Rooting of Choke Cherry Cuttings in Response to a Commercial Liquid Extract of the Marine Macroalga Ascophyllum nodosum

Choke cherry (Prunus virginiana L.) is an ecologically important species in land reclamation because of its extensive lateral root system that readily suckers and thus quickly creates dense thickets. Consequently, choke cherry provides rapid site occupancy that re-establishes nutrient cycling and prevents erosion. Our overall goal is to increase root growth of choke cherry seedlings after spring planting on reclamation sites by improving seedling root system quality in the nursery. Earlier, we reported an increase in the early rooting of choke cherry seedlings in response to a liquid extract of Ascophyllum nodosum (L.) Le Jolis extract (ANE). The objectives of this research were to confirm the optimal rate and to gain an understanding of how ANE elicits rooting. We stratified (2 wk at 21°C/18 wk at 4°C) seeds from a single clone, then placed them in a germination cabinet. After 7 days, we removed germinants, suspended them in vials of deionized water and placed them in a growth chamber (25/20°C D/N, 16-hour photoperiod, 75% relative humidity) to induce a geotropic response.  After 7 days, we grouped germinants into sets such that both mean epicotyl–hypocotyl axis length was equal and leaf number/development was similar. Cuttings were taken 2–3 mm below the hypocotyl–radicle junction and suspended in magenta jars filled with ANE in deionized water at the following rates: 0 mL/L, 0.25 mL/L, and 0.5 mL/L. After 21 days, 100% of cuttings had rooted at the 0.25 and 0.5 mL/L rates. Mean number of total (secondary and tertiary) roots on cuttings was 8.4 and 4.5 at the 0.25 and 0.5 mL/L rates, respectively. No cuttings had rooted at the 0 mL/L rate after 28 d. After 28 days, mean number of secondary roots was 3.6 and 3.1, and mean length of the longest secondary root was 7.4 cm and 7.8 cm at the 0.25 and 0.5 mL/L rates, respectively. On the longest secondary root, mean number of tertiary roots and mean length of all tertiary roots were 7.2 and 4.4 and 10.3 cm and 10.9 cm, respectively, at the 0.25 and 0.5 mL/L rates. Callus formed at the excised surface in 17% of cuttings at the 0.25 mL/L rate and was not associated with root initiation. No callus formed at the 0.5 mL/L rate. Roots arose from the pericycle, later connecting to the vascular stele and extending through the cortical–epidermal layers.