Abstract
Contrary to the established notion that the apical hook of dark-grown dicotyledonous seedlings opens in response to light, we have previously discovered in tomato that the apical hook curvature is exaggerated by light through low- and very low-fluence responses of phytochrome. The novel phenomenon is not limited to tomato, but found also in some other species. This led us to seek the significance of the phenomenon in the field. Here we present the results of field simulation experiments and time-lapse movies, and a hereby-deduced role of the hook exaggeration that it may contribute to the release of the seed coat; i.e., seedlings germinated in soil respond to first photons coming through soil gaps by hook exaggeration which delays the emergence of the seedlings above soil surface. The delay provides the time for the yet small, immature cotyledons to develop and become ready to unroll themselves and eject the seed coat. It also allows the seed coat to stay longer in humid conditions needed for the release of seed coat. Furthermore, we found that the hook exaggeration requires the seed coat and/or endosperm, which was assumed to supply some essential factor to the hook part.
