Abstract: Boron is an essential trace element for plant growth and development, with special functions in pollen tube elongation and the synthesis and transport of carbohydrates. The aim of this study was to determine the effect of applying boron as a foliar spray on flowering and the development of pods and grains, with an overall aim to increase the number of grains per pod and improve yield. The Chinese milk vetch cultivar “Xinzi No.1” was subjected to six different foliar application treatments; water (control) and borate solution (500 mg·L^-1, T₁; 1000 mg·L^-1, T₂; 2000 mg·L^-1, T₃; 4000 mg·L^-1, T₄; 6000 mg·L^-1, T₅), which were applied at the pre-flowering stage. When the plants reached the mature stage, the number of inflorescences, pods, grains per pod for different inflorescence positions (1 lowest on the raceme, 6 highest) and the number of plants and branches per unit area were determined using conventional test methods. The number of inflorescences and pods unit area, grain set rates of pods, and number of grains per pod for different inflorescence positions did not differ significantly between the control and the treatments, except for the T₃ treatment. In the 3rd to 6th inflorescence positions, the number of inflorescences per unit area was increased by 31.87%, 40.79%, 37.49%, and 44.99%, respectively; the number of pods per unit area was increased by 27.89%, 32.69%, 35.92% and 25.84%, respectively; the number of grains per pod was increased by 20.69%, 23.54%, 23.24%, and 22.03%, respectively; and the grain set rates of pods was increased by 2.95%, 2.56%, 3.49% and 4.51%, respectively, in the T₃ treatment as compared with the water control. However, these parameters in the 1st and 2nd inflorescence positions did not differ significantly between the T₃ treatment and the control. Further analyses showed that spraying boron at different concentrations had little effect on first-order branching and 1000-grain weight of Chinese milk vetch. Our results show that, when applied at the pre-flowering stage, a foliar application of boron (2000 mg·L^-1) can significantly increase the number of inflorescences, pods, and grains per pod per unit area at the 3rd to 6th inflorescence positions, thereby increasing the grain-setting rates of pods and increasing the grain yield of Chinese milk vetch.

Key words: Chinese milk vetch, foliar spraying boron, grain-setting characteristic