What makes a plant to become invasive and difficult to control? Do weeds have an innate potential to grow and reproduce rapidly? Why is it that after several years of successful control, a plant tolerates an herbicide rate that used to kill it? Why do good plants go bad? Are weeds “bad to the bone”?
For many years, weed scientists have focused their attention in controlling weeds and invasive plants with the use of herbicides and tillage. Without any doubt, the use of chemical and mechanical control practices have reduced invasive plants and weeds, intensified agricultural production and increased farm labor efficiency. However, the heavy reliance on these practices has, unfortunately, resulted in increased production costs, environmental worries and health concerns. For example, the repeated use of herbicides exerting a strong selective pressure on plants has led, worldwide, to the existence of about 290 documented cases of herbicide resistant biotypes. Also, intensive use of tillage, plowing and cultivation usually results in serious soil degradation problems.
The increased potential for plant movement with the global economy, an improved appreciation of the importance of natural habitats and the growing pressure that exists on agriculture towards environmental stewardship, public health and social well-being is changing our approach to weed science. As a consequence, there is a growing interest in studying weeds and invasive plants within the context of the ecosystem they live. Three articles recently published in the most prestigious scientific journals ("Science" and the "Proceeding of the National Academy of Science") provide a fresh perspective to the study of weeds and invasive species.
For many years, it has been assumed that the main reason for an introduced plant being invasive is that it got placed in a new and favorable environment where there are no insects, parasites, diseases and competitors that suppress its growth. Plant ecologists reasoned that in this new environment plants should have higher chances of reproductive success and growth. This assertion has been challenged in an article recently published in the Aug. 20 issue of "Science." The article summarizes several studies and provides a novel perspective on our understanding of weeds and invasive plants. It proposes that once an organism escapes its natural enemies, it no longer needs the defense mechanisms it had developed to be protected against them. Since these defense mechanisms require precious energy and resources, the intruder may allocate its reserves towards being more competitive. For a plant, this change in allocation strategies translates into having a larger size, a faster growth or a greater reproductive capacity. Therefore, we should blame the circumstances, not just the individuals, since the absence of regulatory mechanisms make interlopers become invasive. Although the evidences are not conclusive, the article describe several species including Chinese tallow tree and St. John's Wort, where a reduction in defensive mechanisms resulted in increased invasive nature.
In a study by Baucom and Mauricion in the "Proceeding of the National Academy of Science," they address the consequences of repeated glyphosate applications on weed populations. The authors evaluated the tall morning glory, a noxious weed affecting the agricultural fields of the southeastern United States. Farmers are reporting that populations of the tall morning glory have begun to develop tolerance to glyphosate as they are able to compensate for the damaging effect of herbicide applications. The authors collected seeds from several fields that have been sprayed consistently with Roundup for eight years. After growing the seeds in a common garden and spraying the seedlings with glyphosate, they evaluated damage and mortality. Baucom and Mauricion observed that there were genetic variations for herbicide tolerance among the different tall morning glory populations. They further concluded that repeated glyphosate applications create a strong pressure to select tolerant biotypes. However, in the absence of glyphosate, tolerant genotypes produced fewer seeds than susceptible genotypes. Thus, they suggest that there is a significant cost of being tolerant, but this cost is overridden by the selection pressure imposed by glyphosate.
Finally, in another study also published in the Proceeding of the National Academy of Science, Kumar and collaborators compared leaf senescence and disease tolerance in tomato plants growing under different weed management scenarios. In one case, weeds were controlled using a cover crop of hairy vetch. In the other case, weed control was achieved by a black polyethylene cover. Using several genetic tools, they assessed the mechanisms underlying beneficial aspects of legume cover crops. Specifically, the authors demonstrated that tomato plants responded to the presence of cover crops by expressing specific classes of genes. The net result of this differential gene expression was that tomato plants growing with the hairy vetch cover crop lived longer, had delayed leaf senescence and were more tolerant to diseases.
These studies are excellent examples on how weed science is progressing towards an increased understanding on the mechanisms that make a plant to become a weed. Certainly the picture that is emerging is complicated. Nonetheless, an improved knowledge on weed biology and ecology will allows us to develop agricultural practices that will secure the long-term sustainability of our agricultural setting, rangelands and natural systems.
Disclosure. Common chemical and trade names are used in this publication for clarity by the reader. Inclusion of a common chemical or trade name does not imply endorsement of that particular product or brand of herbicide and exclusion does not imply non-approval.
Categories: Weed, Integrated Weed Management, Herbicide Resistance
Date: 04/14/2005