Figure 4 . The feedback loop of plant response to waterlogging condition, coupled to plant hydraulics and photosynthesis. It contains general plant responses shown in black arrows that deteriorate plant oxygen status during waterlogging. The “escape” strategy (i.e. , aerenchyma formation and adventitious root development) shown in the pink box and pink arrows imposes an overall negative feedback loop on root oxygen that serves as the self-rescuing mechanism.
In plants that are waterlogging-tolerant, the acclimation processes to waterlogging through ethylene signaling can serve as a negative feedback loop that ameliorates root oxygen condition (pink arrows in figure 4). Root ethylene accumulation that results from the entrapment by the waterlogged soil triggers aerenchyma formation and adventitious roots development. Aerenchyma promotes internal oxygen transport and adventitious roots serve as an oxygenated alternate root system, serving as self-rescuing mechanisms.
Plant responses to complete submergence are shown in figure 5. Again the general submergence responsive processes are displayed with black arrows, including the inhibition of oxygen diffusion from ambient environment not only to root but also to root, the reduced photosynthesis due to turbidity and limited CO2, and a transition from aerobic to anaerobic metabolism (Mommer et al., 2005). Plant internal water transport is hampered as there is no water potential gradient in the plant. Shoot-root oxygen diffusion is low due to not only the resistance but also reduced shoot oxygen level, which mostly comes from the reduced photosynthesis (Colmer & Pedersen, 2008). Therefore, photosynthesis is hampered due to the water turbidity and the limited dissolved CO2. This results in not only a reduction in carbohydrate reserves, but also decreased shoot oxygen level, as shoot oxygen mostly comes from photosynthesis under submergence. Consequently, root oxygen deficit is further exacerbated by limited shoot-root oxygen transport, usually reaches anoxia within 24 hours with shoot hypoxia (Sasidharan & Voesenek, 2015).
Submergence-tolerant plants can, depending on the species and/or ecotype, either self-rescue through “escape” strategy or self-preserve through “quiescence” strategy. Both strategies are mediated by shoot endogenous ethylene, which quickly accumulates due to floodwater entrapment. In plants that typically employ the “escape” strategy, self-rescuing negative feedback loop can be completed through shoot elongation promoting photosynthesis, shoot-derived adventitious roots promoting shoot oxygen level, and aerenchyma promoting internal oxygen transport (pink arrows in figure 5), whereas shoot elongation and adventitious root development consumes carbohydrate and energy reserves, introducing an extra exacerbating positive feedback loop. In plants that typically employ a “quiescence” strategy, self-preservation is introduced by shoot ethylene triggering SUB 1A expression that represses shoot ethylene biosynthesis, gibberellin signaling pathway and energy and carbohydrate consumption (Das et al., 2005) (purple arrows in figure 5).