Effect of arbuscular mycorrhizal fungi on the relationship between root respiration and ultraweak luminescence under salt stress in strawberry

[Objective] Soil salinity poses a significant challenge to global agriculture. Salt stress inhibits root respiration and disrupts physiological and metabolic activities, impairing essential functions such as absorption and transport. Arbuscular mycorrhizal fungi (AMF) enhance plant resistance to salt stress naturally and in an environmentally friendly way. Ultraweak luminescence (UWL) is a natural phenomenon observed in all living organisms, though its mechanisms remain largely unknown. The objectives of this study were to investigate the changes in root respiration, enzyme activities, and UWL intensity after AMF inoculation on the roots of strawberry under salt stress, and to assess the possibility of UWL as a potential indicator of plant physiological status under salt stress. [Methods] The strawberry cultivar Benihoppe was selected for the experiments. The strawberry plantlets were transplanted into the pots in greenhouse. The substrate was composed of peat, vermiculite and perlite (1∶1∶1). The substrate was sterilized by high temperature before treatment. The treatments included AMF inoculation (Glomus mosseae), salt stress, salt stress with AMF inoculation. The sterilized substrate used as control.90 mmol·L-1 NaCl and Na2SO4 salt solution were used for salt stress treatment. Each pot was irrigated with 600 mL of the salt solution, while the control was irrigated with the same amount of water. The samples were taken after 1, 3, 5, 7, and 9 days after the treatments. The main measurements included root respiration rate, enzyme activities (glucose 6-phosphate dehydrogenase, phosphofructokinase, NAD-malate dehydrogenase), and UWL intensity of the roots and leaves. For the measurements of root respiration rate, enzyme activities, and UWL intensity, the samples were taken randomly from the roots. For the measurements of UWL intensity of the leaves, three mature leaves of similar length were selected. Each test was repeated three times for accuracy. The statistical analyses were conducted to observe changes in the indicators and assess AMF's effectiveness in mitigating salt stress effects. [Results] A significant decrease in the root respiration and enzyme activities was observed under the salt stress. Specifically, glucose-6-phosphate dehydrogenase, phosphofructokinase, and NAD-malate dehydrogenase activities decreased under the salt stress, reflecting its negative impact on root respiratory metabolism. Without salt stress, AMF-inoculated plants showed higher and more stable respiration rates, enzyme activities, and UWL intensities compared with those of the control. This suggested that under normal conditions, AMF inoculation could improve the overall physiological status of the strawberry root system. Under the salt stress, AMF inoculation effectively mitigated negative effects, increased respiration rate, enzyme activities, and UWL intensities compared with the control. These findings suggested that AMF would protect root function and metabolic activities under saline conditions. Additionally, there was a positive correlation between the UWL intensity, root respiration, and key respiratory enzyme activities in the leaves and roots of strawberry. [Conclusion] There was a close relationship between strawberry root respiration intensity, related enzyme activities, and UWL of the roots and leaves of strawberry. The salt stress greatly reduced the root respiration rate and respiration-related enzyme activities, associated with oxidative stress processes. The reduced metabolic activities would result in lower UWL intensity, reflecting the adverse effects of salinity on the physiological state of the plants. The AMF inoculation would mitigate the inhibitory effects of salt stress on the root respiration and enzyme activities, thereby slowing the decrease in the UWL intensity. This suggested that under saline conditions, AMF would be helpful in maintaining higher metabolic activities and improve the overall physiological status of the root system, enhancing salt tolerance in strawberry plants. These findings highlight the potential of AMF to enhance strawberry salt tolerance by maintaining root respiratory metabolic activity, while the correlation suggests that the leaf UWL intensity could serve as a reference for non-invasive indicators of physiological status of strawberry under experimental salt stress conditions. This study would provide new insights into AMF's protective mechanisms under sthe alt stress and deepen our understanding of plantstress interactions and AMF's role in enhancing crop stress tolerance.