Velvetleaf

A variety of chemicals may be leached from the aerial portions of plants by rainwater or by fog-drip (16) including organic acids, sugars, amino acids, pectic substances, gibberellic acids, terpenoids, alkaloids, and phenolic compounds. Colton and Einhellig (17) suggested that leaf leachates of velvetleaf (Abutilon theophrasti) may be inhibitory to soybean (Glycine maxT We have recently discovered specialized hairs on the stems of velvetleaf plants which exude toxic chemicals. 

The reported (14) mechanisms of action of allelochemlcals Include effects on root ultrastructure and subsequent Inhibition of Ion absorption and water uptake, effects on hormone-induced growth, alteration of membrane permeability, changes In lipid and organic acid metabolism, inhibition of protein synthesis and alteration of enzyme activity, and effects on stomatal opening and on photosynthesis. Reduced leaf water potential Is one result of treatment with ferulic and p-coumaric acids (15). Colton and Einhellig (16) found that aqueous extracts of velvetleaf (Abutllon theophrastl Medic.) Increased diffusive resistance In soybean fGlycine max. (L.) Merr.] leaves, probably as a result of stomatal closure. In addition, there was evidence of water stress and reduced quantities of chlorophyll In Inhibited plants. 

Pigweed Amaranthus spp., foxtail Setaria spp., velvetleaf Abutilon theophrasti Medikus Carpetweed Mollugo verticillata L., common... 

The remainder of this discussion examines the possibility of a direct feedback mechanism in response to light as an explanation for crop yield reduction from early weed presence. Three types of data will be examined 1) results of our research on velvetleaf interference with light in soybeans 2) a comparison of observed and estimated soybean yield reductions for weed presence versus leaf removal and 3) the poor correlation between weed control and crop yields. 

Velvetleaf interference in soybeans. In 1984, a field study was conducted on the Agronomy farm near Columbia, Missouri to examine the effect of velvetleaf height and duration on soybean yields. 

Figure 8. Effects on Soybean Yields of Three Velvetleaf Heights Maintained for the First Six Weeks after Soybean Emergence Compared to No Height Control (left bar) and Weed Removal for the Entire Season (right bar). Velvetleaf height (15%, 100% and 125%) is relative to soybean height.

Phenolic acids interfere with many major physiological processes of higher plants (35). These disruptions of function include an alteration of plant water balance. We found depression of leaf water potential to be an early indicator of allelochemical stress from ferulic and p-coumaric acids (42). Likewise one mechanism of allelopathic action by cultivated sunflower, velvetleaf Abutilon theophrasti Medic.), Koahia [Koahia saoparia (L.) Schrad.], and several other weeds was water stress (43-45). Since some allelochemicals interfere with plant-water relationships, it seemed logical that their action might be most critical at times when plants are under water stress from other causes. 

Sun JS, Foy CL (1996) Structurally related organosilicone surfactants, their physicochemical properties and effects on uptake and efficacy of primisulfuron in velvetleaf (Abutilon theophrasti Medicus). FRI Bulletin 193 225-230 (Proceedings of the fourth international symposium on adjuvants for agrochemicals, 1995)... 

Gronwald, J.W., R.N. Anderson, and C. Yee (1989). Atrazine resistance in velvetleaf (Abutilon theophrasti) due to enhanced atrazine detoxification. Pestic. Biochem. Physiol., 34 149-163. 

Ritter, R.L. (1986). Triazine resistant velvetleaf and giant foxtail control in no-tillage corn. Proc. Northeast Weed Sci. Soc., 40 50-52. 

Velvetleaf Tumble pigweed Sandhills amaranth Prostrate pigweed Bouchons amaranth Redshank, red amaranth or Italian amaranth Smooth pigweed... 

This characteristic, however, is not universally found in all triazine-resistant weeds. Gray et al. (1995a, b) found that velvetleaf resistance to atrazine in Wisconsin was not associated with a reduction in fitness, productivity, or intraspecific competitive ability. This triazine-resistant species found in Maryland and Wisconsin does not have D1 level resistance in the chloroplasts, but instead has a more rapid metabolic detoxification of triazines in these biotypes. The extent of the rapid metabolic resistance in other velvetleaf-resistant biotypes is unknown. 

Ritter and Menbere (1997) have reviewed the history and control of triazine-resistant weeds - especially common lamb s-quarters, smooth pigweed, bamyardgrass, velvetleaf, and giant foxtail - in the mid-Atlantic region of the United States. They concluded that the factors influencing the presence of the resistant weeds included lack of crop rotation and lack of herbicide rotation. 

Andersen, R.N. and J.W. Gronwald (1987). Noncytoplasmic inheritance of atrazine tolerance in velvetleaf (Abutilon theophrasti). Weed Sci., 35 496 -98. 

Anderson, M.R and J.W. Gronwald (1991). Atrazine resistance in a velvetleaf (Abutilon theophrasti) biotype due to enhanced glutathione 5-transferase activity. Plant Physiol., 96 104-109. 

Balke, N.V. and D.E. Stoltenberg (1998). Differential detoxification of atrazine by velvetleaf biotypes. 9th International Congress of Pesticide Chemistry. London, England International Union of Pure and Applied Chemistry. 

Gray, J.A., D.E. Stoltenberg, and N.E. Balke (1995a). Absence of herbicide cross-resistance in two atrazine-resistant velvetleaf (Abutilon theophrasti) biotypes. Weed Sci. 43 352-357. 

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