Pattern and Amount of Aerenchyma Relate to Variable Methane Transport Capacity of Different Rice Cultivars

Abstract:

Aerenchyma, developed in both root and aboveground parts of rice plants, is predominantly responsible for plant-mediated transfer of methane (CH₄) from the soil to the atmosphere. To clarify the pathways of CH₄ transport through the rice plant and find differences that may determine the large¡¯variation in the patterns of methane transport capacity (MTC) of rice cultivars, we examined the appearance, the distribution pattern, and the density of aerenchyma in different parts of rice¡¯plants of three widely varying rice cultivars during panicle initiation, flowering, and maturity stages. The data on the amount and density of small (> 1 ¡Á 10³ − 5 ¡Á 10³ ŠÌm²), medium (> 5 ¡Á 10³ − 20 ¡Á 10³ ŠÌm²) and large aerenchyma lacunae (> 20 ¡Á 10³ ŠÌm²) were collected using a computer assisted image-analyzing system. The brightfield optical microscopy of roots of all tested rice plants demonstrated the continuity of aerenchyma channels in the roots that function as conduits for bi-directional transport of gases. The aerenchyma channels of primary roots showed direct connection with those of culms. Intercalary meristems were found at the transition zone of rootšCculm aerenchyma connections. Well-developed aerenchyma lacunae present in the internodal region of the culm base were uniformly distributed in the peripheral cortical zone. The nodal region had relatively fewer and smaller aerenchyma lacunae that showed a non-uniform distribution pattern. As a result, few aerenchyma channels continued from the internodal region through to the nodal region. The 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 culmšCleaf sheath intersection showed the continuity of aerenchyma channels from the culm to the leaf. The amount of medium and large aerenchyma lacunae in the leaf sheath was respectively 2 and 33 times greater as compared to those of the tiller. The proportion of the large lacunae in the total amount of aerenchyma in leaf sheath was 75 % as compared to only 8 % in the tiller, revealing higher number and larger size of aerenchyma in the former. There were significant differences in amount and density of aerenchyma between individual cultivars at a given growth stage, as well as in the development patterns. While the amount and density of medium and small aerenchyma lacunae in the internodal region of the culm base did not show any relationship with MTC of rice cultivars, large aerenchyma lacunae exhibited highly significant correlations with MTC of different cultivars, suggesting that the wide variation in MTC of rice plants during different growth stages are related to these structural features.

Key words:

Aerenchyma - rice cultivars - methane emission - brightfield optical microscopy - image analysis

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