Dormancy and Impotency of Cocklebur Seeds. VI. Growth and Ethylene Production of Cotyledon Segments in Response to Oxygen

Abstract

Cotyledon tissue excised from dormant or non-dormant large cocklebur (Xanthium pensylvanicum Wallr.) seeds were examined for growth responses to CO2 or ethylene and for ethylene production under different oxygen tensions. Regardless of the dormancy status, cotyledons responded to CO2 and ethylene in a very similar manner to axes: the maximum growth response to CO2 and ethylene was obtained in quasi-anaerobioses in the case of the non-dormant cotyledons, but in increased oxygen tensions in the case of the dormant ones. A cooperative action of CO2 and ethylene was prominent in the presence of oxygen in suitable amounts. CO2 began to exert an effect first after water imbibition, then ethylene and lastly oxygen was needed to allow normal cotyledon growth. As the ethylene-producing system of cotyledons, active in quasi-anaerobioses, differed from that of axes in requirement for oxygen, the enhancement of ethylene production by exogenous CO2 was greater under reduced rather than increased oxygen tensions. Finally, the possible mechanism of cocklebur seed germination in subterranean habitats was discussed from the likely involvement of these three gaseous factors in axial and cotyledonary growth.