Nitrogen and sugar concentrations in the xylem exudate of field-grown maize at different growth stages and levels of nitrogen fertilization

Abstract

In field-grown maize (Zea mays L.) at different growth stages xylem exudate was collected and analyzed for nitrogen compounds and sugars. The plants were grown at two levels of nitrogen (N) fertilization (low N, high N). In an additional experiment in some plants the cobs were removed after anthesis.The concentrations of nitrate- and reduced-N in the exudate were greater at high than at low N fertilization. At both levels of N fertilization, during plant development the concentrations of nitrate-N declined but not those of reduced-N, leading to an increase in the percentage of reduced-N up to 80-90 per cent of the total N in the exudate. This pattern may reflect either an increase in the proportion of nitrate reduction in the roots, or of N cycling between the shoot and the roots, or both. Increased internal cycling of N was considered to be the main cause of the significant increase in the concentrations of reduced N in the exudate after cob removal.The concentrations of reducing sugars and sucrose in the exudate, at both levels of N fertilization, increased after anthesis to more than 20 mM, and after ear removal to more than 50 mM. Cycling of sugars between shoot and roots and mobilization of sugars previously stored in the roots are discussed as possible causes for the high sugar concentrations in the exudate, which might significantly contribute to the osmotic driving force for the volume flow in the xylem.