The relationship between the rate of entry of ioxynil and effects on photosynthesis in normal and drought‐stressed Stellaria media

Abstract

SUMMARYThe entry of 14C‐labelled ioxynil octanoate into leaves of Stellaria media has been measured for plants grown in dry or moist soil. Of the total herbicide applied, 1–3% entered the leaf by 24 h and 2–5% by 72 h after treatment. Entry into moist‐grown plants proceeded at about twice the rate of that into drought‐stressed plants. Despite the limited rate of entry, the inhibitory action of ioxynil octanoate on photosynthetic carbon dioxide exchange was rapid, inhibition within 24 h reaching 70–90% in moist‐grown plants, and 30–70% in dry‐grown plants. Plants grown under moisture stress contained greater concentrations of the pigments chlorophyll a, carotene and lutein (a xanthophyll) than did moist‐grown plants, and ioxynil‐induced breakdown of these pigments was more rapid in moist‐grown plants. It is suggested that these factors contribute to the greater tolerance of drought‐stressed S. media to applications of ioxynil octanoate.The importance of continuous measurements of herbicide action is discussed in relation to the value and interpretation of 14C uptake data.