The basis of selectivity of 4‐(2, 6‐dichlorobenzyloxymethyl)‐4‐ethyl‐2, 2‐dimethyl‐1, 3‐dioxolan (WL 29, 226), a new pre‐emergence herbicide for use in cereals

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Abstract
A series of pot experiments were undertaken to assess the selectivity of the pre‐emergence herbicide 4‐(2, 6‐dichlorobenzyloxymethyl)‐4‐ethyl‐2, 2, ‐dimethyl‐1, 3‐dioxolan (WL 29,226) against a number of annual weeds in wheat. When applied at dose rates of 0.5–2 kg/ha it gave good control of a number of annual monocotyledonous weeds, including Alopecurus myosuroides (blackgrass), without any adverse effects on the crop. WL 29,226 is relatively immobile in soil, remaining at the soil surface and thus favouring uptake via the emerging shoot. Since WL 29,226 is transported predominantly via the xylem, to reach its site of action in the regions of cell division, and hence to be effective, the compound has to penetrate the shoot either at or below the stem apex. The roots are inhibited only when these come into direct contact with the compound. Selectivity of the herbicide is dependent upon the relative anatomical positions of the stem apices of the weeds and the crop with respect to the soil surface. Mesocotyl elongation in many of the weed species was such that the meristematic tissue was raised to the soil surface and into contact with the compound during the emergence of the shoot. In contrast, the stem apex of wheat remained some distance below the soil surface until considerably later, by which time the leaf sheaths offered protection to the meristematic tissue from direct contact with the herbicide. Selectivity is further enhanced in the field as a result of both the depth of planting for wheat and the tendency of many annual weed species to germinate more readily when near the soil surface. Tolerance of the wheat is lost where it germinates in direct contact with the herbicide, due to the lack of any biochemical selectivity. Under field conditions WL 29,226 gives good control of many dicotyledonous species. In pot experiments, however, these exhibit some tolerance to the compound. Radio‐tracer studies indicate that the tolerance shown by the shoots of these plants is due to limited transport of the herbicide from the shoot to its site of action at the apex. This suggests that control of broad‐leaved weeds occurs predominantly through an inhibition of root growth. However, in species such as sugar‐beet, soyabean and cotton a rapid rate of root elongation confers increased tolerance to the compound. Availability of WL 29,226 for uptake by young seedlings is favoured by soil moisture. Low temperatures further improve performance by reducing the rate of shoot emergence and hence prolonging contact with the compound at the most sensitive stage of growth. After emergence uptake of the compound via the shoot becomes a less efficient mode of entry.