Daniel T. Hahn

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.

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[prime]-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.

Epicuticular wax load and water-use efficiency in bloomless and sparse-bloom mutants of Sorghum bicolor L.

Abstract A mutation breeding approach was utilized to obtain near-isogenic lines of sorghum [ Sorghum bicolor (L.) Moench.] with altered epicuticular wax. Seeds of three bloomless mutants (bm-15, bm-22 and bm-38) and one sparse-bloom mutant (h-19) were obtained from the M 4 generation, and responses of mutants were compared to their corresponding normal , non-mutant sib lines (designated N-15, N-22, N-38 and N-19). Net CO 2 assimilation rate, stomatal conductance, transpiration rate and water-use efficiency were measured on the uppermost, fully expanded leaves in irrigated and non-irrigated 43-day-old plants using a LiCor LI-6200 portable photosynthesis system. Epicuticular wax load (EWL) and cumulative water loss from excised leaves (CWL) were measured, and the relationships among EWL, CWL and water-use efficiency were assessed. Differences in response to irrigation level varied among the four lines. Among irrigated plants, h-19 exhibited a higher water-use efficiency than N-19. Among non-irrigated plants, bm-15, bm-22 and h-19 exhibited lower water-use efficiencies than N-15, N-22 and N-19, respectively. Water-use efficiency measured in non-irrigated bm-22, a mutant line with no visible wax structures on the abaxial leaf surface, was more than 40% below that measured in N-22. The water loss from excised leaves was greater in bloomless and sparse-bloom lines than in normal lines. Water-use efficiency varied linearly with epicuticular wax load under irrigated ( r = 0.72) and non-irrigated ( r = 0.94) conditions.

Leaf water relations and gas exchange in two grain sorghum genotypes differing in their pre- and post-flowering drought tolerance

Summary Two grain sorghum [ Sorghum bicolor (L.) Moench.] lines (TX7078 and B35) of similar maturity but known to exhibit differences in sensitivity to drought in pre- and post-flowering growth stages were evaluated in order to quantify the physiological bases for differences in drought tolerance between lines and groWth stages. The relationships among leaf water status, osmotic adjustment, leaf gas-exchange characteristics, and epicuticular wax load were characterized for irrigated and non-irrigated plants during preand post-flowering growth stages. Osmotic adjustment during pre-flowering stress did not differ between lines, and average adjustment was approximately 0.42 MPa. In contrast, osmotic adjustment occurred only in TX7078 during post-flowering stress, and the extent of adjustment relative to irrigated plants was only 0.16 MPa. Neither osmotic adjustment nor epicuticular wax load appeared to be related to the relative drought tolerance of these genotypes during pre- or post-flowering growth stages. A reversal in the ranking of water-use efficiency (WUE) of TX7078 and B35 occurred between pre- and post-flowering growth stages. Non-irrigated plants of TX7078 exhibited significantly higher WUE than B35 at the pre-flowering stage, but the opposite was true in the post-flowering stage. The reason for the reversal in WUE is not known, but there was no significant relation between osmotic adjustment and WUE. TX7078 and B35 showed a contrasting response for intercellular CO 2 concentration (C i ) in both pre- and post-flowering experiments; C i was strongly reduced by water deficit in TX7078, but it was not significantly affected in B35, suggesting that these genotypes may differ fundamentally in their photosynthetic response to low leaf water potential.

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. Ci 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 Ci. 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.