ABSTRACTAs reported in a previous paper [Lerchlet al. (1995)Plant Cell, 7, 259–270], expression ofEscherichia coliinorganic pyrophosphatase in the cytosol under the control of the phloem‐specificrolC promoter from,Agobacterium rhizogenesresults in decreased growth of transgenic tobacco plants. In this paper we investigate the effect of the phloem‐specific expression of pyrophosphatase on phloem metabolism, and on plant growth and allocation. A small decrease in the hexose phosphate/UDP‐glucose ratio, the ATP/ADP ratio and the respiration rate in the midribs of the transformants provides evidence Hint mobilization of sucrose via pyrophosphate‐dependent reactions is necessary for phloem energy metabolism. The source leaves of the transformants had higher levels of carbohydrates and amino acids and a much higher glutamine/glutamate ratio than the wild type, showing that export was inhibited and that the growth inhibition was not due to a lack of photoas‐similates or organic nitrogen in the leaves. The accumulation of photoassimilates was paralleled by a decrease in photosynthesis, chlorophyll content and ribulose bisphosphate carboxylase/oxygenase (Rubisco) activity, a small increase in hexose phosphates and triose phosphates and a decrease in glycerate 3‐phosphate in the source leaves. There was a decrease of soluble sugars and amino acids in sink leaves of the transformants. In sink leaves amino acids decreased more than carbohydrates and a decrease in the glutamine/ glutamate ratio was observed. This was accompanied by a large decrease of nitrate. Sugars and amino acids were also reduced in the root tips of the transformants. The carbohydrate /amino acid ratio decreased 5‐fold in the root tips, indicating a particularly smile shortage of carbohydrates. Relatively high levels of sugars and amino acids in the basal regions of the root and the increase in sugars in the midrib indicate that there is also increased leakage of assimilates out of the phloem during long‐distance transport. Metabolism is required to maintain phloem function along the transport route, as well as for the initial step of loading. The transformants showed decreased stem and root growth. The growth inhibition was largest in conditions allowing rapid growth of the wild type (high light and