Aerenchyma Formation and Methane and Oxygen Exchange in Rice

Limited information exists on the direct link between the intensity of soil reduction or anaerobiosis and gas exchange in rice (Oryza sativa L.). A laboratory experiment was conducted to determine the extent to which specific levels of soil redox potential (Eh) could influence aerenchyma formation, O2 transport, and CH4 production and emission. Plants were grown in a Crowley silt loam (fine, montmorillonitic, thermic Typic Albaqualf) under controlled Eh values of 200, -200, and −300 mV. Radial O2 loss (ROL) from plant roots was estimated colorimetrically by using Ti3+-citrate solution, a reduced compound. Methane production and its emission increased with decrease in soil Eh; CH4 production increased 10-fold while its emission was enhanced 17-fold when soil Eh dropped from -200 to −300 mV. A positive relationship was established between the intensity of soil anaerobiosis and both aerenchyma formation and ROL. Root and shoot dry weights and root length decreased with decrease in soil Eh. Results of this study demonstrate that soil Eh influences net CH4 flux from rice in two ways: (i) it directly determines the amount and rate of CH4 production in the soil, and (ii) it initiates morphological and physiological changes in the rice plant that affect gas exchange between the soil and the atmosphere. Although the results may not necessarily reflect actual conditions in the field, they provide a theoretical basis for understanding the influence of soil Eh on rice growth, CH4 production, and gas exchange.