Effects of nitrogen application rate on yield, quality, and γ-aminobutyric acid content of giant embryo rice

Objectives Giant embryo rice is highly nutritious because it is rich in γ-aminobutyric acid (GABA). Here, we studied the suitable N application rate for high yield, quality and GABA content of giant embryo rice.

Methods In 2020 and 2021, field experiments were conducted in the Modern Agricultural Scientific Research Park of Chongzhou City, Sichuan Agricultural University, using giant embryo rice cultivar J20 and Koshihikari as test materials. Five N dosages of 0 (N₀), 90 (N₉₀), 135 (N₁₃₅), 180 (N₁₈₀), and 225 (N₂₂₅) kg/hm² were applied. The yield, processing quality, appearance quality, starch RVA spectrum characteristic value, taste value, protein content, 17 hydrolyzed amino acids and γ-aminobutyric acid (GABA) content were investigated.

Results Both rice cultivars reached the maximum yield and higher effective panicle and spikelet number per panicle in N₁₃₅. With increasing N dosage, the brown rice rate, milled rice rate, head rice rate, GABA content, and embryo weight of rice increased at first and then decreased. The chalky grain rate and chalkiness decreased at first and then increased, with the lowest value recorded in N₁₃₅. The N dosage higher than N₁₃₅ decreased the functional component and embryo weight. Increasing N dosage (P<0.05) decreased the peak viscosity of rice, with the average decrease under N application reaching 7.44% (J20) and 9.74% (Koshihikar), respectively. N application rate did not change (P>0.05) the breakdown and setback values and the pasting temperature. The appearance, taste, and taste value of J20 decreased as N dosage increased (P<0.05), with an average decrease of 10.73% in the taste value recorded in N₀ treatment. For Koshihikari, the appearance, taste, and taste value were in the order N₁₃₅ > N₀ > N₉₀ > N₁₈₀ > N₂₂₅. The protein content total amino acid, essential amino acid, and non-essential amino acid content of the two cultivars increased with the increasing N application rate. The suitable range of N application rate for high yield and good quality of the two rice cultivars was estimated at 130–140 kg/hm². Compared with Koshihikari, the yield, head rice rate, breakdown value and taste value of J20 were more sensitive to N application rate. Also, it had a higher protein content, total hydrolyzed amino acids and GABA under the same N application rate.

Conclusions Reasonable application of nitrogen fertilizer can improve yield, processing quality, appearance and nutrition, but excessive nitrogen fertilization will reduce cooking and eating quality of rice. For the giant embryo rice cultivars, applying N 130–140 kg/hm² is recommended for a high yield, quality and good functional characteristics.