Article Cite This: J. Agric. Food Chem. 2018, 66, 1601−1611
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Different Phosphorus Supplies Altered the Accumulations and Quantitative Distributions of Phytic Acid, Zinc, and Iron in Rice (Oryza sativa L.) Grains Da Su,†,‡ Lujian Zhou,‡ Qian Zhao,‡ Gang Pan,‡ and Fangmin Cheng*,‡,§ †
Key Laboratory of Ministry of Education for Genetics, Breeding and Multiple Utilization of Crops; Fujian Provincial Key Laboratory of Agroecological Processing and Safety Monitoring, College of Crop Science, Fujian Agriculture and Forestry University, Fuzhou 350002, P. R. China ‡ Institute of Crop Science, College of Agriculture and Biotechnology, Zhejiang University, Hangzhou 310058, P. R. China § Jiangsu Collaborative Innovation Center for Modern Crop Production, Nanjing 210059, P. R. China ABSTRACT: Development of rice cultivars with low phytic acid (lpa) is considered as a primary strategy for biofortification of zinc (Zn) and iron (Fe). Here, two rice genotypes (XS110 and its lpa mutant) were used to investigate the effect of P supplies on accumulations and distributions of PA, Zn, and Fe in rice grains by using hydroponics and detached panicle culture system. Results showed that higher P level increased grain PA concentration on dry matter basis (g/kg), but it markedly decreased PA accumulation on per grain basis (mg/grain). Meanwhile, more P supply reduced the amounts and bioavailabilities of Zn and Fe both in milled grains and in brown grains. Comparatively, lpa mutant was more susceptive to exogenous P supply than its wild type. Hence, the appropriate P fertilizer application should be highlighted in order to increase grain microelement (Zn and Fe) contents and improve nutritional quality in rice grains. KEYWORDS: rice (Oryza sativa L.), low phytic acid (lpa), grain minerals (Zn and Fe), phosphorus supply
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Zn and Fe.6 However, grain PA content in many cereals and legumes was often greatly variable, depending on different growth environments, such as planting year, geographical location, growing season, water management, and fertilizer application, in addition to the genotype-dependent alteration in grain PA content due to different genetic factors.1,7−9 It was reported that some lpa mutants had adverse impacts on agronomic performance, grain plumpness, and seed viability.2,10−12 A study on rice indicated that the deterioration of seed viability and yield performance of lpa mutant appeared to be negatively correlated with the extent of PA reduction.11 However, there is still controversy about if the genotypes or cultivars with lpa are more susceptible to environments relative to normal crop ones. It was reported that the lpa mutants of rice or maize not only had lower grain PA content but also had the changes in the concentration of some nutrient minerals. Ren et al.13 found that lpa rice cultivars generally had higher grain Zn, Fe and Ca contents than their wild-types. Badone et al.14 revealed that lpa mutation (lpa1-241) altered the accumulation of anthocyan in the maize kernels. But so far, nearly all findings about the relationship of grain PA content with mineral nutrients (Zn and Fe) in cereals and legumes are obtained from a comparison of difference between lpa mutant and its wild-type, and our understanding is relatively poor on the susceptibility of grain PA content to nutrient P supply and
INTRODUCTION Phytic acid (PA, myo-inositol-1,2,3,4,5,6-hexakis phosphoric acid, C6H18O24P6) is the most abundant form of phosphorus (P) storage in cereal and legume grains, accounting for 65%− 80% of seed total P accumulation and approximately 1% of grain dry weight.1 PA is a strong chelator of cations important for nutrition, such as iron (Fe), zinc (Zn), and calcium (Ca).2 The insoluble complexes of PA and these minerals, in the form of mixed-phytate globoids, are almost indigestible in the human and animal intestine, thus causing a marked reduction in bioavailability of nutrient elements, and consequently dietary Fe and Zn deficiency for the populations who mainly live on cereal grains and legumes.3 Accordingly, PA in staple foods is commonly regarded as an antinutritional factor. Moreover, the excreta of nonruminant animals may contain more P when they are fed with high PA feed, resulting in water contamination.2 Therefore, it is important to reduce grain PA content in cereals and legumes in order to improve crop quality and alleviate environmental contamination. Rice (Oryza sativa L.) is one of the most important cereal crops in the world and currently provides food for more than half of global population.4 However, rice grain is relatively lower in the contents of some micronutrients (Fe and Zn) in comparison with other staple crops, such as wheat, maize, and legumes.5 Moreover, the bioavailability of these micronutrients in rice grains is further reduced because of higher PA content.3 Obviously, it is an effective strategy to enhance the bioavailability of Zn and Fe by reducing PA content in rice grains. Actually, development of rice cultivars with low phytic acid (lpa) level has been highlighted.2 Animal feeding tests have proved the potential of lpa seeds in increasing the utilization of © 2018 American Chemical Society
Received: Revised: Accepted: Published: 1601
October 22, 2017 February 2, 2018 February 5, 2018 February 5, 2018 DOI: 10.1021/acs.jafc.7b04883 J. Agric. Food Chem. 2018, 66, 1601−1611
Article
Journal of Agricultural and Food Chemistry
Rice seeds were sown on seedling beds after indoor pregermination. The uniform 30-day old seedlings were subsequently transplanted to plastic pots filled with 3.5 L standard IRRI (International Rice Research Institute) nutrient solution, and three rice seedlings were transplanted for each pot. The pots were placed in a greenhouse under natural light conditions and moderate temperatures (28 °C day/22 °C night). After 30 days of normal growth in the IRRI nutrient solution, all rice plants were randomly divided into three groups (five pots for each group) and then subjected to different P treatments until maturity. Three P levels in the hydroponic culture were designed: 0.16 mmol L−1 (low P level, LP), 0.32 mmol L−1 (medium P level, MP), and 0.64 mmol L−1 (high P level, HP), respectively. The MP treatment (0.32 mmol L−1) was considered by following the standard formulation in IRRI protocol, with the 1/2 and 2× levels of standard P concentration in IRRI solution being set for LP and HP, respectively. NaH2PO4 was used as P sources and for the adjustment of P concentration in nutrient solution. The other essential elements in solution were kept completely similar for different P treatments. The solution in the plastic pots was renewed every 6 days and pH was adjusted to 5.7−5.8. At rice maturity, the grain samples from each pot were individually harvested. Gradient P Concentration Experiment in Detached Panicle Culture System. For the hydroponic experiment described above, the altered range of P concentration in IRRI solution was designed to be relatively narrower with a consideration of the normal growth development of rice plants, because the extremely high level of P concentration or the complete P deficiency in nutrient solution might lead to abnormal plant growth and infertility panicle. To clarify the effect of nutrient P concentration on grain PA and mineral elements (Fe and Zn), a detached panicle culture system was further used to conduct an in vitro experiment with a wider range of gradient P levels in 2015. Prior to this experiment, two rice genotypes (XS110 and lpaXS110) were planted in paddy field under natural growth, with a twenty-row plot for each rice genotype. At the full heading day, the rice panicles with uniformity anthesis day were selected as the experimental materials to impose different P treatments in the detached panicle culture system. The soil was a periodically waterlogged paddy soil, with 1.69 g/kg total N, 24.5 mg/kg available P, and 103.7 mg/kg exchangeable K. Rice plant grown in field was managed according to the recommended local practice. The detached panicle culture system for rice panicles was established according to the method of Singh and Jenner,24 which was initially set for wheat and barley. We redesigned the method with some modification, depending on the morphological and physiological properties of rice panicle. The major procedures were as follows: (1) rice panicle was cut below the penultimate stem node, with only 5 cm flag leaf and leaf sheath being retained, and all other leaves were completely removed; (2) the sterilized detached stems were promptly transplanted into sterilized liquid solution to impose different P treatments and supported with sterilized-cotton plugs; (3) the cultural containers with detached rice panicle were placed in a shallow cold water bath to prevent solution contamination, whereas rice panicles were exposed to temperate air, with temperature maintained at 24 °C−26 °C (day-time) and 20 °C−22 °C (night-time) and 75% of relative humidity. The detached panicles were grown under natural light supplemented to 300 μmol/m2/s, and the light period was set with 16 h a day. The gradient P levels in the detached panicle culture system consisted of 0 mmol L−1 (0P), 1 mmol L−1 (1P), 3 mmol L−1 (3P), 6 mmol L−1 (6P) and 12 mmol L−1 (12P). Here, 3P treatment (3 mmol L−1) was considered as the standard P level in the protocol of N6 liquid medium. NaH2PO4 was used for the adjustment of P concentration. Five flasks were used for each P level, and 4−5 panicles were placed in each flask. All culture containers in the detached rice panicle culture system were kept without distinction in other nutrient components except for P concentration treatments. Based on the liquid N6 medium, the basic nutrient composition was as follows: 1.50 mmol L−1 CaCl2, 1.50 mmol L−1 K2SO4, 0.75 mmol L−1 MgSO4·7H2O, 3.51 mmol L−1 (NH4)2SO4, 27.99 mmol L−1 KNO3,
its relation to the bioavailability of mineral nutrition (Zn and Fe) for lpa genotypes. P is one of most important macronutrients for plant growth and development. Phosphate fertilizer has been widely used to increase crop yield, especially in P-deficient soils and intensive agriculture production.15 In the contaminated soils, P fertilizer has been often used to immobilize some heavy metals (Cd and Pb) and to reduce the absorption of plant to heavy metal elements.16,17 On the other hand, P fertilizer supply had a marked influence on grain PA concentration for many crop species.1,8,10,18,19 However, inconsistent results have been reported for the relationship of nutrient P with grain PA concentration for several types of cereals and legumes. For instance, Jackson et al.8 and Matsuno et al.20 revealed that grain PA and Pi concentrations were not significantly correlated with P supply level, because of the rising level of nutrient P supply increased plant productivity and improved grain filling, which could cause the somatic growth dilution (SGD) of P concentration in plant developing organs. According to Goldberg and Sposito,21 PA concentration in cereal grains depends not only on the dose of P fertilizer application but also on the solubility and availability of P fertilizer in soil, including soil type and fertility, cation exchange capacity, clay content, and organic matter content. Studies on wheat indicated that the overuse of N fertilizer decreased grain PA content, concomitantly with increases in Zn and Fe contents in grains.9,22 In rice, Dang et al.17 revealed that sufficient P supply tended to reduce accumulation of several heavy metals (Pb, As, and Hg) in grains. However, the effect of nutrient P supply on grain PA distribution and its relationship with accumulation and bioavailability of Zn and Fe in rice grains remains largely unknown. In this study, two rice genotypes (XS110 and its lpa mutant) were selected to conduct different treatments of P supply levels by using two kinds of culture systems (hydroponics and detached panicle culture). The objectives of this study were (i) to investigate the effect of P supply level on grain PA content and its distribution in rice grains; (ii) to elucidate whether or not the accumulation of grain PA for lpa rice genotype was more susceptive to the varying P supply level compared with normal rice cultivar; (iii) to clarify the possible relationship of grain PA concentration with the accumulation and bioavailability of nutrient minerals (Zn and Fe) for different rice genotypes imposed to a series of P supply levels. Because the application of P fertilizer in paddy soil can easily adhere to soil particles,21 the nutrient P supply experiment in hydroponic culture and the gradient P concentration experiment in detached panicle culture system were designed to exclude the interference of soil particles to the availability of nutrient P absorption by rice plant. Such results could provide helpful information to improve the grain nutritional quality of crops by appropriate P supply and production management.
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MATERIALS AND METHODS
Plant Materials. The experiments were conducted during rice growing seasons (April to October) from 2013 to 2015 at the experimental station of Zhejiang University (30°18′ N, 120°04′ E), Hangzhou, China. Two rice genotypes, XS110 and its corresponding lpa mutant (lpa-XS110), were used in this study. XS110 was a japonica cultivar widely planted in China, whereas lpa-XS110 was derived by gamma-irradiated XS110 mature seeds as reported previously by Liu et al.23 Nutrient P Supply Experiment in Hydroponic Culture. The hydroponic P supply experiment was conducted in 2013 and 2014. 1602
DOI: 10.1021/acs.jafc.7b04883 J. Agric. Food Chem. 2018, 66, 1601−1611
Article
Journal of Agricultural and Food Chemistry
Table 1. Alterations in Grain Weight and Grain PA and Pi Concentrations (Content) as Affected by Phosphorus Supply (Hydroponic Experiment)a year 2013
genotype XS110
lpa-XS110
2014
P level LP MP HP LP MP HP
source of variation genotype (G) P level (P) P×G XS110 LP MP HP lpa-XS110 LP MP HP source of variation genotype (G) P level (P) P×G
1000-grain weight (g) PA concentration (g/kg) 18.65 21.24 21.04 18.28 20.60 18.36
± ± ± ± ± ±
