One commonly observed effect of phytotoxic compounds is the inhibition or delay of germination of sensitive seeds. Mustard (Sinapis alba L.) seeds were incubated with aqueous extracts of sunflower (Helianthus annuus L.) leaves. Although sunflower phytotoxins did not influence seed viability, extracts completely inhibited seed germination. Inhibition of germination was associated with alterations in reserve mobilization and generation of energy in the catabolic phase of germination. Degradation of lipids was suppressed by sunflower foliar extracts resulting in insufficient carbohydrate supply. The lack of respiratory substrates and decrease in energy (ATP) generation resulted in suppression of the anabolic phase of seed germination and ultimately growth inhibition.
At concentrations in which they occur on the plant surface and retard mitosis, coumarin and xanthotoxin lowered uptake of oxygen (by 60 and 30%, respectively) by meristematic cells ofAllium cepa root tips. They caused changes in the structure of the mitochondrial matrix to become dense, and protrusions of mitochondrial membranes were visible parallelling their hypertrophy, indicating alteration in the structure and physiology of these organelles. Coumarin and, to a lesser extent, xanthotoxin increased succinate dehydrogenase production in mitochondria and also in the cytoplasm, indicating changes in membrane permeability. Changes in oxygen uptake and mitochondrial structure, in addition to the retardation of mitosis, may be the reason these compounds act as allelochemicals after they have been removed from the plant surface and reach the root meristem.
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