Influence of Agar and Sucrose on the Behaviour of Dormant Apple Embryos Cultured in Vitro

SUMMARY Dormant as well as completely after‐ripened embryos of Pyrus malus L. var. Golden Delicious were cultured aseptically on media of different compositions. Distilled water was used as the simplest medium. The influence of the addition of agar (8 g 1−1), of mineral substances (macro‐and micro‐elements of the Heller's medium) and of sucrose (10 or 20 g 1−1) was studied. On the agar‐solidified media, three different modes of culture were used, depending upon the nature of the contact between the embryo and the medium. The criteria used to evaluate the depth of dormancy were the germination percentages and the progression in the greening of the cotyledons. Completely after‐ripened embryos behaved similarly on all the media used whatever the mode of culture. Dormant embryos behaved differently depending upon the composition of the medium and of the mode of culture. Dormancy was always deeper on agar‐solidified media than on liquid media and in the presence of sucrose than in its absence. In order to determine whether these differences in behaviour were due to quantitative differences in the leaching of free abscisic acid (ABA) into the medium, a two‐step culture method was introduced, using [14C]ABA. Dormant embryos were cultured first on water‐agar containing 10−5 M (±) [2‐14C]ABA (355 KBq μmol−1), then on one of the following media deprived of ABA: water‐sucrose‐agar, water‐agar, water. Observations during the first culture indicated that the amount of [14C]ABA absorbed was partially metabolized after only 2 days of uptake. During the course of the second culture, part of the original radioactivity of the embryos (4 days [14C]ABA uptake) was released into the medium. Leaching was much greater and lasted longer on water than on media solidified with agar: it does not depend upon the volume of the liquid used (3 or 15 ml). The presence of sucrose (20 g 1−1) in the agar medium does not modify the amounts of 14C‐radioactivity released. The main part of the radioactivity released in water and in water‐agar was shown to be ABA.