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To investigate the effects of exogenous spermidine (Spd) on metabolism changes under salt stress in cucumber roots, a gas chromatography-mass spectrometry (GC-MS) was performed. The results showed that most of the 142 metabolites responded to salt stress or exogenous Spd treatment. Salt stress reduced carbon consumption, resulted in the transformation of glycolysis and the tricarboxylic acid (TCA) cycle to the pentose phosphate pathway (PPP), and meanwhile increased salicylic acid (SA) and ethylene synthesis, and, thus, inhibited the growth of seedlings. However, exogenous Spd further improved the utilization of carbon, the energy-saving pattern of amino acid accumulation, and the control of hydroxyl radicals. In conclusion, Spd could promote energy metabolism and inhibit SA and ethylene synthesis in favor of root growth that contributes to higher salt tolerance. This study provides insight that may facilitate a better understanding of the salt resistance by Spd in cucumber seedlings.

Drought stress is a prevalent abiotic stress that adversely affects multiple physiological processes in plants, especially their photosynthetic capacity. Application of plant growth–promoting rhizobacteria (PGPR) has been considered as an eco-friendly strategy to ameliorate the deleterious effects of drought stress on plants. The present study was carried out to investigate the effects of Bacillus amyloliquefaciens QST713 on plant growth, leaf relative water content (RWC), photosynthesis processes, photosynthetic pigment content and antioxidant enzyme activities in two alfalfa varieties, Galalxie Max (drought-tolerant) and Saidi 7 (drought-sensitive) under drought conditions. The results showed that drought stress significantly declined plant biomass production, RWC, photosynthetic pigment content (Chl a, Chl b and carotenoids) and photosynthetic gas exchange parameters (transpiration rate (Tr), stomatal conductance (Gs) and intercellular CO2 concentration (Ci)), whereas it enhanced the enzymatic activity of peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) in both cultivars. In contrast, the inoculation of the bacillus strain QST713 was more effective on plant growth, showing higher plant biomass production compared to the non-inoculated plants under drought stress. Moreover, the application of QST713 significantly promoted the content of RWC, the accumulation of chlorophyll content and the activities of antioxidant enzymes as well as enhanced the photosynthetic capacity of alfalfa seedlings under drought stress. These results suggest that QST713 could be considered as a promising bio-inoculant for plants exposed to environmental stresses.

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