Role of Salicylic Acid in Mitigation of Pesticide Toxicity in Cyanobacteria

INTRODUCTION

Any chemical or mixture of chemicals used to kill insects, microbes or invasive species for reducing the damages of crop yield is called a pesticide. Most of these chemicals are non-selective and cause a severe toxic effect on non-target organisms (Yadav and Devi, 2017). All over the world application of pesticide is estimated as 2.5 million tonnes, and it is estimated that only 0.1% of pesticide reaches the target organism and approximately 99% of these chemicals are introduced into the environment (Carriger et al., 2006). Moreover, the manufacturing, formulation and unplanned application of pesticide in the agriculture field leave residues that adversely affect an ecosystem. It leads to environmental pollution, ecological imbalance and residues in crop products (fruit, food, grains, vegetables), soil and water (Gilliom et ah, 2006).

Salicylic acid is the defence hormone that is induced under biotic and abiotic stress (Nazar et ah, 2011). Salicylic acid is a phenolic compound (derivative of benzoic acid) and is also known as 2-Hydroxybenzoic acid. It is a lipophilic crystalline solid powder with a pH of 2.4 (Raskin, 1992a). Salicylic acid is known to regulate physiological activities (photosynthesis, nitrogen metabolism, carbon assimilation, production of heat shock proteins, transcription of defence response gene, regulation of antioxidant compound and production of osmolytes) under stressful conditions in both prokaryotes and eukaryotes (Li et ah, 2002; Raskin, 1992b).

CLASSIFICATION OF PESTICIDE

Pesticides are a chemical ingredient or mixture of chemical compounds that differ in their chemical, physical and other properties from one to another. Pesticides are classified into three broad categories based on their chemical composition, target pest and mode of entry (Yadav and Devi, 2017). On

TABLE 15.1

Classification of Pesticide (Based on Target Pests)

the basis of the target organism pesticides are categorized as algaecide, bactericide, fungicide etc. (Table 15.1) as detailed below.

Classification based on chemical composition is useful in determining the mode and dose of application and precautions that are to be taken. Chemically pesticide may be categorized into inorganic and organic pesticide. Organic pesticides are further classified into four main categories organochlorines, organophosphorus, carbamates and synthetic pyrethroids (Figure 15.1).

Pesticides are also classified according to the mode of entry, i.e. the ways these chemicals come in contact and enter the target pest. For example, through ingestion (stomach poison), contact (contact poison), inhalation (fumigant poison) and plant absorption (systematic poison) (Figure 15.2)

FIGURE 15.1 Pesticide classification based on their chemical composition.

FIGURE 15.2 Pesticide classifications by their mode of entry.

PESTICIDE POLLUTION IN AQUATIC ECOSYSTEMS

Pesticides persist in the environment through bioaccumulation and biomagnification that cause adverse effects on an ecosystem (Parween et al., 2016). Residues of pesticide directly enter the aquatic ecosystem through heavy rainfall, exhaustive irrigation and cause organic pollution. In India, pesticides are mainly used for cotton crops, followed by paddy and wheat. Herbicides are used to kill unwanted plants in the crop field, but via surface runoff it enters into aquatic bodies and causes drastic effects on useful aquatic plants and cyanobacteria (Kumar et al., 2008; Ningthoujam et al., 2013). Many vital pathways in microorganisms are disrupted by herbicides, such as triclopyr blocks the conversion of ammonia to nitrite (Pell et al., 1998), glyphosate retards growth and nitrogen-fixing activity (Santos and Flores, 1995) and artificial auxin 2,4 D shows a negative effect on nitrogen fixation in bacteria and cyanobacteria. Herbicides also block ammonium transforming into nitrates (Arias and de Peretti, 1993). Rice field crops are mainly populated with cyanobacteria and algae that work as a biofertilizer, but on exposure to these chemicals, their vital functions are impaired which results in a loss in rice production (Singh et al., 2016). There is an urgent need for a balanced use of pesticide and to explore sustainable agriculture. Microorganisms and plants synthesize many secondary metabolites such as phenols (including salicylic acid), alkaloids, terpenoids etc., for resisting environment pollution. Salicylic acid is a well-known antioxidant compound, and its expression is induced under stress.