Potassium ions enhances photosynthetic tolerance to salt stress. We hypothesized that potassium ions, by minimizing the trans-thylakoid proton diffusion potential difference, can alleviate an over-reduced photosynthetic electron transport chain and maintain the integrity of the photosynthetic apparatus. This study investigated the effects of exogenous potassium on the transcription level and activity of proteins related to the photosynthetic electron-transport chain of tobacco seedlings under salt stress. Salt stress retarded the growth of seedlings, and caused potassium ion outflow from the chloroplast. It also lowered qP (indicator of the oxidation state of Q A), Y PSII (average photochemical yield of PSII) while increasing Y NO+NF (non-regulatory energy dissipation), accompanied by reduced expression of most light-harvesting, energy-conversion, and electron-transport genes. Interestingly, Lincomycin (a D1 protein-synthesis inhibitor) significantly diminished the alleviation effect of exogenous potassium on salt stress. We attribute the comprehensive NaCl-induced down-regulation of transcription and photosynthetic activities to ROS-induced retrograde signalling. There probably exists at least two types of ROS-induced retrograde signalling, distinguished by their sensitivity to Lincomycin. Exogenous potassium appears to exert its primary effect by ameliorating the trans-thylakoid proton diffusion potential difference caused by salt stress, thereby alleviating over-reduction of the photosynthetic electron transport chain, and maintaining the integrity of photosynthetic proteins.

Mulberry intercropped with alfalfa is a popular agroforestry system in China, which can produce high forage yields with high protein. To investigate the advantages of intercropping as well as the responses of mulberry-alfalfa intercropping systems to nitrogen application, we studied changes in the soil microbial communities and physiochemical properties in the rhizosphere of intercropped mulberry and alfalfa under nitrogen application. Nitrogen application increased available nitrogen contents (AN) and activities of urease (SUR) in rhizosphere soil of mulberry and alfalfa, but reduced soil organic canmatter (OM), irrespective of the cropping system (monoculture or intercropping). Nitrogen application increased soil pH in the rhizosphere of mulberry and alfalfa in monoculture, but reduced soil pH in the intercropped system. Nitrogen application and intercropping enhanced the soil water content (SWC) in mulberry rhizosphere soil and decreased SWC in alfalfa rhizosphere soil. We observed significant differences in the microbial communities inhabiting mulberry and alfalfa rhizosphere soils, indicating that mulberry and alfalfa are strategically complementary in terms of carbon sources. The Shannon-Weaver (H’), Simpson index (D), and McIntosh diversity (U) values were higher in rhizosphere soil of intercropped mulberry compared with mulberry in monoculture without nitrogen supply. There were no significant differences between mulberry in monoculture with nitrogen application (MNE) and mulberry intercropped alfalfa without nitrogen (M0). The results of the principal components analysis showed that MNE and M0 clustered in the scattered plots. We found no significant differences for H’ and D between monoculture and intercropping alfalfa systems, irrespective of the nitrogen application. Nitrogen application, intercropping, and intercropping with nitrogen increased the numbers of carbon sources, and relative use rates exceeded 4%. Nitrogen application and intercropping reduced the numbers of available carbon sources in alfalfa. Redundancy discriminatory analysis results suggest that pH and SWC were positively related with mulberry treatments and negatively with the alfalfa treatments.