Phosphorus nutrition of terrestrial plants

Phosphorus (P) is essential for plant growth and fecundity. It is an integral component of genetic, metabolic, structural and regulatory molecules, in many of which it cannot be substituted by any other elements. Tissue P concentrations in well fertilized plants approximate 0.4–1.5% of the dry matter (Broadley et al. 2004), most of which is present as nucleic acids and nucleotides, phosphorylated intermediates of energy metabolism, membrane phospholipids and, in some tissues (principally seeds), as inositol phosphates. Some P also occurs in phosphoproteins and as inorganic phosphate (Pi) and pyrophosphate (PPi). It has been estimated that small metabolites, nucleic acids and phospholipids contribute approximately equally to leaf P content in P-replete plants (Figure 4.1; Marschner 1995; Dörmann and Benning 2002). Tissue P concentrations show no systematic differences between angiosperm species grown in P-replete conditions, but strong positive correlations occur between shoot P and shoot organic-N concentrations (Broadley et al. 2004). When plants are sampled from their natural environment, shoot N:P mass ratios vary between about 5:1 and 40:1 (e.g. Garten 1976; Thompson et al. 1997; Elser et al. 2000a; Tessier and Raynal 2003; Güsewell 2004; McGroddy et al. 2004; Güsewell et al. 2005; Han et al. 2005; Niklas et al. 2005; Wassen et al. 2005; Wright et al. 2005; Kerkhoff et al. 2006) and leaf N appears to scale as the 3/4 power of leaf P (Niklas et al. 2005; Niklas 2008). Ratios of 10:1 approximate the maximum critical organic-N:P ratios reported for a range of crop plants (Greenwood et al. 1980; Güsewell 2004). In general, leaf N:P ratios below 13.5 suggest N-limited plant growth, whilst leaf N:P ratios above 16 suggest P-limited plant growth (Aerts and Chapin 2000; Güsewell and Koerselman 2002; Tessier and Raynal 2003). Stoichiometric relationships between leaf N and leaf P appear to be a consequence of the requirements of N for proteins and of P for nucleic acids, membranes and metabolism (Elser et al. 2000b; Niklas 2008). Plant relative growth rate (RGR) is positively correlated with rRNA concentration and negatively correlated with protein concentration (Ågren 1988; Elser et al. 2000b; Niklas 2008).