Published online 1 February 2006
Published in Crop Sci 46:544-553 (2006)
© 2006 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
FORAGE & GRAZINGLANDS
Relationships between Emergence and Soil Water Content for Perennial Cool-Season Grasses Native to Iran
Ali Gazanchian*,a,
Nayer A. Khosh Kholgh Simab,
Mohamad A. Malboobic and
Eslam Majidi Heravanb
a Agricultural and Natural Resources Research Center of Khorassan, P.O. Box 91735-1148 Mashhad, Iran
b Agriculture Biotechnology Research Institute of Iran (ABRII) P.O. Box 31535- 1897, Karaj, Iran
c National Research Center for Genetic Engineering & Biotechnology P.O. Box 14155-6343, Tehran, Iran
* Corresponding author (gazanchian{at}kanrrc.ac.ir)
 |
ABSTRACT
|
|---|
Native cool-season grasses are important for improvement of arid and semiarid rangelands in Iran. However, germination and seedling emergence of these species is a critical phase in these areas and are not well understood. The objective of this study was to evaluate the effect of low soil moisture on emergence of 36 populations of native cool-season grasses from 15 species of seven genera including Elymus, Agropyron, Secale, Hordeum, Bromus, Festuca, and Dactylis that were collected from different locations of Iran. A pot experiment was conducted in a greenhouse using a completely randomized design with three replicates. The populations were evaluated under field capacity (FC, –0.33 MPa), 75, 50, and 25% FC. Decreasing soil water content from FC to 25% FC reduced overall mean emergence from 69.3% to zero. Significant differences occurred among species and soil moisture levels for all characters except root/shoot dry weight ratio. Seedling emergence rate and root and shoot growth were decreased by limiting soil water content, while root-to-shoot length ratio (43%) was increased. Minimum water requirement for seedling emergence of all species except Dactylis glomerata L. was 50% FC (–0.6 MPa). Cluster analysis and canonical discriminant analysis separated the populations into five clusters. Populations in Clusters I and III had intermediate, Cluster II the greatest, and Clusters IV and V the least tolerance to low soil moisture. Cluster I and III had greater root and shoot biomass and rapid emergence, whereas decreasing soil moisture caused a severe reduction of root growth of 60% compared with the control. In response to low soil moisture, Cluster II showed high final emergence and the greatest stability (low sensitivity index) of root growth. Clusters IV and V had delayed emergence, low growth potential, and high sensitivity index for shoot and root growth under low soil moisture. In response to low soil moisture, there were differences among species and accessions of the same species collected from different ecological regions. These results suggest that the genetic diversity of Cluster II could be used for plant breeding programs and renovation of pastures in arid and semiarid regions of Iran.
Abbreviations: CRD, completely randomized design • FC, field capacity • PC, principal component • PCA, principal component analysis • RD, root dry weight • RL, root length • SD, shoot dry weight • SL, shoot length • SVI, seedling vigor index • T0, days to emergence initiation • T50, days to 50% emergence
|
INTRODUCTION
|
|---|
IRAN contains both arid and semiarid regions (FAO, 2003), with an estimated 397 native grass species in 115 genera (Mozaffarian, 1996). Perennial grasses are considered as key plants to the economic and environmental sustainability of rangeland for livestock grazing in Iran. Seed germination and emergence are the most critical phases in rangeland renovation, particularly in dry areas. Germination commences with the uptake (imbibition) of water by the quiescent dry seed and is completed when the radicle extends to penetrate the structures that surround it (Bewley, 1997). Low water availability in arid and semiarid regions severely limits seed germination, seedling establishment, and persistence of perennial grasses (Bassiri et al., 1988). Johnson and Asay (1993) reported that water deficit also limits the establishment, growth, and production of cool-season grasses on semiarid rangelands.
Seed germination and seedling establishment of cool-season grasses under water stress conditions are affected by rapid germination, seedling vigor, seed size, early root initiation and rapid root extension (Johnson and Asay, 1993), water quantity and frequency following planting (Cox and Jordan, 1983; Frasier et al., 1987), seedbed preparation and time of seeding (Jordan, 1970), presence of seminal lateral root primordia, increasing the depth of planting (Hassanyar and Wilson, 1978), and pre-germination or osmo-conditioning of seed (Mueller and Bowman, 1989; Mueller, 1996; Hardegree and Van Vactor, 2000; Hardegree et al., 2002). Also, Isselstein et al. (2002) showed that biotic and abiotic factors could limit seedling emergence and establishment of fen-meadow species. They concluded that characterization of the seedling recruitment niches for these species is needed.
Some of the main limitations to adoption of native cool-season grasses for rangeland use in Iran are a lack of knowledge about their water requirements for germination and subsequent establishment. Here, we introduce a modified method for estimating seedling emergence percentage under water deficit that may represent post-seeding rangeland conditions. Our objectives were to: (i) develop simple techniques for screening cool-season grasses that could be used reliably for determining low soil moisture tolerance at seedling emergence, (ii) evaluate the effects of low soil water on 36 perennial cool-season grasses native to Iran and identify low soil water tolerant accessions, and (iii) characterize intraspecific and interspecific diversity for phenotypic traits.
|
MATERIALS AND METHODS
|
|---|
Plant Materials and Treatments
Seeds of 36 populations of perennial native cool-season grasses (15 species of seven genera) were collected from different ecological sites from rangeland habitats of Iran from 1994 to 2000 by the Rangelands and Forestry Research Institute (Table 1). However, according to Mozaffarian (1996), there is a possibility that both Agropyron desertorum Fisch. & Schultes and Bromus inermis Leyss have been introduced to Iran, but they are widely distributed and have been found for than more 50 years in Iran. After collection, seeds were cleaned and germination percentage, purity, and moisture content were determined. Seeds were stored at –20°C until the experiment began in 2003. The effects of four different levels of soil water [percentage of field capacity (FC); 25, 50, 75, or 100%] on seedling emergence, root and shoot length, dry weight and days to initial and 50% of emergence were evaluated in a greenhouse experiment. Each pot (9-cm diameter by 10-cm depth) was filled with 300 g of sifted dry sandy loam soil (722 g kg–1 sand, 147 g kg–1 silt, and 131 g kg–1 clay) which was oven dried at 180°C for 6 h. Twenty intact seeds of each respective species were placed on the dry surface and covered with a 5-mm layer of the soil. Immediately after planting, water was added to dry soil to reach FC, 75, 50, or 25% FC corresponding to 12, 9, 6, and 3 mL water/100 g dry soil, which were approximately equal to –0.03, –0.2, –0.6, and –1.5 MPa (near the permanent wilting point) matric potential of the soil, respectively. After watering, the pots were sealed with transparent nylon bags for the duration of the study to prevent evaporation. Greenhouse conditions were maintained at 15/20°C (night/day) under natural day light and 60 to 70% relative humidity. Soil volumetric water content was determined by weighing the pots during the experimental period for each treatment.
View this table:
[in this window]
[in a new window]
|
Table 1. Descriptions of collection sites and seed characteristics of 36 perennial cool-season grass populations native to Iran evaluated for determining relationship between seedling emergence and soil moisture.
|
|
Measurements
Emergence was monitored daily for 12 d and every other day for the next 20 d. Seedling emergence was recorded at the first leaf appearance from the soil surface for FC, 75% FC, and 50% FC. No seedlings of any species emerged at 25% FC. Therefore, on Day 20, the soil in the 25% FC treatment pots was excavated and germinated seeds counted. Seeds were considered germinated when the radicle was 5 mm long. Days to first emergence (T0) and days required to reach 50% emergence (T50) were determined as described by Usberti and Valio (1997). The shoot length and the maximum root length for each emerged seedling were measured for each pot at the end of experiment. Material from each pot was washed, shoots and roots separated, oven dried (75°C for 48 h), and weighed. Seedling vigor index (SVI) was determined according to Abdul-Baki and Anderson (1973) with some modifications and was calculated by multiplying percent emergence by length (cm) of the seedling (root plus shoot).
Sensitivity index (Fernandez and Reynolds, 2000; Bayuelo-Jimenez et al., 2002) for each trait was determined on the basis of the ratio of the median response at 75% FC or 50% FC to the median response at FC.
Data Analysis
The experiment was conducted as a completely random design (CRD) with three replicates. The normality of data was tested and arcsine square root transformed to normalize the non-normal distributions. Analysis of variance based on a linear model was conducted in SAS (SAS, 1996). Simple correlation coefficients were used to determine the relationships between different characters. Because of a significant interaction between species and soil water content for all traits except one (Table 2), principal component (PC) analysis was used to summarize the variation of the original characters and to graph the populations on the basis of the first two principal components using NTSYS v. 2.02e (Rohlf, 1998). Ward's minimum variance method based on Euclidean distance grouped all 36 populations using measured characters (Ward, 1963). Canonical discriminant analysis was conducted in SPSS v. 11 (2001) to determine optimal number of groups after cluster analysis. Graphical representation of groups obtained in each cutting point of the dendrogram based on the first two canonical variants was used as a criterion to determine the best cutting point in the dendrogram. Multivariate analysis was conducted on means for seven traits (final emergence, T0, T50, shoot length, root length, shoot dry weight, and root dry weight) from the population by soil water content interaction.
View this table:
[in this window]
[in a new window]
|
Table 2. Mean squares from analysis of variance (ANOVA) for measured emergence characteristics of 36 perennial cool-season grass populations native to Iran at four soil moisture levels.
|
|
|
RESULTS
|
|---|
Emergence
Significant differences between soil moisture levels were present for all studied characters except root-to-shoot dry weight ratio. The soil moisture by population interaction was also significant (P < 0.01) for all the traits except root-to-shoot length ratio (Table 2). Overall mean emergence percentage at FC, 75% FC, 50% FC, and 25% FC were 69.3, 62.8, 47.8, and 0.0%, respectively. Although none of the species emerged at 25% FC, some species germinated and averaged 6.5% (Table 3). For instance, at 25% FC average germination for Secale montanum, Elymus elongatum, Bromus inermis, Elymus repense, and Bromus tomentellus were 39.2, 17.3, 6.7, 4.2, and 2.5%, respectively (Table 4). A minimum soil water content of 50% FC (–0.6 MPa) was required for emergence for all species, except Dactylis glomerata which required 75% FC. Soils at FC did not optimize emergence for all species. For example, S. montanum collected from Zanjan, and E. elongatum collected from Brojen and Gorgan had optimum final emergence at 75% FC (Table 4).
View this table:
[in this window]
[in a new window]
|
Table 3. Means of measured emergence characteristics of 36 perennial cool-season grass populations native to Iran at four soil moisture levels.
|
|
View this table:
[in this window]
[in a new window]
|
Table 4. The effects of soil moisture content on final emergence, days to 50% emergence, shoot and root dry weight, and sensitivity index (SI) of 36 perennial cool-season grass populations native to Iran in seedling emergence stage.
|
|
View larger version (11K):
[in this window]
[in a new window]
|
Fig. 2. Ward's cluster analysis classifications of 36 perennial cool-season grass population native to Iran on seven measured traits at three soil moisture levels in seedling emergences stage determined by Euclidian distance. Species names are defined in Table 1.
|
|
View this table:
[in this window]
[in a new window]
|
Table 7. Means of traits used in the identification of five clusters formed from 36 perennial cool-season grass populations native to Iran at three soil moisture levels in seedling emergence stage.
|
|
Low soil water content delayed T0 and T50 emergence by 2.7 and 2.8 d averaged across all species (Table 2). At 50% FC, T50 for S. montanum (Semnan), E. elongatum (Semnan), Agropyron desertorum (Sanandaj), and Festuca arundinaceae (Gochan) were 6.8, 8.2, 12.7, and 13.3 d, respectively (Table 4).
Seedling Growth
Low soil water content decreased (P < 0.01) both root and shoot length by 55% as compared with FC, whereas root-to-shoot length ratio increased by 43% for all of the species (Table 3). Elymus elongatum and F. arundinaceae had the least and the greatest reductions of root (25 and 59%) and shoot (37 and 76%) length, respectively compared with FC (data not shown). Shoot and root dry weight decreased by 42% and 56% at 50% FC averaged across all species. Mean shoot and root dry weight per pot at FC, 75% FC, and 50% FC were 33.0, 27.3, and 19.1 mg, and 23.2, 18.6, and 10.1 mg, respectively (Table 3).
Seedling vigor index (SVI) decreased by 63% with increasing soil water content (Table 3). The highest SVI measurements occurred for S. montanum, E. elongatum, and B. tomentellus and the lowest for F. arundinaceae and F. ovina with low soil water (data not shown).
Estimates of the relative stability of shoot and root dry weight (sensitivity index) of individual populations showed that some species were sensitive to soil water content reduction even from FC to 75% FC. For example sensitivity indices (SI 75%) for shoot dry weight were 57% for S. montanum (Brojen), and 72% for F. ovina (Bojnord) (Table 4). Root dry weight responded in a similar manner. In general, these species were sensitive to decreased soil water content. However, sensitivity indices (SI 50%) at 50% FC for E. elongatum except from the Gorgan and Bojnord sites, had the least reduction in shoot dry weight, and root dry weight either increased (Semnan) or was unchanged (Bojnord) (Table 4). Root dry weight for Bromus tomentellus collected from Bojnord increased by 43% with decreasing soil water content.
Correlation
High correlation coefficients occurred between traits, particularly at 50% FC (Table 5). Negative correlations occurred between final emergence and T0 at 75% FC (–0.42, P < 0.01) and 50% FC (–0.69, P < 0.001). Negative correlations occurred between seedling vigor index (SVI) and T50 at FC (–0.38, P < 0.05), 75% FC (–0.49, P < 0.01), and 50% FC (–0.69, P < 0.001). With decreasing soil water content, associations between characters were increased and species with high vigor emerged earlier under low soil water (Table 5).
View this table:
[in this window]
[in a new window]
|
Table 5. Correlation matrix among seven measured traits on 36 perennial cool-season grass populations native to Iran at three soil moisture levels in seedling emergence stage.
|
|
Principal Component and Cluster Analysis
The PC analysis grouped populations on the basis of traits. The first three PCs explained 83.1% of the total variation observed. The first (PC1) and second (PC2) accounted for 63.6% and 14.3% of the total variation and were used for graphical presentation of the populations (Table 6). Eigenvectors for PC1 and PC2 and measured characters revealed associations between PC1 and high final emergence and emergence rate, and shoot dry weight under low soil water conditions. Whereas characters such as root length at FC and 75% FC showed positive association, shoot length had a negative correlation with PC2 at three soil water levels (Table 6). On the other hand, as shown in Fig. 1
in all species (from the left to right), when PC1 increased, T0 and T50 declined and emergence percentage increased under low soil water. Whereas, with increasing PC2 axes (from the bottom to top), relative stability of root dry weight under low soil water was increased. The PCA biplot also showed that S. montanum and D. glomerata had the highest and lowest variations, respectively (Fig. 1).
View this table:
[in this window]
[in a new window]
|
Table 6. Eigenvectors from the first three principal components axes of the traits used to classify 36 perennial cool-season grass populations native to Iran into five clusters in seedling emergence stage at three soil moisture levels except final emergence.
|
|
View larger version (16K):
[in this window]
[in a new window]
|
Fig. 1. Biplot for diversity of 36 perennial cool-season grass population native to Iran at three soil moisture (FC and 75 and 50% FC) in seedling emergence stage, showing which tolerant species to low soil moisture had higher values PC1 and PC2. PC1 and PC2 are first and second principal component. Species names are defined in Table 1.
|
|
On the basis of the results of cluster and discriminant analysis, the populations were assigned to five clusters (Fig. 2
and 3)
. In Cluster I, the single population was S. montanum collected from Semnan and had on average 4.2 and 3.5 d faster emergence than the overall mean at T0 and T50 at 50% FC, respectively (Table 6). This cluster had the largest germination percentage (71%) at 25% FC (Table 3) and had higher values for all the measured characters as compared with the other populations. However, the relative stability of root dry weight (67% sensitivity index) and length (76%) were 23% and 42% lower than the overall mean, respectively (Table 7). Cluster II, the largest group, contained 13 populations, and five populations were E. elongatum. On average, the populations in Cluster II emerged 0.3 d earlier at T50 and had 29 and 40% longer root and shoot length than the overall mean of populations at 50% FC. Species in this cluster had 23% and 47% higher final emergence at FC and 50% FC, respectively, when compared with the overall mean. Cluster II had higher stability of root dry weight (18% sensitivity index) than shoot dry weight (42%) at 50% FC and had greater stability of root growth in response to low soil water than the other groups (Table 7). Therefore these species (except F. arundinaceae), were the most tolerant to water stress at emergence. The five populations in Cluster III had intermediate final emergence, rapid emergence (1.8 d at T0 and 1.2 d at T50) and increased root length (32%) than the overall mean of all the grasses under low soil water. The populations in this group exhibited higher stability for emergence percentage and shoot dry weight and also had high sensitivity index for T0, T50, and root dry weight. For example, from FC to 50% FC, B. tomentellus collected from Khoramabad had the least reduction ratio (7%) in final emergence, whereas reduction in shoot and root dry weight were 27 and 51%, respectively (Table 4). The seven populations in Cluster IV were characterized by populations with low final emergence percentage, late emergence, low growth potential, and the lowest stability response to low soil water, even at 75% FC. These populations were the most sensitive to low soil water at emergence. Some of the species in this cluster, such as D. glomerata did not emerge at 50% FC. Cluster IV was 98% below the overall mean of all species for dry weight (root and shoot), had a 100% sensitivity index, and was 4.7 d later in T0 and T50 at 50% FC (Table 7). Cluster V included 10 populations, had intermediate final emergence (29%), 1.3 and 0.4 d later emergence for T0 and T50, respectively, and high reduction especially for the shoot and root dry weight (85%) at 50% FC compared with the mean of all grasses (Table 7).
View larger version (12K):
[in this window]
[in a new window]
|
Fig. 3. Canonical discriminant analysis for determining optimal number clusters (dendrogram cutting point). The canonical functions that discriminate among fie different classes are shown.
|
|
|
DISCUSSION
|
|---|
Seed germination and emergence depends on factors associated with genotype, maturation environment, post maturation history and germination environment (Meyer and Allen, 1999), seed dormancy and seed reserves (Forcella et al., 2000), and parental environment during seed development (Fenner, 1991). Germination characteristics are important for the establishment success of grass species (Larson and Bibby, 2004). This investigation indicated that the evaluation method used could effectively to detect variations in seedling emergence of perennial cool-season grasses caused by low soil moisture. The perennial cool-season grasses exhibited high genetic diversity for emergence, emergence rate, and root and shoot growth under low soil water conditions. Considerable variation in response to low soil water occurred among and within species and populations collected from different ecological regions of Iran. Each individual seed within a population has a unique genotype, and this genotype may affect the seed's response to environmental stimuli (Forcella et al., 2000).
Multivariate analysis has been used to group populations in genetic diversity studies (Mohammadi and Prasanna, 2003; Bayuelo-Jimenez et al., 2002; Garcia et al., 1997). Principal component analysis is often used before cluster analysis to determine the relative importance of classification variables (Berdahl et al., 1999; Bregard et al., 2001). In our study, the populations were grouped into five clusters comprised of low soil water tolerant (II), intermediate (I and III), and susceptible (IV and V) clusters in response to low soil water at germination and emergence.
Cluster II species had high final emergence, rapid emergence, and greater stability, particularly in root growth (except F. arundinaceae) under low soil water. Secale montanum collected from Semnan had the largest seed size in cluster I and performed well in high soil water conditions. However, on the basis of relative stability (Fernandez and Reynolds, 2000), low soil water severely affected root (67% sensitivity index) and shoot (22% sensitivity index) dry weight compared with FC. The decline in root growth is incompatible with a high investment in root biomass under water stress, which may be critical for drought resistance (Orians and Solbrig, 1977; Lambers et al., 1998). This species also had the highest final germination percentage (71%) at 25% FC (–1.5 MPa). Although it has a high probability to germinate under severe drought stress, the germinated seeds were not able to emerge and all died. Since arid and semiarid regions have low and unreliable rainfall, high seed germination occurring at low soil water content can result in a low probability of seedling survival (Bassiri et al., 1988). This could result in decreasing the soil seed bank for species such as S. montanum in pastures during wet–dry periods, whereas ungerminated seeds of other species may be able to recover at the end of a drought. Thus, S. montanum in Clusters I and III have both positive and negative consequences for germination and seedling emergence under low soil water, and were characterized as intermediate to low soil water tolerance.
Populations in Clusters IV and V, such as D. glomerata and F. ovina with low final emergence, delayed emergence, and high sensitivity index for root and shoot growth, were the most sensitive grasses at germination and emergence under low soil water. However, Cluster IV was more sensitive to low soil water than Cluster V. This would support the result reported earlier by Otsus and Zobel (2004) that showed F. ovina had the least tolerance to low soil water in a laboratory germination experiment.
A high correlation coefficient among seedling vigor index (SVI) and the other traits in our work, particularly at 50% FC, indicated that it could be used as a selection criterion for improving low soil water tolerance at germination and emergence in cool-season grasses (Table 5). A positive correlation between seed size and seedling vigor of forage grasses has been reported previously (Asay and Johnson, 1983). Our results indicated that small seeded species had lower SVI and dry matter, although there were considerable differences in the geographic origin of accessions. It is possible that seedling establishment under drought stress for populations in Clusters IV and V may be improved by pre-germinating or osmo-conditioning seeds as suggested by several researchers (e.g., Mueller and Bowman, 1989; Mueller, 1996; Hardegree and Van Vactor, 2000; Hardegree et al., 2002).
One of the most important characteristics affected by low soil water was the delay in emergence rate. Populations in clusters I, II, and III required an average of 8.6 d while clusters IV and V need at least 12.8 d to reach T50 emergence at 50% FC. Our results for populations in cluster II, except F. arundinaceae, supported the idea that early root initiation and rapid root extension can help seedlings avoid low soil water conditions (Harris and Wilson, 1970; Johnson and Asay, 1993).
In our study, the effect of low soil water content was more drastic on shoot than root length, particularly at low soil water for all species. Therefore, root growth may be a better indicator of establishment and survival than shoot growth in perennial cool-season grasses for identifying populations tolerant to low soil water conditions. Mueller and Bowman (1989) reported that greater root biomass and root length in crested wheatgrass and western wheatgrass (Pascopyrum smithii Rydb.) should aid in establishment and survival of plants during critical early stages of seedling development. In this study, E. elongatum, and B. tomentellus in cluster II had no reduction in root growth and had greater stability at germination and emergence under low soil water conditions. These traits could be used to further understand the drought tolerance mechanisms for plant breeding programs.
We conclude that great genetic diversity for emergence exists among and within species of perennial cool-season grasses at different soil water levels. Using the methods described in this investigation, we separated low soil water tolerant and low soil water susceptible populations. This could be important for renovating or improving pastures by introducing low soil water tolerant species into arid and semiarid regions. A better understanding of ecological requirements and physiological characterizations of perennial cool-season grasses would be required for improving emergence and, ultimately, seedling establishment.
|
ACKNOWLEDGMENTS
|
|---|
The authors wish to thank Gene Bank of the Rangelands and Forestry Research Institute of Iran for providing seeds, Dr. S. A. Mohammadi for his assistance in statistical analysis and Dr. A.V. Azghandi and Dr. B. Ghareyazi for their critical reviews of the manuscript.
Received for publication June 14, 2004.
|
REFERENCES
|
|---|
- Abdul-Baki, A.A., and J.D. Anderson. 1973. Relationship between decarboxylation of glutamic acid and vigor in soybean seed. Crop Sci. 13:227–232.[Abstract/Free Full Text]
- Asay, K.H., and D.A. Johnson. 1983. Genetic variability for characters affecting stand establishment in crested wheatgrass. J. Range Manage. 36:703–706.
- Bassiri, M., A.M. Wilson, and B. Grami. 1988. Dehydration effect on seedling development of four range species. J. Range Manage. 41:383–386.
- Bayuelo-Jimenez, J.S., R. Craig, and J.P. Lynch. 2002. Salinity tolerance of Phaseolus species during germination and early seedling growth. Crop Sci. 42:1584–1594.[Abstract/Free Full Text]
- Berdahl, J.D., H.F. Mayland, K.H. Asay, and P.G. Jefferson. 1999. Variation in agronomic and morphological traits among Russian wildrye accessions. Crop Sci. 39:1890–1895.[Abstract/Free Full Text]
- Bewley, J.D. 1997. Seed germination and dormancy. Plant Cell 9:1055–1066.[CrossRef][ISI][Medline]
- Bregard, A., G. Belanger, R. Michaud, and G.F. Tremblay. 2001. Biomass partitioning, forage nutritive value, and yield of contrasting genotypes of timothy. Crop Sci. 41:1212–1219.[Abstract/Free Full Text]
- Cox, J.R., and G.L. Jordan. 1983. Density and production of seeded range grasses in Southeastern Arizona (1970–1982). J. Range Manage. 36:649–652.
- FAO (Food and Agriculture Organization). 2003. Statistical data base. The state of Iran Agri-Food country profile. Rome.
- Fenner, M. 1991. The effects of the parent environment on seed germinability. Seed Sci. Technol. 1:75–84.
- Fernandez, R.J., and J.F. Reynolds. 2000. Potential growth and drought tolerance of eight desert grasses: Lack of trade-off? Oecologia 123:90–98.[CrossRef][ISI]
- Forcella, F., R.L. Benech Arnold, R. Sanchez, and C.M. Ghersa. 2000. Modeling seedling emergence. Field Crops Res. 67:123–139.[CrossRef]
- Frasier, G.W., J.R. Cox, and D.A. Woolhiser. 1987. Wet-dry cycle effects on warm-season grass seedling establishment. J. Range Manage. 40:2–6.
- Garcia, E.H., C.B. Pena-Valdivia, J.R. Rogelio Aguirre, and J.S.M. Muruaga. 1997. Morphological and agronomic traits of a wild population and an improved cultivar of common bean (Phaseolus vulgaris L.). Ann. Bot. 79:207–213.[Abstract/Free Full Text]
- Hardegree, S.P., A.J. Thomas, and S.S. Van Vactor. 2002. [(Raf.) Swezey and (J.G. Smith) M.E. Jonse] Variability in thermal response of primed and non-primed seeds of Squirrel tail. Ann. Bot. 89:311–319.[Abstract/Free Full Text]
- Hardegree, S.P., and S.S. Van Vactor. 2000. Germination and emergence of primed grass seeds under field and simulated-field temperature regimes. Ann. Bot. 85:379–390.[Abstract/Free Full Text]
- Harris, G.A., and A.M. Wilson. 1970. Competition for moisture among seedling of annual and perennial grasses as influenced by root elongation at low temperature. Ecology 51:530–534.[CrossRef]
- Hassanyar, A.S., and A.M. Wilson. 1978. Drought tolerance of seminal lateral root apices in crested wheatgrass and Russian wildrye. J. Range Manage. 31:254–258.
- Isselstein, J., J.R.B. Tallowin, and R.E.N. Smith. 2002. Factors affecting seed germination and seedling establishment of Fen-Meadow species. Restor. Ecol. 10:173–184.[CrossRef]
- Johnson, D.A., and K.H. Asay. 1993. Viewpoint: Selection for improved drought response in cool-season grasses. J. Range Manage. 46:194–202.
- Jordan, G.L. 1970. Effective use of available moisture on the San Simon watershed. Bureau of Land Management Progress Report.
- Lambers, H., F.S. Chapin, and P.J. Fonteyn. 1998. Plant physiological ecology. Springer, New York.
- Larson, S.U., and B.M. Bibby. 2004. Use of germination curves to describe variation in germination characteristics in three turfgrass species. Crop Sci. 44:891–899.[Abstract/Free Full Text]
- Meyer, S.E., and P.S. Allen. 1999. Ecological genetics of seed germination in Bromus tectorum L. II. reaction norms in response to a water stress gradient imposed during seed maturation. Oecologia 120:35–43.[CrossRef][ISI]
- Mohammadi, S.A., and B.M. Prasanna. 2003. Analysis of genetic diversity in crop plants-salient statistical tools and considerations. Crop Sci. 43:1235–1248.[Abstract/Free Full Text]
- Mozaffarian, V. 1996. A dictionary of Iranian plant names. Farhang Moaser, Tehran.
- Mueller, D.M. 1996. Germination and root growth of 4 osmoconditioned cool-season grasses. J. Range Manage. 49:117–120.
- Mueller, D.M., and R.A. Bowman. 1989. Emergence and root growth of three pregerminated cool-season grasses under salt and water stress. J. Range Manage. 42:490–495.
- Orians, G.H., and O.T. Solbrig. 1977. A cost-income model of leaves and roots with special reference to arid and semiarid areas. Am. Nat. 111:677–690.[CrossRef][ISI]
- Otsus, M., and M. Zobel. 2004. Moisture conditions and the presence of bryophytes determine fescue species abundance in a dry calcareous grassland. Oecologia 138:293–299.[CrossRef][ISI][Medline]
- Rohlf, F.J. 1998. NTSYS-pc, Numerical taxonomy and multivariate analysis system, version 2.02. Exeter Software, New York.
- SAS. 1996. SAS/STAT User's guide, Release 6.12 ed. SAS Institute, Cary, NC.
- SPSS. 2001. SigmaStat statistical software user's manual, version 11 for windows. SPSS Inc., Chicago, IL.
- Usberti, R., and I.F.M. Valio. 1997. Osmoconditioning effects on germination of Guinea grass (Panicum maximum) seeds. Seed Sci. Technol. 25:303–310.
- Ward, J.H. 1963. Hierarchical grouping to optimize an objective function. Am. Statist. Assoc. J. 56:236–244.
TOP
ABSTRACT
INTRODUCTION
