Plastid Development in Primary Leaves ofPhaseolus vulgaris: THE EFFECTS OF BRIEF FLASHES OF LIGHT ON DARK-GROWN PLANTS

Exposure of dark-grown beans to 1 ms flashes of light (2 × 1014 quanta/cm2/flash) at 15-min intervals induced growth of the primary leaves as shown by increases in fresh weight, dry weight, and total protein. Effects of the flashes on plastid size and fine structure were not obvious until leaf growth was more than half completed, when the prolamellar bodies became consumed and thylakoids were formed. Leaf samples taken after 638 and 922 flashes contained some mesophyll cells with plastids of abnormal appearance which had structures resembling stromacentre fibrils. Flashes of light increased both the chlorophyll content of the leaves and the activities seven enzymes of the photosynthetic carbon cycle and of NAD-linked triosephosphate dehydrogenase (EC 1.2.1.12), these changes being correlated with leaf growth rather than the plastid changes detected by electron microscopy. There was only a small increase in the activity of phosphoribulokinase (EC 2.7.1.19) and no change in the activity of phosphopyruvate carboxylase (EC 4.1.1.31).