In recent years, the increase in crop areas and associated water demands have led to a decrease in fresh water availability. In arid and semiarid areas, where there is an increasing shortage of water, irrigation is fundamental for agricultural development (Feigin et al., 1991). Alternative sources of water such as treated wastewater are therefore required to satisfy the needs of crops and its use is gradually becoming a common practice worldwide (Angelakis et al., 1999).
One of the advantages of treated wastewater is that it acts as an important source of nutrients for crop production (Jimenez-Cisneros, 1995; Kiziloglu et al., 2007) so that it may be possible to reduce fertilizer application when treated wastewater is used (Gori et al., 2000; Wu et al., 2001). In addition, its use can be considered an attractive option as a result of the economic savings involved.
Another advantage in using treated wastewater is that it can be safely used in the production of ornamental plants, which are not destined for human consumption, avoiding the health problems derived from toxicity in vegetables and fruit. However, many research projects have looked at the effects of RW on the physiology of ornamental species with controversial results, probably as a result of the different cultivation techniques used, environmental characteristics, and the different species studied (Acosta-Motos et al., 2014; Bañón et al., 2011; Gori et al., 2000; Schuch, 2005).
The need to obtain ornamental plants of a high quality entails the proper management of irrigation (Weber et al., 1996) and a careful evaluation of its effect on plants. The chemical composition of treated wastewater varies according to the treatment process, source of waters, location, and time of year (Niu and Cabrera, 2010). Although some water reclamation treatment facilities offer periodic laboratory test results, the data are often incomplete for assessing irrigation quality because they are oriented to the “human impact factor” rather than performance (Duncan et al., 2009). As a consequence, it is necessary to take precautions before reusing wastewater (Angelakis et al., 1999).
One of the main factors that determine the suitability of low-quality water for irrigation is its salinity. Among the many effects, salinity generates the decrease in the ψS of the medium, which, as a consequence, increases the difficulty of the plant to take water from the medium (Neumann, 1997). In saline conditions, reductions in the levels of CO2 assimilation have been related, among others, to partial stomatal closure, decreases in the photosynthetic pigments, and changes in the ion concentration of leaves (Demetriou et al., 2007). Furthermore, such physiological responses in plants are also subject to the daily changes in environmental factors such as atmospheric temperature, light, or humidity.
From this point of view, the study of the daily patterns of leaf water relations and gas exchange activity is a good physiological approximation for analyzing the optimum water use by plants (Hsiao, 1993) and can provide basic information on plant responses to irrigation. Moreover, there are recent studies that use other less common methods such as infrared thermography to indirectly determine the plant water status. The physiological basis of this methodology is the decrease in energy dissipation caused by transpiration resulting from the stomatal closure in plants under water stress, the consequence of which is an increase in leaf temperature (Costa et al., 2012; Jackson et al., 1981; Jones et al., 2002).
Euonymus japonica Thunb. (euonymus) is a popular shrub from Japan, which grows naturally in coastal zones where salt accumulates in soils and is widely used as an ornamental plant. The aim of this work was the daily characterization of the water status of euonymus plants irrigated with good-quality water and treated wastewater with a high salinity level to determine the physiological behavior in this plant. We studied the daily and seasonal gas exchange parameters, water relations, and canopy temperature as well as leaf ion accumulation, photosynthetic pigments, and plant growth.
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