Assays for allelopathic interactions among terrestrial plants

Abstract

Plants affect growth of each other through several mechanisms and their combined effect is known as “interference”. Resource competition is major interference mechanism active in nature. In competitive interactions, a plant takes up one or more resources that are also needed for growth of neighbouring plants. Another interference mechanism is allelopathy which comprises the effect of one plant (a donor plant) on other plants (receptor plants) through the release of organic compounds to the environment. These compounds may have a direct action after uptake by receptor plants, acting alone or in mixture with other compounds but they also may have an indirect effect changing the environmental conditions, with negative or positive impact on growth of receptor plants. Allelopathy has been implicated in several interactions among terrestrial plants and has been observed in both natural and managed terrestrial ecosystems, although conclusive proofs are often lacking. Obstacles to prove allelopathy arise from difficulties to simulate natural environmental conditions in laboratory and greenhouse experiments and problems to distinguish other interference mechanisms occurring in natural settings. Although no protocol is available to prove allelopathy, the following criteria have broad acceptance: (i). Evidences suggesting an allelopathic phenomenon must be recognized in the field and alternative explanations to allelopathy must be empirically discarded. Field studies indicate which factors should be considered for further design of assays. Laboratory and greenhouse assays allow analyzing how different components (i.e. microbial activity, soil properties, and climatic parameters) may affect a whole allelopathic process. (ii). Mode of release of allelochemicals from donor plants must be detected (i.e. leachates from aerial parts, root exudates, volatilization from foliage or litter decomposition). Then, allelochemicals involved should be isolated and identified. Assays should consider soil or air, where allelochemcals are released. (iii). Crop plants as radish (Raphanus sativus L.) and lettuce (Lactuca sativa L.) has been broadly used as receptor plants in laboratory and greenhouse assays. However, plant species assayed should be those suspected to be receptor plants in the field. Assays also should consider densities of receptor and donor plants usually found in the field. Different densities of donor plants can determine accumulation of allelochemicals into the soil. Receptor plants exposed to allelochemicals may be inhibited in their growth at low densities, while they can be stimulated or unaffected at high densities (Weidenhamer, 1996). (iv). Since microorganisms often participate in allelopathic interactions, assays to elucidate microbial involvement should be included (Sampietro and Vattuone, 2006a). (v). Appropriate growth parameters and/or physiological processes must be selected for assay. Germination and radicle and/or shoot elongation of receptor plants are commonly measured in both greenhouse and laboratory experiments, radicle being the most sensitive to allelochemicals. Assays involving radicle elongation should be performed with pre-germinated seeds. When germination and radicle elongation are measured in the same assay, response of radicle elongation may be confounded with delays in seed germination. Assessment of the effect of allelochemicals/plant leachates or root exudates on physiological processes of receptor plants may provide insight about how allelopathic substances act on plant growth (Sampietro et al., 2007). Osmolality and pH of plant leachates/root exudates sometimes affect growth of receptor plant in both laboratory and greenhouse experiments. The effect of these parameters should be evaluated on plant growth through the development of proper assays. In this chapter, we propose several assays and criteria to study allelopathic interactions among terrestrial plants. They were selected to provide answers about the biological-ecological aspects of allelopathic interactions and to hold the basis for further studies on isolation and identification of allelochemicals.