Effects of slow-release urea combined with conventional urea on leaf growth and yield formation of indica hybrid rice under direct seeding cultivation

Objectives It is one of the main irreversible trends to adopt mechanized direct seeding intensive cultivation mode progressively in rice plants. Combination of slow-release urea and urea under direct seeding condition could provide theoretical and practical basis that aim to acquire high yield and improve high efficiency fertilization techniques for direct seeding rice.

Methods An experiment was conducted using the split plot design with two factors, and ‘Yixiangyou 2115’ high-quality hybrid indica rice, was used as material. The main plot was two cultivation methods: direct seeding (C_D) and transplanting by hand (C_H), and the subplot was six N application ratios in total amount of 150 kg/hm², the ratio of urea application for basal N, tiller N and panicle N was 3∶3∶4 (U₃₀), urea as single basal application (U₁₀₀), slow-release urea as single basal application (S₁₀₀), the ratios of slow-release urea(basal N) and urea (panicle N) was 8∶2 (S₈₀), 6∶4 (S₆₀), and 4∶6 (S₄₀), and without N application served as a control (CK). In comparison to hand-transplanting rice, changes of leaf color, leaf growth, LAI, photosynthesis of flag leaves and carbon and nitrogen metabolism of leaves and their relationship with yield formation were investigated in direct seeding rice.

Results 1) The yield of the direct seeding rice was slightly higher than that of the hand-transplanting rice, while the former had a significant advantage in effective panicles and the latter advantage was grains per panicle. However, the advantages and disadvantages depended on whether effective panicles and grains per panicle were effective complementary, which reflected in the total number of spikelets. Superior treatment was more coordinated in terms of effective panicles and grains per panicle. 2) Under the combined application of slow-release urea and regular urea, the leaf color of direct seeding rice showed that was alternating with “thrice black and thrice yellow”, the hand-transplanting rice appeared change of “twice black and twice yellow”, the leaf color of direct seeding rice was relatively shallow. The leaf growth of direct seeding rice manifested as early fast and late slow, both LAI at the heading stage and Pn value of the flag leaves after the heading stage were relatively low. However, the more effective leaf area rate indicated that the upper functional leaf area of the later population was larger, and the quality of population was better. The carbon metabolism of direct seeding rice leaves was more vigorous, and the NSC/N ratio was higher at the middle and late stages, and the NSC/N of the high yield treatment was lower, which was similar to the one of hand-transplanting rice at the later stage. But the metabolism pathway was different; the direct seeding rice was strengthened in nitrogen metabolism, while the hand-transplanting rice was mainly regulated by enhancing carbohydrate assimilation. Increased the ratio of panicle N can effectively optimize the leaf quality at the mid-late stage of direct seeding rice. 3) The correlation analysis suggested that leaf growth rate, LAI, flag leaf Pn and leaf NSC/N at the booting stage were significantly or extremely significantly correlated with yield, which indicated that leaf quality at the booting stage was the key to affecting yield formation. 4) Direct seeding rice and hand-transplanting rice had the highest yields at the ratios of 4∶6 and 8∶2 for base fertilizer (slow-release urea) and panicle fertilizer (regular urea) treatments, respectively, reaching 12.11 t/hm² and 11.51 t/hm², and the yields were higher than those of the regular urea treatments, separately by 15.55% and 5.40%.

Conclusions This study indicated that the slow-release urea combined with urea increased the proportion of panicle fertilizer, controlled populations at the early stage and enhanced individuals at the later stage, it can be achieved the coordination between population and individual of direct seeding rice, enhanced the later stage nitrogen metabolism, effectively regulated the leaf carbon-nitrogen ratio, and improved leaf quality and prevented premature aging, so it’s an effective way to increase grain yield.