Effects of girdling on arbuscular mycorrhizal colonization and root hair development of litchi seedlings

Abstract

Litchi (Litchi chinenis Sonn.) is a woody mycorrhizal fruit tree. Hypha and root hairs are the most important structures for nutrition and water absorption in litchi. Girdling is an important technique in litchi cultivation. However, the effects of girdling on arbuscular mycorrhiza (AM) colonization and root hair development remain unknown. To understand the effects of girdling on AM and root hairs in litchi, the carbohydrate content, enzyme activities related to sucrose metabolism, AM colonization, and length and density of root hairs were measured under girdling, wounding and control conditions in the roots of one-year-old seedlings. Results showed that sufficient carbohydrates were available for AM and root hairs in the mycorrhizal roots of litchi in the wounding and control groups. Girdling significantly decreased the concentrations of glucose at 3days (3D), 1 week (1W), 2 weeks (2W), 1 month (1M) and 2 months (2M) after treatment, of fructose at 3D, 1W, 2W, 1M and 2M after treatment, of sucrose at 1W, 2W, 1M and 2M after treatment, and of starch at 2W, 1M and 2M after treatment. Consistent with its effects on carbohydrates, girdling significantly reduced arbuscular colonization (1M and 2M) and root hair length (1M). The correlations of arbuscular colonization and root hair length with concentrations of carbohydrates (fructose, sucrose and starch) were significant only under girdling treatment. Under conditions of hexose starvation caused by girdling, litchi seedlings exhibited significantly upregulated neutral invertase activity in sucrose-cleaving parts and downregulated sucrose phosphate synthase and sucrose synthase (synthesis direction) in sucrose synthesis parts. The correlations of arbuscular colonization with levels of sucrose-cleaving enzymes (neutral invertase and acid invertase) and root hair length with levels of sucrose-cleaving enzymes (neutral invertase and acid invertase) were also significant only under girdling treatment. Our findings indicated that AM colonization was more sensitive to carbohydrates than root hair length in litchi seedlings. Furthermore, girdling likely affected AM colonization and root hair length by decreasing the carbohydrate contents and regulating the activities of sucrose-metabolising enzymes.