Physiological and ultrastructural changes in Stellaria media following treatment with fluroxypyr

SUMMARY The physiological and ultrastructural changes induced by fluroxypyr (4-amino-3, 5-dichloro–6–fluoro–2–pyridyloxyacetic acid) are investigated in a susceptible weed species, Stellaria media. The sequence of symptoms was an initial petiole curvature followed by stem elongation and thickening prior to leaf senescence, stem and petiole necrosis and plant death. Light and electron microscopy revealed leaf tissue containing cells with disrupted and swollen chloroplasts and a disrupted tonoplast. Within the stem tissue there was extensive meristematic differentiation and adventitious root development prior to stem tissue necrosis. Increased levels of reducing sugars and amino acids in treated foliar tissue indicated reserve mobilisation during the initial stages of symptom development. Fluroxypyr induced oat coleoptile elongation with an optimum concentration at 5 × 10-5 m compared to 10-4 m for auxin (IAA). A S. media shoot explant system revealed similar elongation which was maximal during the first 24 h of treatment. Uptake and incorporation of 14C-leucine was stimulated by 96 h after treatment. It is concluded that fluroxypyr activity against S. media is characterised by an initial cell elongation and reserve mobilisation, followed by extensive cell proliferation, increased RNA and protein synthesis, and ultimately plant death.