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Alteration of Growth, Exudation Rate, and Endogenous Hormone Profiles in Mechanically Dwarfed Sunflower1


Growth in stem length, shoot fresh weight, and leaf surface area of sunflower (Helianthus annuus L.) plants was inhibited by periodic shaking (seismo-stress) or rubbing (thigmo-stress), and by drought-stress treatments. Mechanical- and drought-stress treatment of intact plants reduced xylem exudation from detopped plants. This reduction was accompanied by a reduction in root dry weight only in the case of the drought-stress treatment. Exudate from thigmo-stressed plants contained a broad band of extractable inhibitors some of which co-chromatographed with abscisic acid (ABA). ABA-like substances were not detected in exudate from tall or dwarf controls. Enhanced cytokinin (CK)-like substances were detected in exudate collected from seismo-stressed plants. Shoot tip extracts from thigmostressed plants contained no detectable gibberellin (GA)-like substances, whereas those extracts from unstressed controls contained multiple zones of GA-like activity. Extracts of plant parts most disturbed by a given mechanical treatment yielded no detectable GA-like substances. This study provides evidence that, in addition to auxin and ethylene, other endogenous hormones, particularly GA-like substances, are influenced by mechanical stress in sunflower.