Effects of different nitrogen levels and NH4+/NO3- ratios on the growth, nitrogen metabolism and photosynthesis of anther-derived somatic seedlings of Hevea brasiliensis

The effects of four nitrogen (N) levels (1, 4, 8, and 16 mmol/L, using ammonium nitrate as the N source) and five NH4+/NO3- ratios (100/0, 75/25, 50/50, 25/75, and 0/100) on the growth, N metabolism and photosynthesis of anther-derived somatic seedlings of Hevea brasiliensis were studied in a sand culture experiment. The dry weight of stem, leaves, shoot and single plant, the N concentration and N uptake of the roots, stem and leaves, and the chlorophyll concentration increased correspondingly with the increasing N level. The N concentration of the coarse roots, stem and leaves, N uptake by the plant, and the concentrations of chlorophyll a, chlorophyll b and total chlorophyll all differed significantly between treatments. The plant height, stem diameter, dry weight of fine roots and net photosynthetic rate of leaves increased with N level from 1 to 8 mmol/L, but decreased at 16 mmol/L. The nitrate reductase activity of fine roots was highest at the lowest N level. With regard to the effects of NH4+/NO3- ratio, the N concentration and N uptake in different parts of seedlings were highest when grown exclusively with NH4+ nutrition (except N uptake of fine roots and petioles); plant height, stem diameter, and the dry weights of the petioles, leaves, shoot and plant were highest in the 75/25 NH4+/NO3- treatment; the dry weight of the fine roots and stem, the chlorophyll concentration and the net leaf photosynthetic rate were highest in the 50/50 NH4+/NO3- treatment, and all were significantly higher than those of seedlings grown exclusively with NO3- nutrition but were not significantly different from those of the treatments with 50/50 NH4+/NO3- ratios except the N concentration of the fine roots and stem. Nitrate reductase activity of fine roots was highest with NO3- nutrition, but growth of the seedlings was weakest in this treatment. The results indicated that growth of the anther-derived somatic seedlings of Hevea brasiliensis was superior with NH4+ alone or combined NH4+/NO3- nutrition rather than NO3- nutrition alone, and the optimal N level was 8 mmol/L with a NH4+/NO3- ratio exceeding 50/50.