The effect of axis splitting on xylem pressure potentials and water movement in the desert shrub Ambrosia dumosa (Gray) Payne (Asteraceae)

Predawn xylem pressure potential (XPP) measurements were used to investigate the effects of axis splitting on water relations of the Mojave Desert shrub Ambrosia dumosa. Differences among branches within shrubs were significant in all large shrubs with split axes at least once during the summer but not in intermediate or small shrubs with unsplit axes. A subset of plants received 24 liters of water delivered to one side of the plants from buckets every other week during the summer. Two of three large, watered shrubs showed significant differences among branches at least once during the summer, but XPPs were not higher than in unwatered shrubs. No significant differences among branches of intermediate and small, watered shrubs occurred; both intermediate and small shrubs, however, showed increased XPPs and produced small, leafy, terminal shoots in response to being watered. The pathway of water movement was investigated with a dye that ascended sectorially in all plants.