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

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 sandcultured 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 N back to the shoot. In control plants $H2PO4-$ was the principal form of P 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.