Effects of P deficiency on assimilation and transport of nitrate and phosphate in intact plants of castor bean (Ricinus communis L.)

An experimentally-based modelling technique was applied to describe quantitatively the uptake, translocation, storage, and assimilation of NO3- and H2PO4- over a 9 d period in mid-vegetative growth of sand-cultured castor bean (Ricinus communis L.) which was fed 12 mM NO3- and either 0.5 or a severely limiting 0.005 mM H2PO4-. Model calculations were based on increments or losses of NO3- and reduced N or of H2PO4- and organic P in plant parts over the study period, on the concentrations of the above compounds in xylem and phloem sap, and on the previously determined flows of C and N in the same plants (Jeschke et al., 1996). Modelling allowed quantitative assessments of distribution of NO3- reduction and H2PO4- assimilation within the plant. In control plants 58% of total NO3- reduction occurred in leaf laminae, 40% in the root and 2% in stem and apical tissues. Averaged over all leaves more than half of the amino acids synthesized in laminae were exported via phloem, while the root provided 2.5-fold more amino acids than required for root growth. P deficiency led to severe inhibition of NO3- uptake and transport in xylem and even greater depression of NO3- reduction in the root but not in the shoot. Accentuated downward phloem translocation of amino acids favoured root growth and some cycling of transported in xylem with young laminae acting as major sinks. At the stem base retranslocation of P in the phloem amounted to 30% of xylem transport. H2PO4- assimilation was more evenly distributed than NO3- reduction with 54% occurring in leaf laminae, 6% in the apical bud, 19% in stem tissues, 20% in the root; young tissues were more active than mature ones. In P-deficient plants H2PO4- uptake was severely decreased to 1.8% of the control. Young laminae were the major sink for H2PO4-. Considerable remobilization of P from older leaves led to substantial shoot to root translocation via phloem (50% of xylem transport). Young leaf laminae were major sites of H2PO4- assimilation (50%), followed by roots (26%) and the apical bud (10%). The remaining H2PO4- was assimilated in stem and mature leaf tissues. Old leaves exhibited 'negative' net assimilation of H2PO4-, i.e. hydrolysis of organic P exceeded phosphorylation. In young laminae of low P plants, however, rates of H2PO4- assimilation per unit fresh weight were comparable to those of the controls.