Plant Evolution & Ecology (4): Soil
Winder, R.S. and Macey, D.E. 2001. The influence of edaphic and competitve factors on productivity of marsh reed grass (Calamagrostis canadensis) in a cooperative pathosystem. Canada Journal of Botany 79: 805-814.
Weed pathogens refer to virus, bacteria, or fungus that cause disease to their hosts, e.g. weeds or crops, and thus pathogens can be used to be as herbicides. However, such a use is greatly limited by some ecological and environmental factors. For example, some weeds may have developed adaptive shapes or genetic resistance to these pathogens.
Marsh reed grass, Calamagrostis canadensis, is a weedy species that can n typically damage the seedlings of coniferous trees in boreal forests. The tiny seeds of C.canadensis are dispersedprevailed by wind in late summer, and seedlings grow fast with spreading roots. The grass forms a thatch that damages conifer seedlings and is difficult to control. If C.canadensis can be controlled by pathogens at the earliest stage, then conifers will have enough time to grow.
Previous studies show that the cooperative infections of two kinds of pathogens, such as fungi and rhizobacteria (i.e. bacteria living on roots), can result in more damage to marsh reed grass, but the amount of damage they do is influenced by other factors, such as soil fertility or competition. For instance, high Nitrogen availability may improve the growth of plants and thus compensate for the damage or disease caused by pathogens.
The study examined the influence of edaphic and competitive factors on the productivity of C.canadensis exposed to the FPCP pathogens (a fungus, F.avenaceum combined with a bacteria, P.seudomonas) under greenhouse conditions. The experiment focuses on the influence of FPCP on leaves, because the accumulation of leaves makes heavy snow difficult to thaw so that heavy snow causes straw to suppress conifer seedlings. The specific objectives of the study were to determine the interactive effects of autoallelopathy, competition, and nutrient availability on the efficacy of the FPCP for suppressing growth in marsh reed grass seedlings.
*Autoallelopathy: a substancethat plants produce by themselves, such as toxin, and the substance can hurt themselves.
Materials and methods
Grass seeds were collected in late summer, and sown the following spring in plastic pots with coal-like thingsto hold nutrients and plastic dishes to contain water and released nutrients.
The pathogen complex was created using F.avenaceumisolated from wheat, and Pseudomonas isolated fromthe roots of C.canadensis. A group of treatments were inoculated with the pathogen complex, and another group uninoculated are control experiments to compare results.
The study tested the responses of the two group of treatments to individual factor of autoallelopathy(i.e. straw extract), competition, and soil nutrients, and to combinations of any two of the three factors.
Autoallelopathy versus competition
The FPCP complex damaged leaves and inhibited shoot growth under all conditions. Competition had much clearer inhibition on growth of uninoculated plants than on inoculated ones, and increased the leaf damage of inoculated plants; the straw extract caused no change in inoculated plants, and increased leaf damage; a three-way interaction between inoculation, competition and straw extract caused a complicated response: 1) low concentrations of straw extract reduced the inhibition of growth by competition, but the inhibition was enhanced by higher concentrations. 2) increased concentrations of straw extract caused increased damage of plants with highest competition, but the highest increase in damage happened with intermediate concentration.
Autoallelopathy versus N, P, and K
Independently, N addition greatly enhanced the growth of seedlings without pathogen disease, and P, K did not have obvious effects on growth. In the case of inoculation, N, P and K only caused slight changesin growth. Interaction between inoculation, extract, N, P and K showed that extract negatively influenced growth; P-addition? greatly enhanced leaf damage, and P-addition combined with extract doubled leaf-damage.
Competition versus nutrients
Without pathogens, competition inhibited the growth, but nutrient fertilization increased the growth. With pathogens, the inhibition by competition was modified; plants with the lowest competition were more responsive to fertilizer than those with the highest competition, and the degree of the leaf damage decreased as the fertilizers and competition levels increased.
Firstly, in the case of low nutrients, shoot growth increased with leaf damage, which means that the growth of inoculated plants was unaffected by changes of N, P and K. And therefore, the increased leaf damage by P and K imbalances did not limit the early seedling growth.
Secondly, the interaction between straw extract and competition greatly decreased the growth of plants. In addition, straw extract also improved the positive effect of soil nutrients on shoot growth, probably because of its own nutrient content. Therefore, the FPCP pathogens could be more effective where more straw extract has accumulated.
Thirdly, increasing competition decreased seedling growth, and interaction between competition and straw extract improved the decrease, which indicates that densely populated patches of marsh reed grass are already inhibited by both competition and autoallelopathy. Addition of the FPCP pathogens caused further declines in growth in the presence of competition and straw extract, and could therefore be more effective in denser populations, i.e. grass with more competition.
In general, since higher soil nutrients, especially N, can greatly improve the plant growth in the presence of FPCP, fertile soils greatly limit the use of biological control agents, e.g. the FPCP pathogens, as herbicides. The speed of leaf damage is therefore a key determinant of success of the FPCP pathogens. If the speed leaf damage is greater than the speed of shoot growth, the resources produced by photosynthesis will suffer, which will lead to more damage of roots and shoots, i.e. the success of the FPCP pathogens will increase. On the other hand, if the speed of shoot growth is greater than leaf damage, more resources will be allocated to roots and shoots growth, which actually decrease the success of the FPCP. The results indicated that interactions between the three factors only suppress the growth of marsh reed grass, but do not kill it. Such non-lethal suppression plays an important role in making conifer seedlings free to grow, while limiting the negative effects of the ecosystem losing the grass entirely, since marsh reed grass can still provide beneficial effects in the forest.
Suggestions on future studies
Some other factors except autoallelopathy, competition and soil nutrients, should be studied in the future: 1) light can change resources allocation in above- and below-ground tissues; 2) Calcium is another nutrient in soil, and calcium affects the growth of one competitor, C.cpigejos, of the marsh reed grass, and other pathogens, such as some fungi; 3) temperature, humidity, and other environmental factors unrelated to soil, should be considered.
Plant pathogens (i.e. some virus, fungi, or bacteria) can work as herbicides in the field, but their efficacy is limited by some environmental factors. The authors examined the influence of soil conditions and competition on marsh reed grass exposed to pathogens. They grew seeds of the grass in treatments with or without pathogensand estimated the effects of three additional factors: straw extract (i.e. extract from the grass) addition vs. competition; straw extract vs. nutrient-addition; and competition vs. nutrient-addition. They found that high concentration of straw extract and high levels of competition improved the role of pathogens as herbicides, but high nutrients levels tended to decrease such a role. They concluded that these pathogens could be more effective where more extract has accumulated or in denser populations of the grass, and that these pathogens only suppress the growth of the grass, but not kill it..