Treated wastewater (TWW) is used commonly for irrigation in semi-arid and arid zones all over the world (Dragonetti et al., 2020, Rahav et al., 2017). In Israel, most fruit tree plantations are irrigated with TWWs of varying quality (Rahav et al., 2017). In contrast, as compared to irrigation with fresh water (FW), irrigation with TWW, depending on the original water source and level of treatment, might result in increased salinity, high levels of organic and inorganic compounds, and levels of living organisms (Chojnacka et al., 2020, Syed et al., 2020), as well as changes in soil structure (Paudel et al., 2017).
The hydraulic conductance of plants is greatly affected by soil characteristics and water quality, especially salt concentration (Aroca et al., 2012). In roots, hydraulic conductance influences water uptake capacity, which depends on the plant’s root surface area, root anatomy, and root water permeability (Niu et al., 2020, Meunier et al., 2018, canals et al., 2021). The dominating driving force for water uptake is the water potential gradient, which depends on osmotic gradients (Grimm et al., 2020, Bazihizina et al., 2017, Deluliis et al., 2021).
As opposed to FW, TWW contains a variety of organic and inorganic compounds, in addition to suspended and dissolved solids (Kharitonova et al., 2020). The organic substances include peptides, carbohydrates, lignin, fats, detergents, pharmaceuticals, and synthetic industrial waste materials (Jablonsky et al., 2018). The inorganic components include heavy metals, such as arsenic, cadmium, chromium, copper, lead, mercury, and zinc (Rathna et al., 2019). Phytotoxicity can limit the agricultural use of TWW for crop irrigation (Margenat et al., 2017, Werfelli et al., 2021). It can lead to a breakdown in soil structure, reduce the hydraulic conductivity of soil, increase osmotic potential, decrease aeration, and reduce root growth (Skaalsveen et al., 2020). Reductions in root function and water uptake that follow TWW irrigation may be responsible for deceases in the performance of plantations, as has been found in the case of avocado, grapefruit, almond, peach, and other fruit trees species (Syed et al., 2020, Romero-Trigueros et al., 2017).
Recently, some investigators have described the phytotoxic risk posed by polypeptides that have been extracted from plants in laboratory experiments. Some researchers reported that endogenous plant polypeptides known as rapid alkalinity factors (RALFs) rapidly increase the pH of plant suspension cell-culture medium and inhibit root growth (Pearce et al., 2001, Covey et al., 2010, Bergonci et al., 2014, Haruta et al., 2014). Another polypeptide, defensin, which is a small cysteine-rich antimicrobial protein that is an important component of the innate immunity of plants, can inhibit plant growth (Allen et al., 2008). The authors reported that KP4 (a killer toxin from the smut fungus Ustilago maydis) and three plant “defensin” types – MsDef1, MtDef2, and RsAFP2 – all inhibit root growth in germinating Arabidopsis seeds at low micro-molar concentrations (Allen et al., 2008). In a pollen-specific tomato (Solanum lycopersicum), a new phytotoxic polypeptide called RALF (SlPRALF) has been identified (Covey et al., 2010). The SlPRALF gene encodes a pro-protein that appears to be processed and released from the pollen tube as an active peptide. Furthermore, a synthetic SlPRALF peptide based on this putative active peptide did not affect pollen hydration or viability but inhibited the elongation of normal pollen tubes in an in-vitro growth system. Inhibitory effects by SlPRALF were detectable at concentrations as low as 10 nM, and complete inhibition was observed at one µM of peptide. A greater effect was observed in a low-pH-buffered medium. Thus, exogenous SlPRALF acts as a negative regulator of pollen tube elongation within a specific developmental window (Covey et al., 2010).
Another phytotoxic polypeptide, called POLARIS (PLS), was found to regulate indole acetic acid (IAA) transport and root growth via effects on ethylene signaling in Arabidopsis (Chilley et al., 2006). Hydraulic conductivity in roots is an essential factor controlling root growth and plant development (Asli and Neumann 2009). The minimum concentration of IAA that can dramatically reduce root hydraulic conductivity is 10− 6 M (Hose et al., 2000), and such reduction inhibits root growth (Asli and Neumann 2010).
Municipal wastewater contains about 0.5 % protein (Rebhun and Manka 1971). Thus, when TWW is used in agriculture, exogenic polypeptides may reach the plant root zone. These polypeptides may have phytotoxic effects on plant growth and development. Therefore, it has been reported that using irrigation water from a river which is polluted with municipal wastewater reduced the growth of Chinese kale and Dendrobium orchids under greenhouse conditions, as well as the growth of tomatoes and Chinese kale under sterile conditions (Sarawaneeyaruk et al 2014). Moreover, wastewater reduced the amount of rhizosphere microorganisms in Chinese kale to five times less than that in tap water. Thus, the use of wastewater in irrigation may affect whole plant growth and decrease annual yields (Sarawaneeyaruk et al 2014).
The aim of this study is to investigate the effects of a specific fraction from TWW, such as polypeptides, on plant water balance and growth. The hypothesis is that these polypeptides apparently behave as hormone-like molecules and affect cell metabolism. Consequently, root hydraulic conductivity is affected, and plant growth is inhibited.