Availability of high-quality water is becoming a limited resource for both agricultural and public use. When plants are not sufficiently irrigated, they become water stressed, which causes physiological, molecular, and biochemical changes in the plant (Wang et al., 2003). This can adversely affect both yield and quality. Traditional overhead irrigation uses excess water to ensure complete coverage of the entire crop to the point of full saturation of the root medium. Under certain conditions, up to 75% of the water and fertilizer applied by overhead irrigation may be lost through evaporation, runoff, or leached from potted plants (Yelanich and Beirnbaum, 1994). Drippers can deliver water directly to the top surface of the root medium; however, studies have shown over-watering by 10% to 30% is recommended to prevent salinity buildup in the root medium (Mastalerz, 1977). By comparison, subirrigation, and in particular ebb and flow, systems use water more efficiently (Dole et al., 1994; Elliott, 1992; Morvant et al., 2001), and are also more efficient in terms of nutrient (Kent and Reed, 1996; Purvis et al., 2000) and fertilizer use (Strefeler, 1991). The benefits of ebb and flow, as it is currently applied, are that no water is lost to the environment and very little is lost to evaporation. The root medium may take up water to 90% of effective water-holding capacity after each cycle. Geremia Greenhouse (Wallingford, CT), in collaboration with True Leaf (Petaluma, CA), developed an irrigation system for potted ornamental plants that can complete an ebb and flow cycle in 4 min. This short cycle restricts water uptake and achieves partial saturation of the root medium, which we refer to as partial-saturation ebb and flow watering (PSEFW). Partial-saturation subirrigation is a method of regulated deficit irrigation where the medium does not saturate during each irrigation event. Pots may take in 25% less water than those under full saturation (Gent and McAvoy, 2011).
The plant response to salinity is multifaceted and involves changes in plant physiology, morphology, and metabolism (Hilal et al., 1998) and results in reduced plant growth (Rhoades, 1993). Salinity changes the water content in plant cells thereby affecting crop physiology (Hasegawa et al., 2000; Sultana et al., 1999). Water uptake by roots is restricted as excess salts accumulate in the root zone, thereby reducing the capacity for water movement to the shoot. These negative effects of salinity are further exacerbated as the amount of water decreases in the root zone (Warrence et al., 2002). Salinity disrupts various physiological processes: it reduces stomatal conductance (gS), chlorophyll content, photosynthetic rates, and transpiration, and increases membrane permeability (Yurtseven et al., 2005). Plants exert more energy to extract water from the soil, which in turn reduces energy available for normal growth processes, resulting in smaller and fewer leaves, reduced root length, and shorter stature (Shannon and Grieve, 1999).
Salinity and alkalinity can reduce greenhouse crop production up to 50% (Roberts, 1991). Salinity affects the uptake of essential plant nutrients and ultimately leads to deficiency of essential ions and an increase in nonessential ions such as sodium (Greenway and Munns, 1980). With the increase in salinity, leaf Na and Cl content also increase (Niu et al., 2010), thereby affecting the uptake of nutrients like K and Ca, and causing a decrease in plant growth. Sodium can also antagonize root uptake of cations such as K, Ca, and Mg (Kuehny and Morales, 1998). Chloride may become toxic at concentrations in excess of 140 mg·L−1 in the saline irrigation water, resulting in symptoms such as marginal or tip necrosis (Biernbaum, 1994).
Regulated deficit irrigation may control disease (Hong and Moorman, 2005). Elmer et al. (2012) found that the incidence of pythium root-rot diseases in crops under PSEFW was significantly reduced compared with crops under full saturation. Another benefit from regulated deficit irrigation is control of both biomass and height growth of ornamental plants (Alem et al., 2015; Lee and van Iersel, 2008). Gent and McAvoy (2011) reported that ornamental crops grown under PSEFW develop the compact stature, most desired by the industry, by accumulating less shoot biomass compared with plants grown under full-saturation irrigation. Water management techniques that can reliably control growth without adversely affecting plant quality are advantageous, since plant growth regulators are commonly used to control height growth in ornamental species. Compared with full saturation, the oxygen content is higher in substrate irrigated with partial saturation, an environment more beneficial to the metabolic activity of roots (Nemati et al., 2002).
However, the effectiveness of partial- versus full-saturation management on plant growth and quality has not been reported when there is high background salinity in the irrigation water. Therefore, the objective of this research was to characterize the effects of various irrigation management regimes, with or without added salinity, on the growth and tissue nutrient accumulation of poinsettia grown under partial- compared with longer-duration ebb and flow watering, and for plants grown under drip irrigation.
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