How treated plants die

Weed death occurs as a consequence of an auxin-overdose and is due to uncontrolled growth. The exact sequence of events depends on the age and physiological state of the tissues affected and varies considerably between species. Nonetheless, three phases of symptom development are commonly observed.

Phase 1, the first day. Profound changes in membrane permeability to cations can be discerned within minutes of auxin application. For example, rapid and sustained proton-efflux results in measurable cell elongation within an hour, and an enhanced accumulation of potassium ions in guard cells causes increased stomatal apertures and a transient stimulation of photosynthesis (Figure 7.12). Furthermore, a rapid mobilisation of cellular carbohydrate and protein reserves is commonly observed as large rises in soluble reducing sugars and amino acids. This coincides with enhanced mRNA synthesis and large increases in rates of protein synthesis. In addition, ethylene evolution is typically detected from treated plants (see Figure 7.1). Phase 2, within a week. In the days following herbicide treatment major growth changes become apparent as visible symptoms initiated by new genome expression and powered by reserve mobilisation in Phase 1. For example, increased cell division and differentiation in the cambium leads to adventitious root formation at stem nodes in some species (Sanders and Pallett, 1987- , and general tissue swelling caused by the division and elongation of the cortical parenchyma is typically observed in other tissues, such as the petiole. Classic symptoms of stem, petiole, and leaf epinasty are now observed in young tissues in response to ethylene evolution, and abnormal apical growth is sometimes observed (Figure 7.13). Lateral buds may also be released from apical dominance and all other meristems increase in activity.

Figure 7.12 Effect of clopyralid (100 g active ingredient ha 1) on rate of photosynthesis (A) and stomatal aperture (B) in leaves of scentless mayweed (modified from Thompson, 1989).
Figure 7.13 Typical symptom development of auxin-type herbicides. Plants of Tripleurospermum maritimum subsp. Inodora (scentless mayweed) were treated with 0, 50, 100, 200 and 400 g a.i. ha-1 clopyralid, and foliar symptoms were photographed after 7 days.

Phase 3, within ten days. Ultrastructural studies reveal progressive disruption of intracel-lular membranes culminating in the disruption of the plasmalemma, organelle breakdown and tissue collapse. Root disintegration, leaf chlorosis, and senescence are rapidly followed by plant death.

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