Persuading killer plants to turn on each other

Press, 28 May 1983, Page 14

Persuading killer plants to turn on each other

Anybody who has had a brush ■ with stinging nettles knows that plants can produce nasty substances. Because they cannot run away, it has been argued, it is sound evolutionary practice for plants to have developed defence mechanisms to deter predatory animals and insects. Some may also make substances which harm other plants — not as a defence mechanism, but as a weapon of territorial aggression. Allelopathy, as chemical warfare between plants has been dubbed, is arousing commercial interest and scientific hackles. The scientific rows centre on the role of such chemicals in nature. Proponents of allelopathy, such a Professor Alan Putnam of Michigan State University, have no doubts that the plant kingdom is a hotbed of warring tribes. Critics argue that the mere fact that poisons can be isolated from plants does not demonstrate that they are effective weapons against other plants. There is no concrete evidence that they are produced in sufficient quantities to get through the soil undamaged and attack other plants. This controversy does not worry chemical companies (America’s Dow, for instance, is putting money into Professor Putnam’s work). They are interested by the agrument that studies of the toxic chemicals produced by plants could lead to new strategies for .weed control ' Some of the top synthetic herbicides are known to be chemically related to natural plant materials. Many simple organic acids (e.g. acetic acid) inhibit seed generation: one reason why many fruits are acid may be to prevent thenseeds germinating while still in the fruit; '

Related compounds, whose molecules also contain chlorine atoms, were among the first synthetic herbicides. Because the chlo-

rine atoms made the molecules resistant to microbiological degradation in the soil and so made them effective longer than the natural acids, they were labelled “persistent herbicides.” Many of the possible allelochemicals identified so far also inhibit seed germination. Plants also produce compounds which may encourage seed germination. One such is the simple molecule ethylene, better known as a raw material in the production of many petrochemicals. It has been suggested that it could be used to trigger “suicidal” germination. Applied to cultivated land before crops were to be planted, a dose of ethylene could encourage weed seeds to germinate while the weather was still cold. , Professor Putnam points out that sorghum produces allelochemicals which suppress weed growth but do not harm crops — and it

produces larger quantities when put under stress. So it should make sense to grow sorghum in a field, to allow it to be killed off by frost and then to plant crops such as beans among the sorghum residues. There are problems, of course. One is specificity. Ideally, chemical companies would like to identify allelochemicals that attack only unwanted species of plants. As the critics of allelopathy point out, most of the natural chemicals known to date are not specific. Even when the chemicals do apparently discriminate — as in the case of the sorghum substances — difficulties could arise. A plant immune to the deleterious effects of a particular toxic chemical may none the less absorb and store it Many of the plantpoisons are also harmful to animals or, at least, unpalatable, it is no good having a crop that flourishes but cannot be used.