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 of N 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.
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