OXYFERTIGATION: A NEW TECHNIQUE FOR SOILLESS CULTURE UNDER MEDITERRANEAN CONDITIONS

Due to high root density and the relative confinement of the growing medium in soilless culture, rhizosphere oxygen levels drop at an accelerated rate. Conditions may arise that disturb root respiration when the temperature and salinity of the substrate solution are high, as frequently occurs in soilless culture in the agroclimatic context of the Mediterranean coast. Oxygen is the most important macromineral for plants. However, making sure oxygen is available to roots has not received all the attention it deserves. Ensuring substrate aeration is optimal, based on the characteristics of the substrate and careful management of irrigation, is not always enough to guarantee root comfort in soilless culture under Mediterranean coastal conditions. The oxyfertigation technique has been developed to improve rhizosphere oxygen availability in plants grown using soilless culture, and consists of supplying dissolved oxygen in the irrigation water at oversaturated concentrations using fertigation equipment. This article describes the oxyfertigation technique and presents some of the results of different soilless-culture experiments in which this technique was used and which illustrate its agronomic effectiveness. INTRODUCTION The solubility of oxygen in water depends on the water temperature, the partial pressure of oxygen, the atmospheric pressure, the salinity of the water and the area of water exposed to the air. But under normal conditions (20oC, 1 atmosphere of pressure and air with a normal oxygen content), the maximum amount of dissolved oxygen is 9 ppm. This maximum amount goes down as the temperature drops and the concentration of dissolved salts goes down. Likewise, root respiration increases as the temperature goes up and the salt concentration of the rhizosphere solution increases. The most common result of restricted oxygen supply to the rhizosphere is hypoxia, which occurs when root respiration is disturbed by the deficiency of oxygen, not when there is a total lack of oxygen (Drew, 1997). Hypoxia is caused by two things: the plant and the growing medium. Besides the strictly physical aspects of the culture medium or substrate, as mentioned above, the respiratory activity of the substrate microbes also plays a part in the drop in oxygen, in that these microbes actually compete with the plant for the available oxygen. This can happen in organic substrates with considerable microbial activity, and is often associated with the instability or immaturity of the organic matter, as occurs when immature compost is used (Lynch and Harper, 1980). Studies on the evolution of the oxygen content of the rhizosphere solution make it possible to identify three different stages in oxygen-depletion curves. In the first stage, the drop is linear until a partial pressure of between 4% and 6% is reached. In the second, the drop is slower and the root-respiration rate drops gradually until the partial pressure is 1%. Finally, asymptotic evolution occurs around 1%, i.e. the partial pressure cut-off point for the use of oxygen in most plant species (Morard, 1995). At the beginning of the second stage, which can be associated with hypoxia, the amount of CO2 released by the plant increases, as does the concentration of ethylene in the rhizosphere (Pezeshki et al., 1993). The numerous effects of hypoxic conditions on plants have been studied and described in great detail. Morphological-structural changes observed include epinasty, chlorosis, shortening of internodes, growth reduction and root necrosis, a drop in mineral content, smaller harvests, etc. Hormone-based, physiological-metabolic changes also occur, Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilan Acta Hort. 697 ISHS 2005 66 leading to a drop in gibberellin levels, as well as stomatal closure associated with an increase in abscisic acid; an increase in the production of endogenous ethylene is also observed. There is also a reduction in root permeability and radial ion transport is considerably inhibited (Martinez, 1990; Sojka, 1992; William et al., 1995; Drew, 1997). The effects of aeration of the nutrient solution in soilless culture have been described for some species. The effects of aeration on root development, plant growth and the activity of alcohol dehydrogenase (ADH) have been evaluated in roses (Gislerod et al. 1997). The effectiveness of oxygenation on mitigating blossom-end rot (BER) has been studied in the tomato (Tachibana, 1988). The efficacy of solution aeration in terms of productivity has been shown in the cucumber (Yoshida et al., 1996). However, aeration did not improve lettuce grown in a floating hydroponic system (Goto et al., 1996). In soilless culture, particularly in the Mediterranean agroclimatic context, conditions often occur that prompt hypoxia. These conditions consist of the restriction of growingmedium volume, high root density, low air-filled porosity levels, high salt concentrations, high temperatures and nutrient solutions with low concentrations of dissolved oxygen. It is therefore advisable to adopt practices and/or technical solutions that can help reduce the onset of these conditions. The most common solution has been managing the amount and frequency of watering, depending on the physical and hydrological properties of the substrate. Under soilless growing conditions, dissolved oxygen is rapidly depleted as a result of root respiration. Moreover, replacing the oxygen concentration through diffusion is slow, especially when air-filled porosity is low. Therefore, in the nutrient-film technique (NFT) and similar methods, or in substrates with limited air-filled porosity or with a high root density, it is advisable to aerate the nutrient solution to saturate it with oxygen. The advantages of aerating the nutrient solutions used in soilless culture are described in the literature (Lesaint, et al., 1983; Jackson et al., 1984; Morard, 1996). Aeration systems consist of mechanically stirring the nutrient solution or bubbling air into it. These practical methods can be effective when root-respiration levels are high, when irrigation water contains little dissolved oxygen, such as when the water temperature is high (Riviere et al., 1993), under conditions of low evapotranspiration, when it is very common for the substrate to become waterlogged (Ansorena, 1994), or when the salinity of irrigation water is high (Schwarz, 1989). Although some studies have shown that aerating nutrient solutions can be beneficial in soilless culture, little information is available on the effects of aeration and even less on the application of solutions oversaturated with oxygen and their agronomic effects. The use of ozone to disinfect nutrient solutions has been shown to be effective, and such solutions are known to be slightly oversaturated with oxygen (Vanachter et al., 1988). Measurements of the daily evolution of the dissolved-oxygen content in the rhizosphere of different soilless cultures have shown that dissolved-oxygen levels drop to values as low as 3.5 mg L at solar noon or slightly later (Marfa and Guri, 1999). The results of our research group and those of other authors support the hypothesis that the oxygen content goes down in the root medium in soilless culture, particularly at midday, and that such decreases result in hypoxic conditions or take intensive plant cultures out of the comfort zone in which they are able to produce high yields (Riviere et al., 1993). MATERIALS AND METHODS Because the pressure of the water circulating through the tubes used for irrigating soilless culture systems is greater than atmospheric pressure, oxygen can be dissolved in the pressurized nutrient solution until it reaches concentrations above saturation at atmospheric pressure. Drip systems can be used to locate this oversaturated oxygen solution at the base of each plant. The high watering frequency common in most soilless cultures under the climatic conditions of the Mediterranean coast make it possible to ration out this supplementary oxygen supply. When the water coming out of the drip system is once again at atmospheric pressure, the oxygen in excess of the saturated concentration is released. This process takes some time and logically occurs inside the substrate for the most part. The oxygen gradually released inside the substrate will contribute to increasing the partial pressure of the oxygen in the atmosphere of the rhizosphere and a fraction of the oxygen released will probably be adsorbed in gas form on the generally extensive surface of the substrate particles. Thus, the porous substrate, whose tortuosity and air-filled porosity are generally high, keeps oxygen from diffusing into the outside atmosphere and thus helps store applied oxygen. This