The regulation of reserve mobilization in cotyledons of lipid-rich Cucumis sativus L. seeds

Conflicting results in the literature regarding the possible regulatory role of the embryonic axis on reserve mobilization in lipid-rich seeds prompted this study on cucumber (Cucumis sativus) seed. Two existing models supporting control by the axis, namely the hormonal control- and source-sink relationship models, were tested by incubating detached cotyledons in plant hormone solutions and axes extracts as well as in the presence of artificial sinks. This approach was an attempt to alleviate any effect that axis removal might have on reserve mobilization in cotyledons measured in terms of changes in fresh and dry weight, respiratory capacity and isocitrate lyase activity. No clear evidence that lipid mobilization in cucumber seeds was controlled by the axis through either a hormonal- or a source-sink mechanism was obtained. However, axis removal prevented the natural shedding of the testa by germinating cucumber seeds resulting in clear differences between intact and detached cotyledons in terms of fresh and dry weight, respiratory and lipid metabolism. Moreover, weakening of the testa on detached cotyledons, by cutting longitudinally, only partially alleviated the inhibitory effect that axis removal had on these metabolic events while total removal alleviated this effect completely. The latter was more pronounced in an elevated oxygen atmosphere indicating that the testa played a more pronounced role in controlling the metabolism of cucumber cotyledons during the post-germinative seedling establishment phase than did the growing axis.