16.3.2  Quantitative requirements

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For practical use in crop production, plant nutritionists need to know whether a plant’s internal supply of a nutrient is optimal for growth and, if not, how best to make it so. Answers to these two questions characterise a plant’s nutrient requirements.

Unfortunately, discussions about a plant’s quantitative nutrient requirements frequently fail to differentiate between the two quite distinct entities of internal and external requirements. Such confusion can be avoided with appropriate terminology: ‘internal requirement’ is the minimal concentration of a nutrient required within plant tissues and organs to sustain optimal metabolism and growth; ‘external requirement’ is the amount of a nutrient which must be supplied to soil or other culture medium for a plant to meet the internal requirement.

Estimates of internal nutrient requirements of actively metabolising leaves of dicotyledonous plants (Table 16.2) are minimal concentrations for maximal growth. Internal require-ments of roots for Mn and Mg could be expected to be lower than those for leaves, while those of N2-fixing nodules for Co, which is not required by leaves, is around 0.1 mg kg–1.

Internal nutrient requirements of grasses and other monocotyledons appear similar to those of dicotyledons, except for a halving of their Ca and B requirements. With these exceptions and differences in the extent to which plants can use Na to replace K, variation in internal nutrient requirements offers little scope for improving nutrient use efficiency of metabolic processes by selection or genetic mani-pulation. By contrast, wide variation in shoot tissue concentrations of micronutrient ions acquired by different genotypes does occur (Figure 16.4) and most notably for Mn (Figure 16.4). Such gene-based differences in acquisition of soil nutrients does constitute a ready source of variation in uptake efficiency that can be put to good effect in breeding cultivars better adapted to deficient soils (see also the frontispiece to Chapter 16).


Figure 16.4 Trace elements represent only a few parts per million of plant shoot tissue dry mass, but nevertheless vary between species, and according to varieties within species. These concentrations of Cu, Fe, Mn and Zn in tops of 24 annual crop and pasture species grown in filed plots at Gidgegannup, Western Australia, emphasise such variation and highlight an especially wide range of leaf Mn (Based on Loneragan 1976 from data of Gladstones and Loneragan 1970)

External requirements of seedlings may be modified by seed nutrient content, and in some cases seeds may contain sufficient Mo and Co for the life cycle of annual plants.

Soil nutrient resources will vary in their availability to plants due to physico-chemical conditions in root zones, as well as fluctuations in root activity. Such intermittency in supply is buffered within plants by redistribution of internal nutrient resources as excess amounts in older tissues are retranslocated to growing regions. Significantly, not all nutrients can be recycled in this way, and in many plants Ca, B and sometimes Fe and Mn must be supplied continuously. Very little Ca or B moves from leaves to growing regions. Indeed, roots cannot grow into environments lacking Ca or B, even when other parts of the same root system have ample supplies of these nutrients. Internal remobilisation of Fe and Mn embedded within plant tissues is similarly restricted.

This failure of Ca, B, Fe and Mn to move from leaves to growing regions has been attributed to their low phloem mobility. While this is probably true for Ca, restricted movement of B, Fe and Mn is better attributed more broadly to their immobility after deposition in leaves. Immobile nutrients such as Ca contrast sharply with highly mobile nutrients such as N, P and K. Several other nutrients including S, Cu and Zn display variable mobility from leaves, being immobile from actively metabolising leaves but moving out of senescing leaves (Table 16.2). B physiology remains an enigma with some species showing B mobility in association with sorbitol translocation. Those species translocating sucrose typically show low B mobility.