Orchids have long been attractive plants to many people because of their wide variety of shapes and patterns, exotic colors, long-lasting flowers, and fragrance. The U.S. Department of Agriculture (USDA) reported that in the United States, the wholesale value of potted orchids increased from $47 million in 1996 to $144 million in 2005 (USDA, 2006). Dendrobium Sw. was reported to be the second most valued orchid genus sold in Japan in 2002, which had a market share of 20%, only behind Phalaenopsis Blume (Laws, 2004; Wang, 2004). Dendrobium has also been reported to be the most economically important flowering pot orchid genus sold in Hawaii with a wholesale value of $6 million in both 2005 and 2006 (USDA, 2007).
Dendrobium nobile is one of the most commonly cultivated Dendrobium species (Baker and Baker, 1996) that has been grown for decades. It is native to habitats ranging from the Himalayas to southeast Asia and much of southern China. This sympodial epiphyte produces lateral flower buds (Baker and Baker, 1996). Flowers open nearly simultaneously from leaf nodes of the long pseudobulbs that matured the previous year (Ichihashi, 1997; Rotor, 1952, 1959; Wood, 2006).
Although D. nobile has been grown for years, its hybrids, the nobile dendrobiums, are relatively new commercially mass-produced orchids having a high market potential. Commercial growers are recommended to apply nutrients to 1-year-old nobile dendrobium liners soon after they have been planted in February, then reduce the rate or stop applying mineral nutrients completely in late July or August to “avoid potential flower buds turning into aerial shoots” (Yamamoto Dendrobiums, 2007). In a commercial nursery, mature plants that produce aerial shoots instead of flowers are unmarketable and become a total financial loss to the grower. Wood (2006) recommended withholding nitrogen after 1 Aug. in the northern hemisphere for D. nobile and its hybrids. Baker and Baker (1996) suggested reducing watering after new growth matures in autumn and withholding nutrients until the next spring. However, none of these claims were supported by solid research data or explained the cause and effect in detail.
Bichsel et al. (2008) investigated how various rates of nitrogen (N), phosphorus (P), or potassium (K) and nutrient termination dates (1 Sept., 1 Oct., or 1 Nov. 2005) affected growth and flowering of Dendrobium Red Emperor ‘Prince’ in a warm climate. It was found that ending nutrient application on 1 Oct. resulted in the best vegetative and reproductive characteristics. Plants were taller with more nodes, leaves, and flowers when nutrient application was ended on 1 Oct. than 1 Sept. or 1 Nov. regardless of P rates. Plants produced more flowers with 1 Oct. nutrient termination than the earlier or later date when given 0.1 g·L−1 N. However, when N rate was 0.2 or 0.4 g·L−1, more flowers were produced at 1 Sept. or 1 Oct. than the later nutrient termination date. Plants with all N rates and nutrient termination dates took similar time to reach anthesis, except for a delay when ending nutrients containing 0.2 or 0.4 g·L−1 N on 1 Nov.
Optimum timing of nutrient termination for quality flowering varies with orchid genera. For the cultivar Pendragon Sikkim of a Cymbidium Sw. hybrid, nutrient application was suggested to be ended before flower initiation began for increased inflorescence/shoot ratio along with earlier flowering (Arnold Bik and van den Berg, 1983). For Phalaenopsis TAM Butterfly, maintaining N fertilization until the completion of flower initiation increased flower number (Wang, 2000). The best time to end nutrient application in nobile dendrobiums remains to be defined.
After flower initiation in orchids, resumption of nutrient supply may be needed for optimum flower development because reproductive growth may represent a strong nutrient sink. However, nutrient reapplication might cause detrimental aerial shoot formation or reversion of floral primordia to vegetative shoots. The most ideal stage of development for nutrient reapplication still needs to be defined. To observe if the bud reversion occurs in nobile dendrobiums, it is necessary to know when an initiated floral bud is distinguishable under a microscope and to identify histological differences between vegetative and reproductive primordia.
The objectives of this study were to investigate the effect of nutrient termination date and reapplication time on growth and flowering, to study the development of axillary buds by histological examination, and to ascertain if nutrient reapplication would cause aerial shoot formation or reversion of reproductive to vegetative buds.
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