Oxygen is essential for the functioning of roots, hence, its deficiency is an important concern. Problems with oxygen supply may periodically appear in soil conditions after rains. Also, in soilless systems, water and nutrients are supplied continuously and these wet conditions limit diffusion of oxygen to the root system (Veen, 1988). Oxygen deficiency stress in plants is distinguished by three physiologically different states: transient hypoxia (insufficient supply of oxygen), possible anoxia (complete lack of oxygen) and reoxygenation (Blokhina et al., 2003). An inadequate management of irrigation may lead to temporal hypoxia conditions caused by inadequate aeration in some parts of the root system (Morard and Silvestre, 1996). In contrast, anoxia is rare in soilless culture (Klaring and Zude, 2009; Morard and Silvestre, 1996).
In order to avoid oxygen deficiency in the root environment, it is essential to provide the nutrient solution with enough O2. Possibilities for accurate control of root oxygen supply are more easily achieved in soilless cultures than in soil systems (Olympios, 1999). The best oxygenation system of the root environment is the aeroponic system, which allows the roots to grow in air with a plentiful supply of oxygen, hence, no extra mechanism is needed. In liquid systems, aeration might be needed by means of pumps if the solution culture is static. However, in continuous flow solution culture like the nutrient film technique, there is an abundant supply of oxygen to the roots of the plants if the system is well designed. In substrate systems, it is essential to choose a substrate that has a correct distribution of particle size, a low bulk density, a high porosity and a stable structure so that the supply of air to the roots is sufficient (Abad et al., 2004). If more aeration is needed, Urrestarazu and Mazuela (2005) have observed that the addition of potassium peroxide as chemical oxygenation improves water uptake and yield of different vegetables as sweet pepper, melon and cucumber. Also, the application of exogenous nitrate to plants under oxygen deprivation has been observed to improve their survival through the mechanism of "nitrate respiration" (see ahead) (Morard et al., 2004).
In addition to the capacity of the substrate to provide the roots with enough aeration, the availability of oxygen in the root environment also depends on O2 consumption by roots and microorganisms (Naasz et al., 2008). O2 consumption increases with increasing nutrient solution temperature, root weight and photosynthates concentration in the roots, which leads to an increase in the relative CO2 concentration in the root environment if root aeration is not adequate. The increased CO2 concentration leads to an increase of anaerobic respiration, which continues to release CO2. Therefore, oxygen depletion is linked to the increase in the relative CO2 concentration in the root environment (Morard and Silvestre, 1996).
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