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 (Almeria). 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) than for the O2treatment (average value of 5.9 mg L). 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 for O2+ and the O2treatments, respectively. In both treatments, [O2] values were above 3 mg L, except for a short time interval in the O2treatment. 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 for the O2+ and O2treatments respectively. Additionally, none of the quality parameters of seedless fruits were significantly affected by the nutrient solution oxygen content. INTRODUCTION The Mediterranean coast of South-east Spain, mostly the Almeria coast, constitutes one of the largest greenhouse areas in the world, with approximately 35,000 ha of greenhouses dedicated to intensive vegetable production. In this area, substrate cultures, mostly open systems, have been used increasingly in the last fifteen years and the perlite culture, mostly in bags, is the main commercial soilless system (Perez-Parra and Cespedes, 2001). Numerous reports demonstrate that oxygen deficiency in the rhizosphere can induce lower root growth and ion and water uptake (Armstrong and Drew, 2002). The main advantage of most substrates, such as rockwool and perlite, over soil cultivation is their ability to provide sufficient levels of both water and oxygen to the roots. Oxygen exists in porous media in both gaseous and dissolved forms, but plant roots consume only dissolved oxygen in water. Oxygen availability to roots usually depends on the oxygen diffusion rate (ODR) in the media, which is much lower in dissolved forms than in air. Oxygen deficiency can occur even in well-aerated substrates as the result of various factors (Raviv et al., 2004). Most substrate-grown plants, cultivated in greenhouses under supposedly near-optimal production conditions, have high growth rates associated with high root respiration rates and water demands. This situation requires the supply of ample water, which makes it harder to allow enough air into the medium. Moreover, the higher greenhouse temperatures, compared to crops grown outdoors, increase root respiration Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilan Acta Hort. 697 ISHS 2005 26 rates and could decrease dissolved oxygen concentrations in the substrate solution. Lower productivity or quality of substrate-grown horticultural crops induced by oxygen deficiency has been described for ornamentals (Bass et al., 1991) and vegetable crops (Bowen, 1983; Guri and Marfa, 1999). In crops grown in perlite bags, where the substrate is wrapped in a polyethylene film with little surface open to the atmosphere, the oxygen content may be depleted quite rapidly at high root respiration rates and substrate temperatures. This situation may occur in spring cycles of greenhouse vegetable crops on the Almeria coast. Several methods of nutrient solution oxygen-enrichment (bubbling, agitation, forced aeration, etc) have been used to improve the oxygen supply to the roots in hydroponics systems. The supply of pure, pressurized, oxygen gas to the nutrient solution is another oxygen-enrichment method mainly used for research purposes (Chun and Takakura, 1994; Goto et al., 1996). The latter has been improved and adapted in order to be used in commercial horticultural greenhouses on the Spanish Mediterranean coast and has been called oxifertigation (Marfa and Guri, 1999). This work studied the behaviour of a watermelon crop (Citrullus lanatus L.) grown in perlite bags and irrigated with a nutrient solution whose dissolved oxygen concentration was maintained at super-saturated levels (oxifertigation facility). Watermelon is one of the main greenhouse spring crops on the Almeria coast. MATERIALS AND METHODS The experiment was carried out from February to May 2001 in a commercial Parral-type greenhouse in El Ejido (Almeria). Iris, a seedless watermelon cultivar, was interplanted with Sweet Marvel, a standard seeded pollinator cultivar, at 0.77 plants m. Plants were grown in two-year-old reused perlite bags of 40 L and the most common perlite type (< 5 mm particle size) was used. A crop irrigated with a nutrient solution super-saturated with dissolved oxygen (O2+) was compared with a standard irrigated crop for which the dissolved oxygen concentration was below or about saturation values (O2-). The experiment was arranged in a randomised complete-block design. During each irrigation event, pure, pressurized, oxygen gas was dissolved in the nutrient solution of the O2+ treatment with a gas injector within the irrigation pipe. Irrigation water of 1.5 dS m electrical conductivity was supplied. Dissolved oxygen concentration [O2] was measured with an oxygen probe (Handylab OX 1/SET, Schott-Gerate/Glas, Germany) of ± 0.1 mg L resolution. [O2] values in the nutrient solution supplied were measured every 2-3 days at the dripper outflow. Moreover, the diurnal [O2] dynamic in the substrate solution was measured on a sunny day (28 April 2001) at the end of the crop cycle. Solution was extracted from the lower and central part of the perlite bags with a soil moisture sampler (Rhyzon SMS, Eijkelkamp, Giesbeek, The Netherlands) after irrigation. Total and marketable watermelon yield was measured separating seedless from seeded fruits. Four seedless fruits were randomly selected from each replicate plot for quality analysis. Morphology parameters (transversal and longitudinal perimeter) of each fruit were measured. The fruit was then cut in half, and the rind thickness (cm) and core firmness (kg cm) were determined. Watermelon juice was extracted from a piece of core and total soluble solids (%) and pH were measured. Finally, a representative fruit sample was dried at 70 oC for a week to determine percentage of dry-matter (%). Aerial or shoot biomass and its partitioning into fruits and vegetative parts were also measured at the end the crop cycle. RESULTS AND DISCUSSION Nutrient and Substrate Solution The watermelon crop was managed following common local practices. In both oxygen treatments, daily values of percentage of drainage water were higher at the beginning of the cycle (Fig. 1), maintained between 30 and 45% during flowering, fruit setting and fruit growth, and were decreased progressively by 5-10% at the end of the