Diurnal variation in ion content of the solution bathing roots of two plants growing together in sand culture was analysed for three pairs of grass-legume species (Lolium multiflorum and Trifolium pratense; Zea mays and Glycine hispida; Avena sativa and Vicia sativa) and their monospecific controls. Biomass and nitrogen content of plants were determined.Ion concentration (NO~-, NO{, NH~, and K § and pH of root solutions were measured for LoliumTrifolium plant pairs and controls at 6 hours intervals over 36 h, starting at 8 am within a circadian cycle. Root solutions were regularly depleted in NO; by the grasses (Lolium-Lolium control) throughout the cycle. For associations involving the legume (Lolium-Trifolium and Trifolium-Trifolium), NO; depletion was followed by NO; enrichment at night, from late afternoon to early morning; the enrichment was more marked for the Lolium-Trifolium association. Solutions which did not contain NO2 ions, were enriched by trace amounts of NH2-ions, largely depleted in K § and alkalanized for all associations throughout the cycle.Repeating the experiment with the three pairs of species at the vegetative phase of development confirmed the previous results: NO; enrichment during the night for associations with legumes. When the experiment was repeated with older plants which had almost completed their flowering stage, depletion only was observed and no NO; enrichment.These data suggest that NO~-enrichment results from N excretion from active nodulated roots of the legume, accounting for the increase in both biomass and nitrogen content of the companion grass in grass-legume association. The quantitative importance and periodicity of nitrogen excretion as well as the origin of nitrate enrichment are discussed.

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(fig 1a, b). On the primary tillers, the mean degree-day duration of a phyllochron depends on tiller order. Under the imposed constraints, the effects of intraspecific (density) and interspecific (association) competition were evidenced. Potassium deficiency increased the duration of the phyllochron for the 2 species (fig 2). The rate of leaf emergence presented a high interaction between the rank of the phyllochron and the levels of nitrogen and potassium. Intraspecific competition slowed down the rhythm of leaf emergence and reduced leaf longevity in both species (figs 5, 6). There was a strong effect of the water regime on vetch (fig 3). When potassium availability decreased, leaf emergence was slowed down. Increasing the intensity of the water constraints resulted in prolonged leaf longevity. There was a clear effect of potassium on the longevity of vetch leaves (fig 7). Potassium

le 17-8-1988, accepté le 1-2-1989) Résumé — Ce travail analyse la variation de la surface foliaire et de l'indice foliaire sous l'influence de la densité de peuplement et de la fertilisation azotée. Le paramètre surface foliaire a réagi différemment chez les deux espèces aux variables étudiées. La fertilisation azotée, ayant augmenté la croissance de l'avoine, a exercé un effet dépressif sur la croissance de la vesce.Une relation entre l'indice foliaire global et la matière sèche produite par les parties aériennes, en association, a été mise en évidence. Toujours en association, la compétition pour la lumière a été étudiée à partir de l'analyse de la structure des profils de surface foliaire, d'une part, et par la contribution de la vesce à l'indice foliaire total, d'autre part. Il se dégage de cela que la compétition pour la lumière est non seulement déterminée par la position des feuilles dans les différentes strates du couvert, mais aussi par la contribution de chaque espèce à l'indice foliaire global. association vesce-avoine -compétition pour la lumière -croissance -profil de surface foliaire -indice foliaire Summary — Variability of the phenomena of interferences between Vicia sativa L. and Avena sativa L. I. Growth dynamics of the vetch in a mixed stand of vetch and oat. The work analyses the variation of the leaf area and of the leaf area index (LAI) under the effects of stand density and nitrogen supply. The leaf area parameter has differentially responded for the 2 species to the studied variables. The nitrogen fertilization, increasing the growth of oat, has determined a depressive effect on the growth of vetch.A relationship between the global LAI and the biomass (DM) of the shoots, in mixture, has been evidenced. In mixture, the competition for light has been studied with an analysis of the structure of the leaf area profiles, on one hand, and, on the other hand, through the contribution of the vetch to the global LAI. It results that the competition for light is not only induced by the position of the leaves in the several strata of the cover, but also by the contribution of each species to the global LAI. l. vetch-oat mixtures -competition for light -growth -leaf area profile -leaf area index