Fig. 7.

Rates of radial O₂ loss, and assessment of ‘blue halos’ formed with methylene blue treatment, for adventitious roots of rice grown in aerated or stagnant deoxygenated agar nutrient solution. The basal region of root, 11–50 mm below the root–shoot junction (i.e. 30–60 mm behind the root apex), was measured for ROL and then observed for staining with methylene blue. Before measurements, plants were raised for 8 d in aerated nutrient solution and then grown in either aerated or stagnant deoxygenated nutrient solution for 14–21 d. ROL was measured using cylindrical root-sleeving O₂ electrodes in an O₂-free agar solution with shoots in air, and subsequently the measured adventitious roots were transferred into O₂-free agar solution containing methylene blue. Methylene blue is colourless when reduced and blue when oxidized. Measurements and observations were taken at 28 °C using intact adventitious roots 57–107 mm in length (three plants, n = 6) grown in aerated nutrient solution and 60–81 mm in length (four plants, n = 9) grown in stagnant deoxygenated agar nutrient solution. Dashed line: apparent threshold of methylene blue staining for 30–60 min.