MAGNESIUM FUNCTION IN PLANTS
SYMPTOMS OF DEFICIENCY AND EXCESS
Symptoms of Deficiency
In a physiological sense, magnesium deficiency symptoms are expressed first as an accumulation of starch in the leaves ), which may be associated with early reductions in plant growth and decreased allocation of carbohydrates from leaves to developing sinks . This process is followed by the appearance of chlorosis in older leaves, patterns of which can be explained by the physiological processes associated with magnesium uptake, translocation, and metabolism in plants . Magnesium is physiologically mobile within the plant. Therefore, if insufficient magnesium is available from the rhizosphere, magnesium can be reallocated from other plant parts and transported through the phloem to the actively growing sinks. Because of this mobility within the plant, symptoms of deficiency will first be expressed in the oldest leaves . Early symptoms of magnesium deficiency may be noted by fading and yellowing of the tips of old leaves which progresses interveinally toward the base and midrib of leaves, giving a mottled or herringbone appearance . In later stages of development, deficiency symptoms may be difficult to distinguish from those of potassium deficiency. Under mild deficiencies, a ‘V’-patterned interveinal chlorosis develops in dicots as a result of magnesium dissociating from the chlorophyll, resulting in chlorophyll degradation. In conifers, minor magnesium deficiency symptoms are browning of older needle tips (0.10% magnesium concentration) and in more severe deficiencies, the enter needle turns brown and senesce (0.07% magnesium concentration) . In some plants, a reddening of the leaves may occur, rather than chlorosis, as is the case for cotton (Gossypium spp.) , since other plant pigments may not break down as quickly as chlorophyll. The loss of protein from magnesium-deficient leaves, however, usually results in the loss of plastic pigments from most plants . On an individual leaf, as well as on a whole plant basis, deficiency
FIGURE 6.1 Symptoms of magnesium deficiency on (left) pepper (Capsicum annum L.) and (right) cucumber (Cucumis sativus L.). (Photographs by Allen V. Barker.) (For a color presentation of this figure, see the accompanying compact disc.)
Symptoms of Excess
During the early 1800s, symptoms of ‘magnesium’ toxicity in plants were described; however, during this time, manganese was called magnesium and magnesium was referred to as magnium or magnesia . Because of the confusion in nomenclature, early reports regarding magnesium and manganese should be read carefully. At the present time, no specific symptoms are reported directly related to magnesium toxicity in plants. However, relatively high magnesium concentrations can elicit deficiency symptoms of other essential cations. Plant nutrients that are competitively inhibited for absorption by relatively high magnesium concentrations include calcium and potassium and occasionally iron . Therefore, symptoms of magnesium toxicity may be more closely associated with deficiency symptoms of calcium or potassium.
NUTRIENT IMBALANCES AND SYMPTOMS OF DEFICIENCY
Magnesium deficiency symptoms may be associated with an antagonistic relationship between magnesium ions (Mg2 ) and other cations such as hydrogen (H ), ammonium (NH4 ), calcium (Ca2 ), potassium (K ), aluminum (Al3 ), or sodium (Na ). The competition of magnesium with other cations for uptake ranges from highest to lowest as follows: K NH4 Ca Na (85,86). These cations can compete with magnesium for binding sites on soil colloids, increasing the likelihood that magnesium will be leached from soils after it has been released from exchange sites. Within the plant, there are also antagonistic relationships between other cations and magnesium regarding the affinity for various binding sites within the cell membranes, the degree of which is influenced by the type of binding site (lipid, protein, chelate, etc.), and the hydration of the cation (87). These biochemical interactions result in competition of other cations with magnesium for absorption into the roots and translocation and assimilation in the plant (88-92).
1 Potassium and Magnesium
Increased potassium fertilization or availability, relative to magnesium, will inhibit magnesium absorption and accumulation and vice versa (34,35,90,93-99). The degree of this antagonistic effect varies with potassium and magnesium fertilization rates, as well as the ratio of the two nutrients to one another.
2 Calcium and Magnesium
High rhizosphere concentrations of calcium, relative to magnesium, are inhibitory to the absorption of magnesium and vice versa . In the early 1900s, the importance of proper ratios of magnesium to calcium in soils was emphasized through studies conducted by Loew and May (4) on the relationships of lime and dolomite. High calcium concentrations in solution or in field soils sometimes limit magnesium accumulation and may elicit magnesium deficiency symptoms
3 Nitrogen and Magnesium
4 Sodium and Magnesium
5 Iron and Magnesium
6Manganese and Magnesium
7 Zinc and Magnesium
8 Phosphorus and Magnesium
9 Copper and Magnesium
10 Chloride and Magnesium
11 Aluminum and Magnesium
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