Assimilates macronutrients

The level of plant mineral nutrients available to trees is known to affect fruit quality, but its relative effect is often overestimated compared to other factors such as fruit load and light (and associated assimilate supply to fruit) (see sections above) (for review see Neilsen and Neilsen, 2003). The mineral nutrition of trees and fruits is complex. Uptake of the macronutrients nitrogen... 

Malabsorption is defined as an inadequate assimilation of dietary substances due to defects in digestion, absorption or transport. Malabsorption can affect macronutrients (proteins, carbohydrates, fats), micronutrients (vitamins, minerals) or both, causing excessive faecal excretion and producing nutritional deficiencies and GI symptoms. Digestion and absorption occur in three phases, namely (i) the intra-lumen hydrolysis of fats, proteins and carbohydrates by enzymes, and emulsification by bile salts, (ii) digestion by brush-border enzymes and uptake of end-products and (iii) lymphatic transport of nutrients. Malabsorption can occur when any of these phases is impaired. 

As we have seen, trace metals are involved as cofactors of metalloenzymes and proteins, in all general metabolic processes of phytoplankton, including photosynthesis and respiration, and in assimilation of macronutrients. The vertical profiles of trace metal concentrations in open oceans (Figure 10.9) are like those of macronutrients that is, they show surface depletion resulting from algal uptake and partial release at greater depth due to mineralization. 

The vertical distribution of dissolved iron in the vast majority of the ocean is similar to that of other biologically-assimilated nutrients, notably nitrate and phosphate, and characterized by the following key features (a) depletion at the surface, driven by biological uptake (b) a mid-depth maximum, correlated to the rate of export production from the photic zone and the result of microbial regeneration of exported material (c) reasonably stable, invariant concentrations at depth [71]. Iron, however, dilfers from the macronutrients in notable, sometimes surprising ways, which raise basic questions about how its nutrientlike profile is maintained (Fig. 3). 

The average needs of the three major fertiliser elements per acre, per cropping season, lie within the ranges 50-200 lbs of N, 10-40 lbs of P and 30-150 lbs of K. Assimilation of these macronutrients by the plant is mainly as... 

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