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).