Cultivation management

An automatic plastic tunnel-type greenhouse, was used to carry out this experiment. This facility is located in Instituto Tecnológico de Torreón. Melon seedling (Cruisier hybrid) of 30 days old were transplanted into twenty L black polyurethane bags (a single seedling was transplanted per bag), containing 80% river sand and 20% perlite as growing medium. Each bag had six holes underneath to allow drainage. Before use, river sand was sanitized using tap water with a concentration of 2700 mg L^-1 of sodium hypochlorite. Plants were then placed in greenhouse with a density of four plants for square meter. An automatic drip irrigation system watered plants three times a day, adding a volume enough to drain 30%. Throughout the growing cycle, the drainage was kept between 20 to 30%. In the Beginning at flowering stage, and until fruit development, polinization was performed mechanically by hand from noon to 14:00 h. Plants were kept at only one stem per plant and were attached by plastic string to the greenhouse frame. All fruits were hanging from the greenhouse structure using plastic mesh pockets.

Experimental design and treatments

In a completely randomized design, three ecological nutrient solutions and a control were evaluated:1) compost tea; 2) vermicompost tea; 3) vermicompost leachate (collected from vermicompost obtention); and 4) Steiner universal solution (^{Steiner, 1984}), with ten replicates per treatment. Bovine manure was the source for the compost and vermicompost teas used, and were prepared according to the method proposed by ^{Ingham (2005)}. The ecological solutions pH was adjusted using citric acid (^{Capulín et al., 2007}) and maintained at a range from 5.5 to 5.8. The electrical conductivity (EC) was kept at 2 dS m^-1 by diluting the ecological solutions with tap water to evade phytotoxicity (^{Oliva et al., 2010}) (Table 1).

Analytical tests

The commercial quality of the fruit (fresh fruit weight, equatorial and polar diameter, peel and pulp width, and soluble solids content), and the nutritive quality (antioxidant capacity by the ABTS⁺ and DPPH⁺ methods and phenolic content) were evaluated.

Extract’s preparation for phenolic content and antioxidant capacity

One-gram sample (lyophilized pulp) was mixed in 10 mL methanol in plastic tubes with a twist cap, that were kept for 6 h at 20 rpm and 5 °C in a shaker (ATR Inc., USA). Then, Tubes were set in the centrifuge for 10 min at 3000 rpm, and the supernatant was removed for posterior analytical tests.

Total phenolic content

Total phenolic content was obtained by a modified method of Folin-Ciocalteau (^{Esparza et al., 2006}). Results were reported in % of gallic acid per dry weigh in grams (% AG/g DW).

Antioxidant capacity in trolox equivalent (ABTS⁺ method)

The antioxidant capacity was determined using method described in ^{Esparza et al. (2006)}. The results were informed in mg g^-1 equivalent Trolox of dry weight.

Antioxidant capacity in trolox equivalent (DPPH⁺ method)

The antioxidant capacity also was determined by a modified DPPH⁺ method devised by ^{Brand-Williams et al. (1995)}, to report the results in mg g^-1 equivalent Trolox of dry weight.

Statistical analysis

All variables described above were evaluated by one-way ANOVA by the GLM method of SAS statistical package version 9.1 (^{SAS Institute, 2009}). Tukey simultaneous test was used for comparing statistical means, at (P ≤ 0.05).

Yield and fruit marketable quality

Weight and fruit size are important because it affects yield and influences the final selection of product by the consumers (^{Montaño-Mata and Méndez, 2009}). Our results show that treatment with compost tea affected yield and fruit quality, being statistically similar to the Steiner solution which showed the highest yield value. The treatments with vermicompost tea and leachate were statistically lower (Table 2). These results agreed with ^{Budiastuti et al. (2012)} who reported that fruits obtained under organic or natural fertilization generally have smaller size and lower weight and yield than those produced using conventional chemical fertilization. This, due to a lower nutrient concentration in organic solutions (^{Preciado et al., 2011}). According with ^{Tapia et al. (2010)} it should be noted that the fruit weight of melons produced in our study was within marketing acceptable values. Except for vermicompost-tea treatment which showed the lowest yield and weight value. The highest yield values were obtained with the fertilization treatments using the more nitrogen-rich Steiner solution, since this macro nutrient is directly involved in yield and final fruit weight in vegetables (^{Sainju et al., 2003}; ^{Rodríguez et al., 2005}). Hence, the slightly lower yields are understandable, considering that the organic nutrient solutions had to be diluted to avoid salt phyto-toxicity in plants (^{Oliva et al., 2010}) (Table 1). Hereafter, the suggestion is that the lower fruit size and weight in melon grown under the natural nutrient solutions treatments was caused by nitrogen deficit. ^{Rembiałkowska (2007)} states that high yield in conventional crops are often associated to a high nitrogen availability. Hence, some alternatives that can be used to increase nitrogen and other nutrients content in natural solutions include the mixture of organic nutrient sources (^{Ingham, 2005}), in addition to using less diluted fertilizer solutions (^{González Solano et al., 2013}). Nevertheless, those natural solutions must be assessed to meet the specific crop mineral requirements (^{Alam and Chong, 2010}). In contrast fruits from high yielding plants are usually characterized by their low biochemical content (^{Hallmann and Rembiałkowska, 2012}).

Compost-tea treatments, were superior with respect to very important quality criteria, such as soluble solids content (^{Budiastuti et al., 2012}). ^{Montaño-Mata and Mendez (2009)} explain that consumers prefer melon fruits with a soluble solids content ranging 8-10 ^oBrix. In our study plants fertilized with compost-tea showed the highest soluble solids content (9.98; Table 2). These outcomes are similar to those published by ^{Preciado et al. (2011)}, who informed a higher soluble solid content in tomato organically fed compared to conventional chemical fertilized fruits rich in nitrogen. A negative correlation occurs between nitrogen supply by chemical fertilizer application and production of soluble solid content (^{Parisi et al., 2004}). Soluble solids content in fruits may increase due to a low level of water absorption, hence, fruits store solutes as simple sugars, such as glucose, fructose and sucrose (^{Goykovic and Saavedra, 2007}). Another cause of soluble solids accumulation in fruits is salt buildup in the nutrient solution, particularly an increase of sodium and chloride content (^{Wu and Kubota, 2008}). These findings suggest that the application of ecological solutions, particularly compost-tea, should be considered as a feasible fertilizer alternative that may replace traditional chemical fertilization; since the decrease in yield can be remedied by obtaining premium quality and higher price as organic product (^{De la Cruz-Lázaro et al., 2010}).

Fruit nutraceutical quality

The quantity and type of antioxidants contained in fruits and vegetables are affected by agricultural practices and post-harvest produce handling (^{Schreiner and Huyskens, 2006}; ^{Huyskens et al., 2006}). The results found from this study suggest that the treatments applied affected nutraceutical quality of melon fruits.

In our study (Table 3) fruits with the treatment with compost tea and vermicompost leachate reached statistically higher values by the DPPH⁺ method, than fruits fed with chemical nutrient solution. Fruits with the treatment of vermicompost leachate reached statistically higher values by the ABTS⁺ method, than fruits fed with chemical nutrient solution. In regards to total phenolic content plants fed with vermicompost leachate got the highest value statistically similar to control plants fed with the chemical Steiner nutrient solution (Table 3). Differences among treatments concerning antioxidant capacity and phenolic content could be associated with the low concentration of nitrogen in the ecological solutions applied (Table 1). When plants are subjected to nitrogen deficiency create higher quantities of sugars and secondary metabolites such as organic acids, vitamins, pigments, phenolic compounds and terpenoids (^{Herms and Mattson, 1992}). The antioxidant capacity of melon produced in the current experiment is even higher than those values found by ^{Salandanan et al. (2009)}, who reported as well a higher content of phenolics and antioxidant capacity in organic fruits than in chemically fed fruits. Hence, the higher antioxidant capacity of organic melons could be attributed to the increase in phenolic compounds production due to the nitrogen deficit in the ecological solutions applied. Hence, we endorse the use of natural nutrient solutions, as the ones studied in this experiment, as fertilizer substitutes for production of soilless melon fruits with enhanced quality.