Inorganic orthophosphate for diagnosing the phosphorus status of wheat plants

Winter wheat (Triticum aestivum L) was grown to the 6-leaf stage hydroponically under wide-ranging conditions of phosphorus (P) supply. Tissue concentrations of inorganic orthophosphate (Pi) were measured in different plant parts and related to growth. The Pi was extracted from microwave-dried plant material which is an effective and practical way of preserving Pi at levels close to those prevailing in fresh tissue. Extremely deficient wheat contained low levels of "metabolic" Pi (1.4 mM), while wheat abundantly supplied with P accumulated Pi to concentrations of 40 mM without adverse effects on growth. The main effect of P on shoot growth was to increase the number and weight of tillers. The maximum weight of different organs was attained with different internal Pi concentrations. Critical leaf Pi (last fully expanded main-shoot leaf) and critical whole shoot Pi for maximum whole shoot growth were 2.8 mM (0.043%) and 2.6 mM (0.040%), respectively. The Pi in mature main-shoot leaves was closely related to whole shoot Pi. The diagnosis of plant nutrient status using P "storage pool" concentrations is discussed.