Introduction
During the winter season, different
chemical and abrasive materials are being spread for de-icing road and
sidewalk pavement, with cheap sodium chloride (NaCl) most commonly used
(Dobson, 1991, Cunningham et al., 2008, Fayun et al., 2015,
Ordóñez-Barona et al., 2018, Dmuchowski et al., 2019). Alone, moistened
or mixed with sand, the amounts of NaCl reach 19.6 tons per lane km in
New York State (1.1 million tons year-1, Cunningham et
al., 2008), more than 200’000 tons in Poland (Marosz, 2011),
16’000-31’500 tons in Edmonton (Equiza et al., 2017), 2 kg
m-2 in Denmark (Pedersen et al., 2000) and 4.06 kg
m-2 in Riga (Cekstere et al., 2008) annually. Such
high amounts cause substantial environmental contamination by leaching
and uptake in the surrounding soil and vegetation during the subsequent
vegetation seasons (Pedersen et al., 2000, Bryson & Barker 2002;
Ordóñez-Barona et al., 2018, Nikolaeva et al., 2019). The contaminants
absorbed by roots and translocated to foliage cause leaf necrosis, crown
defoliation, twig dieback and street tree decay (Dobson, 1991, Bryson &
Barker, 2002, Paludan-Müller et
al., 2002, Cekstere et al., 2008, Dmuchowski et al., 2014,
Milewska-Hendel et al., 2017). In turn, the important ecosystem services
provided by street trees, such as cooling of urban climate, mitigation
of air pollution, reduction of street runoff or promotion of
biodiversity can be seriously affected (Moser et al., 2015, Nowak et
al., 2017; Bouraoui et al., 2019).
In street greeneries, NaCl contamination affects the whole urban
ecosystem. In the soil compartment, Na accumulation destroys the soil
aggregates, thus reducing the soil porosity and promoting puddling in
the case of fine-textured soils (Marschner & Marschner, 2012; Bryson &
Barker, 2002). The effects on soil chemistry include the degradation of
soil organic matter, increase in pH, dislodging of cations within
absorption complexes or nitrate leaching as a consequence of enhanced
nitrification rates (Cunningham et al., 2008; Dmuchowski et al. 2014;
Eimers et al., 2015). Directly or indirectly, the soil salinization can
depress the biological processes and alter the microbial communities in
the rhizosphere (Ke et al., 2013). Tree lines planted on
salt-contaminated soils can suffer under osmotic and ionic stress (Munns
& Tester, 2008), similar to ‘physiological drought stress’ (Dobson,
1991), which can reduce the leaf gas exchanges and growth rates (James
et al., 2006, Munns & Tester, 2008, Cekstere et al., 2015). Within leaf
tissues, the accumulation of salt ions can impair several enzymatic
activities, inhibit membrane functions, promote nutrient imbalance,
decrease chlorophyll concentration, and significantly affect essential
physiological activities (Muszynska et al., 2014; Negrão et al., 2017).
Molecular responses and tolerance mechanisms have been extensively
investigated and reviewed (Munns & Tester, 2008; Polle & Chen, 2015;
Flowers et al., 2015; Munns et
al., 2016; Wu et al., 2018). Structurally, various studies, so far not
yet comprehensively reviewed, have reported about cellular responses
such as enhanced oxidative stress (Hernandez et al., 1995; Benzarti et
al., 2012) and numerous alterations in cytoplasm and e.g.chloroplast or mitochondria (Naidoo et al., 2011; Yamane et al., 2012;
Ivanova et al., 2016;). However, quantitative evidence relating the
levels of NaCl contamination to structural injury is still largely
missing and the mechanisms of NaCl toxicity in foliage thus remain
partly elusive (Munns & Tester, 2008). Moreover, most reports
illustrating microscopic injury miss microlocalisation evidence,
complicating the distinction between direct and indirect effects by salt
contaminants.
Adaptations to elevated salinity show similarities with those in the
case of infertile soils (Chapin,
1980). They may have evolved in the framework of exclusion or inclusion
strategies, the latter conferring lower tolerance to chronic salt stress
(Chen et al., 2018). Tolerance mechanisms in includers may encompass
vacuolar allocation of contaminants whilst keeping a high
K+/Na+ ratio, allocation to
non-photosynthetic tissues, leaf succulence or translocation to older
foliage organs (Ottow et al., 2005; Flowers et al., 2015; Polle & Chen,
2015; Wu et al., 2018). Safe allocation of salt contaminants is thus
crucially important with a view to salt tolerance in salt includers
(Polle & Chen, 2015; Munns et al., 2016). Given their high mobility
however, the microlocalisation of Na+ and
Cl- ions within tissues and cells is demanding
methodologically (Frey & Zierold, 2003; Wu & Becker, 2012) and still
limited evidence is so far available. The latter has been obtained
applying various microscopical techniques, but control evidence for
excluding dislodging artefacts is generally missing.
After characterizing the extent of soil pollution by salt contaminants
(Cekstere & Osvalde, 2013) or the toxic impact of high salt
concentration in foliage (Cekstere et al., 2008), the present study has
focused on mechanistic aspects of chronic salt stress in foliage of
urban lime trees (Tilia spp. ) in the street greenery of Riga,
Latvia. Its main objectives included to 1) relate the allocation of salt
contaminants within leaf tissues and cell compartments to injury at
cell, tissue and whole leaf level, 2) identify the most characteristic
symptoms and cellular responses to salt stress and 3) distinguish direct
and indirect salt stress effects from those by other environmental
factors. We had the following working hypotheses: 1) by changing the
nutrient balance in the soil, the de-icing salt contaminants cause
nutrient imbalance within foliage of street trees; 2) the salt
contaminants are primarily allocated to safe storage compartments,
including the epidermis at leaf (2a) and vacuole at cell (2b) level; 3)
salt accumulation in leaves causes specific structural changes relating
to storage (3a) and toxicity (3b).
Using 8 street sites forming a
gradient of foliage contamination representative of the salt pollution
in Riga’s street greenery (Cekstere et al., 2008), we 1) characterized
the soil and foliage chemistry, 2) analyzed the distribution of salt
contaminants and main nutrients within foliar tissues and cells and 3)
quantified structural and ultrastructural markers of salt stress within
assimilating tissues.