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Douglas A. Cox

Paclobutrazol (PBZ) was applied to `Mustang' geranium (Pelargonium × hortorum L.H. Bailey) as a single growth-medium drench at 0.06 mg a.i./pot or as a single foliar spray at 100 mg·liter-l when the plants had three to four expanded true leaves (34 days after sowing). At these rates, PBZ caused excessive growth suppression but plants flowered earlier than untreated controls. A single foliar spray of gibberellic acid (GA) at 100 mg·liter-l applied 0 (same day), 7, 14, or 21 days after PBZ reversed the growth suppression caused by PBZ. Plants treated with GA30 or 7 days after PBZ were as tall or taller and flowered at the same time as or later than the untreated (no PBZ, no GA3) controls. Plants treated with GA, 14 or 21 days after PBZ were shorter and flowered earlier than untreated controls but were taller than plants treated with PBZ alone. Response to GA3 was similar whether PBZ was applied as a drench or as a spray. Chemical name used: (+)-(R*,R*)-β([4-chlorophenyl]methyl)-α-(1,1-dimethylethyl)-1 H -1,2,4-triazole-1-ethanol (paclobutrazol).

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Douglas A. Cox

Six cultivars of poinsettia (Euphorbia pulcherrima Willd. ex Kl.) were grown in unlimed growth medium consisting of equal volumes of sphagnum peat and perlite. Plants received fertilizer solutions supplying Mo at either 0.0 mg·liter-1 (-Mo) or 1.0 mg·liter -1 (+Mo). In the –Mo treatment, moderate to severe symptoms of Mo deficiency (marginal and interveinal chlorosis, marginal necrosis, and downward curling of the margins) developed on the middle-aged and some recently matured leaves of `Annette Hegg Brilliant Diamond' and `Eckespoint Lilo', while some interveinal chlorosis occurred on the same leaves of `Gutbier V-17 Angelika'. No symptoms appeared on `Gross Supjibi', `Peace Regal Velvet', and `Peace Noel'. All cultivars were symptomless in the + Mo treatment. In the –Mo treatment, upper, recently matured leaves of the symptomatic cultivars and two of three symptomless cultivars had Mo concentrations at or near the critical level for deficiency; however, nitrate reductase enzyme activity was higher and NO3-N was lower in the leaves of symptomless cultivars than of symptomatic cultivars.

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Douglas A. Cox

`Annette Hegg Brilliant Diamond' poinsettia (Euphorbia pulcherrima Willd. ex Kl.) was grown in an unlimed growth medium consisting of equal volumes of sphagnum peat and perlite and received a fertilizer solution supplying all micronutrients except Mo. Plants were untreated or foliage was sprayed with solutions of 1, 10, or 100 mg Mo/liter 5, 8, or 11 weeks after pinching. Untreated plants developed foliar symptoms of Mo deficiency (marginal and interveinal chlorosis, marginal necrosis, and downward curling), and leaf tissue contained Mo below the critical level of 0.5 μg·g-1 and NO3-N > 1.0%. Treatment at 5 or 8 weeks largely prevented deficiency symptoms, increased tissue Mo, and reduced tissue NO3-N. Molybdenum deficiency symptoms were in the early stages of development on the day of treatment when plants were sprayed at 11 weeks. Molybdenum sprays at this time increased tissue Mo and reduced tissue NO3-N but did not eliminate the symptoms. However, when the experiment was completed 15 weeks after pinching, the number of leaves showing symptoms was about one-half that of untreated plants. Increasing the concentration of Mo in the spray solution increased the concentration of Mo in the leaves but had no effect on NO3-N.

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Douglas A. Cox

Nine cultivars were grown in a 1:1 sphagnum peat and perlite medium with no limestone or trace element fertilizer. Fertilizer solutions of 300 ppm N and K (calcium nitrate and potassium nitrate) and 24 ppm Mg were applied at every watering. Solutions supplied all trace elements and either 0 or 1 ppm Mo. Moderate to severe foliar symptoms of Mo deficiency developed on `Annette Hegg Brilliant Diamond' and `Eckespoint Lilo' with 0 ppm Mo. Symptoms did not occur with 1 ppm Mo. No Mo deficiency symptoms developed on the other 7 cultivars which included `Supjibi', `Gutbier V-17 Angelika', `Peace Regal Velvet', and `Cheers!'. With 0 ppm Mo these cultivars generally maintained higher levels of nitrate reductase enzyme activity and lower tissue nitrate levels than the 2 showing symptoms.

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Douglas A. Cox

`First Lady' marigold (Tagetes erecta L.) and `Selenia' New Guinea impatiens (NGI; Impatiens sp. hyb.) were grown in solution culture for 60 days. At 10-day intervals, plants received low N for 10 days (marigold) or 20 days (NGI). Low-N treatment was 5% and 10% of the control, respectively, for marigold and NGI. After each low-N period, FW of treated and control plants was measured and N uptake by the controls was determined by solution depletion. Nitrogen uptake by marigold reached a peak 40 days after planting, and then decreased somewhat during the final 20 days of the experiment. In contrast, N uptake by NGI increased gradually after planting, reaching its highest level at the end of the experiment (60 days). Low-N periods 10 to 20 and 20 to 30 days after planting reduced the FW of marigold about 35% vs. control. FW reductions resulting from earlier or later low-N periods were much smaller or did not occur. Reductions in NGI FW resulted from low-N periods 20 to 40, 30 to 50, and 50 to 60 days after planting. While short periods of low N reduced the growth of both species, these reductions were desirable and not excessive, and no foliar symptoms of N deficiency were apparent at any time. Results of these experiments have implications for efficient fertilizer use and growth suppression using short periods of low nutrition.

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Douglas A. Cox

'Annette Hegg Brilliant Diamond' plants were cultured under Mo stress conditions by using an unlimed sphagnum peat and perlite potting medium and by supplying all trace elements but Mo in the fertilizer solution. Plants were untreated or sprayed with solutions of 1, 10, or 100 μg Mo·liter-1 5, 8, or 11 weeks after pinching. Untreated plants developed foliar symptoms of Mo deficiency (interveinal chlorosis, marginal necrosis, and marginal curling) and leaf tissue contained Mo below the critical level of 0.5 μg·g-1 and NO3-N above 1.0%. At 5 or 8 weeks all Mo spray concentrations prevented deficiency symptoms, increased tissue Mo, and reduced tissue NO3-N. Some symptoms were visible when plants were treated at 11 weeks. Mo sprays at this time did not eliminate the symptoms but reduced the number of leaves showing symptoms to about one-half that of untreated plants when the experiment was ended 15 weeks after pinching.

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Douglas A. Cox

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Laura K. Judd and Douglas A. Cox

New Guinea impatiens (Impatiens sp. hybrids) are particularly sensitive to growth medium soluble salts level during the first four to six weeks after potting. Results of this experiment show that this response is affected by the interaction of fertilizer rate and application frequency. Solutions containing 20N-4.3P-16.6K at 0.5, 1.0, 1.5, or 2.0 g·liter-1 were applied to `Selenia' growing in 520 ml pots 4, 8, 12, or 16 times (evenly-spaced) during a 70 day experiment. A significant interaction occurred between fertilizer rate and application frequency. Shoot dry weight (DW) increased linearly with application frequency at 0.5 g·liter-1. Overall 16 applications of 0.5 g· liter-1 resulted in the most growth of all rate and frequency combinations. Maximum DW at 1.0 g·liter-1 was achieved with 12 applications and 8 applications resulted in the most DW with 1.5 and 2.0 g·liter-1. In treatments where growth was inhibited, growth medium EC exceeded 1.0 dS·m-1. EC did not exceed 0.4 dS·m-1 at any application frequency with 0.5 g·liter-1.

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Laura K. Judd and Douglas A. Cox

To test the effects of fertilizer concentration and growth medium electrical conductivity (EC) on the growth of New Guinea impatiens (Impatiens sp. hybrids or I. platypetala Lindl.), plants of `Selenia' were grown 70 days in a commercial soilless medium and irrigated with solutions of 20N-4.3P-16.6K at concentrations of 0.5, 1.0, 1.5, or 2.0 g·liter-1. In a fifth treatment (“delay”), no fertilizer was applied for the first 14 days after planting, then 0.5 g·liter-1 was applied for the next 14 days, followed by 1.0 g·liter-1 until the end of the experiment. Measurements of shoot dry weight and growth medium EC were made at 14-day intervals. Differences in dry weight between fertilizer treatments became significant (P = 0.0001) 42 days after planting. Over the 70-day experiment, plants grew most with 0.5 g·liter-1 and delay treatments. High fertilizer concentrations (1.5 and 2.0 g·liter-1) caused the most growth suppression and resulted in increasing growth medium EC with time. An EC of ≈1.5 dS·m-1 or higher was associated with suppressed growth beginning 42 days after planting. A mild chlorosis developed on the leaves of some plants at the two highest fertilizer concentrations.