Structural responses to salt accumulation in foliage
Notwithstanding concomitant Mn and K deficiency and likely physiological
drought stress, the visible and microscopic injuries in lime tree
foliage from salt-polluted street lines of Riga could be attributed to
the sole effects of salt stress. Visible injury was similar to that
reported in lime trees or other broadleaved species from Warsaw
(Dmuchowski et al., 2013), Opole (Czerniawska-Kusza et al., 2004),
London (Gibbs & Palmer, 1994) or coastal ecosystems under salt stress
(Vollenweider & Günthardt-Goerg, 2005). Symptoms appeared partly
similar to those resulting from drought stress or K deficiency but
clearly distinct from Mn deficiency injury (Vollenweider &
Günthardt-Goerg, 2005; Hartmann et al., 2007; Papadakis et al., 2007).
However, several microscopic traits (i.e. cell hypertrophy /
normal chloroplastic grana stacks / ubiquitous starch grains) clearly
excluded any significant contribution to injuries by environmental
constraints other than salt stress (Fink, 1999; Vollenweider et al.,
2016). Moreover, changes in the foliar concentration of Na/Cl explained
79%/65% of variation in the necrosis percentage area within leaves.
The salt-driven increase in the vacuole/cell size and microscopic injury
in cytoplasm were indicative of sink adjustments and degenerative
responses. Larger vacuome and cells form structural hallmarks of
so-called leaf succulence trait within salt-exposed foliage (Ottow et
al., 2005; Benzarti et al., 2014; Polle & Chen, 2015). By contrast with
healthy succulent plants however (e.g. Kondo et al., 1998) and as
indicated by LMEM and RDA models, larges vacuoles in lime tree mesophyll
did not appear for ontological reasons but resulted from phenotypic
adjustments to increasing salt accumulation. Moreover, they were
associated to a syndrome of typical but unspecific degenerative traits
in adjacent cytoplasm (i.e. cytoplasm and organelle condensation
/ autophagic activity / plastoglobule size increase and extrusion into
vacuoles). Indeed, most of these injuries are being observed not only in
the case of salt (Hernandez et al., 1995; Naidoo et al., 2011; Yamane et
al., 2012; Ivanova et al., 2016) but also ozone (Vollenweider et al.,
2019) or drought stress (Fink, 1999; Vollenweider et al., 2016) or with
ontological senescence (Mikkelsen & Jorgensen, 1996; Inada et al.,
1998). Similar to latter studies, they indicated an acceleration of cell
senescence (ACS; Günthardt-Goerg & Vollenweider, 2007). The lack of
more specific e.g. swelling of chloroplastic thylakoids (Fink,
1999), as commonly observed during experimental salt exposure (Hernandez
et al., 1995; Guan et al., 2013; Bejaoui et al., 2016), may relate to
the primarily vacuolar allocation of salt contaminants. Acclimated
halophytes from saline environments can also miss this trait (Naidoo et
al., 2011).
The drawback of higher vacuolar osmolarity, caused by steady NaCl
accumulation, may comprise impeded leaf conductance, leading in turn to
observed ACS in synergy with disturbed hormonal balance. Mesophyll
succulence-like, as a consequence of concomitant NaCl accumulation and
autophagy, may decrease leaf conductance by reducing the circadian
variation of cell volume and water exchanges (Canny & Huang, 2006)
driven by evapotranspiration and causing diel changes in the leaf
thickness (Westhoff et al., 2009; Ehrenberger et al., 2012). Within
epidermis and together with K+,
NO3- and malate2-,
Cl- is one of the main osmolytes which fluctuations
actuate guard cell/vacuome size variation and stomatal opening (Daloso
et al., 2017). Interestingly in some adapted halophytes, better
salt-tolerance appear to be conferred by the lower NaCl and steady
K+ concentration to be found in guard versusepidermal neighbor cells (Perera et al., 1997; McCully et al.,
2010). Furthermore,
Cl- > Na+ are abscisic
acid inducers, the main hormone regulator of stomatal closure (Geilfus
et al., 2018) and lower stomatal conductance forms an ecophysiological
hallmark of salt stress (Munns & Tester, 2008). Lowered stomatal
conductance is consistent with enhanced senescence, as indicated by the
aforementioned ACS responses (Bond, 2000, Munns & Tester, 2008;
Daszkowska-Golec & Szarejko, 2013). Farther, it can enhance
photoinhibition and photo-oxidative stress, especially in high light
environment as in the outer tree canopy or at many saline habitats
(Foyer et al., 1994; Suzuki et al., 2012). Enhanced oxidative stress is
another consequence and hallmark of salt accumulation in foliage
(Hernandez et al., 1995; Benzarti et al., 2014; Polle & Chen, 2015),
and its structural markers (Günthardt-Goerg & Vollenweider, 2007; Moura
et al., 2018) show many similarities with those observed in lime tree
foliage.