Introduction
The major cereal crops in Egypt are wheat and rice, and they are directly associated with Egyptian food security. Both of these are considered main stable foods for the Egyptians. Rice is an important cereal crop, so several experiments were carried out to increase the yield production of rice [1-7]. NPK are the main nutrients for growth and limiting factors under flooding conditions. Nitrogen is required in large quantities, and it has a positive effect on the tiller number and yield components of many plants [8-10]. The appropriate level of NPK fertilizer is very important for the better growth and yield of rice [11-14]. Due to population growth, climate change, and urbanization, the demand for wheat and rice has increased year after year, so Egyptian rice breeders have improved high-yielding rice varieties that have adapted to biotic and abiotic stresses. The importance of N fertilizer was studied in many plants, such as wheat [15, 16], cowpea, sunflower [17], and sugar beet [18]. The improved varieties should have high nutritional value and good grain quality characteristics to meet Egyptian consumers’ needs. The genotypic variations in grain quality and nutritional value of milled rice are predominantly driven by the genetic and environmental aspects, which have been notified by several investigations. Singh et al. [19] indicated genotypic differences in the physicochemical, cooking, and textural properties of milled grains of rice cultivars (Japonica type). Metwally et al. [20] found wide variations among rice varieties in terms of physical grain characteristics. Metwally et al. [21] indicated that the Egyptian rice varieties have different nutritional values and grain quality characteristics. They found that hulling, milling, head rice, gelatinous temperature, kernel elongation, amylose content, protein content, carbohydrate content, lipid content, ash, phosphorus, potassium, and energy composition are varied among the rice varieties. Application of an optimum agronomic package of recommendations for each variety is important to obtain a high yield with acceptable grain quality characteristics. Nitrogen fertilization is one of the most important factors that control rice grain quality and quantity. Zhou et al. [22] noted that rising nitrogen fertilizer rates from 0 to 220 kg N/ha boosted the percentages of brown rice, head rice, chalky-kernel percentage, setback, and peak time values, but reduced the length/width ratio, chalkiness, apparent amylose content, gel consistency, and peak-, trough-, and final-viscosity values. These outcomes suggest that nitrogen fertilizer has a useful influence on milling and nutritional quality, but a detrimental effect on appearance, cooking, and eating quality. So, the objective of the current investigation was to determine the effect of nitrogen fertilizer rates on the nutritional value and grain quality characteristics of three Egyptian improved rice varieties.
Experimental
Materials and Methods
A two-year field experiment was conducted to study the behavior of three improved Egyptian rice varieties under different rates of nitrogen fertilizer. The experiment was located at the Experimental Farm of RRTC (Rice Research & Training Center), Sakha, Kafrelsheikh Governorate, Egypt (31°5′17″N, 30°56′44″E altitude). The selected tested varieties were Sakha109, Giza179, and Giza182. Sakha109 is a recently registered Japonica variety, Giza179 is a newly Indica/Japonica variety, and Giza182 is an Indica variety (Fig. 1.). The parentage, type, and origin of the studied genotypes are presented in Table 1.
Soil representative samples were collected from the experimental sites at a depth of 0-30 cm from the soil surface, then air-dried, ground to pass through a 2mm sieve, and well mixed. The technique of soil analysis was followed by the procedure of Black et al. [23]. Soil chemical analysis in the two studied seasons is presented in Table 2.
Cultural Practices
Seed Bed Preparation and Raising Nurseries
The seedbed had been established and sufficiently plowed, dry-grounded, irrigated by water, then wet-leveled. Calcium monophosphate (15.5% P2O5) was added to dry soil at a rate of 37 kg P2O5/ha just before plowing. Nitrogen in the form of urea (46.5% N) was applied and incorporated into the soil just before flooding. All seeds of the three rice genotypes were submerged in fresh water for one day and then incubated for two days in order to accelerate germination. Germinated seeds had been uniformly planted in the recommended area of 624.8 m2/ha for all rice genotypes. The nursery sowing dates were the 6th and 8th of May for 2020 and 2021, respectively.
Table 1.
Name, origin and Subspecies group of the studied rice genotypes.
| Genotype | Parentage | Origin | Subspecies group |
|---|---|---|---|
| Sakha109 | Sakha101/Sakha105 | Egypt | Japonica |
| Giza179 | GZ1368/IRAT112 GZ1368/GZ6296 | Egypt | Japonica/Indica |
| Giza182 | Giza181/IR39422//Giza181 | Egypt | Indica |
Table 2.
Some chemical analyses of the experimental sites before planting in 2020 and 2021 seasons.
The Permanent Field
The permanent plots were sufficiently set as plowing two times, harrowing one time, removing the crop residues of the previous crop, and dry leveling using a laser device. All plots received the optimum phosphorus and potassium fertilizer doses. Phosphorus was applied just before the second plowing at 36.9 kg P2O5kg/ha in the form of super-monophosphate Ca(H2PO4)2 which contains 15.5% P2O5. The recommended dose of 57 kg K2O/ha as potassium sulfate (48% K2O) was used in the permanent field and incorporated into the soil before the second plowing. Regarding nitrogen fertilization, urea (46.5% N) as a recommended source of nitrogen has been used and added according to the treatment schedule (0, 50, 100, 150, and 200 kg N ha-1) as two splits. The first split equaled two-thirds of the total amount and was applied and incorporated into the soil just before the first irrigation. The second split equaled one-third and was top-dressed 30 days after the application of the first split. 30-day-old seedlings had been uprooted from the seedbed and sufficiently spread via the experimental units. The transplanting operation was done manually at a planting density of 20 X 20 cm to maintain 25 hills per meter square. Three seedlings per hill were used for transplanting all rice varieties.
Grain Quality Characteristics
The milling recovery, gelatinization temperature, kernel elongation, and amylose content were estimated according to Cruz and Khush [24]. 150 (g) of cleaned rough rice at 14% moisture content was dehulled using an Experimental Huller Machine (Satake - Japan). The brown rice was separated and weighed, then the hulling percentage was calculated. The brown rice was milled using MC GILL Rice Miller No.2. (S.K. Appliances – India). The total milled rice (whole milled grains + broken milled grains) was weighed, and the milled rice percentage was calculated. Whole milled grains were separated from the total milled rice using a rice sizing device SKU: 61- 220-50 (Seedburo – USA). The percentage of head rice (whole grains) was calculated.
Six grains of whole milled rice were placed in boxes containing 1.7% KOH and arranged so that the kernels did not touch each other. The boxes were covered and incubated for 23 h at 30 °C. The appearance and disintegration of endosperm were graded visually according to the numerical scale of the gelatinization temperature. Kernel elongation was measured using the Micrometer: the length of five milled grains was measured (mm), and their average was determined for each treatment (before cooking). Grains were left in a test tube filled with 30 ml of distilled water for 30 min, then for another 10 min in a 98 °C water tub. After that, the tubes were placed in cold water until reaching room temperature. Grains were lifted from the distilled water, dried (by filter paper), and measured again by graph paper (after cooking). Kernel elongation percentage was calculated as the percentage of grain expanding before and after cooking. Amylose content was determined by accurately weighing 100 mg of sample into a 100 ml volumetric flask, then carefully adding 1 ml of 95% ethanol and 9 ml 1 N NaOH. The mixture was heated for 10 min in a boiling water bath to gelatinize the starch, then cooled, and the content made up to a volume of 100 ml with water. A 5 ml pipette portion of the gelatinization starch solution, 1 ml of 1 N acetic acid, and 2 ml of iodine solution were added and made up to a volume of 100 ml with distilled water. The content was shaken and stood for 20 min before reading the transmission at 620 nm by the Spectronic 1201 Spectrophotometer (Milton Roy, USA).
Protein, Carbohydrate, Lipids, Ash, Phosphorus, and Potassium Determination in Rice Grain
Plant samples were taken from the grain after milling (50 g of milled rice). All plant samples were placed in paper bags and oven-dried at 70 °C for 48 h. Grain samples were ground to powder and digested according to the method of Chapman and Pratt [25] before chemical analysis as follows: The nitrogen content of milled grains was determined by using the Microkieldahl method [26] to calculate protein content. Total carbohydrate was calculated by difference, as mentioned by Fraser and Holmes [27]. Lipids were determined according to A.O.A.C. [28]. The phosphorus content of milled grain was determined using the Spectronic 1201 Spectrophotometer (Milton Roy, USA) following the procedures of Watanabe and Olsen [29]. The potassium content of grain was determined using an Elico CL378 Flame Photometer (RHYS international LTD, India) according to the Peterpurgski method [30]. The data was subjected to analysis of variance (ANOVA), and the differences among treatments’ means were compared by Duncan’s Multiple Range Test (P < 0.05) and multiple F. test according to Duncan [31].
Results and Discussion
The grain yields of studied varieties, as affected by various nitrogen levels and their interaction in the 2019 and 2020 rice seasons, are presented in Tables 3 and 4. There are highly meaningful variations among various varieties in terms of grain yield in both seasons of investigation. Giza182 delivered the most elevated grain yield, followed by Giza179 without any significant differences between both of them in the second season only, while Sakha109 presented the lowest grain yield. Boosting nitrogen rates up to the highest rate caused a significant increase in grain yield.
The data in Table 4 asserts that highly significant variations in grain yield were due to the interaction between the variety and nitrogen level in both seasons. Giza179, treated with the highest nitrogen rate, recorded the highest grain yield values. While the lowest grain yield had been achieved at zero nitrogen with Sakha109. These results suggested that the studied cultivars were positively efficacious in nitrogen use at the highest rate, causing a relative boost in vegetative growth characteristics as well as yield attributes. The expansion in grain productivity at a higher nitrogen rate may also be attributed to the accumulation of more dry matter and translocation from source to sink. Moreover, Chu et al. [32] noted that the increase in sink ability is a fundamental way for plant breeders to improve rice productivity. They also reported that the number of unproductive tillers declined with increasing nitrogen application, and the highest weight of 100-grain was obtained from the lowest rate of nitrogen. Zhou et al. [33] suggested that under the application of a high dose of nitrogen fertilizer, a higher post-anthesis biomass could be gained and greater advantages from heading to maturity than the low nitrogen in the aspects of more dry matter accumulation at the late growth stage, a coordinated organ growth ratio, and a higher matter translocation rate from the stem and leaf to the panicle, thus benefiting yield formation.
Milling recovery characteristics were significantly affected by the studied treatments. Generally, Sakha109 produced the highest percentage of hulling, milling, and head rice. The varietal distinctions in hulling recovery could be attributed to their genetically inherited variants. Zhao et al. [34] reported that rice grain milling and physical characteristics are commanded by the genetic backdrop of rice genotypes. They also reported that gene GS9 regulates grain shape and hull thickness by altering cell division. It means rice grain milling recovery characteristics are mainly controlled by the genetic background.
Increasing the nitrogen level to the highest parameter gradually increased the milling recovery characteristics. The boost in milling recovery by raising nitrogen levels could be associated with the enlargement in assimilates translocation to the rice spikelets at the filling stage. The highest percentages of those characteristics were recorded when the Sakha109 rice variety was fertilized with the highest rate of nitrogen fertilizer.
Data in Tables 5 and 6 revealed significant differences due to nitrogen fertilizer application among rice varieties for carbohydrate, oil, ash, and amylose percentages in milled grains. Carbohydrate, oil, ash, and amylose percentages were significantly higher in the milled grains of Giza182 than the other varieties. Singh and Singh [35] found genotypic variation in carbohydrate, oil, ash, and amylose percentages among different rice varieties. The application of nitrogen fertilizer significantly increased the contents of carbohydrate, oil, ash, and amylose in milled grains. Liao et al. [36] concluded that nitrogen application regulated the grain yield, nitrogen metabolism, and antioxidant responses of different rice genotypes.
Table 3.
Grain yield and milling recovery characteristics of studied rice varieties under different levels of nitrogen fertilizers in 2020 and 2021.
Table 4.
Grain yield and milling recovery characteristics as affected by the interaction between rice variety and nitrogen level in 2020 and 2021.
Table 5.
Carbohydrate, oil, ash and amylose percentages in milled rice grains of studied varieties under different levels of nitrogen fertilizers in 2020 and 2021.
Nitrogen, phosphorus, and potassium concentrations in milled rice grains of the studied varieties under different levels of nitrogen fertilizer varied significantly in both seasons. Giza179 and Sakha109 recorded the highest percentage of nitrogen content in both seasons. Giza182 recorded the highest percentage of phosphorus content in the first season and for potassium content in the second season, while Sakha109 recorded the highest percentage of phosphorus content in the second season and for potassium content in the first season. Application of nitrogen fertilizer significantly increased the NPK content in milled rice grains of all varieties, up to the highest level. Gosal et al. [37] found a positive effect of increasing the dose of inorganic N on NPK uptake by rice plants that might be due to its direct contribution towards the available N pool and the enhancement of the decomposition of the organic nitrogenous material.
Table 6.
Carbohydrate, oil, ash and amylose percentages in milled rice grains as affected by the interaction between rice variety and nitrogen level in 2020 and 2021.
Table 7.
Nitrogen, phosphorus, potassium, and protein concentrations in milled rice grains of studied varieties under different levels of nitrogen fertilizers in 2020 and 2021.
Data in Tables 7 and 8 revealed significant differences among rice varieties, nitrogen levels, and their interactions with protein content percentage in both seasons. Sakha109 and Giza179 recorded higher values of protein content than Giza182, which came in the last rank and recorded the lowest value of protein content. Zhao et al. [34] indicated that the GS9 gene on Chromosome 9 acts as a transcriptional activator to regulate rice grain protein content and appearance quality. Singh and Singh [35] found genotypic variation in protein content percentage among different rice varieties. The application of nitrogen fertilizer significantly increased the contents of protein over the control treatment. Also, increasing the level of nitrogen fertilizer increased the available ammonium concentration in rice soil, which might enhance the uptake of nitrogen by rice plants, as well as increase the protein content. Moreover, Liao et al. [36] found that N fertilizer application regulates rice yield by affecting yield formation and biomass accumulation and modulating the activities of enzymes involved in N metabolism, the antioxidant enzyme activity, and the content of malondialdehyde.
Table 8.
Nitrogen, phosphorus, potassium, and protein concentrations in milled rice grains as affected by the interaction between rice variety and nitrogen level in 2020 and 2021.
Conclusions
The studied varieties showed different behavior under the various nitrogen levels. Giza182 (Indica Type) recorded a high grain yield with better nutritional value (protein, carbohydrate, lipids, ash, phosphorus, and potassium contents). Higher grain quality characteristics (milling recovery, gelatinization temperature, kernel elongation, and amylose) were obtained from Sakha 109. Generally, increasing the nitrogen level up to 200 kg N/ha increased rice grain yield, hulling %, milling %, and head rice % as well as carbohydrate %, oil %, ash %, amylose %, nitrogen %, phosphorus %, potassium %, and protein % in milled rice grains.
