A SURVEY OF ROOT PRESSURES IN VINES OF A TROPICAL LOWLAND FOREST

Pre-dawn xylem pressures were measured with bubble manometers attached near the stem bases of 32 species of vines on Barro Colorado Island, Panama, to determine if pressures were sufficient to allow for possibl e refilling of embolized vessels. Of 29 dicotyledonous species 26 exhi bited only negative xylem pressures, even pre-dawn during the wet seas on. In contrast, three members of the Dilleniaceae exhibited positive pre-dawn xylem pressures, with a maximum of 64 kPa in Doliocarpus majo r. A pressure of 64 kPa is sufficient to push water to a height of 6.4 m against gravity, but the specimens reached heights of 18 m. Thus, i n all 29 dicotyledons examined, the xylem pressures were not sufficien t to refill embolized vessels in the upper stems. In contrast, two of the smaller, non-dicotyledonous vines, the climbing fern Lygodium venu strum and the viny bamboo Rhipidocladum racemiflorum, had xylem pressu res sufficient to push water to the apex of the plants. Therefore, a r oot pressure mechanism to reverse embolisms in stem xylem could apply to some but not to most of the climbing plants that were studied.