Aims Low temperature at the seedling stage is the key factor constraining growth and yield in spring-sowing soybeans (Glycine max) in North China. The objectives of this research were to explore responses of soybean cultivars of different eco-types in Shanxi Province to low temperatures at the seedling stage, to develop indices and approaches for comprehensive evaluation of low-temperature tolerance in soybeans, and ultimately to provide theoretical basis for breeding soybean cultivars of low-temperature tolerance in order to maintain stable yield under the conditions of low temperature stress at the seedling stage.
Methods We selected three soybean cultivars (‘Jinda 53’, ‘Jinda 70’ and ‘Jindou 24’) bred and grown widely in Shanxi Province. At the seedling stage, the plant materials were subjected to stress treatments at three levels of low temperatures and a control treatment in growth cabinets. The low temperature treatments were set for 14, 17 and 20 °C, respectively, and the control treatment was maintained at 23 °C. Upon completion of the temperature treatments, the seedlings were transplanted into larger pots and grown in a greenhouse. Values of photosynthetic traits (including net photosynthetic rates (Pn), intercellular CO2 concentration (Ci), stomatal conductance (Gs) and transpiration rate (Tr)) were determined on seedlings. The yield traits (plant height, plant mass, pods per plant and yield per plant) were assessed after harvesting. Furthermore, the genotype main effect plus genotype-environment interaction (GGE) biplot was employed to analyze the effects of various traits on different cultivars and correlations among the traits. Then, subordinate function was applied for comprehensive evaluation of low-temperature tolerance of the three cultivars studied.
Important findings (1) The seedling stage of all three cultivars was extended by 1–12 days by the low temperature treatments. (2) Values of the four photosynthetic traits were reduced in seedlings subjected to low temperature treatments; the rate of reduction was significantly greater for Gs and Tr than for the other two traits. Reductions in the values of photosynthetic traits were significantly smaller in the cultivar ‘Jinda 53’ than in other two cultivars, suggesting the stronger low temperature tolerance of this cultivar in terms of photosynthetic capacity. Additionally, ‘Jinda 70’ maintained higher Pn values than the other two cultivars, which was beneficial to greater dry matter accumulation in this cultivar. (3) Values of the four yield traits were all significantly reduced in plants subjected to low temperature treatments at the seedling stage. The rates of reduction were significantly greater for plant mass and pods per plant than for the other two traits. Among the three cultivars tested, the rates of reduction in plant mass and pods per plant were significantly smaller in ‘Jinda 70’ than in other two cultivars. Furthermore, the yield per plant in ‘Jinda 70’ was reduced by less than 50% as a result of low temperature treatments at the seedling stage, indicating that the low temperature stress at the seedling stage had the least effect on the yield in ‘Jinda 70’. (4) GGE biplot analysis revealed the rates of reduction in Tr and Gs had the most apparent effects on ‘Jinda 70’, the rate of reduction in Pn had the most apparent effects on ‘Jindou 24’, and the rate of reduction in Ci affected both ‘Jinda 70’ and ‘Jindou 24’, and that ‘Jinda 53’ was not affected by reductions in any of the photosynthetic traits. Meanwhile, the biplot indicated that there were significant positive correlations among the reduction rates of Tr, Gs, Ci, and Pn. Analysis of the yield traits using the biplot showed that the reduction rates of plant height and plant mass had the most apparent effects on ‘Jinda 53’, the reduction rate of yield per plant had the most apparent effects on ‘Jindou 24’, and the reduction rate of pods per plant affected both ‘Jinda 53’ and ‘Jindou 24’, and that the reduction rates of yield trait values had no apparent effects on ‘Jinda 70’. There were significant positive correlations among the reduction rates of plant mass, plant height, pods per plant, and yield per plant. Hence, the photosynthetic and yield traits can be used as indices for evaluation of low-temperature tolerance. Using subordinate function, the ranking of low-temperature tolerance of the three cultivars at the seedling stage is in the ascending order of ‘Jinda 70’, ‘Jinda 53’, and ‘Jindou 24’.