ABSTRACT
A nutrient is defined as a substance that cannot be synthesised by an organism in sufficient amounts to maintain its normal functions. Plants can synthesise all the organic compounds they require, provided they have access to sufficient carbon dioxide, water and 14 essential mineral elements (White and Brown, 2010). The mineral elements essential for plant nutrition comprise six macronutrients – nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S) – which are required at tissue concentrations greater than 1 mg g−1 dry matter (DM), and eight micronutrients – chlorine (Cl), boron (B), iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), nickel (Ni) and molybdenum (Mo) – which are required in much smaller amounts (Table 1). Every essential mineral element is required for specific biochemical or physiological functions in which it cannot be replaced by any other element. Thus, a deficiency of any essential mineral element will result in slower growth, impaired development and loss of yield, even if the supply of all other elements is sufficient. The symptoms of deficiency are generally related to the role of that particular mineral element. For example, N deficiency in potato results in small plants with pale green or yellow leaves; K deficiency results in short, flaccid plants whose older leaves are yellow at the edges; and Ca deficiency results in internal browning and deformed, hollow tubers (Bould et al., 1983; Ulrich, 1993). The location of deficiency symptoms is related to the mobility of an element within the plant. Symptoms of deficiencies of mineral nutrients
that are mobile in the phloem are manifested initially in older leaves, whereas those of mineral nutrients that are less mobile in the phloem are observed first on younger leaves and developing tissues. In potatoes, N, P, K, S and Cl are relatively mobile, whereas Mg, Fe, Zn, Cu and Mo are less mobile, and B, Mn and Ca are relatively immobile in the phloem
Table 1 The principal roles and minimal concentrations of mineral nutrients in diagnostic leaves (fourth leaf from the top) of potato plants associated with optimal growth during tuber bulking. Information from White et al. (2007), Hawkesford et al. (2012) and White and Greenwood (2013). ND = not determined
Nutrient Principal roles Minimal leaf concentration
Nitrogen (N) Constituent of amino acids, proteins, enzymes, secondary metabolites and nucleic acids; photosynthesis, respiration and cellular metabolism; transcription, translation and replication of genetic information; responses to biotic and abiotic stresses.
