Phalaenopsis orchid is currently the world’s most important potted flowering crop (Chang et al., 2013). It is the most highly valued indoor plant in flower auctions in the Netherlands (FloraHolland, 2013), whereas in the United States, orchids, constituted mainly by Phalaenopsis, rank first in wholesale value in the potted flowering plant market (U.S. Department of Agriculture, 2012). Phalaenopsis is an epiphytic plant with succulent leaves and roots and is noted for its long blooming period and great resilience against stresses, including nutrient deficiency stress (Hou et al., 2010; Hung, 2012; Lei, 2007). Compared with other crops, orchids have similar mineral requirements, but they respond more slowly to fertilizer application (Hew and Yong, 2004). Phalaenopsis is a typical orchid in this respect; it is slow-growing and can continue to grow and bloom even under long periods of suboptimal substrate fertility. Experimentally, it is very difficult to induce nutrient deficiency symptoms when starting with healthy Phalaenopsis plants, even after several months have passed without additional fertilization (Lei, 2007). We hypothesize that the fleshy leaves of Phalaenopsis have nutrient storage function, which likely accounts for its resilience against nutrient deficiency. Studies on mineral nutrition in Phalaenopsis done by different workers have yielded varying and at times inconsistent results (Lei, 2007; Wang, 2000, 2007; Yoneda et al., 1997; Yu, 2012). We think that the conflicting experimental results may be a result of how much nutrients have been stored in the plants used. Although the ability of some orchids to store nutrients in thickened stems called pseudobulbs has been demonstrated (Ng and Hew, 2000), Phalaenopsis lacks such a structure, and the capability of the succulent leaves to store mineral nutrients has not been definitively shown with research.
Nitrogen is one of the essential macronutrients having great influence during the cultivation of Phalaenopsis. Nitrogen fertilization regimen significantly affects both the vegetative and reproductive growth of Phalaenopsis (Lei, 2007; Yu, 2012) and can be adjusted to manipulate the timing of flowering (Ichihashi et al., 2010). Despite the great importance of N in Phalaenopsis production, little has been studied on the absorption and partitioning of N in Phalaenopsis. Conventionally, plant N requirements are analyzed by measuring the concentration and content of N in plant structures to determine absorption and use of this element. However, whether the measured N comes from fertilizer or other sources cannot be determined with this method. Nitrogen absorption from fertilizer application is thus often overestimated as a result of the presence of non-fertilizer sources such as previously stored N (Westerman and Kurtz, 1974). The presence of a hypothesized pool of stored N in the succulent leaves of Phalaenopsis would aggravate the aforementioned difficulties encountered when studying use of N fertilizer using traditional methods.
Nitrogen-14 and nitrogen-15 (15N) are the two stable isotopes of N with atmospheric natural abundances of 99.6337% and 0.3663%, respectively. The latter is an important tracer element in chemistry, medicine, and agriculture research and is often used to study the movement of N in plants (Lajtha and Michener, 1994). Using a stable isotope as a tracer, the uptake of N and its absorption efficiency can be determined more accurately compared with conventional analytical methods (Sandrock et al., 2005). Although 15N has been used to trace the fate of N in numerous fruit crops (e.g., Feigenbaum et al., 1987; Munoz et al., 1993; Retamales and Hanson, 1989), it has not been a common research tool used in floriculture crops (e.g., Cabrera et al., 1995; Trepanier et al., 2009). Because Phalaenopsis has emerged as the world’s number one potted flower crop, and considering its specific characteristics that make nutritional study difficult, investigating N absorption and partitioning in Phalaenopsis with 15N is of great relevance.
In this study, we used 15N labeling to compare the absorption and partitioning of fertilizer N applied to Phalaenopsis leaves and roots, analyze the fate of fertilizer N after application to the roots, and trace the partitioning of fertilizer N at different growth stages to compare the sink-source relationships between the various organs.
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