THE BASIS OF THE DIFFERENTIAL PHYTOTOXICITY OF 4‐HYDROXY‐3,5‐DI‐IODOBENZONITRILE*

Abstract

Summary. Ioxynil added to culture solution was of comparable toxicity to mustard, pea and barley at the end of a 2‐week treatment period. When the destruction of chlorophyll and the decrease in reduction of 2,3,5‐triphenyl tetrazolium chloride were studied in leaf discs infiltrated with ioxynil solutions, three to five times the concentration of ioxynil was required for barley compared with mustard to produce a similar response. Pea was intermediate.Recovery of ioxynil from sprayed barley plants (following washing) was highest I day after spraying; 2 days after, it had decreased to 37% and 9 days after, to 25% of the maximum. Recovery from mustard increased for the first 2–3 days and remained constant I'm the next 28 days. Separation of necrotic areas of treated mustard foliage confirmed that almost all the ioxynil remained in sprayed leaves which had subsequently been killed. Disappearance of ioxynil from barley was probably due to degradation, though only small amounts of ioxynil derivatives could be detected chromatographically.It is concluded that the differential phytotoxicity between barley and mustard is largely based on retention, supported by small differences associated with plant morphology, rates of uptake of ioxynil, its effects on plant metabolism and the ability of barley to degrade or inactivate it. Almost all the differences between mustard and pea were dependent on large differences in retention and small differences in rate of uptake and interference by ioxynil with plant metabolism. The first two were removed by adding a surfactant, and the last was sufficient only to delay the onset of phytotoxicity.