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-
The effect of plant status on net assimilation and translocation of “C‐labeled assimilates in cacao (Theobroma cacao L.) was evaluated. As plant water potential (ψ) decreased from −0.5 to −1.0 MPa, neither net assimilation nor the rate of label translocation out of the l4CO,‐fed leaf were affected, but as iji fell between −1.0 and −1.5 MPa, net assimilation decreased sharply and label retention increased greatly. Translocation out of source leaves was strongly correlated with net assimilation (r =−0.93). Translocation velocity, assessed by detection of labeled assimilates in sink leaves, was sensitive to plant water deficit, and it declined linearly (r = 0.97) throughout the range of leaf water potentials observed. The results may be explained by reduction in the velocity of assimilate movement within the sieve elements, reduction in supply of labeled assimilates from source leaves, reduction in sink strength or diversion of assimilates to sites of storage or utilization.
1989. Effects of plant water deficit on the daily carbon balance of leaves of cacao seedlings. -Physiol. Plant. 77: 407-412.The daily carbon balance of individual source leaves of Theobroma cacao L. seedlings was measured at 2-to 3-d:ay intervals during a 19-day period of increasing plant water deficit and during an 8-day period of recovery following rewatering. In each case, responses of stressed seedlings were compared to those of irrigated controls. Leaves of irrigated cacaO' seedlings assimilated approximately 41 mg carbohydrate clm~d uring 12-h photoperiods, and exported an average of 34 tug carbohydrate dm"' during 24-h measurement cycles. The rate of carbon export from cacao leaves was sharply reduced as leaf water potential (il)) declined between -0.8 and -2.0 MPa. Further, the rate of export was closely associated with the net assimilation rate (A), with export capacity being severely reduced as A fell to near zero. Net accumulation of dry matter occurred as long as A remained greater than approximately 20 mg carbohydrate dm"^ over the 12-h photoperiod, but at lower assimilation rates, export exceeded concomitant assimilation. Carbon export continued at the expense of leaf carbon reserves as photoassimilation fell to near zero during periods of severe water stress (ij) < -2.0 MPa). Night respiration rate was independent of plant water status.
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