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The form of nitrogen (N) in fertilizer can influence plant growth, nutrient uptake, and physiological processes in the plant. However, few studies have been conducted on the effects of N form on tall bearded (TB) iris (Iris germanica L.). In this study, five NH4:NO3 ratios (0:100, 25:75, 50:50, 75:25, and 100:0) were applied to investigate the response of TB iris to different N form ratios. NH4:NO3 ratios in fertilizer did not affect the leaf, root, and rhizome dry weight, or total plant dry weight. Plant height and SPAD reading were affected by NH4:NO3 ratios in some months, but not over the whole growing season. Neither spring nor fall flowering was influenced by NH4:NO3 ratios. Across the whole growing season, leachate pH was increased by higher NH4:NO3 ratios. At the end of the growing season, concentrations of phosphorous (P), iron (Fe), manganese (Mn), zinc (Zn), copper (Cu) in leaf; calcium (Ca), magnesium (Mg), Mn, boron (B) in root; and N, P, Mg, Fe, Mn, and Zn in rhizome tissues were affected by NH4: NO3 ratios. Greater NH4:NO3 ratios increased the uptake of Fe, Mn, and Zn. The net uptake of N was unaffected by NH4:NO3 ratios, which indicates TB iris may not have a preference for either ammonium or nitrate N. Nitrogen is an important macronutrient needed by plants and often required in the highest amount of all the mineral elements. Normally, N is supplied to plants in form of nitrate (NO3 ), ammonium (NH4 ), or urea [CO(NH2)2]. Both the rate and the form of N fertilizer can influence plant growth and must be managed appropriately to maximize plant growth and development (Bar-Yosef et al., 2009; Bernstein et al., 2005; Niu et al., 2011). Usually a large proportion of urea from fertilizer would be converted to ammonium in soil or medium and then absorbed by plants. In other words, urea may be considered to be the same as ammonium during the uptake process. Thus, the two major N forms taken up by plants are NO3 – and NH4 . Many fertilizers provide nitrogen in one or both of these forms. The optimal NH4:NO3 ratio depends on many factors, such as plant species, age of the plant, application timing, climate, and location (Marschner, 2012). The responses of plant growth to N forms or NH4:NO3 ratio vary with species (BarYosef et al., 2009; Bernstein et al., 2005; Hewins and Hyatt, 2010; Mendoza-Villarreal et al., 2015; Niu et al., 2011). Usually, plants adapted to acid soils prefer NH4 , whereas plants adapted to high pH soils prefer NO3 – (Marschner, 2012). Solutions with 67:33 NH4:NO3 ratio produced greater biomass than other ratios in mesquite (Prosopis velutina) (Hahne and Schuch, 2006). Seventyfive percent of NO3 – in total N is preferable for improving growth and flowering in hybrid phalaenopsis orchid (Phalaenopsis) (Wang, 2008). Rohozinski et al. (1986) found supplying ammonium to apple trees (Malus pumila) increased floral initiation. In some plants, N form has no significant effects on plant growth, for instance, the dry weight of shoot and root, and root-to-shoot ratio in texas mountain laurel (Sophora secundiflora) were unaffected by NH4:NO3 ratio (Niu et al., 2011). Chlorophyll content can be affected by NH4:NO3 ratios. In endive (Cicorium endivia), chlorophyll content increased with increasing NH4:NO3 ratios due to its tolerance to ammonium nutrition (Bonasia et al., 2008). In apple (Malus domestica), sole ammonium nutrition led to the lowest chlorophyll content (Sotiropoulos et al., 2005). This negative effect of high ammonium ratio on chlorophyll content could be caused by low pH in the medium reducing the enzyme activity and cell growth (Mashayekhi-Nezamabadi, 2000) or ammonium accumulation increasing leaf sensitivity to ethylene which enhanced chlorophyll loss (Hsu et al., 2003). But, in some plants, NH4:NO3 ratio had no effect on chlorophyll content or SPAD reading, such as in garlic mustard (Alliaria petiolata) (Hewins and Hyatt, 2010). The process of up taking NH4 + or NO3 – has a strong impact on the uptake of other cations and anions and rhizosphere pH.When roots take up NO3 , which has a negative charge, and NH4 , which has a positive charge, they typically release an identically charged molecule to maintain a balanced pH inside the plant cells. For example, the assimilation process of one molecule of NH4 + produces one proton which will be excreted into the external rhizosphere, reducing rhizosphere pH (Marschner, 2012). Since NO3 – has a negative charge, the process of NO3 – uptake is associated with an uptake of protons from the rhizosphere that leads to increasing pH (Hinsinger et al., 2003). High levels of NH4 + can also inhibit the uptake of cations such as calcium and magnesium from the substrate and thus induce a deficiency of those elements in the crop (Adams, 1966; Siddiqi et al., 2002). This decreased uptake of essential cations could cause more problems for plant growth and metabolism. For instance, calcium deficiency led to ‘‘toppling’’ disorder in tulip (Tulipa) (Nelson and Niedziela, 1998). However, ammonium applications can also benefit uptake of nutrients. For instance, in calcareous soils, ammonium application reduces the incidence of iron (Fe) deficiency (Mills and Jones, 1997). In addition, ammonium-fed plants accumulate more phosphate and sulfate due to acidification of the rhizosphere, whereas nitrate depresses the uptake of those essential anions (Marschner, 2012). Thus, most of the time, supplying proper NH4:NO3 ratio results in the highest growth rates and plant yields (Kafkafi, 1990; Santamaria and Elia, 1997). The objective of this study was to investigate the effects of NH4:NO3 ratios on plant growth, flowering, and uptake of nutrients in ‘Immortality’ TB iris. Materials and Methods This study was conducted under natural conditions in Starkville, MS (lat. 33 27# N, long. 88 47# W). Starkville has a humid temperate climate with long summers and short, mild winters. The monthly average air temperature in Starkville is 17, 21, 25, 26, 26, 23, 17, 11, and 6 C in April, May, June, July, August, September, October, November, and December, respectively. The monthly average relative humidity in Starkville is 66%, 69%, 70%, 73%, 73%, 73%, 70%, 70%, and 74% in April, May, June, July, August, September, October, November, and December, respectively. Rhizomes (average caliper = 4.7 cm and length = 5.8 cm) of ‘Immortality’ TB iris Received for publication 7 Jan. 2016. Accepted for publication 14 June 2016. Contribution of the Mississippi Agricultural and Forestry Experiment Station Journal article no. 12745. This work was supported by the Mississippi Agriculture and Forestry Experiment Station, the USDA National Institute of Food and Agriculture Hatch projects MIS-249120 and MIS-212050, and the China Scholarship Council. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by Mississippi State University and does not imply its approval to the exclusion of other products or vendors that also may be suitable. Current address: Hebei Agricultural University, Baoding, Hebei 071000, China. Corresponding author. E-mail: gbi@pss.msstate.edu. HORTSCIENCE VOL. 51(8) AUGUST 2016 1045 (Schreiner’s Iris Gardens, Salem, OR) were potted in Aug. 2012 into 3.78-L (23 cm diameter; 16 cm height) round plastic pots (one rhizome per pot) containing a commercial substrate with no starter fertilizer (Fafard growing mix 2; Sun Gro Horticulture, Agawam, MA). From 28 Aug. to 28 Sept. 2012, plants were supplied twice per week with 400 mL of modified Hoagland’s solution (Hoagland and Arnon, 1950) containing 10 mM N from NH4NO3 to provide basic nutrients for fall growth. On 5 Apr. 2013, before the start of the NH4:NO3 ratio treatments, five plants were harvested for background dry weight and nutrient composition. The experiment was a completely randomized design with five treatments and 16 replications in each treatment. Five treatments of NH4:NO3 at 0:100, 25:75, 50:50, 75:25, and 100:0, which had the same concentrations of N (12 mM), K (10 mM), and PO4 3– (5 mM), were used. The nutrient solutions were prepared by adding analytical grade chemicals KNO3, NH4NO3, Ca(NO3)2, CaCl2, (NH4)2SO4, Na2SO4, K2SO4, KH2PO4, KCl, and MgSO4 to tap water with the composition shown in Table 1. Other micronutrients, including Fe (0.1 mM), Mn (0.01 mM), Zn (10 mM), Cu (10 mM), and B (0.05 mM), were also added to all nutrient solutions. Plants were supplied with 400 mL solution containing one of five NH4:NO3 ratio twice per week from 8 Apr. 2013 to 17 Sept. 2013. Throughout the experiment, plant height, leaf SPAD readings (SPAD-502; Minolta Camera Co., Japan, one of the first two fully expended leaves was selected to measure SPAD reading), and pH and electrical conductivity (EC) in the leachate (using the pourthrough extraction method) were measured monthly. Plants were watered before collecting the leachate. After the container has drained for 30 min, 100 mL water was poured on the surface of substrate to extract 50 mL leachate. At the end of the growing season, four plants from each treatment were randomly selected and destructively harvested on 5 Dec. 2013. Each plant was divided into leaves, roots, and rhizomes. All samples were oven-dried at 60 C until constant weight and dry weights were recorded by tissue type. All samples were ground to pass a 40-mesh sieve using a Wiley Mill (Thomas Scientific, Swedesboro, NJ) for tissue nutrient analyses. Net uptake of nutrients in 2013 was estimated by subtracting the nutrient content in the plant on 5 Apr. 2013 from nutrient content on 5 Dec. 2013. The N content of each structure (leaf, root, and rhizome) was calculated by multiplying the dry mass by its N concentration. Nitrogen allocation to leaves, roots, and rhizomes was calculated by dividing N content in each tissue by the total plant N. Data were analyzed by using the fiveNH4: NO3 ratio treatments as a one-factor study. Continuous response data were ana

Effects of Different NH4:NO3 Ratios on Growth and Nutrition Uptake in Iris germanica ‘Immortality’