Dormancy and Impotency of Cocklebur Seeds. V. Growth and Ethylene Production of Axial Segments in Response to Oxygen

Abstract

Growth responses to exogenous CO₂ or ethylene and the production of CO₂ or ethylene in embryonic axes excised from dormant or non-dormant seeds of cocklebur (Xanthium pensylvanicum Wallr.) were examined in relation to oxygen tension. There were two ethylene-producing systems in the axes, one anaerobic and one aerobic. The former was active in dormant seeds and the latter was active in non-dormant seeds. The axes from non-dormant but impotent small seeds, incapable of germinating under ordinary conditions, showed higher activities for both systems. However, there was no qualitative difference in the oxygen response as to CO₂ output between the dormant and non-dormant axes, but CO₂ output under aerobic conditions was greater with non-dormant seeds.

Regardless of dormancy status, growth stimulation of axes by CO₂ occurred when it was applied during the beginning of incubation and in atmospheres with oxygen concentrations insufficient to permit normal axial growth. Nevertheless, the stimulation of ethylene production by CO₂ was parallel with the increase of oxygen tension, suggesting that the CO₂-stimulated ethylene production could not be the result of CO₂-stimulated axial growth.

The growth response was most sensitive to ethylene after the CO₂-sensitive period and before an oxygen-requiring period. Similarly to the action of CO₂, the most striking effect of ethylene on the growth of non-dormant axes was obtained in oxygen-deficient atmospheres. However, the synergistic interaction between CO*2 and ethylene occurred only in aerobic conditions. Unlike the non-dormant axes, the dormant ones, whose aerobic ethylene productivity was very small, responded to ethylene with increased growth in proportion to increasing oxygen tension.