Control by the Embryo and Embryonic Axis

It will be recalled that food mobilization in barley, rice and wheat does not occur in endosperms from which the embryo has been removed. This is an easy way to demonstrate that the embryo is responsible for the initiation (and subsequent control) of reserve breakdown. The same simple method has been applied for other seeds, with variable success, but in many cases removal of the embryo has been found to prevent or retard mobilization and adversely to affect development of the requisite enzymes. 

Let us look first at those seeds in which the cotyledons are the storage organs. A number of researchers have used pea seeds (a hypogeal germinator) for their experiments. Unfortunately, variable, sometimes contradictory results have been found, possibly because different varieties have been employed or, more likely, because the experimental conditions were not standardized. Respiration, differentiation of mitochondria, and other sub-cellular changes are all impaired in detached pea cotyledons [10, 112]. The cotyledons therefore seem to require a factor from the axis for the development and maintenance of respiratory metabolism—this can be supplied even by a small piece of attached axial tissue [112, 119]. As far as protein and starch mobilization in peas are concerned it is unclear if the presence of the axis is necessary. In some cases protein and starch breakdown proceed equally well in detached and attached cotyledons [10] but in others both the development of proteinase activity and 

In addition to the above findings there is nevertheless a body of evidence which raises doubts concerning the relationship between axis and cotyledons in food mobilization. We should recall, in this context, that a- and j8-amylase have been reported to reach higher levels in detached pea cotyledons [118], that starch and protein breakdown occur in detached pea cotyledons [10] and that a-amylase development in P. vulgaris cotyledons is unaffected by excision, at least over the first four days. Similarly, removing the axis has no deleterious effect on isocitrate lyase activity in cotyledons of Arachis hypogaea (peanut) and the enzyme reaches a level identical to that in intact seeds [80]. 

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