4.2. Anatomical adaptive strategy i

4.2. Anatomical adaptive strategy involved in roots
Anatomical adaptive strategy to floodingwas widely reported in
wetland plants (Armstrong, 1971; Koncalová, 1990; Evans, 2003).
Diffusivity of oxygen in water is approximately 10, 000 times
slower than in air, thus direct exchange of oxygen between
submerged tissues and the environment is strongly impeded under
waterlogging (Colmer, 2003). Success of wetland plants, including
mangroves, growing in such an anaerobic condition, is generally
ascribed to anatomical adaptations that allow sufficient oxygen can
be transported to submerged roots (Kludze et al., 1993; Youssef and
Saenger, 1996; Pi et al., 2009). Developed aerenchyma within plants
provides a low-resistance internal pathway for oxygen transport
between shoots and roots (Jackson and Armstrong, 1999; Colmer,
2003; Bodegom et al., 2005). Besides, lignification and suberization
within exodermis in subapical root regions contributed to the
formation of oxygen diffusion barrier on root surface (Soukup et al.,
2007; Garthwaite et al., 2008), which directly reduced radial
oxygen loss (ROL) and oxygen leakage during long-distance of
oxygen transport within plants (Armstrong and Wright, 1975;
Colmer, 2003; Colmer and Pedersen, 2008).
Our previous studies (Cheng et al., 2010, 2012) and the present
study showed that both metals and salt could directly promote the
levels of lignification within exodermis and endodermis. Armstrong
and Armstrong (2001, 2005) also reported a similar anatomical
response in Phragmites and rice when they were exposed to other
toxins, such as sulfide and organic acids. These anatomical symptoms
were considered as a barrier property to prevent further
ingress and the spread of toxins within the plants and possible
fungal infection (Degenhardt and Gimmler, 2000; Hose et al.,
2001). Such anatomical adaption of roots was related to the fact
that they were in direct contacted with the surrounding environment,
and so served as the first guard for preventing excessive
biotic and abiotic toxins entering into the plants. Deng et al. (2009)
and Cheng et al. (2012) also reported that the plants with significant
lignified (impermeable) exodermis tend to intake fewer metals
(e.g., Fe and Zn) than the plants with a relative high permeability in
exodermis. Our previous study (Cheng et al., 2010) have investigated
the transverse distribution of heavy metals (e.g., Zn) in roots
by scan electron microscope and X-ray microanalysis (SEM X-ray
microanalysis), and the results showed that lignified (impermeable)
exodermis acted as one of the major barriers to metal
movement and is considered as the first line of defence against
metal entering into the roots. Most heavy metals were located in
plant cell walls, existing in ion form or chelating with structural
materials such as lignin (Nishizono, 1987). A significantly increased
lignification induced by salt suggests that more metals were
combined within cell walls, and hence, delayed the further transfer
of excessive metals among cells or tissues and mitigated the harm
to plants. Additionally, lignified exodermis could directly reduce
radial oxygen loss (ROL) and rhizosphere oxidation. Most heavy
metals in mangrove sediments are combined with sulphur (S) and
formed steady non-biotoxic chelation (Chen and Jiao, 2008). It has
been confirmed that a reductive condition is advantageous to this
chelation process (Youssef and Chino, 1989; Ye et al., 1997; Jacob
and Otte, 2003; Tao et al., 2003). Thus, lignified exodermis
induced by salt would maintain a moderately reductive rhizosphere,
leading to less metal bioavailability in the rhizosphere.
More detailed studies on the dynamics of metals in mangrove
sediments related to root activities should further be conducted.
As for N deficiency, decreased lignification was observed. Here
as a strategy it can be speculated that the plants tried to increase N
uptake and translocation under N deficient conditions. Decreased
lignification within exodermis would promote ROL from roots
(Soukup et al., 2007). The promotion of ROL could then increased
iron plaque formation on root surface and enhanced rhizosphere
nitrification, leading to an enhancement of N bioavailability in
rhizosphere (Li et al., 2008).