Relationships between nitrogen (N) content and growth are routinely measured in plants. This study determined
the effects of N on the separate morphological and physiological components of plant growth, to assess how N-limited growth is effected through these components. Lettuce (Lactuca sativa) plants were grown hydroponically
under contrasting N-supply regimes, with the external N supply either maintained continuously throughout the
period of study, or withdrawn for up to 14 d. Richards' growth functions, selected using an objective curve-fitting
technique, accounted for 99.0 and 99.1% of the variation in plant dry weight for control and N-limited plants
respectively. Sublinear relationships occurred between N and relative growth rates under restricted N-supply
conditions, consistent with previous observations. There were effects of treatment on morphological and
physiological components of growth. Leaf weight ratio increased over time in control plants and decreased in N-
limited plants. Shoot:root ratio followed a similar pattern. On a whole-plant basis, assimilation of carbon
decreased in N-limited plants, a response paralleled by differences in stomatal conductance between treatments.
Changes in C assimilation, expressed as a function of stomatal conductance to water vapour, suggest that the effects
of N limitation on growth did not result directly from a lack of photosynthetic enzymes. Relationships between
plant N content and components of growth will depend on the availability of different N pools for remobilization
and use within the plant.