Internal Gas Transport Mechanism

Despite the extreme conditions in waterlogged soils, most wetland plants are able to survive, grow and reproduce there. In contrast to typical land plants, wetland plants have anatomical and physiological attributes necessary for their long-term survival under waterlogged soil conditions [17-20, 44]. The degree of adaptation is specific to individual species and their survival varies from a few hours to several months [45]. This adaptation is based on an extensive gas (oxygen) transport mechanism that enables the plants to provide their roots with atmospheric or internal photosynthetic oxygen for respiration and to oxidize reduced compounds in the rhizosphere [17]. This process has been considered as a major mechanism for coping with soil anaerobiosis [17-20, 22, 36, 38-43, 46-48]. Gas transport of air (oxygen) from aboveground parts of the plants to the underground parts for respiration and release into the flooded rhizosphere occurs by special internal tissues forming open channels with low flow resistance, known as aerenchyma [17]. Depending on the degree of adaptation, aerenchyma can account for as much as 60% of the total tissue volume [49]. In addition, this tissue enhances the potential for oxygen to be transported to the remote underground parts of the plant by diminishing the internal volume of respiring tissues and oxygen consumption [17-20, 50]. The genesis of aerenchyma structures by cell lysis or cell formation as well as their anatomical peculiarities has been thoroughly studied physiologically and anatomically [17, 42, 51-53].

Diffusion is one mechanism for moving the oxygen within the plants [54]. In addition, intensive convective gas flow induced by pressure gradients and known as "ventilation" or "through flow" exists in many wetland plants [18-20, 55-60]. The pressure gradients are formed by low pressure in oxygen-consuming tissues of the plants caused by different solubilities of the oxygen consumed and the carbon dioxide formed in this process, and by high pressure in the plant's leaves resulting from the inflow of atmospheric gases. The higher pressure in the leaves causes air to flow throughout the entire body of the plant, including the whole root system, by entering the aerenchyma tissues and leaving the plant through older leaves with lower stomatal conductance [49,

61]. The types and combination of the transport mechanisms (diffusion and/or convection) involved are specific to each plant species. Intensive through flow has been observed in Typha latifolia (cattail) and Phragmites australis (reed) for example [57,

62]. The inflow of air into the leaves to produce high internal pressure is mainly induced by temperature and humidity gradients between the inside of the leaves and the ambient air [18, 20, 49, 52, 58, 61-64].

Different mechanisms of oxygen transport to and through the plants, including the processes of the temperature and humidity induced turnover of air and its effects, are described in detail in [50] and [51]. The processes involved in ventilation in plants have been studied since the mid-19th century [65]. This interest has been increasingly revived since the 1980s, at least in connection with the growing attention paid to the biotechnological usage of helophytes to clean wastewater in constructed wetlands [65].

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