Rhythmic Variations of the Growth Rate in Rice Embryo Revealed by Time Series Analysis

Abstract

To analyze the growth of embryo of Oryza sativa cultivar, , Nihonbare in detail, caryopses were collected at every ninety minutes. At c.ach time, (relative) growth rate was calculated for the length and the thickness of embryo, and the time series analysis was applied to the variations of the growth rate. At first, correlogram analysis was made on the stationary growth rate and the apparent periodicity was detected as a result of the calculation. In the next place, as a pcriodogram, the correlation coefficient between the autocorrelogram and the cosine function was employed. Periodogram shows two sharp peaks and reveals the existence of two rhythms in the variations of the growth rate. In other words, the variations of the growth rate are represented by the summation of two cosine functions. Periods of two cosine functions are 5. 16 and 5.69 hrs for the growth rate in length and 5.13 and 5.67hrs in thickness. The difference of periods between the growth rate in length and in thickness is not significant. In the next place, to reveal the biological nature of two rhythms of the growth rate at the cellular level, the increasing rate in cell number on the central longitudinal section of embryo was calculated. On the carefull comparison, it was confirmed that the two rhythms of growth rate agreed very well with the variations of the increasing rate in cell nurnber except at the early stage of the development. At the early stage, both the growth rate ancl the increasing rate in cell number seem to show arhythmicity. Therefore, it is considered that the two rhythms of growth rate reflect the rhythinicity of cell divisions. However, tho two rhythms do not exist at the beginning, and appear in the early period of the development. The existence of the two rhythms indicates that the cells divide synchronously in two regions. The authol- supposes that the two rhythms correspond to the synchronous cell divisions at the two growing points in plumule and radicle.