Diurnal cycles of embolism formation and repair in petioles of grapevine (Vitis vinifera cv. Chasselas).

The impact of water deficit on stomatal conductance (g(s)), petiole hydraulic conductance (K(petiole)), and vulnerability to cavitation (PLC, percentage loss of hydraulic conductivity) in leaf petioles has been observed on field-grown vines (Vitis vinifera L. cv. Chasselas). Petioles were highly vulnerable to cavitation, with a 50% loss of hydraulic conductivity at a stem xylem water potential (Ψ(x)) of -0.95 MPa, and up to 90% loss of conductivity at a Ψ(x) of -1.5 MPa. K(petiole) described a daily cycle, decreasing during the day as water stress and evapotranspiration increased, then rising again in the early evening up to the previous morning's K(petiole) levels. In water-stressed vines, PLC increased sharply during the daytime and reached maximum values (70-90%) in the middle of the afternoon. Embolism repair occurred in petioles from the end of the day through the night. Indeed, PLC decreased in darkness in water-stressed vines. PLC variation in irrigated plants showed the same tendency, but with a smaller amplitude. The Chasselas cultivar appears to develop hydraulic segmentation, in which petiole cavitation plays an important role as a 'hydraulic fuse', thereby limiting leaf transpiration and the propagation of embolism and preserving the integrity of other organs (shoots and roots) during water stress. In the present study, progressive stomatal closure responded to a decrease in K(petiole) and an increase in cavitation events. Almost total closure of stomata (90%) was measured when PLC in petioles reached >90%.

Differential attachment by conidia of the endophyte, Discula umbrinella (Berk. & Br.) Morelet, to host and non-host surfaces.

Differential attachment of conidia to leaves of different species (host and non-host) has been observed in the endophytic fungus, Discula umbrinella (Berk. & Br.) Morelet, a deuteromycete known to cause occasional epidemics of anthracnose in beech stands. On beech leaves, attachment gradually increased over 24 h and reached a maximum after 16-24 h; there were small differences in attachment between conidia suspended in water and those in HEPES buffer. The range of variation of results obtained with conidia suspended in HEPES was smaller but the adhesion was clearly enhanced. The adhesion of conidia to the host surface was highest at pH 5, and decreased with increasing pH. Heat and chemical treatments significantly reduced attachment. Two factors (or groups of factors) are likely to be involved in the recognition process. The first is heat-labile, the second one could be a non-protein molecule present on the conidial surface and actively recognized by the host. The physiological similarity between endophytes and other symbionts is briefly discussed.

Ecology, metabolite production, and substrate utilization in endophytic fungi.

Endophytic fungi are a taxonomically and ecologically heterogenous group of organisms, mainly belonging to the Ascomycotina and Deuteromycotina. The isolation methods affect the species composition of the endophyte assemblage in a given host. The number of endophyte taxa isolated from a host species is usually large; however, only few, normally host specific species or strains are dominant. Endophyte assemblages are specific at the host species level, but species composition and frequencies are significantly affected by site-specific conditions. Moreover, the relative importance and number of endophytic species vary among individuals within sites. In some cases, each individual could be considered a separate ecosystem. In general, however, 40 individuals with 30 to 40 sampling units per organ and individual should be enough to detect 80% of taxa present in a given host at one site. Endophytes usually produce the enzymes necessary for the colonization of plant tissues. Substrate utilization studies and isozyme analysis have demonstrated that most endophytes are able to utilize most plant cell components. The production of growth promoting factors and of metabolites useful in the pharmaceutical and agricultural industry is widespread among endophytic fungi. The usefulness of endophytes in agricultural and pharmaceutical research is briefly discussed.