After a period of equilibration, the relative growth rate (RGR) adjusts to match the RAR, and the mean N concentration of the whole plants remains constant. In these, plants in the exponential phase of growth are fed with nitrate at a range of exponentially increasing rates, each corresponding to a different constant relative addition rate (RAR). Previous investigations of the effects of external N supply on plant growth have focused largely on steady‐state approaches, using techniques developed previously ( Ingestad and Lund, 1979 Ingestad, 1979 a, b ). However, there is still some confusion about the exact mechanisms controlling responses to N, which appear to vary depending on the relative availability of N from internal reserves and from external sources of nitrate ( Burns, 1994 a ). These in turn could increase the efficiency of N use to the benefit of the environment. It is widely accepted that improved information on the factors controlling the acquisition and utilization of N by crops will help to identify the constraints to developing more effective strategies of N fertilization. Studies of the interrelationship between N uptake, plant N concentration and growth rate are central to an understanding of the role of N within plants. Lettuce, N uptake, N reserves, nitrate N, organic N, relative addition rate, relative growth rate. Evidence is presented to show that the rate of remobilization of N depends on the size and type of the N pools within the plant, and that changes in their rates of remobilization and/or transfer between pools are the main factors influencing the form of the relationship between RGR and N concentration. By comparison, plants forced to rely solely on their internal reserves were never able to mobilize and redistribute N between tissues quickly enough to prevent reductions in growth rate as their tissue N reserves declined. As a result, nitrate only tended to accumulate in plant tissues when its supply was essentially adequate. Newly acquired N was channelled directly to the sites of highest demand, where it was assimilated rapidly. These results show that when sub‐optimal sources of external N were available, RGR was maintained at a rate which was dependent on the rate of nitrate uptake by the roots. These differences resulted from the ability of the plant to use external sources of N more readily than their internal N reserves. The relationship was curvilinear when the external N supply was interrupted, but linear when N was supplied by either RAR methods or as a supra‐optimal external N supply. The resulting relationships between whole plant relative growth rate (RGR) and N concentration varied between linear and curvilinear (or possibly bi‐linear) forms depending on the treatment conditions. Hydroponic experiments were carried out with young lettuce plants ( Lactuca sativa L.) to compare responses to either an interruption in external N supply or the imposition of different relative N addition rate (RAR) treatments. This paper investigates the effects of uptake of nitrate and the availability of internal N reserves on growth rate in times of restricted supply, and examines the extent to which the response is mediated by the different pools of N (nitrate N, organic N and total N) in the plant.
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