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a UFR Génétique et Amélioration des Plantes, ENSA-INRA, F-34060 Montpellier, France
b Laboratorio de Relacion Suelo-Agua-Planta, Facultad de Ciencias Agrarias y Forestales, Universidad de Chile, Casilla 1004, Santiago, Chile
c CEA/Cadarache, DSV, DEVM, Laboratoire d'Ecophysiologie de la Photosynthèse, F-13108 Saint-Paul-lez-Durance, France
* Corresponding author (m.zaharieva{at}cgiar.org)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: BIOM, plant biomass • CHL, total chlorophyll content • DH, days to heading • 
, carbon isotope discrimination • EPT, epidermal transpiration rate • GW, grain weight per plant • LA, leaf area • LCO, leaf colour • LL, leaf length • PTD, plant temperature depression • RWC, relative water content • SLDW, specific leaf dry weight
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Escape and avoidance traits are likely to play an important role in adaptation to specific environments (Monneveux and Belhassen, 1996). Escape from water, heat, and high radiation stresses can be achieved under Mediterranean conditions by shortening of the growing cycle. Earliness, however, reduces light absorbed by the crop and consequently total biomass and potential yield. Drought avoidance comprises mechanisms involved in water potential maintenance, such as the reduction of stomatal conductance and leaf area. Stomatal closure, which is the most efficient way to reduce water loss, negatively affects CO2 assimilation (Yin et al., 1995). Stomatal transpiration also plays a major role in plant temperature regulation (Chetti et al., 1997; Singh et al., 1997). In wheat, stomatal conductance correlates with canopy temperature depression in a wide range of climatic conditions (Amani et al., 1996; Lu et al., 1998). Epidermal transpiration comprises both a nonstomatal (cuticular) and a stomatal component. The first corresponds to water loss through the leaf epidermis, and the second to transpiration due to incomplete closure of stomates (Kirkham et al., 1980; Muchow and Sinclair, 1989). Epidermal transpiration has proved to be an important trait for adaptation to very dry environments (Clarke et al., 1991). Leaf color, glaucousness, and pubescence also contribute to stress avoidance, by reducing radiation absorbed by the plant and increasing crop albedo (Blum, 1988). Van Oosterom and Acevedo (1992) observed that barley landraces adapted to harsh Mediterranean environments have leaves lighter in color, attributable to decreased chlorophyll content (Watanabe et al., 1995).
Water-use efficiency is closely related to C isotope discrimination () in numerous species including cereals (Hubick and Farquhar, 1989; Araus et al., 1993; Condon et al., 1993; Matus et al., 1996). is a measure of the ratio of stable carbon isotopes (13C/12C) in the plant dry matter compared to the same ratio in the atmosphere (Farquhar and Richards, 1984). It is determined by the intercellular partial pressure of CO2, which is largely regulated by variation in stomatal aperture (Farquhar et al., 1989). Under drought stress, can consequently be considered as a good predictor of stomatal conductance (Condon et al., 1990). Carbon isotope discrimination has been shown to be positively correlated with yield and growth cycle duration (Araus et al., 1997) and negatively correlated with leaf temperature (Acevedo, 1993). In cultivated cereals, wide genotypic variation has been noted for while little is known about variation in wild wheats. Among the diploid Aegilops spp., the lowest mean value was observed for Ae. sharonensis, followed by Ae. searsii (Waines et al., 1993). To our knowledge, no information is available on the variation of and its relationship to other physiological traits in Ae. geniculata.
The main objectives of the present study were (i) to examine the variation in leaf temperature, chlorophyll content, water-status related traits and leaf morphology among accessions of the wild wheat relative Ae. geniculata Roth, (ii) to analyze relationships among those traits and the evolutionary strategies developed by different accessions to regulate leaf temperature and adapt to water deficit, and (iii) to evaluate the consequences of those strategies on water-use efficiency and biomass production. This work forms part of a large wheat improvement program focusing on the use of Ae. geniculata to expand genetic variability, develop alternate plant types and physiological processes, and increase drought stress and high temperature tolerance in durum (T. durum Desf.) and bread wheat (T. aestivum L.).
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Experimental Conditions
Field experiments were carried out at ENSA-INRA Montpellier (4°29'E, 48°46'N, elevation 45 m above sea level) during two successive years (1996–1997 and 1997–1998). The seeds were sown in Jiffy pots and the plants were grown to the three-leaf stage. Seedlings (six per accession for the first year and two for the second) were transplanted to the field using a complete randomized block design with two replications (50 cm inter-plant spacing) at the end of November. The plants received 80 kg ha-1 of N (one-half after transplanting and one-half at tillering). No pesticides were applied and weeds were manually eliminated.
The soil was a sandy-loam (organic matter content 2.1%, pH 7.8) with a depth of about 0.6 m. Total rainfall between 1 November and 31 July was 777.5 mm in 1996 to 1997 and 595 mm in 1997 to 1998 (Fig. 1). The winter was very dry in 1997 to 1998, while in 1996 to 1997 a total of 206.5 mm fell in January. In 1996 to 1997, the balance between rainfall and evapotranspiration was mostly negative from February through June. In 1997 to 1998, the balance was negative in March and from May through harvest. For the month of June, the rainfall was lower and the temperature and evapotranspiration were higher in 1997 to 1998 as compared to 1996 to 1997.
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Plant temperature depression (PTD), total chlorophyll content (SPAD) and relative water content (RWC) were assessed before initiation of leaf senescence. Measurements were made on sunny days, around midday, on 21 May 1997 and 4 June 1998. Mean daily irradiance was 19.2 and 21.6 106 J m-2, and air temperature during measurements around 28.8 and 34.1°C, respectively. Aegilops geniculata is a tufted plant with many tillers and plant temperature was evaluated on individual plants by using a focused infra-red thermometer (Mikron M80G, Wyckoff, NJ, USA). In a preliminary experiment, flag leaf temperature was assessed on 22 accessions using a thermo-couple thermometer and a positive correlation was noted between the two methods (r = 0.72, P < 0.001). Total chlorophyll content was estimated in intact flag leaves in the field using a portable chlorophyll meter (SPAD-502, Soil-Plant Analysis Development (SPAD) Section, Minolta camera, Osaka, Japan). At least five flag leaf blades were measured per plant. On a subset of 38 accessions, photosynthetic pigments were extracted by acetone and the total (a + b) chlorophyll content was determined by colorimetry using the equations of Lichtenthaler (1987). A close correlation (r = 0.77, P < 0.001) was noted between (a + b) chlorophyll content and SPAD values. For measurement of relative water content (RWC), leaf area (LA) and specific leaf dry weight (SLDW), three flag leaf blades per plant were sampled. The leaves were weighed just after sampling (fresh weight, FW) and placed in tubes (containing 2 mL distilled water) with their cut ends dipped in the water. The tubes were then stored in the refrigerator (4°C) for at least 24 h (overnight) to achieve turgidity of the leaves. The leaves were removed from the tube and the saturated fresh weight was determined (SFW). The leaf area (LA) was measured using a Li-Cor (LI-3000, Lincoln, NE, USA) leaf area meter. Finally, the leaves were oven-dried (48 h at 80°C) to obtain dry weight (DW). The specific leaf dry weight (SLDW) was calculated as leaf dry weight per leaf area. The leaf relative water content (RWC) was determined as: [(FW - DW)/(SFW - DW)] (Barrs and Weatherley, 1962).
In 1997 to 1998, epidermal transpiration (EPT) and C isotope discrimination (), determined from a grain sample also were measured. Epidermal transpiration was evaluated on three leaves per plant. The leaves were hydrated, as described above, before the start of the wilting experiment. They were weighed (SFW) and placed horizontally on Petri dishes, in a darkened growth cabinet set at temperature 20°C and relative humidity of 40% (Araus et al., 1991). They were weighed again after 2 h of dehydration (DHW). Epidermal transpiration was then calculated as [(SFW - DHW)/(LA)]/2 (Clarke and McCaig, 1982) and expressed in mg cm-2 h-1.
Carbon isotope (13C/12C) ratio was determined by mass spectrometry at Isotope Services, Inc. (Los Alamos, NM 87544, USA). From the total harvested seeds, a 5 g sample was collected for each plant and ground to a fine powder. An aliquot of 2 mg was used for C isotope analysis. Carbon isotope composition of the samples was calculated as 
13C(
) = [(R sample/R standard) - 1] x 1000, where R is the 13C/12C ratio. Carbon isotope discrimination () values were obtained from a and p according to the formula (Farquhar and Richards, 1984): () = (a - p)/(1 + p), where a and p refer to air and plant, the C isotope composition of air was taken as -8 (Farquhar et al., 1989). Replicate samples differed by <0.20.
Statistical Analysis
The data were analyzed using appropriate procedures of the SAS software package (SAS Institute, 1988). An analysis of variance was performed for each trait using the GLM procedure (Freund et al., 1985) to test the significance of differences among accessions, years and accession by year interaction. Accession effect was considered random and the associated variance 
2 (accession) was estimated using the VARCOMP procedure. To study the influence of the eco-geographical origin of the accessions on expression of phenological, morphological, and physiological traits, we partitioned the accession effect into an origin effect and an accession within origin effect. The accession within origin term was declared random and used as an error term to test the significance of the origin main effect. Correlation analysis was used to determine associations between the traits measured.
To assess the relationship between physiological traits and the eco-geographical origin of Ae. geniculata accessions, a principal component analysis (PCA) was performed on the means of the 15 regions of origin for earliness and six physiological characters. The data of the second year of experimentation (1997–1998), which involved more physiological traits (as epidermal transpiration and C discrimination analysis), were used in the PCA analysis.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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) was positively correlated with plant temperature depression, biomass (Fig. 3a) and grain weight per plant (Fig. 3b). No correlation was found between and leaf chlorophyll content (SPAD). Positive correlations were also observed between days to heading (DH) and each of the measured physiological traits, except epidermal transpiration.
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, r = 0.73, P < 0.001. Specific leaf dry weight was correlated with the second principal component PC2 (r = 0.85, P < 0.001). The first principal component separated Jordanian, Libyan, Tunisian, and Cypriot accessions from the Lebanese and North Mediterranean ones. Accessions of the first group were characterized by earliness, pale green leaves (low chlorophyll content), high water-use efficiency (low ), and low epidermal transpiration rate. In this group, accessions from Jordan were clearly distinguished by their higher SLDW and lower values. The North Mediterranean group was characterized by high values for the studied traits, except for SLDW.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Leaf color has been reported to be an important trait in heat avoidance. Aase (1971) postulated that pale leaf color is likely to be a passive adaptation to high light intensities. Low chlorophyll content would reduce the heating effects of high light intensities by decreasing the light absorbance of the leaf. In our study, however, a positive correlation was found between plant temperature depression and chlorophyll content (Fig. 2a). This correlation was not strongly modified (r = 0.42, P < 0.001) by using DH as a covariable in the analysis. We thus suggest that the role of chlorophyll content (and consequently leaf color) in leaf temperature regulation is rather limited under the experimental conditions of this work (mild heat and drought stress). In some accessions originating from very hot regions (e.g., Jordan), chlorophyll content and leaf color could significantly influence plant temperature (Fig. 2b), as already suggested by Van Oosterom and Acevedo (1992) for barley (Hordeum sativum L.) genotypes originating from the Middle-East. According to Havaux and Tardy (1999), chlorophyll loss also is associated with higher lipid/protein ratio and increased lipid fluidity of the thylakoid membrane, which could confer an increased drought tolerance (Quartacci et al., 1995). The pale green trait is mainly constitutive, since it was expressed in the mild stress conditions of our experiment, and associated with the eco-geographical origin of the accessions (Fig. 4). Chlorophyll loss may be partially induced by elevated temperatures (lower SPAD values in 1997–1998 than in 1996–1997), as already suggested by Tardy et al. (1998). Accessions originating from hot and dry regions also exhibited low epidermal transpiration and leaf area. These traits both contribute to a reduction of water loss and maintenance of leaf water potential. Under mild stress conditions, small leaves could, however, represent an important disadvantage in terms of photosynthetic activity (Rawson et al., 1983).
The two described heat avoidance strategies, namely low chlorophyll content (pale-green character) and high transpiration, could have different effects on water-use efficiency and final yield. Under mild drought conditions, high biomass and grain production was achieved with the dark green accessions from France, Greece, Croatia, Lebanon, Bulgaria, Italy, Portugal, Spain and Turkey. These accessions also were characterized by a low water-use efficiency (high ) which was reported to be associated with high stomatal conductance (Condon et al., 1990; Morgan et al., 1993). Thus, in these accessions, plant temperature could be essentially regulated by stomatal transpiration. Together with late maturity, this trait presumably leads to a greater total CO2 assimilation and hence to a higher biomass and grain production. Relative water content also was found to be high in these accessions. This trait is largely dependent on the capacity to adjust osmotically (Rekika et al., 1998b), which is associated with the maintenance of high stomatal conductance (Rekika et al., 1998a). Conversely, low yields were noted in the pale green accessions from Cyprus, Jordan, Libya and Tunisia under the environmental conditions of this study. Positive correlations between chlorophyll content and grain production were observed in cultivated wheat under high temperature stress by Reynolds et al. (1998), but a direct effect of chlorophyll content on biomass and grain yield through photosynthesis is questionable (Tardy et al., 1998). Low production observed in the pale-green accessions could likely result from stomatal closure which simultaneously reduces CO2 assimilation (and then photosynthetic rate) and transpiration. The decreased transpiration would then result in increased leaf temperature, that may lead to chlorophyll degradation (Tardy et al., 1998). Consequently, low chlorophyll content could be an adaptation to decrease leaf absorbance and reduce leaf heating, as suggested by Havaux and Tardy (1999).
Greater variation among accessions was observed for C isotope discrimination () in Ae. geniculata (7) as compared with most cultivated cereals where the range is around 4 (Deléens et al., 1994). Carbon isotope discrimination was positively correlated to plant temperature depression as previously noted in barley (Acevedo, 1993). Carbon isotope discrimination also was correlated positively with BIOM and GW, which supports the potential interest in using as a predictive criterion for biomass and grain production.
The few reported studies on adaptation to water and heat stresses in Aegilops (Mayoral et al., 1981; Shimshi et al., 1982; Benveniste-Levkovitz et al., 1993; Waines et al., 1993; Rekika et al., 1997, 1998b) were focused on specific traits and did not analyze corresponding adaptive strategies. Furthermore, genetic variation of these traits was generally not described. Our study shows that Ae. geniculata accessions are highly variable for traits related to plant temperature and water status regulation. As a consequence, different strategies can be distinguished according to the climatic constraints and stress intensities encountered by the accessions in their region of origin. Accessions that originated from areas with mild climate (North Mediterranean regions and Lebanon) developed adaptive strategies based on maintenance of high relative water content and stomatal conductance, leading to low water-use efficiency (high ) and higher photosynthetic activity at the leaf and plant levels. All these traits contribute to high biomass production. Conversely, accessions originating from drought and high temperature prone areas of the Middle-East (e.g., Jordan) and North-Africa exhibit low epidermal conductance, low transpiration, and high water-use efficiency (low ). In addition, their pale green color and low chlorophyll content could play an important role in leaf temperature regulation, as suggested for barley (Havaux and Tardy, 1999) while their earliness could permit them to complete their growth cycle before the hottest and driest period. All these traits contribute to a conservative strategy (Blum, 1988) based on escape and avoidance mechanisms. This strategy may contribute to plant survival, but leads to low productivity levels.
The adaptive strategies suggested in our study need further analyses to clarify the relationships among traits and assess their environmental dependency. In the Mediterranean region, Ae. geniculata has an area of distribution very similar to that of cultivated wheat (Van Slageren, 1994) and has been subject to several thousand years of selection. Consequently, it constitutes a good model to study the physiological responses to high temperature and water stress affecting cultivated cereals. The collected information is also useful to promote the use of Ae. geniculata for wheat improvement by identifying traits for introgression and the accessions to be used in wide hybridization programs. Some traits, such as pale green character, low epidermal transpiration, or small leaf area, play an important role in the adaptation of wild species to high temperatures, high irradiance, and drought stress (Blum, 1996). In the case of crop species however, it is also required to examine and estimate the value of stress adaptive traits for yield. The successful utilization of wild relatives in crop improvement for abiotic stress tolerance will depend on our knowledge of their association with productivity. The above-mentioned traits may be associated with low productivity, since they lead to reduced photosynthetic activity. Priority probably needs to be given in introgression programs to traits influencing both production and tolerance to abiotic stress, such as high or high RWC. These traits were observed in Ae. geniculata accessions originating from regions with the most temperate climates. A crossing program between selected Ae. geniculata accessions and wheat has been initiated. We believe that the progeny of these crosses may permit more precise evaluation of the stress adaptive traits and their importance for grain yield.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Received for publication May 1, 2000.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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