Effect of nitrogen and sulfur interaction on growth and pungency of different pseudostem types of Chinese spring onion (Allium fistulosum L.)

Effects of N and S supply on the growth and pungency (estimated as pyruvic acid levels) of Chinese spring onion (Allium fistulosum L. var. giganteum Makino) were investigated in two pot experiments using soilless growing media. In the first experiment the effects of S supply (0.01 and 4.00 mmol L⁻¹ SO₄²⁻) on the growth and pungency of Chinese spring onion were investigated among four cultivars with fleshy root type or long pseudostem type. In the second experiment the effects of different S (0.01 and 4.00 mmol L⁻¹ SO₄²⁻) and N (1.5, 3.0, 6.0, 12.0 and 24.0 mmol L⁻¹ N) supply levels on the growth and pungency of Chinese spring onion were studied. Fleshy root spring onion had stronger pungency and larger pseudostem diameter than long pseudostem spring onion, and the pungency of fleshy root spring onion was regulated to a greater extent by N and S supply compared with long pseudostem spring onion. Increasing S supply level significantly increased the biomass, N and S uptake and pungency of all cultivars tested. The biomass of Chinese spring onion of fleshy root type (cv Longyao) and long root type (cv Zhangqiu) was more influenced by N supply than it was by cultivar or S supply. Low S supply decreased the pungency of the two cultivars with increasing N supply. No significant differences in N or S uptake or pungency were observed in the two cultivars with different S supply at the N supply level of 1.5 mmol L⁻¹ N, however, cultivar differences in N and S uptake and pungency were investigated at high N supply (12.0 mmol L⁻¹ N) and S supply (4.0 mmol L⁻¹ SO₄²⁻). Excessive N supply (24.0 mmol L⁻¹) significantly inhibited plant growth, retarded S assimilation, and decreased pungency. It is therefore essential to apply the optimum recommended rate of N fertilizer in Chinese spring onion production.