| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a Dep. of Crop and Soil Sciences, The Pennsylvania State University, University Park, PA 16802
b Dep. of Plant, Soil and Insect Sciences, 12F Stockbridge Hall, Univ. of Massachusetts, Amherst, MA 01003
c Dep. of Plant Science, Univ. of Connecticut, Storrs, CT 06269
* Corresponding author (sebdon{at}pssci.umass.edu)
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Abbreviations: ADF, acid detergent fiber • ANOVA, analysis of variance • INTOL, wear intolerant genotypes • NDF, neutral detergent fiber • NTEP, national turfgrass evaluation program • TCW, total cell wall content • TOL, wear tolerant genotypes
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
The relationship between specific plant mechanisms and wear tolerance has been explored at the interspecies level for cool-season grasses (Shearman and Beard, 1975a, 1975b, 1975c) and at the intraspecific level in warm-season grasses (Trenholm et al., 2000). Wear tolerance has been suggested to correspond with various anatomical and morphological plant characteristics. These characteristics include total cell wall content (TCW), quantity of schlerenchyma fibers, leaf width and leaf tensile strength, and shoot density (and verdure) (Shearman and Beard, 1975b, 1975c).
Studies by Shearman and Beard (1975c) and Trenholm et al. (2000) have focused on the constituents of cell walls as a principle means of explaining turfgrass wear tolerance. Cell wall components include cellulose, hemicellulose, and lignin (Taiz and Zeiger, 1972). Cellulose is a tightly packed group of polysaccharide chains that provides plant tissues with a high tensile strength. This suggests that plants with higher percentages of cellulose may be more tolerant to wear stress. Hemicelluloses are a heterogeneous group of polysaccharides that bind to cellulose to further strengthen cell walls. Lignin is a highly branched polymer of phenylpropanoid groups that possesses high mechanical rigidity and therefore strengthens stems and vascular tissues (Taiz and Zeiger, 1972). Because of its physical toughness, lignin deters feeding by animals (van Soest, 1994) and therefore may play a role in wear tolerance.
Anatomical characteristics unique to the cell walls of wear tolerant species have been analyzed by Shearman and Beard (1975b). At the interspecies level, no single cell wall constituent (cellulose, hemicellulose, lignin, and lignocellulose) when considered on an individual dry weight basis was found to be significantly correlated with wear tolerance. However, over 96% of the total variation in interspecies wear tolerance was accounted for by the combined effects of these cell wall constituents (Shearman and Beard, 1975b). Morphological characteristics among turfgrasses species subjected to wear stress have also been investigated by Shearman and Beard (1975c). Verdure (shoot biomass), load bearing capacity, leaf blade tensile strength, and relative turgidity were evaluated at the interspecies level. Variation among species were observed in the different plant characteristics; however, only the combined effect of increased leaf tensile strength and increased leaf width accounted for a significant amount of the variation in interspecies wear tolerance (97%).
An intraspecies analysis of wear tolerance within seashore paspalum (Paspalum vaginatum Swartz.) genotypes, as well as hybrid bermudagrass genotypes (Cynodon dactylon L. x C. transvaalensis Burtt-Davy), was conducted by Trenholm et al. (2000). Within both seashore paspalum and bermudagrass genotypes, plant water content and shoot density increased with wear tolerance. At the interspecies level, however, Shearman and Beard (1975c) reported no relationship between plant water content and wear tolerance, although shoot density was found to be positively correlated with wear tolerance.
Relationships between cell wall constituents and wear tolerance were identified within seashore paspalum and bermudagrass genotypes (Trenholm et al., 2000). Within seashore paspalum genotypes, wear tolerance decreased as leaf TCW increased. Within bermudagrass genotypes, reduced cellulose and increased lignin contents were associated with improved wear tolerance. Increased leaf TCW expressed on a weight per unit area basis was associated with superior wear tolerance at the interspecies level (Shearman and Beard, 1975b). According to Beard (1973), greater leaf TCW will lead to a decrease in leaf elasticity. Within seashore paspalum genotypes, the concept of increased leaf blade elasticity was associated with superior wear tolerance (Trenholm et al., 2000). As was true at the interspecies level (Shearman and Beard, 1975b), within bermudagrass genotypes decreased elasticity (higher rigidity) was significantly correlated with superior wear tolerance (Trenholm et al., 2000).
Plant characteristics associated with superior wear tolerance vary greatly, both at the interspecies and intraspecies level. Relationships developed between plant characteristics and wear tolerance observed at the interspecies level (Shearman and Beard, 1975b, 1975c) may not be relevant within a particular species (Trenholm et al., 2000). Furthermore, wear mechanisms important within species may not have universal application to other species and genotypes. While wear evaluations among cool-season grasses (and genotypes) have been conducted (Bonos et al., 2001; Minner et al., 1993), these studies did not emphasize specific plant mechanisms of wear tolerance.
In developing grasses for turf use, breeders routinely evaluate plant characteristics from spaced-planted nurseries and mowed turf (Bourgoin and Mansat, 1977). Plant measurements from mowed turf can be cumbersome, obstructed by diminutive tillers and high stand densities (Brede and Duich, 1982). Measurements from unmowed, spaced-plants are not always the most reliable in predicting turfgrass performance of mowed turf stands (Kramer, 1947; Bourgoin and Mansat, 1977). However, occasionally a relationship between turfgrass performance and single plant morphology can be found (van Wijk, 1989; Ebdon and Petrovic, 1998). Accordingly, the objective of this research was to identify anatomical and morphological characteristics in Kentucky bluegrass from mowed turf and unmowed, spaced-plants representing diverse wear tolerance, which then may serve as important selection criteria for breeding wear tolerance within this species.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
The 10 most wear tolerant (TOL) genotypes and the 10 most wear intolerant (INTOL) genotypes were selected after the application of wear treatments to the NTEP plots in the fall of 2002. Wear treatments were applied as a strip within replicates (split block) with a differential slip-wear device fitted with metal football cleats. The wear simulator was developed according to the design by Canaway (1976). The wear simulator was designed to create a scuffing action while minimizing pressure to the soil, therefore limiting soil compaction. Thus, the majority of the stress was applied to the foliage. At the termination of the study, two undisturbed soil cores with thatch removed (5.3-cm diameter by 7.5-cm length) were obtained from nine field plots (three genotypes selected at random) for determination of soil bulk densities according to the methods of Blake (1965).
The effects of wear should be assessed immediately after the application of wear treatments to minimize the effects of recuperative potential (Canaway, 1983). Because of the large number of plots and numerous passes needed to achieve sufficient separation in wear injury among genotypes, several days of wear treatments were required. Therefore, the late fall period was selected to minimize the potential for regrowth and recovery during the application of wear. A cumulative total of 75 passes were applied with the differential slip-wear device from 25 to 31 Oct. 2002. Wear tolerance was measured by visually rating the percentage of the surface covered by the turfgrass foliage after wear was applied. The percentage ground cover ratings were made by a 1-to-9 scale (9 = no injury or 100% ground cover) 1 d after wear applications were completed. The 10 genotypes with the highest percentage of ground coverage were classified as wear TOL, while the 10 genotypes with the lowest percentage of ground coverage were identified as wear (susceptible) INTOL. Visual wear tolerance ratings were made by three different evaluators. Analysis of variance (ANOVA) was conducted to test the effects due to genotype as well those due to the interaction of genotype and evaluator. No significant interaction between genotype and evaluator wear rating was detected. Therefore, final TOL and INTOL wear groupings were based on the pooled data to form one grand (evaluator) mean for each genotype. The 10 TOL genotypes ranged in wear tolerance (1–9 scale) from 7.4 to 8.1 while INTOL genotypes ranged from 4.4 to 5.2. Genotypes selected for further study were distinctly different in percent ground cover because of the effects of wear. However, in the untreated wear portions of NTEP plots, wear TOL and INTOL genotypes were at 100% ground cover (9 on the wear rating scale). The same methods used in 2002 for wear treatments and visual ratings were repeated in 2003 to assess year-to-year variation in wear tolerance among genotypes. Wear was applied in 2003 from 8 to 12 November.
Greenhouse and Field Measurements
Seeds for greenhouse evaluations were obtained from NTEP. Seeds were germinated for 4 wk beginning 1 Feb. 2003, and again on 2 Dec. 2003. After coleoptile emergence, genotypes were seeded in Pro-Mix BX growing medium and placed in the greenhouse at the University of Massachusetts, Amherst, MA, on 24 Feb. 2003, and 17 Dec. 2003. Seedlings were maintained under mist heads and received daily irrigation. The day/night temperature in the greenhouse was maintained at 25°C.
On 1 April 2003 and 22 Jan. 2004, single (plant) seedlings were transplanted into clear polyethylene tubes and maintained in the greenhouse as unmowed spaced-plants for 8 wk. Plant breeders typically evaluate plant characteristics from space planted nurseries (Bourgoin and Mansat, 1977), and therefore space plants have some relevance to evaluations used by turfgrass breeders. Clear polyethylene tubing (3 mil wall thickness, 3.2-cm outside diameter) was cut to a length of 70 cm and heat sealed at one end. Small holes were made in the heat-sealed end of each tube to allow for adequate drainage. Medium grade sand (67.3% 0.5 to 0.25 mm diameter) weighing 879 g (±10 g) was evenly mixed with 0.55 g of 28-2.2-9.9 fertilizer analysis and 0.28 g of dolomite, then poured into the tube. Polyvinyl chloride (PVC) pipe, 4.3-cm inside diameter, was cut to 70-cm lengths. A wire grid was positioned in one end for support of the sand column. The sand-filled tube was inserted into the PVC pipe, which formed a sleeve around the sand-filled tube.
Genotypes were replicated four times in a randomized complete block design, and were fertigated daily to saturation with combinations of 17-2.2-19.9 and 15-0-12.4 fertilizer analysis at 200 µg g–1 nitrogen. A thermocouple (Model 422314; Extech Instruments, Cole Palmer, Vernon Hills, Ill.) recorded the daily maximum, minimum and average temperatures. In 2003, the overall average temperature was 23.2°C (±3.8°C), with a mean maximum temperature of 31.0°C (±6.7°C) and a mean minimum temperature of 15.5°C (±3.5°C). In 2004, the overall average temperature was 22.2°C (±2.4°C), with a mean maximum temperature of 28.0°C ( ± 4.8°C) and a mean minimum temperature of 16.2°C (±0.4°C).
A total of 12 plant attributes were evaluated in the unmowed spaced-plants. Measurements were made on the 80 spaced-plants (20 genotypes by 4 replications) by sampling genotypes within a replicate over several days. Leaf character measurements included leaf number per shoot, leaf width, leaf angle, leaf strength, leaf turgidity, and leaf fiber analysis for cell wall constituents. Whole plant characteristics measured at the time of harvest from all shoot biomass (verdure) included shoot water content, tiller density, and shoot fresh and dry weights.
All leaf characteristics were measured on three shoot samples per genotype. Leaf width and leaf strength were measured at the midpoint of the second subtending leaf from the budleaf, which has been reported to vary most between genotypes while minimizing variation within a genotype (Sheffer et al., 1978; Brede and Duich, 1982). Leaf strength was defined as a measure of the tension (in grams) required to reach the breaking point and tear a leaf blade in half. Leaf strength was measured using Shimpo Digital Force Gauge (Model FGS-50H; Nidec-Shimpo America Corporation, Itasca, IL). Leaf number per shoot was defined as the number of green leaves per tiller or lateral shoot. Leaf angle was rated on a scale of 1 to 4 with the budleaf as the vertical axis, a score of 1 indicating a horizontal leaf orientation (0–22.5°), 2 indicating a semi-horizontal orientation (22.5–45°), 3 indicating a semi-vertical (45–67.5°), and a score of 4 indicating a vertical leaf orientation (67.5–90°).
Leaf turgidity was determined on all available nonsenescing, fully developed leaves by the formula [(fresh weight – dry weight)/(turgid weight dry weight)] x 100. Turgid weight was measured after soaking leaves in distilled water for twelve hours. Leaf fiber analysis assessed the amount of total cell wall content (entire fibrous portion), lignocellulose, and hemicellulose according to the methods of Goering and van Soest (1970). Polyester bag technology (Contreras Lara, 1999; Komarek et al., 1994) was used for cell wall analysis. The procedure required acid and neutral detergent testing with different reagents to measure quantities of cell wall constituents. The neutral detergent fiber (NDF) procedure was used to determine the percent total cell wall content (TCW) on a dry weight basis. Lignocellulose content was determined on a dry weight basis by the acid detergent fiber method (ADF). The difference between the quantity of NDF and ADF served to estimate the percentage hemicellulose (NDF-ADF).
Filter bags (ANKOM Technology, Macedon, NY) were used in both fiber procedures. The polyester bags had a uniform pore size of 30 µm. Bags were weighed and filled with approximately 0.1 g of dried leaf sample. Bags were then placed in an 11 ball flask, and depending on the analysis, moistened with 70 mL of the appropriate detergent solution (neutral detergent solution or acid detergent solution). All solutions were prepared according to the methods of Goering and van Soest (1970). The flask was heated keeping temperature between 95 and 100°C, and continuously agitated. After sixty minutes for neutral detergent fiber analysis and seventy minutes for acid detergent fiber analysis, the bags were removed from the flask and washed with boiling water to remove any detergent solution. They were then soaked in acetone for three minutes and oven dried for 60 h at 70°C. Oven dry weights were then recorded and converted to percentages as {[(initial weight – final weight)/(initial weight)] x 100} with the percentage of neutral detergent fiber representing TCW, the percentage of acid detergent fiber representing the lignocellulose content, and the difference between the two (NDF-ADF) representing the hemicellulose content.
Whole plant characteristics measured included shoot water content, tiller density, and shoot fresh and dry weights. Shoot water content was derived from shoot fresh and dry weights at harvest as [(fresh weight – oven dry weight)/fresh weight] x 100. Tiller density at harvest was defined as the total number of primary lateral shoots per plant.
All plant measurements made on the 20 Kentucky bluegrass spaced-plants were also obtained from the mowed field plots. Three 2.25-cm diameter plugs were taken from each field plot and measured in the same manner as greenhouse samples. Greenhouse and field measurements of plant characteristics were determined at different times from wear evaluations conducted in the field to partition the workload. Greenhouse measurements were made from 2 June through 20 June 2003, and 25 March through 3 April 2004. Field measurements were made from 6 May through 23 May 2003, and 1 May through 11 May 2004. Time was used as a blocking variable for both greenhouse and field experiments.
Statistical Analysis
Subsamples that were taken on the various plant characteristics were averaged and ANOVA was performed on those averages. Genotype sum of squares were partitioned into single degree of freedom (df), orthogonal contrasts, to test for the difference between the combined means of wear TOL and INTOL genotypes. Contrasts were also performed to test for differences within wear TOL and INTOL groupings. Least significant difference (LSD) values are reported for other comparisons at the 0.05 level. Correlation coefficients between genotype means (n = 20) were calculated to investigate the relationship between wear tolerance and greenhouse and field measured plant characteristics. Interactions using orthogonal contrasts for genotype with year, as well as group (TOL vs. INTOL) and year were tested. No interaction between genotype (and TOL vs. INTOL genotypes) with year was detected (excepted where noted), so pooled data (averaged across year) are reported here.
|
RESULTS AND DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
0.001) and between all 173 NTEP entries (r = 0.69, P 0.001). Furthermore, after 2 yr of wear treatments, no difference was detected in soil bulk densities between wear treated and untreated (check) plots. Wear plots averaged 1.21 g cm–3 while check plots averaged 1.20 g cm–3, suggesting that soil bulk density (i. e., soil compaction) was not a confounding factor contributing to injury imposed by the differential slip-wear machine. Our wear evaluations of Kentucky bluegrass genotypes were consistent in year-to-year wear tolerance. However, comparing wear tolerance results from this study to previous evaluations of Kentucky bluegrass (Minner et al., 1993; Bonos et al., 2001), inconsistencies were detected in genotype performance. Minner et al. (1993) ranked ‘Trenton’ and ‘Wabash’ as excellent wear tolerant genotypes, while Bonos et al. (2001) found Trenton and Wabash to be very poor in terms of wear tolerance. Minner et al. (1993) reported the genotype ‘Amazon’ as having poor wear tolerance while Bonos et al. (2001) ranked Amazon as very good in its tolerance to wear. Bonos et al. (2001) also ranked ‘Limousine’ to be poor in wear tolerance, Limousine was a top performing (wear TOL) genotype in our wear evaluations (Table 1). Additionally, Bonos et al. (2001) found the genotype ‘Unique’ to have superior wear tolerance, while our study classified Unique as wear INTOL (Table 1) because of its poor tolerance to wear.
|
Greenhouse Spaced-Plants
Genotype differences were present in all attributes measured on greenhouse spaced-plants except the number of leaves per shoot (Table 1). Large genotype differences were present in most plant attributes indicated by a large percent range value calculated as [(maximum value – minimum value)/(maximum value) x 100]. Leaf angle (Table 1) and shoot fresh weight and dry weight (Table 2) varied by as much as 53.5, 67.3, and 65.6%, respectively. Minimal variation was observed between genotypes for shoot water content (7.2%, Table 3). The coefficient of variation (CV) was calculated for all attributes measured. The CV was the smallest for shoot water content (2.4%, Table 3) and the largest for leaf angle (23.6%, Table 1) and shoot fresh weight and dry weight (32.1 and 31.7%, respectively, Table 2). Ebdon and Petrovic (1998) reported similar ranges and CV in Kentucky bluegrass for the same plant characteristics and methods.
|
|
|
Only three attributes in the greenhouse (spaced-plant) morphological study were significantly correlated to wear tolerance (Table 5), leaf angle (r = 0.95, P 0.001), shoot water content (r = –0.51, P 0.05), and leaf turgidity (r = –0.43, P 0.10). A significant genotype x year interaction was detected with leaf angle in the greenhouse (Table 1), indicating significant year-to-year variation in leaf angle among genotypes. However, the correlation (within year) between leaf angle in the greenhouse and wear tolerance was also highly significant in 2003 (r = 0.82, P 0.001) and 2004 (r = 0.95, P 0.001). Low water content and low relative turgidity in spaced-plants was associated with superior tolerance to wear in the field. The combination of low water content and low leaf turgidity has been proposed to increase elasticity in the leaf blade (Beard, 1973) and may be a mechanism active in Kentucky bluegrass wear tolerance. This relationship between superior wear tolerance and the concept of an "elastic leaf blade" was also observed by Trenholm et al. (2000) within seashore paspalum genotypes. Sun and Liddle (1993) also concluded that leaf flexibility (i.e., elasticity) was of greater importance than leaf strength in imparting wear tolerance. In this greenhouse study, leaf strength was not important in Kentucky bluegrass wear tolerance (Tables 1 and 5). Shearman and Beard (1975c), however, found leaf tensile strength to be positively correlated with improved interspecies wear tolerance in cool-season turfgrass. Thus, results developed at the interspecies level (Shearman and Beard, 1975c) are not necessarily relevant to these studies conducted at the intraspecific level in Kentucky bluegrass.
|
Genotypes with a more horizontal leaf orientation may expose greater leaf surface area to wear. Conversely, genotypes with a more vertical leaf orientation will have less tissue on a horizontal plane exposed to the forces present in wear stress. Measurements such as leaf angle may be a useful selection criteria because (i) it can be measured directly (physically) or indirectly (visual rating), (ii) leaf angle is routinely assessed by turfgrass breeders because leaf orientation is an important turf forming characteristic, and (iii) it is correlated with wear tolerance.
Field Plots
As with the greenhouse spaced-plants, wear TOL and INTOL genotypes differed in leaf angle (Table 1), shoot fresh weight (Table 2), shoot water content (Table 3), and TCW and lignocellulose content (Table 4). Although evident in greenhouse spaced-plants, no difference in leaf turgidity was observed between wear TOL and INTOL genotypes (Table 3).
Wear TOL genotypes had lower fresh weights compared with INTOL genotypes, 0.0683 g cm–2 compared with 0.0764 g cm–2, respectively (Table 2). No significant group (TOL vs. INTOL) x year interaction was detected, differences between TOL and INTOL wear groups remained consistent in 2003 and 2004. The difference in fresh weight present between groups may have been related to shoot water content, as was the case with greenhouse spaced-plants no difference on a shoot dry weight basis was observed in field plots (Table 2). The wear TOL group in the field had significantly lower shoot water content than the INTOL genotypes, 80.7 to 81.9% (Table 3). No significant interaction with year was detected, so the relationship between superior wear tolerance and low moisture content was observed in both test years.
Another relationship observed in greenhouse spaced-plants that was detected in field plots occurred with leaf angle. As with the greenhouse experiment, leaf angle was the single most important characteristic in separating wear TOL and INTOL genotypes, indicated by having the largest variance (F) ratio among all field measurements (85.1). Wear TOL genotypes had a 29% steeper leaf angle from horizontal than INTOL genotypes, 2.0 to 1.5, respectively (Table 1). A significant group (TOL vs. INTOL) by year interaction was detected; however, the relationship between wear TOL and INTOL genotypes remained consistent during the 2003 and 2004 growing seasons. In 2003, the TOL group possessed a steeper leaf angle than the INTOL group, 1.8 and 1.7, respectively. This difference was greater in 2004, with wear TOL genotypes exhibiting a mean leaf angle of 2.2 compared with 1.2 for wear INTOL Kentucky bluegrass. Tolerant and intolerant genotypes also exhibited differences in cell wall constituents similar to those observed in the greenhouse. Wear groups differed in TCW and lignocellulose content (Table 4). Wear TOL genotypes had significantly greater TCW and lignocellulose content than INTOL genotypes, 71.0 to 69.8% and 32.1 to 31.3%, respectively (Table 4). No interaction with year was detected with comparisons contrasting wear TOL vs. INTOL genotypes.
Leaf angle and shoot water measurements sampled from mowed field plots were also important predictors of wear tolerance. Leaf angle determined from mowed field plots was correlated with wear tolerance (r = 0.69, P 0.001, Table 5), as was shoot water content (r = –0.48, P 0.05, Table 5). As was the case in the greenhouse study, a significant genotype x year interaction was observed with leaf angle measured in the field (Table 1), indicating that leaf angle (averaged over year) was not an effective summary of the 2003 and 2004 data. Accordingly, the correlation between leaf angle and wear tolerance, while significantly correlated in 2004 (r = 0.87, P 0.001), were uncorrelated during the 2003 season (r = 0.24, P > 0.10). Generally, as leaf angle from horizontal increased and shoot moisture content decreased in mowed field plots and greenhouse spaced-plants, a corresponding increase in wear tolerance was observed (Table 5). Leaf angle measured in the field accounted for almost 50% of the total variation in wear tolerance, where as leaf angle in greenhouse experiments accounted for as much as 90% of the variation. As already suggested, one possible biological explanation for the relevance of vertical leaf orientation to improved wear tolerance is the potential to minimize the extent of leaf surface area that is exposed to wear stress. Another plausible explanation is leaf angle and TCW measured in the field were observed to covary significantly in 2003 (r = 0.71, P 0.001); wear tolerance increased with TCW, which was associated with increased leaf angle. However, no significant relationship between leaf angle and TCW was detected in the field during 2004 or in greenhouse experiments. So, there is no consistent evidence that breeding for a more vertical leaf orientation indirectly selects for greater TCW. No significant covariation between leaf angle with other plant characteristics relevant to wear tolerance was observed. Greenhouse (unmowed) spaced-plant and mowed field plot measurements were correlated for many plant characteristics found to be important in separating wear TOL and INTOL Kentucky bluegrass. For example, leaf angle (r = 0.62, P 0.01), TCW (r = 0.39, P 0.10) and shoot water content (r = 0.39, P 0.10) were found to be correlated when comparing between field and greenhouse experiments. Therefore, spaced-planted nurseries may be relevant in selecting turfgrass for wear under mowed conditions.
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
ACKNOWLEDGMENTS |
|---|
Received for publication August 27, 2004.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | 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 | |||
