THE DISTRIBUTION AND IDENTITY OF ASSIMILATES IN TOMATO WITH SPECIAL REFERENCE TO STEM RESERVES

Much of the work on the distribution of 14C-labelled assimilates in tomato has been done in winter under low light intensities, and consequently the reported distribution patterns of 14C may not be representative of plants growing in high light. Further, there are several somewhat conflicting reports on patterns of distribution of 14C-assimilates in young tomato plants. We sought to clarify the situation by studying the distribution of 14C-assimilates in tomato plants of various ages grown in summer when the light intensity was high. In addition, the role of the stem as a storage organ for carbon was assessed by (a) identifying the chemical fractions in the stem internode below a fed leaf and monitoring 14 C activity in these fractions over a period of 49 d, and (b) measuring concentrations of unlabelled carbohydrates in the stem over the life of the plant. The patterns of distribution of 14C-assimilates we found for tomato grown under high light intensity confirmed some of those described for plants grown under low light, but export of 14C by fed leaves was generally higher than reported for much of the earlier work. Lower leaves of young plants exported over 50% of the 14C they fixed, although export fell sharply as the plants aged. Initially, the roots and apical tuft were strong sinks for assimilates, but they had declined in importance by the time plants reached the nine-leaf stage. On the other hand, the stem became progressively more important as a sink for 14C-assimilates. Older, lower leaves exported more of their 14C-assimilates to the upper part of the plant than to the roots, whereas young leaves near the top of the plant exported more of their assimilates to the roots. The stem internode immediately below a fed leaf had about twice the 14C activity of the internode above the leaf. Mature leaves above and below a fed leaf rarely imported much 14C, even when in the correct phyllotactic relationship to the fed leaf. In the first 3 d after feeding leaf 5 of nine-leaf plants, the organic and amino acid pools and the neutral fraction of the internode below the fed leaf had most of the 14C activity, but by 49 d after feeding, the ethanolic-insoluble, starch and lipid fractions had most of the 14C activity. Glucose, fructose and sucrose were the main sugars in the stem. Although concentrations of these sugars and starch declined in the stem as the plants matured, there was little evidence to indicate their use in fruit production. Stems of plants defoliated at the 44-leaf stage had lower concentrations of sugars and starch at maturity, and produced less fruit than the controls. It was concluded that tomato is sink rather than source limited with respect to carbon assimilates, and that the storage of carbon in the stem for a long period is possibly a residual perennial trait in tomato.