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CROP PHYSIOLOGY & METABOLISM

Grain Position Affects Grain Macronutrient and Micronutrient Concentrations in Wheat

^a Dep. Producción Vegetal, Facultad de Agronomía, Univ. de Buenos Aires, Argentina
^b CIMMYT (International Maize and Wheat Improvement Center), Apdo. Postal 6-641, 06600 Mexico, D.F., Mexico

* Corresponding author (danielcalderini{at}uach.cl)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Little is known about nutrient distribution within the spike of bread wheat (Triticum aestivum L.). This knowledge is important for determining breeding strategies aimed at increasing grain yield without affecting nutritional quality. The objective of this study was to gain a better understanding of how grain position affects nutrient concentration, dry matter distribution, and water dynamics of grains. An experiment using two sowing dates was performed under field conditions. Dry weight and concentrations of macronutrients (Ca, Mg, K, P, and S) and micronutrients (Cu, Fe, Mn, and Zn) in grains from the basal (BS), central (CS), and apical spikelets (AS) of two cultivars and one synthetic hexaploid line were determined. Grain water dynamics and nutrient and dry matter concentrations were also measured throughout the grain-filling period for the second sowing date. Genotypes showed different distributions of dry matter in different grain positions. Grain macronutrient and micronutrient concentrations in all genotypes decreased at grain positions more distal from the rachis. This reduction was as great as 30% (Ca) but varied by nutrient (e.g., Zn = -18%; S = -10%; K = +1%). Grain water content did not differ between grains. The observed differences in grain weight and nutrient concentration between grain positions could have important implications for wheat breeding. They suggest that it might be more effective to select for higher grain yield by increasing individual grain weight rather than grain number, a strategy that, in addition, would be less likely to affect the balance of nutrient concentrations within the spike.

Abbreviations: AS, apical spikelets • BS, basal spikelets • CS, central spikelets • G_1–4, grain positions 1 to 4 • S_1–2, sowing dates 1 and 2

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
DURING THE TWENTIETH CENTURY, plant breeding and improved crop management techniques successfully increased grain yield of bread wheat (Slafer et al., 1994; Calderini and Slafer, 1998; Calderini and Slafer, 1999) and, in so doing, effectively reduced food shortages and malnutrition. The future poses a similar challenge: by 2020, the demand for wheat is expected to be 40% greater than its current level of 552 million tons per year (Rosegrant et al., 1997). Now, however, a unique opportunity exists for agriculture to invest in developing more nutrient-dense staple food crops (Underwood, 2000) that could help reduce not only energy but also nutrient malnutrition. Welch and Graham (1999) proposed a new balanced nutrition paradigm for crop production. They point out that although the world's food supply in recent years has been sufficient, it does not promote an adequate nutritional balance (Anonymous, 1992; World Bank, 1994).

Breeding staple crops with high nutrient concentration in the grain has been proposed as a low cost, sustainable strategy for reducing mineral deficiencies in humans (more than 2 billion people suffer from deficiencies worldwide) and crops (Welch and Graham, 2000). To explore this possibility, the International Maize and Wheat Improvement Center (CIMMYT) and other international centers and research institutions have been participating in the Consultative Group on International Agricultural Research (CGIAR) micronutrient project since 1995. The project is studying the feasibility of developing micronutrient-dense cultivars of wheat, rice (Oryza sativa L.), maize (Zea mays L.), phaseolus beans (Phaseolus vulgaris L.), and cassava (Manihot esculenta Crantz). Its objectives are to determine: (i) available genetic variability that could be exploited in future breeding programs; (ii) the genetics and physiology of the selected traits; and (iii) the bioavailability of Fe and Zn in the best wheat selections (Bouis et al., 2000). This study, part of the broader CGIAR project, focused on the physiology of nutrient concentration in wheat grain. A better understanding of nutrient physiology could help design wheat breeding strategies aimed at increasing grain yield without reducing grain nutrient content. Nutrient-rich grain will, in turn, ensure a balanced nutrition in consumers, especially those who rely on wheat for a major portion of their calorie intake.

Wheat breeding has produced significant reductions in N (Austin et al., 1980; Slafer et al., 1990; Calderini et al., 1995; Ortiz-Monasterio et al., 1997) and P (Calderini et al., 1995) concentrations in the grain due to biomass dilution. Reductions in grain N concentrations as a result of plant breeding have also been reported for maize (Duvick, 1996) and sunflower (Helianthus annuus L.) (López Pereira et al., 2000). Therefore, it could be expected that nutrient concentration in individual grains has been affected similarly by dilution, that is, the heavier the grain the greater the dilution effect. Individual grains of modern wheat cultivars, however, have shown significant variation in N concentration depending on their position within the spike. Simmons and Moss (1978) and Herzog and Stamp (1983) found that lighter grains (mainly those in distal positions on the spikelet) had lower N concentrations than heavier grains (generally in proximal positions on basal-central spikelets), which suggests that grain position can have considerable impact on nutrient concentration. Therefore, future breeding strategies aimed at increasing grain yield while maintaining or increasing grain nutrient concentrations should take into account both the biomass dilution effect and nutrient distribution within the spike.

In the past, wheat yield gains have been associated with increases in the number of grain rather than in grain size (Loss and Siddique, 1994; Slafer et al., 1994; Calderini et al., 1999). Increases in grain number have been due mainly to improved grain set in more distal positions of the spikelet (Slafer et al., 1994). Miralles and Slafer (1995) showed there was a greater proportion of grains in distal positions in semidwarf wheat than in standard-height isogenic lines. The weight of these distal grains was clearly lower than that of grains from florets more proximal to the rachis. This trend suggests that continued selection for additional grains at more distal positions on the spikelets without parallel selection for higher nutrient concentration will result in continued reductions in nutritional quality and a greater imbalance in nutrient concentrations among grains. The effect of grain position on N concentration has been evaluated, but the manner and extent to which the position within the spikelet affects grain concentration of other macronutrients have not been reported.

The finding that grain N concentration is variable within the spike (Simmons and Moss, 1978; Herzog and Stamp, 1983) supports the hypothesis that grain concentration of other macronutrients (e.g., P) is also affected by grain position. This hypothesis, however, might not hold for micronutrients, which are less demanded by growing grains (Kochian, 1991). Ortiz-Monasterio and Graham (2000) found a very weak association (r² < 0.33) between Fe and Zn grain concentrations and year of release of CIMMYT cultivars, and a strong (r² = 0.72) linear relationship between grain yield and year of release in the same set of cultivars. To the best of our knowledge, there are no reports on how grain distribution within the wheat spike affects grain micronutrient concentrations.

It has been proposed that dry matter distribution influences N partitioning within the spike (Herzog and Stamp, 1983), since distal grains achieve less weight than those located closer to the rachis (Bremner and Rawson, 1978; Slafer and Savin, 1994; Miralles and Slafer, 1995; Kruk et al., 1997). At the same time, the dry matter and water dynamics of growing grains are closely related in wheat cultivars (e.g., Sofield et al., 1977; Schnyder and Baum, 1992; Calderini et al., 2000). Little is known, however, about the relationship between dry matter distribution and macro- and micronutrient concentration in grains in different positions and how they accumulate during grain filling. Synthetic hexaploid wheats (T. durum x Aegilops tauschii) can be used to study these relationships, since they have shown differences in dry matter distribution compared with modern cultivars (Calderini and Reynolds, 2000). Synthetic wheats may also be a source of variability for grain nutrient concentration, given that A. tauschii, a parent of the synthetics, has shown higher grain concentrations of micronutrients than T. aestivum (Ortiz-Monasterio and Graham, 2000).

The objectives of this work were (i) to study the concentration of macronutrients (Ca, Mg, K, P, and S) and micronutrients (Cu, Fe, Mn, and Zn) in grains set at different positions within the spike in two currently available wheat cultivars and a synthetic hexaploid line (T. durum x A. tauschii), and (ii) to evaluate macro- and micronutrient accumulation in grains together with dry matter distribution and water dynamics within the spike during grain filling.

	MATERIALS AND METHODS

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This study was performed under field conditions at CIMMYT, El Batán, Mexico (19°31' N, 98°50' W; 2249 m above sea level) in 1998. The treatment design was a factorial combination of two sowing dates, 22 May (S₁) and 17 June (S₂), and three wheat genotypes, two high yielding CIMMYT cultivars (Bacanora T88 and Rayón F89) and one hexaploid synthetic line [68.111/RGB-4//WARD/3/FGO/4/RABI/5/Ae. sq. (878)], also developed at CIMMYT. The synthetic was chosen because its phenology and 1000-grain weight are similar to those of Bacanora T88 and Rayón F89.

Experimental plots consisted of two 5-m long raised beds spaced 80-cm apart, with two rows 20-cm apart on each bed. Seeding rate was one seed per 3 cm. Plots were arranged in a split-plot design with three replications. Main plots were assigned to sowing dates and subplots to genotypes. Mineral N was applied at 150 kg N ha^-1 as urea and incorporated during land preparation (see results of presowing soil chemical analysis in Table 1). The plots were surface irrigated at sowing, and irrigation was continued as required until physiological maturity. Weeds were removed periodically by hand, while diseases (powdery mildew) and insects (aphids) were controlled by applying Tebuconazole {-[2-(4 chlorophenyl) ethyl--(1-1-dimethylethyl)-1H-1,2,4 triazol-1-ethanol} and Triadimephon [1(4 chloro phenoxyl-3,3 dimethyl 1-(1H-1,2,4 thriazol-1-il)-2-butomene), respectively.

Spikes were divided into three fractions containing five fertile spikelets each to study the effect of grain position upon grain weight and macronutrient and micronutrient concentrations. Two spikelets were excluded (the lowest and uppermost) because they generally carry only one grain instead of two or more as the other spikelets. Spikelets were pooled for the BS, CS, and AS fractions, and grains were sub-divided into first (G₁) and second (G₂) floret positions for all spikelets. In CS, grains of the third (G₃) and fourth (G₄) florets were also evaluated. Macronutrient and micronutrient concentrations were quantified by digesting samples with nitric/perchloric acid and analyzing them using ARL inductively coupled plasma spectrometry (Inductively Coupled Plasma Optical Emission Spectrometer ARL 3580 B, Switzerland) (Zarcinas et al., 1987).

For the purpose of comparing grain weight and grain nutrient concentration between grain positions, relative grain weight and relative grain nutrient concentration (percentage change from CS-G₁) were calculated as follows:

[1]

where RGW and RNC are relative grain weight and relative grain nutrient concentration, respectively; G_i is grain weight or grain nutrient concentration at a particular grain position; and CS-G₁ is grain weight or grain nutrient concentration in G₁ of CS.

Data were subjected to an analysis of variance utilizing a split-split-plot design, where sowing date was the main plot, genotype was the subplot, and grain position was the sub-subplot. Bacanora T88 and Rayón F89 set four grains in CS, while the synthetic line set just three. Therefore, two analyses of variance were performed. ANOVA 1 evaluated all grain positions shared by the three genotypes (i.e., all grain positions except G₄ in CS). ANOVA 2 evaluated all grain positions including G₄ for those genotypes that set it (i.e., all grain positions in Bacanora T88 and Rayón F89).

	RESULTS

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General
Sowing date modified date of anthesis; S₂ delayed anthesis by about one month with respect to S₁ (Table 2). Within each sowing date, however, genotypes reached anthesis on similar dates. Bacanora T88, Rayón F89, and the synthetic line had comparable grain-filling periods and reached physiological maturity on similar dates. The only exception was Rayón F89 in S₁, which showed slightly longer grain filling than did Bacanora T88 and the synthetic. Average temperature during grain filling did not differ between the two sowing dates.

Grain Weight
ANOVA 1 revealed that grain weight was affected by sowing date and grain position (Table 3). This analysis included all grain positions except G₄, which was not present in the synthetic line, for the three genotypes.

Results showed that P and K were the macronutrients with the highest grain concentration in all three genotypes; S and Mg reached intermediate levels, and Ca showed the lowest concentration (Table 6). Grain concentration of Mg and P was 20% higher in the synthetic line, and there were no differences between Bacanora T88 and Rayón F89 for these macronutrients. The synthetic line showed the lowest concentration of Ca, while Bacanora T88 showed the highest (16 and 24% higher than Rayón F89 and the synthetic line, respectively). There was little variation among genotypes for grain K and S concentrations.

The clearest effect of grain position on macronutrient concentration was found between grains in CS (Table 6). In Fig. 1, the values were expressed relative to G₁ (data shown are averages of S₁ and S₂) to facilitate viewing the differences in macronutrient concentration among grain positions. Except for K, grain macronutrient concentration decreased as distance from the rachis increased. The magnitude of the reduction, however, varied by nutrient; for example, Ca, Mg, P and S decreased by 30, 25, 17, and 10% in G4 of Bacanora T88 and Rayón F89, while K showed similar or higher concentrations in distal positions of CS (Fig. 1).

View larger version (17K): [in this window] [in a new window]   Fig. 1. The concentration of macronutrients (Ca, Mg, K, P and S) in wheat grain at grain positions G2, G3 and G4 of central spikelets. Data are presented relative to G1 concentrations in ‘Bacanora T88’, ‘Rayón F89’, and the synthetic line. Closed symbols indicate significant (P < 0.05) differences from G1.

For all genotypes, Fe and Mn achieved the highest concentrations; Zn was intermediate, and Cu was the lowest. The most consistent effect of genotype on grain micronutrient concentration was evident in the difference between the synthetic line and cultivars Bacanora T88 and Rayón F89. The synthetic line had higher concentrations of Cu, Fe, Mn, and Zn than the other genotypes in almost all grain positions, while differences in concentration between Bacanora T88 and Rayón F89 were rare (Table 7). The synthetic line accumulated around 15, 20, 30, and 20% more Cu, Fe, Mn, and Zn, respectively, in the grain than the two cultivars (averages for sowing date and grain position, excluding G₄). Differences between Bacanora T88 and Rayón F89 were always <5% regardless of whether G₄ was included in the analysis. Sowing date had a smaller effect on grain micronutrient concentrations. The difference between sowings was <6% even for Cu (3.8%) and Zn (5.3%), the only micronutrients affected by sowing date (Tables 3 and 7).

The values of all grain positions were expressed relative to G₁ CS (averages of S₁ and S₂, shown in Table 7) in Table 8 to facilitate comparing genotypes and grain positions for grain micronutrient concentration. The effect of spikelet position was most evident in Bacanora T88, in which grain concentrations of Cu, Fe, and Zn were lower in AS than in CS. Almost no effect of spikelet position was detected in Rayón and the synthetic line, yet micronutrient concentration was greatly reduced in distal grains of CS. This effect of position was similar among genotypes despite differences in grain set at G₃ and G₄, and dry matter distribution between grains (Tables 4 and 5). Although there were no clear differences among micronutrients trends, the effect of grain position was greater for Mn and Zn (where G₄ decreased 20% relative to G₁ in Bacanora T88 and Rayón F89) than for Fe and Cu concentrations (where the decrease in Bacanora T88 and Rayón F89 was 15%).

View larger version (33K): [in this window] [in a new window]   Fig. 2. Water (W), Ca, Mg, K, P, and S concentrations in developing grains of ‘Bacanora T88’, ‘Rayón F89’, and the Synthetic. Samples were from the second sowing date. G1, G2, G3, G4 indicate grain positions within the CS, G1 is the grain position closest to the rachis. Values in parentheses indicate relative differences (%) between grain positions G3 or G4 (depending on genotype) and G1 averaged across all sampling dates.

In agreement with data obtained for macronutrients, the effect of grain position on micronutrient concentration in grains at maturity was also found during grain filling (Fig. 3). In addition, micronutrient concentration in the synthetic line tended to be higher than in the two cultivars during development and at maturity (Fig. 3). Interestingly, Fe showed a different dynamic compared with other micronutrients. Concentrations of Cu, Mn, and Zn decreased (by an average of 30, 38, and 38%, respectively), while Fe concentration was nearly constant during grain filling (Fig. 3). This result implies that Fe unloading in grains had a similar relative rate as dry matter, while Cu, Mn, and Zn were unloaded at lower relative rates.

View larger version (35K): [in this window] [in a new window]   Fig. 3. Copper, Fe, Mn, and Zn concentrations in developing grains of ‘Bacanora T88’, ‘Rayón F89’, and the synthetic line. Samples were from the second sowing date. G1, G2, G3, G4 indicate grain positions within the CS, G1 is the grain position closest to the rachis. Values in parentheses indicate relative differences (%) between grain positions G3 or G4 (depending on genotype) and G1 averaged across all sampling dates.

	DISCUSSION

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Genetic Variability
As expected, genotype had a significant effect on grain macronutrient (Ca, Mg, and P) and micronutrient (Cu, Fe, Mn, and Zn) concentrations. This effect was mainly the result of the higher concentrations reached in the synthetic line. Similar results have been reported for micronutrients evaluated in a wide range of synthetic lines (Ortiz-Monasterio and Graham, 2000). The present study confirms that there is variability for nutrient concentration in wheat grains, and that synthetic hexaploids may be a useful source for increasing nutrient concentrations. Nonetheless, exceptions to this general pattern were found for some macronutrients (Ca, K, and S).

Vertical vs. Horizontal Axis Spike Variability in Grain Nutrient Concentration
Grain position significantly affected grain weight in both the vertical and horizontal axes of the spike. Grains located in AS and the G₃ and G₄ positions of CS were particularly low in weight. Clear differences in grain nutrient concentrations both between and within spikelets were observed. The present study showed that the greatest differences in grain nutrient concentrations were found within spikelets. Thus, grains along the rachis were less affected in their nutrient concentration than those along the rachilla (Tables 6 and 8). Similar results have been found for grain N concentration between wheat spikelets (Simmons and Moss, 1978; Herzog and Stamp, 1983). Differences in grain N concentration between spikelets were also found in two-rowed barleys, where each spikelet carries only one grain (Ellis and Marshall, 1998). Although significant, these differences were quite small. In wheat, inter-spikelet differences in N concentration were smaller than those between proximal and distal grains within a spikelet (Simmons and Moss, 1978; Herzog and Stamp, 1983).

Breeding progress had different effect on macronutrient (N and P) and micronutrient (Fe and Zn) concentration in grains as N and P concentrations of high yielding wheat cultivars were reduced (Austin et al., 1980; Slafer et al., 1990; Calderini et al., 1995; Ortiz-Monasterio et al., 1997) while Fe and Zn were not significantly affected by the dilution effect (Ortiz-Monasterio and Graham, 2000). However, modern cultivars evaluated in the present study showed similar patterns of distribution within the spike for most of macronutrients and micronutrients; that is, distal positions within spikelets showed lower nutrient concentration at maturity and during all grain filling period. Moreover, water dynamics during this period showed that grain water concentration did not vary with grain position (Fig. 2), in agreement with previous reports (Calderini et al., 2000). In addition, the synthetic line showed similar nutrient and water patterns to cultivars. These results suggest that the lower nutrient concentrations found in distal grains could be related with the transport of nutrients from the rachis to distal positions. The ratio of phloem vs. xylem concentration of nutrients and the mobility of the different nutrients within the phloem (see Kochian, 1991) may prompt studies aimed at understanding the causes of the observed differences in nutrient concentration in individual grains.

Breeding Implications
The differences in grain weight and nutrient concentration between grain positions within the spike observed in this study could have important implications for future wheat breeding strategies. For example, if plant breeders continue to strive for increased wheat yields by selecting for grain set in distal positions of the spike, the inherent lesser grain weight potential at these distal positions will likely limit advances in grain yield. Moreover, the added grains will have progressively lower mineral concentrations compared with proximal grains of CS. This conclusion is supported by the positive associations (P < 0.05) between nutrient concentration and grain weight found in Bacanora T88 and Rayón F89 for most macronutrients: Ca (r = 0.41), Mg (r = 0.68), P (r = 0.82), and S (r = 0.77); and micronutrients: Cu (r = 0.56), Fe (r = 0.71), Mn (r = 0.65), and Zn (r = 0.59). The magnitude of this effect, however, would likely be different for each nutrient. For example, a greater imbalance between grain positions would be expected for Ca, Mg, and Mn than for S and Cu, considering the differences in nutrient concentrations among grain positions. On the other hand, no effect is expected for K.

From a practical perspective, a greater number of smaller grains would have negative consequences for test weight and milling efficiency. In addition, small seeds (Richards and Lukacs, 2002) and low seed Zn (Rengel and Graham, 1995) and Mn (Longnecker et al., 1991) contents negatively affect seedling vigor and resistance to infection during the seedling stage (Genc et al., 2000; McCay Buis et al., 1995).

A promising breeding strategy would be to select for grain yield by increasing average grain weight. This would take advantage of the fact that yield of modern wheat cultivars is sink—rather than source—limited during grain filling (Slafer and Savin, 1994; Richards, 1996; Kruk et al., 1997). The distribution of grain nutrient concentration within the spike was similar among the genotypes examined in this study (Fig. 1 and Table 8). But only a few significant (P < 0.05) associations were found between nutrient concentrations and grain weight in the synthetic line (Cu: r = 0.39; S: r = 0.43), which implies that any imbalance in nutrient concentration between grain positions is probably more closely related to spike architecture than to dry matter distribution among grains. If so, the alternative strategy of increasing individual grain weight rather than grain number would be less likely to impact the balance of grain nutrient concentrations within the spike. To this end, the use of synthetic hexaploids in breeding programs could offer a potent source for increasing grain weight without sacrificing grain nutritional quality (Villareal et al., 1994; Mujeeb-Kazi and Hettel, 1995; Calderini and Reynolds, 2000).

	ACKNOWLEDGMENTS

 
We especially thank Dr. A. Mujeeb-Kazi (CIMMYT) for providing the synthetic hexaploid line used in this study, as well as Dr. A. Barneix (University of Buenos Aires), Dr. R. Graham (University of Adelaide), and Dr. R. Welch (Cornell University) for critically reviewing the manuscript. We also thank Alma McNab (CIMMYT) for revising the English usage, and Magda Lobnik, Tirso Rojo, José Luís Miranda, and CIMMYT's Wheat Physiology Lab for their important technical assistance. This work was supported by an overseas scholarship of the University of Buenos Aires, Argentina (Programa Thalmann).

Received for publication July 20, 2001.

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