Axial variation of xylem conduits in giant cacti

Giant columnar cacti store massive amounts of water in their parenchymous storage tissues in order to persist under conditions of extreme aridity. Nevertheless, the relationship between stem water storage capacity and the maximum efficiency to deliver water from the roots to stem storage tissues via xylem vessels remains largely unknown. Indeed, the relationship between the axial water flow in xylem and the lateral flow through the storage tissue may affect the xylem structure and, therefore, the plant water conduction strategies. Since the axial structure of vascular conduits has been demonstrated to be universal (i.e. in a broad spectrum of plant species xylem conduits widen basipetally at the same rate), we wanted to determine if both the vessel size and wall thickness in giant cactae follow the same general rule in spite of the buffer action of water storage tissue. To address these hypotheses, we are investigating anatomical variation in xylem structural traits and storage volume in the stems of giant cacti species belonging to different phylogenetic lineages that are native to both the Northern and Southern hemisphere (e.g.Pachycereus weberi, Echinopsis terschekii, Carnegiea gigantea). We collected cross-sections from 6 to 13 samples along the stem of each plant. We found that vessel lumina increased basipetally following a widening rate similar to what has been documented by the theoretical model (WBE model) and from existing surveys on a wide range of tree species. The conduits double wall thickness (t) and its span (s) ratio decrease basipetally and interplay to reduce the risk of cell collapse. We concluded that the xylem architecture of columnar cacti in this study was not influenced by the buffering action of the surrounding storage tissue, and that axial water transport efficiency is maintained for the length of the path as in many other plant species.