Effect of two main rootstock species on cold resistance of Shatangju mandarin saplings under low temperature stress

Abstract: 【Objectives】This work aimed to explore the effects of two rootstocks mainly used in the citrus production of Zhaoqing, Guangdong on the cold resistance of the famous local citrus variety Shatangju mandarin (Citrus reticulate‘Shatangju') in order to provide a scientific basis for optimizing its introduction, cultivation and propagation of grafted seedlings.【Methods】The three-year-old Shatangju mandarin plants grafted on trifoliate orange [Poncirus trifoliate (L.) Raf.] stock and sunki mandarin (C. sunki Hort. ex Tanaka) stock were used as experimental materials. The plants were placed in a lowtemperature climate chamber, cooling down in steps, and suffered from low temperature stress under(3±0.5) ℃ and(-3±0.5) ℃ for 24 h, respectively. Then, the room temperature was raised to 10 ℃ in steps,and after 24 hours, they were transferred to plastic greenhouses for conventional cultivation. When cultivated for 90 d, all the plants were harvested and tested separately. During the period, the test and analysis of the physiological characteristics were carried out. The plants not subjected to low temperature stress in each stock-scion combination were used as controls.【Results】The number of fallen leaves per plant of Shatangju mandarin grafted on sunki mandarin (Shatangju/sunki combination) decreased significantly, compared with the plant grafted on trifoliate orange stock (Shatangju/trifoliate combination) during 90 d of cultivation after stress at (3±0.5) ℃ and (-3±0.5) ℃, respectively, being 412.2% and134.5% larger, and the weight reduction of the fallen leaves was 546.6% and 1 098.0% larger than that of Shatangju/trifoliate combination, respectively; after 90 d of stress, the reduction of plant height of Shatangju/sunki combination was 5.0% and 16.3% larger than that of Shatangju/trifoliate combination,and the reduction of ground diameter was 75.0% and 8.8% larger than that of Shatangju/trifoliate combination, respectively. The reduction of crown width was 231.6% and 52.2% larger than that of Shatangju/trifoliate combination, respectively; and the reduction of root biomass was 91.6% and 97.3%larger than that of Shatangju/trifoliate combination, respectively. The decrease of stem and branch biomass was 141.4% and 27.3% larger than that of Shatangju/trifoliate combination, respectively, and the decrease of leaf biomass was 47.5% and 11.7% larger than that of Shatangju/trifoliate-combination, respectively. The decrease of biomass was 465.4% and 45.6% larger than that of Shatangju/trifoliate combination, respectively. Low temperature stress significantly decreased leaf chlorophyll content(Chl c)and photosynthetic rate(Pn) of Shatangju mandarin. After 90 d of stress at(3±0.5) ℃ and(-3±0.5) ℃,the reduction of Chl c of Shatangju/sunki combination was 101.1% and 27.0% larger than that of Shatangju/trifoliate combination, respectively, and the reductions of leaf Pn were 65.8%, 52.2% larger respectively; the leaf transpiration rate(Tr) increased significantly under low temperature stress for 30 d,and then tended to decrease but was still significantly higher than that of the control plant in the same period. The increase of Trof Shatangju/sunki combination under(3±0.5) ℃ and (-3±0.5) ℃ stress for90 d was 44.1% and 99.1% larger than that of Shatangju/trifoliate combination, respectively. Stomatal conductance(Gs) decreased significantly under low temperature for 30 d, and then increased but still was significantly lower than that of the control at the same period. The increase of leaf Gsof Shatangju/sunki combination was 41.3% and 39.7% larger than that of Shatangju/trifoliate combination respectively after(3±0.5) ℃ or(-3±0.5) ℃ stress for 90 d. The activities of key enzymes of Rubisco, FBP,Ald and PEPC in the carbon assimilation of leaves of Shatangju mandarin significantly decreased by low temperature stress compared with the control plants at the same period. 90 d after(3±0.5) ℃ stress,the reduction degree of activities of the four enzymes of Shatangju/sunki combination was 216.3%,46.7%, 168.7% and 47.2% lower respectively than that of Shatangju/trifoliate combination, and 90 d after(-3±0.5) ℃ stress, the reduction degree was 49.9%, 52.6%, 73.8% and 58.0% larger respectively.Low temperature stress led to a significant increase in water saturation deficit(WSD) and malondialdehyde(MDA) content in leaves of Shatangju mandarin; 90 d after(3±0.5) ℃ stress, the increase of the WSD and MDA contents of Shatangju/sunki combination was 28.0% and 82.0% larger than that of Shatangju/trifoliate combination respectively, and the increase range 90 d after (-3±0.5) ℃ stress was49.3% and 79.5% larger than that of Shatangju/trifoliate combination respectively.【Conclusion】These results fully demonstrated that the cold resistance of Shatangju/trifoliate combinationwas significantly stronger than that of Shatangju/sunkicombination, and the grafted saplings of Shatangju/trifoliate combination should be mainly propagated and cultivated in Shatangju mandarin production. The mechanism of Shatangju/trifoliate combination having significantly stronger cold resistance under low temperature stress was mainly as follows: the degree of defoliation was lighter, the root system was more developed,the leaves were thicker, the photosynthetic efficiency was higher, and the leaves were resistant to dehydration and membrane lipid peroxidation.