A plant's roots must find oxygen in their immediate environment (Drew, 1983, 1992, 1997). Early studies of oxygen content in nutrient solutions in water systems demonstrate that inadequate aeration may cause hypoxia in plant roots, and this phenomenon is especially relevant when the oxygen concentration is the limiting factor for growth and forced aeration is expensive (Zeroni et al., 1983). In addition, water logging of the pore space in a substrate leads to a reduction or interruption in gas exchange between the atmosphere and the rhizosphere. In this case, the oxygen concentration required for the respiration of the root system becomes a limiting factor (Morard and Silvestre, 1996; Urrestarazu and Mazuela, 2005). Oxygen is critical in obtaining the energy required for growth and root survival as a result of its role as the final electron acceptor in the respiratory chain (Morard and Silvestre, 1996).
Temperature affects oxygen solubility; increases in the NS temperature are accompanied by a decrease in its solubility and a linear increase in the plant's physiological requirements of oxygen (Bartholomeus et al., 2008; Carrasco and Izquierdo, 1996). Root respiration doubles for every 10 °C increase in temperature up to 30 °C. In warm climates, where the NS can easily reach temperatures up to 25 °C, the nutrient film technique (NFT) is negatively impacted.
Researchers have tested NFT channel slopes of 0.5%, 1.0%, 2.0%, and 4% and observed the best growth for the 2% slope (Cooper, 1996). Schwarz (1995) suggests the use of a 1.33% slope (one in 75) to improve the aeration in the flowing solution by NFT. However, the slope should never be less than 1% (one in 100) or greater than 2% (one in 50). Consequently, many users of NFT systems in temperate regions worldwide use a 2% slope (e.g., Molyneux, 1988; Schwarz, 1995; Winsor and Schwarz, 1990, in the northern hemisphere and Carrasco and Izquierdo, 1996; Romer, 1993, in the southern hemisphere). Furthermore, the inclusion of gaps in the NS in the gutter itself is an important method of increasing the dissolved oxygen available to the roots.
The aim of this study was to evaluate the effect of different slopes (2% and 4%) and number of gaps (zero, one, two, and three) in the NFT gutter on dissolved oxygen in the NS and on the production from a tomato cultivar in a warm climate.
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