Pb-N complex stress mitigates the physiological damage of a single stress (Pb or N) on bryophytes

Nitrogen (N) deposition levels and the frequency of lead (Pb) contamination events are increasing globally. In an effort to improve our understanding of plant responses to these stressors, we investigated moss responses to single and combined Pb and N stress. Three mosses from different habitats (Syntrichia caninervis, Bryum argenteum and Plagiomnium acutum) were studied and simulated Pb/N single and complex stresses were applied to them indoors. The chlorophyll (Chl) content, osmotic adjustment substances content, and antioxidant enzyme activities were measured at 7, 14, 21, and 28 days. The results revealed that the tolerance of the three bryophyte species to Pb or N stress was in the order of P. acutum > B. argenteum > S. caninervis, which was closely related to the conditions of their respective natural habitats. S. caninervis and B. argenteum were stress tolerant for 7 days and P. acutum for 14 days. The bryophytes were tolerant to Pb or N stress after the contents of osmoregulatory substances and antioxidant enzyme activities increased; however, as toxicity accumulated over time, all three species suffered irreversible damage, as indicated by an abrupt decrease in the Chl content and osmoregulatory substances, as well as a sudden drop in antioxidant enzyme activities. Under the combined effects of Pb-N stress, the Chl content, osmoregulatory substance contents, and antioxidant enzyme activities were significantly higher in the N-loving P. acutum (N produced significant benefits) than in P. acutum exposed to Pb stress alone. This phenomenon is likely because Pb and N have antagonistic effects on the growth of P. acutum; thus, their recombination generates a counter-balancing effect. In the N-sensitive species, S. caninervis and B. argenteum (N caused obvious toxicity), the indicators were slightly better than under N tress alone (indicated by the reduction of membrane lipid peroxidation and increased osmoregulatory substance contents and enzyme activities), suggesting that there is a certain antagonistic effect exerted by the simultaneous addition of Pb and N. Therefore, the detrimental effects of a single abiotic stress (Pb or N) on bryophytes may be diminished under the combined conditions of N deposition and presence of the heavy metal, Pb.