Physiological responses of mosses to combined stress of low temperature and elevated nitrogen deposition
Due to the rapid development of industry and agriculture, increasing nitrogen deposition becomes a global environmental problem. In addition, extreme weather conditions, such as abnormally low winter temperatures have been more frequently occurred in recent years. Thus, what will happen when plants suffered high N deposition and low temperature stress (LTS) simultaneously? Will N deposition affect the recovery of plants from LTS? Results from the growth experiments were controversial, whereas, the underlying physiological mechanisms have been seldom studied. In the study, two moss species common in south China, Pogonatum cirratum subsp. fuscatum and Hypnum plumaeforme, were chosen to investigate the physiological responses of moss to low temperature stress and subsequent short-term recovery under varied nitrate (KNO3) or ammonium (NH4Cl) supply rates (0, 20, 40 and 60 kg N hm-2 yr-1). The temperature of LTS was set to 1 ºC (dark)/3 ºC (light) vs 7 ºC (dark)/15 ºC (light) control. We monitored 24 indices relative to carbon and nitrogen metabolisms immediately after the stress and after 10 days of recovery respectively. The indices included contents of sugars, nitrogen, FAAs and activities of ribulose-1,5-bisphosphatecarboxylase (RuBPC), sucrose-phosphate synthase (SPS), sucrose synthase (SS), glutamine synthethase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH) etc.
Both high nitrogen and LTS caused serious damages to photosynthetic carbon assimilation in the mosses, and the adverse effects of LTS were exacerbated by combined N application, with ammonium being more toxic. N applications stimulated the activities of almost all tested enzymes and increased the contents of non-protein nitrogen (NPN), total free amino acids (FAA), proline and some N-rich amino acids, such as arginine and histidine. Under LTS conditions, activities of SPS and SS maintained relatively high, whereas the activities of GS/GOGAT pathway in both mosses were dramatically inhibited and those of IDH (isocitrate dehydrogenase) and GDH were almost not affected. LTS also caused an increase in starch contents. After subsequent recovery from LTS, photosynthetic carbon assimilation in both mosses significantly increased and the scope of increase decreased with increasing N application rates. SS/SPS activities, GS/GOGAT activities as well as starch content also increased after recovery, whereas IDH activity decreased at most cases. Meanwhile, contents of total FAA and NPN in both mosses were significantly decreased after recovery. Our results indicated that compensatory effects existed in both species during the recovery from stress, though the extent of compensation was closely related to N treatment.