Intervessel connectivity and relationship with patterns of lateral water exchange within and between xylem sectors in seven xeric shrubs from the great Sahara desert.

The main objective of this study was to evaluate the role of intervessel contacts in determining the patterns of hydraulic integration both within and between xylem sectors. The degree of intervessel contacts and the lateral exchange capability within and between sectors were examined and correlated in different xeric shrubs. A dye injection method was used to detect the connections between vessels; an apoplastic dye was sucked through a known number of vessels and its distribution in the xylem network was followed. Hydraulic techniques were used to measure axial and tangential conductivity both within and between xylem sectors. The intra- and inter-sector integration indexes were then determined as the ratio of tangential to axial conductance. Species differed significantly in the degree of intervessel contacts, intra- and inter-sector integration index. In all cases, hydraulic integration was observed to be higher within sector than between sectors. From the correlation analyses, the intervessel contacts showed a very weak relationship with inter-sector integration index and a strong positive relationship with intra-sector integration index. Results suggested that (1) the factors affecting patterns of lateral flow within xylem sectors might be relatively different from those between sectors. (2) The degree of intervessel contacts was a major determinant of hydraulic integration within the same xylem sector. (3) Intervessel connectivity alone was a poor predictor of hydraulic integration between different sectors, implying a significant contribution of other anatomical, physiological and environmental factors in determining the patterns of integrated-sectored transport within woody stems.