A sharp temperature optimum is observed at 21.5°C when the incorporation of ["4Clsucrose into starch is measured with discs cut from developing tubers of potato (Solanum tuberosum L. cv Desiree). By contrast, increasing temperatures over the range 9 to 31°C only enhance release of '4C to respiratory CO2 and incorporation of '4C 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°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, NADdependent 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 I'4Cltriose-phosphate generated from phosphate 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°C observed with tissue discs is not observed with amyloplasts.The yield of potatoes in the warm tropics is limited by the continuously high soil temperatures (1). Growth of the potato tuber is closely linked to starch synthesis, and physiological studies with whole tubers still attached to the parent plant have indicated that higher temperatures could reduce yields by inhibiting starch synthesis in the tuber (17,24,28,33). However, interpretation ofthe results of these studies has been complicated by the possible influence of a temperature-dependent redistribution of plant growth regulators, especially gibberellin ( 17,28), and the sensitivity of starch synthesis per se to higher temperatures has remained unknown. Thus, the present work was carried out to determine the temperature optimum for starch synthesis in excised potato tuber tissue supplied with sucrose and in cellfree amyloplasts supplied with triose-P. were taken from plants grown in Reading, either in the open ground during the summer or in a glass house with supplementary heating (15-20°C) and lighting (13 h daylength) during winter. Developing corms of cocoyam (Colocasia esculenta L. Schott) were taken from plants grown under natural lighting in a heated glasshouse. Tubers and corms were washed, surface sterilized, and peeled before use. In the glasshouses, plants were grown in potting compost contained in I0-L polythene tubs and watered daily. MATERIALS AND METHODSPreparation of Discs and Measurement of Sucrose Uptake into Different Fractions. Potato tubers or cocoyam corms were sliced to 1 mm thickness,...

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