Effect of nitrate and ammonium on long distance transport in cucumber plants

Cucumber plants (Cucumis sativus L. cv. 'Chinese Snake') were grown on culture Solutions containing nitrate or ammonium as nitrogen source. The carbon and nitrogen fluxes from shoot to roots and vice versa have been investigated by analysis of plant material, xylem and phloem sap and by measurement of biomass production and root respiration. Despite the markedly reduced growth on ammonium, the C-flux from shoot to roots and root respiration were equivalent on a single plant basis for both treatments. Root respiration consumed about 30% of the carbon delivered through the phloem. Plants on ammonium used only 20% of the carbon for root biomass production, while 50% was recycled to the shoot through the xylem. In cucumbers fed with nitrate nearly 50% of the carbon went into root biomass, and only about 25% was recirculated to the shoot. From xylem sap analyses it became evident that plants fed with ammonium assimilated nitrogen in the roots and transported it to the shoot in organic form (bound to C-skeletons), while nitrate-fed plants transferred the nitrogen taken up mainly in unaltered form into the xylem. With ammonium nutrition, root biomass production and the high demand for C-skeletons used in ammonium assimilation compete for the phloem-derived carbon. The alterations in the long-distance transport must be considered as possible factors causing the weaker growth of ammonium-fed plants.