Flowering of the Orchid Miltoniopsis Augres ‘Trinity’ is Influenced by Photoperiod and Temperature

The production of flowering potted orchids has increased dramatically throughout the world in the past decade. For example, production value in the United States has increased 147% in the past six years, and in 2002 the estimated wholesale value was $105 million. Scheduling orchid species to flower on specific dates requires knowledge of how environmental parameters regulate plant development from propagation to flowering. In a preliminary experiment, we observed that flowering of Miltoniopsis Augres ‘Trinity’ was promoted by cool temperatures (14 to 20 C), short photoperiods (9 hours), or a combination of both. We performed additional experiments to determine the minimum durations of short photoperiods and cool temperatures (vernalization at 14 oC) required for rapid and uniform flowering. To quantify the effect of photoperiod before exposure to cool temperature, Miltoniopsis were placed under 9or 16-h photoperiods at 20 oC for 0, 4, 8, 12, or 16 weeks, then were transferred to growth chambers at 14 °C with a 9-h photoperiod for 8 weeks. To determine the optimal cooling duration, a separate experiment was performed in which plants were placed under 9or 16-h photoperiods at 20 oC for 8 weeks and then transferred to 14 °C with a 9-h photoperiod for 0, 3, 6, 9, or 12 weeks. Following treatments, plants were grown in a common environment at 20 °C with a 16-h photoperiod. The flowering percentage of plants that were not exposed to short days was only 40%. Flowering percentage was greatest when exposed to short days for 4 to 8 weeks before cooling. The optimum vernalization treatment was for eight weeks under short days. INTRODUCTION Orchids have become the second most valuable flowering potted crop in the United States (USDA, 2003). According to the American Orchid Society, over 75% of all orchids sold in the United States are Phalaenopsis, commonly known as the moth orchid (Griesbach, 2002). Currently, the wholesale price of flowering Phalaenopsis in a 15-cm pot in the U.S. is $8 to $12, with a retail price of $15 to $25 on the mass market (Wang, 2003). As the market becomes saturated with Phalaenopsis due to the widespread knowledge of its flower induction requirements, the price will likely decrease and the plant may become a commodity. Consequently, commercial growers and retailers are already seeking other orchids that have consumer appeal and can be programmed into flower for specific markets dates. Miltoniopsis, or the pansy orchid, produces inflorescences that are adorned with three to six flat, and large (7 cm) fragrant and showy flowers, which range in color from cream to pink, magenta, scarlet, or yellow. The flowers often last on the plant for four to eight weeks at temperatures from 14 to 20 oC (Robinson, 2002). Due to these attributes, Miltoniopsis has become the fifth most valuable potted orchid produced commercially in the Netherlands, with 797,000 pots sold in 2001 (Barendse, 2002). In its native habitat, Miltoniopsis is an epiphytic and lithophytic genus of six species distributed throughout the wet cloud forest regions (610 to 2,100 m) from Costa Rica to Peru (Baker and Baker, 1993; Morrison, 2000). The sympodial growth habit of this compact plant is distinguished from Miltonia by the presence of a single leaf at the 175 Proc. V IS on New Flor. Crops Eds.: A.F.C. Tombolato and G.M. Dias-Tagliacozzo Acta Hort. 683, ISHS 2005 pseudobulb apex, which is surrounded by distinct leaf-like sheaths. Several outstanding hybrids have been developed in recent years, which are mainly enjoyed by hobbyists. However, little information is currently available on flowering induction of Miltoniopsis. In a previous study, we found that flower initiation of Miltoniopsis Augres ‘Trinity’ was uniform and complete only when plants were exposed to 9-h photoperiods before and during vernalization at 11 to 14 oC (unpublished data). A similar response was observed in Cattleya warscewiczii, C. gaskelliana, and C. mossiae; flower induction occurred under continuous 9-h photoperiods at 13 oC, and flowering was inhibited when the photoperiod was 16 h (Rotor, 1952; Rotor, 1959). Like Miltoniopsis, Cattleya are epiphytes native to the humid and wet forests of Central and South America. Further investigation is necessary to minimize the production time of Miltoniopsis. This information will assist greenhouse growers with developing a production schedule for complete, rapid and uniform flowering of Miltoniopsis Augres ‘Trinity’ for specific market dates. We performed experiments to determine how the duration of prevernalization photoperiods and vernalization duration influence flowering of Miltoniopsis. Another objective of this study was to determine if short days are indeed critical for flowering prior to vernalization. MATERIALS AND METHODS Plant Material and Culture Miltoniopsis Augres ‘Trinity’ were grown by a commercial greenhouse (Calif.) at 16 to 26 oC under natural photoperiods (37 oN lat.) with a maximum photosynthetic photon flux (PPF) of ≈350 μmol⋅m⋅s. They were planted in a bark and perlite-based media in June 2002 (10-cm pots) and were received in East Lansing, Mich. on 22 July 2002. Plants were maintained at ≈23 oC in a glass-glazed greenhouse until experiments began. The photoperiod was a constant 16 h, consisting of natural daylengths (42 oN lat.) with day-extension lighting from high-pressure sodium (HPS) lamps, which delivered a supplemental PPF of ≈50 μmol⋅m⋅s at plant height [as measured with a line quantum sensor (Apogee Instruments, Inc., Logan, Utah)]. Light transmission through the greenhouses was reduced using a permanent woven shade curtain that reduced light by ≈55% (OLS 50; Ludvig Svensson, Charlotte, N.C.) and by applying whitewash (up to 50%) to the glass as needed so that the maximum PPF was ≈350 μmol⋅m⋅s. Plants were irrigated as necessary with reverse osmosis water supplemented with water-soluble fertilizer to provide the following (mg⋅L): 125 N, 12 P, 100 K, 65 Ca, 1.0 Fe and Cu, 0.5 Mn and Zn, 0.3 B, and 0.1 Mo (MSU Special, Greencare Fertilizers, Chicago, IL). Pre-vernalization Duration Experiment (Expt. 1) Plants were grown at 20 oC under 9-h short days (SD) or 16-h long days (LD) beginning on 26 July 2002 for 0, 4, 8, 12, or 16 weeks in a glass-glazed greenhouse. Photoperiods were created by pulling opaque blackout cloth over plants between 1700 and 0800 HR, and for the 16-h photoperiod, light from incandescent lamps (delivering ≈2 μmol⋅m⋅s) was provided between 1700 and 2400 HR. From 0800 to 1700 HR, HPS lamps provided a supplemental PPF of ≈40 μmol⋅m⋅s at plant level when the ambient greenhouse PPF was <120 μmol⋅m⋅s. The plants were then cooled for eight weeks at 14 oC under SD in a controlled-environment chamber. The photoperiod was provided by a combination of cool-white fluorescent (VHOF96T12, Philips, Bloomfield, NJ.) and incandescent lamps from 0800 to 1700 HR at 150 μmol⋅m⋅s. After the cooling treatment, plants were grown at 20 oC under a 16-h LD in the greenhouse. Vernalization Duration Experiment (Expt. 2) Plants were grown at 20 oC under SD or LD (as described above) beginning on 26 August 2002 for eight weeks. The plants were then cooled for 0, 3, 6, 9 or 12 weeks at 14