Mind the bubbles: achieving stable measurements of maximum hydraulic conductivity through woody plant samples

The maximum specific hydraulic conductivity (kmax) of a plant sample is a measure of the ability of a plants’ vascular system to transport water and dissolved nutrients under optimum conditions. Precise measurements of kmax are needed in comparative studies of hydraulic conductivity, as well as for measuring the formation and repair of xylem embolisms. Unstable measurements of kmax are a common problem when measuring woody plant samples and it is commonly observed that kmax declines from initially high values, especially when positive water pressure is used to flush out embolisms. This study was designed to test five hypotheses that could potentially explain declines in kmax under positive pressure: (i) non-steady-state flow; (ii) swelling of pectin hydrogels in inter-vessel pit membranes; (iii) nucleation and coalescence of bubbles at constrictions in the xylem; (iv) physiological wounding responses; and (v) passive wounding responses, such as clogging of the xylem by debris. Prehydrated woody stems from Laurus nobilis (Lauraceae) and Encelia farinosa (Asteraceae) collected from plants grown in the Fullerton Arboretum in Southern California, were used to test these hypotheses using a xylem embolism meter (XYL'EM). Treatments included simultaneous measurements of stem inflow and outflow, enzyme inhibitors, stem-debarking, low water temperatures, different water degassing techniques, and varied concentrations of calcium, potassium, magnesium, and copper salts in aqueous measurement solutions. Stable measurements of kmax were observed at concentrations of calcium, potassium, and magnesium salts high enough to suppress bubble coalescence, as well as with deionized water that was degassed using a membrane contactor under strong vacuum. Bubble formation and coalescence under positive pressure in the xylem therefore appear to be the main cause for declining kmax values. Our findings suggest that degassing of water is essential for achieving stable and precise measurements of kmax through woody plant samples. For complete rehydration of woody samples, incubation in water under vacuum for 24 h is suggested as a reliable technique that avoids bubble problems associated with flushing under high positive pressure

Wound-induced and bacteria-induced xylem blockage in roses, Astilbe and Viburnum

We previously concluded that the xylem blockage that prevents water uptake into several cut flowers is mainly due to the presence of bacteria, whilst in chrysanthemum and Bouvardia we observed a xylem occlusion that was mainly due to a wound-reaction of the plant. We have further tested which of these two mechanisms was dominant in Astilbe,Viburnum and rose flowers. Astilbe x arendsii (cvs. Erica and Glut) flowers were stored dry in plastic bags (24 h at 5 degreesC, 100% RH) and placed in water at 20 degreesC without recutting the steins. The dry storage treatment considerably hastened a wounding-induced xylem occlusion in the stems. A 5 h pulse treatment with inhibitors of peroxidase (hydroquinone) and catechol oxidase (tropolone and 2,3-dihydroxynaphtalene), prior to dry storage, considerably delayed the xylem blockage. The 24 h dry storage treatment had no effect in rose (Rosa x hybrida cv. Red One), and Viburnum opulus (cv. Roseum). These flowers were therefore directly placed in water, with and without enzyme inhibitors. Except hydroquinone, all tested enzyme inhibitors reduced bacterial growth in the vase water. The latter chemicals could therefore not be used to distinguish between a plant-induced and a bacterial occlusion of the xylem. Hydroquinone had no effect on the time to wilting in roses, nor in Viburnum. It considerably delayed wilting in Astilbe flowers that were directly placed in water after harvest. It is concluded that the blockage in Astilbe is mainly due to the plant-induced xylem occlusion. The xylem occlusion in the tested rose and Viburnum cultivar was apparently not due to this mechanism. (C) 2004 Elsevier B.V. All rights reserved

Dispositif de recherche du plan médio-labyrinthique adapté au nouveau vestibulographe

Fenart R., Blain A., Deblock R., Loubaud J.P. Dispositif de recherche du plan médio-labyrinthique adapté au nouveau vestibulographe. In: Bulletins et Mémoires de la Société d'anthropologie de Paris, XIII° Série. Tome 4 fascicule 1, 1977. pp. 19-21

Réutiliser les eaux usées traitées : quelles perspectives pour l'agriculture française ?

Article à paraître dans le numéro 394 d'Économie Rurale (octobre-décembre).International audienceUnder specific circumstances, treated wastewater reuse (REUSE) for irrigation can save water for other uses. But REUSE projects remain marginal in France. The article analyze why REUSE is struggling to establish itself in the socio-technical regime of irrigation in France. The instability of the regulatory framework, environmental uncertainty and the lack of a stabilized economic model are hampering its development and the commitment of farmers. The introduction of integrated risk management to guarantee the technical and economic feasibility of REUSE projects for agricultural use, and the transition to more water-efficient farming practices, are key developments for the transition of irrigation in France.Dans certains contextes, la réutilisation des eaux usées traitées (REUT) pour l’irrigation peut permettre d’économiser de l’eau pour d’autres usages. Mais les projets de REUT demeurent marginaux en France. L’article analyse pourquoi la REUT peine à s’imposer dans le régime sociotechnique de l’irrigation en France. L’instabilité du cadre réglementaire, l’incertitude environnementale et le manque de modèle économique stabilisé freinent son développement et l’engagement des agriculteurs. La mise en place d’une gestion intégrée des risques pour garantir la faisabilité technico-économique des projets REUT à usage agricole, et la transition vers des pratiques agricoles plus économes en eau sont des évolutions clés pour la transition de l’irrigation en France