As oxygen-breathing, food-eating critters, it's safe to say that humans depend on plants for staying alive. But, there's one large and growing group of leafy things that we simply can't stand.
Weeds.
But there's more to those tricky little weeds.
Nature isn't above playing dirty, and there are several weeds that are just as dirty cheaters. Vulpia myuros, a species of invasive grass, pumps chemicals into the soils it grows (An et al., 2000). These chemicals aren't just some benign plant product we can write off, they have an insidious effect on this grass's neighbors.
This grass is salting its own earth.
V. myuros has evolved to make its own phytotoxin, or plant poison, to sabotage the competition. Much like the allegation of Tonya Harding hiring someone to break the leg of a rival ice skater, this grass uses what ecologists call "interference competition, where a plant going out of its way to keep competing plants from using its resources (Weston & Duke, 2003). One might think that the plant would succumb to its own poison, but this plant and other individuals of its species have evolved to be resistant. The idea is that after generations of V. myuros have lived and died in a particular location, that soil will only be suitable for other V. myuros (An et al., 2000). This strategy of filling the soils with certain chemicals to alter how other plants and the microbes they associate with (Cheng & Cheng, 2015) use is called allelopathy (Li et al., 2010), and it's surprisingly common in the plant world, especially with weeds and invasives.
Referencing the Research
Referencing the Research
Last post, we discussed phenolic compounds, a broad class of chemicals synthesized by plants, and how they can be used to attract beneficial microbes and defend themselves against herbivores. Phenolic compounds also contribute to a significant portion of allelopathic chemicals and have a multitude of nasty little tricks they can play on rival plants. In fact, most allelopathic compounds are actually phenolic chemicals (Li et al., 1993)! The effects of these attacks include inhibiting germination for seeds, outright reducing plant mass, and lowering growth rate, and these allelopathic phenolics are employed by even the most innocent-seeming plants (Li et al., 2010; Yu et al., 2003).
Despite sunflowers' innocuous appearance, they are capable of some potent allelopathy and can use phenolic compounds to sabotage their local plant communities. In Pakistan, sunflowers are an important crop that used for their seeds and oils and is often grown near other staple crops such as wheat (Ghafar et al., 2000). As research developed, however, scientists began to note how sunflowers play dirty with allelopathic chemicals and wanted to see if they had an effect on wheat growth. To test this, they grew wheat plants using water treated with varying concentration of dried, crushed sunflowers.
This figure (Ghafar et al., 2000) from these researchers' study categorizes the different morphological characteristics of the wheat plants into different significance groups--so if two measurements are in the "a" group they wouldn't be significantly different, but if one was in "a" and one was in "b" there would be a statistically significant difference between those two measurements. According to this figure, germination, shoot length, and root length significantly decreased with the treatment, which in this case refers to the powdered sunflower mixed with the water the wheat plants were given. The data shown in this figure suggests that sunflowers contain allelopathic compounds that are detrimental to wheat and likely many other plants.
Another set of researchers was interested in how allelopathic compounds affected different morphological characteristics of soybeans (Patterson, 1981). Soybeans, much like wheat, is a staple crop, especially for animal and food industries and by answering which compounds affect it the most negatively, scientists can determine what sort of weed control is necessary! These researchers evaluated which allelopathic compounds affected soybean morphology in nearly the same way as the previously referenced study--in that they exposed plants to varying concentrations of allelopathic solutions.
This figure (Patterson, 1981) displays how 10 different treatments affected total dry weight, leaf area, plant height, and leaves present of soybeans. In the same way as the last figure, the letters beside each value determine significance categories. While some allelopathic compounds were more potent than others in negatively impacting these soybean traits, some numbers here particularly stand out. Regardless of the allelopathic chemical tested, low concentrations of treatment had little effect on plant morphology, but higher concentrations had varying effects, such as the effect of the higher-concentration treatments of vanillic acid and coumaric acid, whose soybeans had nearly half the total dry weight of the control. While moderate levels of allelopathic compounds seem to be tolerable by soybeans, environments rife with plant chemical warfare aren't conducive to soybean growth.
Implications and Theory
These studies demonstrate just how important allelopathy, and consequently, phenolic compounds, are to plant growth and diversity, especially in an agricultural setting. Because producers want to optimize the growth of their crops, allelopathic relationships between crops, weeds, and even other crops should be considered before planting and in weed control. Taking this in stride, humans have devised an ingenious approach to manipulating allelopathy in agriculture that you're probably familiar with:
Many plants, such as rice, naturally produce chemicals that can be used to suppress neighboring weeds, which they can produce through their roots and through their litter (Weston & Duke, 2003). This allelopathic property might be used to explain the viability of certain crops and why we decided to domesticate them anyway. For instance, if rice didn't naturally fend off weeds, would ancient people have bothered to cultivate it at all?
While we haven't known about allelopathy for very long and scientists are still trying to understand the mechanisms and evolutionary implications behind it, it's easy to say that there's an evolutionary basis for it. It makes sense for plants to want to do whatever they can to nab the resources they can, and passively producing allelopathic chemicals to inhibit the growth of other species is a strong way to pave the success of individuals and species groups.
Another point to make is that allelopathy, to an extent, could be present in species that aren't harmed as much by intraspecific competition, competition between individuals of the same species, than interspecific competition, or competition between individuals of different species. This would make sense for communities that are limited in their ability to reproduce, as close-knit populations of plants could benefit from allelopathy to ensure that they're able to bear viable offspring.
Conclusion
As human populations rise, we're going to increasingly depend on controlling our environment to support ourselves. By studying and manipulating the allelopathy of crops and the weeds that plague them, we can learn how to best propagate the plants that we depend on while excluding the ones that we'd rather not have to deal with. Implementing optimal growth periods will depend on both farmers and scientists collaborating by sharing knowledge of allelopathic relationships and considering the evolutionary development and resource acquisition strategies of the plants common (introduced and invasive) in agricultural environments.
TL;DR
Plants engage in allelopathy, a form of interference competition wherein they produce chemicals (mostly phenolic compounds) that are dispersed into the soil, to sabotage rivals in their nutrient uptake. Allelopathy is present in many plants, including crops and the weeds that compete with them, and can affect plants in several forms, including reducing biomass and growth rate. Allelopathy can be explained evolutionarily and likely has evolutionary limitations. Humans have likely exploited allelopathy since the domestication of plants and continue to use it for agriculture as agricultural demand increases.
References:
Like sunflowers!
This figure (Ghafar et al., 2000) from these researchers' study categorizes the different morphological characteristics of the wheat plants into different significance groups--so if two measurements are in the "a" group they wouldn't be significantly different, but if one was in "a" and one was in "b" there would be a statistically significant difference between those two measurements. According to this figure, germination, shoot length, and root length significantly decreased with the treatment, which in this case refers to the powdered sunflower mixed with the water the wheat plants were given. The data shown in this figure suggests that sunflowers contain allelopathic compounds that are detrimental to wheat and likely many other plants.
Another set of researchers was interested in how allelopathic compounds affected different morphological characteristics of soybeans (Patterson, 1981). Soybeans, much like wheat, is a staple crop, especially for animal and food industries and by answering which compounds affect it the most negatively, scientists can determine what sort of weed control is necessary! These researchers evaluated which allelopathic compounds affected soybean morphology in nearly the same way as the previously referenced study--in that they exposed plants to varying concentrations of allelopathic solutions.
This figure (Patterson, 1981) displays how 10 different treatments affected total dry weight, leaf area, plant height, and leaves present of soybeans. In the same way as the last figure, the letters beside each value determine significance categories. While some allelopathic compounds were more potent than others in negatively impacting these soybean traits, some numbers here particularly stand out. Regardless of the allelopathic chemical tested, low concentrations of treatment had little effect on plant morphology, but higher concentrations had varying effects, such as the effect of the higher-concentration treatments of vanillic acid and coumaric acid, whose soybeans had nearly half the total dry weight of the control. While moderate levels of allelopathic compounds seem to be tolerable by soybeans, environments rife with plant chemical warfare aren't conducive to soybean growth.
Implications and Theory
These studies demonstrate just how important allelopathy, and consequently, phenolic compounds, are to plant growth and diversity, especially in an agricultural setting. Because producers want to optimize the growth of their crops, allelopathic relationships between crops, weeds, and even other crops should be considered before planting and in weed control. Taking this in stride, humans have devised an ingenious approach to manipulating allelopathy in agriculture that you're probably familiar with:
Naturally allelopathic plants!
Many plants, such as rice, naturally produce chemicals that can be used to suppress neighboring weeds, which they can produce through their roots and through their litter (Weston & Duke, 2003). This allelopathic property might be used to explain the viability of certain crops and why we decided to domesticate them anyway. For instance, if rice didn't naturally fend off weeds, would ancient people have bothered to cultivate it at all?
While we haven't known about allelopathy for very long and scientists are still trying to understand the mechanisms and evolutionary implications behind it, it's easy to say that there's an evolutionary basis for it. It makes sense for plants to want to do whatever they can to nab the resources they can, and passively producing allelopathic chemicals to inhibit the growth of other species is a strong way to pave the success of individuals and species groups.
Another point to make is that allelopathy, to an extent, could be present in species that aren't harmed as much by intraspecific competition, competition between individuals of the same species, than interspecific competition, or competition between individuals of different species. This would make sense for communities that are limited in their ability to reproduce, as close-knit populations of plants could benefit from allelopathy to ensure that they're able to bear viable offspring.
Conclusion
As human populations rise, we're going to increasingly depend on controlling our environment to support ourselves. By studying and manipulating the allelopathy of crops and the weeds that plague them, we can learn how to best propagate the plants that we depend on while excluding the ones that we'd rather not have to deal with. Implementing optimal growth periods will depend on both farmers and scientists collaborating by sharing knowledge of allelopathic relationships and considering the evolutionary development and resource acquisition strategies of the plants common (introduced and invasive) in agricultural environments.
TL;DR
Plants engage in allelopathy, a form of interference competition wherein they produce chemicals (mostly phenolic compounds) that are dispersed into the soil, to sabotage rivals in their nutrient uptake. Allelopathy is present in many plants, including crops and the weeds that compete with them, and can affect plants in several forms, including reducing biomass and growth rate. Allelopathy can be explained evolutionarily and likely has evolutionary limitations. Humans have likely exploited allelopathy since the domestication of plants and continue to use it for agriculture as agricultural demand increases.
References:
An, M., Haig, T., and Pratley, J. E. 2000b. Phytotoxicity of vulpia residues. II. Separation, identification, and quantitation of allelochemicals from Vulpia myuros. J.Chem. Ecol.26, 1465–1476.
Cheng, Z.H. Cheng. Research progress on the use of plant allelopathy in agriculture and the physiological and ecological mechanisms of allelopathy. Frontiers in Plant Science, 6 (2015), p. 1020
Ghafar, A.; Saleem, B.; Qureshi, M.J. Allelopathic effects of sunflower on germination and seedling growth of wheat. Pak. Pak. J. Biol. Sci. 2000, 3, 1301–1302.
Li, H.H.; Inoue, M.; Nishimura, H; Mizutani, J.; Tsuzuki, E. Interaction of trans-cinnamic acid, its related phenolic allelochemicals, and abscisic-acid in seedling growth and seed-germination of lettuce. J. Chem. Ecol. 1993, 19, 1775–1787.
Li, Z. H., Wang, Q., Ruan, X., Pan, C. D., and Jiang, D. A. (2010). Phenolics and plant allelopathy. Molecules 15, 8933–8952.
Patterson, D.T. Effects of allelopathic chemicals on growth and physiological response of soybean(Glycine max). Weed Sci. 1981, 29, 53–58.
Leslie A. Weston & Stephen O. Duke (2003) Weed and Crop Allelopathy, Critical Reviews in Plant Sciences, 22:3-4, 367-389,
Yu, J.Q.; Ye, S.F.; Zhang, M.F.; Hu, W.H. Effects of root exudates and aqueous root extracts of cucumber (Cucumis sativus) and allelochemicals, on photosynthesis and antioxidant enzymes in cucumber. Biochem. Syst. Ecol. 2003, 31, 129–139.
