Differential growth responses to N sources have been reported for many crops. Although Rhododendrum obtusum (Lindl.) Planch ‘Hexe’ (azalea) prefers NH4-N (Colgrave and Roberts, 1956), Viburnum plicatum Miq. tomentosum Thunb. had improved growth with NO3-N over NH4-N at low pH but not at high pH (Dirr, 1975). Plant species have different sensitivities to NH4-N. Spinacea oleracea L. is highly sensitive to NH4-N and requires NO3-N to grow well, whereas Pisum sativum L. is tolerant to NH4-N (Lisa et al., 2001).
Growth of the tropical foliage species Maranta leuconeura did not respond to N being 5% to 75% as NH4-N (Strojny, 1999). Brassia actinophylla Endl., Calathea makoyama E. Morr., and Philodendrum selloum C. Koch had best growth and quality with NH4-N or urea than with NO3-N (Conover and Poole, 1982). Similarly, growth and grade of Aglaonema Schott. × Silver Queen and Philodendrum scadens oxicardium (Schott) Bunt. were improved when the N they received contained 25% NH4-N or 100% urea as opposed to NO3-N (Conover and Poole, 1986). However, Dieffenbachia maculata (Lodd.) G. Don. ‘Camille’ and Nephrolepis exaltata (L.) Schott. grew equally well regardless of their N sources. There exists a wide diversity in plants' responses to N sources, and how one species responds to various N sources or ratio of N sources cannot be used to predict how another species would respond.
Hybrids of Phalaenopsis Blume. have become an economically important pot orchid on all continents and its production is fast increasing (Wang, 2004). This orchid requires high fertility for optimum growth and flowering (Wang, 1994, 1996; Yoneda et al., 2000). Although high N is required, there have been no efforts to examine how NO3-N and NH4-N regulate the performance of this important crop under greenhouse production conditions. A recent study showed that Phalaenopsis had good vegetative growth under K-deficient conditions, but plants started dying after they became reproductive (Wang, 2007). Therefore, it is critical to study how nutrients affect plant growth in both phases.
The objective of this study was to characterize how a hybrid Phalaenopsis responded to several NO3-N to NH4-N ratios during vegetative growth and flowering in each of two root substrates that differed in their physical and chemical properties.
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