WHEN GROWING POTATO-TUBERS ARE DETACHED FROM THEIR MOTHER PLANT THERE IS A RAPID INHIBITION OF STARCH SYNTHESIS, INVOLVING INHIBITION OF ADP-GLUCOSE PYROPHOSPHORYLASE

Labelling experiments in which high-specific-activity [U-C-14]sucrose or [U-C-14]hexoses were injected into potato (Solanum tuberosum L. cv. Desiree) tubers showed that within 1 d of detaching growing tubers from their mother plant, there is an inhibition of starch synthesis, a stimulation of the synthesis of other major cell components, and rapid resynthesis of sucrose. This is accompanied by a general increase in phosphorylated intermediates, an increase in UDP-glucose, and a dramatic decrease of ADP-glucose. No significant decline in the extracted activity of enzymes for sucrose degradation or synthesis, or starch synthesis is seen within 1 d, nor is there a significant decrease in sucrose, amino acids, or fresh weight. Over the next 7 d, soluble carbohydrates decline. This is accompanied by a decline in sucrose-synthase activity, hexose-phosphate levels, and the synthesis of structural cell components. It is argued that a previously unknown mechanism acting at ADP-glucose pyrophosphorylase allows sucrose-starch interconversions to be regulated independently of the use of sucrose for cell growth.