Effect of phosphorus application on carbohydrate distribution in roots, stems and leaves of alfalfa and resistance to thrips (Thysanoptera: Thripidae)

The aim of this research was to clarify the distribution of carbohydrates in the roots, stems, and leaves of alfalfa with different phosphorus application rates, and the alfalfa's resistance to thrips. Two alfalfa varieties, 'Gannong No. 3' and 'Gannong No. 9, ' were used in this study. Odontothrips loti, which was the dominant pest of thrips in the alfalfa planting area of northern China, was selected as the research subject. Five soil phosphorus levels 0 (P1), 27(P2), 54(P3), 81(P4), and 108 mg(P₂O₅)·kg^-1soil (P5), were set. And alfalfa was infested with O. loti of 3 head·plant^-1 at the 6 leaf stage. The damage index was evaluated; the biomass of the leaves, stems and roots were measured; and the soluble sugar and starch contents of the leaves, stems, and roots were determined after the alfalfa was damaged by O. loti for 7, 14, and 21 days. The result showed that the damage index of 'Gannong No. 3' and 'Gannong No. 9' decreased gradually with increased phosphorus level, the lowest being at P3 level. The damage index of both alfalfa varieties decreased significantly after 14 and 21 days. The leaf, stem and root biomass of both alfalfa varieties increased significantly with increased phosphorus level. More biomass was distributed to the leaves in the early and middle stages (7 and 14 days), while it was distributed to the roots in the period of serious injury (21 days). However, the biomass allocated to stems was not significant. In 'Gannong No. 9', the biomass allocated to the leaves was more than that in 'Gannong No. 3' at the same phosphorus level. The soluble sugar and starch contents of the leaves, stems, and roots increased significantly with phosphorus level increasing. The soluble sugar content of the roots, stems and leaves increased, while the starch content decreased as the period of damage increased. The soluble sugar and starch contents of the leaves and roots of 'Gannong No. 9' were significantly higher than those of 'Gannong No. 3', which was damaged by thrips for 14 and 21 days. Phosphorus application enhanced the tolerance of alfalfa to thrips, with greater effect on 'Gannong No. 3'. Phosphorus application promoted the compensatory growth of the aboveground part at the moderate pest pressure and helped maintain root growth for the survival of alfalfa under severe pest pressure. Correspondingly, with the increase in damage time, the starch content of the root, stem, and leaves tended to decrease, and the soluble sugar content increased, showing that more resources were used to develop the photosynthetic and storage organs. P3 81 mg(P₂O₅)·kg^-1 soil was the best fertilization level for enhancing alfalfa's resistance to thrips in this research.