DORMANCY IN SEEDS OF CHARLOCK .4. INTERRELATIONSHIPS OF GROWTH, OXYGEN SUPPLY AND CONCENTRATION OF INHIBITOR

Respiration measurements were made over a period of 24 h at 25 °C on seeds and excised embryos maintained in Warburg flasks with partial pressures of oxygen ranging from 0 to 1 atm. In the initial phase (0 to 4 h), the rate of oxygen uptake (QO2 of excised embryos increased linearly with external oxygen concentration (CO from 0 to 0.1 atm O2 from 0.1 to 0.2 atm O2 the relation was curvilinear, and from 0.2 to 1.0 atm O2 uptake was independent of concentration. In later stages the relation between QO2 and Co changed, and from 20 to 24 h the rate of oxygen uptake increased with concentration to 1.0 atm O2. The changes with time were associated with increase in rate of respiration, increase in cell size and cell number, and the oxidation of fats.The decline in concentration of oxygen from the surface to the centre of embryos was calculated to be relatively small at each external oxygen concentration. Althugh the rate of diffusion failed to keep pace with consumption, the main parameters which determined the internal oxygen status of the embryos were the surface concentrations and the permeability of the seed coat. The resistance of the seed coat to diffusion of oxygen was found to be very high, the coefficient of diffusion being about 10-7 mm2 s-1. The concentration of oxygen and in air were estimated to be approximately 0.04 and 0.02 atm O2, respectively. Since a smaller concentration of oxygen (0.012 atm O2) in the tissues was found to be sufficient for growth, the dormancy of the seeds was not due to lack of oxygen.Dormancy appeared to be due to the activity of growth-inhibiting substances, the concentration of which increased with decrease in oxygen supply; below 0.1 atm O2 their rate of production increased with decrease in the oxygen concentration of the tissues. They accumulated within the testas of dormant seeds and prevented cell elongation. Extracts of the inhibitory substances were partially purifield by partitioning the aqueous fraction with ether and separating chromatographically. The active principle(s) was not abscisic acid ((+)-Abscisin II, 'Dormin') or the mustard oil, allylisothiocyanate. © 1969 Oxford University Press.