Effect of reduced oxygen pressure on rice germination

In the 3 years during which this work was conducted in the field rice sown on the surface of the soil, 1/2 inch, and 1 inch deep, then continuously submerged, produced an average of 78.14, 20.00, and 2.66% of seedlings, respectively. No seedlings were obtained from seed sown 1/2, 2, or 2 1/2 inches deep and continuously submerged either in the field or in pots in the greenhouse, except when oxygen was forced into the soil. All seed sown on the surface of the soil either germinated and produced normal seedlings or failed to germinate. Of the seed sown 1/2 inch deep in the field an average of 25.78% produced plumules only. With increased depth of seeding, however, the percentage of seed that produced plumules only decreased and the percentage of seed that failed to germinate increased materially. The results presented indicate that when continuously submerged a 1/2-inch layer of Stockton clay adobe soil materially reduced the oxygen pressure and a layer 1 inch or more in depth reduced the oxygen pressure to a point at which normal germination did not occur. Apparently there was insufficient oxygen under such conditions to initiate growth of the radicle. These results indicate that seeding in the water (surface seeding in these experiments) is the best practice when rice is grown by continuous submergence. The results presented from germination in pots into which oxygen was forced, especially in sand, indicate that a deficiency of oxygen is probably the principal factor causing a lack of normal germination in rice seed sown at various depths and continuously submerged with water. The development of the radicle is initiated by a suitable supply of oxygen. Early seeding of rice is preferable to late seeding because temperatures of the atmosphere and water are lower early in the spring than they are late in the spring, and more oxygen is dissolved in water at low temperatures than at high temperatures. The dissolved oxygen at the lower temperatures results in a larger percentage of seedlings than can be obtained at higher temperatures.