Effects of exogenous calcium on nitrogen absorption and utilization of Malus hupehensis Rehd. seedlings

Abstract:【Objective】This study was to examine the effect of calcium application on nitrogen absorption and utilization in Malus hupehensis Rehd. and to screen the optimum amount of calcium for appli- cation. Calcium and nitrogen are both macronutrients needed by fruit trees. There are many studies on the effect of a single element on the growth of fruit trees, but there are few studies on the interactions of calcium and nitrogen. This study aimed to provide some data for improving the nitrogen use efficiency in apple.【Methods】The seedlings of M. hupehensis Rehd. were applied with 3 mmol·L-1 NO3-. On this basis, there were four calcium treatments: 0 (CK), 0.1, 1 and 3 mmol·L-1 Ca2+. The calcium solutions ap- plied were prepared with analytical pure CaCl2 and adjusted to pH 6.0. Nitrogen and calcium were ap- plied by soil drenching. Twenty-four seedlings with even growth were selected for each treatment, and the effect of calcium on the biomass of different parts of the seedlings was analyzed by drying and weighing the samples. The difference in leaf area among different treatments was analyzed by image J software. The plant height was measured with a ruler and the stem diameter with an electronic Vernier caliper. A root morphology scanner (Epson V 700) was used to collect the root images, which were ana- lyzed with software WinRHIZO and data of total root length, total root area and root tip number in dif- ferent treatments were obtained. The content of nitrate nitrogen was determined with salicylic acid method. The activity of nitrate reductase was determined by sulfonamide colorimetry, and the changes in nitrate reductase activity were compared between calcium treatment and the control. Glutamine syn-thetase activity was determined by glutamine synthetase (GS) kit purchased from Suzhou Keming Tech- nology Co., Ltd.. After the underground and aboveground parts of the seedlings were dried at 80 °C, they were digested using H2SO4-H2O2 method, and the contents of total nitrogen, total calcium and phos- phorus, potassium, iron, magnesium, zinc and molybdenum were determined by automatic Kjeldahl ni- trogen meter, atomic absorption spectrophotometer or Icp-Ms【. Results】The aboveground indexes ofM. hupehensis Rehd. seedlings was the highest level in the treatment with 1 mmol · L- 1 Ca2 + . The fresh weight and dry weight of the seedlings increased by 31.5% and 33.5%, respectively, and the seedling height increased by 129.3% compared with the control. The total leaf area of the seedlings increased by 49.2% compared with the control. In the treatment with 3 mmol · L- 1 Ca2 + , the stem diameter was the highest, which was 22.8% higher than that of the control. At the same time, calcium increased root length, root surface area, root tip number and root activity, which were highest level in the treatment with 0.1 mmol·L-1 Ca2+, where these indexes increased by 24.1%, 26.6%, 17.3% and 129.9% respective- ly compared with the control. The application of calcium promoted the absorption of nitrate nitrogen in different parts and reduced the content of ammonia ion in the plant. Calcium content in the plant in- creased with the increase of the calcium concentration applied. The activities of nitrate reductase (NR) and glutamine synthetase (GS), the key enzymes of nitrogen assimilation, increased with the increase in calcium concentration. Ca2+ treatments significantly affected the nitrogen assimilation in M. hupehensisRehd seedlings, and the nitrogen assimilation amount in the treatment with 1 mmol · L- 1 Ca2 + was the highest under normal nitrogen level. In addition, we found that different concentrations of calcium led to changes in the contents of phosphorus, potassium, iron, magnesium, zinc and molybdenum in the plant. With the increase in calcium concentration, the aboveground phosphorus content decreased gradu- ally, and phosphorus content in roots increased at first and then decreased, and was the highest at 1 mmol · L- 1, while the contents of potassium, iron, magnesium, zinc and molybdenum increased at low calcium concentrations, but decreased at higher concentrations. Calcium application not only affected the absorption of nitrogen, but also influenced the absorption of the other mineral elements. It is sug- gested that calcium might play an important regulatory role in the process of nitrogen absorption and as- similation【. Conclusion】Exogenous calcium promotes nitrogen absorption and utilization in M. hupe- hensis seedlings. Under normal nitrogen treatment, 1 mmol·L-1 Ca2+ may be the best calcium concentration for nitrogen absorption and utilization.