PHYSIOLOGICAL-STATE AND CLONAL VARIABILITY EFFECTS ON LOW-TEMPERATURESTORAGE OF IN-VITRO SHOOT CULTURES OF ELMS (ULMUS SP)

As a means of protecting elms against Dutch Elm Disease (DED), in vitr o storage of shoot cultures of some elm genotypes under reduced temper atures was investigated. At 7-degrees-C under 8 h of fluorescent light ing, survival rate remained high (> 82%) during the storage periods (2 4-30 months), provided the shoots used for the storage were already ro oted or about to root (1-2 months). Necrosis of some axillary ramifica tions at the end of the storage period reduced the multiplication rate from 3-4 to 1. According to the clone, explants taken from the stored plantlets were not as easy to root as the control (20-30% for U. camp estris, OCBi). In order to store elm clones free of DED and to reduce in vitro transfers, an optimal storage period of 18-24 months could be retained. It may be possible to extend the period by improving rootin g and growth during the pre-culture phase.