No abstract

Studies were undertaken to evaluate the effects of mechanical impedance on root exudation by maize (Zea mays L., var Dea) and to examine the importance of these effects in relation to the stage of plant development. Plants were grown under sterile and hydroponic conditions. Mechanical impedance was simulated using glass beads of 1 mm diameter. This treatment was compared with a control without beads. Results demonstrated that plant growth was influenced by mechanical impedance. Mechanical impedance markedly affected the growth of the shoot, whether this was measured as leaf area or total dry matter. Besides increasing root/shoot biomass ratios, mechanical impedances also stimulated root exudation of organic and inorganic compounds. Stressed plants lost more nitrogenous compounds than control plants. Otherwise, the percentage of released carbon decreased. Depending on the developmental stage of the plant, there was a large variation in the magnitude and time course on mechanical impedance effects. The effects of mechanical impedance persist and accentuate with time.

Plant roots release in the rhizosphere diverse organic materials which may have different effects on soil structure. We have evaluated the effect of natural and modelled root-released materials on soil aggregates and the biodegradation of carbon from roots in the soil. The effects of root mucilage from maize and of a modelled soluble exudate were compared with those of simple compounds (glucose, polygalacturonic acid). For all treatments, soil was amended with 2 g C kg ±1 soil and incubated for 30 days at 25°C. The biodegradation of mucilage was similar to that of polygalacturonic acid, and slower than the decomposition of modelled exudates and glucose. Addition of all substrates increased the stability of aggregates, but the duration of this effect depended on the chemical nature of the material. Compared with the control, the proportion of stable aggregates after 30 days of incubation was multiplied by 3.8 for root mucilage, by 4.2 for modelled soluble exudates, by 2.5 for polygalacturonic acid and by 2.0 for glucose. The different fractions of root exudates in the rhizosphere evidently affected the aggregate stability.In¯uence d'exsudats racinaires mode Âlise Âs et de mucilages sur la structure du sol Re Âsume  Les racines des plantes libe Árent dans la rhizosphe Áre divers mate Âriaux organiques qui peuvent avoir des effets variables sur la structure du sol. La biode Âgradation et l'effet sur l'agre Âgation du sol de diffe Ârents mate Âriaux organiques ont e Âte  e Âtudie Âs. Pour cela, nous avons compare  du mucilage issu de racines de maõ Ès et des exsudats solubles mode Âlise Âs (chimiquement reconstitue Âs) a Á des compose Âs simples (glucose, acide polygalacturonique). Pour tous les traitements, le sol a e Âte  incube  pendant 30 jours a Á 25°C apre Ás l'apport de 2 g C kg ±1 de sol. La biode Âgradation du mucilage est semblable a Á celle de l'acide polygalacturonique, et plus lente que la de Âcomposition des exsudats mode Âlise Âs et du glucose. L'ajout de tous les substrats a augmente  la stabilite  des agre Âgats, mais la dure Âe de cet effet a e Âte  variable selon la nature chimique du mate Âriel. Compare  au te Âmoin, la proportion d'agre Âgats stables apre Ás 30 jours d'incubation a e Âte  multiplie Âe par 3.8 pour le mucilage de racine, par 4.2 pour les exsudats solubles mode Âlise Âs, par 2.5 pour l'acide polygalacturonique et par 2.0 pour le glucose. Les diffe Ârentes fractions d'exsudats racinaires dans la rhizosphe Áre ont e Âvidemment affecte  la stabilite  globale.