THE UPTAKE AND FLOW OF C, N AND IONS BETWEEN ROOTS AND SHOOTS IN RICINUS-COMMUNIS L .2. GROWN WITH LOW OR HIGH NITRATE SUPPLY

Seedlings of Ricinus communis L. cultivated in quartz sand were supplied with a nutrient solution containing either 0.2 mol m(-3) NO3- or 4.0 mol m(-3) NO3- as the nitrogen source to obtain insufficiently (low supply, nitrogen-limited) or well-fed plants (high supply, control). During the period between 41 and 51 d after sowing, the flows of C, N and inorganic ions between root and shoot were modelled on the basis of empirical observation and expressed on a fresh weight basis. With a low nitrate supply the biomass production was decreased while the root/shoot ratio was drastically increased and the water content in the shoot was slightly reduced. Nitrogen was transported in the xylem mainly in the form of nitrate in both treatments. However, in nitrate-limited plants the ratio of nitrate to total nitrogen was lower, indicating a higher fraction of whole-plant nitrate reduction occurring in the root. The spectrum of amino acids in phloem sap was changed due to N-limitation. Nitrate and cation uptake, as well as photosynthesis was strongly decreased in nitrate-limited plants. The partitioning of C, N and ions was shifted in favour of the root compared to well-fed plants. Transport of C, N, and cations in the xylem was decreased. Flows of ions and elements in the phloem were increased relative to uptake and xylem transport. In contrast, the chloride flows were nearly the same in low- and well-fed plants, pointing to a role of chloride as a compensating ion for nitrate.