Pairs of homozygous near-isogenic glycinebetaine-containing (Betl/Betl) and -deficient (betlhetl) F, lines of Zea mays L. (maize) were tested for differences in salt (1 50 mM NaCl or 127.25 mM NaCl plus 22.5 mM CaCI,) tolerance. The Betl/Betl lines exhibited less shoot growth inhibition (as measured by dry matter accumulation, leaf area expansion rate and/or, plant height extension rate) under salinized conditions in comparison to their nearisogenic betl/betl sister lines. These growth differences were associated with maintenance of a significantly higher leaf relative water content, a higher rate of carbon assimilation, and a greater turgor in Betl/Betl lines than in b e t l h e t l lines under salinized conditions. These results strongly suggest that a single gene conferring glycinebetaine accumulation (and/or a tightly linked locus) plays a key role in osmotic adjustment in maize. Yancey (1994) has recently discussed the roles of betaines and their sulfonio analogs as compatible solutes and in cell volume regulation. These solutes are excluded from the hydration sphere of proteins and tend to stabilize the tertiary structure of proteins (Yancey, 1994). They also prevent or reverse the disruption of the tertiary structure caused by noncompatible (perturbing) solutes such as urea (Bateman et al., 1992). It is probable that these compounds have similar functions in higher plants (Wyn Jones and Storey, 1981;Grumet and Hanson, 1986;Robinson and Jones, 1986; Rhodes and Hanson, 1993), but rigorous genetic experiments with higher plant mutants defective in betaine synthesis are needed to verify this point.Genetic tests for the role of glycinebetaine in osmotic stress resistance in Zea mays L. (maize) are now possible because of the development of a series of near-isogenic F, pairs of glycinebetaine-containing and glycinebetaine-deficient lines (Yang et al., 1995). Here we report the growth, water relations, gas-exchange characteristics, and solute compositions of these glycinebetaine-containing and gly-

A series of near-isogenic glycinebetaine-containing and -deficient F, pairs of Zea mays 1. (maize) lines were developed. The pairs of lines differ for alternative alleles of a single locus; the wild-type allele conferring glycinebetaine accumulation is designated Betl and the mutant (recessive) allele is designated betl. The nearisogenic lines were used to investigate whether glycinebetaine deficiency affects the pool size of the glycinebetaine precursor, choline, using a new method for glycinebetaine and choline determination: stable isotope dilution plasma desorption mass spectrometry. Glycinebetaine deficiency in maize was associated with a significant expansion of the free choline pool, but the difference in choline pool size was not equal to the difference in glycinebetaine pool size, suggesting that choline must down-regulate its own synthesis. Consistent with this, glycinebetaine deficiency was also associated with the accumulation of the choline precursor, serine. A randomly amplified polymorphic DNA marker was identified that detects the betl allele. In 62 F, families tested the 10-mer primer 5'-GTCCTCGTAG produced a 1.2-kb polymerase chain reaction product only when DNA from Betl/betl or betl/betl lines was used as template. AI1 26 homozygous Betl/Betl F, families tested were null for this marker.It is now well established that betaines and their sulfonio analogs can play important roles in osmotic adjustment and/or osmoprotection in bacteria (Csonka and Hanson, 1991), cyanobacteria (Borowitzka, 1986), marine algae (Blunden and Gordon, 1986), and mammals (Garcia-Perez and Burg, 1991) (see Yancey [1994] for a recent review of the role of betaines and their sulfonio analogs as compatible solutes). It is probable that these compounds have similar functions in higher plants (Robinson and Jones, 1986;Rhodes and Hanson, 1993). Toward the goal of genetically testing the role of glycinebetaine in osmotic stress resistance in maize (Zea mays L.), we have developed a series of near-

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