used to measure paclobutrazol activity and distribution in two growing media following drench or subirrigation applications. The bioassay exhibited a saturation-type response curve for paclobutrazol concentrations up to 1000 μg·L-1 in solution and 100 μg·L-1 in the media. The concentration of paclobutrazol required to achieve one-half of the maximum observed bioassay activity was 3-fold as high in bark-based commercial potting medium as in a peat-based medium. Less than 2% of applied paclobutrazol leached out during the drench application despite the collection of up to 50 mL of leachate per 120 mL of the solution (1000 μg·L-1) that was applied per 15-cm pot. Immediately following drench application, paclobutrazol concentrations in both media were highest in the uppermost 2.5 cm and decreased downward. By 3 weeks after treatment, drench-applied paclobutrazol had moved into lower depths. Distribution of paclobutrazol was limited to the bottom 2.5 cm of media when applied as a subirrigation soak. Chemical name used: (±)-(R*,R*)-β-[(4-chlorophenyl)methyl]-α-(1,1-dimethyl)-1H-1,2,4-triazole-1-ethanol (paclobutrazol).