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.