PHOSPHORUS-NUTRITION OF SPRING WHEAT (TRITICUM-AESTIVUM L) .3. EFFECTS OF PLANT NITROGEN STATUS AND GENOTYPE ON THE CALIBRATION OF PLANT-TESTS FOR DIAGNOSING PHOSPHORUS DEFICIENCY

The influence of plant nitrogen (N) status and plant genotype on plant test criteria for diagnosing phosphorus (P) deficiency in wheat was e xamined in 2 glasshouse experiments. Criteria for both total and labil e P in leaf blades of standard physiological age are, to only a minor extent, affected by variations in N supply and by genotypic diversity. Interactions between N and P supply had marked and complex effects on shoot and root yield, P and N uptake in shoots and concentrations in leaf blades, and on the distribution of P and P fractions within wheat shoots. Thus, whilst the external P requirement (i.e. P level require d for 90% maximum shoot yield) more than doubled as N supply was raise d, variations in N supply had only minor effects on internal P require ment (i.e. the tissue P concentration required for 90% maximum shoot y ield). On the other hand, the external P requirement for root yield va ried markedly with plant age and N supply. N deficiency increased tota l P concentrations in leaf blades at all P levels, primarily by increa sing the concentration of the labile P fraction. Also, N concentration s increased to adequate levels in the shoots of P-deficient plants but only at the 2 lower levels of applied N. Plant N status also affected the shape of diagnostic relationships between relative shoot yield an d P concentrations in young and mature leaf blades by constricting P c oncentration in the adequate-luxury zone and increasing the slope of t he relationship in the zone of deficiency. Whilst the asymptotic grain yield and external requirement. for P for the tall cultivar (Halberd) was substantially less than for the semi-dwarf cultivars (Condor and Durati), consistent P x cultivar interactions on shoot yield and P upt ake during vegetative growth, were largely absent. For leaf blade clas ses examined, the shape of the diagnostic relationship for total and l abile P was essentially similar for each cultivar. As a result, differ ences in estimated critical P concentrations for total and labile P be tween the cultivars for leaf blades during vegetative growth, or crite ria for grain, glumes, and straw at maturity, were relatively small.