One of the major problems in closed hydroponic systems is the accumulation of salt ions, especially Na+ and Cl−, in reused solutions (Savvas et al., 2005). For most plant species, Na+ appears to reach a toxic concentration before Cl− (Munns and Tester, 2008), resulting in Na+ toxicity from a combination of osmotic and ionic stresses (Kronzucker and Britto, 2011). To prevent high Na+ levels in the rhizosphere, growers may periodically permit salt leaching (Varlagas et al., 2010). For example, in The Netherlands, discharge from closed hydroponic systems is allowed if Na+ concentration reaches 6 mm for cucumber or 8 mm for tomato during production (Van Os, 1998). Discharging used nutrient solution can pollute the environment and sometimes even results in soil salinization (Varlagas et al., 2010). One way to address this problem is to use these solutions for the cultivation of another economically valuable crop, which has a high salt tolerance and the ability to accumulate sodium (Grieve and Suarez, 1997).
Purslane (Portulaca oleracea L.) is not only a salt-tolerant plant (Kilic et al., 2008; Shannon and Grieve, 1999), but also a good salt-removing crop (Aksoy et al., 2003; Grieve and Suarez, 1997), capable of removing up to 65 kg·ha−1 of Na+ in one growing season (Kilic et al., 2008). It has been recommended as a potential intercrop to remove salt in orchards (Kilic et al., 2008, 2010) and an excellent candidate for inclusion in saline drainage water reuse systems (Grieve and Suarez, 1997). However, these studies have been carried out under soil or sand culture conditions (Grieve and Suarez, 1997; Kilic et al., 2008, 2010), and the related information is not available under hydroponic conditions, because Na+ uptake in plants can be affected by culture medium (Liu, 2002).
In addition, purslane is a traditional food crop in some Mediterranean, Central American, and Asian countries (Cros et al., 2007; Grieve and Suarez, 1997) and has now become a potential key vegetable crop worldwide because of its high nutritive and antioxidant properties (Uddin et al., 2012b; Wenzel et al., 1990). Recently, hydroponic production of purslane, as the easiest and cheapest growing method, has been gaining more and more attention as a result of the shorter cultivation cycles, higher planting densities, and clean and easily packed products compared with soil-based culture (Cros et al., 2007; Kaşkar et al., 2008). Furthermore, previous studies indicated that shoots had considerably higher Na+ content than roots (Aksoy et al., 2003; Tester and Davenport, 2007) and, also, a tight coupling has been observed between plant growth (especially shoot growth) and Na+ uptake (Lv et al., 2012). So it may be possible to accumulate Na+ in purslane shoots at reasonably high levels without adversely affecting biomass productivity or, in other words, to hydroponically produce purslane as a sodium-removing vegetable.
Previous related studies in purslane were carried out separately on salt removal (Grieve and Suarez, 1997; Kilic et al., 2008) and vegetable production (Cros et al., 2007; Lara et al., 2011), and the NaCl concentrations in the solution used for growing purslane were totally different in these two categories of studies. In the salt removal studies (Grieve and Suarez, 1997; Kilic et al., 2008), the concentrations of NaCl treatment were usually higher up to more than 50 mm, but in the vegetable production studies (Cros et al., 2007; Lara et al., 2011), NaCl was not added to solutions and its concentrations were not known (should be very low). Currently, there is a lack of information regarding the effects of moderate NaCl concentrations (≈6 to 10 mm) on biomass production and sodium removal in purslane.
An appropriate selection of cultivars is vital to evaluate the potential of hydroponic production of purslane as a sodium-removing vegetable. Commercially available purslane cultivars are either green- or golden-leafed genotypes (Seedaholic, 2013). In studies aimed at evaluating growth and yield responses of purslane, significant differences were observed in plant growth between green- and golden-leafed genotypes in hydroponic solutions without added NaCl (Lara et al., 2011; Palaniswamy et al., 2000). However, in salt removal studies on purslane, only one genotype was used to evaluate Na+ removal capacity under high NaCl concentrations (Grieve and Suarez, 1997; Kilic et al., 2008). Consequently, comparisons of sodium removal and biomass accumulation between green- and golden-leafed purslane genotypes are needed under moderate NaCl concentrations (≈6 to 10 mm).
Determining how growth stage affects purslane biomass accumulation and sodium removal is important to predict the ideal harvest time of hydroponically produced purslane when used as a sodium-removing vegetable crop. A long production time (delaying the harvest time) would result in high biomass (i.e., increased yield); however, most crops exhibit higher Na+ uptake efficiency during vegetative vs. reproductive growth (Subbarao et al., 2003). Uddin et al. (2012a) found that Na+ concentration in dried purslane leaves decreased with plant maturity when evaluated over 60 d after transplanting young plants from the field into potted soil. In contrast, Kilic et al. (2008) reported that percent Na+ in dry purslane shoots was highest at the last harvest, but it was evaluated only 12 to 38 d after germination in a sand culture system. Therefore, further clarification is needed to evaluate how growth stages affect sodium removal and biomass accumulation of hydroponically grown purslane during a longer growth period.
The objectives of the present study were to assess 1) the potential of producing purslane hydroponically under moderate NaCl concentrations (≈6 to 12 mm); 2) whether purslane can be used to remove sodium from hydroponic solution with moderate NaCl salinity; and 3) how NaCl concentrations, cultivar type, and growth stage affect purslane growth and sodium removal from a hydroponic system.
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