Role of Pressure in Xylem Transport of Coconut and Other Palms

The significance of root pressure in the transport of xylem sap has been investigated in Cocos nucifera L. and a few other palms. Despite the fact that excised palm roots can generate considerable pressures in situ, the quantity of water transported is only a small fraction of the demand resulting from transpiration. Most water transport is induced by negative pressure gradients, as in other higher plants. The development of considerable negative pressures has been demonstrated both directly and indirectly. Acoustic detection was used for the first time to monitor cavitation in water-stressed Cocos leaves. Its detection implies the ready disruption of xylem sap under these tensions. We suggest that root pressure might serve to refill cavitated xylem conduits when water is abundantly available and transpiration practically zero. However, little or no positive pressure could be demonstrated in intact palms subjected to low water stress: experimentally.