Co-ordination of sugar transport and metabolism between developing seeds and their enclosing fruit tissues is little understood. Liu et al. examine physiological mechanisms using two genotypes of asparagus bean (Vigna unguiculata ssp. sesquipedialis) differing in pod wall and seed growth rates, and find differences in sucrose metabolism in relation to the pathway of phloem unloading in developing seed coats, and to pod and seed growth. Low invertase activity in the pod wall corresponds to a shortened and weak growth period; by contrast, the apoplasmic path in the seed coat is associated with a high cell wall invertase activity and strong early seed growth.