CONCLUSIONS
On dystrophic anthroposoils and in increasingly warming-up and polluted
urban environment of e.g. Riga’s street greeneries, the large
environmental tolerance of also culturally important lime tree
ornamentals makes them especially suited with regard to effective
ecosystem services direly needed. The studied Tilia x vulgaris in
Riga’s street greeneries remediated essential nutrient deficiencies (N,
S, B) or tolerated them (K, Mn) rather well, similar to the
anthropogenically-enhanced metals (Fe, Cu). Their observed sensitivity
to salt contamination appears to relate to 1) poor or missing exclusion
uptake strategies leading to superior foliage concentration than in
similarly or more tolerant ornamentals, 2) missing allocation
management, with massive salt accumulation in stress-prone mesophyll and
3) substitution of K+ by Na+ in
vacuoles, at least of mesophyll and possibly guard cell sinks. These
sensitivity traits can provide improvement targets with a view to
selecting more tolerant Tilia species and cultivars. Findings in
this study led us to only partially validate our working hypotheses.
Nutrient deficiencies in foliage primarily related to other soil
pollution (Mn) or bedrock (K) issues, with salt pollution playing a
lesser role (rejection of Hyp. 1). Within foliage, the allocation of
salt contaminants primarily to mesophyll vacuoles was stress-prone at
tissue (rejection of Hyp. 2a) but safe at cell (validation of Hyp. 2b)
level. Structural changes in relation to storage (vacuole and cell size
increase) and toxicity (ACS symptoms) reactions to foliar accumulation
of salt contaminants were characteristic but not specific (partial
validation of Hyp. 3). Altogether, they suggested hindrance of circadian
cell size variation and evapotranspiration because of salt osmotic
effects, then indirectly enhancing photoinhibition and photo-oxidative
stress and promoting ACS. Direct interference by salt contamination on
the water exchanges between the foliar tissues and atmosphere form a so
far less frequently considered stress mechanism, which contribution to
the still partly elusive NaCl toxicity in tree foliage may be of
significant importance.