Arsenic is a carcinogenic metalloid with the symbol ‘As’ and atomic number ‘33’. This phytotoxic metalloid, found naturally in soils (Smedley and Kinniburgh, 2002) belonging to Group 15 or V-A in the periodic table, and can exists in four oxidation states: (–3), 0, (+ 3) and (+ 5). Arsenite (As-III) and arsenate (As-V) are the two predominant oxidation states (WHO, 2001; IARC, 2004) of which As(III) is more toxic than As(V) for both humans and plants (Zhao et al., 2009). As toxicity inhibits various kinds of physiological as well as biochemical metabolisms in plant cells among those most severe conditions may occur during photosynthesis (Anjum et al., 2017; Srivastava et al., 2013). Rice is the major crop for nutrition in Pan-Asiatic countries consumed worldwide by humans. Rice cv. MTU-1010, possesses high yielding capacity and extensive amount of groundwater is needed for crop irrigation. So, As could easily get accumulated in different tissues including grains of the test cultivar by As polluted groundwater.
Silicon (Si), a trace element present in the soil, belongs to Group 14 or IV-A in the periodic table. Plants can absorb Si as uncharged silicic acid (H4SiO4). Si possesses the ability to reduce the stress induced toxic effects during Al, Cd and Zn stress in rice (Singh et al., 2011). The Si mediated transporter, Lsi1, is a member of NIP subfamily that helps to transport Si in plant cells (Mitani et al. 2008). There is another transporter Lsi2, known to efflux Si/As(III) towards xylem in plants (Zhao et al. 2009). According to Wattanapayapkul et al. (2011), Si fertilization is beneficial for plants. Therefore, Si administration will be a low-cost approach to combat As induced toxicity by lessening As uptake and accumulation within plant cells. In our previous study, we also reported that in the presence of Si, As accumulation was reduced in rice seedlings (Das et al. 2018).
Selenium (Se) is a trace element and a Group 16 or VI-A metalloid in the periodic table. Se prevails in nature as both inorganic forms viz., selenate (SeO42−), selenite (SeO32−), selenide (Se2−), elemental Se and organic forms viz., Selenocysteine (SeCys) and Selenomethionine (SeMet) (Wu et al., 2015). SeO42− is the most ubiquitous form of bioavailable Se in agricultural fields as well as more water soluble than SeO32− (Missana et al., 2009). In previous studies, it has been demonstrated that at low concentration (0 µM – 5 µM), Se has the ability to protect plants from various environmental stresses viz., cold (Chu et al., 2010), drought (Hasanuzzaman and Fujita, 2011) and also metal stresses (Kumar et al., 2012; Pandey and Gupta, 2015). Generally, Se concentration lower than 1 mg Kg− 1 in soils can enhance plant growth in non-accumulating plants. Se increases plant resistance against oxidative stress caused by generation of free radicals in soybean (Djanaguiraman et al., 2005). As(V) uptake in Pteris vittata was suppressed by the addition of Se, indicating the antagonistic effects of selenium on arsenate uptake (Feng et al., 2009). Se application in the form of fertilizer will be a low-cost way to reduce the toxicity in rice grown in arsenic prone soil due to its antagonistic nature.
In chloroplast, chlorophylls operate photosynthesis by assimilating and converting light energy into chemical energy to drive all kinds of cellular activities. But the formation and activation of this pigment is suppressed under stress induced oxidative difficulties in the environment (Agathokleous et al., 2020). There is another pigment, carotenoids provide defence to chlorophyll during biotic and abiotic stresses (Drezkiewica and Basznzki, 2010). They have photoprotective role and protect the photosynthetic machinery from the harmful effects of free radicals. As stress impedes the biogenesis of chlorophyll that let down the potency of PS-II and thus interrupts the photosynthesis process (Bankaji et al., 2014). This metalloid contamination restricts the stomatal gateway from entering the CO2 which is correlated with the depletion in transpiration rate as there is limitation of gas interchange from environment to plants (Milivojevic et al., 2006; Anjum et al., 2016). If the stomata have been closed for a long time, water levels and its potentiality are being enhanced which results in the reduction of photosynthesis (Ohashi et al., 2006). In some previous reports, it was demonstrated that Si application enhanced the rate of photosynthesis in wild type rice plant (Sanglard et al., 2014). Sil et al., (2019) also reported that Si was able to improve photosynthetic activity under As stress in wheat seedlings. On the other side, Se reduced the generation of oxidative stress by alleviating malondialdehyde levels and elevated the photosynthetic activity as well as accumulation of sugars in Solanum tuberosum L. cv. Sante under both As and Cd stresses (Sahid et al., 2019). Therefore, the present investigation is emphasized on the comparative effect of Si and Se to lessen As induced toxicity on the basis of pigment levels, photosynthetic parameters and carbohydrate metabolism in hydroponically grown rice seedlings.
A seedling in flourishing stage generally face endangered situations concerning instability of carbohydrates in the cells which is a perfect phase to examine the impact during biotic and abiotic stresses (Hanley and May, 2005). During photosynthesis, plants promote carbohydrate production which is carried out to different parts in the form of soluble sugars or stored in the form of starch and sugar. It was demonstrated in the report of Rosa et al., (2009) that the growth and development of plant seeds to seedlings stage is fundamentally rely on carbohydrate storage that is transported to different plant parts from the repository organ which is essential to balance an osmotic equilibrium in plants. Higher levels of photosynthetic products in plant tissue can maintain the proficiency of plants to revive them under various kinds of environmental stresses (Bagheri and Sadeghipour, 2009; Naureen and Naqvi, 2010). Accumulation of sugars under metal toxicity predominantly protect the concentration of amino acids, nucleic acids, proteins and also provides osmo-protection in the plant cell. Assimilated carbon produce sugar which can modulate toxicity level as well as gene regulation during environmental stresses (Lemoine et al., 2013). Joint application of As(V) with Si and As(V) with Se was an attempt to reduce the metal induced toxic effects by improving pigment levels, photosynthetic parameters and regulating carbohydrate metabolism to develop tolerance against toxicity.
In the present study, we have documented how exogenous Si and Se amendments are individually potential to improve the chloroplast pigments level, photosynthetic parameters and carbohydrate metabolic processes in rice cv. MTU-1010 seedlings during As(V) application. The current study will assist to develop some techniques in soil condition which will be a low-cost way to cultivate rice with improved growth and yield in As subdued agricultural soils.