Food Safety Focus (60th Issue, July 2011) – Food Safety Platform
What are Plant Growth Regulators?
Reported by Ms. Joan YAU, Scientific Officer,
Risk Assessment Section, Centre for Food Safety
We noticed the recent hot topics are "cucumber with flowers" and "exploding watermelon" which raised public concerns about the use of plant hormones and plant growth regulators. What are plant hormones and plant growth regulators? Are they hazardous to human health?
From Plant Hormone to Plant Growth Regulator
Plant hormones are produced naturally by plants and are essential for regulating their own growth. They act by controlling or modifying plant growth processes, such as formation of leaves and flowers, elongation of stems, development and ripening of fruit.
In modern agriculture, people have established the benefits of extending the use of plant hormones to regulate growth of other plants. When natural or synthetic substances used in this manner, they are called Plant Growth Regulators.
The application of plant growth regulators in agriculture has started in the1930s in the USA . Ethylene, a naturally occurring substance, is one of the first plant growth regulators being discovered and used successfully for enhancing flower production in pineapple. Its toxic effects to human beings are low. Synthetic substances that mimic such naturally occurring plant hormones were also produced, since then the use of plant growth regulators has been growing significantly and becoming a major component in modern agriculture.
Three Common Plant Growth Regulators
Over the years, ethylene has continued to be among the best known examples of plant growth regulators. It is a gaseous plant hormone playing a key regulatory role in ripening of many types of fruits, including banana, apple, pear and melons. It can be produced naturally by ripening fruit or from synthetic sources such as ethephon. Try this experiment: put raw bananas and a ripening fruit (e.g. apple) in the same paper bag and cover it up. Ethylene produced by the ripening apple will speed up the ripening process of bananas, giving you the ripe bananas next morning.
Another major class of plant growth regulators are auxins and related compounds. The earliest study on auxins was intended for the initiation and acceleration of the rooting of cuttings. The natural auxin, indole-3-acetic acid, was identified in 1930s. Later on, synthetic auxins such as indolebutyric acid and naphthylacetic acid were developed. Synthetic auxins have a wide range of applications including the prevention of fruit drop in apples.
The recently reported "suspect" causing "exploding watermelon" is also a plant growth regulator, a chemcial called forchlorfenuron. It is a synthetic plant growth regulator under the group called phenylurea type cytokinin, which can induce cell division and cell differentiation. Forchlorfenuron is known to increase size and yield of fruits such as grapes, kiwifruits and watermelon. Proper use of forchlorfenuron (i.e. following the good agricultural practice (GAP)) will result in minimal residue in food and hence low food safety risk.
Ripening experiment: put a raw banana into a paper bag together with a ripening apple, cover it up, place over night. Next morning you will find the banana ripened as compared with one that is left in air. The effect is due to the banana's exposure to ethylene released by the ripening apple or other ripening fruits that may be present.
Plant Growth Regulators and Food Safety
From the regulatory control perspective, plant growth regulators are classified under " pesticides ". The same classification applies both internationally, under the Codex systems and competent authorities of developed countries, as well as locally in the Pesticides Ordinance, Cap 133.
As a well accepted principle, all pesticides, including plant growth regulators, have to be registered with the competent authority before application in agriculture. Their safety and efficacy will be thoroughly assessed during the registration process. Proper use of these registered pesticides including plant growth regulators in accordance with GAP will result in minimal residue in food of insignificant food safety risk. GAP is a set of nationally authorised conditions to use the pesticide safely (e.g. in relation to public health and environmental safety concerns) for effective and reliable pest control. Various conditions, such as the type of commodities authorised for using the pesticide, the recommended application rates, frequencies, and amount as well as the duration between the last application of the pesticide and harvest, are prescribed in the GAP.
In conclusion, plant growth regulators are a group of chemicals for controlling and enhancing the natural plant growth processes to better meet the requirements of food supply in general. Mechanisms are in place under the Codex system to oversee residues of pesticides (including plant growth regulators) in food for setting standards and public health protection.