1,5-Diphosphate ribulose carboxylase/oxygenase (EC 4.1.1.39, Rubisco) is the key enzyme in the metabolism of photosynthetic carbon in higher plants, and it catalyzes CO₂ fixation and reduction. Rubisco controls the metabolic rate of photosynthesis and photorespiration and has a critical effect on the net photosynthetic rate. The enzyme comprises eight major subunit (rbcL) and eight small subunit (rbcs) L8S8 polymer composites. The expression of small subunits is important for regulating the expression of large subunits. Enzyme activity is important in photosynthetic electron transport and photochemical quenching.The influence of low temperature on plant physiological metabolism is multifaceted, especially with respect to the key factors that influence the photosynthetic system. The major manifestations are the blocking synthesis in the chloroplast, the destruction of chloroplast structure, the decrease in photosynthetic electron transport activity, and the decline in related enzyme activity in the process of photosynthesis. Under low temperature stress, reactive oxygen species accumulate and damage the photosynthetic apparatus through oxidation. In contrast, the light-harvesting ability, Photosystem II maximal photochemical efficiency (Fv/Fm), and other indices decrease. In addition, enzyme activities that catalyze the photo dark reaction also decline.Saussurea involucrata is a typical extreme low temperature-resistant higher plant. After a long period of natural selection in extreme environmental conditions, S. involucrata formed a series of mechanisms to adapt to harsh natural conditions, and developed a unique mechanism to maintain high photosynthetic activity at low temperatures. Previous research by our laboratory isolated a full-length cDNA encoding a Rubisco small subunit gene, named sikRbcs2, from a cDNA library of S. involucrata leaves induced by cold stress. Sikrbcs2 showed high identity of 57.54% to that of rbcs from Helianthus annuus, and could noticeably improve the cold resistance ability of tobacco.Tobacco is used as one of the model organisms for studying plant molecular biology. Low temperature has a clear effect on tobacco photosynthesis. To study the effects of the S. involucrata sikRbcs2 gene on plant photosynthesis under cold conditions, sikRbcs2-transgenic tobacco and non-transgenic tobacco plants were exposed to various temperatures (16, 10, 6,and 4 ℃) under low light (70 μmol m^-2s^-1) for 72 h, and the chloroplast pigment content, photosynthetic rate, and chlorophyll fluorescence parameters were measured in the leaves. The contents of chlorophyll a, chlorophyll b, and chlorophyll a+b were significantly higher in transgenic tobacco than in non-transgenic tobacco after cold stress treatment. Furthermore, the Fv/Fm, photosynthetic electron transport (ETR), and photochemical quenching coefficient (qP) of transgenic tobacco decreased more slowly than those of non-transgenic tobacco under low temperature stress. In comparison, the nonphotochemical quenching coefficient (qN) increased faster in transgenic than non-transgenic tobacco. Measurements of the photosynthetic parameters showed that, as temperature decreases, the net photosynthesis rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and intercellular CO₂ concentration (Ci) of both transgenic tobacco and non-transgenic tobacco declines. However, the Pn of transgenic tobacco exhibited a noticeably variable trend, initially falling sharply and then rising steadily. After cold treatment, the leaf plastochron index, shoot length, stem diameter, fresh weight of the shoot and root, and dry weight of the shoot and root were higher for transgenic than non-transgenic tobacco. The results indicate that low temperatures damage non-transgenic tobacco. In contrast, the damage to transgenic tobacco containing large numbers of sikRbcs2 proteins, which act as molecular chaperones, was reduced in response to cold stress, with better low temperature stress tolerance and higher biomass accumulation. Through the preliminary analysis ofchlorophyll fluorescence and photosynthesis characteristics of sikRbcs2-transgenic tobacco and non-transgenic tobacco plants under low temperature conditions, this study provides the experimental and theoretical grounding for the further study of the mechanism of how the sikrbcs2 gene acts in photosynthesis.