The Action of Plant Growth Retardants at the Biochemical Level

Abstract

Xenobiotics which reduce plant growth without affecting the pattern of development have been available for almost 40 years. A group of nicotinium growth retardants was described by Mitchell et al, in 1949 [36], followed shortly by the quaternary ammonium carbamates, including AMO-1618 [47]. Partial reversal of the dwarfing effect of these compounds by GA₃ [32] and the demonstration that the retardants reduced GA production by the fungus Fusarium moniliforme [30] implicated GA biosynthesis as a site of action. This was confirmed by Dennis et al. [10], who showed that AMO-1618 and other retardants inhibited the cyclization of geranylgeranyl pyrophosphate (GGPP) to ent-kaurene in cell-free homogenates from Marah macrocarpus endosperm. A large number of quaternary ammonium, phosphonium and sulphonium retardants have been described [11], although only chlormequat chloride (CCC) and mepiquat chloride have found large-scale application in agriculture, due predominantly to their use on wheat and cotton, respectively. Ironically, the precise mode of action of CCC is in considerable doubt [11].