INDUCTION OF COMPLETE DESICCATION TOLERANCE IN CARROT (DAUCUS-CAROTA) EMBRYOIDS

Successful induction of complete desiccation tolerance in carrot embryoids (Daucus carota L.) depends on at least four different factors: developmental stage, abscisic acid (ABA), drying rate and rehydration mode. Embryoids may become desiccation tolerant when they have reached the torpedo stage of development. In contrast, at the earlier globular and heart stages, the embryoids never germinated after any drying treatment. Addition of at least 20 mu M ABA after 7 days of culture of pro-embryogenic masses in 2,4-dichlorophenoxyacetic acid-free B-5 medium was necessary to induce complete desiccation tolerance. Less ABA resulted in desiccation tolerance of the roots only, while higher ABA concentrations retarded developmental growth. Slow drying is essential for the acquisition of complete desiccation tolerance. Slowly dried embryoids (moisture content 0.05 g H2O.g(-1) dry weight) had a germination rate of 100%, while rapidly dried ones germinated only 0-30%. Initially, viable dry embryoids may suffer injury when they are imbibed in water without prehydration in water vapour. One hundred percent germination was reached by prehydration of the embryoids in moist air for 4 to 8 hours at 24 degrees C before imbibition in B-5 medium. With the optimized protocol we were able to produce desiccation tolerant embryoids of two genotypes having completely different genetic backgrounds.