The Mechanism of Axial and Embryonic Control

Returning now to those seeds which are reputed to exhibit embryonic or axial control of enzyme activity and food mobilization we can examine the possible mechanisms of these effects. They fall essentially into two groups (1) Hormonal control (2) Sink effects. 

Stimulated, no doubt, by the elucidation of the mechanism in cereals, several attempts have been made to demonstrate that a factor from the embryo or axis is also implicated in other seeds. This has been approached in two ways. Firstly, the promotive capacity of extracts or diffusates of embryos and axes has been investigated secondly, it has been argued that if a known hormone such as gibberellin or cytokinin can replace the beneficial effect of the embryo or axis this raises the possibility that an endogenous hormone might naturally be involved. 

In only a few instances has the possibility of a diffusible or extractable factor been demonstrated. Protein hydrolysis in detached cotyledons of C. maxima is somewhat enhanced when these organs are incubated in a liquid medium together with excised embryonic axes [116]. The inference from this result is that the axis produces a hormonal factor which promotes metabolism in the cotyledons. A hormonal factor also seems to diffuse into the incubation medium bathing P. ponderosa embryos since this medium, when applied to isolated megagametophytic tissue, stimulates 44% more isocitrate lyase than in the control tissue [13]. Similarly, a promotive extract obtained from Ricinus (castor bean) embryos increases the activity of extractable fructose-1,6-diphos-phatase in isolated endosperm of this seed [100]. It should be recognized, however, that these effects are nowhere near as dramatic as the action of cereal embryonic diffusates and extracts upon their endosperm. Moreover, some of the experiments whose results have just been described were not rigorously carried out and are open to serious criticisms (see below, Sect. 7.2.3). It can be stated, in summary, that no embryonic or axial hormonal influence comparable to that exhibited by certain cereal grains has yet been found in other seeds. 

Evidence for a diffusible or extractable hormonal factor in embryos and axes has been mentioned above with respect to three seeds—C. maxima, Ricinus communis, and P, ponderosa. If these promotive factors were gibberellins one would expect applied GA to substitute for the embryo or axis. In Cucurbita and Pinus the axis and embryo cannot be replaced by GA and thus the stimulatory factor in the embryo is not likely to be GA alone. Isolated endosperm of Ricinus, however, does respond to GA by showing enhanced activity of fructose-1,6-diphosphatase, as it does to embryonic extracts. 

Three enzymes in isolated, cytokinin-treated cotyledons of C. maxima— soc -trate lyase, proteinase and di peptidase—all reach levels comparable to those found in cotyledons of intact embryos [91, 92, 105]. Since cytokinin (benzylade-nine) thus apparently replaces the lost axis the conclusion has been made, and quoted in the research literature, that the axis in Cucurbita embryos produces cytokinin which regulates enzyme activity or formation. No direct confirmation of this conclusion has yet been made there is no experimental evidence that the axis does indeed export cytokinins to the cotyledons let alone at levels similar to those which have been applied from an exogenous source. Moreover, the experiments using external cytokinin are themselves open to criticism on grounds that are discussed later (Sect. 7.2.3). 

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