Potential for Greenhouse Aeroponic Cultivation of Urtica dioica

Resent studies investigating aeroponic cultivation of medicinal plants have provided encouraging results for increasing yields, shortening time to maturity, and improving consistency and overall quality of produce over field production. The goal of the current study was to determine the applicability of aeroponic technology for the cultivation of the traditionally field grown herbaceous medicinal plant Urtica dioica. In addition, we investigated if control of nutrient delivery and repeated harvesting practices could be utilized to increase and direct yield of desired plant parts. Comparison of root and shoot dry weights between treatments revealed; (1) U. dioica cultivated in soil-less medium yielded equal shoot biomass and greater root biomass than aeroponically cultivated plants, (2) potassium and phosphorus ratios within the nutrient solution had no significant impact on yield or biomass allocation, and (3) multiple harvesting of aeroponic roots and shoots yielded greater total biomass of both roots and shoots than a multi-crop replanting strategy. Results suggest aeroponic technology could be a powerful tool for the cultivation U. dioica as well as a variety of other important herbaceous medicinal plants. However, further optimization of the plant growing environment is required to maximize and direct growth. INTRODUCTION Medicinal plants are a primary or supplementary element of 80% of the world’s health care practices (World Health Organization, 2003). Worldwide use of medicinal plants has risen significantly over the past two decades, fueling development of an important agricultural market valued at over US $60 billion annually (World Health Organization, 2004). This rapid rise in demand, however, has not been matched by equal increases in cultivation. Unsustainable collection from wild plant populations has led to the endangerment of nearly 1/5 of the world’s medicinal plants (Edwards, 2004). The rise in medicinal plant use is also correlated with an increase in reported adverse reactions, poisonings, and even deaths linked to quality control problems such as misidentification of species, weed adulteration, and contamination from soil or soil microorganisms (Edwards, 2004). Aeroponic cultivation of important medicinal plants may offer solutions to these production and quality control issues. Recent studies investigating aeroponic cultivation of Echinacea purpurea and Articum lappa L. (Burdock) have demonstrated significant advantages over traditional field practices, including; (1) accelerated plant growth rates and shorter time to maturity, (2) significant increases in yield, (3) higher levels of plant to plant biomass and secondary metabolite consistency, and (4) contaminant free product (Pagliarulo and Hayden, 2002). Results reported here is one part of a four part investigation, supported by the National Center for Complementary and Alternative Medicine, with the goal of determining the universal applicability of aeroponic cultivation for a variety of medicinal crops. This research evaluated aeroponic cultivation of Urtica dioica (Stinging nettles) a herbaceous perennial plant commonly found in forest understory or newly disturbed moist Proc. VII IS on Prot. Cult. Mild Winter Climates Eds. D.J. Cantliffe, P.J. Stoffella & N. Shaw Acta Hort. 659, ISHS 2004 62 areas (Srutek, 1995). U. dioica was chosen for these studies because it matures quickly, is easily cultivated, is an important herbal dietary supplement, and could be used for aeroponic vs. field cultivation yield comparisons. Both aerial and below ground portions are sold commercially; below ground portions are popular for treatment of benign prostate hyperplasia (non-cancerous prostate enlargement), and aerial portions are commonly used as a remedy for arthritis and allergies (Belaiche and Lievoux, 1999.). U. dioica root biomass allocation is inversely proportional to nitrogen availability (Srutek, 1995). Restriction of nitrogen can improve root to shoot biomass ratios, but at the direct expense of overall biomass. Nitrogen to phosphorus and potassium ratios are also critical effecters of biomass partitioning (Pigott, 1971, Srutek, 1995, Linkohr et al., 2001). Hydroponic solutions with standard leafy crop nitrogen levels and varying phosphorus and potassium levels were tested to determine if optimum yields can be obtained while directing biomass accumulation toward root growth (Resh, 1998). Multiple shoot coppicing of plants that fall into similar ecological niches as U. dioica stimulates compensatory growth, generating the potential for greater seasonal biomass accumulation over the course of a growing season (Van der Meijden, 2000). Repeatedly pruned aeroponic U. dioica above and below ground yields were compared with single harvest multi-crop treatments to determine if multiple harvest practices can improve total yields. The specific goals were to (1) investigate the response of an herbaceous perennial model medicinal plant to aeroponic cultivation in comparison to a simulated soil control, (2) to determine if the control and flexibility of the plant growing environment offered by aeroponics can be used to improve yield and direct plant growth to maximize production of desired organs, (3) to determine if modification of nutrient ratios within the hydroponic fertigation solution and if repeated harvesting practices affect biomass allocation and total yield. MATERIALS AND METHODS Controlled environment cultivation of U. dioica was completed at the University of Arizona’s Campus Agriculture Center in Tucson, Arizona from August 2003 to February 2004. The 7 m x 15 m polycarbonate greenhouse was covered by 30% shade cloth and climate controlled to maintain minimum (20°C) and maximum (33°C) air temperature. Split plot blocking of treatments was utilized to account for differences in light availability and shading patterns within the north-south oriented structure. The 3-factor experimental design included the following treatments; (1) aeroponic vs. greenhouse soil-less medium cultivation, (2) high phosphorus and potassium (PK) nutrient solution vs. low PK, and (3) multiple harvested aeroponic plants vs. multi-crop single harvest aeroponic plants. Design of the 4 A-frame aeroponic units was similar to the novel knee-wall system detailed in Hayden (2002) with the addition of rigid plywood constructed internal reservoirs and greenhouse frame supported “skyhooks” that hold the apex of the A-frame units from above. Each growing surface was inclined 60° and supported 24 plants spaced evenly on the 1.5 m x 1.5 m, or approximately 2.25 m growing area. Soil-less treatments utilized 12 elevated beds supporting 8 plants apiece. Each bed was 88 cm x 69 cm x 28 cm (L x W x D) in dimension and filled with steam sterilized Sunshine Mix #1 and sand in a 2:1 ratio, respectively. Nutrient solution treatment formulations were modified from Resh (1998), and pH was maintained between 5.5 and 6.2. Lab measured formulations of treatment samples are described in Table 1. Identical high and low PK nutrient solutions were used to irrigate both the aeroponic and soil-less medium treatments. U. dioica seed stock was acquired from Horizon Herbs, LLC, Williams, OR, lot #1101, 2003. Seeds were germinated in autoclaved rubberized peat plugs in an ebb and flow hydroponic system utilizing half strength low PK treatment nutrient solution. Initial plugs were transplanted directly into aeroponic and soil-less treatments 33 days after seeding.