Evaluation of drought resistance of eight apple germplasm resources

Abstract: 【Objective】Drought is one of the main factors restricting agricultural production, which would cause a large scale yield reduction. The Loess Plateau is the largest apple producing area in China. However, the Loess Plateau is faced with perennial drought and water shortage, and most of the apple planting areas are located in mountainous areas short of irrigation conditions. Drought and water shortage are the main limiting factors for the development of apple industry in the Loess Plateau of China. Therefore, it is of great significance to breed rootstocks and varieties with strong drought resistance. In the previous study, 8 apple germplasm resources with utilization value were found in our laboratory. This study evaluated their drought resistance in order to provide reference for the utilization and resistance breeding.【Methods】In this study, P5 (Malus asiatica), L51 (M. robusta), L37 (M. hybrid‘Dwarf Tree’), LC36 (M. hybrid‘Cranberry’), L7 (M. soulardii), LC54 (M. domestica‘Oekonomierat Echtermeyer’), ZN18 (M. domestica, Sciros × Scifresh) and C31 (M. domestica‘Trail’) were used as experimental materials, and M. prunifolia and M. sieversii were used as controls. In the spring of 2022, the bud grafting method was used to graft them on the M. hupehensis Rehd. When the height of all test materials reached 70-80 cm, the plants with the same height were selected for experiment. The treatment group was watered thoroughly the day before the treatment and stopped watering until the 9th day of the treatment. The control group was watered normally every day, and the soil relative water content was maintained at 75%-85%. From the 0th day of treatment, the net photosynthetic rate, chlorophyll content, relative water content and relative conductivity of leaves were measured every other day. Completely mature leaves were collected from 7-15 leaves below the top of the stem, wrapped in the tin foilpaper, immediately frozen in liquid nitrogen, and stored at −80 ℃ for the determination of the malondialdehyde content, hydrogen peroxide content, superoxide anion (O2 - ) content, antioxidant enzyme activity, proline, ABA content and the expression of the synthesis-related genes of each apple germplasm resource. The drought resistance of each apple germplasm resource was evaluated by membership function method.【Results】(1) After natural drought stress, the leaves of the apple germplasm resources wilted to varying degrees. The leaves of LC54 wilted most seriously, and the leaves of LC36 wilted most lightly. After drought treatment, the leaf relative water content of each apple germplasm resource decreased significantly, and the leaf relative water content of LC54 decreased most apparently. (2) After drought treatment, the relative conductivity, MDA content and proline content of the leaves of the apple germplasm resources increased significantly. On the 9th day of the drought stress, the net photosynthetic rate and chlorophyll content of various germplasm resources decreased significantly. (3) The O2 - content of the apple germplasm resources increased significantly after drought stress, and the increase range of the O2 - content of the apple germplasm resources was between 84.31% and 197.97%. The content of H2O2 was lower on the 0th day of drought stress, and significantly increased on the 9th day of the drought stress. (4) The ABA content of the apple germplasm resources increased significantly after the drought stress. The gene expression of the MdNCED1 and MdNCED3 remained at a low level on the 0th day of the drought treatment, and increased significantly on the 9th day of the drought treatment, which was consistent with the change of the ABA content in the leaves. (5) The comprehensive net photosynthetic rate, chlorophyll content, leaf relative water content, relative conductivity, malondialdehyde content, hydrogen peroxide content, superoxide anion (O2 - ) content, superoxide dismutase (SOD) activity, peroxidase (POD) activity, proline, ABA content, a total of 11 indicators, were used to calculate the average membership function value of each apple germplasm resource. The results showed that the average membership function value of LC36 was the largest, indicating that the relative change degree of each index of LC36 was the smallest under the drought stress, and the drought resistance was the strongest among the 8 apple germplasm resources. The average membership function value of LC54 was the smallest, indicating that its drought resistance was the weakest.【Conclusion】The results of this study showed that under the drought stress, the net photosynthetic rate of plants decreased, the membrane integrity was destroyed, and the contents of ABA and proline increased significantly. However, due to the different resistance of the apple germplasm resources to drought, the changes of each index before and after the drought stress were also different. According to the membership function value, we concluded that the drought resistance of each apple germplasm resource is: LC36＞L7＞M. prunifolia＞M. sieversii＞L51＞C31＞P5＞ZN18＞L37＞LC54. The drought resistance of LC36 and L7 germplasm resources is greater than that of M. prunifolia and M. sieversii, while the drought resistance of other resources is lower than that of M. prunifolia and M. sieversii. Therefore, LC36 and L7 are important resources for improvement of the drought resistance of apple.