Hippeastrum (Hippeastrum hybridum), a native of Central and South America, is a bulbous ornamental flowering plant in the Amaryllidaceae family [1] . It features large and showy flowers that come in a variety of bright colors. Hippeastrum is a popular flower for Christmas and the New Year because of its combination of red petals and dark green leaves [2] . Bulbs are produced by the Netherlands, South Africa Israel, Japan, Brazil and the United States (Florida). Hippeastrum production in Florida for the 2012 season was valued at seven million dollars; centered mainly in Highlands County [3] .

Ammonium as a sole source of nitrogen can result in a variety of negative effects to most plant species. These include but are not limited to a reduced photosynthetic rate, lower dry weights, reduced root growth, and a reduced rate of water uptake [4] -[7] . These studies were mostly on field crops where nitrification is presumed to be rapid. Nursery crops are often grown in soilless media that maybe pasteurized. The growth of ornamental shrub, “Doublefile viburnums” (Viburnum plicatum var. tomentosum), shoot and root was significantly lower with NH₄ (ammonium) supplied alone than that with NO₃ (nitrate) or a combination of NH₄ and NO₃ [8] . Hydroponically grown date palm seedlings had the greatest leaf area and root weight with NO₃ as the sole nitrogen (N) source but had a lower root:shoot ratio than with NH₄- or urea-N sources [9] . Lilium root growth was poor in liquid medium containing a high NH₄/NO₃ ratio, however, a high NH₄ content increased bulb enlargement [10] . In contrast, Lilium bulblets supplied with NH₄NO₃ (ammonium nitrate) depleted a hydroponic media of NH₄ very rapidly while NO₃ removal was at a steady state [11] . Uniform availability of ions was thought to be important for optimum bulblet growth. A NH₄/NO₃ ratio of approximately 30:70 is thought to produce the best growth in most plants [4] .

In Hippeastrum, bulbs grew best when fertilized with 25 mM N (as NH₄NO₃) and enriched with CO₂ (carbon dioxide) [12] . Hippeastrum bulbs are a strong sinks for nutrient accumulation during the initial stages of bulbil development. Once leaves begin to supply assimilates for plant growth, bulbs become a source of nutrients for the developing plant [13] . Similar results were reported for Odontoglossum hybrids [14] . When the N concentration was lower than that supplied to the control, the order of N content was leaves > roots > bulbs, but when the N concentration was higher than the control, the order was bulbs > leaves > roots. However, the correct balance of NH₄ to NO₃-nitrogen in a fertilizer mix for Hippeastrum plants is largely unknown.

The study was conducted at the Subtropical Horticulture Research Station (SHRS) in Coral Gables, FL (Latitude: N 25˚38.577815', Longitude: W 80˚17.545062').

Hippeastrum bulbs were grown in a greenhouse from seed collected from a self-pollinated tetraploid hybrid clone. Seed were germinated on a mist table in two groups planted approximately 30 d apart. The first (group 1) group of 150 plants was grown in the SHRS greenhouse 3 which has evaporative cooling; the second (group 2) group of 150 plants was grown in SHRS greenhouse 2 which is cooled by fans. Greenhouse 3 is approximately 10˚C cooler (mean daily temperature) than greenhouse 2. In addition, plants in greenhouse 3 received a starter fertilizer treatment resulting in larger bulbs at the onset of the study. Therefore, each group was treated separately. Soil was completely removed from plants in group 1 when bulbs were transplanted. On March 26, 2012, all plants were transplanted into 10 × 10 cm square, 450 mL pots containing a mix of 50% pine bark, 10% sand, 40% coir pith, fortified with 4.4 kg dolomite, and 3.2 kg of a micronutrient mix per cubic meter (6% Ca from calcium carbonate, 3% Mg from magnesium carbonate, 12% S, 0.10% B from sodium borate, 1% Cu from copper sulfate, 17% Fe from ferrous sulfate, 2.5% Mn from manganese sulfate, 0.05% Mo from sodium molybdate, 1.0% Zn from zinc sulfate). Phosphorus (P) and potassium (K) were supplied at a rate of 1 g triple super phosphate and 3 g K₂SO₄ per pot at planting and again every four months.

Nitrogen was supplied 2x weekly following irrigation with a syringe containing a solution with 0.241 M N for High concentration enough N to supply 0.6 g (high), 0.121 M N for Medium concentration 0.3 g (medium) or 0.060 M N for Low concentration 0.15 g (low) total N over a four month period. Nitrogen was supplied in different combinations of NO₃ and/or NH₄. Nitrate:NH₄-N ratios were 100% NO₃:0% NH₄ (100NO₃), 70% NO₃:30% NH₄ (70NO₃), 50% NO₃:50% NH₄ (50NO₃) (second group only), 30%NO₃:70%NH₄ (30NO₃), or 0% NO₃:100% NH₄ (100NH₄). Plants were irrigated with in-pot emitters 2x weekly to saturation plus a 10% leaching requirement during the hot months and once weekly when temperature cooled.

Initially measurements of bulb diameter were recorded on 17 Dec. 2012. Bulb number, diameter and weight were determined after 400 days of growth and again after 730 days. Growth is defined as the change in bulb diameter between measurements from the initial measurement to the diameter after 400 days.

A completely randomized design was utilized with five levels of ammonium-N to nitrate-N ratios, three different application rates, with eight plants per treatment for the first group. The second group had a completely randomized design with four nitrate-N to ammonium-N ratios with 12 plants per treatment. Each group was analyzed separately. Analysis of variance was performed on the data using the Mixed Procedure of Statistical Analysis System (SAS Institute, Cary, NC) and Least Squares Means values were separated by Tukey’s test. Statistical tests were run of the model y = nitrogen source x application rate x source by application interaction x replication, where nitrogen source and application rate were fixed effects.

Table 1 shows the weight, diameter and number of bulbs produced for Group 1. Treatment 100NO₃M, followed by treatments 100NO₃H, 70NO₃M, 30NO₃H and 70NO₃H, produced the greatest bulb weights. Within a given NO₃ to NH₄ nitrogen source ratio, no significant differences between high and medium application rates occurred for bulb weight. The low application rate produced bulbs that were similar to (30NO₃L) or less than the medium rate. At both the high and medium application rate all treatments including NO₃ as a nitrogen source outperformed treatments using NH₄ as the sole source of nitrogen. Bulb diameter followed the same general trend as bulb weight. No differences existed between the high and medium application rates. Treatments receiving a portion of their nitrogen as NO₃ yielded larger bulb diameters than those receiving NH₄ as the sole nitrogen source at the high and medium application rate. Treatments receiving nitrogen only in the form of NH₄ produced the least number of bulbs.

In Group 2 the largest diameter bulbs were produced in the 70NO₃ and 50NO₃ high N treatments; however, for plants receiving a portion of their N as NO₃, no significant differences occurred between treatments at the high N rate (Table 2). Plants treated with 100% NH₄-N at the high rate produced bulbs similar in diameter to plants treated with a mix of NO₃ and NH₄-N supplied at the medium rate. Within any NO₃/NH₄-N ratio grouping, fertilization at the high N rate resulted in larger diameter bulbs than the medium or low application rate. At the medium (or low) application rate 100NO₃ produced larger bulbs than any other N treatment at that application level.

Bulb weight followed the same general trend as bulb diameter; at the high N rate a mixture of NO₃ and NH₄-N produced the greatest bulb weights, within any N application level, 100NH₄-N treatments produced the

^zNumbers in an individual column followed by the same letter are not significantly different at P = 0.05; ^ynot significant

^zNumbers in an individual column followed by the same letter are not significantly different at P = 0.05; ^ynot significant.

smallest bulbs, and bulb size increased with increasing N fertilization rates. Secondary bulbs were generally small and added very little to the total bulb weight; total bulb weight followed the same trend as main bulb weight. No significant differences occurred between treatments in the number of bulbs produced. However, the 100NO₃ treatments tended to produce the greatest number of bulbs and 100NH₄ the lowest within N application levels.

The number of bulb scales determines flowering in Hippeastrum. A flower bud is produced with every eight scales. Ephrath et al. [15] and Silberbush et al. [12] reported that bulb diameter was the best predictor of flowering capability and timing. Figure 1 shows a box plot of growth in bulb diameter after one year of fertilizer treatments. An unfertilized treatment (zero N) is included for comparison. Zero N treatment only provided enough fertility to maintain initial bulb size with very little additional increase in bulb diameter. Since initial size of a bulb should not affect its increase in relative growth from one date to another, Groups 1 and 2 data are combined in Figure 1. Bulb growth responded not only to increase from 0.15 to 0.6 g N (low to high level) but also to the form of N supplied to the plant. The median increase in bulb growth was greater with a portion of N supplied as NO₃ than with NH₄-N alone. Treatments producing the five largest median bulb sizes all received a portion of their N as NO₃ applied at the high rate or with 100NO₃ treatment high and medium application rates. Bulbs from Group 1 are arranged in order of high N level treatment diameter mean (Figure 2). Root density was rated on visual appearance as low, medium or high. Plants receiving 50% or more of their N as NO₃ produced a denser root mass than plant receiving a majority of their N in the form of NH₄ (Figure 2(a)). Below 50% NO₃-N root density began to thin out. Root density was thinner at the low N fertility than at the high level (Figure 2(b)).

During the second year greenhouse temperatures were excessive and as a result bulb growth was inhibited. Although nitrogen source affected bulb diameter (Table 3) the 50NO₃L treatment was the only treatment with a significantly larger increase in diameter. All other treatments resulted in a similar change in bulb diameter from the previous year (data not shown). In all treatments except for 50NO₃L less than one cm of growth was recorded. Nitrogen source and application rate had a significant effect on changes in bulb weight.

All five high nitrogen application rates and the 50NO₃M and 50NO₃L treatments were the only treatments that resulted in increased bulb weight after the second year (Figure 3). The reduction in bulb weight for the remaining treatments was attributed to extended periods of elevated summer greenhouse temperatures. Fertilization at the medium to low rates did not supply enough nutrients to support plant maintenance and additional growth with greater respiratory demand at high summer greenhouse temperatures. Bulb diameter and weight gain in year two were similar to that of the first year; 50NO₃H and 70NO₃H outperformed all other treatments.

Nitrogen is a nutrient element that plants require in amounts larger than that of any other soil supplied element. Most plants can utilize both and, however, at high concentration can be toxic to plants [4] . Plants differ in their sensitivity to toxicity. Onion, a member of the Amaryllidaceae Family, is known to be fairly tolerant to high concentrations of [6] [16] . In our study Hippeastrum, a member of the Amaryllidaceae, suffered reduced bulb size when fertilized with ammonium as the sole source of N. The ammonium applied was not considered excessive, however, temperatures in the greenhouse were high compared to conditions that Amaryllidaceae were normally grown under in South Florida. High concentrations of ammonium reduce plant growth in a number of ways, including but not limited to: reduced stomatal conductance, increased utilization of reserve carbohydrates and the ability to neutralize reactive oxygen species [6] [17] -[19] . Each of these mechanisms can be enhanced at high temperatures. A long-term exposure to ammonium combined with elevated temperatures could easily account for the reduction in growth from ammonium as the sole source of N compared to other N sources with different nitrate concentrations. Based on these results, application of N as NH₄NO₃ at 0.6 g per 4 months produced the largest increase in bulb diameter.

CarlosVazquez,StewartT. Reed,ChristopherDunn, (2015) Nitrogen Fertilization as Ammonium or Nitrate-N on Hippeastrum hybridum Bulb Growth. Agricultural Sciences,06,1547-1554. doi: 10.4236/as.2015.612148