Plant growth and development, as well as the responses to environmental factors, are highly regulated by the complex and coordinated action of the five classical hormones: gibberellins (GAs), abscisic acid (ABA), cytokinins (CKs), auxins (IAA) and ethylene. In addition, some other molecules, such as brassinosteroids (BRs), polyamines (PAs), jasmonates (JAs) and salicylic acid (SA), and some polypeptide hormones have shown to be involved, both directly or indirectly, in such processes. The recent characterization of the main steps of hormone biosynthesis, perception and signal transduction allowed to tune up the use of chemicals, called “Plant Growth Regulators” (PGRs), to interfere with plant growth and development. In nursery conditions, CKs (i.e. 6- benzyladenine, 6-BA), auxins (i.e. 3-indolebutyric acid, IBA) and inhibitors of auxins polar transport (i.e. morphactin, 1-N-naphthylphthalamic acid, 2, 3, 5-triiodobenzoic acid) are used to control branching and morphogenesis during propagation and micropropagation. In open field conditions, PGRs are effective in controlling both vegetative and reproductive activity of fruit trees. The most widely used growth-retardant (i.e. chlormequat and mepiquat chlorides, triazoles, prohexadione-calcium) reduce internode length by blocking different steps of GAs biosynthesis. GAs and inhibitors of GAs biosynthesis are also employed to control phase transition and fruit set, whereas chemical thinning is usually performed by means of molecules that induce the drop of flower buds or fruitlets, such as hydrogen cyanamide, dinitro-orthocresol (DNOC), ammonium thiosulphate (ATS), naphthaleneacetic acid (NAA), BA, Carbaryl®, and 2-chloroethylphosphonic acid (CEPA). Inhibitors of ethylene biosynthesis (aminoethoxyvinylglycine, AVG) or action (1-methylcyclopropene, 1-MCP) are used in pre- and post-harvest phase, respectively, to slow down climacteric fruit ripening. PAs and JAs are also effective in modulating fruit ripening and improving quality. Different developmental stages and the environmental conditions may determine different responses to the same levels of PGRs. As a consequence, a correct use of PGRs relies upon the deep comprehension of biochemical and biomolecular processes which regulate plant growth and development, and upon the complete elucidation of hormone biosynthesis and action.
Keywords: flower and fruit thinning, fruit development, fruit ripening, growth regulators, plant hormones