M.C. Dussi
The use of growth regulators is an important tool in pear production. Fruit trees are complex perennial structures that differ in many respects from annual or herbaceous plants. Growth and reproductive potential of the tree can be manipulated using different compounds. Plant bioregulators (PBRs) have the peculiarity that in some cases the same active ingredient can induce different responses depending on the application time and rate used. Their effectiveness is influenced by various factors like pear cultivar, tree vigor and yield, weather factors during, before and after the application and application method, e.g., water volume and/or adjuvant usage among others. One of the important aspects that contribute to increasing or maintaining the sustainability of fruit tree agroecosystems is the use of substances in a rational way, with high efficiency and low environmental impact to improve fruit quality without toxic residues, preserving human health and productive areas. Many of the growth regulators are hormones or hormone-like substances that promote, inhibit or affect biological or biochemical processes in plants. Endogenous plant hormones have shown the ability to affect different developmental processes in a positive or negative manner but in some processes (e.g., flower induction) there is still a need for a better understanding of these signal interactions, their origin, transport and perception to reliably manipulate them with PBRs. In recent years, progress has been made testing molecules for different purposes. Vegetative growth control using prohexadione-calcium, a shoot growth retardant, may result in different responses depending on the pear cultivar and rate used. Exogenously applied gibberellins (GA) with prohexadione calcium was reported to induce larger fruit than GA alone. Dormancy release and bud break agents were also evaluated on European and ‘Nashi’ pears especially in warm winter pear production regions to satisfy the chilling requirement, and to induce newly formed meristems to develop into lateral shoots (feathers) using products that modify auxin activity. Ethylene inhibitors can delay harvest and control fruit abscission in various pear cultivars. 6-benzyladenine (BA) has shown consistency in thinning pears and increasing fruit size. Abscisic acid can potentially overcome plant stress, enhance fruit color, reduce sunburn and thin fruits but its effectiveness in pears needs to be confirmed. Molecular biology research to identify regulating genes will help to find new uses for PBRs. A more holistic approach to the use of growth regulators should be made taking into consideration the physiological status of the plant, irrigation management, fertilization, pruning, training systems, light interception and distribution in the tree and weather conditions. In this way, models should be developed to predict and explain tree responses.
DOI: 10.17660/ActaHortic.2011.909.40
Pyrus communis, hormones, thinning, fruit ripening and abscission, bud break, shoot growth control

Acta Horticulturae