This study was conducted to evaluate the application of carbon isotope discrimination (CID) as a selection criterion for improving water use efficiency (WUE) and productivity of barley (Hordeum vulgare L.) under field and droughtstress conditions in a greenhouse. A total of 54 genotypes were screened for variability in CID under field conditions, while 23 genotypes were evaluated under water-deficit conditions in the greenhouse. A survey of leaf CID of 54 genotypes at two field locations showed more than 2.14& difference between extreme genotypes. Significant (P £ 0.05) genotypic variation was found in WUE and CID that had a negative strong correlation. There was a negative correlation between leaf CID and aerial biomass in the greenhouse and among six-row genotypes in the field. Correlations between leaf CID across field locations and across irrigation regimes in the greenhouse were significant (experiment 1, r = 0.79 and 0.94 for six-and two-row genotypes), suggesting stability of the CID trait across different environments. Overall, these results indicate the potential of leaf CID as a reliable method for selecting for high WUE and productivity in barley breeding programmes in the Canadian prairies. Further work is currently underway to determine heritability ⁄ genetics of leaf CID and application of molecular marker-assisted selection for the traits in barley breeding programmes.

1996. Aluminum-induced alterations in lipid composition of microsomal membranes from an aluminum-resistant and an aluminum-sensitive cuitivar of Tritieum aestivum. -Physio]. Plant. 96: 683-691.We have studied tiie effect of aluminum (Al) on the lipid composition of microsomal membranes isolated from 5-mm root tips of an Al-resistant (FT 74!) and an Al-sensitive (Katepwa) cultivar of Tritieum aestivum L. Exposure of both genotypes to 10 and 50 nM AlCl, for 1 day bad no effect on lipid composition: however, decreases in pbospholipids and increases in monogalactosyl diacyiglycerols, free sterois. fi"ee fatty acids and triacylglycerols were obsen^ed with prolonged exposure '[3 days) to 50 ^M AlClj. Several genotype-specific changes were also obsen'ed under these conditions. The content of digalactosyl diacylglycerols increased by 66.7% in Katepwa. but decreased slightly in PT 741. Tbus. the ratio of monogalactosyl diacylglycerols to digalactosyl diacylglycerols increased by 46.2'?'r in PT 741, but decreased by 2I.37r in Katepwa. Genotype-specific differences were aiso observed in ster>'I iipids. Treatment with Al induced a 70.2% increase in sterylglucosides and a 23.3% increase in aeylated sterylglucosides in Katepwa. In contrast, a 18.9% decrease in aeylated steryiglucosides and no cbanges in sten'lglueosides were observed in PT 741. Our limited understanding of the effect of membrane composition on membrane structure and function makes it difficult to predict iiow these ciianges relate to At toxicity and resistance. Wbile it is possible thai many changes reflect the toxic effects of Al. we believe that cbanges obsen.'ed only in the Al-resistant genotype could contribute to continuotis growth in the face of Ai stress.

1997. Alteration of plasma membrane lipids in aluminum-resistant and aluminum-sensitive wheal genotypes in response to aluminum stress. -Physiol Plant. 99; 302-308.We have studied Ihe effect of aluminum (Al) on lipid composition of plasma membranes from roots of an Al-resistant(Pr741)and an Al-sensitive (Katepwa)cullivarof Triticum aestivum L. Several genotype-specific changes were observed in phospholipids and steryl lipids. While exposure to 20 \}M AlCl, for 3 days had no effect on total phospholipids in either genotype, the most abundant phospholipid, phosphatidyicholine. increased significantly in the .Al-resistant Pr741 with corresponding decreases in other phospholipids. In comparison, no change in phosphatidylcholine was observed in the Al-sensitive Katepwa. Aluminum also decreased steryl lipids (mainly free sterols) in PT741. Such changes were not observed in Katepwa. As a result of differential changes in lipid composition, the relative abundance of one iipid class to another changed. The ralio of steryl iipids to phospholipids decreased in Pr741. with no change in Katepwa. While limited information on the relationship between membrane function and lipid composition makes it difficult to relate these changes lo Al toxicily and resistance, changes observed only in Ihe A)-resistant genotype could contribute lo continued plant growth in the face of Al stress.

To estimate rates of AI accumulation within the symplasm, all apoplastic pools of AI need to be eliminated or accounted for. We have developed a revised kinetic protocol that allows us to estimate the contribution of mucilage-bound AI to total, nonexchangeable AI, and to eliminate the mucilage as an apoplastic pool of AI. By comparing the AI content of excised root tips (2 cm) of wheat (Trificum aestivum L.) with and without the removal of the mucilage (using a 1 O-min wash in 1 M NH,CI), we found that AI bound to the mucilage accounted for approximately 25 to 3 5 % of AI remaining after desorption in citric acid. The kinetics of AI uptake into mucilage were biphasic, with a rapid phase occurring in the first 30 min of uptake, followed by a linear phase occurring in the remainder of the experimental period (1 80 min). By adopting a step for removal of mucilage into our existing kinetic protocol, we have been able to isolate a linear phase of uptake with only a slight deviation from linearity in the first 5 min. Although we cannot unambiguously identify this phase of uptake as uptake into the symplasm, we believe this new protocol provides us with the most accurate quantitative estimate of symplastic AI yet available.A number of recent studies have emphasized the importance of the root tip in the expression of A1 toxicity and resistance in plants. This was perhaps most elegantly demonstrated by Ryan et al. (1993), who showed that A1 must be supplied to the terminal 2 to 3 mm of the root apex of Zea mays for symptoms of A1 toxicity to be expressed. This observation is consistent with an array of less direct evidente, which also supports the role of the root tip as the primary site of Al-related lesions. For example, in Allium cepa and Vigniu unguiculata, decreased rates of mitosis have been associated with accumulation of A1 in the root apex (Clarkson, 1965;Horst et al., 1982Horst et al., , 1983. A1 has also been shown to bind to cell nuclei in root tips of Z. mays (Galsomies et al., 1992) and, more specifically, to DNA in roots of Pisum sntivum and A. cepa (Matsumoto et al., 1976;Morimura et al., 1978). If the root tip is indeed the site where toxicity is most clearly expressed, we would expect potentia1 resistance mechanisms to be most clearly expressed in this region as well. Although mechanisms of A1 resistance are poorly understood, Delhaize et al. (1993b) demonstrated that the terminal 3 to 5 mm of root tips of an Al-resistant cultivar of Triticum aestivum L. were the pri-

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