Nitrogen distribution within a lucerne canopy during regrowth: relation with light distribution

The distribution of nitrogen between stem height classes of a lucerne stand was studied on both leaf and stem components during spring growth and summer regrowth. The N content of stems expressed in mg cm-1 is constant from the top to the base for a given stem height class and increases with the size of the stem in relation to its diameter. So an expression of the N content per stem area accounts for the distribution of N between the different stem height classes in the canopy. The N content of leaves expressed in mg cm-2 is constant within each 10-cm stratum of the canopy, and depends neither on the stem class nor on the leaf age. New leaves are initiated at the top of the canopy at a relatively constant maximum N content cm-2, which decreases regularly as these leaves become displaced to shaded horizons. A relationship between the relative diminution of N content per leaf area and the relative extinction of light from the top to the bottom of the canopy was established. This relationship suggests that the diminution of N content of leaves is influenced by the level of light. One process could be concerned with the adjustment of the photosynthetic capacity of leaves to the light level and the other could correspond to the senescence which leads to the death of the leaves. The major part of this phenomenon is avoided if individual stems are prevented from becoming shaded. About 80% of the leaf N is lost by an individual leaf before abscission, and 70% of the initial stem N has been lost by an individual stem by the time the last leaf drops. The results are related to self-thinning processes which occur during the regrowth dynamics of the canopy. The diminution of the nitrogen content of individual stems and the death of stems observed during regrowth can be considered as reflections of the competition for light.