Plants utilize nutrition in the form of inorganic minerals for growth and development. Plant nutrients can be divided into two groups and these are macronutrients and micronutrients. Macronutrients are elements required by plants in relatively large quantities and these include Carbon (C), hydrogen (H), oxygen (O), phosphorus (P), potassium(K), nitrogen (N), sulfur (S), magnesium (Mg) and calcium (Ca) (D.R. Decotean, 2005). These elements play different but equally important roles in plant growth and lack of any one of them affects the plant in one way or another. Deficiency symptoms for any element depend primarily on the function of the element and whether or not the element is readily translocated from old leaves to younger leaves (Purohit, 2003). Carbon, hydrogen and oxygen are the three non-mineral plant macronutrients. These are taken in from the air and water and are essential for respiration and photosynthesis and they also enter into the composition of practically all organic compounds in the plant and account for a major part of the dry weight (Pandey and SInha, 1995). Nitrogen is present in plants in the largest concentration of any of the mineral nutrients. It is a component of many organic molecules of great importance, including chlorophyll (Decotean, 2005). Nitrogen is a component of amino acids, which are the molecular subunits from which proteins are synthesized. The nucleic acids DNA and RNA also contain nitrogenous bases (adenine, guanine, cytosine and thymine) and these make up the genetic coding sequences (McIntosh, 2010). According to Uchida (2000), nitrogen is also responsible for the improvement of the quality and quantity of dry matter in leafy vegetables and protein in grain crops. Nitrogen deficiency in plants is characterized by chlorosis, which is a yellowing of the leaves. Nitrogen is a mobile element in plants, and can be moved around as needed. Therefore older leaves tend to be the first plant parts to show signs of nitrogen deficiency as nitrogen is transported to support new growth (Salisbury and Ross, 1999). Chlorosis is often evident when other minerals are deficient as well and depending on the severity of deficiency, the chlorosis could result in the death and/or dropping of the older leaves (Uchida, 2000). Plants grown in poor nitrogen conditions tend to have stunted growth which may be as a result of reduced cell division. They may also have the characteristic of abnormally thin shoots. (McIntosh, 2010). Nitrogen deficiency can also cause early maturity in some crops, which results in a significant reduction in yield and quality (Uchida, 2000).
According to Purohit (2003), potassium is best known in the opening of stomata. Potassium serves a critical role in the turgor mediated opening of guard cells stomatal aperture which is determined by guard cell turgor. In photosynthesis, K has the role of maintaining the balance of electrical charges at the site of ATP production (Uchida, 2000). It is also thought to be essential for the conformational stability of the enzyme pyruvate kinase, also known as ATP (Glass, 1989). It helps to adjust water balance, improves stem rigidity and cold hardiness, enhances flavor and color on fruit and vegetable crops, increases the oil content of fruits and is important for leafy crops (Dorothy Morgan, n.d). Like nitrogen, potassium is highly mobile in plants resulting in deficiency symptoms appearing in the oldest foliage first. The first observable symptom of potassium deficiency is mottled or marginal chlorosis, which then develops into necrosis primarily at the leaf tips, at the margins, and between veins (Taiz and Zeiger, 2002). Since K+ ions are important in maintaining water balance and turgor, potassium deficient plants are sensitive to water stress and wilt easily. (McIntosh, 2010)
Phosphorus is absorbed from the soil as the monovalent phosphate ion H2PO4- or as the divalent ion HPO4- (Purohit, 2003). It is a part of the energy molecules...
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