Aeroponic Cultivation of Ginger (Zingiber officinale) Rhizomes

Ginger (Zingiber officinale Rosc.) rhizomes are popular as a spice and an herbal dietary supplement. The anti-inflammatory and anti-nausea qualities of ginger have applications in the pharmaceutical industry. Conventionally grown as a tropical field crop, ginger is plagued by soil-borne disease and nematode problems. Aeroponic cultivation of ginger can provide high-quality rhizomes that are free from pesticides and nematodes and can be produced in mild-winter greenhouses. An experiment involving 34 ginger plants grown in aeroponics was performed in a temperature controlled greenhouse in Tucson, Arizona. The unique aeroponic growing units incorporated a “rhizome compartment” separated and elevated above an aeroponic spray chamber. Bottom heat was supplied to one half of the plants. Accelerated growth was observed in plants receiving bottom heat. One third of the plants were grown in units where the rhizome compartment was filled with perlite, one third in sphagnum moss, and one third without any aggregate medium. Those plants grown in perlite matured faster than the other treatments. The aeroponic units without aggregate medium provided an opportunity to photograph the growth habit of rhizomes over a three month period. Those images were incorporated into a 60-second digital movie that dramatically illustrates how underground rhizomes develop and grow. INTRODUCTION Ginger (Zingiber officinale, Rosc.) is a major spice crop, grown primarily in central Asia for export worldwide. The species is not found in the wild, it has been cultivated for so long in China and India that its exact origins are unclear (Indian Institute of Spices Research, 2004). Recent interest in ginger as a potential anti-nausea and antiinflammatory constituent in pharmaceutical preparations has opened new markets for consistent, high-quality ginger rhizomes (Mustafa, et al. 1993). The perennial rhizome of the ginger plant is a specialized segmented stem structure that grows horizontally just under the soil surface. Upright-growing shoots are produced from the tips of lateral rhizome branches. Adventitious roots and lateral growing points emerge from the nodes of the rhizome stem. In ginger, the roots emerge from the lower rhizome sections. For commercial purposes, ginger is grown as an annual crop, the rhizomes are harvested after seven to nine months (Wilson and Ovid, 1993). Ginger is commercially propagated vegetatively from “seed pieces” of rhizomes, which limits the reproductive and harvest productivity of the crop while perpetuating many devastating crop diseases. In field culture, ginger is susceptible to a number of pathogens and soil-borne diseases, such as Cucumber Mosaic Virus, bacterial wilt (Ralstonia solanacearum), Erwinia soft rot, Fusarium yellows, and rootknot nematodes (Inden and Asahira, 1988; Stirling, 2002; Vilsoni, McClure and Butler, 1976). These disease problems cause producers in infested areas to acquire virgin land for their crops every year, or undergo long crop rotations with unaffected crops. In Hawaii, ginger fields are fumigated with methyl bromide prior to planting in an attempt to control nematodes, Fusarium yellows and weeds (Kratky, 1998). Hydroponics can be an alternative horticultural system for crops susceptible to soil-borne diseases. The uniform growing environment in a controlled greenhouse may Proc. VII IS on Prot. Cult. Mild Winter Climates Eds. D.J. Cantliffe, P.J. Stoffella & N. Shaw Acta Hort. 659, ISHS 2004 398 produce crops with more consistent levels of secondary metabolites, which is of concern to the phytopharmaceutical industry. Unfortunately, there are few hydroponic or aeroponic production systems suitable for rhizome crops. Most hydroponic systems are designed for crops that produce fruit or leaf products and have fibrous root systems and a predictable crown size at the soil line. Rhizome-producing crops have special requirements, in that the horizontal growth habit of the rhizome needs room to expand and produce vertical shoots and secondary roots as needed, uninhibited by physical barriers. Most commercial hydroponic systems utilize an aggregate growing medium, such as perlite or rockwool, contained in a plastic wrap or bag and are drip irrigated with a fertilizer solution. These systems provide sufficient aeration for the roots while physically supporting the plants. Non-aggregate systems, such as Nutrient Film Technique (NFT), Deep Flow or Ebb-Flood systems, are also popular commercially, but tend to minimize root growth and are dependent on a rigid plastic structure to support the plant at the crown. Aeroponics is another type of non-aggregate hydroponics, where the roots of the plants are suspended in an enclosed chamber and sprayed periodically with a fertilizer solution by means of a timer and pumps. Aeroponics offers several advantages over other hydroponic systems, particularly for root crops. The roots are easily accessible for monitoring, sampling, and harvesting. Without the buffering capacity of a solid or aggregate growing medium, the air/liquid medium of aeroponics permits precise control of the nutrient solution mineral composition and temperature. Finally, the common use of A-frame growing structures in aeroponics permits twice the growing area surface in the same size greenhouse, potentially doubling the economic yield for a grower. However, all aeroponic systems previously described in the literature require a rigid structure at the crown of the plant to support the plants while their roots are suspended in the fertilizer spray (Massantini, 1985; Weathers, 1992; Leoni, et al., 1994). This rigid support would restrict the horizontal growth habit of the rhizome. A new aeroponic system was needed to accommodate the horizontal nature and growth habit of a rhizomatous crop. The study presented here describes a new aeroponic design that has not been previously reported. In this system, the rhizomes can be grown in an aggregate medium supported above the spray chamber by a porous layer that protects the rhizomes from direct contact with the nutrient salt solution, while permitting the roots to grow downward into the aeroponic spray chamber. Alternatively, the rhizomes can be supported just above the porous layer, but without the aggregate medium, under a layer of slotted plastic to protect the rhizomes from sunlight. This unique design allowed the rhizomes to grow horizontally and send up shoots at will, permitted the feeder roots to grow in an aeroponic environment, and provided easy access to both roots and rhizomes for sampling and monitoring. MATERIALS AND METHODS Initial planting stock was obtained from three different sources. Sixteen rhizome pieces were obtained from greenhouse-grown stock from S.P. McLaughlin at the Southwest Center for Natural Products Research and Commercialization at the University of Arizona. Since this was a very limited amount of material, an additional 26 pieces were obtained from local grocery stores in Tucson, Arizona. It is unknown if any post-harvest storage treatments, other than chilling, were applied to the ginger purchased from the retail stores. All rhizome pieces were prepared for planting by cleaning with soap and water to remove any visible soil particles, rinsing well in fresh tap water, followed by soaking in a 10% solution of “Ultra bleach” (final concentration 0.6% sodium hypochlorite) for five minutes. The pieces were then rinsed in distilled water and soaked in warm water (50°C) for 10 minutes to reduce nematodes (Trujillo, 1963). All rhizome pieces were transplanted into the aeroponic growing units between March 20 and April 20, 2002. Six identical aeroponic units were constructed using slotted angle iron and 2.5cm expanded polystyrene insulation board for structural support (See Fig. 1). The units were