Plant Hormones

Plant Hormones

Strictly, this term applies to certain substances produced by plants which regulate growth but it is also used for some synthetic chemicals producing similar effects. A better term for these is ‘growth regulators’. Often these have an effect quite disproportionate to the concentration at which they are used. For example, alpha-indole-acetic acid has been used at dilutions of 1 part in 50,000 to include rapid root formation on tomato cuttings. Beta-naphthoxyacetic acid used in extremely weak solution as a spray on tomato flowers causes fruits to set irrespective of normal pollination. Roottforming hormones can be purchased in proprietary brands both as liquids and as powders. The former are diluted with water according to manufacturers’ instructions. Cuttings are stood erect in this solution for twenty-four hours (longer for hard cuttings). Only the basal part of each cutting is immersed. Powders are mixed with the soil in which the cuttings are to be inserted. The root-forming hormones principally used are alphatnaphthalenetacetic acid, indoletacetic acid and indolebutyric acid, but there are many other effective substances.

Betatnaphthoxyacetic acid also has the property of causing some fruits to swell and mature even though unfertilized. This chemical has been used with excellent effect on tomatoes, the method being to apply as a very fine spray to the open blossoms. The dilution is 50 to 60 parts per million of water, but proprietary brands are already much diluted, so manufacturers’ instructions should be followed for these.

A third use for growth regulators has been found in the prevention of pretharvest drop of apples by spraying with alphatnaphthalenetacetic acid. This is applied at a strength of 10 to 20 parts per million of water in August or early Septtember, five to ten days before the date at which the apples normally start to fall badly. A second application may be given ten days later. By this means the ripening period is extended and much fruit is saved.

Growth regulators in excess are toxic to plants and this has suggested their use as weedtkillers. Some are used to kill annual and perennial broadtleaved weeds, without harming grasses and similar plants, and are sprayed on to the plants so that the solution is absorbed through the leaves and stems.

These weed-killers are also absorbed through the roots. Plants sprayed with them curl and become distorted, gradually turn brown and eventually die.

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