Allelopathic substances from seed leachates can mediate plant interspecific relationships. ( +)-Catechin is the main phytotoxin leached by the seeds of Sesbania virgata with effects on native and crop species. In this article, we hypothesised that S. virgata seed leachates affect mobilization of seed storage carbohydrates in crop species. We used tomato and rice as target species, which store mannan and starch as seed polysaccharides, respectively. Experiments of seed-to-seed co-germination with S. virgata and germination of tomato and rice with of S. virgata seed leachates or commercial catechin showed that S. virgata affected tomato germination and the root and shoot growth in both species. Commercial catechin only reduced seedling growth. The reducing sugar content decreased in both rice and tomato and the total sugar content in rice only when seeds germinated with S. virgata seed leachates. The endo-β-mannanase activity was inhibited in tomato seeds germinated with S. virgata seed leachates but not with commercial catechin. These results suggest that although catechin interferes with germination and seedling growth of agronomic species, the inhibition of storage carbohydrate mobilization by S. virgata seems to be related to other allelochemicals present in the seed leachates rather than to catechin.