EFFECT OF INCREASING THE DISSOLVED OXYGEN IN THE NUTRIENT SOLUTION TO ABOVE-SATURATION LEVELS IN A GREENHOUSE WATERMELON CROP GROWN IN PERLITE BAGS IN A MEDITERRANEAN AREA

Oxygen deficiency can occur even in well-aerated substrates as the result of various factors. In crops grown in perlite bags, where perlite is wrapped in a polyethylene film with little surface open to the atmosphere, the dissolved oxygen concentration [O₂] may be depleted quite rapidly at high growth rates and high substrate temperatures. This situation may occur in spring cycles of greenhouse vegetable crops on the Spanish Mediterranean coast. This work studied the behaviour of a watermelon crop (Citrullus lanatus L.) grown in perlite bags in a commercial greenhouse in El Ejido (Almería). A crop irrigated with a nutrient solution super-saturated with dissolved oxygen (O₂+) was compared with a standard irrigated crop for which the [O₂] in the nutrient solution was below or about saturation values (O₂-). [O₂] values measured at the dripper outflow were higher for the O₂+ treatment throughout the whole crop cycle (average value of 13.5 mg L^-1) than for the O₂- treatment (average value of 5.9 mg L^-1). Moreover, [O₂] values in the nutrient solution extracted from perlite bags at the end of the watermelon cycle were slightly higher for the O₂+ treatment during the whole diurnal period measured, and the average values were 4.5 and 3.7 mg L^-1 for O₂+ and the O₂- treatments, respectively. In both treatments, [O₂] values were above 3 mg L^-1, except for a short time interval in the O₂- treatment. No significant differences were found between oxygen treatments for total and marketable production of watermelon, although it was slightly higher for the O₂+ treatment. Mean values of final marketable production were 7.2 and 6.9 kg m^-2 for the O₂+ and O₂- treatments respectively. Additionally, none of the quality parameters of seedless fruits were significantly affected by the nutrient solution oxygen content.