Effect of temperature on starch synthesis in potato tuber tissue and in amyloplasts.

A sharp temperature optimum is observed at 21.5 degrees C when the incorporation of [(14)C]sucrose into starch is measured with discs cut from developing tubers of potato (Solanum tuberosum L. cv Desirée). By contrast, increasing temperatures over the range 9 to 31 degrees C only enhance release of (14)C to respiratory CO(2) and incorporation of (14)C into the ethanolsoluble fraction. By comparison, starch synthesis in discs from developing corms of cocoyam (Colocasia esculenta L. Schott) is increased by raising the temperature from 15 to 35 degrees C. The significance of a relatively low temperature optimum for starch synthesis in potato is discussed in relation to the yield limitations imposed by continuously high soil temperatures. Amyloplasts isolated from protoplasts prepared from developing potato tubers contain activities of alkaline pyrophosphatase, NAD-dependent glyceraldehyde-3-phosphate dehydrogenase, fructose-1,6-bisphosphatase, and phosphoglucomutase in addition to ADP-glucose-pyrophosphorylase, starch phosphorylase and starch synthase. Cell-free amyloplasts released by thinly slicing developing potato tubers synthesize starch from [(14)C]triose-phosphate generated from [(14)C]fructose-1,6-bisphosphate in the reaction medium. This starch synthesis is inhibited by addition of 10 millimolar inorganic phosphate and requires amyloplast integrity, suggesting the operation of a triose-phosphate/inorganic phosphate exchange carrier at the amyloplast membrane. The temperature optimum at 21.5 degrees C observed with tissue discs is not observed with amyloplasts.