sterile filtrates of the fungus. The active substances were later identified as gibberellic acid. F. Skoog and his co-workers observed that from the internodal segments of tobacco stems the callus (a mass of undifferentiated cells) proliferated only if, in addition to auxins the nutrients medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA.
Miller et al. ( ), later identified and crystallised the cytokinesis promoting active substance that they termed kinetin. During mid-1960s, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin. Later all the three were proved to be chemically identical.
It was named abscisic acid (ABA). H.H. Cousins ( ) confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. Later this volatile substance was identified as ethylene, a gaseous PGR.
Let us study some of the physiological effects of these five categories of PGRs in the next section. . . Physiological Effects of Plant Growth Regulators .
. . Auxins Auxins Auxins Auxins Auxins Auxins (from Greek ‘auxein’ : to grow) was first isolated from human urine. The term ‘auxin’ is applied to the indole- -acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties.
They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. Auxins like IAA and indole butyric acid (IBA) have been isolated from plants. NAA (naphthalene acetic acid) and , -D ( , -dichlorophenoxyacetic) are synthetic auxins. All these auxins have been used extensively in agricultural and horticultural practices.
They help to initiate rooting in stem cuttings, an application widely used for plant propagation. Auxins promote flowering e.g. in pineapples. They help to prevent fruit and leaf drop at early stages but promote the abscission of older mature leaves and fruits.
In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance . Removal of shoot tips (decapitation) usually results