Introduction
Plants require 14 essential nutrients of which the macronutrients nitrogen (N) and the minerals potassium (K), calcium (Ca), magnesium (Mg), phosphorous (P) and sulfur (S) are present in plant tissues in relatively large amounts [1]. By contrast, inorganic macronutrients are usually present at low concentrations in the soil and often need to be accumulated against steep concentration gradients. Although generally low, soil availability can fluctuate greatly in both space and time due to factors such as precipitation, temperature, wind, soil type and soil pH. As sessile organisms, plants therefore have had to develop adaptive and flexible strategies for the acquisition of nutrients and these are mechanistically similar for all macronutrients. Further mechanisms are present for (re)distribution throughout the plant. A more detailed description of the availability and distribution can be found elsewhere in this volume for N, P and S [2] and K, Ca and Mg [3].
Issues of availability, uptake and distribution pertain to all macronutrients and it is therefore not surprising that many of the adaptive and molecular mechanisms recur when different nutrients are discussed. For example, uptake mechanisms at the root-soil boundary are typically multiphasic with varying affinities to accommodate different substrate supplies. Localised deficiency or surplus for many nutrients induces morphological root adaptations such as proliferation of lateral roots in the soil. When excess nutrients are available, these are typically stored in the central vacuole and deficiency leads to depletion of vacuolar stores in order to maintain cytoplasmic requirements.