Activation of acid growth in germinating horse chestnut seeds

The validity of the acid-growth hypothesis is proved for the case of cell elongation initiation in germinating seeds of horse chestnut (Aesculus hippocastanum L.), the embryo axes of which are known to extend during the first stages of germination only by cell elongation. During seed imbibition, H⁺-ion excretion was firstly low; it increased several times prior to radicle emergence and was maintained at a high level during growth initiation and further cell elongation. Cell wall acidification and radicle emergence were enhanced in the presence of 0.02 mM fusicoccin, thus indicating the involvement of the plasma membrane H⁺-ATPase in the execution of acid growth. The presence of this enzyme and its activator (14-3-3 protein) in microsomal fractions obtained from radicles and hypocotyls of the embryo axes during and after initiation of cell elongation was demonstrated immunochemically. It is supposed that the initiation of cell elongation at early germination occurs via the activation of the plasma membrane H⁺-ATPase and results in the acidification of cell walls, leading to their higher extensibility, in accordance with the hypothesis of acid growth.