J Chem Ecol DOI 10.1007/s10886-014-0464-7

COMMENTARY: REFLECTIONS ON 40 YEARS

Allelopathy - The Solution is Indirect Ren Sen Zeng

# Springer Science+Business Media New York 2014 Allelopathy is one important branch of chemical ecology that studies chemical mediated plant-plant interactions. It has been extensively studied over the past four decades. In fact, in the first two issue of the Journal of Chemical Ecology, despite a predominance of pheromone papers, three addressed this phenomenon. Definition of allelopathy (any direct and indirect effects of one plant, including microorganism) on the survival, growth, and reproduction of another plant through the release of natural products (named allelochemicals) into the environment still holds today. Although the ecological phenomenon has been known and used in agriculture since ancient times, its recognition by other chemical ecologists and use in modern agriculture has been limited. The complexity and interacting nature of allelopathic phenomena make it difficult to demonstrate its role in plant interactions and community organization. As one working on both plant allelopathy and chemical-mediated plant-insect interactions, I affirm that allelopathy is much more difficult to demonstrate. However its importance in natural and agricultural ecosystems and its potential application in agriculture cannot be ignored. There is no denying that allelopathy plays a prominent role in structuring plant communities, maintaining biodiversity, and determining agricultural and forest productivity. Its application helps find environmentally friendly alternatives of weed control in agriculture, and solutions to replant problems and soil sickness in horticulture. It is a well-known fact that plants produce thousands of secondary metabolites and these allelochemicals are released into the environment by root exudation, leaching, volatilization, and residue decomposition. In nature, all plant-produced compounds return finally to the environment. The question is what functions do these chemicals have in plants’ interactions with their neighbors. Hundreds of plants have been shown to have phytotoxic effects in the last five decades. However, the majority of such studies lack convincing evidence to show the natural existence of allelopathy. Plant extracts and isolated pure compounds often show strong phytotoxicity, but they are not necessarily responsible for allelopathy. Such compounds may be

R. S. Zeng (*) College of Crop Science, Fujian Agriculture and Forestry University, Fuzhou 350002, People’s Republic of China e-mail: [email protected]

either immediately diluted in soil solution, absorbed by soil particles, or rapidly degraded by soil microbes. They may not reach sufficient concentration and duration in soils to display direct inhibitory effects on their neighbors. It follows that direct inhibitory effects of plant allelochemicals, on which most studies concentrate, may not be so important, and raises the likely possibility that the indirect mediator effects of allelochemicals on plant interactions are more important. In many cases, the study of such effects has been underestimated, and even ignored. One indirect way that allelochemicals from a plant may act in the natural environment is to change the availability of soil nutrients. The soil environment can be modified through root exudates and litters that affect soil structure, biochemical properties, and nutrient mobilization. The release of organic acids from faba bean (Vicia faba L.) roots, for example, enhances phosphorus availability in low-phosphorus soil and thereby improves the growth of intercropping maize (Zea mays L.) (Li et al. 2007). Tannins and other plant phenolics increase microbial activity, often resulting in immobilization of nitrogen, phosphorus, and other nutrients. Other effects on microbial activity are common. Some invasive plants produce toxins that disrupt mutualistic associations between native plants and belowground arbuscular mycorrhizal fungi, leading to growth inhibition of the native plants (Stinson et al. 2006). Soil microbes are targets and important mediators of plant allelopathy and they are recognized as an essential predictor of the outcome of allelopathic interactions between plants. Continuous monocropping often results in severely poor growth and a crop yield decline known as soil sickness. Autotoxicity has been believed to be responsible for the continuous agricultural obstacle faced by many crops e.g., Rehmannia glutinosa, Panax ginseng, Pnn notoginseng, Cucumis sativus, etc. However, no highly phytotoxic compounds have been found in these crops. In most cases, phenolics with only mild phytotoxicity have been claimed implausibly to be responsible for the observed growth stunting. This has raised a debate about the role of autotoxins as cropping obstacles. Recent studies, however, demonstrate that accumulating autotoxins in soils can recruit detrimental microbes, which indirectly may lead to poor growth of the same crop in a subsequent season (Huang et al. 2013).

J Chem Ecol We have reasons now to believe that indirect effects of allelopathy as a mediator of plant-plant interactions are more important than the direct effects of an inhibitor (s). Our understanding will rapidly increase as these indirect effects are unraveled. This means not only better appropriate experiment design, but also the use of sophisticated biochemical methodology and the incorporation of molecular biology and genetic modification studies. Such research on the indirect effects must be conducted and will promote our understanding of allelopathy beyond phytotoxicity in an ecosystem and evolutionary context. I hope that the Journal of Chemical Ecology will become the major forum for accelerating our understanding of the indirect effects of allelopathy.

References Huang LF, Song LX, Xia XJ, Mao WH, Shi K, Zhou YH, Yu JQ (2013) Plant-soil feedbacks and soil sickness: from mechanisms to application in agriculture. J Chem Ecol 39:232–242 Li L, Li SM, Su JH, Zho LL, Ba XG, Zhan HG, Zhang FS (2007) Diversity enhances agricultural productivity via rhizosphere phosphorus facilitation on phosphorus-deficient soils. Proc Natl Acad Sci U S A 104:11192–11196 Stinson KA, Campbell SA, Powell JR, Wolfe BE, Callaway RM, Thelen GC, Hallett SG, Prati D, Klironomos JN (2006) Invasive plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms. PLoS Biol 4(5):e140. doi:10.1371/journal.pbio. 0040140

Allelopathy - the solution is indirect.

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