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.