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Evaluating wheat cultivars for waterlogging tolerance.

Environmental factors in the Gulf Coast region consistently depress wheat yields below the national average, yet wheat acreage has increased in Louisiana seven-fold since 1970, making it the state's sixth major crop. Soil waterlogging is common in this area of high rainfall, level topography, and poorly drained silty-clay soils. Soil oxygen deficits can restrict plant performance directly through root metabolism or indirectly by changed plant nutrient availability (Trought and Drew, 1980). While estimates of crop loss due to waterlogging range from 15 to 20% for mid-winter waterlogging in Britain (Cannell et al., 1980; Belford, 1981), these estimates are highly temperature dependent (Luxmoore et al., 1973; Sojka et al., 1975; Labanauskas et al., 1975). Previously, we estimated that waterlogging caused a yield loss of 40 to 50% compared with wheat grown on well-drained soil (Musgrave, 1994; Ding and Musgrave, 1995). Efforts to assess the extent of yield loss in wheat due to waterlogged soils on a larger scale have used remote sensing of stress-induced changes in foliar reflectance (McFarlane and Wheaton, 1990; Wallace et al., 1993). Boyer (1982) estimated that 12% of the agricultural soils in the USA are affected by waterlogging.

Genotypic differences exist for tolerance to waterlogging in cereal crops (Davies and Hillman, 1988; Bourget et al., 1966; Thomson et al., 1992; Ding and Musgrave, 1995; Huang et al., 1994a,b; McKersie and Hunt, 1987; Gardner and Flood, 1993). Several studies have located the chromosomes where the genes responsible for waterlogging-tolerance traits are in wheat and its close relatives (Poysa, 1984; Taeb et al., 1993). Nevertheless, the physiological trait(s) responsible for waterlogging tolerance remains elusive.

Physiological attributes of both the shoot and root have been examined by ourselves and others for potential correlation with differential waterlogging tolerance. Box (1986) suggested that limitations on wheat photosynthesis through stomatal closure caused by waterlogging could restrict the amount of assimilate available for grain fill, and the potential importance of photosynthetic limitation by waterlogging was emphasized in a review by Sojka (1992). Huang et al. (1994a,b) reported different recovery rates for stomatal conductance among six wheat genotypes with differential waterlogging tolerance. Root aerenchyma formation is another trait that has been implicated in waterlogging tolerance in wheat (Varade et al., 1970; Huang et al., 1994a,b; Ding and Musgrave, 1995; Thomson et al., 1990; Box, 1986; Erdmann and Wiedenroth, 1988). In the present study, we sought to explore the relationship between differential physiological performance and reductions in grain yield when wheat cultivars were subjected to waterlogging stress.

In a previous survey of eight cultivars grown in Louisiana, waterlogging tolerance, as assessed by yield from full-term greenhouse experiments, was uniformly poor (Musgrave, 1994). The studies described herein explored ways of evaluating waterlogging tolerance in the field using more genotypes. Two types of studies were conducted in the field: an irrigated yield trial and a rain-excluding shelter test done to determine extent of waterlogging depression of yield. We also expanded the scope of our greenhouse tests to determine other traits predictive of waterlogging tolerance of wheat in Louisiana. All three studies provide information useful for breeding improved waterlogging tolerance in wheat.

MATERIALS AND METHODS

Pot Study in the Greenhouse

Seeds of eight winter wheat cultivars and breeding lines (listed in Fig. 1) obtained from Dr. Steve Harrison, Agronomy Department, Louisiana State University Agricultural Center, were planted in 9-L plastic pots containing 10.9 kg of sieved, sterilized river silt amended with 1.65 g 8-8-8 (N-P-K) fertilizer. After emergence, the seedlings were thinned to three in each pot. Pots were held on racks in 120- by 60- by 20-cm rubberlined tanks equipped with float-controlled valves to maintain the desired water level. A randomized complete-block design, with treatments in a split-plot arrangement and four blocks, was used. Waterlogging levels were the main plots and cultivars were allocated as sub-plots. In the non-waterlogged (control) treatment, water level was maintained at 2 cm above the bottom of the pots. The waterlogged treatment was initiated after plant establishment (2 wk after seeding) by raising the level of the deionized water in the tanks to the soil surface of the pots. At 8 wk after planting, 1.4 g [NH.sub.4][NO.sub.3] and 100 mL deionized water were applied to each pot. Four weeks after this top-dressing, chlorosis of the leaves in the control treatment indicated the need for additional fertilizer, so 100 mL of liquid fertilizer (Miracle-gro, 15-30-15,5.5 g [L.sup.-1]; Sterns', Port Washington, NY) was added to every pot at 2-wk intervals until dry-down for harvest. The tanks were on benches in a ground-to-ground Quonset-type greenhouse with open ends, that supplied the wheat with similar temperature and day length regimes to those in the field during the winter months.

[Figure 1 ILLUSTRATION OMITTED]

Soil redox: potential was quantified weekly throughout the waterlogging periods. Three pots in each tank were randomly assigned for use in these measurements at the beginning of the experiment. Details on construction and use of the redox electrodes were presented previously (Musgrave, 1994).

Methods used for the measurement of apparent net photosynthesis and harvesting were fully described by Musgrave (1994). Briefly, apparent net photosynthesis was measured on fully expanded flag leaves of each plant using an Analytical Development Model LCA2 infra-red gas analyzer (Analytical Development Col, Ltd., Hoddesdon, UK) equipped with a Parkinson leaf chamber. Twelve measurements for each cultivar in each treatment were performed in the unheated greenhouse under saturating light conditions ([is greater than] 1500 [[micro]mol [m.sup.-2 [s.sup.-1]) provided by an industrial high-intensity-discharge metal halide lamp (1000 W). A compressed gas (350 [micro]L [L.sup.-1] [CO.sub.2], balance air; Scott's Specialty Gases, Inc., Plumsteadville, PA) was used as the air supply. At harvest, heads in each pot were hand-harvested and threshed. Seeds were weighed and counted.

Information on methods used for root color scoring and mineral content determination is given by Ding and Musgrave (1995). Washed root samples were evaluated for color differences using a scale of 1.0 to 4.0, with 1.0 = light (color 161D) and 4.0 = dark (color 200C) (Royal Horticultural Society Colour chart numbers). Values obtained from four pots of each cultivar and treatment were averaged to obtain a visual score rating for comparison with mineral analyses. Mineral content, defined here as the sum of Fe + Mn + P, was determined from root samples which had been thoroughly washed, dried in a mechanical convection oven at 65 [degrees] C for 3 d, and then ground with a Wiley mill (20 mesh sieve). From each sample, 0.15 to 0.50 g was weighed, put into a 150-mL volumetric flask and digested in 20 mL of 7:1 nitric/perchloric acid. The samples were brought to volume with deionized water and analyzed on an Inductively Coupled Plasma-emission Spectrometer (PS 3000, Leeman Labs Inc., Lowell, MA).

Field Study

Twenty wheat genotypes (Table 1) were evaluated from 1990-1995 for waterlogging tolerance at the Louisiana State University Agricultural Center Ben Hur Research Farm at Baton Rouge, LA, (30 [degrees] N latitude, 91 [degrees] W longitude). Soil type was a Mhoon silty clay (fine-silty, mixed nonacid, thermic Fluventic Haplaquepts). The field was fertilized pre-plant with 224 kg-[ha.sup.-1] 8-8-8 (N-P-K), and later top-dressed with 101 kg-[ha.sup.-1] N as urea applied in mid-February. Seeds were drilled at a rate of 67 kg [ha.sup.-1]. Chlorsulfuron [[2-chloro-N-{[(4-methoxy-6-methyl-1,3, 5-triazin-2-yl)amino]carbonyl] benzenesulfonamide]] herbicide was applied pre-emergence at 53 g ad. [ha.sup.-1]. At the first appearance of rust in the spring, Tilt fungicide [[1-{[2-(2,4-dichlorophenyl)-4-propyl-1, 3-dioxolan-2-yl]methyl}-1H-1,2,4-triazole]] was applied at 120 ml a.i. [ha.sup.-1]. A randomized block design with four blocks of 20 cultivars was laid out in four rows of five cultivars within each block. Each plot consisted of six 1-m long rows spaced 25 cm apart. Drip irrigation hose (Turbulent Twin-Wall, 15 mil, 23-cm outlets, flow 0.1 Ls.sup.-1][(100 m).sup.-1]; Chapin Watermatics Inc., Watertown, NY) ran along the outside of each row of plots and within the plots on alternate rows. Drip tape was connected to 1.3-cm-diam. source pipe and irrigation was supplied via timer-controlled solenoid valves. Irrigation began after stand establishment and continued for approximately 3 mo. In 1992-1993, the study was lost to late winter flooding of a nearby bayou that caused the plants to be completely under water for 4 to 5 d, so only the data from 1991-1992, 1993-1994, and 1994-1995 are included here. At maturity a 0.5-[m.sup.2] area was harvested from the center of each plot.

Table 1. Yield characteristics for 20 wheat cultivars in a 3-yr waterlogged field study in Baton Rouge, LA.
                       Grain      Kernel    Kernels
                       weight     weight    [m.sup.-1]  of row

                      g [m.sup.-1] row    mg   -- no. --
Genotype
Pioneer 2548             77.7      24.5      3213
LA 8564A80-3-1-X         79.3      23.6      3094
Terral 877               79.6      25.7      2971
Florida 303              80.5      27.1      2772
McNair 1003              83.5      27.1      2940
LA 862AI6-3-1-X          89.9      23.3      3657
Bayles                   91.4      23.9      3629
Florida 304              91.6      26.7      3266
Coker 9105               97.1      26.7      3689
Coker 9766               97.2      24.6      3815
Coker 9543               97.8      30.3      3718
Terra] 101               98.7      24.7      3827
FFR 525W                103.3      27.7      3393
Buckshot's DS 2368      106.0      26.0      3958
LA 861AI8-4-1-X         109.4      30.0      3513
LA 8576A53-2-1-X        113.0      24.7      4425
Savannah                113.3      29.6      3802
Coker 9835              114.9      25.4      4437
Coker 9877              131.2      26.7      4765
LA 862A16-3-3-X         132.4      27.1      4894
Mean                     99.4      26.3      3689
Std. Error               13.1      1.6        434
Significance level    ([dagger])              (*)

                        Heads                Kernels
                        [m.sup.-1]  of row   [head.sup.-1]

Pioneer 2548            145.4                  20.9
LA 8564A80-3-1-X        152.2                  18.9
Terral 877              138.1                  19.9
Florida 303             123.7                  21.6
McNair 1003             132.2                  21.4
LA 862AI6-3-1-X         188.8                  19.4
Bayles                  140.8                  25.2
Florida 304             145.9                  21.7
Coker 9105              157.4                  21.6
Coker 9766              172.6                  21.0
Coker 9543              164.1                  22.5
Terra] 101              166.5                  22.7
FFR 525W                150.3                  21.3
Buckshot's DS 2368      165.3                  22.9
LA 861AI8-4-1-X         150.8                  22.0
LA 8576A53-2-1-X        182.6                  22.9
Savannah                134.0                  27.2
Coker 9835              169.8                  24.0
Coker 9877              164.6                  26.4
LA 862A16-3-3-X         197.5                  25.3
Mean                    157.1                  22.5
Std. Error               12.9                   1.6
Significance level       (**)                  (**)


([dagger]) Significant (P [is less than or equal to] 0.10, 0.05, or 0.01, respectively) effect of cultivar was detected by analysis of variance.

Rainshelter Study in the Field

To compare cultivar performance under waterlogged and well-drained field-like conditions, two rain-excluding shelters were constructed at the Ben Hur farm on the site described above. The shelters followed the general design described by Bittman et al. (1987), with a plastic barrier buried in the soil to a depth of 1 m to prevent lateral water flow between two planting areas (approximately 19 [m.sup.2]). Seeds were hand-planted, and rates and timing of seeding, herbicide and fungicide treatments were the same as for the field study. In each rain shelter, a waterlogged and a control treatment were randomly assigned to each side and were maintained by irrigating two different zones for different lengths of time with the drip system described previously. During the 3-yr study (1991-1994), the cultivars evaluated were `Florida 303', `Terral 877', `Coker 9105', `McNair 1003', `Coker 9877', and experimental line LA8564 A80-3-1-X. Harvesting and sample processing were as for the field study.

Soil Oxygen Measurement

Soil oxygen content was monitored in the field and rainshelter studies using oxygen samplers constructed of porous sintered bronze cups attached to sampling taps as described by Dowdell et al. (1972). After planting the rhizosphere gas samplers were buried in the soil between rows at a depth of 10 cm, with four samplers for each treatment. Samples (20 mL) were withdrawn through the sampling taps with a 50-mL syringe. Oxygen concentration of the withdrawn sample was determined by using an oxygen probe (DO-166, Lazar Research Laboratories, Los Angeles, CA).

Data Analysis

Data from these experiments were analyzed by analysis of variance (Hintze, 1987). Yearly data were initially analyzed by considering block effects random and treatment and cultivar effects fixed. After homogeneity of error variances was evaluated, yield data from multiple years were combined and analyzed considering block and year effects random and treatment and cultivar effects fixed.

RESULTS

Rain Shelter and Field Experiments

Soil oxygen concentrations produced by the irrigation treatments are shown for the rainshelter (Fig. 2A) and field experiments (Fig. 2B). After cessation of irrigation, oxygen content returned to non-waterlogged levels. The rainshelter treatments had soil oxygen concentrations similar to those measured in the irrigated field site.

[Figure 2 ILLUSTRATION OMITTED]

Analysis of yield components from 3 yr of the rain shelter studies showed a 45% decrease in grain weight, largely due to decreased kernel number, but also accompanied by decreased kernel weight (Table 2). Cultivars had a wide range in grain yield reduction (as percent of control): `Coker 9877', 9%; `McNair 1003', 28%; `Coker 9105', 50%; `Florida 303', 54%; `Terral 877', 58%; LA8564 A80-3-1-X, 69% (Fig. 3). The cultivar effects on yield components in the 3-yr field study comparing 20 cultivars and breeding lines under irrigated field conditions are in Table 1.

[Figure 3 ILLUSTRATION OMITTED]

Table 2. Yield characteristics for wheat grown in waterlogged and control treatments in a 3-yr rainshelter study in Baton Rouge, LA.
                  Grain     Kernel    Kernels
                  weight    weight    [m.sup.-1] of row

                g [m.sup.-1] row    mg  -- no. --

Treatment        (***)      (*)         (**)

Control          104.9      32.5        3344
Waterlogged       57.2      30.7        1841

Cultivar        ([dagger])   (***)       (***)

FL 303            66.6      35.1        1899
Terra] 877        82.2      10.4        2786
LA 8564
  A80-3-1-X       77.0      29.5        2504
Coker 9105        76.6      25.7        3054
McNair 1003       93.6      35.5        2594
Coker 9877        90.2      33.2        2721

Treatment x
  Cultivar       (*)        (*)         (**)

                Heads               Kernels
                [m.sup.-1] of row   [head.sup.-1]

Treatment        (**)                (**)

Control          93.0                36.1
Waterlogged      62.8                28.9

Cultivar          (*)               (***)

FL 303           70.3                25.4
Terra] 877       88.7                29.3
LA 8564
  A80-3-1-X      85.9                28.4
Coker 9105       83.8                35.2
McNair 1003      68.1                37.9
Coker 9877       70.5                38.6

Treatment x
  Cultivar       (*)                 ([dagger])


([dagger]),(*),(**),(***) Significant (P [is less than or equal to] 0.10, 0.05, 0.01, or 0.001, respectively) effect of treatment, cultivar, or treatment x cultivar interaction was detected by analysis of variance.

Greenhouse Experiments

Redox potential differed between the waterlogged and control treatments for 1992-3 (92.6 mV vs. 363.0 mV; P = 0.025) and 1993-4 (-45.9 mV vs. 345.8 mV; P = 0.011), indicating the intensity of waterlogging. In the 2-yr greenhouse study using eight cultivars and breeding lines, treatment effects were highly significant for all yield components except number of heads per pot (Table 3). Grain weight decrease due to waterlogging averaged 43% for 1992-1993 and 28% for 1993-1994. Performance by individual cultivars in the 2 yr is shown in Fig. 1.

Table 3. Effect of waterlogging treatment and cultivar on performance on wheat cultivars in the greenhouse studies.
                 Grain    Kernel    Kernels
                 weight   weight    [pot.sup.-1]

                   g        mg       -- no. --

1992-3
Treatment         (*)      (*)        (**)
  Waterlogged    36.5      26.7       1358
  Control        64.1      32.9       1988
Cultivar          0.31     (***)      (***)

Treatment x
  Cultivar        0.37     (**)          0.23

1993-4
Treatment         0.40     (*)         (*)
  Waterlogged    24.7      26.4        934
  Control        34.3      29.4       1173
Cultivar          0.21    (***)        (*)
Treatment x
  Cultivar        0.31    (*)            0.16

                 Heads          Kernels
                 [pot.sup.-1]   [head.sup.-1]

1992-3                   -- no. --
Treatment         0.13             (*)
  Waterlogged    33.0              41.6
  Control        37.3              54.2
Cultivar        (***)              (***)

Treatment x
  Cultivar        0.17              0.88

1993-4
Treatment         0.40             (**)
  Waterlogged    30.0              31.0
  Control        28.9              40.7
Cultivar         (*)               (***)
Treatment x
  Cultivar        0.41              0.09


(*),(**),(***) Significant (P [is less than or equal to] 0.05, 0.01, or 0.001, respectively) effect of cultivar was detected by analysis of variance.

Figure 4 summarizes relationships between grain weight and physiological measures of performance by the shoots (A, B) and roots (C, D) for each cultivar. Assimilation (A) and conductance (B) were very weakly predictive of yield ([r.sup.2] = 0.61 and 0.52 respectively). Mineral content (C) and a score based on a visual scale of root color (D) had a strong negative relationship with yield ([r.sup.2] = 0.94 for both). Biomass and harvest index (Fig. 5A and B) were positively related to yield ([r.sup.2] = 0.84 and 0.81 respectively).

[Figures 4-5 ILLUSTRATION OMITTED]

Yields of waterlogged cultivars were not proportionally related to control yields ([r.sup.2] = 0.14) (Fig. 6A), reflecting differential response by cultivars to stress (Fig. 1). Furthermore, the range in yield under waterlogged conditions was about twice the range under control conditions. This same pattern occurred for biomass ([r.sup.2] = 0.26) (Fig. 6B) and root mineral content ([r.sup.2] = 0.23) (Fig. 6C). Interestingly, when plotted against the same parameter under well-drained conditions, assimilation rate, harvest index, and kernel weight showed linear relationships ([r.sup.2] = 0.87; 0.71; 0.89 respectively) (Fig. 6, D, E, F). Furthermore, the range of values for these parameters was approximately the same under waterlogged and control conditions. This suggests that for these parameters the cultivars are not responding differentially to waterlogging stress, but rather are all simply restricted to a similar extent by waterlogging.

[Figure 6 ILLUSTRATION OMITTED]

DISCUSSION

One goal of these ongoing experiments has been to determine how waterlogging tolerance can be studied in an agriculturally meaningful setting. To study waterlogging stress tolerance, it is necessary first to impose the stress in a controlled manner and to measure it in a way that allows comparison across different experiments. In these field studies, use of irrigation lines between alternate rows of the crop maintained a constant oxygen deficit both in the field study intended to compare performance among cultivars and in rain-excluding shelters used to compare yield under waterlogged and drained conditions. In the greenhouse, float-controlled waterlogging tanks were used to impose the stress on pot-grown plants and provided a convenient environment for physiological studies. Yield depression by waterlogging in these different regimes was about 40%, in agreement with our previously published studies which used different methods and cultivars (Musgrave, 1994; Ding and Musgrave, 1995). While the net result in yield was very similar, yield components responded somewhat differently in the greenhouse study than in the field-like conditions in the rainshelter study. In the greenhouse study, the number of heads per pot was not significantly reduced by the waterlogging treatment whereas in the rainshelter study, heads per meter of row was reduced by 32%. Except for this caveat, the field and greenhouse methods used gave very similar results.

Waterlogging is a difficult stress to study in a field setting because of the need to carefully maintain a waterlogged soil environment and at the same time provide a non-waterlogged control for comparison. Where waterlogging is a significant agronomic problem, as in Louisiana, a non-waterlogged control is impossible to achieve without manipulations such as the rain excluding shelter used in this study, or subsurface water-level control systems (Carter et al., 1990). Greenhouse experiments allow for a split-plot design to examine performance under both drained and waterlogged regimes, but suffer from other problems that may contribute to variability in yield data. Experiments in the greenhouse and under rain-excluding shelters have size limitations which restrict the number of cultivars that can be evaluated. An alternative approach is to provide maximum waterlogging stress in a field setting to identify which entries from the state's standard variety trials have exceptional performance under these conditions. These data, coupled with information from split-plot trials in the greenhouse and rain-excluding shelter, will identify a cultivar expressing tolerance.

Our results attest to the importance of developing waterlogging tolerant cultivars for areas prone to excess soil moisture. Despite the large yield reduction that generally occurs due to waterlogging, some cultivars already seem more tolerant (such as Coker 9877) and the highest yielding line tested in the waterlogged field experiment was an advanced breeding line LA 862A163-3-X. These results corresponded qualitatively to the performance by six different cultivars under waterlogged and drained conditions in the rainshelter, where Coker 9877 showed only a 9% decrease in yield under waterlogged conditions relative to the control. Both the rainshelter and the field studies identified line LA8564 A80-3-1-X (an [F.sub.5]-derived line from the cross McNair 1003/`Coker 762') as a poor performer under waterlogged conditions. The variation in these advanced breeding lines relative to waterlogging points out the value of conducting specific screening through imposition of controlled stress conditions.

Because total plant biomass (Fig. 5A) and harvest index (Fig. 5B) were correlated with yield ([r.sup.2] = 0.84 and 0.81, respectively) we investigated the relationship between assimilation rates and grain yield to understand if photosynthetic performance might be related to differences in yield under waterlogging stress. A previous report relating photosynthetic parameters and waterlogging tolerance (Huang et al., 1994a) was not borne out in this study. Apparent net assimilation (Fig. 4A) and conductance (Fig. 4B) were only weakly related to yield ([r.sup.2] = 0.61 and 0.52 respectively). Assimilation rate under waterlogging stress was correlated with assimilation under well-drained conditions (Fig. 6D) ([r.sup.2] 0.87), showing that cultivars responded similarly (y 1.57x - 7.18) to this stress.

This linear relationship between performance under control and waterlogged conditions was also observed for harvest index (Fig. 6E; [r.sup.2] 0.71) (y = 0.76x + 0.04) and kernel weight (Fig. 6F; [r.sup.2] 0.89)(y = 0.74x + 5.03). Thus, assimilation and subsequent biomass partitioning are depressed similarly across cultivars by waterlogging. This situation is in contrast to grain yield, which responds differentially to waterlogging depending on cultivar.

In this greenhouse study, waterlogging stress- led to broad differences in yield performance (a 20 g [pot.sup.-1] range) compared with the well-drained controls (10 g [pot.sup.-1] range) (Fig. 1A and 6A). Yield by cultivars under waterlogged conditions was not related to their performance under well-drained conditions ([r.sup.2] = 0.14). This same differential performance by cultivars under control and waterlogged conditions was observed for total biomass (Fig. 6B). Assimilation and partitioning were shown earlier to not account for these cultivar-specific responses in yield and biomass production.

This makes the finding that attributes of the root system (mineral content, Fig. 4C; [r.sup.2] = 0.94 and visual score Fig. 4D, [r.sup.2 = 0.94) correlate well with yield very important because like yield and biomass, mineral content of waterlogged roots is not related to that measure for the same cultivars under control conditions (Fig. 6C; [r.sup.2] = 0.23). This strongly suggests that factors related to changes in root mineral content during waterlogging also influence total biomass and grain yield.

The mineral content measure used for this work is the sum of Fe + Mn + P in ground root samples as determined by ICP analysis. This measure was chosen because of our previous work which described the constituents of mineral plaque that forms on waterlogged wheat roots (Ding and Musgrave, 1995). Through a combination of ICP elemental analysis, iron-specific staining, and ion-mapping by scanning electron microscopy using an X-ray detector, we found that iron was the primary component of these coatings. Of 11 elements quantified by ICP spectroscopy, we found that six were significantly affected by a waterlogging treatment (Fe, Mn, P, K, Na, S), and that three of these, Fe, Mn, and P, were well correlated negatively with yield (Ding and Musgrave, 1995).

In plants that are adapted to grow in waterlogged or even flooded soils, such as rice (Oryza sativa L.), mineral plaque has been found to be positively correlated with yield (Chen et al., 1980). The amount of plaque that forms on a root is dependent on the amount of oxygen that can leak out of the root to oxidize and precipitate ions in the soil solution (McKevlin et al., 1987). The positive relationship between root mineral content and yield in rice is thought to occur because both would be augmented by superior supplies of oxygen to the root system (Chen et al., 1980). There is evidence that the better yielding and higher Fe-precipitating cultivars possess better developed aerenchyma gas transport passages in the roots. Higher yield is therefore attributed to greater root metabolism possible because of higher oxygen supply and/or better capability to detoxify minerals in solution in anaerobic reduced soils by precipitating them at the root surface.

Given this background, the strong negative correlation between mineral content and yield in wheat is very interesting, and suggests that factors encouraging buildup of precipitates around the roots during waterlogging are not beneficial. One possibility is that following lysigenous aerenchyma formation in wheat roots, some degree of root function is lost even though the capacity to precipitate ions is retained (just as minerals will precipitate around a pipe connected with a more highly oxygenated area). The mechanism behind the strong negative correlation between root mineral content and yield is still unknown. Understanding this process and how it influences root function will make it possible to improve wheat performance under waterlogged conditions above the level of our current tolerant cultivars. In the meantime, however, it is useful to note that root color and mineral content (Fe + Mn + P) are good indicators of waterlogging tolerance in wheat that allow differentiation at the cultivar level. Mineral content of roots determined by ICP is an easier assay to perform than root porosity or even whole plant biomass and may prove to be useful in breeding efforts to improve waterlogging tolerance in wheat. Full-term studies evaluating waterlogging tolerance in a field setting are important companions to greenhouse studies where individual traits such as root color and mineral content can be more readily studied.

ACKNOWLEDGMENTS

Funded in part by grants from the Louisiana Soybean and Grain Research and Promotion Board. The technical assistance of A.G. Hopkins, Jr., is appreciated. Dr. Steve Harrison, Agronomy Department, LSUAC, is thanked for providing seeds and assistance with planting the field studies, and Mr. David A. Wall of the Feed and Fertilizer Lab is thanked for the ICP elemental analyses.

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M. E. Musgrave(*) and N. Ding

Dep. of Plant Pathology and Crop Physiology, Louisiana Agric. Exp. Stn, Louisiana State Univ. Agric. Center, Baton Rouge, LA 70803. Approved for publication by the Director of the Louisiana Agric. Exp. Stn. as paper No. 96-38-0176. Received 5 Aug. 1996. (*)Corresponding author (xp3031a@lsuvm.sncc.lsu.edu).

Published in Crop Sci. 38:90-97 (1998).
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Author:Musgrave, M.E.; Ding, N.
Publication:Crop Science
Date:Jan 1, 1998
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