Water Regulation of Unrooted Cuttings in Propagation

Water demand of unrooted cuttings changes during the first week in propagation prior to the visible appearance of callus issue or adventitious roots. This phenomenon is well-known by commercial propagators who alter their mist schedule daily during the first week in propagation to adjust to the changing water requirements of the cuttings. A series of experiments were conducted to quantify this phenomenon and to elucidate the mechanism for water regulation of unrooted cuttings. The effect of cutting age on water loss was determined using Osteospermum cuttings that were placed on a propagation bench for 1, 3, 5, or 7 days, then removed from the bench and allowed to wilt. Time to wilt decreased as the age of the cuttings increased from 1 to 7 days. Water loss rates were confirmed with stomatal conductance measurements that demonstrated a 7-fold increase in water loss of 1-day old cuttings compared to 7-day old cuttings. Stomatal aperture changed from 11.5 to 8.5 µm following transfer of cuttings from the mist propagation bench to a drier environment, however the effect of cutting age was minimal. Exogenous application of an abscisic acid analog caused a 2-3 µm difference in stomatal aperture in both 1 and 7-day old cuttings. These results suggest that stomatal functioning does not explain the large changes in the rates of water loss. Leaf water potential increased with time following transfer to a dry environment in both 1- and 7-day old cuttings; however, the 7-day old cuttings had a higher leaf water potential indicating that hydraulic conductivity of the cutting’s vascular tissue decreased with cutting age. The base of the cutting stems was removed prior to measure leaf water potential with a pressure bomb to confirm that the changes in the anatomy of the stem base were not associated with the changes in leaf water potential. These data suggest that the capacity to regulate water loss of unrooted cuttings increases during the first week that the cuttings are in propagation. Water regulation was not primarily regulated through a change in stomatal functioning but rather due to a change in hydraulic conductivity in the vascular tissue.