The Amount of γ‐Amino Butyric Acid and the Activity of Glutamic Decarboxylase in Aging Leaves

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Abstract
The amount of γ‐amino butyric acid as measured by thin‐layer chromatography, and the specific activity of glutamic decarboxylase as measured both by Warburgapparatus (CO2) and by thin‐layer chromatography (γ‐ABA) were determined in leaves of different ages. Datura suaveolens, Medicago sativa and Salvinia natans were used as experimental plants.The amount of γ‐ABA increases in the leaves of Salvinia natans, Medicago sativa and Datura suaveolens in proportion to the age of the leaves. In the case of Salvinia natans, the amount of γ‐ABA in old leaves is about twice as much as in young ones: in the case of Medicago sativa and Datura suaveolens the increase is slightly less. In Salvinia natans the amount of γ‐ABA reaches its greatest value in the third to fifth leaf, in Medicago sativa and Datura suaveolens later. This increase in the amount of γ‐ABA with respect to the age variation of the leaves, is reflected in the growth of leaves. In Salvinia natans the third or fourth leaf is full‐grown, in Medicago sativa and Datura suaveolens this stage is reached in the seventh or eighth leaf.The activity of glutamic decarboxylase parallels the γ‐ABA accumulation in that it increases when the leaves grow. The increase of glutnmic decarboxylase activity takes place more rapidly in the aging leaves of Medicago sativa than in those of Datura suaveolens. The activities observed are somewhat higher when the amounts of γ‐ABA are analysed than when the CO2 evolved is measured.KM‐values for glutamic decarboxylase were calculated using various substrate concentrations to test the enzyme activity. The KM‐values obtained were 7.0 · 10−3M in the case of Medicago sativa, and 7.9 · 10−3M in the case of Datura suaveolens.