Effects on the Germination of Seeds and the Growth of Seedlings of Erigeron sumatrensis Retz. produced by the Relative Illuminance and its Own Dead Roots

As to the secondary succession of weed vegetation in no-cultivated well-drained paddy fields, communities dominated by E. sumatrensis were established in the second year of no-cultivation. However, this species began to disappear rapidly in the third year. In order to elucidate the above mentioned phenomena, effects of the relative illuminance and dead roots of this species on the germination of seeds and the growth of seedlings were investigated. (1) The relative illuminances (that is, 100, 15.5, 6.0, 2.5, 1.0 percent) did not affect in the germination but affected the growth of seedlings. The growth of seedlings was maximum under 15.5% illuminance. Thirty days after germination, seedlings under 15.5% and 100% illuminances developed into rosette type. Under the other illuminances, seedlings did not grow to rosette type but spindly growth type and they were stunted in growth extremely. (2) There appear to be existed some phytotoxic substances in the roots or the root decompositing products of E. sumatrensis. These substances inhibited the growth of seedlings, especially radicles, of E. sumatrensis and E. linifolius, and induced the negative geotropism of radicles. The higher the concentration of these substances was, the stronger these effects were. Moreover, under the same concentration, the substances of adult plant roots included in soil showed the inhibitive effect on the growth more than the inductive effect of negative geotropism of radicles. The effects, however, were reverse in substances of rosette type plant roots. Therefore, these two effects may be controlled by two different substances. These substances did not affect in the germination. There found same effects above on the tests when added to the seeds with aqueous substances which extracted from the dead roots, E. linifolius also contained the same substances in its roots. (3) Besides, when more than 4% of the dead roots were addes to soil weight, all the seeds (achenes) which germinated early in October developed into small seedlings with 2-3 leaves of non rosseted at the end of December, and then these individuals with 2-3 leaves had died outdoors in winter by the middle of February. (4) In E. Sumatrensis-community which is established in the second year of no-cultivation, the relative illuminance decreases and its own roots accumulate at the time of germination of next generation. Therefore, the growth of seedlings of next generation is inhibited and the nagative geotropism is induced, t