In <a title="Growing Tomatoes in Soilless Culture" class="intexttaglink" data-tid="10517" href="/soilless-culture.html">soilless culture, an accurate and dynamic control of the water supply is needed to meet plant water requirements due to the low water holding capacity of the system (De Boodt and Verdonck, 1972). Optimum water supply should fulfil plant demand and also prevent salt accumulation in the substrate area surrounding the root. However, under conditions of high transpiration (e.g. at midday in summertime), supply of water may be often insufficient leading to temporal water stress in the plant. In order to avoid this, sometimes excess water is supplied. This results in excessive ion lixiviation within the root environment and loss of unabsorbed water, which should be avoided from an environmental standpoint because water is a scarce resource. For review about the environmental impact of irrigation see Stockle (2001).
In order to carry out an effective management of irrigation, precise information of water status of the group substrate-plant-environment is needed. Different methods try to approach this objective through measurements in the plant, in the substrate or by means of climatic sensors. An indetail review of these methods is included in Medrano (1999). At present, most soilless systems rely on the measurement of a single sensor, normally a radiometer to determine solar radiation or a tensiometer to determine substrate water potential. When the level of water potential or cumulated radiation reaches a threshold, an irrigation event is activated. A higher level of precision, though, may be obtained through the integration of a more complex model in the irrigation computer control system, which estimates water demand according to several parameters. Many models have been developed with different levels of complexity (Medrano, 1999) but currently, most of them are based on Penman-Monteith equation, which include radiation, VPD and leaf area, among other parameters (Monteith and Unsworth, 2007).
Due to water scarcity, new irrigation scheduling approaches designed to ensure the optimal use of water have come up. Deficit irrigation and partial root-zone drying are two ways of maximizing water use efficiency for higher yields per unit of irrigation water supplied. The expectation is that any yield reduction will be insignificant compared with water saved (Kirda, 2002). Although certain water stresses might be suffered by plants irrigated through those strategies, sometimes a mild water stress may be advisable for obtaining a high quality of the product. For example, water stress conditions significantly affected xylem anatomy and functioning of two Zinnia elegans cultivars, which resulted in a longer vase life (Twumasi et al., 2004). For fruit quality, solute accumulation is a recognized physiological response to water stress. Accordingly, moderate water stress improved the quality of kiwi (Miller et al., 1998) of "Merlot" grapes and wine (Peterlunger et al., 2005) and of wheat kernel (Ozturk and Aydin, 2004). Withholding irrigation water during certain periods of time may be a useful management tool to manipulate some quality attributes of the produce (Miller et al., 1998), but it is important to study when to apply water stress to avoid a significant yield reduction.
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