WATER TRANSPORT THROUGH THE ROOTS OF ORNAMENTAL SHRUB CUTTINGS

The use of leafy cuttings to propagate ornamental shrubs such as Coryl us maxima cv. Purpurea is often hampered by ''weaning'' problems. Thes e arise when the rooted cuttings are removed from the supportive envir onment designed to promote rooting and are transferred to a more natur al, but relatively stressful, environment. The shrivelling of leaves w hich ensues points to some failure of normal plant water relations, wh ich may be caused either by excessive water loss from the leaves, or a n inadequate water supply from the roots, or a combination of these fa ctors. To understand this problem more fully, the root hydraulic condu ctance of C. maxima cv. Purpurea cuttings was investigated, and compar ed with that of Weigela florida cv. Variegata, a shrub which does not exhibit weaning problems. The root hydraulic conductance for both spec ies increased with the size of the root system, and root fresh weight was shown to provide a useful basis for relative measurements of root conductance. On this basis, cuttings of W. florida exhibited a higher root hydraulic conductance than comparable C. maxima cuttings (1.5 x 1 0(-4) and 9.7 x 10(-5) g s(-1) MPa-1 g(root)(-1) respectively), but th e difference was not sufficiently large to account for the differences in weaning of these two species. It is suggested that the adventitiou s root system of C. maxima is functioning normally, but that it cannot supply enough water to balance uncontrolled water loss from the leave s.