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 qu antitatively the uptake, translocation, storage, and assimilation of N O3- and H2PO4- over a 9d period in mid-vegetative growth of sand-cultu red castor bean (Ricinus communis L.) which was fed 12 mM NO3- and eit her 0.5 or a severely limiting 0.005 mM H2PO4-. Model calculations wer e 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- r eduction and H2PO4- assimilation within the plant. In control plants 5 8% of total NO3- reduction occurred in leaf laminae, 40% in the root a nd 2% in stem and apical tissues. Averaged over all leaves more than h alf 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 th e 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 transport ed 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 o nes. In P-deficient plants H2PO4- uptake was severely decreased to 1.8 % of the control. Young laminae were the major sink for H2PO4-. Consid erable 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 roo ts (26%) and the apical bud (10%). The remaining H2PO4- was assimilate d in stem and mature leaf tissues. Old leaves exhibited 'negative' net assimilation of H2PO4-, i.e. hydrolysis of organic P exceeded phospho rylation. In young laminae of low P plants, however, rates of H2PO4- a ssimilation per unit fresh weight were comparable to those of the cont rols.