During the fifties were the shedding of fruits and leaves, also called abscission, and the dormancy of buds intensely studied. This lead to the discovery of a hormone called abscisic acid. It showed that this substance is, too, wide-spread in the plant kingdom. Cotton fruits became a suitable source for the isolation amounts large enough to elucidate the chemical structure (F. F. ADDINCOTT and collaborators 1961, 1963, 1969, B. W. MILBORROW, 1967).
Abscisic acid (ABA) is identical with a substance that causes bud dormancy in wooden perennial plants. It was therefore at first also called dormin. In maple and birch buds causes the change from long-day to short-day conditions a marked increase in the activity of dormin (=ABA) and consequently stops the growth of buds.
ABA-containing maple and birch leaf extracts from plants grown under short-day conditions inhibit leaf growth and induce even in fast growing shoots dormant buds. When the formula of ABA was known the production of a number of derivatives began none of which attained the effect of ABA.
In some plant tissues (especially in young shoots) occurs a related compound called xanthoxine.
Whether xanthoxine is an intermediate of the ABA-biosynthesis or whether it is an independent product remains unknown. The structure indicates that both ABA and xanthoxine are terpene derivatives. This was proven when it could be shown that radioactively labelled mevalonic acid is integrated into ABA though it does not elucidate which intermediates are produced. Two alternative biosyntheses have been discussed:
- ABA is a degradation product of xanthophyll (especially of violaxanthin).
- ABA is produced from a C15 precursor using a separate pathway and is thus independent from the carotenoid/xanthophyll metabolism.
The first idea seemed initially more plausible since the structures of xanthophylls and ABA correspond to a large degree. In vitro occurs conversion only upon exposure to strong light and with an extremely low yield, though. This and supplementing in vitro observations called the first assumption into question again.
Biological activities:
ABA is an efficient inhibitor of germination and occurs in high concentrations in dormant seeds. Just as in sprouting buds decreases its content also during seed germination, an indication that germination is controlled by an equilibrium of auxin(s), gibberellin(s), and cytokinin(s) on one and ABA on the other hand. The role ABA has during the abscission of fruits and leaves is largely unknown. Though both cases seem to be governed by similar mechanisms has ABA nearly no effect on the abscission of leaves while it shows a clear effect on fruit abscission. Moreover was a regulating effect of abscisic acid on the water balance observed.
As soon as the water supply of cut wheat leaf blades is interrupted and the cell turgor decreases raises the concentration of ABA forty-fold within four hours. Comparable data have been collected for other plant species, too. These effects were also observed in rooted shoots. A water loss of 5 – 10 percent (of the green weight) was sufficient to increase the ABA level. The raise is based on a new synthesis and not on the release of an inactive state as could be shown by W. MILBORROW. The concentration of ABA remains high if the plant’s situation improves slightly or even decreases dramatically. It induces the stomata to close thus inhibiting further loss of water.
Though it cannot be said too much about the mode of action does it seem certain that ABA inhibits the guard cells’ uptake of potassium ions. Potassium ions are essential for the opening mechanism of the guard cells (K. RASCHKE, 1975).
ABA reverses the effect of growth-stimulating hormones (auxin, gibberellins, cytokinin) in several tissues. The synthesis of hydroxylases within germinating wheat seeds, for example, does not take place after the application of ABA.
In summary can the importance of the ABA effect be interpreted as an effector that has the ability to close down certain parts of the plant metabolism for a period of time. Since ABA is easily removed from tissues, is its effect reversible. An example is the inhibition of seed germination in berries (like tomatoes). The germination does not occur even though the seeds are in a humid surrounding. When isolated and transferred to a normal, humid milieu do they immediately start to germinate. The failure of the inhibition of seed germination leads usually to vivipary.
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de
