Conclusions

Since phenolic allelochemicals are broadly found throughout the plant kingdom, their concentration at a point in time and differences in the sensitivity of receiving species are keys to their allelopathic action. At very low concentrations they may be stimulatory, whereas higher concentrations inhibit functions in the receiving species. Whether by chance or selection, derivatives of cinnamic and benozic acid, coumarins, and various polyphenolic compounds like the tannins have multiple target sites whereby they alter the physiology of plants or microorganisms. There is sufficient evidence to conclude that these compounds first act by altering permeability and protein functions of the cell membrane. Subsequently, they impact major physiological processes that normally act to maintain a favorable ion balance, water content, photosynthetic rate, respiratory metabolism, and hormone regulation. One of the consistent effects of phenolic acids on higher plants is water stress that reduces cell expansion and causes more stomatal closure and a reduction in photosynthesis. However, we conclude that it is the general cytotoxicity of the phenolics and the resultant loss of efficiency in several cellular functions that reduce growth. In contrast to the phenolic acids, the mode of action of allelopathic flavonoids is less clear. Flavonoids inhibit electron transport in mitochondria and chloroplasts, yet other physiological effects cannot be ruled out.

A holistic view regarding the mode of action of phenolic allelochemicals recognizes the effects of these compounds as one of several stresses on plants. Complexes of the individual phenolic compounds become the effective unit causing allelopathy. In summary, explanations of the mode of action of phenolic allelochemicals must take into account that these compounds act in concert, the different compounds vary in toxicity but they have similarities in their mechanisms of action, and they all appear to disrupt cellular functions at multiple target sites.

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