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 (CH 4 ) from the soil to the atmosphere. To clarify the pathways of CH 4 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 × 10 3 -5 × 10 3 µm 2 ), medium (> 5 × 10 3 -20 × 10 3 µm 2 ) and large aerenchyma lacunae (> 20 × 10 3 µm 2 ) 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 rootculm 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 culm-leaf 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 aeren-chyma 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.

Reviews and syntheses: Four decades of modeling methane cycling in terrestrial ecosystems

“…The complicated effects induced by a few key factors in CH 4 processes have been mathematically described and incorporated into many CH 4 models, for example, direct factors such as soil temperature, moisture, oxygen availability, soil pH, and soil redox potential (Grant, 1998;Riley et al, 2011;Zhuang et al, 2004). The indirect factors such as nitrogen input (Banger et al, 2012), irrigation , and agricultural practices were not reviewed in this study as their impacts are indirect and were modeled through impacts on vegetation and hydrology (Li, 2000;Ren et al, 2011;Xu et al, 2010).…”

“…The parameters for plant species capable of transporting gas (i.e., aerenchyma) are poorly constrained (Riley et al, 2011), although plant-mediated transport has been identified as the dominant pathway for CH 4 emission in some natural wetlands (Aulakh et al, 2000;Colmer, 2003).…”

“…The dominant direct factors controlling methanogenesis and methanotrophy in most ecosystems include oxygen availability, dissolved organic carbon concentration, soil pH, soil temperature, soil moisture, nitrate and other reducers, ferric iron, microbial community structure, active microbial biomass, wind speed (Askaer et al, 2011), plant root structure (Nouchi et al, 1990), etc. Indirect factors include soil texture and mineralogy, vegetation, air temperature, soil fauna, nitrogen input, irrigation, agricultural practices, sulfate reduction, and carbon quality (Banger et al, 2012;Bridgham et al, 2013;Hanson and Hanson, 1996;Higgins et al, 1981;Mer and Roger, 2001). The complicated effects induced by a few key factors in CH 4 processes have been mathematically described and incorporated into many CH 4 models, for example, direct factors such as soil temperature, moisture, oxygen availability, soil pH, and soil redox potential (Grant, 1998;Riley et al, 2011;Zhuang et al, 2004).…”

Greenhouse gas emissions from rice microcosms amended with a plant microbial fuel cell

“…The main pathway of gas exchange with the rhizosphere under waterlogged conditions is by transport through the aerenchyma of plants (90%). Other mechanisms include diffusion and ebullition (Aulakh et al 2000;Bazhin 2010). Therefore a promising route for mitigation of CH 4 emissions is to prevent the formation of CH 4 in the rhizosphere.…”

“…Therefore a promising route for mitigation of CH 4 emissions is to prevent the formation of CH 4 in the rhizosphere. Next to introduction of alternative electron acceptors, other mitigation strategies include fertilizer management, water management, biochar addition to the soil and choosing crop varieties with little aerenchyma or little rhizodeposition (Aulakh et al 2001;Aulakh et al 2000;Conrad 2002;Majumdar 2003;Singh et al 1999;Zhang et al 2012).…”