Orchids are currently the top-selling potted flowering plant in the United States because of their long flower life and compact growth (Banks, 2005; Fitch, 2004; USDA, 2016). While the popularity of many potted plants is declining, the number of orchids that are sold each year is increasing. Most commercial orchid sales are Phalaenopsis sp., also called moth orchids (Griesbach, 2002). The USDA Floriculture Crops Summary reported the wholesale value of potted orchids (including Phalaenopsis) to be $288 million in 2015, an increase of $15 million over the 2014 value (USDA, 2016). The introduction of improved hybrids and the development of large-scale production protocols that reduce the time required to produce flowering plants have allowed potted orchids to be grown for mass-market consumers (Griesbach, 2002).
Phalaenopsis species, which originate from tropical and subtropical climates, have a long flower display life in the typical temperatures and low light levels of a home (Banks, 2005). These orchids are epiphytic in their native environments and grow on trees with their roots exposed to the air (Banks, 2005). The roots of Phalaenopsis orchids contain chlorophyll and are photosynthetic (Dycus and Knudson, 1957; Lopez and Runkle, 2005). Commercial production of Phalaenopsis is usually in a bark-based potting media that allows for good aeration and drainage (Griesbach, 2002). In the home environment, proper irrigation is the largest challenge to maintaining a healthy potted orchid. Underwatering the plant will result in wrinkled, flaccid leaves, whereas overwatering or letting the orchid sit in water can result in damaged roots (Cullina, 2004). Healthy roots of Phalaenopsis are vivid green when the plant is well-watered, and they have a silvery hue when dry. Unhealthy roots are tan or brown (Cullina, 2004).
The technique of irrigating Phalaenopsis orchids with ice cubes has been recommended to orchid owners to reduce the likelihood of over- or underwatering the plants (Onofrey, 2009). Ice cubes provide a convenient way to apply a set volume of water, which is released slowly as the ice melts. The idea is that the ice melt will move slowly through the porous bark media, allowing more water to be absorbed by the bark and roots and resulting in less water accumulation in the bottom of pots where it may cause root damage. Concerns about this technique include whether the melting ice will provide enough water for proper plant growth and whether the ice will cause low temperature damage or reduce the display life of the plants (Cullina, 2004).
Even brief exposure to low temperatures can cause chilling injury (CI) in tropical species (Wang, 2007). Mesophyll cell collapse is a physiological disorder that can occur in orchid leaves exposed to low air or water temperatures (Cating and Palmateer, 2009; Sheehan, 2002). Symptoms include the development of sunken, light-green to yellow areas on the upper surfaces of the leaves, which then turn brown and necrotic. The severity of the CI is dependent on the temperature, duration of exposure, and physiological age of the leaves (McConnell and Sheehan, 1978). Exposure to 2 °C for 8 h causes mesophyll cell collapse in Phalaenopsis leaves. Water soaked spots on the upper surface of the leaves can be observed 0.5 h after removal from the chilling temperature treatment and these progress to deeply sunken dark brown spots within 3 weeks (McConnell and Sheehan, 1978).
While CI can be induced in the leaves of some Phalaenopsis species at temperatures below 10 °C, there is no information about root damage or what temperatures the roots experience when plants are irrigated with ice cubes. The objectives of this research were to 1) evaluate the effect of ice cube irrigation on the flower longevity, display life, and quality of potted Phalaenopsis orchids growing in bark; 2) measure the temperature of the bark media after irrigating orchids with ice cubes or water; 3) determine if there were any visual or physiological symptoms of low-temperature damage in the roots or leaves of orchids irrigated with ice cubes; 4) identify the temperature at which the photosynthetic apparatus in orchid roots is damaged; and 5) determine the internal temperature of roots that are in direct contact with ice cubes.
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