Water movement and storage in a desert succulent: anatomy and rehydration kinetics for leaves of Agave deserti

Anatomic and kinetic aspects of water storage were investigated for the succulent leaves of the desert CAM plant, Agave deserti. An approximately linear relationship was found between the number of vascular bundles and leaf surface area, both for leaves of different sizes and also along the length of a single leaf. The bundles, which were distributed throughout the leaf cross-section, were separated from each other by about eight water-storage cells. Even though the cell walls of the water-storage ground tissue made up only 2·5 percent of the cell volume, they provided about 10 percent of the total cross-sectional area available for water transport radial to the xylem because cell-cell contact in such a direction averaged 25 percent of the cell surface area. The rehydration kinetics of partially dehydrated leaf segments were resolved into three phases: (1)a relatively rapid movement into the vascular tissue (half-time of 2 min); (2)water movement into storage in the ground tissue (half-time of 59 min); and (3)water movement into the intercellular air spaces (half-time of about 10 h). Using the observed kinetics for water movement into the storage tissue and standard diffusion theory, the bulk-averaged diffusivity of water in the relatively homogeneous ground tissue (D1)was 2·0 × 10?10 m2 s?1Using this (D1)and pathway analysis, most of the water moving from the xylem into storage in the massive leaves of A. deserti apparently occurred from cell to cell across the cell membranes rather than through the cell walls.