16.3.1  Essential mineral nutrients

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Solution culture experiments have established a number of facts basic to plant growth on soils — that plant roots must have a supply of oxygen, that solid soil particles and micro-organisms, while sometimes beneficial, are not essential, that in addition to C, H and O some 14 or 15 chemical elements are essential for plant growth, and that all of these essential elements may be supplied to plant roots as simple ions of inorganic salts in solution and must be supplied in adequate but non-toxic amounts.

A chemical element is regarded as essential if, in its absence, a plant cannot complete its life cycle. An alternative criterion has also been suggested — that the element is part of an essential plant constituent or molecule. So far, only the first criterion has been used to establish essentiality although the latter has proved useful as a guide to further research, as, for example, in the association of Mn with laccase activity and Ni with urease activity.

‘Essential mineral nutrients’ or simply ‘essential nutrients’ include all those chemical elements which are normally absorbed from the soil solution by higher plants. They exclude C, H, and O which comprise more than 99% of metabolically active leaves and some 90–95% of their dry matter, but, paradoxically, they include N which does not occur in any soil mineral and is supplied to some plants from air. Essential mineral nutrients comprise less than 1% of fresh mass in active leaves and 5–10% of their dry matter; yet within this small proportion, all 14 or 15 elements must be present in adequate amounts for healthy growth and effective reproduction.

Functions of nutrients

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Table 16.2

Historically, essential nutrients have been classified in one of two groups based on amounts required by plants, namely macronutrients or micronutrients, and that convention has been adopted for convenience in Table 16.2. This distinction reflects historical sequence and experimental difficulties in discovering essential nutrients. Following advances in nine-teenth century chemistry, essentiality of macronutrients was relatively easy to prove, but essentiality of micronutrients (except Fe) was elusive, requiring great care in eliminating contamination from macronutrient salts, from water and from other environmental sources (see Feature essay 16.1). The classification remains in common use, and was therefore used in Table 16.2 but the distinction between macro and micro is somewhat arbitrary. In terms of chemistry and function, an alternative grouping is outlined below.

Group 1: N, S

N and S are covalently bound in organic compounds in reduced states. They are essential components of proteins providing reactive groups for interaction with metabolites and other nutrients. They are generally absorbed as oxyanions which, except for small amounts of sulphate in a few organic com-pounds, must be reduced before use.

Group 2: P, B

P and B are covalently bound in organic compounds in their fully oxidised states. P is present in phospholipids of cell membranes, nucleic acids of chromosomes and a large number of intermediary metabolites including many in which it plays a crucial role in bioenergetics. P is always present as mono- or polyorthophosphate which behaves as a weak acid. B, like P, is thought to function only in its fully oxidised form; B oxyacid is much weaker and is largely undissociated at the pH of cells. B complexes strongly with hydroxyls in adjacent cis-diol configuration in sugars and their derivatives such as those in the hemicellulose of cell walls, but key metabolic or structural functions still remain unknown.

Group 3: K, Ca, Mg, Na, Cl

K, Ca, Mg, Na and Cl are present primarily in ionic form as either free ions in solution or as entities reversibly adsorbed by electrostatic forces on charged sites. K, Ca and Mg are required in relatively large amounts while Na and Cl, though often present in large amounts, are required only in trace amounts.

Group 3 nutrients function in osmotic adjustment of cell activities and in enzyme activation, probably by modifying the shape and orientation of substrates and enzymes. Ca and to a lesser extent Mg also stabilise cell structures such as membranes and cell walls, probably via their divalent positive charge which forms cross-links with negative charges on cell structures. Some Ca binds strongly and reversibly to the protein calmodulin which in turn activates several enzymes and controls Ca transport. Some Mg is structurally bound in chlorophyll (a strict 1:1 stoichiometry of Mg atoms to chlorophyll molecules is universal)

Group 4: Mn, Fe, Co, Ni, Cu, Zn, Mo

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Figure 16.3 Induction kinetics for in vivo chlorophyll a fluorescence change dramatically according to leaf Mn status of wheat leaves. Constnat yield fluorescence (F0) increases but variable fluorescence (Fv; where Fv = PF0) decreases with decrease in leaf Mn. A ratio of F0/Fv increases abruptly as leaf Mn drops to a critical level, and provides an early indication of impending Mn deficiency. Preplanting soil dressings corresponding to transients on leaf samples were, left to right, 10.6, 1.9 and 0.0 kg MnSO4 ha-1 (Based on Kriedemann et al. 1985)

Fe, Co, Ni, Cu, Zn, Mo and, to some extent, Mn are tightly bound in proteins as metalloproteins or, in the case of Co, which is only required in plants fixing N2, in a coenzyme. All are transition series elements, and with the exception of Zn all show variation in valency state with attendant variation in physiological effect. Group 4 elements govern a wide range of reactions including many oxidation–reductions in which all except Zn can act as electron carriers by undergoing reversible oxidation–reduction. All are micronutrients, and all except Mn form strong complexes with organic and amino acids. Mn is exceptional in that it resembles Group 3 nutrients in being present largely as a free divalent ion which can replace Mg
in the activation of a number of enzymes. But some Mn is tightly bound within the water-splitting apparatus of photo-system II and in superoxide dismutase (Section 1.2.3), an enzyme system that helps dissipate harmful effects of singlet oxygen formation. Mn deficiency results in highly charac-teristic induction kinetics for in vivo chlorophyll a fluorescence (Figure 16.3) and can be used as a diagnostic tool in combination with chemical assay.

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