Improved rooting methods for walnut (Juglans) microshoots.

Modified rooting procedures using etiolated microshoots, extended induction times, and reduced auxin concentrations were examined for rooting a variety of walnut genotypes including Persian walnut (Juglans regia) cultivars and several hybrid selections tolerant to cherry leaf roll virus. These methods generally increased rooting percentages, root number, and root elongation, reduced basal callusing, and reduced the length of time required for root emergence. Etiolation was particularly effective in improving the rooting of the most difficult genotypes but introduced new challenges. Leaves of etiolated microshoots did expand well on the vermiculite-supplemented medium routinely used for root development of green shoots, resulting in poor greenhouse survival of shoots rooted on this medium. Methods were tested to improve leaf expansion concurrently with root development in vitro, to reduce brittleness of microshoot stems, and to reduce damping-off during greenhouse acclimatization. Efforts to improve root quality and to pre-harden shoots before greenhouse acclimatization are continuing.