Abstract:

Carbohydrates serve as food reserves for grasses at times when photosynthesis cannot supply sufficient carbon for maintenance and growth such as when grasses are completely defoliated during the winter and early spring. L. chinensis, a rhizomatous graminoid that commonly dominates the steppe in semiarid areas of northern China, stores enough carbohydrates in the rhizome before winter to ensure that it can survive the hard environmental conditions of the winter. Nitrogen (N) fertilization is a common technique used to increase forage productivity in many countries; however, fertilization is not widely used in the steppe grasslands of northern China. Nitrogen fertilization significantly affects the non-structural carbohydrates during both growth and dormant periods of some grass species, but the effect of N fertilization on carbohydrate storage in the L. chinensis rhizome remains unclear. We conducted an N addition experiment with different rates and different application times of N to L. chinensis in fenced plots at the Inner Mongolia Grassland Ecosystem Research Station, Chinese Academy of Sciences. In the first experiment, NH4NO3 was applied at six treatment levels (0, 1.75, 5.25, 10.5, 17.5, 28 g N·m－2) on July 5 every year. In the second experiment, the timing of N application was studied by applying 17.5 g N·m－2 NH4NO3 on two dates, April 20 and July 5. In both experiments, 9 replicate plots (5 m x 5 m) were used for each treatment in a complete randomly designed experiment. L. chinensis rhizome samples were collected from the field on Sep. 20. Rhizomes were washed with cold water, oven-dried at 80 ℃ for 24 hours and ground to pass through a 100 μm mesh screen. Stored carbohydrates in the rhizome of L. chinensis were analyzed using a Sugar-PAKTM I column, a specific column for carbohydrate components assay, in High Performance Liquid Chromatography (HPLC). The results showed that the stored carbohydrates of the L. chinensis rhizome consisted of fructan, mannitol, sucrose, fructose, and glucose. Fructan, which contributed 60% of total non-structural carbohydrates (TNC), was the predominant stored carbohydrate followed by mannitol, which contributed 20% of TNC. Both fructose and glucose were less than 15% of TNC and sucrose was only 6% of TNC. Hence, sucrose, a main component of stored carbohydrates in other grasses, was only a minor component of L. chinensis. Nitrogen rates significantly influenced the contents of fructan, mannitol and total carbohydrates, but had little effects on glucose, fructose and sucrose content in the L. chinensis rhizome. The total carbohydrate, fructan and mannitol concentrations increased in the rhizome with increasing N application rates from 0 to 17.5 g N·m－2, but carbohydrate reserves were reduced at application rates of 28 g N·m－2. The timing of N application also significantly influenced the contents of carbohydrate reserves; L. chinensis stored more carbohydrates in the rhizome when N was applied in July than that in April. In conclusion, fructan and mannitol are important stored carbohydrates in the rhizome of L. chinensis. Both N application rates and timing of application significantly affect the levels of carbohydrate reserves, especially fructan and mannitol in the L. chinensis rhizome.