Pattern and amount of aerenchyma relate to variable methane transport capacity of different rice cultivars

Aerenchyma, developed in both root and above-ground parts of rice plants, i s predominantly responsible for plant-mediated transfer of methane (CH4) fr om the soil to the atmosphere. To clarify the pathways of CH4 transport thr ough the rice plant and find differences that may determine the large varia tion in the patterns of methane transport capacity (MTC) of rice cultivars, we examined the appearance, the distribution pattern, and the density of a erenchyma in different parts of rice plants of three widely varying rice cu ltivars during panicle initiation, flowering, and maturity stages. The data on the amount and density of small (> 1 x 10(3) - 5 x 10(3) mu m(2)), medi um (> 5 x 10(3) - 20 x 10(3) mu m(2)) and large aerenchyma lacunae (> 20 x 10(3) mu m(2)) were collected using a computer assisted image-analyzing sys tem. The brightfield optical microscopy of roots of all tested rice plants demonstrated the continuity of aerenchyma channels in the roots that functi on as conduits for bi-directional transport of gases. The aerenchyma channe ls of primary roots showed direct connection with those of culms. Intercala ry meristems were found at the transition zone of root-culm aerenchyma conn ections. Well-developed aerenchyma lacunae present in the internodal region of the culm base were uniformly distributed in the peripheral cortical zon e. The nodal region had relatively fewer and smaller aerenchyma lacunae tha t showed a non-uniform distribution pattern. As a result, few aerenchyma ch annels continued from the internodal region through to the nodal region. Th e aerenchyma in the cortex zone of the culm expanded along with the growing secondary tiller, developing continuity between the culm and the secondary tiller. The micrographs of longitudinal sections of different specimens of culm-leaf sheath intersection showed the continuity of aerenchyma channels from the culm to the leaf. The amount of medium and large aerenchyma lacun ae in the leaf sheath was respectively 2 and 33 times greater as compared t o those of the tiller. The proportion of the large lacunae in the total amo unt of aerenchyma in leaf sheath was 75 % as compared to only 8 % in the ti ller, revealing higher number and larger size of aerenchyma in the former. There were significant differences in amount and density of aerenchyma betw een individual cultivars at a given growth stage, as well as in the develop ment patterns. While the amount and density of medium and small aerenchyma lacunae in the internodal region of the culm base did not show any relation ship with MTC of rice cultivars, large aerenchyma lacunae exhibited highly significant correlations with MTC of different cultivars, suggesting that t he wide variation in MTC of rice plants during different growth stages are related to these structural features.