The amount of endogenous ABA in cell clusters and embryos remained low during the first 7 days of culture regardless of the presence or absence of 2,4-D in the medium, but it increased toward the 10th day of culture. The ABA content in developing embryos reached a maximum on the 10th day then decreased. But, in cell clusters grown in a medium containing 2,4-D (in which embryo formation and development were suppressed) the ABA content increased continuously until it reached the maximum value of 80 ng/g fr wt on the 13th day, then it decreased to a low level on the 17th day of culture.The effects of ABA on embryo formation and development were investigated in a medium containing ABA at a concentration of 100 ng/ml, a level comparable to the endogenous levels of ABA which we had measured. Due to continuous treatment with ABA, both the total number of embryos and the number of abnormal embryos (those with extremely elongated roots) decreased. A similar, but weaker effect, was obtained when ABA was applied only during the first 7 days of culture (embryo-forming stage). When ABA was applied during the last 7 days of culture (embryo-developing stage), its effect was marked especially on the suppression of abnormal embryo formation.
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