Energy relations of solute loading in sieve elements of willow.

Experiments are described which attempt to clarify the quantitative relationship between sieve-tube loading of sucrose, and ATP-turnover rates in the phloem of willow (Salix viminalis L.). Two experimental approaches have been made towards the solution of this problem. In the first of these the respiratory breakdown of (14)C-sugars was measured in segments of willow stem when no sieve-tube transport was taking place, and also under conditions where transport was occurring in response to the feeding of individuals of the aphid Tuberolachnus salignus (Gmelin). An increase in respiratory activity, measured by the output of (14)CO2, was found to occur as a consequence of transport. Since the rate of sieve-tube sugar loading could be measured by the production of honeydew from the aphids, and by making assumptions concerning the production of ATP in respiration, it was concluded that the stoichiometry of sucrose loading was 1.9 mol ATP · (mol sucrose)(-1). A somewhat higher value of 2.5 mol ATP · (mol sucrose)(-1) was found using the second approach. In this, attempts were made to measure ATP turnover rates using [(32)P]orthophosphate supplied to strips of willow bark which bore exuding aphid stylets.