Changes in lysine content in rice caryopses during grain filling and in response to mid-season nitrogen management

Context and problem: Lysine (Lys) is the first limiting essential amino acid in rice, and its biosynthesis in caryopses is regulated by nitrogen (N) management. However, the temporal dynamics of Lys content in earlier-flowering superior caryopses (SCs) and later-flowering inferior caryopses (ICs), as well as its distribution across caryopsis layers during grain filling, remains poorly understood. Moreover, variations in Lys anabolism between SCs and ICs, among rice varieties, and in response to mid-season N application have not been fully elucidated.

Objective: The study aimed to (1) characterize changes in Lys content in both SCs and ICs during grain filling across different rice varieties; (2) examine the relationship between Lys content in different layers of a caryopsis during grain filling and final Lys content in brown and milled rice; and (3) identify N management practices that enhance Lys biosynthesis in rice grains.

Methods: Two field experiments were conducted for three years. The first experiment involved six rice varieties grown under conventional N management. The second tested four mid-season N treatments, i.e., no N application (N0), N application at panicle initiation (N1), pistil and stamen differentiation (N2), and heading initiation (N3), using two representative rice varieties.

Results: Compared to SCs, ICs exhibited a lower peak increase rate (PIR) of Lys content during grain filling, with varietal differences observed. Varieties with higher PIR had higher Lys accumulation at maturity, especially in ICs. Lys content in the mid and inner layers of developing grains was closely associated with the proportion of Lys in milled rice relative to brown rice at maturity. Enhanced activities of enzymes involved in Lys biosynthesis, rather than catabolism, contributed to higher PIR and Lys retention. Mid-season N application (N1 - N3) significantly increased PIR, Lys biosynthesis enzyme activities, and Lys and total amino acid contents in both brown and milled rice, while also increasing grain yield. Notably, N1 and N2 increased the proportion of Lys in milled rice and enhanced taste quality, whereas N3 reduced both.

Conclusions: Boosting the PIR of Lys and its accumulation in the middle and inner caryopsis layers during grain filling is key to increasing Lys content in milled rice. N application during panicle differentiation effectively promotes this strategy while enhancing both grain yield and eating quality.

Implications: This study offers a practical approach to improving the nutrient quality of rice through optimized mid-season N management focused on enhancing Lys biosynthesis in developing grains.