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Morphological Development of Rice Seedlings in Water at Controlled Oxygen Levels¹

Germination and seedling establishment of rice (Oryza sativa L.) is usually less than optimum in water saturated and flooded soils apparently because of a deficiency of available oxygen (O₂). However, the effect of low O₂ level on rice germination and seedling development is not well defined. To evaluate the effect of varying O₂ level on rice seedling development and to identify rice cultivars which may be more tolerant to the low O₂ concentrations associated with water saturated rice soils, seeds of four rice cultivars were incubated in water containing six O₂ concentrations. Gas flow meters and regulators allowed mixing of air and N₂ gas to obtain O₂ levels of 21, 10.5, 5.3, 2.0, 1.0, and 0% (99.995% N₂ gas) which were bubbled through special water filled containers placed in a growth chamber providing 400 microeinstein m^–2 sec^–1 light intensity, 12-hour day, and 27 C constant temperature.

Oxygen concentration had little effect on coleoptile emergence from rice seed but low O₂ levels enhanced coleoptile elongation. Maximum coleoptile length (20 mm) occurred at 0% O₂, making it appear that rice germinated at very low O₂ levels. However, it was apparent that root and shoot (leaf) growth were inhibited at 0% O₂, providing evidence that low O₂ levels can restrict rice stand establishment.

The critical O₂ level for maximum shoot elongation appeared to be about 5%. The critical O₂ level for root elongation was less evident. Roots of ‘Labelle’ and "Bellemont’ seedlings reached maximum length at O₂ levels as low as 5% whereas roots of ‘Calrose 76’ and ‘M 101’ seedlings continued to increase in length as the O₂ level was maintained at 10.5 and even 21%. Differences in cultivar tolerance to low O₂ levels were observed among the 10 cultivars evaluated. Also, rice cultivars exhibited tolerance to low O₂ levels in terms of one parameter (root, shoot, or coleoptile length) without showing tolerance in terms of the other two parameters.

Key Words: Stand establishment • Coleoptile • Root and shoot elongation • Germination • Oryza sativa L.

² Associate professor, research assistant, and assistant professor, respectively.

Received for publication August 11, 1980.

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