Acclimatization of In Vitro Derived Plantlets of Walnut Rootstock Clones

Survival of in vitro derived plantlets of 40 walnut rootstock clones was greatly enhanced by ex vitro rooting and provision of an intermittent fog derived high humidity. The high humidity chamber consisted of a 1.25x1.25x4 m polyethylene enclosure on a 21 degrees C minimum temperature greenhouse bench. Fog was provided intermittently by a single pneumatic, venture-type nozzle controlled by a QCOM computer program, based on solar energy during daylight hours and a fixed cycle during darkness. A small fan at the end opposite the nozzle pulled evaporatively cooled air through the chamber and provided more uniform distribution of fog along the length of the chamber. Thermostatically controlled heating mats provided bottom heat as needed. In vitro derived microshoots were rooted ex vitro in the fog chamber described above. The microshoots were given a basal treatment with 10 mg/L KIBA in a gellan-gum-solidified Driver and Kuniyuki medium for 4 d in the dark and then stuck in coarse vermiculite in small containers placed on heating mats at 28 degrees C in the fog chamber. After 21 d, an average of 58% of the microshoots of 40 clones rooted, with fibrous roots 2 to 4 cm long. After transplanting to soil in tree tubes (2.5 cm diameter) and acclimatizing in the fog chamber for 14 d, 81% of the rooted microshoots survived and grew on a greenhouse bench at 21 degrees C minimum night temperature. After 3 months, plantlets were large enough to plant in a nursery row.