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Ornithine is one of the precursors of 2-acetyl-1-pyrroline, the key compound of fragrant rice aroma, in fragrant rice. However, the application of ornithine in fragrant rice production has never been reported. The present study was conducted with a 2-year field experiment in order to study the effects of foliar application of ornithine on fragrant rice yield formation, quality characters and aroma. At the heading stage, ornithine solutions at three concentrations (0.5, 1.0, 2.0 g L−1) were foliar applied to fragrant rice plants in paddy field and treatment applied with distilled water was taken as control. Our results showed that foliar application of ornithine significantly improved fragrant rice grain yield as well as the seed-setting rate and 1000-grain weight. Higher contents of photosynthetic pigments (chlorophyll a, chlorophyll b and carotenoid) and stronger net photosynthetic net were recorded in ornithine treatments compared with control. Foliar application of ornithine also increased grain protein content and reduced the grain amylose content, chalky rice rate, chalk area ratio and chalkiness. Moreover, we observed higher grain 2-AP concentrations were recorded in ornithine treatments and the ornithine treatments significantly reduced the transcript level of BADH2 which is the key gene controlling the 2-AP biosynthesis in fragrant rice. Overall, 1.0 g L−1 was considered as the optimal amendment for foliar application of ornithine in fragrant rice production.

Organic cultivation is an eco-friendly planting strategy. To explore the effects of organic cultivation on yield, grain quality, and volatile organic compounds of fragrant rice, the present study conducted a field experiment with organic cultivation and inorganic cultivation treatments over two subsequent growing seasons. The late season rice was sent for the determination of untargeted flavoromics using GC × GC-ToF-MS. Meanwhile, the GC × MS were used for the quantification of 2-acetyl-1-pyrroline. Our results showed that compared with inorganic cultivation, organic cultivation increased grain 2-AP and amylose contents by 21.19-26.10 % and 4.51-7.69 %, respectively. 5.89 %-15.55 % lower crude protein contents were recorded in organic cultivation. The results of untargeted flavoromics showed that organic cultivation caused the absence of 56 volatiles and the exclusive presence of 10 new volatiles compared with inorganic cultivation. In conclusion, organic cultivation produced fragrant rice with lower yield and higher 2-AP content as well as different flavor profiles.

The influences of γ-aminobutyric acid (GABA) and nitrogen on grain yield, yield-related traits, plant growth, grain–leaf ratio and NSC contents in different fragrant rice genotypes are not fully understood. A pot experiment was conducted with four nitrogen rates, i.e. 0, 0.87, 1.75, and 2.61 g pot−1, two GABA levels, i.e. 0 and 250 mg L−1, and three fragrant rice cultivars, i.e. Yuxiangyouzhan, Yungengyou 14 and Basmati. Results depicted that GABA treatment significantly increased the mean grain yield by 11.84% as a consequence of the increment of grain weight, spikelet per pot, grain weight per leaf area, stem sheath dry weight at HS, panicle dry weight at HS, panicle dry weight at MS and total dry weight at MS. Besides, significant effects of nitrogen and cultivar on yield and yield-related traits and some rice growth parameters were observed. Moreover, significant positive correlation relationships between yield and panicle number per pot, leaf area and dry weight for all the cultivars were observed. GABA application could alleviate the effects of nitrogen on grain yield in fragrant rice. These results suggest that GABA is feasible to increase grain yield and improve rice plant growth for fragrant rice varieties grown under different nitrogen conditions.

Proline is one of the precursors of the biosynthesis of 2-acetyl-1-pyrroline (2-AP) which is the key and characteristic volatile component of fragrant rice aroma. In order to study the effects of exogenous proline on 2-AP biosynthesis and other grain quality attributes in fragrant rice, two indica fragrant rice cultivars, “Meixiangzhan-2” and “Xiangyaxiangzhan”, and one japonica fragrant rice, “Yunjingyou”, were used in present study. At initial heading stage, proline solutions at 0 (CK), 0.10 (Pro1), 0.20 (Pro2) and 0.50 (Pro3) g L-1 were applied as foliar spray solution to fragrant rice plants. Compared with CK, Pro1, Pro2 and Pro3 treatments significantly increased the grain 2-AP content. The significant up-regulation effects due to proline treatments were observed in the contents of proline, △1-pyrrolidine-5-carboxylic acid (P5C) and △1-pyrroline which involved in 2-AP formation. Exogenous proline application also significantly decreased the grain γ-aminobutyric acid (GABA) content. Furthermore, proline treatments enhanced the activity of proline dehydrogenase (ProDH) as well as transcript level of gene PRODH. On the other hand, the transcript level of gene BADH2 and activity of betaine aldehyde dehydrogenase (BADH) decreased under proline treatments. Proline treatments (Pro2 and Pro3) also increased the grain protein content by 3.57–6.51%. Moreover, 32.03–34.25% lower chalky rice rate and 30.80–48.88% lower chalkiness were recorded in proline treatments (Pro2 and Pro3) for both Meixiangzhan and Xiangyaxiangzhan whilst for Yunjingyou, foliar application of proline had no significant effect on chalky rice rate and chalkiness. There was no remarkable difference observed in grain milled quality (brown rice rate, milled rice rate and head rice rate) and amylose content between CK and proline treatments. In conclusion, exogenous proline enhanced the 2-AP biosynthesis and promoted some grain quality characters of fragrant rice.

Salinity was shown to impact the formation of 2-acetyl-1-pyrroline (2-AP), the key flavour compound in aromatic rice cultivars. To study the effects of saline application to seeds on 2-AP biosynthesis in aromatic rice and to further investigate the enzymatic and molecular mechanisms involved, the present study was conducted with nine treatments. The seeds of three aromatic rice cultivars were primed or soaked in 0.01, 0.02, 0.05 and 0.10 mM sodium chloride solutions, and the seeds that were both primed and soaked in distilled water were considered the control group. Then, the seedlings were allowed to grow in an artificial climate box. The results showed that both priming and soaking seeds with saline significantly increased the 2-AP content in aromatic rice seedlings. The highest 2-AP content was recorded in the seeds primed with 0.10 mM sodium chloride. In the seeds treated with saline, increased amounts of proline, P5C and Δ1-pyrroline were observed and resulted in enhanced 2-AP biosynthesis through induced gene expression (P5CS1, P5CS2 and PRODH) and activities of enzymes including PDH and P5CS. In summary, application of saline to seeds increased 2-AP content in aromatic rice seedlings by up-regulating the related precursors contents, enzymes activities and genes expression.