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
G. Lemaire1, J-C. Avice2,*, T-H. Kim3and A. Ourry2
1INRA, Unité d'Ecophysiologie des Plantes Fourragères, F-86600 Lusignan, France
2UMR INRA/UCBN 950 EVA (Ecophysiologie Végétale, Agronomie et Nutritions N,C,S), Institut de Biologie Fondamentale et Appliquée, Université de CAEN-Basse Normandie, F-14032 Caen Cedex, France
3Department of Animal Science and Institute of Agricultural Science and Technology, College of Agriculture and Life Science, Chonnam National University, Buk-Gwangju PO Box 205, Gwangju, 500-600, Korea
* To whom correspondence should be addressed. Fax: +33 2 31 56 53 60. E-mail: [email protected]
Received 26 July 2004; Accepted 15 November 2004
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 (Nsh) and shoot mass (Wsh): Nsh=a(Wsh)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 [dNsh/(Nshdt)] and the relative growth rate [dWsh/(Wshdt)], 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 Nsh and leaf area (LA): Nsh=1.7 g m–2LA, 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 Nsh 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 multispecific plant stands. Therefore, the sharing of N resources among plant species could partially be the result of the sharing of light within the canopy.
Key words: Leaf area, leaf:stem ratio, Medicago sativa L., N dilution, plant density, shoot N accumulation
Journal of Experimental Botany 2005 56(413):935-943. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. Open Access article.