MICROSPECTROPHOTOMETRY OF NEWLY SYNTHESIZED STARCH IN SITU

Many of the cells in stem sections of several Commelinaceae species synthesized much new starch when incubated in buffered 1% glucose 1-phosphate solution. The starch appeared in the cytoplasm rather than in the plastids. Although the starch I2-KI color was uniform within any one cell, there was considerable variation from cell to cell, even in the same section. The colors with I2-KI ranged from blue, through purples to magenta and mahogany. Tests with α-amylase and with β-amylase showed the starch to be amylose. Microspectrophotometrically determined extinction curves, based on the new starch in situ, varied with the visualized color. As expected, starch which stained blue with I2-KI had an absorption maximum in the orange-red wavelengths above 600 mµ; increasingly red I2-KI colors were characterized by shifts of the absorption maximum further into the shorter wavelengths. The course of new starch digestion by α-amylase and by β-amylase was followed visually and with the microspectrophotometer. Similarities and differences between these spectral curves and those published for in vitro studies are pointed out. The difficulties met with in using the microspectrophotometric method are discussed.