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Stylo [Stylosanthes guianensis(Aublet) Sw.] is an important forage legume and cover crop in tropical and southern subtropical regions. Its growth and survival are largely affected by low temperature in winter in subtropical regions. It is important to improve its chilling tolerance. S‐adenosylmethionine synthetase (SAMS) is the key enzyme catalyzing the formation of S‐adenosylmethionine (SAM), a precursor of polyamines synthesis. To improve chilling tolerance in stylo, MfSAMS1from Medicago falcatawas overexpressed in stylo plants. DNA gel blot hybridization and quantitative reverse transcription PCR analysis showed that MfSAMS1was inserted into the genomes of transgenic plants and expressed. Compared to the wild‐type, transgenic plants had enhanced SAM and spermidine levels and increased chilling tolerance with higher level of the maximum photochemical efficiency of photosystem II (Fv/Fm) and lower ion leakage after chilling treatment. Polyamine oxidase activity and H2O2levels were increased in transgenic plants compared with the wild‐type. In addition, higher activities of superoxide dismutase and catalase were observed in transgenic plants than in the wild‐type. The results suggest that overexpression of MfSAMS1promoted polyamine synthesis and oxidation, which in turn improved H2O2–induced antioxidant protection and protected transgenic plants against chilling stress. Core IdeasTransgenic stylo plants overexpressing MfSAMS1were produced.Plants showed promoted polyamine synthesis and oxidation.Transgenic plants had increased chilling tolerance with higher antioxidant levels. Transgenic stylo plants overexpressing MfSAMS1were produced. Plants showed promoted polyamine synthesis and oxidation. Transgenic plants had increased chilling tolerance with higher antioxidant levels.

Stylosanthes guianensis(Aublet) Sw., an important forage legume in tropical and subtropical regions, is sensitive to chilling stress. To understand its physiological mechanism of chilling resistance, physiological responses to chilling in antioxidants and photosynthesis in three γ‐ray‐irradiated chilling‐resistant mutants and the parent cultivar CIAT184 were analyzed and compared. Activities of superoxide dismutase, catalase, and ascorbate peroxidase and contents of ascorbic acid and glutathione increased in all plants within 4 d after chilling treatment at 6°C, followed by a gradual decrease. Chilling decreased the net photosynthetic rate, the maximum photochemical efficiency, quantum efficiency of photosystem II (PSII) photochemistry, photochemical quenching efficient, and excitation energy allocated to PSII photochemistry, while higher levels were maintained in the mutants than in CIAT184. Thermal energy dissipation increased during chilling and showed no difference between the mutants and CIAT184. Chilling resulted in an increase in the excess of energy excitation (Ex), but the mutants had lower levels of Exthan CIAT184. The results suggest that the higher levels of the antioxidant defense system induced by low temperature in the mutants may improve protection of photosynthesis against chilling‐induced oxidative damage and increase chilling resistance.