Abstract
Contrary to the established notion that the apical hook of dark-grown dicotyledonous seedlings opens in response to light, we found in tomato that the apical hook curvature is exaggerated by light through phytochrome actions. Based on some simulation experiments for germination in the field and anatomical observations of the seed part of germinating seedlings, we have proposed that light-induced hook exaggeration (LIHE) plays an important role in survival by securing the seed coat release in the field. Interestingly, for the LIHE to occur, the presence of the seed coat and/or endosperm is essential. Once the seed coat carrying the yet unconsumed endosperm has been released, no LIHE occurs any more. Furthermore, LIHE is not limited to tomato, but also found in several other plant species, cotyledons of which are enclosed with unconsumed endosperm in the seed coat. Evidence such as these has led to the test of the ability of endosperm to induce LIHE and we found that the water extract from tomato endosperm applied to the tomato cotyledons free from seed coat and endosperm causes hook exaggeration under red light. Endosperm seems to provide some factor essential for LIHE.
