Integrated Root Responses to Variations in Nutrient Supply

Abstract

Plants require 15 essential elements – C, H, O, N, P, K, Ca, Mg, S, Fe, Cl, B, Mn, Mo and Cu – plus several others such as Si, Co and Na (Marschner 1986, p. 5). In land plants, all of these elements except C and O are obtained largely from soil via roots and (usually) associated mycorrhizas (see Chap. 9, this Vol.). The elemental compositions of plants can vary within restricted limits. This variation has environmental and genetic components. The first component reflects the nutrient-supplying characteristics of a particular habitat, the second, the evolutionary history of a species. Two examples illustrate this. First, plants growing on saline soils usually accumulate more Na in their tissues than they would when growing under non-saline conditions, but members of the Chenopodiaceae such as Beta accumulate more Na than do cereals and legumes under the same conditions (Marschner 1986, p. 536). Second, plants growing on calcium-rich soils tend to contain more Ca than those from acidic soils, but dicots contain more Ca per unit dry weight than do monocots when on the same soil (Rorison and Robinson 1984). Such variation in elemental composition may have functional significance. Na accumulation in leaves reduces the internal water potential, thereby partly compensating for the salt-induced increase in external water potential. By contrast, Ca2+ accumulation probably has little direct physiological significance because Ca2+ is required physiologically only in meagre amounts, and much of the Ca2+ in plant tissue is adsorbed onto the negatively charged surfaces of cell walls external to plasma membranes. Ultimately, however, variations in plant elemental compositions are limited by the metabolic systems which have evolved, and by the range of ions and molecules which these systems require as substrates and cofactors in biochemical processes (MacDonald and Davies 1996; Elser et al. 2000; Magnani and Grace 2000). This requirement constitutes a plant’s demand (see also