Late-season height control of poinsettia (Euphorbia pulcherrima) is difficult since most chemical growth retardants adversely reduce bract size when applied after first bract color. Paclobutrazol (Bonzi) controls stem elongation late in poinsettia crop development but can excessively reduce bract size if improperly applied. Two experiments were conducted to quantify how paclobutrazol application influenced height and bract area of `Freedom' poinsettia. In the first experiment, paclobutrazol was applied at 1 mg·L-1 (ppm) in 118-mL (4.0-fl oz) volumes per pot [(a.i.) 0.12 mg/pot (28,350 mg = 1.0 oz)] as a drench to a new group of plants weekly from the initiation of short days until 1 week before anthesis. Maximum reduction in height and bract area was obtained when paclobutrazol was applied immediately after short days, and the response to paclobutrazol decreased as application time was increasingly delayed toward anthesis. In the second experiment, paclobutrazol was applied weekly after first bract color as either a drench or subapplication at various concentrations. Plant height and bract area were reduced by 23% when 2 mg·L-1 [(a.i.) 0.24 mg/pot) paclobutrazol was applied through subapplication at first color. The effects of paclobutrazol on height and bract area reduction decreased as application time was progressively delayed. Concentrations lower than 1 mg·L-1 had no significant effect on height or bract area reduction, regardless of application time or method. Generally, the reduction in height and bract area was larger when paclobutrazol was applied through subapplication. The combined results from both experiments indicate that paclobutrazol drench applications after flower initiation concomitantly reduce plant height (internode extension) and bract area. Therefore, drench applications should be delayed as long as possible to limit reduction in bract size.
Genhua Niu, Royal Heins and Will Carlson
Roar Moe, John E Erwin and Will Carlson
The role of irradiance and/or ethylene in inducing mortality and self-branching disorders in Gerbera jamesonii Bolus. seedlings was studied. Seedling mortality increased from 8% to 57% when seed was covered with vermiculite than left uncovered during germination. Supplemental lighting for 30 days after germination decreased seedling mortality and decreased the time to visible bud compared to seed germinated under natural light only. In subsequent experiments, seeds were germinated and then seedlings were water logged or sprayed with ethephon (0.69, 3.45, or 17.25 mM) at four different stages of seedling development. Half of the ethephon-treated seedlings were sprayed with silver thiosulfate (STS). Seedling mortality was greatest after cotyledon expansion but before expansion of the first tree leaf. The highest ethephon concentration caused reduced seedling dry weight after 42 days. Applying STS did not overcome self-branching or meristem necrosis.
Genhua Niu, Royal D. Heins, Arthur Cameron and Will Carlson
The effects of temperature on flower size and number of flower buds of Campanula carpatica Jacq. 'Blue Clips', 'Deep Blue Clips', and Campanula 'Birch Hybrid' were investigated in four temperature and light-transfer experiments. In year 1, 'Blue Clips' and 'Birch Hybrid' plants were grown initially at 20 °C and then transferred at visible flower bud (VB) to 14, 17, 20, 23, or 26 °C until flower (Expt. 1). In Expt. 2, 'Blue Clips' and 'Birch Hybrid' plants were transferred from 14 to 26 °C or from 26 to 14 °C at various intervals after flower induction. Flower size of both species was negatively correlated with average daily temperature (ADT) after VB; flowers on plants grown at 14 °C were 35% larger than those on plants grown at 26 °C. In contrast, temperature before VB had only a small effect on final flower size in both species, although flower diameter of 'Birch Hybrid' plants grown at constant 26 °C was 20% smaller than that of the plants grown initially at 20°C and then transferred to VB to 26 °C. For both species, the longer the exposure to high temperature after VB, the smaller the flowers. Number of flower buds at flower in 'Birch Hybrid' decreased as ADT after VB increased. In year 2, 'Deep Blue Clips' plants were grown at constant 20 °C under high or low daily light integral (DLI, 17 or 5.7 mol·m-2·d-1) until VB, and then transferred to 14, 17, 20, 23, or 26 °C under high or low DLI (Expt. 3). In Expt. 4, 'Deep Blue Clips' plants were grown at 14, 17, 20, 23, or 26 °C until VB, and then transferred to constant 20 °C under high or low DLI until flower. Flower size (petal length) was negatively correlated with ADT both before and after VB, while flower bud number was negatively correlated with the ADT only after VB, regardless of DLI. In both experiments, petal length decreased by 0.3 to 0.5 mm per 1 °C increase in ADT before or after VB. Flowers were larger and more numerous under high than under low DLIs after VB, regardless of the DLI before VB.