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PLANT GENETIC RESOURCES

Aegilops Species from Semiarid Areas of Lebanon: Variation in Quantitative Attributes under Water Stress

Faculty of Agricultural and Food Sciences, American Univ. of Beirut, P.O. Box 11-02361, Beirut, Lebanon

^* Corresponding author (riadbaal{at}aub.edu.lb)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Aegilops species in semiarid areas constitute potentially important gene sources for abiotic stress resistance of domesticated Triticum species. Drought tolerance of several Aegilops species was evaluated, and the structure and extent of variation in quantitative attributes of those species when subjected to different degrees of water stress was evaluated. Twenty-one populations belonging to six species were collected: Ae. biuncialis Vis., Ae. cylindrica Host, Ae. geniculata Roth, Ae. markgrafii (Greuter) Hammer, Ae. triuncialis L., and Ae. vavilovii (Zhuk.) Chennav. Quantitative attributes, namely above-ground dry weight, plant height, tillers per plant, days to maturity, productive tillering, spike length, kernels per spike, seed number, seed weight, and yield, were evaluated at three levels of moisture stress. On the basis of changes in measured quantitative attributes under different levels of drought stress, Ae. geniculata and Ae. markgrafii were found to be the most drought tolerant species. Attributes accounting for most of the variation under different levels of water stress were total seed number, seed weight per spike, total number of tillers, and productive tillering capacity, although the extent of each attribute's variation depended on water stress level. Under severe water stress, the ability of plants to produce many fertile tillers with few large seeds seems to be an important adaptive mechanism which should be considered in evaluating plants for drought tolerance.

Abbreviations: PCA, principal component analysis

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
THE GENUS Aegilops (Poaceae) is characterized as Mediterranean–western Asiatic (Blumler, 1994; Hegde et al., 2002; Valkoun, 2001; van Slageren, 1994), and its center of diversity, including regions of Lebanon and Syria, comprises semiarid areas subject to frequent droughts. Aegilops is closely related to Triticum (Kerby and Kuspira, 1988) and an important genetic resource for bread (Triticum aestivum L.) and durum (T. turgidum L.) wheat improvement (Hegde et al., 2002; Valkoun, 2001; Zaharieva et al., 2003). Aegilops cylindrica is a probable D genome donor to hexaploid wheat (AABBDD), and a homologous relationship was found between chromosomes of Ae. geniculata and wheat (Asghar et al., 2001; Farooq and Azam, 2001). Other Aegilops species, such as those belonging to the Sitopsis group (Ae. speltoides Taush, Ae. bicornis Jaup et Sp., Ae. sharonensis Eig, Ae. longissima Schwein, and Ae. searsii Feld. and Kislev) are thought to be the most probable B genome donors to wheat (Khlestkina and Salina, 2001). Aegilops species adapted to growth under limited water availability in semiarid areas are therefore a potential reservoir of genes for improving drought tolerance of cultivated wheats.

Aegilops genetic resources have been intensively collected for more than 30 yr, with more than 22000 accessions of 22 known species in genebanks around the world (van Slageren, 1994). While these genetic resources have been successfully exploited to improve disease resistance in wheat (e.g., Barloy et al., 2000; McIntosh and Lagudah, 2000; Thiele et al., 2002), little use has been made of them for physiological improvement (Skovmand et al., 2001) with limited economic benefits achieved by transferring drought and heat resistance traits to wheat from species such as Ae. tauschii, Ae. geniculata, and Ae. cylindrica (Farooq et al., 1996; Mujeeb-Kazi et al., 1996; Valkoun, 2001; Zaharieva et al., 2001a). A thorough morphological and phenological characterization of qualitative and quantitative traits of Aegilops species under varying moisture stress is needed. Such characterization should lead to more accurate identification of target traits for incorporation in breeding programs. The imprecise nature of qualitative morphological characters as indicators of genetic potential has justifiably led many researchers to concentrate on biochemical and molecular approaches in evaluating wild populations (Dvorak et al. 1998a,1998b; Hegde et al., 2000; Pestsova et al., 2000; Sasanuma et al., 2002). The same limitation applies to quantitative traits, largely avoided when evaluating germplasm because of their high susceptibility to environmental influences. However, evaluating quantitative traits can provide the background against which genetic as well as qualitative phenotypic variation is compared. Valuable insight into the processes determining the phenotype, similar adaptation patterns, presence of useful genes, as well as evolutionary and taxonomic significance can be gained by studying the sources and extent of quantitative variation under different environmental conditions (Ortiz et al., 1998; Pfenninger and Magnin, 2001). When investigating the quantitative variation of a number of related species, Keddy (1992) recommended measuring many traits on a large number of species. An alternative approach is to select groups of traits of potential functional significance, each group reflecting a different emphasis on plant processes of interest, such as vegetative and reproductive growth under varying moisture stress. The latter approach combines large and complex data sets into smaller and more easily interpreted sets of attributes, including traits of known or potential adaptive value (Willby et al., 2000).

The objectives of this investigation were to evaluate drought tolerance of several Aegilops species, to study the structure and extent of variation in quantitative attributes of those species when subjected to different degrees of water stress, and to identify quantitative attributes that can be used in evaluating Aegilops germplasm for breeding purposes.

	MATERIALS AND METHODS

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Plant Materials
Seeds from 21 populations of six Aegilops species were collected: Ae. biuncialis, Ae. cylindrica, Ae. geniculata, Ae. markgrafi, Ae. triuncialis, and Ae. vavilovii. A single patch of conspecific individuals separated from other individuals of the same species by more than 100 m was considered as one population. Seed samples from each population consisted of bulked seed from one spike collected from each of 20 plants at least 1 m apart. Seeds were collected in the spring–summer period from 14 locations representing typical variation within the semiarid Beqa'a region of Lebanon (Table 1), with an annual yearly precipitation ranging from 400 to 600 mm, almost all of which fell in the period from October to April, and a dry season, coinciding with elevated temperatures, from late May to early September. The lowest temperatures, which normally occur in January, are around 0°C with a monthly average of 5°C, while the highest temperatures, normally recorded in July, sometimes exceed 35°C with a monthly average of 26°C. Depending on occurrence, one or more species were collected from each location. The total collection area did not exceed 100 km².

Days to maturity were the number of days from germination to physiological maturity, when 50% of culms per plant had yellow uppermost internodes. Plant height was measured as the distance from the soil surface to spike tip of the main stem, excluding awns, and plant dry weight was the total above ground dried biomass. Productive tillering capacity was the total number of fertile tillers per plant at maturity, and the total number of tillers per plant was the sum of fertile and non-fertile tillers. Spike length was the average of five randomly selected spikes per plant, from the base to the tip of the highest spikelet, excluding awns, and the number of kernels per spike was the average seed number based on the same five spikes. Total seed number per plant was obtained after mechanically threshing all fertile spikes from a single plant. Seed weight per spike was the total yield per plant divided by the total number of fertile tillers per plant.

Analysis of variance was performed with species and moisture levels as fixed factors replicated in three blocks, and populations within species effects were partitioned into five sources of variation, one for each species, excluding Ae. cylindrica, which had only one population. Since population effects were mostly not significant for all species (p 0.05), reported means are the averages of blocks and populations. Differences among the three water level treatment means within each specie were determined for each attribute using Bonferroni's test (p 0.05) (Miller, 1991). Pearson's simple linear correlation coefficients were calculated for each water stress treatment using attribute values averaged over all populations and blocks. Principal component analysis (PCA) was performed on species for each water stress treatment. Calculations of eigenvectors were made using correlation matrices with Kaiser's varimax rotation of axes (Kaiser, 1958).

	RESULTS

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Analysis of variance revealed that population effects within species in general affected quantitative attributes less than water stress level and species (Table 2). To facilitate the interpretation of results, quantitative attributes were divided into two groups. The first group was composed of vegetative growth attributes (days to maturity, plant height, tiller number, and above-ground dry weight). The second was composed of reproductive attributes (productive tillers, kernels per spike, seed number, spike length, seed weight per spike and yield). Above-ground dry weight of all species decreased as moisture stress increased (Table 3). However, with one exception, reductions were only significant under severe moisture stress. Species did not respond uniformly to water stress in terms of plant height: height of Ae. markgrafii, Ae. geniculata, and Ae. triuncialis did not respond to water stress. Height of Ae. biuncialis, Ae. cylindrica, and Ae. vavilovii was reduced under severe moisture stress. Aegilops cylindrica was the only species to show height increase under moderate water stress. Soil moisture level had no effect on total tiller number of Ae. markgrafii, Ae. cylindrica, or Ae. triuncialis, while tiller number of Ae. biuncialis and Ae. vavilovii decreased with increased moisture stress. Aegilops geniculata tillers increased under moderate water stress compared with the control but decreased under severe stress. Maturity was delayed with increasing stress only for two species: Ae. biuncialis and Ae. markgrafii. While none of the vegetative attributes were correlated in the absence of stress, height, and above-ground dry weight were positively correlated under moderate and severe stress, as were days to maturity and tiller number (Table 4).

To gain a better understanding of the sources of variation across species, and the change in attribute variation with changing water stress, PCA analysis was performed (Table 5). Three principal components were sufficient to describe the main features of the data and reduce its dimensionality at all stress levels, with little loss of information. In the absence of stress, the main explanatory variables were total seed number and seed weight per spike (PC1), total number of tillers and productive tillering capacity (PC2) and above-ground dry weight (PC3). Subjecting plants to moderate stress reversed the main explanatory variables, with total tillers and productive tillering capacity accounting for most of the observed variation (PC1), followed by seed number and seed weight (PC2). Only three attributes differed among species under severe water stress, productive tillering capacity (PC1), seed weight per spike (PC2) and above-ground dry weight (PC3). A plot of PC1 and PC2 based on the no stress treatment did not clearly separate species or populations within species (Fig. 1 ). In contrast, under moderate moisture stress, two groups were clearly distinguishable along PC1. The first, with negative coordinates, included Ae. biuncialis, Ae. markgrafii, and Ae. vavilovii. The second, with positive coordinates, included populations of Ae. cylindrica, Ae. geniculata, and all but one of the populations of Ae. triuncialis (Fig. 2 ). PC2 further separated the second group into two sub groups, with Ae. cylindrica and Ae. triuncialis in one and Ae. geniculata in the other. Under severe moisture stress, populations of Ae. geniculata and Ae. triuncialis differed mostly with respect to PC1, with very little variation along the PC2 axis and with no distinct clustering (Fig. 3 ). In contrast, most Ae. biuncialis populations formed one group, along with Ae. cylindrica, with negative coordinates, and Ae. vavilovii and Ae. markgrafii populations clustered near the center.

View larger version (15K): [in this window] [in a new window]   Fig. 1. Associations among different populations of six Aegilops species, Ae. biuncialis, Ae. cylindrica, Ae. geniculata, Ae. markgrafii, Ae. triuncialis, and Ae. vavilovii, as revealed by principal component analysis from quantitative attribute data under no moisture stress. Numerals next to species symbols are population numbers.

View larger version (16K): [in this window] [in a new window]   Fig. 2. Associations among different populations of six Aegilops species, Ae. biuncialis, Ae. cylindrica, Ae. geniculata, Ae. markgrafii, Ae. triuncialis, and Ae. vavilovii, as revealed by principal component analysis from quantitative attribute data under moderate moisture stress. Numerals next to species symbols are population numbers.

View larger version (15K): [in this window] [in a new window]   Fig. 3. Associations among different populations of six Aegilops species, Ae. biuncialis, Ae. cylindrica, Ae. geniculata, Ae. markgrafii, Ae. triuncialis, and Ae. vavilovii, as revealed by principal component analysis from quantitative attribute data under severe moisture stress. Numerals next to species symbols are population numbers.

	DISCUSSION

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Vegetative growth of all species was largely unaffected by moderate water stress, with no reduction in above-ground dry weight for any of the species except Ae. triuncialis (Table 3), reflecting the species' adaptation to semiarid environments with limited water availability. However, above-ground dry weight of all species significantly declined under severe stress. On the basis of changes in vegetative attributes with increasing water stress, Ae. geniculata appeared to be the most drought tolerant species. Its vegetative attributes, especially above-ground dry weight, were the least affected by severe water stress (Table 3), in agreement with conclusions by Rekika et al. (1998). Zaharieva et al. (2001a, 2003) also observed that Ae. geniculata achieved high biomass under drought and was more tolerant than Ae. cylindrica. While Guzy et al. (1989) found that biomass variation of several species of wild and cultivated Triticum accessions under a range of conditions was due to differences in tillering, our results indicated that the decrease in biomass of tested Aegilops species under drought was associated with plant height rather than tiller number (Table 4). Gupta et al. (2001) also reported that plant height rather than tiller number of differentially drought tolerant wheat cultivars decreased with imposed water stress. Our results therefore demonstrate that plant height is a better indicator than tiller number for drought tolerance of Aegilops species, and the same might also apply to other wild grasses.

Water stress is reported to cause a reduction in number of days to heading of many grass species such as Ae. geniculata (Zaharieva et al., 2001a, 2001b), indicating a quicker overall development rate under stress. However, using days to maturity as a measure of growth rate showed that drought slowed the development of Ae. biuncialis, Ae. markgrafii, and Ae. triuncialis (Table 3). Tillers, although stunted and slender, remained succulent and retained a pale green color for a longer period under stress compared with healthy control plants, but the lack of correlation with yield demonstrated that this increase did not improve yield (Table 4). Blum (1996) reviewed the responses of several crops to drought and found that water stress more often than not delays flowering. In some cases, such as that of corn (Zea mays L.) (Byrne et al., 1995), drought caused a delay in silking but not anthesis, increasing the anthesis-to-silking period. In light of the above, the interrelationship among days to maturity, heading and anthesis under drought should be further investigated to determine the validity and utility of using days to maturity as an indicator of drought tolerance.

Seed yield as well as biomass should be considered when evaluating wild accessions. On the basis of changes in reproductive attributes under increasing water stress, Ae. markgrafii appeared to be drought tolerant. Although the yield of Ae. markgrafii and Ae. triuncialis was not affected by water stress, none of the reproductive attributes of Ae. markgrafii were affected by stress level (Table 3). Aegilops markgrafii has also been found to be heat tolerant in terms of biomass, yield and seed weight (Khanna-Chopra and Viswanathan, 1999). In contrast, all reproductive attributes of Ae. cylindrica were significantly reduced with increased moisture stress, indicating its low tolerance to drought. The importance of yield stability of wild species under varying levels of water stress should not be exaggerated and should be evaluated in conjunction with yield potential. In this study, species with relatively high yield under no stress were the ones that suffered significant yield reductions (Table 3). Aegilops geniculata's yield was significantly reduced but was still higher under severe stress than Ae. markgrafii's, making it a more useful source of genes for wheat improvement since it was also the most tolerant in terms of vegetative attributes. Other studies have shown yield stability of wheat wild relatives to be associated with poor yield potential (Basnal and Sinha, 1991; Khanna-Chopra and Viswanathan, 1999). Spike length and kernels per spike were less subject to change under varying water availability, but were not correlated with yield or its components, suggesting their inadequacy as potential indicators of drought tolerance. In contrast to cultivated wheat, which shows a positive correlation between biomass and yield, even under contrasting soil moisture levels (Khanna-Chopra and Viswanathan, 1999; Villegas et al., 2001), we found no correlation between above-ground dry weight and yield of the six Aegilops species. This was probably because biomass was mainly dependent on height, an attribute unrelated to either yield or its components.

PCA analysis reduced the nine original variables to three new explanatory variables or components. In the absence of stress, the main explanatory attributes belonged to two groups, a "seed yield" group (seed number and seed weight per spike), which accounted for most of the variation, and a "tiller" group (number of tillers and productive tillering capacity). Subjecting plants to moderate stress reversed the main explanatory attributes, with the tiller factor accounting for most of the variation, and the seed yield factor becoming a less important source of variation. The ability to develop productive tillers rather than high tillering capacity was the most important factor contributing to variation under severe stress, with the first component accounting for 41.4% of the total variation, followed by the plants' ability to produce large or heavy seeds (Table 5). Under all stress levels, biomass, expressed either as above-ground dry weight or height, was a minor source of variation across species. These results illustrate the change in attribute variation and shift in adaptive strategy as water becomes less available and support conclusions by Zaharieva et al. (2003) that part of the observed morphological variation in three studied Aegilops species was due to adaptation to environmental constraints. When soil water does not limit vegetative growth, seed number and weight are the most important characters contributing to plant differences. As water availability decreases, the main differentiating feature becomes tillering capacity, and as stress becomes severe and survival is at stake, total tillers and seed number are reduced across species, with the ability to produce fertile tillers with few large seeds becoming a distinguishing feature, as also suggested by Blum (1996).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Improvements in wheat yield through enhanced drought tolerance can be achieved by exploiting genetic resources of adapted wild relatives as well as identifying useful traits to be included in breeding programs. Grouping quantitative traits into vegetative and reproductive attributes proved to be an effective way of identifying tolerant germplasm. Two drought tolerant species were identified, Ae. geniculata and Ae. markgrafii. Aegilops geniculata had a high seed yield compared with other species and its vegetative attributes were least affected by water stress. On the other hand, none of the reproductive attributes of Ae. markgrafii, including yield, was reduced because of water stress. Plant height, seed weight, seed number, and productive tillers were the most useful single traits for inclusion in a set of selection criteria for drought tolerance. However, a better approach to screening wild species would be to identify groups of traits (equivalent to new variables) with the highest contribution to overall variation under different water stress conditions. On the basis of our results, two new variables were identified and should be considered in selection programs, a seed yield and a tiller variable. While the seed yield variable, composed of seed number and weight, was a more important factor to consider under conditions of adequate moisture, the tiller variable, which included number of total and productive tillers, proved to be of higher relevance as a selection criterion under conditions of drought stress. Under severe water stress, the ability of plants to produce fertile tillers with few large seeds was identified as a distinguishing characteristic that should be considered in evaluating plants for drought tolerance.

	ACKNOWLEDGMENTS

 
We gratefully acknowledge support of the Conservation and Sustainable Use of Agrobiodiversity in the Drylands project, GEF/UNDP and the Lebanese Agricultural Research Institute.

Received for publication February 5, 2005.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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