Agronomy Journal Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 13 May 2005
Published in Agron J 97:924-934 (2005)
DOI: 10.2134/agronj2004.0309
© 2005 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Wiersma, J. V.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Wiersma, J. V.
Agricola
Right arrow Articles by Wiersma, J. V.
Related Collections
Right arrow Soybean
Right arrow Seed Treatment
Right arrow Nutrient Management
Right arrow Plant Nutrition

Soybean

High Rates of Fe-EDDHA and Seed Iron Concentration Suggest Partial Solutions to Iron Deficiency in Soybean

John V. Wiersma*

Univ. of Minnesota, Northwest Res. and Outreach Cent., Crookston, MN 56716

* Corresponding author (jwiersma{at}mail.crk.umn.edu)

Received for publication December 16, 2004.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Plants require a continuous supply of iron (Fe) to maintain proper growth. Low rates of Fe chelates applied to reduce Fe deficiency in soybean [Glycine max (L.) Merr.] probably do not satisfy this requirement. Our objective was to evaluate the effectiveness of high rates of Fe-EDDHA in reducing Fe deficiency when applied to susceptible and resistant cultivars grown on soils where soybean historically has exhibited mild to severe Fe deficiency. Four cultivars (two resistant, two susceptible) and six rates of Fe-EDDHA (0, 2.24, 4.48, 6.72, 8.96, and 11.20 kg ha–1) were evaluated at one location in 2002 and two locations in 2003. Severity of Fe deficiency varied markedly across environment and cultivars. Unifoliolate relative chlorophyll concentrations indicated that Fe deficiency can occur early in plant development and that planting seed Fe concentration (seed [Fe]) may be insufficient for early growth. Responses to higher rates of Fe-EDDHA were environment and cultivar specific and occurred over an extended period, manifest at maturity. At lower rates (<6.72 kg Fe-EDDHA ha–1), resistant cultivars exceeded susceptible cultivars in plant height, seed number, and grain yield, whereas at higher rates, susceptible cultivars often had values similar to resistant cultivars. Both resistant and susceptible cultivars exhibited linear responses to increasing rates when grown in harsh or intermediate environments, suggesting that even at very high rates of Fe-EDDHA, Fe deficiency limited plant growth and grain yield. Seed [Fe] changed very little in response to added Fe. Plotting relative grain yield versus seed [Fe] for each environment illustrated the narrow range of seed [Fe] associated with wide ranges in relative yield and the large difference between resistant and susceptible cultivars regardless of relative yield.

Abbreviations: Env. 1, 2, and 3, Environments 1, 2, and 3 • IDC, iron deficiency chlorosis • seed [Fe], seed iron concentration • SPAD reading, relative chlorophyll concentration • VCS, visual chlorosis score(s)


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
ALTHOUGH THE MOST abundant micronutrient in surface soils (Fageria et al., 2002), Fe is the most limiting to agricultural production throughout the world (Kochian, 2000) and to soybean production in much of the North Central United States (Hansen et al., 2003, and references therein). Iron deficiency is a complex disorder and occurs in response to multiple soil, environmental, and genetic factors. Iron deficiency chlorosis (IDC) is symptomatic of the disorder and commonly observed in soybean fields in southern and western Minnesota every year. Cultivars tolerant to IDC are considered essential for profitable production on chlorosis-prone soils although even the best cultivars can suffer yield losses when harsh conditions prevail (Froehlich and Fehr, 1981). Nonetheless, there are few management practices other than cultivar selection that will reduce the incidence and/or severity of Fe deficiency in soybean. Other management tactics that have been suggested include increasing plant populations (Penas et al., 1990; Goos and Johnson, 2001) and applying various seed, soil, or foliar Fe chelates or fertilizers. These practices strive to improve the availability of Fe, but generally they are effective only for short periods of time.

Plants require a continuous supply of Fe to maintain proper growth (Brown et al., 1972) since very little accumulated Fe is mobilized from older to younger tissues (Ham and Dowdy, 1978; Karlen et al., 1982; Sojka et al., 1986; Sadler et al., 1991; Zhang et al., 1996; Burton et al., 1998). Because Fe is necessary for several metabolic processes, yet potentially toxic, a plant's Fe uptake and homeostasis are tightly controlled (Motta et al., 2001; Hell and Stephan, 2003). Thus, in Strategy I plants such as soybean, Fe stress response mechanisms are turned on or off as needed to maintain adequate Fe concentrations in plant tissues (Romheld, 1987; Marschner et al., 1989; Jolley et al., 1996). Soybean genotypes vary in the magnitude and timing of Fe stress responses; more resistant (tolerant) genotypes reduce Fe3+ to Fe2+ sooner and in larger quantities than more susceptible genotypes (Jolley et al., 1992; Ellsworth et al., 1998). In calcareous soils with high pH and high-pH buffering capacity (elevated bicarbonate levels), severe impairment of Fe stress response mechanisms can occur (Romheld, 1987), and repeated applications of foliar sprays may be necessary to supply adequate Fe for either susceptible or resistant soybean cultivars (Niebur and Fehr, 1981). Although mildly chlorotic soybean often achieves a greener color, this does not necessarily mean that Fe is no longer limiting. The requirement of a continuous supply of Fe complicates the search for resistant genotypes as well as management practices that will improve Fe nutrition on chlorosis-prone soils.

Research to reduce or alleviate IDC in soybean by applying various seed, soil, or foliar Fe chelates or fertilizers has been conducted for decades. Although the results have been mixed (Mortvedt, 1986) and are seldom directly comparable, positive responses to foliar (Randall, 1981), seed (Karkosh et al., 1988), and soil (Penas et al., 1990) treatments have been reported. Other researchers have observed only small, if any, response to similar treatments (Goos and Johnson, 2000; Goos and Johnson, 2001; Heitholt et al., 2003). Lack of consistent results may be related to inconsistent levels of chlorosis severity, soil, environmental, or genetic differences, and/or the low rates of Fe often applied to ensure economic feasibility. Low rates of Fe probably do not satisfy the requirement of a continuous supply of Fe as plant development progresses (Goos and Johnson, 2001).

Penas et al. (1990), using seed-furrow-applied Fe chelate solutions, reported that some genotypes required rates as high as 4.5 kg Fe-EDDHA ha–1 to achieve maximum yields in the presence of moderate to severe Fe deficiency. Averaged across 15 cultivars, maximum economic return occurred at 3.5 kg Fe-EDDHA ha–1. On the other hand, Karkosh et al. (1988), using seed-applied Fe chelate, recommended that low (0.56 kg ha–1) rates of Fe-EDDHA be applied only to more resistant genotypes because lower rates did not provide adequate levels of Fe for sustained growth of more susceptible genotypes and higher rates were not economic. Goos and Johnson (2001) reported that the effectiveness of seed-applied Fe chelate rates of 0.56 and 1.12 kg ha–1 had diminished by the fifth to sixth trifoliolate stage and that higher rates may have been more reliable. The economic feasibility of applying higher rates of chelated Fe will depend on the magnitude of the yield response the cost of the chelate and the price of soybean. Using a combination of soil- and seed-applied Fe chelates to provide higher rates of Fe may reduce Fe deficiency by extending the period of time Fe is available for plant development. This practice may provide a partial solution to IDC, especially in production areas with a history of moderate to severe Fe deficiency and especially if less expensive chelates become available. The objective of this study was to evaluate the effectiveness of high rates of Fe-EDDHA in reducing Fe deficiency when applied to susceptible and resistant soybean cultivars. Some of the questions we sought to address included: (i) is there a linear response to increasing rate? (ii) do susceptible cultivars respond differently than resistant cultivars? (iii) is there a maximum effective rate when harsh conditions prevail? (iv) do higher rates provide Fe later in the season? (v) do higher rates influence seed [Fe]? and (vi) what is the relationship between relative yield and seed [Fe]?


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Experiments were conducted at one location in 2002 [the Northwest Research and Outreach Center (NWROC) at Crookston, MN) and at two locations in 2003 (NWROC and Fisher, MN) on soils where soybean has historically exhibited mild to severe Fe deficiency. To facilitate further description and discussion, year x location combinations are renamed as follows: Env. 1 is Crookston, 2002; Env. 2 is Crookston, 2003; and Env. 3 is Fisher, 2003. Soil samples were collected from each experimental area and analyzed by Agvise Laboratories, Northwood, ND (Table 1). Each year, trials were done using calcareous, high-pH soils. Trial areas also received additional N fertilizer to reduce nodulation and enhance Fe deficiency (Aktas and van Egmond, 1979; Lucena, 2000; Terry et al., 1991). Urea fertilizer (392 kg ha–1, Env. 1; 168 kg ha–1, Env. 2; and 200 kg ha–1, Env. 3) was broadcast and incorporated before planting. Weeds were controlled by hand weeding and application of selected herbicides for control of specific weed species and intensities. Bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] at 0.84 kg a.i. ha–1, sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 0.31 kg a.i. ha–1, and thifensulfuron {3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]-amino]sulfonyl]-2-thiophenecarboxylic acid} at 4.3 g a.i. ha–1 were applied in Env. 1. Bentazon (1.12 kg a.i. ha–1) and sethoxydim (0.31 kg a.i. ha–1) along with imazamox {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-(methoxymethyl)-3-pyridinecarboxylic acid} at 44 g a.i. ha–1 were applied in Env. 2 and 3. Insect pests and diseases were either absent or considered inconsequential. Weather variables were recorded at an official National Weather Service station located within 2 km of the experimental areas in Env. 1 and 2 (Table 2).


View this table:
[in this window]
[in a new window]
  		Table 1. Selected properties of soils from three experimental areas.{dagger}

 

View this table:
[in this window]
[in a new window]
  		Table 2. Summary of growing season climatic data for Env. 1 and 2 and 30-year averages for Crookston.

 
Trials were planted 20, 15, and 16 May in Env. 1, 2, and 3, respectively. A split-plot arrangement of a randomized complete block design with four replications was used for each environment. Cultivars were whole plots, and Fe-EDDHA chelate rates were subplots. Four, early-maturity (Group 00) cultivars were selected to represent moderately tolerant (‘MN0302’ and ‘Norpro’) and susceptible (‘Gold Country Seeds 3104’ and ‘World Seeds 2020’) genotypes (unpublished data, 2001). Subplots were four rows wide and 6.1 m long, with an interrow spacing of 0.56 m. Plots were seeded at approximately 470000 seeds ha–1. Six Fe-EDDHA (6% Fe, CIBA Specialty Chemicals, Suffolk, VA) rates (0, 2.24, 4.48, 6.72, 8.96, and 11.2 kg product ha–1) were used. For the 2.24 and 4.48 kg ha–1 rates, all of the chelate was seed-applied, whereas for the higher rates, 4.48 kg ha–1 was seed-applied and the remainder was seed furrow-applied. A mixture of Fe-EDDHA product (140 g), gum arabic (80 g), and water (130 g) was prepared, and 4.8 (12) and 9.6 (24) mg Fe-EDDHA product (mixture) seed–1 were applied to establish the 2.24 and 4.48 kg ha–1 rates, respectively. Preliminary tests indicated that about 72% of the mixture adhered to the mixing container. Small batches of 3800 seeds were prepared, adjusting for the loss during preparation. Additional chelate for the higher rates was pumped into the seed furrow at planting using three solutions of varying concentration and a constant volume of 93.3 L ha–1. Treated seed was air-dried before planting. Only the two middle rows were treated with Fe-EDDHA.

Relative chlorophyll readings were recorded throughout the growing season in each environment to evaluate the effectiveness of higher rates of Fe-EDDHA to supply Fe as plant development progressed. Although we were not able to record these measures at exactly the same stage of development in each environment, these readings were considered to represent seedling and early-, mid-, and late-season stages of plant development. A seedling estimate of the ability of the seed and young seedling to absorb and utilize Fe supplied by Fe-EDDHA was determined at the V1 stage (Ritchie et al., 1988) in Env. 2 and 3. SPAD meter (Minolta, Ramsey, NJ) readings were recorded at three locations on each unifoliolate leaflet of 10 plants plot–1 and averaged to give a plot value. Similar procedures were used to record readings of the first trifoliolate in Env. 1 and 2, the third trifoliolate in Env. 3, and the sixth trifoliolate in all three environments. A late-season estimate was measured in Env. 1 and 3. At the two- to three-trifoliolate stage in each environment, one observer recorded visual chlorosis ratings using the procedure described by Goos and Johnson (2000).

Plant height was recorded at R8, and grain yields were determined after harvesting 5.17 m of the two center rows (5.79 m2) with a small-plot combine. Moisture concentration of plot samples was recorded and used to adjust grain yields to a moisture concentration of 130 g kg–1. Weight per seed was determined by counting and weighing 2000 seeds from samples of grain from each plot, and seed number was calculated from yield and weight per seed.

After grinding [Tecator Cyclotec Mill (Fisher Scientific, Itasca, IL) with a 0.048-mm (32-mesh) screen], 1-g subsamples of grain were dry-ashed (Miller, 1998) and assayed for Fe concentration using a modification of a ferrozine procedure developed by Stookey (1970). One milliliter of 6 M HCl was added to the residue remaining following dry ashing, and the sample was shaken until homogeneous (clear to yellow solution). Next, 26 mL of water was added, followed by 1 mL of 6 M NaOH. A stir bar was added, and the solution was stirred slowly for 3 to 4 s. Then, 0.5 mL of FerroZine Iron Reagent (Hach Chemical Company, Loveland, CO) was added, followed by stirring for 5 to 10 s until a clear solution was evident. Color was allowed to develop for 30 min; absorbance was read at 562 nm. Serial dilutions of an iron standard solution (Hach Chemical Company) were used to develop a standard curve. Aliquots of the standard solution added to subsamples of grain demonstrated 99 to 101% recovery. Check tissue was included every 10 samples to monitor precision.

Data were analyzed using standard procedures for an analysis of variance of a split-plot arrangement of a randomized complete block design (Gomez and Gomez, 1984). Separate analyses were done for each environment, considering cultivars (C) and Fe-EDDHA rates (R) as fixed effects. Although trials were done using calcareous, high-pH soils each year, the severity of Fe deficiency varied substantially across environment. Bartlett's test was used to assess homogeneity of error variances, and results of nearly all of these tests were highly significant (P < 0.001). Thus, combined analyses across environment were not computed. Orthogonal, single degree-of-freedom contrasts were formulated for comparisons among cultivars and seed-applied Fe chelate rates, and products of contrasts were used to evaluate interactions. The cultivar contrast was (C1) moderately tolerant vs. susceptible, and the Fe-EDDHA rate contrasts were (R1) linear and (R2) quadratic.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
The severity of Fe deficiency can vary dramatically within small field distances, within and across years and locations, and in response to numerous environmental and edaphic factors (Godsey et al., 2003; Hansen et al., 2003). Indeed, if consistent responses to treatments are predicated on having consistent severities of Fe deficiency, it is not likely they will occur. In this study, monthly precipitation, average daily temperature, and several soil factors associated with Fe deficiency varied across environments (Tables 1 and 2), and the severity of Fe deficiency and error variances often differed substantially. Responses to treatments were similar but not entirely the same.

Relative Chlorophyll Concentration
With high-pH, highly calcareous soils, Fe-EDDHA is a superior Fe chelate, facilitating Fe availability over extended periods in laboratory studies (Goos and Germain, 2001). When used in field trials, researchers (Karkosh et al., 1988; Goos and Johnson, 2001) have reported only short-lived benefits of low rates of Fe-EDDHA. In this study, higher rates of Fe-EDDHA improved the availability of Fe throughout plant development and reduced Fe deficiency. Resistant cultivars exhibited higher relative chlorophyll concentrations (SPAD readings) than susceptible cultivars at all stages. At seedling and early-season stages (Fig. 1) , much of the increase in SPAD readings occurred in response to the first increment (2.24 kg ha–1) of chelate. Additional Fe caused little increase in SPAD readings, probably because this supply of available Fe during early development far exceeded demand. Linear and quadratic contrasts accounted for most of the response to increasing rates of chelate for both resistant and susceptible cultivars (Fig. 1 and 2 ; Table 3). Although occasionally statistically significant, cultivar x rate interactions and interaction contrasts were associated with a small percentage (usually < 8%) of the total sums of squares and were considered inconsequential for seedling and early-season measures.



View larger version (24K):
[in this window]
[in a new window]
  		Fig. 1. Increase in relative chlorophyll concentrations at seedling and early-season stages of plant development in three environments in response to increasing rates of Fe-EDDHA.

 


View larger version (22K):
[in this window]
[in a new window]
  		Fig. 2. Increase in relative chlorophyll concentrations at mid- and late-season stages of plant development in three environments in response to increasing rates of Fe-EDDHA.

 

View this table:
[in this window]
[in a new window]
  		Table 3. Summary of analyses of variance for seedling, early-, mid-, and late-season relative leaf chlorophyll concentrations and visual chlorosis scores recorded at V3 in each environment.

 
As rates of Fe-EDDHA increased, unifoliolate SPAD readings (Fig. 1) also increased, suggesting that Fe deficiency can occur early in plant development and that planting seed may be impoverished, i.e., that planting seed [Fe] may be insufficient for early growth. Other researchers have described the importance of micronutrient concentrations in seed to be used for planting. Molybdenum deficiency in corn (Zea mays L.) (Weir and Hudson, 1966), Zn deficiency in barley (Hordeum vulgare L.) (Genc et al., 2000), Mn deficiency in wheat (Triticum aestivum L.) (McCay-Buis et al., 1995), and B deficiency in soybean (Rerkasem et al., 1997) have been shown to be related to inadequate seed reserves when these crops are planted in soils with low concentrations or availabilities of these nutrients. Similar research with seed Fe reserves in soybean has not been reported. Only small amounts of Fe are stored within embryos and seed coats of soybean (Laszlo, 1988, 1990), yet these fractions may be especially important in delaying the onset of IDC during seedling establishment and early plant growth. Improved chlorosis ratings and small increases in yield observed when seed is treated with low rates of Fe chelate (Karkosh et al., 1988; Goos and Johnson, 2001) may result because the Fe chelate supplements an inadequate seed reserve early in plant development.

As plant development progressed to mid- and late-season stages, quadratic responses to Fe-EDDHA rates were not detected, suggesting that even high rates of chelate were insufficient for maximum chlorophyll production, especially for susceptible cultivars (Fig. 2; Table 3). Rates of increase were substantially higher with susceptible cultivars, whereas resistant cultivars showed only small increases in SPAD readings at midseason and no response at R6 in Env. 1. Although high rates of Fe-EDDHA improved the availability of Fe throughout plant development, susceptible cultivars probably benefited more than resistant cultivars. Nonetheless, susceptible cultivars rarely had SPAD readings that equaled those of resistant cultivars, even at high rates of Fe-EDDHA.

Visual Chlorosis Score
In the absence of added Fe-EDDHA, visual chlorosis scores (VCS) recorded at V3 indicated that moderate to severe IDC occurred across environments. Mean VCS at 0 kg Fe-EDDHA ha–1 were 3.0, 4.2, and 3.4 in Env. 1, 2, and 3, respectively. High (6.72 kg ha–1) rates of Fe-EDDHA were required in each environment to reduce VCS to values (1.0) thought to represent nondeficient plant growth (Fig. 3) . SPAD readings recorded during seedling and early-season stages (Fig. 1) also indicated that similar rates should be adequate for early growth and development. However, at mid- and late-season stages, SPAD readings (Fig. 2) continued to increase as Fe-EDDHA rates increased, suggesting that despite early-season "regreening," latent Fe deficiency continued as plant development progressed, especially with susceptible cultivars. Presumably, high rates of Fe-EDDHA were required to provide a continuous supply of available Fe.



View larger version (22K):
[in this window]
[in a new window]
  		Fig. 3. Decline in visual chlorosis score in response to increasing rates of Fe-EDDHA recorded at V3 for resistant and susceptible soybean cultivars in three environments. Cultivar means at higher chelate rates occasionally overlap and are indistinguishable from each other.

 
Unlike results of SPAD readings, resistant cultivars did not always express less visual chlorosis than susceptible cultivars, and cultivar x rate interactions and interaction contrasts usually were statistically significant (Table 3). This may reflect the difficulties encountered with a subjective, visual scale, but it also is important to remember that because we evaluated visual chlorosis on a 1 to 5 scale, in 0.5 increments, differences less than 0.5 are meaningless, even if statistically significant. For example, VCS recorded in Env. 3 (Fig. 3) suggested that resistant cultivars were more chlorotic than susceptible cultivars, especially as Fe-EDDHA rates increased. Yet these differences were less than 0.5 at 6.72 and 8.96 kg Fe-EDDHA ha–1 and less than 0.6 at the highest rate. Visually, there was little chlorosis at rates higher than 4.48 kg Fe-EDDHA ha–1 and no meaningful difference among cultivars. Hansen et al. (2003) reported good agreement between SPAD readings and VCS when both were recorded on the same day at later stages of development.

Plant Height, Seed Number, and Grain Yield
Chlorotic soybean plants often are substantially shorter than nonchlorotic plants, have reduced biomass, produce fewer seeds, and have lower yields (Froehlich and Fehr, 1981; Fehr, 1982; Fehr and Trimble, 1982; Penas et al., 1990; Hansen et al., 2003). In this study, we observed only small differences in plant height and number of nodes at R1 (data not shown) but large differences in plant height at maturity (Fig. 4) , seed number (Fig. 5) , and grain yield (Fig. 6) . The response to higher rates of added Fe-EDDHA appeared to occur over an extended period, manifest at maturity. At lower rates (<6.72 kg Fe-EDDHA ha–1), resistant cultivars exceeded susceptible cultivars for these three agronomic characters, whereas at higher rates, susceptible cultivars often had similar values. The significant cultivar x rate interaction contrasts (Table 4) generally indicated substantially larger responses to increasing rates with susceptible cultivars. Nonetheless, the magnitude and direction of differences between resistant and susceptible cultivars varied with harshness of the environment, i.e., responses were environment specific.



View larger version (21K):
[in this window]
[in a new window]
  		Fig. 4. Increase in plant height in response to increasing rates of Fe-EDDHA for iron deficiency chlorosis resistant and susceptible soybean cultivars grown in three environments.

 


View larger version (22K):
[in this window]
[in a new window]
  		Fig. 5. Increase in seed number in response to increasing rates of Fe-EDDHA for iron deficiency chlorosis resistant and susceptible soybean cultivars grown in three environments.

 


View larger version (21K):
[in this window]
[in a new window]
  		Fig. 6. Increase in grain yield in response to increasing rates of Fe-EDDHA for iron deficiency chlorosis resistant and susceptible soybean cultivars grown in three environments.

 

View this table:
[in this window]
[in a new window]
  		Table 4. Summary of analyses of variance for plant height, seed number, grain yield, and seed iron concentration (seed [Fe]) of iron deficiency chlorosis resistant and susceptible soybean cultivars grown at six rates of Fe-EDDHA in three environments.

 
Among environments, Env. 2 was especially harsh with limited rain during seed filling (Table 2) and high soluble salts, nitrates, P, and K (Table 1), resulting in stunted plants with few seeds and low grain yield (Table 4). In this environment, most (>85%) of the response to Fe-EDDHA rate was linear and resistant, and susceptible cultivars exhibited nearly parallel responses (Env. 2; Fig. 4–6). The linearity of response indicates that even at very high rates of Fe-EDDHA, Fe deficiency limited plant growth and grain yield. In Env. 1, despite harsh soil conditions, high yield potential apparently was provided by adequate rain throughout the growing season and especially during seed filling. In this environment, resistant cultivars had small, linear responses to Fe-EDDHA rate, whereas susceptible cultivars exhibited large, quadratic responses, having similar plant height, seed number, and grain yield values at higher rates (Env. 1; Fig. 4–6). Apparently, high (>4.48 kg ha–1) rates of Fe-EDDHA provided adequate Fe for a prolonged period of time, allowing susceptible cultivars to yield as well as resistant cultivars when growing conditions were favorable. Grain yields in Env. 3 were intermediate. In this environment, susceptible cultivars exhibited large, linear responses to Fe-EDDHA rate while resistant cultivars had linear but smaller responses. As in Env. 1, susceptible cultivars had similar plant height, seed number, and grain yield values at higher rates; yet, as in Env. 2, the linearity of response for both resistant and susceptible cultivars indicated that Fe deficiency may have limited agronomic performance (Env. 3; Fig. 4–6).

Economic Feasibility
The economic feasibility of applying high rates of Fe-EDDHA was environment specific as well as cultivar specific. To determine economic feasibility, environments were characterized as high yielding, mild Fe deficiency (Env. 1); intermediate yielding, moderate to severe Fe deficiency (Env. 3); and low yielding, severe Fe deficiency (Env. 2) while cultivars were considered resistant or susceptible. Separate feasibilities (Table 5) were calculated for resistant and susceptible cultivars in each environment based on the significance of cultivar x rate interaction contrasts from analysis of variance for grain yield (Table 4) and regressions of grain yield on Fe-EDDHA rate for resistant and susceptible cultivars (Fig. 6). For linear responses (all but susceptible cultivars in Env. 1, Fig. 6), the current price of Fe-EDDHA ($24.26 kg–1; 6% Fe, Libfer SP, Ciba Specialty Chemicals, Suffolk, VA; W.C. Broadway, personal communication, 2004) was divided by the appropriate regression coefficient (slope) of the linear equation. The dividend provides the soybean price ($ kg–1) required to match the cost ($ kg–1) of each increment of added Fe-EDDHA, assuming the cultivars respond as suggested by the regressions. For example, using the slope (207.8) of the regression for susceptible cultivars in Env. 3, and $24.26 kg–1 as the cost of the chelate, soybean prices would have to be about $0.12 kg–1 ($3.26 bu–1) to equal the cost of each increment (kg) of added Fe (Table 5). The linearity of response indicates that any additional increment (within the range from 0 to 11.2 kg ha–1) of Fe-EDDHA will at least pay for the cost of the chelate once the soybean price is > $0.12 kg–1. For resistant cultivars in Env. 3, soybean would have to be worth substantially more, or about $0.33 kg–1 ($9.00 bu–1) to equal the cost of each increment. Overall, generalizations are difficult. With the exception of low-yielding, severely Fe-deficient conditions (Env. 2), resistant cultivars required higher soybean prices than susceptible cultivars to justify increasing rates of added Fe-EDDHA (Table 5). This price, however, declined as environments became less favorable. On the other hand, susceptible cultivars required lower soybean prices as yield levels increased and Fe deficiency lessened or as environments became more favorable. Cultivars susceptible to IDC are not usually recommended, but for producers who select resistant cultivars to plant where harsh conditions are likely to prevail, adding Fe-EDDHA may be economically feasible. Although Karkosh et al. (1988) also concluded that resistant genotypes were more likely to benefit from added Fe-EDDHA, it is important to remember that neither our results nor those of Karkosh and coworkers will apply to all circumstances. Cheaper chelates are needed.


View this table:
[in this window]
[in a new window]
  		Table 5. Economic feasibility of adding increasing rates of Fe-EDDHA to iron deficiency chlorosis resistant and susceptible soybean cultivars grown in three environments.

 
Seed Iron Concentration
Planting seed [Fe] may be especially important in preventing or delaying the onset of IDC during seedling establishment and early plant growth (Tiffin and Chaney, 1973). Assuming that plants require a continuous supply of Fe, that little accumulated Fe is mobilized from older to younger tissue, and that high rates of soil-applied Fe-EDDHA improve the availability of Fe later in plant development, it is reasonable to think that higher rates of Fe-EDDHA would increase seed [Fe] of harvested seed. This did occur in our study but to a very limited extent (Table 4; Fig. 7) . Small (<1 µg Fe g–1), statistically significant, linear responses to increasing Fe-EDDHA rates were observed in Env. 1 and 2, yet in Env. 3, seed [Fe] did not change as Fe rates increased. Differences between resistant and susceptible cultivars were large and consistent within and across Env., and neither the cultivar x rate interaction nor interaction contrasts were significant (Table 4; Fig. 7). Compared with late-season relative chlorophyll concentration and grain yield, seed [Fe] changed very little in response to added Fe. For example, with susceptible cultivars in Env. 3, late-season SPAD measures nearly doubled (Fig. 2), and grain yields more than doubled (Fig. 6) with the highest rate of Fe-EDDHA, yet seed [Fe] only increased about 11% (Fig. 7). Soybean seed [Fe] has been reported to change very little, if at all, after about R5 to R6 (Sale and Campbell, 1980; Dornbos and McDonald, 1986; Sojka et al., 1986). Late-season SPAD measures and grain yields indicate that Fe availability should not have limited seed Fe accumulation in our study; rather, it is likely that seed [Fe] was limited genetically. Even when susceptible cultivars had equivalent yields at higher rates of added Fe (Env. 1 and Env. 3 in Fig. 6), seed [Fe] of resistant cultivars far exceeded that of susceptible cultivars (Fig. 7). Soybean may be similar to corn in that genetic factors which determine agronomic response are different than those which determine seed deposition (Gorsline et al., 1964), i.e., response curves may differ between vegetative and reproductive organs.



View larger version (20K):
[in this window]
[in a new window]
  		Fig. 7. Increase in seed Fe concentration (seed [Fe]) in response to increasing rates of Fe-EDDHA for iron deficiency chlorosis resistant and susceptible soybean cultivars grown in three environments.

 
Critical levels of mineral nutrients in vegetative or seed tissue often are calculated to determine nutrient concentrations associated with maximum yields or improved seed quality (Jones et al., 1980; Hitsuda et al., 2004). In this study, relative grain yield was not highly correlated with seed [Fe]; values were 0.54 (not significant) (Env. 1), 0.60 (significant at the 0.05 probability level) (Env. 2), and 0.58 (significant at the 0.05 probability level) (Env. 3). We did not attempt to calculate critical levels. Nonetheless, we plotted relative grain yield versus seed [Fe] for each environment (Fig. 8) to illustrate the narrow range of seed [Fe] associated with wide ranges in relative yield and the large differences between resistant and susceptible cultivars regardless of relative yield. Collectively, results from this study provide strong evidence that seed [Fe] is tightly controlled genetically. Similar results were reported recently by Moraghan et al. (2002). Working with dry bean (Phaseolus vulgaris L.), they noted that genetic differences in seed [Fe] were expressed (maintained) across a wide range of soil types and environments and in the presence or absence of added Fe chelates. Beebe et al. (2000), also working with dry bean, concluded that while the degree of expression of seed [Fe] may vary across environments, genotypes having superior seed [Fe] at one site/year should maintain that superiority at other sites and years. In wheat (Shen et al., 2002), genetic differences in resistance to Fe deficiency have been associated with differences in seed [Fe]. If soybean seed [Fe] is tightly controlled genetically, then there are at least two possible implications: (i) producers should use planting seed that has high (>80 µg g–1) seed [Fe] for chlorosis-prone soils since it is likely that higher seed [Fe] is associated with resistance to Fe deficiency; and (ii) plant breeders may not require specifically designed chlorosis nurseries to select for resistance to Fe deficiency since susceptible genotypes will not likely accumulate high seed [Fe] even when excess Fe is available. These suggestions, however, may not apply to work done using noncalcareous, low-pH (<5) buffering capacity soils where the expression of genetic differences in Fe stress response mechanisms are not required to obtain adequate supplies of Fe throughout plant development. Although Longnecker and Welch (1990) documented the capacity of resistant soybean to accumulate more Fe than susceptible soybean in root and shoot tissue, even in the presence of adequate Fe, additional research involving a wide range of seed [Fe] and genotypes will be required to confirm the importance of seed [Fe] for improved agronomic performance and efficient genotype selection.



View larger version (14K):
[in this window]
[in a new window]
  		Fig. 8. Relative grain yield versus seed Fe concentration (seed [Fe]) at harvest. For resistant (susceptible) cultivars, grain yield was relative to the highest mean grain yield of resistant (susceptible) cultivars within each environment.

 

   ACKNOWLEDGMENTS
 
The author acknowledges the excellent technical assistance of Robert J. Bouvette, Jr., Mark A. Hanson, and Eugene L. Peters, without whom this research would not have been possible.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Research supported in part by the Minnesota Soybean Research and Promotion Council and the Northwest Research and Outreach Center.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




This article has been cited by other articles:


Home page
 Soil Sci.Home page
A. de Santiago, I. Diaz, M. d. C. del Campillo, J. Torrent, and A. Delgado
Predicting the Incidence of Iron Deficiency Chlorosis from Hydroxylamine-Extractable Iron in Soil
Soil Sci. Soc. Am. J., August 20, 2008; 72(5): 1493 - 1499.
[Abstract] [Full Text] [PDF]

Home page
 Agron. J.Home page
J. V. Wiersma
Iron Acquisition of Three Soybean Varieties Grown at Five Seeding Densities and Five Rates of Fe-EDDHA
Agron. J., June 5, 2007; 99(4): 1018 - 1028.
[Abstract] [Full Text] [PDF]

Home page
 Agron. J.Home page
S. L. Naeve
Iron Deficiency Chlorosis in Soybean: Soybean Seeding Rate and Companion Crop Effects
Agron. J., October 3, 2006; 98(6): 1575 - 1581.
[Abstract] [Full Text] [PDF]

Home page
 Agron. J.Home page
S. L. Naeve and G. W. Rehm
Genotype x Environment Interactions within Iron Deficiency Chlorosis-Tolerant Soybean Genotypes
Agron. J., May 3, 2006; 98(3): 808 - 814.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Wiersma, J. V.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Wiersma, J. V.
Agricola
Right arrow Articles by Wiersma, J. V.
Related Collections
Right arrow Soybean
Right arrow Seed Treatment
Right arrow Nutrient Management
Right arrow Plant Nutrition

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome