| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a Dep. of Hortic., Poole Agric. Center, Box 340375, Clemson Univ., Clemson, SC 29634-0375
b Dep. of Genetics and Biochemistry, Clemson Univ., Clemson, SC 29634-0324
c Univ. of Georgia, 1109 Experiment Street, Griffin, GA 30223-1787
* Corresponding author (vbaird{at}clemson.edu)
 |
ABSTRACT |
|---|
 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
|---|
Abbreviations: DBI, double bond index • GC, gas chromatography • MS, mass spectrometer
|
INTRODUCTION |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
|---|
Whereas most temperate perennial plants can survive extensive exposure to subfreezing conditions, most tropical and a number of subtropical plants have little or no capacity to withstand freezing temperatures. A plant's ability to alter its physiology in response to low temperature, such that it can survive otherwise lethal temperatures, is called cold acclimation (hardening). The process of cold acclimation occurs seasonally when plants are exposed to low, nonfreezing temperatures before the onset of winter (Levitt, 1980). Some C4 grasses have been shown to cold acclimate (Anderson et al., 1988, 1993). The biochemical changes accompanying acclimation to low temperature include alterations in soluble carbohydrate content, synthesis, and conformation of proteins and changes in membrane lipid composition and fatty acid saturation (Li, 1984; Sakai and Larcher, 1987; Hallgren and Oquist, 1990). Many of the new compounds, synthesized during cold acclimation, have been variously associated with improved tolerance to freezing temperatures by avoiding or mitigating the deleterious effects of cellular dehydration (Gusta et al., 1996).
On the other hand, surviving chilling stress requires maintenance of the structural and functional integrity of cellular membranes. Biological membranes exist as a lipid bilayer in which proteins are embedded or associated. The major classes of lipids present in plant cell membranes are the phospholipids and glycolipids (Lea and Leegood, 1993). Phospholipids consist of two fatty acyl side chains esterified to the first and second hydroxyl groups of glycerol. A polar head group is linked through a phosphodiester bond to the third carbon of glycerol. The most common glycolipids found in plants are monogalactosyl diglyceride and digalactosyl diglyceride, having one or two sugar molecules at the Sn-3 position and fatty acyl groups occupying the Sn-1 and Sn-2 positions of glycerol.
The fatty acyl chains of phospholipids and glycolipids play a very important role during chilling stress. The fatty acids may either be saturated or unsaturated. Saturated fatty acids lack a double bond between carbon atoms. Unsaturated fatty acids, on the other hand, have one or more double bonds, either in the cis or trans configuration. The presence of the more common cis double bond helps maintain membrane fluidity by introducing bends or kinks in the fatty acyl chains, thereby inhibiting tight packing of adjacent lipid molecules (Lehninger, 1977; Vigh et al., 1998).
At low temperatures, membranes undergo a phase transition from a highly fluid, liquid crystalline phase to a more rigid gel phase. In the gel phase, lipids are closely packed and more highly ordered, which hinders normal physiological functions and can render the membrane more permeable and prone to rupture. Many of the biochemical and biophysical changes associated with cold acclimation help to prevent membrane phase changes (i.e., homeophasic adaptation) (Cossins, 1994). For example, the synthesis of unsaturated acyl chains is thought to reduce the transition temperature of membrane lipids (Lynch and Steponkus, 1989). Furthermore, changes in phospholipid composition, such as increases in phosphatidylcholine, also appear to reduce the membrane transition temperature. Thus, the degree of unsaturation or the class of lipid can significantly affect the temperature range in which membranes undergo deleterious phase transition (Vigh et al., 1998).
With soil salinity being a major problem in many regions of the USA and around the world, salt-tolerant grasses (e.g., seashore paspalum) are being more widely utilized than in the past (Duncan, 2000). Although some salt-tolerant grasses are moderately cold-tolerant, most are subject to winter kills, especially in the northern boundaries of their zone of adaptation. For seashore paspalum in the continental USA, this boundary roughly coincides with an area south of a line from Raleigh, NC, to Chattanooga, TN, to Little Rock, AR, to Dallas, TX, to Albuquerque, NM, to south of San Francisco, CA (Duble, 1998; Outsidepride.com, 2002)—anywhere plants experience prolonged cold that freezes the ground to below rhizome depth. Investigations of low-temperature tolerance in seashore paspalum are limited to a few cultivars and primarily address electrolyte leakage and regrowth assays (Ibitayo and Butler, 1981; Cardona et al., 1997). The goals of this study were to (i) investigate whether exposure to low temperatures has an effect on membrane lipid fatty acid saturation levels in seashore paspalum genotypes differing in their tolerance to low temperature (i.e., SeaIsle1, Adalayd, and PI 299042); (ii) determine if any observed changes in fatty acid saturation differed significantly between treated and untreated plants of the same genotype; and (iii) determine if the response to low temperature differed among the three genotypes.
The data from this study showed that the concentration of the triunsaturated linolenic acid increases during exposure to low temperatures. Also, the cold-tolerant genotype SeaIsle1 demonstrated a greater increase in linolenic acid content than did the relatively cold-susceptible genotypes Adalayd and PI299042. This suggests that the increase in linolenic acid in seashore paspalum is part of an adaptive response when exposed to low temperatures, and that this is a fundamental part of the overall mechanism of cold acclimation in warm-season turfgrass species.
|
MATERIALS AND METHODS |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
|---|
Cold Treatment
After two to three months in the greenhouse, Rootrainers containing either the control or the experimental plants were transferred to growth chambers (model E-15, Conviron, Asheville, NC). The plants were preconditioned in growth chambers at 28°/22°C (day/night) temperatures for 1 wk before the initiation of each experiment. In a single chamber, treated plants were exposed to 8°/4°C (day/night temperatures) at 250 µmol m-2 s-1 photosynthetic photon flux density (PPFD) over a 10-h photoperiod for 3 to 4 wk (Anderson et al., 1988, 1993). Control plants were maintained under a similar PPFD and photoperiod, but at 28°/22°C (day/night temperatures) in the second chamber. Air and soil temperature was monitored throughout the experimental period. The plants were watered once daily but were not fertilized during the treatment period. The experiment was replicated a second time using the same chambers.
Sample Collection
Tissue samples were collected from the control plants and the cold-treated plants at 7-d intervals during a 3-wk period (i.e., 0, 7, 14 and 21 d). For each genotype (three) and treatment (two) at each time point (four), plants in randomly chosen Rootrainer compartments were removed from the growth chamber and the crowns and rhizome nodes were quickly cut away from shoots and roots, which were discarded. This tissue was washed in cold water and the excess moisture was removed by blotting on paper towels. The samples were frozen in liquid N and stored at -70°C for later use. At each collection time point for each cultivar and each temperature treatment, 
10 to 20 g of tissue were collected.
Total Lipid Extraction
For each biochemical analysis (i.e., three genotypes, each exposed to either of two temperature treatments, sampled four times during each 3-wk experiment), 1 g of tissue was ground in liquid N to a fine powder using a mortar and pestle. The powdered tissue was transferred to a test tube for extraction of total lipids. Three milliliters of extraction solution [chloroform:methanol:water (1:2:0.8; v/v/v)] were added to each sample. The samples were incubated at room temperature for 20 min. One milliliter of 1% (w/v) aqueous sodium chloride and 3 mL chloroform were added, mixed, and centrifuged at 3000 x g for 5 min to separate the aqueous and organic phases. The lower organic layer containing the lipid was transferred to a new test tube. The remaining aqueous layer was reextracted twice (as before), and the organic layers containing the lipids were pooled (9 mL total) and dried at 45°C under a stream of N. The extracted lipid was resuspended in 250 µL of chloroform and stored at 4°C (Kates, 1972). Each 1-g tissue extraction, representing an individual genotype, treatment, and time point, was repeated four times.
Polar Lipid Purification and Fatty Acid Isolation
The extracted total lipid fraction was applied to a 20- by 20-cm thin layer chromatography plate (Type K6F, Whatman, Clifton, NJ) as a spot about 2 cm long, 2 cm from the bottom of the plate. The polar lipids were separated from the nonpolar lipids by placing the loaded thin layer chromatography plate in a chromatography chamber containing nonpolar solvent (hexane:ethyl ether:acetic acid at 80:20:1), and developing until the solvent front migrated about three-fourths the length of the plate (e.g., 1 to 2 h). The silica at the origin (containing the polar lipids) was placed in a 5-mL plastic syringe containing 0.2 µm nylon Acrodisc (Gelman Sciences Inc., Ann Arbor, MI). Fifty micrograms of nonadecanoic acid (C19:0) was added into the syringe as an internal standard. The polar lipids were extracted from the silica with 5 mL chloroform:methanol (2:1; v/v) followed by 3 mL ethyl ether. The eluted polar lipids from each extraction sample were split and each half was dried at 45°C under a stream of N. The split samples were subsequently processed simultaneously as an internal control for loss of lipid material, contamination, and consistency of fatty acid identification.
The isolated polar lipids were saponified with 1 mL 15% sodium hydroxide in 50% aqueous methanol at 100°C for 30 min and allowed to cool to room temperature. The free fatty acids were then converted to fatty acid methyl esters by adding 2 mL acidified methanol (6 M aqueous hydrochloric acid:methanol, 54:46; v/v) and heating at 80°C for 10 min. The solution was then cooled to room temperature. Free fatty acid methyl esters were recovered from the acidic aqueous phase by organic extraction with 1.25 mL hexane:methyl-tertbutyl ether (1:1; v/v). The lower aqueous phase was discarded and the upper organic phase was purified by partitioning with 3 mL alkaline solution (1.5% aqueous sodium hydroxide; w/v). The solution was mixed by inversion for 5 min at room temperature and the upper organic phase was transferred into an amber glass vial with Teflon-coated septa and sealed for GC analysis.
Fatty Acid Quantification
Fatty acids were analyzed using a Hewlett-Packard gas chromatograph (model GC 5890, Avondale, PA) equipped with Sherlock microbial identification software (MIDI, Inc., Newark, DE). Two-microliter samples were injected into an Ultra2 GC column (5% phenyl methyl silicone, 25 m by 0.2 mm, 0.33 um film; Agilent Technologies, Palo Alto, CA) and analyzed utilizing the FASTWO method. The column temperature was maintained initially at 170°C, followed by a 5°C min-1 ramp to 295°C and a secondary ramp of 30°C min-1 to 300°C. The final column was held at 300°C for 7 min. Injector temperature was 250°C and detector temperature was 300°C. The carrier gas was hydrogen at a flow rate of 0.5 mL min-1 (55.2 x 103 Pa). The individual peaks were identified based on comparisons to a peak table associated with the Sherlock microbial identification software after calibration using a FASTWO calibration mix (MIDI Inc., Sasser, 1990). Sample peaks that could not be positively identified due to coelution of two or more fatty acids were further analyzed on a mass spectrometer (MS). Mass spectrometer analysis was conducted on an HP5890 GC with a quadropole HP5891A MS (Hewlett-Packard) operating in total ion monitoring mode (at 50 to 600 amu). Gas chromatography and column conditions were the same as above, but 3 µL of each sample was injected (helium carrier gas) and the transfer line was set at 300°C. The spectra obtained were compared with spectra of a Wiley database for identification of fatty acids. Individual fatty acids were measured as a percentage relative to the total of the four major fatty acids identified by comparing their individual peak areas. The absolute amounts (µg) of each fatty acid were quantified by comparing their peak areas to that of the known internal standard. Overall, each biochemical analysis and quantification was performed eight times for each sampling time (i.e., four 1-g samples processed for each sampling time point in the replicated experiment).
Through evaluation of the double bond indices (DBIs), changes in the composition of unsaturated fatty acids among SeaIsle1, Adalayd, and PI 299042 during the cold treatment period could be compared. The DBI was calculated as follows: The relative percentage of each unsaturated fatty acid was multiplied by the number of double bonds in that fatty acid and the products were summed and divided by 100 [e.g., (di-unsaturated fatty acid x 2) + (triunsaturated fatty acid x 3)/100].
Statistical Analysis
The replicated experiment consisted of three genotypes, two temperature treatments, and four sampling times, and the tissue analyzed for fatty acid content was a mixture of rhizome nodes and crowns. Tests of significance were performed in an analysis of variance for mixed-effects model using a SAS program (SAS Institute, 2001). The significance values are based upon changes throughout the 3-wk treatment period. Three hypotheses were tested using the data: (i) If cold exposure had an effect on fatty acid composition across time—the null hypothesis that the treatment had no effect on the fatty acid composition was rejected when the P value was less than P = 0.05; (ii) the cold acclimated and control plants were compared to test the hypothesis of equal responses—the responses were considered different when the null hypothesis was rejected at P = 0.05; and (iii) to determine if the response to low temperature exposure was different between the three genotypes, SeaIsle1, Adalayd, and PI299042—the response of each genotype was compared with that of the other. The response between any two genotypes were considered different if the P value was less than the hypothesized value of P = 0.05.
|
RESULTS AND DISCUSSION |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
|---|
95% of the total polar fatty acids identified. The remaining 5% were accounted for by at least seven different fatty acid species, and these were not considered for further study because of their relatively low abundance and variability.
The saturated fatty acid, palmitic acid, accounted for 25% of the total in all three genotypes studied. All three genotypes showed only minor changes in palmitic acid content during cold acclimation (Fig. 1
and Table 1)
. In SeaIsle1, palmitic acid decreased from 24.1% of the total fatty acid content to 23.5% by the end of the third week of cold treatment. In Adalayd, a minor increase from 25.1% to 25.5% was observed. In PI 299042-1, palmitic acid decreased from 27.2% of the total fatty acid content to 25.4% by the end of the treatment period. Analysis of variance showed that the observed changes were not significant in SeaIsle1 (P = 0.249), Adalayd (P = 0.664), or PI 299042 (P = 0.178). As expected, in untreated (control) plants of all three genotypes, the fatty acid content remained statistically the same throughout the experimental period (Table 2)
. Taken together, these results show that the cold treatment had no direct effect on palmitic acid content.
|
|
|
5% of the total polar fatty acid content. As was observed for palmitic acid, changes in stearic acid content during cold treatment were not statistically significant within or among genotypes (Fig. 2)
. For example, in both SeaIsle1 and Adalayd, the measurable change was <1% (i.e., an increase from 3.4% to 4.2%, P = 0.8471; and from 3.8% to 4.7%, P = 0.2419, respectively).
|
50% of the total fatty acid content in SeaIsle1, Adalayd, and PI299042 at the start of the experiment. Among the three genotypes, SeaIsle1 showed the greatest measurable decrease in linoleic acid when compared with Adalayd and PI 299042. Linoleic acid content decreased significantly in SeaIsle1 and Adalayd during cold acclimation (Fig. 3)
. In the cold-tolerant genotype, SeaIsle1, although no significant change occurred during the first week of acclimation (i.e., decrease from 49.7 to 48.1% of the total fatty acid content); by the second week, the fatty acid content decreased significantly (i.e., from 48.1 to 43.3%; P = 0.001). Similarly, in Adalayd, the intermediately tolerant genotype, a statistically significant decrease in linoleic acid content was observed by the second week of low temperature exposure (i.e., from 48.2 to 45.4%; P = 0.0001). On the other hand, the comparatively cold-sensitive genotype, PI 299042, did not show a significant change in linoleic acid content (P = 0.1782). In all three genotypes, after the second week of low temperature exposure, no further significant change in linoleic acid was observed.
|
In the crown and rhizome node tissues of seashore paspalum, the triunsaturated fatty acid, linolenic acid, accounted for 25% of the total polar lipid fatty acids (Fig. 4)
. In all three genotypes, linolenic acid was found to increase significantly during cold treatment. In SeaIsle1, during the second week of cold treatment, the amount of linolenic acid continued to increase, and showed a significant change by Day 14 (i.e., from 23.9 to 30.2%; P = 0.001). After the second week of cold treatment, there was no change in the amount of linolenic acid. In Adalayd and PI 299042, linolenic acid also increased during cold treatment, with the greatest change coming during the second week of cold treatment. However, in both genotypes the magnitude of change was not as large as that observed for SeaIsle1. In Adalayd, linolenic acid content increased, from 22.6 to 24.4% (P < 0.0001). In PI 299042, linolenic acid increased from 21.5% of the total fatty acid content at the start of the experiment to 23.9% (P = 0.001) by Day 14. No significant change was observed in either genotype after the second week of cold treatment. In addition to the difference in magnitude, the rate of increase of linolenic acid was much greater for SeaIsle1 than for either Adalayd or PI 299042 (3.1 vs. 1.2 vs. 0.7 µg g-1 d-1, respectively).
|
The number of double (unsaturated) bonds in membrane lipid fatty acids is an indirect measure of the overall fluidity of those membranes. At the start of the experiment, SeaIsle1 had a higher DBI than either Adalayd or PI 299042 (1.67 vs. 1.64 vs. 1.6, respectively; Fig. 5) . During low temperature exposure, a statistically significant increase in DBI was observed in SeaIsle1 (P < 0.001) when compared with untreated control plants. The DBI increased during the first 2 wk of cold acclimation and then leveled off by the third week. Adalayd and PI 299042 did not demonstrate significant changes in their DBI when compared with untreated control plants (P = 0.6012 and P = 0.7864, respectively).
|
The results from this study are consistent with earlier findings on biochemical changes accompanying prolonged exposure to low temperature. A similar change in fatty acid content was reported recently for another warm-season turfgrass, bermudagrass [Cynodon dactylon (L.) Pers.]. As was observed for seashore paspalum, cold-tolerant bermudagrass genotypes showed a greater magnitude of increase in linolenic acid than did the relatively cold-susceptible genotypes (Samala et al., 1998; Cyril et al., 2001).
Similarly, chilling-resistant cucumbers (Cucumis sativus L.) and sweet peppers (Capsicum annuum L.) have a greater amount of 18-carbon unsaturated fatty acids than saturated fatty acids (Wang and Baker, 1979). Even findings from studies of freeze-tolerant species reveal alterations in the degree of fatty acid saturation, but as part of a more complex response involving changes in membrane lipid composition (not observed in the present study), biosynthesis of osmoprotectants and hydrophilic proteins (Gerloff et al., 1966; Latsague et al., 1992; Palta et al., 1993). Furthermore, mechanistic investigations in cyanobacteria, protozoa, and plants confirm roles for fatty acid desaturation and acyl-lipid desaturases in cold adaptation (Cossins, 1994; Nishida and Murata, 1996; Tasaka et al., 1996). For example, transformation experiments with a 
-12 (
-6) desaturase (Wada et al., 1990) or inducing expression of the wild type -12 desaturase (Avery et al., 1995) resulted in increased membrane unsaturation, as well as increased chilling tolerance and restored normal membrane function, respectively. Conversely, saturated fatty acids have been associated with chilling sensitivity. For example, by engineering an Escherichia coli gene for glycerol-3-phosphate acyltransferase into Arabidopsis thaliana (L.) Heynh., the composition of plastidal membrane lipids was altered to contain elevated levels of saturated fatty acids. The transgenic plant demonstrated an increased sensitivity to chilling temperatures (Wolter et al., 1992).
Current evidence indicates that the biophysical state of membrane lipids plays an important role in maintaining membrane integrity during cold stress (Vigh et al., 1998). According to Lyons (1973), the primary injury during chilling is due to membrane lipid transition from a relatively fluid state to a more gel-like state. Unsaturated fatty acids have a pivotal role in maintaining membrane fluidity for proper functioning. Unsaturated fatty acids lower the transition temperature at least partly by introducing bends in the otherwise linear fatty acyl chains. This prevents tight packing of the acyl chains of adjacent lipids, thus maintaining molecular interactions critical for normal membrane functioning.
In the three seashore paspalum genotypes studied here, the amount of the triunsaturated fatty acid, linolenic acid, increased during exposure to low temperature. In addition, the magnitude and rate of increase was greater in the cold-tolerant genotype, SeaIsle1, than in the more cold-sensitive genotypes, Adalayd and PI 29042. On the basis of the observed changes, it can be hypothesized that the biosynthetic pathway leading to the synthesis of linolenic acid may be regulated in response to low temperature. Linolenic acid is synthesized from stearic acid (C18:0) by a series of desaturase enzymes. Stearic acid is converted to oleic acid (C18:l) by stearoyl-ACP desaturase. Formation of the second double bond is catalyzed by a another enzyme, oleoyl-PC desaturase, to form linoleic acid (C18:2). The third double bond is synthesized by linoleoyl-PC desaturase to form linolenic acid (C18:3). It is possible that either the activity of the desaturase(s) or the expression of the gene(s) responsible for the synthesis of one or more of the desaturase enzymes may be cold temperature regulated.
Molecular studies in several plant species indicate that linoleoyl (-3) desaturase has a role in improving cold tolerance. In transgenic tobacco (Nicotiana tabacum L.), overexpressing a plastid localized -3 desaturase (fad7) gene, the levels of two triunsaturated fatty acids increased and the plant demonstrated a cold-tolerant phenotype (Kodama et al., 1994). The fad8 -3 desaturase gene is up-regulated in response to low temperature in A. thaliana (Gibson et al., 1994). Conversely, A. thaliana mutants deficient in -3 desaturase activity demonstrated a chilling-sensitive phenotype (Browse et al., 1989; Miquel et al., 1993). Similar results were observed in Synechocystis where saturation of plasma membrane fatty acids was artificially increased (Vigh et al., 1993).
Studying the enzyme activity or expression of the genes encoding the desaturase enzymes in seashore paspalum will further address the role of unsaturated fatty acids in cold tolerance, and will help in efforts aimed at identifying genes associated with the process of cold acclimation. This information will be useful for germplasm improvement in seashore paspalum and other turfgrass species, using selection, breeding, and recombinant DNA methodologies.
|
ACKNOWLEDGMENTS |
|---|
|
NOTES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
|---|
Received for publication September 5, 2001.
|
REFERENCES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
|---|
-3 fatty acid desaturase in transgenic tobacco. Plant Physiol. 105:601–605.[Abstract]Related articles in Crop Science:
This article has been cited by other articles:
|
|
 |
|
  R. Valluru and W. Van den Ende Plant fructans in stress environments: emerging concepts and future prospects J. Exp. Bot., August 1, 2008; 59(11): 2905 - 2916. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 X. Zhang, K. Wang, and E. H. Ervin Bermudagrass Freezing Tolerance Associated with Abscisic Acid Metabolism and Dehydrin Expression during Cold Acclimation J. Amer. Soc. Hort. Sci., July 1, 2008; 133(4): 542 - 550. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. J. Patton and Z. J. Reicher Zoysiagrass Species and Genotypes Differ in Their Winter Injury and Freeze Tolerance Crop Sci., July 30, 2007; 47(4): 1619 - 1627. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Bakht, A. Bano, and P. Dominy The role of abscisic acid and low temperature in chickpea (Cicer arietinum) cold tolerance. II. Effects on plasma membrane structure and function J. Exp. Bot., November 1, 2006; 57(14): 3707 - 3715. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. C. Munshaw, E. H. Ervin, C. Shang, S. D. Askew, X. Zhang, and R. W. Lemus Influence of Late-Season Iron, Nitrogen, and Seaweed Extract on Fall Color Retention and Cold Tolerance of Four Bermudagrass Cultivars Crop Sci., January 24, 2006; 46(1): 273 - 283. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 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 | |||
