Potted orchids, whose wholesale value reached $160 million in 2009, up 26% from the previous year (U.S. Department of Agriculture, 2010), have become the most valued potted flowering plant in the United States. As a relatively new mass-produced commercial crop, the nobile dendrobiums, hybrids made from Dendrobium nobile, have a high market potential because they produce large amounts of flowers and inflorescences simultaneously (Rotor, 1952). A mature pseudobulb of the nobile dendrobium may have 10 to 15 nodes each with a leaf and an axillary inflorescence that may develop two to six flowers.
The nobile dendrobiums need a period of vernalization after maturation to induce flowering. Ichihashi (1997) reported that 15 °C chilling was required for flower bud differentiation of nobile-type dendrobium; lack of chilling caused blind buds, aerial shoots, or no flower formation. Yen et al. (2008) determined that 3 weeks at constant 13 °C saturates the cooling requirement for Den. Sea Mary ‘Snow King’. Conditions such as inadequate vernalization or excessive nutrient supply may cause aerial shoots rather than flowers to form in the leaf axils (Yen, 2008).
Thus, temperature is the most important factor to control flowering of the nobile dendrobium. Rotor (1952) reported that cooling under 18 °C for 4 months delayed flowering of Dendrobium nobile until Easter; however, plants kept under 13 °C started to flower in February or March. The Chinese growers start cooling (night temperature is controlled in the 9 to 13 °C range) from late September or October to advance the flowering time of the nobile dendrobium to meet the New Year and Chinese New Year market time (Li and Wang, 2005). Higher temperature during vernalization and longer cooling durations (up to 6 weeks) were reported to delay flowering of Den. Sea Mary ‘Snow King’ (Yen et al., 2008). For example, counting from the day cooling starts, Den. Sea Mary ‘Snow King’ cooled at 18 °C for 3 weeks took 93 d and those cooled at 15 °C for the same duration needed only 80 d to flower. Plants that received 6 weeks of 15 °C cooling took 75 d and those that got 2 weeks of cooling at the same temperature needed only 60 d to reach flowering.
Response to low temperature in orchid is further complicated by an interaction between temperature and light; thus, light intensity can also be used to manipulate the timing of flowering for the nobile dendrobium. Wang (1995) reported that the number of days in darkness or at a low PPF delayed inflorescence initiation (spiking) of Phalaenopsis orchids by that duration, even when plants were exposed to inductive temperatures. A large-scale test conducted in Taiwan showed that 5 d of heavy shading (complete darkness or PPF 20 μmol·m−2·s−1 or less) alternating with 2 d of light is an effective and inexpensive alternative to heating to a constant 28 °C for inhibiting Phalaenopsis spiking in commercial operations (Wang et al., 2006). Similarly, the appearance of flower buds in Odontioda and Cymbidium decreased under low light intensity (Kubota et al., 2005; Yamaguchi et al., 1977). Rotor (1952) found that the degree of Cymbidium flower response to low temperature (21 °C) was determined by light intensity; however, response to low-temperature induction in Dendrobium nobile was unaffected by light.
The results from a preliminary study suggested that unlike Phalaenopsis, which requires adequate light while being cooled to induce spiking, the nobile dendrobium can be cooled in complete darkness to induce flower initiation. Additional research is needed to obtain more data to study if induction in darkness or under low PPF would negatively impact flower quality of the nobile dendrobium. The objective of this study was to determine the effect of light intensity during vernalization and duration of vernalization on the subsequent flower development and quality of the nobile dendrobium.
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