Significance of light-induced hook exaggeration as reinforced by the concomitant anatomical change of germinating tomato seeds

Progression of the apical hook of tomato, Solanum lycopersicum, exaggerated by phytochrome mediation at the early germination stage, is followed in detail macroscopically and anatomically, and its proposed significance, i.e., survival by securing the seed coat release in the field, is reinforced by new findings. Furthermore, after self-release or artificial removal of the seed coat and the endosperm, no hook exaggeration occurs any more. Similar light-induced hook exaggeration (LIHE) is also found in carrot, parsley and Cryptotaenia japonica, which share some seed characteristics with tomato. These findings also support the above-stated significance.Key words: apical hook, carrot, cotyledons, Cryptotaenia japonica, endosperm, field germination, light action, parsley, phytochrome, seed coat, Solanum lycopersicumContrary to many other seed species,^1–3 the apical hook of germinating tomato seeds is exaggerated by red (R) and far-red light (FR) given in a pulse or continuously, mediated by the very low and low fluence responses of phytochromes.⁴ Also, an R high-irradiance response is probably involved.⁴ Based on some simulation experiments for germination in the field, we have proposed that LIHE may play a role, not in breaking through compacted soil surface, but in securing to release the seed coat at some depth of soil in response to light coming through soil gaps.⁴ Here we present additional observations and experimental data not published yet and reinforce the proposed significance of the recently discovered photo-response.