Germination of Cocklebur Seeds: Interactions Between Gibberellic Acid, Benzyladenine, Thiourea, KNO₃ and Gaseous Factors

Abstract

The germination response of small, upper seeds of cocklebur (Xanthium pensylvanicum Wallr.) was examined with respect to the germination stimulants oxygen, CO₂, ethylene, gibberellic acid, benzyladenine, thiourea and KNO₃. Thiourea, benzyladenine, ethylene and oxygen-enriched air (50% O₂) stimulated germination, but gibberellic acid was only slightly effective and KNO₃ had little effect. In contact with thiourea or CO₂, seeds usually germinated by extrusion of the radicle without any change of germination pattern, but while in O₂-enriched air the seed coat was predominantly ruptured at the cotyledon end. In about half of the seeds germinated with ethylene, benzyladenine and gibberellic acid, the seed coat split at the cotyledon side.

Trapping of endogenously evolved ethylene and CO₂ from the ambient atmosphere did not affect the actions of benzyladenine and gibberellic acid, but the action of thiourea was significantly reduced by trapping CO₂. Except for thiourea, with which the CO₂ production was enhanced, benzyladenine, gibberellic acid and KNO₃ did not increase CO₂ and ethylene production from the seed in the germination period. The maximum germination percentage was obtained by a combi- nation of CO₂, ethylene, gibberellic acid and benzyladenine, but the interaction of gibberellic acid and benzyladenine was not significant. In contrast, the interaction of gibberellic acid and ethylene was very effective, and further addition of CO₂ to this combination hastened the germination in air and also facilitated it under the semi-anaerobiosis assumed to exist in a natural underground habitat, although the effect of benzyladenine alone was nearly completely suppressed by semi- anaerobiosis. Thus the particular importance of CO₂, ethylene and gibberellic acid in the normal germination regulation of this seed is suggested.