Histogenetical Studies on Cortical Disintegration in Rice Roots

Abstract

Several lines of investigation have been carried on the formation of lysigenous lacunae of rice plant roots. The results obtained so far, however, do not agree with each other on the mechanism of formatlon and physiological significance of the lacunae. It was reported by many investigators that the development of the lacunae of rice plant roots growing under anaerobic soil conditions was remarkable as compared with that of aerobic soil conditions. It was assumed that oxygen needed for root respiration would be supplied mainly by leaves via stems and lysigenous lacunae when the rice plant was kept in anaerobic soil. On the other hand, an investigator asserted that the physiological role of lysigenous lacunae of rice plant roots should be reexamined from that the formation of lysigenous lacunae had relations with the development of lateral roots, that the fomation of lateral roots in the soil of paddy field was generally accelerated when it was in oxidative state and that the development of lacunae, on the contrary, was slowed down when the soil was reductive. In an attempt to throw the light on the differences observed in these works, the histogenesis and the changes in the volume of lacunae and lateral roots were studied in rice seminal roots growing under two kinds of soil conditions in oxidation-reduction potentials. The phenomenon of cortical disintegration may be consisted of two phases, emergence and enlargement. With the exception of roots, 2 days old growing under reductive soil conditions, the emergence of cortical disintegration was always observed with the primordia of lateral roots. In two days roots growing under reductive soil conditions, the emergence of cortical disintegration was found, but the initiation of lateral root primordia was observed nowhere. While the enlargement of cortical disintegration was noticeable more distinctively in the roots growing under oxidative soil conditions (hereinafter referred to as O) than in the roots growing under reductive soil conditions (hereinafter referred to as R) for 4 days after seeding. Though there were little differences in the density of lateral roots and the primordia on the same aged parts (e. c. was 0.9 cm length from root base in O and 0.6 cm in R), for 4 days after seeding O exceeded distinctly R in the volume of lateral roots per centimeter and the growth of lateral roots in length of which the longest lateral roots represented of each region abovementioned. On and after the 4th, R was superior to O in the percentages of lacunae, the volume of lateral roots and the growth of lateral root in length. On the basis of such results, it was assumed that the emergence of cortical disintegration and the initiation of lateral roots have a close relationship with each other in time and position, and the enlargement of cortical disintegration was restricted by the velocity and the volume of the growth of lateral roots. Anyhow, the results of this experiment seem to indicate that whether the development of cortical disintegration will become well in R or O, depends mainly on the growth of lateral root in volume or velocity. A difference, as previously stated, in observations among investigators with refference to the extent of the development of cortical disintegration in R and O may be consolidated by these results.