PHOSPHORUS-NUTRITION OF SPRING WHEAT (TRITICUM-AESTIVUM L) .2. DISTRIBUTION OF PHOSPHORUS IN GLASSHOUSE-GROWN WHEAT AND THE DIAGNOSIS OF PHOSPHORUS DEFICIENCY BY PLANT ANALYSIS

In 2 glasshouse experiments, the effects of variations in phosphorus ( P) level on concentration and distribution of total and labile P withi n wheat plants were examined to compare the sensitivity of various pla nt tests for assessing the P status of wheat. Total P was distributed unevenly within the plant: shoots had higher total P concentrations th an roots, and concentrations in grain were markedly higher than in glu mes or straw. Within wheat shoots, the concentration of total and labi le P varied with stage of plant growth and the type and physiological age of the plant part analysed. In general, leaf blades had higher con centrations than their subtending sheaths. During the vegetative phase , the highest P concentrations were measured in immature tissues and t he concentrations decreased progressively in older plant parts, except at luxury supply where total P was invariably higher in old leaf blad es. Critical total P concentrations (CTPC) estimated for whole shoots decreased with advancing plant age beyond Zadoks Scale 13.5. Circumsta ntial evidence suggests that this may be caused by stems of lower P co ncentration progressively constituting a larger proportion of shoot bi omass with increasing age. Critical concentrations defined for leaf bl ades of known physiological age also decreased with plant age beyond Z adoks Scale 13.5 for both labile and total P. These decreases may resu lt from either an increase in the size of the sampled individual leaf blades as plants age or shifts in distribution of P within the shoot f rom the main culm to developing tillers (or reproductive structures). Total P is the preferred and easier method of analysis, and in most ca ses, diagnostic and predictive CTPC were similar in defined individual leaf blades. The estimated critical value for the labile P:total P ra tio of about 30% appeared to be independent of leaf age or stage of pl ant ontogeny. These data suggest that the existence of P deficiency in wheat can be confirmed by analysing whole shoots or recently matured leaf blades for total P. In the period up until early tillering, the P test criteria are constant but thereafter must be related to stage of growth.