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

The 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.