Watercress (Nasturtium officinale R. Br.) is a perennial aquatic plant native to Europe that belongs to the Brassicaceae family [1]. Soilless cultivation is often employed in the production of this vegetable, because it is considered a useful technique for the development of leafy vegetables when early maturity is sought [2]. Moreover, this plant is very light sensitive, though it can acclimate to low-light regions by augmenting its physiology and morphology in order to increase photosynthetic efficiency [3]. The production of watercress in Chile is concentrated primarily in the Santiago Metropolitan Area, including 68.5 ha in greenhouses [4]. It is a good source of essential vitamins and minerals and beneficial phytochemicals, such as lutein and zeaxanthin [5].

Green chemistry aims to reduce the flow of chemicals into the environment. To this end, the chemical industry must see to it that chemical processes are not toxic and do not create byproducts that contain contaminants; moreover, the industry should use biodegradable raw materials [6]. The incorporation of green chemistry strategies in protected horticultural practices and in the food industry could lead to the release of biodegradable substances that do not contaminate the environment. In hydroponics, the use of appropriate doses of peroxyacetic mixtures achieves disinfection and improves root oxygenation, which has a positive effect on yield, as well as improving the quality of post-harvest horticultural products [7].

Root aeration is an important factor in obtaining a larger number of plants. Among the methods used to oxygenate the nutritive solution are the use of compressed air and the recirculation of the solution, both of which require pumps to bubble air into the solution. An elevated level of oxygenation favors rapid root growth, which will result in higher yields [8].

Some experiments have obtained good results using various sources of peroxides in soilless cultures [8]. On the other hand, other experiments have reported phytotoxicity at concentrations of 80 mg L⁻¹ [9]. This is the case with equilibrium mixtures containing peracetic acid, acetic acid, hydrogen peroxide and water in variable proportions, according to predetermined quantities of acetic acid and hydrogen peroxide [10,11], where the chemical compositions are detailed in Table 1. The peroxyacetic acid (PAA, C₂H₄O₃) forms a mixture of hydrogen peroxide (H₂O₂) and acetic acid (CH₃COOH) dissolved in an aqueous solution, which acts as an oxidizing agent that is more effective than chlorine or chlorine dioxide. However, one of the principal applications of peroxyacetic acid is as a disinfectant used in the food processing industry to control odors and bioparticulates on tool and structural surfaces; for this reason, moreover, it is used as surface protection on post-harvest fresh fruit and vegetables [9]. Peroxyacetic acid is an effective bactericide, sporicide, fungicide and virucide, because it acts across cellular membranes, oxidizing molecules and destroying essential enzymatic systems [12]. For this reason, it is used in agricultural applications, because its breakdown yields no nocive secondary compounds: green chemistry. Disinfection using hypochlorites yields byproducts such as chlorine vapor and chloroform, among others, which have proven to be carcinogenic [7].

Therefore, given that the application of peroxyacetic mixtures enables increased aeration of the nutritive solution and, consequently, increased production, it is necessary to determine the specific dose that will result in higher yield without phytotoxicity. The goal of this project was to determine the effect on watercress of the application of 0, 20 and 40 mg L⁻¹ of peroxyacetic acid.

The research was carried out in the greenhouse on the campus of Talca University in Chile’s Maule Region (35°25′59″ South Latitude, 71°40′00″ West Longitude; altitude: 102 masl) between August 20 and December 14, 2009.

The vegetative material used was watercress plants (Nasturtium officinale R. Br.) var. Cresson de Fontaine. The watercress seeds from which the seedlings were obtained were sowed on a mixed substrate: pine compost (organic substrate) and perlite A6 (inorganic substrate), combined at a 3:1 ratio. A solid, crystalline complete fertilizer (18-6-18, N-P-K), sold under the trade name Ultrasol Desarollo [13], was used. In addition, we used other essential elements: magnesium, sulfur, iron, zinc, manganese, molybdenum, boron and copper. Tradecorp AZ and Tradecorp Fe were also applied.

The peroxyacetic mixture described in Table 1 was used. The culture was established on a substrate board with a mixture of composted pine and perlite at a 3:1 ratio. The board measured 1 m × 2 m. The seeds were sowed on moistened substrate. Spacing was 10 cm between rows and 1 cm within rows. The time from sowing to transplant was 40 days; the average height at transplant was 3 cm. In the nutrient solution two commercial doses of a peroxyacetic acid mix (20 and 40 mg L⁻¹, equivalent to 2 and 4 mg L⁻¹ of pure PAA respectively) were evaluated (Table 1), along with a control treatment. Applications took place in the beginning of experiment and once a week when all the hydroponic nutritive solution was changed.

Twelve baths were set up with the following dimensions: 90 cm long, 90 cm wide and 10 cm deep. Next, a 4-cm layer of nutritive solution was added. The pH, CE and dissolved oxygen were measured weekly.

An entirely random design with four blocks per treatment was used [14]. Vegetative growth and yield parameters were measured and quality variables evaluated by a sensory panel. Each plot (experimental unit) had 280 plants (140 plants for growth and the sensory parameters, respectively).

Treatment effects were evaluated on the basis of three harvests from 36, 49 and 67 days after transplant, respectively. Growth parameters were evaluated at 60 days since transplant. Harvest standard was an average plant height of 15 cm from the base. On the day of the last measurement of each harvest, a sensory panel evaluated general appearance, color, texture and taste of each treatment. In all, there were three sensory panels corresponding to the three harvests of the trial: each panel comprised 40 panelists.

A sensory analysis (Table 2) was carried out with the samples contained in perforated, low-density polyethylene bags corresponding to the three treatments. Sensory panels took place on the ninth day following each harvest in order to evaluate general appearance, color, texture and taste for each treatment. The sensory panels used vegetative material that had been held in a system of perforated, low-density polyethylene bags for eight days.

The growth, yield and sensorial quality variables were subjected to variance analysis using the Statgraphics Centurion statistical program. In those cases where statistical differences existed, Tukey’s HSD test (P ≤ 0.05) was used to separate the means.

The application of different peroxyacetic mixtures positively influenced vegetative growth and yield parameters in water cress, including plant height, leaf length and stem diameter. Better results were obtained using a dose of 40 mg L⁻¹). Compared to the control, there were increases in plant height, leaf length and stem diameter of 31%, 25% and 31%, respectively. The application of 40 mg L⁻¹was the treatment producing the best results for plant weight at a particular harvest, resulting in 62% and 67% increases in fresh and dry weights, respectively. In general, the application of peroxyacetic mixtures did not affect the organoleptic qualities of the watercress.