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.