Near-isogenic lines of maize differing for glycinebetaine.

A series of near-isogenic glycinebetaine-containing and -deficient F8 pairs of Zea mays L. (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 Bet1 and the mutant (recessive) allele is designated bet1. The near-isogenic 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 bet1 allele. In 62 F8 families tested the 10-mer primer 5'-GTCCTCGTAG produced a 1.2-kb polymerase chain reaction product only when DNA from Bet1/bet1 or bet1/bet1 lines was used as template. All 26 homozygous Bet1/Bet1 F8 families tested were null for this marker.

Salt Tolerance of Glycinebetaine-Deficient and -Containing Maize Lines.

Pairs of homozygous near-isogenic glycinebetaine-containing (Bet1/Bet1) and -deficient (bet1/bet1) F8 lines of Zea mays L. (maize) were tested for differences in salt (150 mM NaCl or 127.25 mM NaCl plus 22.5 mM CaCl2) tolerance. The Bet1/Bet1 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 bet1/bet1 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 Bet1/Bet1 lines than in bet1/bet1 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.

Net CO(2) assimilation of cacao seedlings following dark chilling.

Overnight exposure of cacao (Theobroma cacao L.) seedlings to chilling temperatures between 4.7 and 15.8 degrees C reduced net CO(2) assimilation rate (A) and stomatal conductance to water vapor (g(s)), with temperatures below 10 degrees C causing severe inhibition. Net CO(2) assimilation rates of chilled seedlings recovered to those of nonchilled plants within 7 days. No differences in daytime intercellular CO(2) concentration (c(i)) with overnight temperature were observed on the first day after the chilling treatment, which indicates that the reduction in photosynthesis was not caused by the reduction in stomatal conductance. However, c(i) of chilled plants was much less than that of nonchilled plants on the second day after treatment, which suggests that chilling caused a change in stomatal response to CO(2) concentration. Even 7 days after treatment, when A had recovered to control values, g(s) of chilled leaves was only approximately 70% that of controls. Chilling did not inhibit A through an effect on leaf water potential, which was higher in chilled plants than in unchilled plants.

Net CO2 assimilation of cacao seedlings during periods of plant water deficit.

The effect of leaf water potential (Ψ) on net CO2 assimilation rate (A), stomatal conductance (g), transpiration (E) and water-use efficiency (WUE) was measured for three cultivars of cacao (Theobroma cacao L.) seedlings during three recurrent drought cycles. Net assimilation varied greatly at high water potentials, but as Ψ dropped below approximately -0.8 and -1.0 MPa, A was reduced to less than 1.5 µmol CO2 m(-2) s(-1). The relation between g and A was highly significant and conformed to an asymptotic exponential model, with A approaching maximal values at stomatal conductances of 55-65 mmol H2O m(-2) s(-1). Net assimilation varied linearly (r=0.95) with transpiration, and the slope of the A-E relation (WUE) was approximately 3.0 µmol CO2 mmol(-1) H2O throughout the range of stomatal conductances observed. C i was insensitive to water stress, even though both g and A were strongly affected. Under the experimental conditions used here, mesophyll photosynthesis did not appear to control g through changes in C i. As stress intensified within each drying cycle, WUE of nonirrigated seedlings did not decline relative to that of controls even though CO2 and water vapor exchange rates underwent large displacements. The effect of seed source was highly significant for WUE, and the basis for observed differences among genotypes is discussed.