Effect of seed hardening on the seedling emergence and α-amylase activity in the grains of wheat and rice sown in dry soil

Seed hardening, wetting and redrying of the seed before sowing, increases the rates of germination and seedling emergence of wheat and rice sown in dry soil. We examined the effect of the seed hardening on the water potential of seed, a driving force of water absorption from soil, and the activity of alpha-amylase, a key enzyme for soluble carbohydrate supply. In wheat variety Norin-61, the water and osmotic potentials at the time of sowing were -7.2 and -12.3 MPa., respectively, in the hardened seed, and was -4.8 and -9.9 MPa, respectively, in non-hardened seed. However, in rice variety Nipponbare, neither water nor osmotic potential was clearly affected by seed hardening. In hardened seeds, the activity of alpha-amylase at 12 hours after sowing in dry soil was 2.7 and 2.8 times as high as that in non-hardened seeds in both Norin-61 and Nipponbare, respectively, In several other Japanese varieties and Korean native varieties also, seedling emergence rate in dry soil and alpha-amylase activity before sowing were increased by hardening treatment, and alpha-amylase activity of hardened seeds before sowing was 1.9similar to8.3 times as high as that of non-hardened seeds. It was assumed that the rates of germination and seedling emergence in dry soil were increased by seed hardening through the increase of soluble carbohydrate supply owing to increased alpha-amylase activity. However, hardening treatment did not decrease osmotic potential of rice seed. Redrying during the hardening treatment may have kept the activities of some enzymes in addition to alpha-amylase at the levels just before germination, and thus promoted germination.