A Database for Triticeae and Avena
1 Institute of Evolution
University of Haifa
Mount Carmel
Haifa 31905, Israel
2Plant Physioloy Lab.
Faculty of Agricultural Engineering
Technion - Israel Institute of Technology
Haifa 32000, Israel
Water deficit occurs when water potentials in the rhizosphere are sufficiently negative to reduce water availability to sub-optimal levels for plant growth and development. On a global basis, it is a major cause limiting productivity of agricultural systems and food production (Boyer 1982). In cereal crops which provide the major carbohydrate staples for humans, even intermittent water stress at critical stages may result in considerable yield reduction (Ludlow & Muchow 1990) and crop failure (Elmourid et al. 1995). However, the wild progenitors of crop species are often found to be relatively drought-resistant since they grow in environments that are far more adverse than crop environments (Hancock 1992; Nevo 1992; Richards 1993). For example, wild barley (Hordeum spontaneum) inhabiting the arid Negev, Judean and Samaria Deserts, has to complete its life cycles under severe and unpredictable drought conditions (Gutterman 1993; McKersie & Leshem 1994). Moreover, wild cereals have been shown to be a repository of characteristics important for drought resistance since they possess vast genetic diversity which may be missing in crop species (Nevo et al. 1984, 1986; Nevo 1992).
When plants are subjected to drought stress, a number of physiological responses have been observed (Ludlow & Muchow 1990; Fukai & Cooper 1995). In some cultivated cereals, osmotic adjustment has been found to be one of the most effective physiological mechanisms underlying plant resistance to water deficit (Turner & Jones 1980; Morgan 1984; Blum 1988; Zhu et al. 1997). Osmotic adjustment, as a process of active accumulation of compatible osmolytes in plant cells exposed to water deficit, may enable (1) a continuation of leaf elongation, though at reduced rates (Turner 1986); (2) stomatal and photosynthetic adjustments (Morgan 1984); (3) maintained root development and soil moisture extraction (Morgan & Condon 1986); (4) delayed leaf senescence (Hsiao et al. 1984); (5) better dry matter accumulation and yield production for crops in stressful environments (Boyer 1982; Blum 1988). Recently, mapping the single gene and/or quantitative trait loci (QTLs) for osmotic adjustment in cultivated cereals has been conducted in wheat (Morgan & Tan 1996), rice (Lilley et al. 1996) and barley (Teulat et al. 1998). The data obtained by means of combining the approach of stress physiology with molecular genetics look very promising. However, up to now the physiological mechanisms underlying more pronounced drought-resistance of wild cereals and the genes involved have not been elucidated. The ultimate aims of this project are to identify the specific physiological mechanisms at the whole-plant and cellular levels responsible for drought resistance in Hordeum spontaneum, and then map the candidate genes associated with these mechanisms by use of DNA-based molecular markers.
We briefly report here results of the first stage of this project. These concern the differences between wild desert barley and a modern cultivar in resistance to a uniform water deficit and the underlying mechanisms. Accession 23-39 of wild barley (Hordeum spontaneum) was collected from Wadi Qilt, an area of low rainfall (100-250 mm) in the Judean Desert, Israel. Hordeum vulgare cv. Mona is an elite spring barley cultivar grown in Sweden. The seedlings were cultured hydroponically in the aerated 0.1-strength nutrient solution containing extra 5 mM calcium chloride (Kirkham et al. 1969). Experimental conditions in the growth chamber were: 12 h photoperiod (35 W m-2 PAR); 25ñ1 °C; R.H. 35% day, 60% night. Water deficit at -0.4 MPa external water potential was gradually imposed by adding polyethylene glycol (PEG 6000) to the root nutrient solutions. PEG was used as an osmolyte because it is an inert, non-toxic and non-penetrating solute in plant research, unlike other osmolytes such as mannitol, sodium chloride and sugar (Kramer & Boyer 1995). All the measurements were made at 37 d after the onset of PEG treatments. Leaf water relations (water potential, osmotic potential and turgor) were psychrometrically assayed in the middle mature tissues of the last fully expanded leaf, as described in Lu & Neumann (1998). Leaf relative water content was determined according to Matin et al. (1989) and water loss rates of excised-leaves as in Clarke & McCaig (1982). Growth at the whole-plant level was measured by: shoot height, elongation rate per week, number of tillers, number of expanded leaves on the main tiller, maximal root length, fresh and dry weights of shoot and root, and ratio of root to shoot dry matter. The genotypic ability to resist drought was evaluated based on the two criteria: - absolute biomass production under drought stress (mg per plant) and relative reduction in biomass by water deficit (%; calculating as (1- Bd/Bc) 100%, where Bd = biomass under water deficit and Bc = control biomass).
Although the tiller number and the shoot fresh weight of the unstressed control plants differed between the two genotypes, other physiological parameters assayed were similar. For example, after 5-week growth under control conditions, each shoot in cv. Mona weighed 307.9 ñ 22.0 mg DW-1, not different from 344.2 ñ 26.6 mg DW-1 in Wadi Qilt 23-39 (means ñ SE, n=10). However, when subjected to -0.4 MPa root water deficit, the shoot growth in cv. Mona (on the basis of dry weight) decreased by 85.2%, as compared with the control plants; while the shoot growth in Wadi Qilt 23-39 was significantly less inhibited (74.8%) by the same root water deficit. In addition, the stressed plants of Wadi Qilt 23-39 produced much more biomass at the whole-plant level than those of contrasting cv. Mona (P < 0.01), as shown in Figure 1. Thus, both criteria for the resistance evaluation indicated that wild desert progenitor is more drought-tolerant than cultivated cv. Mona. By comparing and contrasting the cellular responses to water deficit in intact plants, the mechanisms underlying drought-tolerance strategy in Wadi Qilt 23-39, appeared to be related to the higher ability of osmotic adjustment. Figure 2 shows that in response to -0.4 MPa water deficit, full osmotic adjustment was achieved in the last fully expanded leaf tissues of Wadi Qilt 23-39. In contrast, the expanded leaf tissues of cultivated barley cv. Mona compensated for only 62.5% of the 0.4 MPa change in external water potential during the same period. As a result, the osmotic potential gradient between the last fully expanded leaf and the root water source in Wadi Qilt 23-39 was 1.33 MPa, significantly higher than 1.09 MPa in cv. Mona (P < 0.05) under water deficit. Leaf relative water contents in the stressed plants of Wadi Qilt 23-39 and cv. Mona, were respectively 87.5 ñ 1.1% and 82.0 ñ 0.9%. Both values were also different at the level of p=0.01.
In conclusion, significant differences existed between wild desert barley and cultivated barley in resistance to a uniform root water deficit. These differences appeared to be primarily related to their differing genetic abilities of osmotic adjustment under drought conditions. The findings suggest that further genetic mapping and marker-assisted transfer of the osmotic-adjustment genes harboured in the wild progenitor could improve resistance of cultivated barley grown in water-limited environments.
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