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This study investigated how spring nitrogen (N) application affects N uptake and growth performance in tall bearded (TB) iris ‘Immortality’ (Iris germanica L.). Container-grown iris plants were treated with 0, 5, 10, 15, or 20 mM N from NH4 NO3 through fertigation using a modified Hoagland’s solution twice a week for 6 weeks in Spring 2013. Increasing N rate increased plant height, total plant dry weight (DW), andN content. Total N content was closely related to total plant DW. The allocation of N to different tissues followed a similar trend as the allocation of DW. In leaves, roots, and rhizomes, increasing N rate increased N uptake and decreased carbon (C) to N ratio (C/N ratio). Leaves were the major sink for N derived from fertilizer (NDFF). As N supply increased, DW accumulation in leaves increased, whereas DW accumulation in roots and rhizomes was unchanged. This indicates increasing N rate contributed more to leaf growth in spring. Nitrogen uptake efficiency (NupE) had a quadratic relationship with increasing N rate and was highest in the 10 mM N treatment, which indicates 10 mM was the optimal N rate for improving NupE in this study. TB iris (Iris germanica) is a perennial plant belonging to the family Iridaceae. Hundreds of TB iris hybrids exist representing every color from jet black to sparkling white. TB iris is a popular garden plant with potential as a cut-flower crop. In spring, TB iris produces great amounts of shoot growth, which requires sufficient nutrient supply from both internal and external sources. Usually, fertilization in early spring and after spring flowering is recommended for growing TB iris (Lockatell and Spoon, 2011). However, limited information is available regarding how N rate affects spring N uptake and use efficiency in TB iris. Nitrogen plays an important role in plant growth and development. Insufficient N supply restricts plant growth. Increasing N application rate influences plant growth (Bi et al., 2007), leaf CO2 assimilation (Cheng and Xia, 2004), and uptake and allocation of other nutrients (Scagel et al., 2008, 2012). However, excessive N fertilizer application results in higher root zone electrical conductivity, which causes lower gas exchange rates, shoot DW, and SPAD readings (Niu et al., 2011). Increasing N supply may decrease NupE and lead to more N runoff to the environment (Syvertsen and Smith, 1996). Understanding a plant’s N requirement and the way N affects production and quality of plants is important to both the environment and crop production (Bi et al., 2008; Dong et al., 2004; Lea-Cox et al., 2001; Scagel et al., 2012). Nitrogen use efficiency (NUE) is estimated as the amount of dry matter fixed in plant biomass per unit ofN applied (Marschner, 2012), which integrates two components: plant NupE and use efficiency of absorbed N (NaUE) by the plant (Benincasa et al., 2011). NupE is the ability of the plant to take up N from supplied fertilizer. NaUE demonstrates the ability of the plant to use the absorbed N to produce dry biomass. Considering mean residence time of N in plant tissue affected NUE responses to increasing N availability, NupE showed a more dynamic response to N availability from applied N (Iversen et al., 2010). C/N ratio of biomass may indicate relative availability of C and N sources (Herms and Mattson, 1992). Carbon constitutes 50% of plant DW and provides the structural basis for plants (Agren, 2008) and C compounds provide both energy and the C skeletons for amino acid assimilation. If C supply is insufficient, it will cause decreased N uptake and assimilation (Zhang, 2009). On the other hand, insufficient N supply reduces photosynthetic output, such as, sucrose and glucose (Coruzzi and Zhou, 2001). By controlling N application, C/N ratios can be adjusted in crops to enhance yield and quality. The objectives of this study were to investigate influences of N rate on plant growth, N concentration, content, allocation, and C/N ratio, and to evaluate the effects of increasing N rate on N uptake, NUE, NupE, and NaUE during the spring growth period. Materials and Methods This study was conducted under natural conditions in Starkville, MS (lat. 33 27# N, long. 88 47# W). In Aug. 2012, rhizomes (average caliper = 4.7 cm and length = 5.8 cm) of TB iris ‘Immortality’ (Schreiner’s Iris Gardens, Salem, OR) were potted one rhizome per pot into 3.78-L (23 cm diameter; 16 cm height) round plastic pots filled with commercial substrate with no starter fertilizer (Fafard growing mix 2; Sun Gro Horticulture, Agawam, MA). Fertigation was applied to plants twice per week from 28 Aug. to 28 Sept. in 2012 with plants receiving 400 mL of modified Hoagland’s solution (Hoagland and Arnon, 1950) containing 10 mM N from NH4NO3 to provide basic nutrient supply for fall growth. On 25 Mar. 2013, before the start of spring N treatments, five plants were harvested for background biomass and nutrient composition. Remaining plants were fertigated twice per week from 25 Mar. to 3 May 2013 with 250 mL of modified Hoagland’s solution containing one of five N concentrations (0, 5, 10, 15, or 20 mM N) from NH4 NO3. The monthly average air temperature was 11.6, 17.7, and 21.9 C in Mar., Apr., and May 2013, respectively. The experiment was arranged as a randomized complete block design with five blocks. In each block, four plants in one group was an experimental unit receiving one of five N rates. Five plants from each N rate were randomly selected and destructively harvested on 7 May 2013 and the remaining plants were continually treated with the same N rate treatments from NH4NO3 until Sept. 2013. During the 2013 growing season, number of inflorescences, inflorescence stem length, plant height, and leaf SPAD reading (SPAD502; Minolta Camera Co., Japan, one of the first two fully expended leaves was selected to measure SPAD reading) data were collected. During harvesting on 7 May 2013, plant height and number of fans were recorded. Each plant was divided into leaves, roots, and rhizomes. All samples were oven dried at 60 C until constant weight and DWs were recorded by tissue type. All samples were ground to pass a 40-mesh sieve using a Wiley Mill (Thomas Scientific, Swedesboro, NJ). Received for publication 29 Dec. 2015. Accepted for publication 8 Mar. 2016. Contribution of the Mississippi Agricultural and Forestry Experiment Station Journal article no. 12746. This work was supported by the Mississippi Agriculture and Forestry Experiment Station, the USDA National Institute of Food and Agriculture Hatch projects MIS-249120 andMIS-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. Corresponding author. E-mail: gbi@pss.msstate.edu. HORTSCIENCE VOL. 51(5) MAY 2016 563 | SOIL MANAGEMENT, FERTILIZATION, AND IRRIGATION

Spring Nitrogen Uptake, Use Efficiency, and Partitioning for Growth in Iris germanica ‘Immortality’