Interaction between nitrogen deficit of a plant and nitrogen content in the old leaves

We examined changes in nitrogen content of the first leaves in relation to growth and nitrogen status of sunflower (Helianthus annuus L.) plants that were raised hydroponically at two irradiance levels (high and low light, HL and LL) and at two nitrogen concentrations (high and low nitrogen, HN and LN). Initial increases in total dry mass and total nitrogen of the whole plant were faster in HL-plants than in LL-plants irrespective of nitrogen supply, but in LN-plants the increase in total nitrogen was soon blunted, When plants grown under the same irradiance were compared, nitrogen content of the first leaves (leaf N) decreased faster in LN-plants than in HN-plants, while for the plants grown at the same nitrogen concentrations, it decreased faster in HL-plants than in LL-plants. Since these changes in leaf N were not explained solely by the changes in plant dry mass or plant nitrogen, we introduced an index, 'nitrogen deficit (ND*)', to quantify nitrogen deficit of the whole plant, ND* was expressed as ND*(t)=[N-max-N(t)] xDM(t), where N-max and N(t) were nitrogen contents in the young, expanding leaves that had just unfolded to expand, at the initial stage with sufficient nitrogen and at time t, respectively, and DM(t), plant dry mass at t, The decrease in leaf N was expressed as a liner function of ND* irrespective of the growth conditions, which indicates validity of this index, Limitation of the use of ND*, and mechanisms by which leaves sense nitrogen demand are also discussed.