Developmental studies of NAD-malate dehydrogenase isozymes in the cotyledon of cucumber seedlings grown in darkness and in light.

The total activity of NAD-malate dehydrogenase (E.C. 1.1.1.37; MDH) in the cotyledons of cucumber (Cucumis sativus L.) seeds and seedlings increased 10fold during the first 3 days of germination in darkness and then declined gradually to one third of the peak activity after 10 days. Exposure of the seedlings to light at day 3 sharply reduced the activity to one third of the peak activity within 1 day but the residual activity remained fairly constant from day 4 to day 10. In extracts of cotyledons of 2-day-old, dark-grown seedlings and of cotyledons of 6-day-old, light-grown seedlings MDH activity was resolved into 5 isozymes by starch gel electrophoresis. Three of these isozymes were identified as cytosol isozymes, one each as a mitochondrial isozyme and a microbody (glyoxysome or peroxisome) isozyme. During germination in darkness, the activities of the five isozymes increased at different rates. However, they all peaked together at day 3 and then declined gradually at a similar rate. Light applied at day 3 selectively eliminated two cytosol isozymes but did not affect the subsequent developmental pattern of the third cytosol isozyme, the mitochondrial isozyme and the microbody isozyme. Such a selective elimination of two cytosol isozymes, together with their absence in roots and green leaves, indicates that these two isozymes participate in the mobilization of food reserves during germination. The glyoxysomal and peroxisomal isozymes were not resolved in starch gel electrophoresis. Their similarities are discussed in connection with the relationship of the two enzymes and of the two types of microbodies during the greening process.