Disappearance of IAA in the presence of tissues of sunflowers infected by Plasmopara halstedii

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Abstract
Stem elongation of sunflower plants inoculated at the two-leaf stage through apical buds with Plasmopara halstedii was greatly inhibited. Systemically infected plants did not show normal phototropic and negative-geotropic responses. There was a positive correlation between stunting of infected plants and ability of stem slices to remove indoleacetic acid (IAA) from test solutions. IAA disappearance in the presence of stem slices of infected plants was also highly correlated with area of mildew symptoms on leaves. Although IAA disappeared in the presence of all diseased tissues, leaf tissues were least active. No IAA disappeared in the presence of sporangia of the pathogen. These results explain reduced levels of IAA observed in mildew-infected sunflowers in earlier studies and may account for the characteristic stunting of systemically infected plants.

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