Hydraulic architecture and water relations of Spartium junceum branches affected by a mycoplasm disease
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 1 ) and a decline of minimum leaf water potentials (ψ 1 ) below the turgor loss point. FD twigs had nearly twice the leaf area of the healthy controls as well as high g 1 values and ψ 1 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 (Σr 4 ) to total surface area (A 1 ), so that the efficiency of the stem in supplying the whole transpiring area with water is strongly reduced.