Fig. 2.

Incidence of shoot tip necrosis (STN) in in vitro cultures of walnut (Juglans regia L.) Paradox rootstock during micropropagation in Driver and Kuniyuki walnut medium (DKW; Driver and Kuniyuki, 1984) (unpublished results). (1) if 3-week-old shoots were used, the incidence of STN was high (20–30%), most likely because tissue is soft (non-lignified), but the use of 4-week-old shoots, which are more lignified, have a lower incidence of STN, even reduced to 0%; (2) initial “Vlach” [a selection of Paradox (J. hindsii x J. regia)] material is from a 110-year-old mother tree, located near Modesto (CA, USA) for which in vitro cultures were originally established by John Driver in 1985; (3) walnut tends to be somewhat recalcitrant to rooting, so occasionally high concentrations of IBA (8–10 mg/l) are added to rooting medium. If IBA is transported to the shoot tip, especially soft shoots that may take up excessive amounts of IBA, this may result in the death to the shoot tip, a condition we coin as “IBA burn”, which is visually similar to STN. However, this does not take place if more mature shoots are used and this can be achieved by increasing the subculture interval from 3 to 4 weeks. Black arrows indicate STN. Scale bars indicate 3 cm (top), 2 cm (middle) and 2 cm (bottom)