Assessing the phosphorus status of winter wheat crops: inorganic orthophosphate in whole shoots

Winter wheat (Triticum aestivum L., cv Mercia) was grown at Rothamsted over three seasons (1992/94/95) with plant-available soil P (Olsen-P) ranging f rom very deficient to superoptimal(1-52 mg P/kg; ADAS soil P Index 0-4). Cr itical Olsen-P for 95% maximum grain yield changed little between years and averaged 9 mg P/kg. Total-P and inorganic-P (Pi) were measured in whole sh oots several times between late tillering and anthesis to see if Pi was a b etter indicator than total-P of the adequacy of P supply for maximum grain yield. Inorganic-P accumulated in shoot tissue water with increasing Olsen- P to a maximum concentration of c. 15 mM. Inorganic-P was a better indicato r of crop P status as there was less variation in critical values between s easons and with crop growth stage. Expressing Pi concentrations on a tissue water basis (Piw) reduced the effect of differences in soil water supply a nd also offers the prospect of in situ testing. Over the three seasons, cri tical shoot Piw for 95% maximum grain yield averaged 4.7 mM (range 4.3-5.8 mM) during stem elongation.