Hydraulic architecture and water relations of Spartium junceum branches affected by a mycoplasm disease

Abstract

Plant water relations, xylem anatomy and the hydraulic architecture of 1-year-old twigs of Spartium junceum, both healthy and affected by a phytoplasm disease, were studied. The disease causes twigs to be either shortened (witches broom disease, WBD) or flat (fasciate disease, FD). WBD twigs show a sevenfold increase in total leaf area, smaller and shorter xylem conduits, a higher stomatal conductance (g(l)) and a decline of minimum leaf water potentials (Ψ(l)) below the turgor loss point. FD twigs had nearly twice the leaf area of the healthy controls as well as high g(l) values and Ψ(l) values below the turgor loss point. Moreover, significant differences between healthy and affected twigs in stem stomatal conductance (g(s)) and in the total stem area were recorded. Affected twigs die back under drought stress, which is explained by a pronounced loss of hydraulic conductivity of the infected stems (40 and 60%) in FD and WBD as well as by the unfavourable ratio of weighted conduit radius (Σr4) to total surface area (A(t)), so that the efficiency of the stem in supplying the whole transpiring area with water is strongly reduced.