Ecological significance of root tip rotation for seedling establishment ofOryza sativa L.

How plant seeds secure root penetration into soil to obtain good seedling e stablishment is one of the basic ecological problems. In this study, semina l mot growth was investigated to clarify the cause of varietal difference o f seedling establishment in direct seeding of rice in flooded paddy fields, with special reference to root tip rotation. In a field experiment, seedli ng establishment percentage had a weak correlation with seminal root elonga tion rate but was not correlated with apparent seedling weight in water, wh ich has been reported to be the cause of floating seedlings resulting in po or seedling establishment. Root tip rotation was analyzed for indoor-grown seedlings using spectrum analysis: the maximum entropy method (MEM) was use d. Maximum entropy method power spectrum analysis clarified that maximum ME M power density (practically corresponds to spiral angle) detected in the f requency range above 0.1 cycles mm(-1) was highly and positively correlated to seedling establishment percentage in the field experiment. Maximum MEM power density in high correlation with seedling establishment was mostly fo und around frequencies of 0.2 cycles mm(-1), which corresponded to 2.0-3.4 cycles of root tip rotation per day. From these results, root tip rotation (circumnutation) with a larger spiral angle was suggested to play an import ant role in the establishment of rice seedlings on flooded and very soft so il. A possible explanation for why a larger spiral angle was advantageous f or seedling establishment is that if buoyancy and seedling weight are const ant, a larger pushing force of the seminal root is available without causin g floating of a seedling, due to the upward force being a reaction of the s eminal root pushing force.