Developmental changes in shoot N dynamics of lucerne (Medicago sativa L.) in relation to leaf growth dynamics as a function of plant density and hierarchical position within the canopy.

Shoot N concentration in plants decreases as they get bigger, due to the fact that N accumulates less rapidly than dry matter in plants during the plant growth process, leading to an allometric relationship between shoot N content (N(sh)) and shoot mass (W(sh)): N(sh)=a(W(sh))b. The results obtained on lucerne plants growing either under controlled low density conditions or in dense stands under field conditions show that the value of the allometric coefficient b that represents the ratio between the relative N accumulation rate in shoots [dN(sh)/(N(sh)dt)] and the relative growth rate [dW(sh)/(W(sh)dt)], decreases from 0.88 for a low plant density to 0.72 for a dense stand. Therefore, the fractional increase of shoot N per unit of shoot dry matter is lower when plants are in competition for light in dense canopies. This decrease can be entirely explained by the parallel decline in the leaf area per unit of shoot mass. Thus, a remarkably constant linear relationship can be established between N(sh) and leaf area (LA): N(sh)=1.7 g m(-2) LA, regardless of the conditions (low versus high density, controlled versus field conditions). Moreover, in a field dense stand, the comparison of plants with contrasting positions between the top and the bottom of the canopy (dominant, intermediate or suppressed plants), also shows that the difference in N(sh) at similar shoot mass is explained by the proportion of leaf mass to shoot mass. These data support the idea that leaf growth drives the dynamics of shoot N accumulation. These results also indicate that competition for light among individual plants within a dense canopy induces developmental changes in plant morphology (leaf:stem ratio) that explain the differences observed in shoot N concentration. This last observation could be extrapolated to multi-specific plant stands. Therefore, the sharing of N resources among plant species could partially be the result of the sharing of light within the canopy.

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