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.
A study of root selective adsorption of Ca, Mg, K and Na was made with rootlets of 20 species of Gramineae (from natural vegetation) exhibiting low cation exchange capacities (CEC) scattered over a narrow range 7.6-16.7 meq 100 g-t dry matter). For each sample of rootlets successive measurements were made of its CEC (amount of Ca adsorbed from 0.05 N calcium nitrate solution) and the amounts of Ca, Mg, K and Na adsorbed from a tetracationic nitrate solution having equal concentrations (0.05 N) of each cation. Rootlet selectivity for bivalent cations increased with CEC and was greater for Ca than for Mg. In contrast, the selectivity for monovalent cations increased with decreasing CEC and was greater for K than for Na. A root's selective adsorption may therefore be governed by its CEC, in conformation with the exchange laws, as is the case with clays and resins.
Dittrichia (ex Inula) viscosa is a ruderal species that has recently become an invading plant in the northwest Mediterranean basin. A previous study failed to demonstrate the occurrence of morphologically differentiated ecotypes among populations of the species but suggested the existence of nutritional ecotypes. This latter possibility is examined here by comparing the ability of plants from contrasting habitats to control cation accumulation balance. Dittrichia viscosa plants, from eight siliceous habitats and nine calcareous habitats of southern France and neighbouring Spain and Italy, were cloned and grown together hydroponically with a solution simulating an acid soil with an aluminium constraint. Two independent hydroponic units containing solution supplemented with two levels of Al were used (2 Al levels x17 populations x3 genotypes x3 replicates). The growth and cation content (K, Ca, Mg and Na) of plant shoots and the chemical composition of the soil of each habitat were analysed. At the high Al level (1.1MM), populations differed in K, Ca and Mg plant proportions. Two groups could be distinguished: one containing all but one siliceous populations and the other containing all but one calcareous populations. Plants of the siliceous group accumulated proportionally more K and less Ca, and had better growth, than plants of the calcareous group, in the same way as calcifuge and calcicole species when grown on acid soil. At the lowest Al level (0.37MM), differences between siliceous and calcareous populations were less marked. The results suggest that differences in the ability of plants to control K and Ca balance, which appear to be of adaptive significance, could have arisen through selection, and that Dittrichia viscosa has evolved calcifuge and calcicole nutritional ecotypes in siliceous and calcareous habitats respectively. Various degrees of calcifugy, and to a lesser extent of calcicoly, can thus be suggested to occur among the studied populations, some in relation to the intensity of mineral stress in the natural habitats. So far, only functional traits have provided evidence of ecotypic differentiations within Dittrichia viscosa.
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