The control of food mobilisation in seeds of Cucumis sativus L. : I. The influence of embryonic axis and testa on protein and lipid degradation

The control of food mobilisation in seeds of Cucumis sativus L. : V. The effect of light on lipid degradation

The degradation of neutral lipid and the development of lipase activity in cucumber cotyledons is stimulated by white light. Malate synthase and isocitrate lyase activities show no stimulation. Lipase activity and neutral lipid breakdown are also enhanced by red light, far-red light proving ineffective. Far-red light reverses the effect of red light indicating the involvement of phytochrome in the control of lipase activity. Although light stimulates neutral lipid degradation it appears that much of the additional lipid lost is used in the synthesis of polar lipid constituents. Furthermore, the influence of light on lipid degradation appears to be species dependent.

The control of food mobilization in seeds of Cucumis sativus L. : III. The control of protein degradation

An analysis of the in vitro activities of proteolytic enzymes from cotyledons of germinating cucumber seeds has been carried out and the effects of protein degradation products on such activities monitored. Aminopeptidase activity is substantially inhibited with either L-leucine or L-phenylalanine and trypsin activity with L-arginine. Aminopeptidase activity was also markedly reduced in the presence of individual di- and tripeptides. Of the peptides tested, however, only L-tryptophyl-L-phenylalanine inhibited the degradation of native cucumber seed protein by the endogenous cucumber seed protease(s) (autodigestive activity).

The control of food mobilisation in seeds of Cucumis sativus L. : II. The role of the embryonic axis

Removal of the embryonic axis from germinated cucumber seeds, either on the second of fourth day after imbibition, subsequently results in reduced rates of lipid and protein degradation and in the accumulation of free sugars and amino acids in the cotyledons. These isolated cotyledons show an inherent capacity for expansion growth which apparently results from an increased rate of water uptake. When water uptake is inhibited by incubating samples in polyethylene glycol the rate of lipid degradation is further reduced. This is accompanied by an additional increase in the reducing sugar and sucrose content of the cotyledons. Protein degradation in isolated cotyledons is inhibited to the same extent whether samples are incubated in water or polyethylene glycol. Furthermore, amino acid levels show appreciable and almost identical increases in both incubation media. Evidently an inverse correlation exists between rates of reserve mobilisation and levels of end products. It is suggested that the axis controls food mobilisation through a sink effect by reducing the levels of end products in the cotyledons.