During waterlogging, the oxygen concentration in soils decreases, and plant roots experience hypoxia. Plants with roots with low tolerance to waterlogging do not prosper well in wet soils due to decreased root respiration and mineral absorption [7–11]. Wetland plants and some upland plants can survive under flooding conditions because of the well-developed aerenchyma tissue in their roots [12–17]. A white, spongy tissue with substantial gas space covers the stem base, roots, and nodules of some flood-tolerant leguminous plants, such as Viminaria juncea [18], Lotus uliginosus [19, 20], Lotus corniculatus, Lotus glaber [21], and Glycine max [14, 15, 22]. Under waterlogging conditions, this spongy tissue is connected with the aerenchyma tissues at the nodes. Therefore, this tissue is regarded as a continuous aerenchymatous link and is called secondary aerenchyma [15, 23]. Plants growing in waterlogged conditions have shallow roots that are distributed near the soil surface [6, 24–26]. Swelling of the submerged portions of the lower shoot [27] and development of hypertrophic lenticels [27, 28] facilitate oxygen entry into the aerenchyma of nearby adventitious roots [29]. The swelling of the submerged portions of the lower shoot is probably a consequence of cell expansion promoted by endogenous ethylene formed in the submerged tissue by water [28, 29]. A more widespread response is the emergence, from the shoot base, of aerenchymatous adventitious roots to replace longer and deeper roots damaged or killed due to anoxia [29, 30]. In grains such as maize, ethylene promotes the outgrowth of root primordia from the stem base [29]. The survival of some species under flooding conditions has been attributed to shallow rooting [31]. The wheat lines with shallow roots show higher tolerance to waterlogging than those with deep roots [
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