Rapid urbanization and industrialization are among the major causes of pollutant contamination in the environment. In agricultural areas, plants can uptake toxic elements present in the soils and accumulate them in edible parts consumed by animals and humans. The intake of contaminated crops and vegetables has become a major source of public health problems in several countries (Chaney 2015). Rice is a staple food of more than half of the world’s population. Unfortunately, the contamination of rice by toxic elements, particularly cadmium (Cd), has been shown to be an important factor contributing to acute and chronic diseases in humans (Chaney 2015; Wang et al. 2021). Cadmium contamination in soil occurs from both natural and anthropogenic sources. The major anthropogenic sources of Cd include mining activities, phosphate fertilizers, and industrial emissions (He et al. 2005). Cd enters freshwater from industrial sources and is mostly absorbed in sediments. In a paddy field, sediments and irrigating water originating from a contaminated water reservoir cause contamination, since the land is often continuously irrigated by local surface water (Charoenpanyanet and Huttagosol 2020).
In addition to its effects on human health, the presence of Cd in the soil causes many toxic symptoms in plants. The severity of Cd toxicity varies among plant species and varieties. In general, Cd interferes with photosynthesis by slowing down the activity of photosynthetic enzymes (Ying et al. 2010), by competing with Ca+ 2 entering guard cells and triggering stomata closure (Perfus-Barbeoch et al. 2002), and by inhibiting chlorophyll biosynthesis (Skrebsky et al. 2008). In addition, Cd also causes oxidative stress and interferes with nutrient uptake and carbohydrate metabolism, resulting in a reduction in biomass and yield (Kibria et al. 2006). Uptake of Cd in soil solution into the root occurs through symplasm, driven by the electrochemical potential gradient across the plasma membrane of root cells. After entering plant roots, most metals form phosphate, sulfate, or carbonate precipitates which are immobilized into extracellular and intracellular compartments such as cell walls and vacuoles. In rice, transporters such as OsIRT1 (iron-regulated transporter1), OSIRT2, and OSNramp1 are potentially involved in Cd uptake (Uraguchi and Fujiwara 2012). Xylem is a major pathway of Cd translocation from root to shoot. The translocation of Cd is a relatively fast process by which substantial Cd can be detected in shoot tissues after Cd treatment of roots for only one hour. In contrast, phloem is the major Cd transport route into grains (Tanaka et al. 2007). Among rice cultivars, Cd accumulation in shoots and grains is potentially higher in indica than japonica cultivars. Moreover, some specific cultivars of indica rice accumulate much higher Cd in vegetative tissues and grains (Kato et al. 2010).
In Thailand, Cd contamination has been reported in Mae Sot District of Tak Province (Sriprachote et al. 2012a; Meeinkuirt et al. 2019). It has been demonstrated that the high level of Cd concentrations in the soil are linked with significant health problems for local villagers who consume crops grown in the area (Simmons et al. 2005; Sriprachote et al. 2012a). Agricultural soils in Mae Sot District have a diverse spatial distribution of Cd concentrations. Soil Cd concentrations there were found to range from 0.5 to 284.0 mg kg− 1. Rice grown in this region has been reported to accumulate Cd at a rate of more than 0.2 mg kg− 1 (Saengwilai and Meeinkuirt 2021), classified as unsafe for human consumption according to the standards of the Codex Committee on Food Additives and Contaminants (CCFAC 2005). As a result, the local government has initiated several policies to reduce health risks by promoting the cultivation of alternative crops such as sugarcane and rubber trees. However, these efforts have been proven to be unsuccessful, and the farmers have returned to cultivating rice for their own consumption (Sriprachote et al. 2012a). It is, therefore, necessary to seek appropriate technologies to reduce Cd concentration in rice grain.
Moringa oleifera Lam. is native to the north of India and grows well in tropical regions. It has been extensively studied because it is rich in amino acids, antioxidants, phytohormones, and minerals (Araújo et al. 2013). The content of compounds in Moringa extract depends on extraction technique. A solution of M. oleifera leaf extracted by 70% ethanol contains as many as 44 compounds, including pentacosane (17.41%), hexacosane (11.20%), (E)-phyton (7.66%), and 1-[2, 3, 6-trimethyl-phenyl]-2-butanone (3.44%). These compounds are beneficial for fighting against dermatophytes (Vongsak et al. 2014). In addition, Chuang et al. (2007) discovered that leaves of M. oleifera contain essential oils which have anti-fungal activities against dermatophytes. Apart from its medicinal benefits, Moringa extract has been shown to remove heavy metals and contaminants in water by a significant amount (Araújo et al. 2013). This is possibly due to some components such as thiol-containing proteins present in the extracts that interact with heavy metals, resulting in ion adsorption and charge neutralization. Recently, Kerdsomboon et al. (2021) reported that aqueous Moringa oleifera leaf extract (AMOLE) reduced intracellular reactive oxygen species (ROS) levels in yeast cells grown in the presence of As(III), Cd, Ni, and Pb. It was shown in that study that gallic acid, a component in the extracts, physically bound with As(III) via its hydroxyl and carboxyl groups, thereby preventing As(III) accumulation and As(III)-induced ROS generation (Kerdsomboon et al. 2021). In plants, the application of Moringa oleifera leaf extract improved the antioxidant enzyme system of wheat plants under salt stress, improving their performance significantly (Yasmeen et al. 2013). In a Cd contaminated environment, not only did Moringa coagulate Cd in the soil and thereby prevent its absorption and accumulation in wheat plants, but it also influenced the expression of genes associated with alleviation of metal toxicity (Hassanein et al. 2016).
The aim of the present study is to assess the effect of soluble AMOLE on toxicity and accumulation of Cd in Thai jasmine rice, Khao Dawk Mali 105 (KDML105). Plants were grown in Petri dishes, a hydroponic system, and a pot system under different Cd concentrations, with and without AMOLE treatments.