242 W. ARMSTRONG Physiol. Plant., 24, 242-247, Oxygen Diffusion from the Roots of Rice Grown under Non-waterlogged Conditions By W. ARMSTRONG Botany Department, University of Hull, U.K. (Received July 7, 1970) Abstract Three rice varieties, cv. Norin 36, cv. Norin 37 and cv. Yubae, were grown in a loam with a 20 cm water-table which gave aerobic conditions to a depth of not less than 15 to 17 cm. Under these conditions Norin 36 grew more vigorously and tillered more frequently than the other two varieties. The rates of oxygen diffusion at 23°C from roots up to 11 cm in length were however appreciably lower for Norin 36 (4.3 X 10-** g • cm - of root surface • min-') than for Norin 37 or Yubae (c. 7.8 x 10-» g). A considerable increase (up to 200 "/o) in the oxygen dif- fusion rate (ODR) from the roots occurred if they were cooled to 3^'C, and at this temperature differences in ODR between the varieties were not significant. For a purely physi- cal system, because of the decrease in the diffusion coeffi- cient of oxygen m water, and the increase in oxygen solubility, a drop of c. 20 "Io in ODR should accompany the above 20°C drop m temperature. A 16 "/o drop was recorded for artifi- cial 'roots' under these conditions. It was concluded that respiratory activity at the higher temperature must have been responsible for the low readings and intervarietal differences observed at 23'^C. By increasing the 3°C values by 25 "/o a mean value of 14.2x10-'* g • cm-- of root surface • min-' was recorded for the three varieties, being the probable ODR at 23^C in the absence of a respiratory factor. Calculations show that respi- ratory activity removed enough oxygen tc reduce the ODR for Norin 36 by more than 9 x 10-* g, and for Norin 37 and Yubae by c. 6.7 x 10"* g • cm-- of root surface • min '. Anatomical investigations showed that cortical breakdown was always extensive at 4 to 4.5 cm from the apex of the roots. In some cases however breakdown had not occurred in the basal segment of the root. No opinion could be formed as to whether differences in the amount of cortical breakdown between the varieties might have occasioned the respiratory differences observed. An interesting feature of the root anatomy was the failure of breakdown in those regions of the roots through which lateral roots emerged. Introduction Root adaptation to a waterlogged soil environment often includes the formation of lacunae from the break-down of cortical cells, and all previous investiga- tions by the author on root oxygen "secretion' have been on plants grown in media chosen for their induction of aerenchyma formation. Heide et al. (1963) found that a certain amount of cortical break-down could be induced in Barley by anaerobic media. Oygen diffusion into anaerobic media was detected from the root systems of the plants treated in this way, but not from Barley previously grown in aerobic solutions and lacking aerenchyma. The fact that the roots from plants in aerated solutions did not re- lease oxygen was taken to imply that the internal rate of supply of oxygen to the roots was not in excess of that required for respiration rather than that no oxygen was being transferred along the roots. The following investigation was designed to investi- gate the effect of aerobic soil conditions on the oxygen "secretory power' of rice roots, and to make some quan- titative estimate of the influence of root respiration on the process. Three rice varieties were used and grown in soil with a 20 cm water table in the hope that the aerobic soil conditions would not mduce aerenchyma formation. Materials and Methods Plant material Seeds were germinated on moist blotting paper in trays, before transferring to a moist loam in 10 in. plastic pots — 15 seedlings per pot. The final number of seedlings to survive varied from 10 to 13. For the duration of the experiment the pots stood in a tray of