Role of Pesticides in Produce Production, Preservation, Quality, and Safety 369
Table 11.11 gives examples of compounds used as PGRs. PGRs may be naturally
occurring plant hormones (phytohormones) or synthetic organic compounds often
derived from them [79]. There are five major types of phytohormones: auxins,
gibberellins, cytokinins, ethylene, and abscisic acid [80]. The synthetic PGRs may
mimic natural phytohormones, may interfere with them, or may affect plant growth
in some other way.
The term auxin is derived from the Greek word auxein, which means to grow.
Compounds are generally considered auxins if they are able to induce cell elongation
in stems and otherwise resemble physiological activity of the main naturally occur-
ring auxin indolyl-3-acetic acid (IAA) biosynthesized from the amino acid tryp-
tophan [81]. In addition to elongation of stem cells (internode elongation), auxins
usually affect other processes, such as differentiation of phloem and xylem; root,
leaf, and fruit growth; suppression of lateral buds below the apical bud (apical
dominance); bending in response to gravity or light (gravitropism and phototropism);
fruit setting and ripening; or fruit and leaf abscission.
IAA and its more stable analog indolyl-3-butyric acid (IBA) are used to induce
rooting of cuttings. Examples of synthetic auxins include aryloxyalkanoic acids (e.g.,
2,4-D), which are applied mainly as herbicides; however, their sublethal doses may
prevent fruit drop or promote fruit set. Other synthetic auxins, 1-naphthylacetic acid
(NAA) and 2-(1-naphthyl)acetamide (NAD or NAAm), are used to improve fruit
setting and prevent premature flower and fruit drop in apples and many other fruits,
to induce flowering in pineapples, or as fruit thinning agents [5].
Fruit thinning means the removal of a portion of the crop before it matures. It
is usually necessary for apples, pears, peaches, nectarines, and plums. The benefits
of fruit thinning include improvements in size and quality (color, flavor, and sugar
content) of the remaining fruit, promotion of return blooms, and reduction of limb
breakage from excessive crop loads. Hand thinning is accurate and selective but also
laborious and very difficult on large trees. Chemical thinning represents a viable
alternative, although it lacks the ability to selectively position fruit and may result
in removal of too many or too few fruit. The most frequently applied fruit thinning
TABLE 11.11
Examples of Compounds Used As Plant Growth Regulators (PGRs)
PGR Group Representatives
Auxins Indolyl-3-acetic acid (IAA), indolyl-3-butyric acid (IBA), 1-naphthylacetic acid
(NAA), 2-(1-naphthyl)acetamide (NAD or NAAm), aryloxyalkanoic acids
(2,4-D, dichlorprop, etc.)
Gibberellins Gibberellic acid, gibberellins
Cytokinin 6-Benzylaminopurine, kinetin
Ethylene releasers Ethephon
Defoliants Dimethipin, endothal, thidiazuron, tribufos
Growth inhibitors Maleic hydrazide, chlorpropham, propham, tecnazene
Growth retardants Chlormequat, trinexapac-ethyl