The Effect Of Nitrogen On Rice Quality And Its Physicochemical Foundation

Rice (Oryza sativa L.) is one of world’s most widely grown cereals and plays a critical role in food security especially in South Asia and East Asia. A continuous improvement in rice production is a challenging task for crop scientists to keep abreast of the world’s population growth. Quality of grain, next to yield, is the most important factor for rice production, and among its key components, nutritional quality has historically been considered as one of the main objectives for rice improvement.Nitrogen (N) is one of the most important elements required for obtaining high rice yields. However, the current irrigated rice system is characterized as low fertilizer N use efficiency due to overuse of N fertilizers, which also poses potential adverse environmental and health concerns. In addition, N has closely relation to rice quality. Obvious bifurcations existed in many current researches about the effect of N on rice quality due to different cultivars, amount of N application and cultivation environment. So, the present research was conducted and aims to clarify the effect of nitrogen on rice quality and its physicochemical foundation.Field experiment involving six japonica rice cultivars with contrasting agronomic traits and seven N fertilizer treatments was performed at Jiangning (2007 and 2008) and Danyang (2009 and 2010) experimental station, to determine the effects of N and genotype on grain yield and its components, processing quality, appearance quality, cooking and eating quality, physicochemical properties of starch, and chemical constituents of rice grain. The main results were shown as follows:(1) Grain yield:With increased N level, grain yield progressively increased. Among yield components, panicles increased significantly with N level and then did the spikelet per panicle, to a less extent. Seed-setting percentage decreased with N level, while 1000-grain weight was less influenced by increasing N level. Grain yield of cultivars with different type of panicle had different sensitively to N level, such as small panicle Wuyujing3, large panicle Xudao4 and Wuyunjing7 exhibited relatively more stable under contrasting N treatments compared to large panicle Ningjing2, small panicle Zaofeng9 and Ningjingl. Genotype difference in response of yield to N was significant under CK, but showed a decreasing trend with increasing N level.(2) Processing quality:Head rice rate decreased with increasing N level, brown rice rate and milled rice rate changed slightly. Processing quality of rice was mainly controlled by genotype, while the effect of different N level was not significant. Brown rice rate and milled rice rate of Wuyunjing7 were improved under MN55 compared to MN82. Significant genotype difference was found among four cultivars. Brown rice rate was highest for Wuyujing3 and lowest for Wuyunjing7, milled rice rate and head rice rate was highest for Ningjing2 and lowest for Wuyunjing7.(3) Appearance quality:With increased N rate, PRK percentage increased significantly under HN55. WDRK decreased, GRK increased and WCRK, WBRK, ORK and Others changed uncertainty with increasing N level. WCRK, WDRK and ORK were highest for Ningjing1; WBRK and Others were highest for Wuyujing3; GRK was highest for Wuyunjing7; WCRK, WBRK, and ORK were lowest for Ningjing2; WDRK and Others were lowest for Wuyunjing7, GRK was lowest for Wuyujing3. N application showed a positive effect on appearance quality except for Ningjingl due to its BRK was increased with N level.(4) Eating quality:Applied N fertilizer had an adverse effect on rice cooking and eating quality that evaluated by a taste analyzer using a Japanese cultivar Koshihikari as reference. The taste value of four cultivars was 69,63,63 and 57 for Wuyunjing7, Ningjing2, Wuyujing3 and Ningjingl, respectively. Interestingly, Wuyujing3 was famous for its good eating quality but the score was lower than Wuyunjing7, which own bad eating quality. This indicated that estimate of cooking and eating quality by a taste analyzer was rather limited.(5) Accumulation of chemical constituents of rice grain:Total starch concentration especially amylopectin concentration decreased and ratio of amylose to amylopectin increased with N level increasing, while amylose concentration changed unconspicuous.With increased N level, grain phytic acid concentrations and ratios of prolamin to glutelin progressively reduced whereas concentrations of the 4 proteins and ratios of glutelin to total protein increased, indicating that N level has a beneficial effect on rice nutritional quality. For brown and milled rice, albumin and globulin were controlled more by genotype than N treatments, whereas prolamin and glutelin were largely determined by N. Substantial genotypic differences in response of milled/brown (M/B) ratios of proteins to N treatments were detected. In comparison with large panicle cultivars, small panicle cultivars such as Wuyujing3 had the lower ratio and exhibited more stability under contrasting N treatments. N had significant influence on amino acid composition of brown and milled rice, with concentrations of the 17 amino acids measured increasing with elevated N rate. However, cysteine and methionine in brown rice and lysine and methionine in milled rice were not significantly affected by N.(6) Accumulation of Macronutrient and micronutrient:For brown rice, variance analysis showed no significant effect of N on concentrations of macronutrients, but N tended to promote calcium whereas lower magnesium and sodium deposition. However, N showed significant effect on micronutrients, especially lowered zinc accumulation. For milled rice, significant effect of N on micronutrients, being positive for iron and manganese whereas negative for zinc. In addition, genotype difference in response of macronutrients and micronutrients to N was detected, which also varied with growing years. Our results suggest different mechanism between N effect on macronutrients and micronutrients, and are of significance for coping with "hidden hunger" both in human and crops through agronomical practices.(7) Physicochemical properties of starch:Rapid viscosity analyzer (RVA) is widely used to identify cultivars that possess prominent or poor eating quality. In our sduty, RVA profile of milled rice starch showed a descend trend when N rate was increasing. RVA parameters were more influenced by genotype than N fertilizer. With increased N level, PKV, HPV, CPV and CSV decreased; PT increased; BDV decreased in 2008 and 2009 but increased in 2010; SBV decreased first and then increased. PKV and BDV of Wuyujing3 were relatively stable and higher than wuyunjing7, while CSV and PT were lower.Overall, application of N promoted grain yield but had negative effect on grain processing quality, cooking and eating quality. Therefore, more investigations should be carried out to elucidate the physicochemical foundation of deterioration of rice quality in relation to high N input.