Dormancy and impotency of cocklebur seeds III. CO2- and C2H4-dependent growth of the embryonic axis and cotyledon segments

Growth of segments of embryonic axes and cotyledons excisedfrom dormant or nondormant cocklebur (Xanthium pennsylvanicumWallr.) seeds and CO₂ and C₂H₄ production in these segmentswere examined in relation to the effects of temperature, CO₂and C₂H₄. Both the nondormant axes and cotyledons grew evenat low temperatures below 23°C, but the dormant ones failedto grow. There was only little difference in the CO₂ evolutionbetween the nondormant and dormant ones, but both the axis andcotyledon segments from the dormant seeds exhibited little orno C₂H₄ productivity, unlike the nondormant ones, at low temperatures.However, a high temperature of 33°C caused rapid extensiongrowth and C₂2H₄ production even in dormant axes and cotyledons. The inability of dormant axes and cotyledons to grow disappearedcompletely in the presence of C₂H₄ at fairly low concentrations.Removal of endogenous CO₂ and C₂H₄ reduced the growth in bothaxes and cotyledons, while exogenous CO₂ mainly enhaced axialgrowth although exogenous C₂H₄ strongly stimulated the growthof both organs. Regardless of the dormant status, however, maximumgrowth of these organs occurred when C₂H₄ was given togetherwith CO₂. We suggest that dormancy in cocklebur seeds is dueto the lack of growing ability in both organs, caused by thelack of C₂H₄ productivity in both dormant axes and cotyledons,particularly in the former. (Received December 2, 1974; )