Plant-growth regulators

Some examples of plant growth regulators, IA A, 2,4-D, MCPA and 2,4,5-T, all plant growth stimulants, have already been discussed. 

Many synthetic compounds are available, and one of the earUest examples was the use of ethylene to promote ripening of fruit. Compounds are now available which when sprayed on to crops slowly decompose to give ethylene. An example is ethrel which is used in orchards where it is sprayed onto trees and the ethylene it produces promotes ripening and loosening of the fruit. 

Interestingly, 2,4-D at a concentration of 12 ppm is used to prevent fruit drop and increase fruit size in citrus trees, e.g. grapefruit, lemon and orange. 

Cytokinins, which are adenine derivatives, are used to prolong the storage life of green vegetables, cut flowers and mushrooms. A well-known example is zeatin. 

Equally, compounds are available which retard plant growth. Again both naturally-occurring and a variety of synthetic compounds exist. Examples of the naturally occurring group are gallic acid (3,4,5-trihydroxybenzoic acid) and cinnamic acid, both of which prevent sprouting in stored onions and potatoes. 

The notion of using exogenous chemicals to control plant growth, and hence productivity, is not new. Many of the first commercial herbicides (auxin herbicides) were developed as a direct consequence of plant growth regulator research (see Chapter 1, Section 3.3). 

---

Recently, however, new interest has developed due to their chemotherapeutic and bacteriostatic usefulness in animal husbandry. They also have been patented as herbicides and plant-growth-regulating agents. 

COLORANTSFORFOOD,DRUGS,COSTffiTICSANDTffiDICALDEVICES] (Void) -plant growth regulators for [GROWTH REGULATIONS - PLANT] (Vol 12)... 

Growth rate dispersion Growth regulator, plant Growth regulators... 

Plant growth regulators Plant gum exudates Plant layout... 

The ideal plant growth regulator should leave no harmful persistent residue in a finished product or crop and the paradigm compounds are ones that have high specific activity, are target specific, and are environmentally biodegradable. 

Fig. 2. Natural product derivative plant growth regulators. See Table 1.

Ethylene. Ethylene [74-85-1/, (7) is considered to be one of the principal plant growth regulators in the natural products family. However,... 

Until other gibbereUins (- 80) can be either fermented or synthesized in bulk, their future is uncertain for use as industrial plant growth regulators except in very special circumstances. 

Table 2. Authorized Synthetic Plant Growth Regulators ...

Fig. 3. Synthetic plant growth regulators authorized for use in the United States and elsewhere. See Table 2.

Ghlormequat Chloride. 2-CHoioethyltrimethylammonium chloride [999-81-5] (chlormequat chloride), known as CCC (20), is an onium-type plant growth regulator that has variable use depending on where it is used. CCC by itself is registered for use in the United States by the Environmental Protection Agency, but the mixture of CCC with choline chloride has not been approved. 

Naphthalene Acetic Acid and Naphthalene Acetamide. Naphthalene acetic acid [26445-01-2] (38) is historicaHy one of the first plant growth regulators. Reports concerning its activity in crops and plants have been a subject in much of the eady Hterature (57). Consequently, it has been used as a starting matedal for other compounds, eg, vide infra Sevin. Naphthaleneacetamide [31093-43-3] (39) has been used as a standard matedal to evaluate abscission pnor to 1953 and its effect on apple drop was reported in 1953 (58). The substance is used as an internal standard in the abscission bioassay (59). 

---