Morphological Development of Rice Seedlings in Water at Controlled Oxygen Levels

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.