| The Loess Plateau region is one of apple planting favorable areas in China, however, the local faced with water scarcity and deterioration of ecological environment and other outstanding issues, meanwhile, most orchards in this area are still using surface irrigation which water use efficiency is low. Those problems seriously hampered the healthy development of the region’s apple industry. Water storage pit irrigation is a new water-saving irrigation technology, which has broad application prospects in the Loess Plateau. Exploration suitable lower limit of irrigation under water storage pit irrigation condition for young apple trees and fully exploiting the water-saving and drought-resistance potential of young apple trees have an important theoretical and practical significance for scientific and reasonable formulating optimum water storage pit irrigation system for young apple trees and accelerating the local area apple orchard water-saving, high quality and efficient construction etc.Based on the three years old dwarf red Fuji as the experimental material, the lower limit of irrigation and the water storage pit depth as the control factors and the surface irrigation as the control, through the field experiment and theoretical analysis, this paper analyzed the response of the dynamic changes of apple orchard soil moisture and young apple trees water-requiring characteristics, plant growth, physiological characteristics of leaf photosynthesis, leaf water use efficiency on different lower limit of irrigation. Then, depended on the Logistic equation to construct the growth model of young apple trees’ growth indexes, and combined with theoretical analysis and principal component analysis method to determine the area suitable lower limit of irrigation under water storage pit irrigation condition for young apple trees. The main results are as follows:1. The dynamic change of soil moisture in water storage pit irrigation condition under different treatments.The soil moisture content of the planning moisture layer in different treatments was in the range of the lower and upper limits of the experimental design. From May to July, less rainfall, the soil moisture content mainly controlled by irrigation, the difference between different treatments significantly; From August to September, rainfall was sufficient to meet the needs of young apple trees, soil moisture content of different treatments tended to synchronize; From September to October, young apple trees water consumption decreased, soil moisture content of different treatments were basically the same. During the year, the average soil moisture content of the planning moisture layer arranged in the order of large to small was treatment 70%?Fã€treatment 60%?F and treatment 50%?F。The more close to the surface, soil water movement more active. Soil moisture content was not affected by the experimental treatment in 140 cm depth below the surface, and it could be considered that this depth does not occurring deep percolation.2. The young apple trees water-requiring characteristics in water storage pit irrigation condition under different treatments.(1) Water-requirement of young apple trees in different treatments was big in the middle, small on the both sides of the "spindle" distribution in the whole growth period, the peak of water-requirement appeared in July(treatment 50%?F) or August(treatment 60%?F and treatment 70%?F). During the whole growth period, the total amount of water-requirement arranged in the order of large to small was treatment 60%?Fã€treatment 70%?F and treatment 50%?F, and the differences among the different treatments occurred mainly in May to August. From July to September, water requirement modulus in different treatments was in the range of 68.77%~73.41%, this stage was the critical period of water requirement of young apple trees. When the lower limit of irrigation was same, the young apple trees water-requirement of the treatment pit depth of 20 cm was greater than the treatment pit depth of 30 cm.(2) Single crop coefficient of young apple trees under different treatments during the year was basically a bimodal distribution, each treatment reached the main peak value in September, except treatment 50%?F reached the second peak value in July, the rest reached the second peak value in June. Single crop coefficient of young apple trees of the treatment pit depth of 20 cm was generally greater than the treatment pit depth of 30 cm.3. The growth dynamics of young apple trees in water storage pit irrigation condition under different treatments.(1)The different lower limit of irrigation leaded to different soil moisture content of the planning moisture layer, thereby affected the young apple trees growth rhythm. The difference among different treatments in young apple trees plant height, stem diameter was significant, but new shoot length, new shoot diameter and new shoot leaf number was not significant. Under water storage pit irrigation condition, the growth dynamics indices of young apple trees in response to different lower limit of irrigation was not exactly the same. When pit depth was 20 cm, tree height and new shoot leaf number of young apple trees increased with higher lower limit of irrigation, but stem diameter, new shoot length and new shoot diameter were the best in the treatment 50%?F; When pit depth was 30 cm, tree height of young apple trees increased with higher lower limit of irrigation, but stem diameter, new shoot length, new shoot diameter and new shoot leaf number were the best in the treatment 60%?F; And the treatment pit depth of 20 cm outperformed the treatment pit depth of 30 cm on the growth dynamics index of young apple trees. Because of a certain degree of water stress, young apple trees growth was significantly inhibited under the treatment 50%?F and pit depth of 30 cm.(2)The Logistic growth model can be accurately simulated in young apple trees plant high cumulative growth, stem diameter cumulative growth, new shoot length growth, new shoot diameter growth, new shoot leaf number growth, and the fitting degree of the model was excellent.4. Photosynthetic Characteristics of young apple tree leaves in water storage pit irrigation condition under different treatments.(1)Variation of net photosynthetic rate of young apple tree leaves during the year were basically the same, the difference among different treatments was not significant. From August to September was the most vigorous period for photosynthesis of young apple trees, the net photosynthetic rate showed a unimodal distribution in the growth period, and in September 12 reached its peak. When pit depth was 20 cm, the net photosynthetic rate of young apple tree leaves arranged in the order of large to small was treatment 50%?Fã€treatment 70%?F and treatment 60%?F; When pit depth was 30 cm, the order was treatment 60%?Fã€treatment 50%?F and treatment 70%?F; And the treatment pit depth of 20 cm outperformed the treatment pit depth of 30 cm on the net photosynthetic rate of young apple tree leaves. The results of path analysis showed that net photosynthetic rate of young apple tree leaves was mainly affected by soil moisture content, photosynthetic active radiation(PAR) and stomatal conductance, and this response progress may be achieved through the adjustment of stomatal conductance of young apple tree leaves.(2) The difference among different treatments in young apple tree leaves stomatal conductance, transpiration rate and intercellular CO2 concentration was not significant. The results of path analysis showed that the stomatal conductance was mainly affected by soil moisture content and stomatal conductance increased with higher soil moisture content. Stomatal conductance was the main factor influencing the change of transpiration rate and intercellular CO2 concentration of young apple tree leaves. When pit depth was 20 cm, stomatal conductance, transpiration rate and intercellular CO2 concentration of young apple tree leaves got the maximum under the treatment 70%?F, then pit depth became 30 cm, those indexes got their maximum under the treatment 60%?F. And the treatment pit depth of 20 cm outperformed the treatment pit depth of 30 cm on those indexes.(3) The opening of the quadratic parabola between developmental period and leaf water use efficiency of young apple trees was up. In July, the leaf water use efficiency was lowest in all treatments, the highest in October, and the difference between different treatments was significant. During the whole growth period, the leaf water use efficiency of young apple trees increased with decreasing lower limit of irrigation under the condition of different depths. And the treatment pit depth of 30 cm outperformed the treatment pit depth of 20 cm on leaf water use efficiency.Based on principal component analysis method and all the results of the test showed that, in conditions of typical local climate and water storage pit irrigation, when upper limit of irrigation was 90% of field water-holding rate, the high-quality, water-saving, efficient and suitable lower limit of irrigation for three years old dwarf red Fuji was 50% of field water-holding rate(pit depth of 20 cm) or 60% of field water-holding rate(pit depth of 30 cm). |