Crop Science 41:1405-1407 (2001)
© 2001 Crop Science Society of America
CROP BREEDING, GENETICS & CYTOLOGY
Genetic Control of High Temperature Tolerance in Wheat as Measured by Membrane Thermal Stability
Amir M. H. Ibrahim*,a and
James S. Quickb
a Plant Science Dep., South Dakota State Univ., Brookings, SD 57007
b Dep. of Soil and Crop Sciences, Colorado State Univ., Fort Collins, CO 80523
* Corresponding author (amir_ibrahim{at}sdstate.edu)
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ABSTRACT
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Heat stress is an important production constraint of wheat (Triticum aestivum L.) affecting many plant biological activities in the cell membrane. This study determined the genetic control of heat tolerance through diallel analysis of selected wheat germplasm. Heat-induced damage of plant membranes was assayed by the membrane thermal stability (MTS) assay, which measures electrolyte leakage from leaf tissue after exposure to high temperature. Six wheat genotypes (‘TAM 107’, ‘TAM 108’, ‘Arlin’,' Kauz', ‘Glennson 82’, and ‘Siete Cerros’) were hybridized in a complete diallel, and MTS was measured on 12 d old F1 seedlings. The mean square for general combining ability (GCA) was four times that of specific combining ability (SCA), indicating the importance of additive gene effects in acquired thermal tolerance. Maternal effects accounted for 67% of reciprocal variation, suggesting that maternal seed-source effects may be important in hybrid seed. These results suggest that heat tolerance based on MTS can be improved using the existing genetic variability available within the germplasm evaluated in this study.
Abbreviations: GCA, general combining ability • SCA, specific combining ability • MTS, membrane thermal stability
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INTRODUCTION
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WHEAT IS GROWN as a winter season crop in the tropics and subtropics despite the relatively high temperatures that occur during the growth cycle. Heat stress is also a common constraint during anthesis and grain filling stages in many temperate environments in South and West Asia, North Africa, Australia, and the central and southern Great Plains of the USA (Reynolds et al., 1994). Wheat producers are seeking new heat-tolerant germplasm suited to these stressed areas. Plant physiological processes differ in their response to heat stress from one phenological stage to another (Fischer, 1985). The complex physiological–genetic relationships conditioning heat tolerance must be combined with genes necessary for superior agronomic performance. Therefore, breeders use empirical selection with visual screening traits such as biomass, tillering ability, and leaf senescence (Ortiz-Ferrara et al., 1993). Breeders evaluate many lines during selection for heat tolerance because identification of a plant with all the required genes is difficult (Ortiz-Ferrara et al., 1993). Yield selection is difficult in large breeding programs, with thousands of segregating lines. In fact, wheat varieties are developed with heat stress tolerance without a complete understanding of the selective effects of the environments in which selection took place. Yet, physiological and biochemical screening techniques, as a complement to empirical methods could increase selection efficiency, securing heat tolerance genes that may be lost during empirical selection (Reynolds et al., 1994).
Several heat stress related traits have received considerable attention; in particular, canopy temperature depression (Blum et al., 1982), cell viability measured by the reduction of tetrazolium triphenyl chloride (TTC) (Porter et al., 1995), and electrolyte leakage measured by conductivity meters (Saadalla et al., 1990). Information on the genetic control of these traits would aid in choosing selection protocols for germplasm enhancement efforts. Porter et al. (1995) determined the genetic control of acquired high temperature tolerance in common bread wheat cultivars using the TTC assay. Using a diallel mating design including five bread wheat parents, they concluded that only the GCA component effect was significant, accounting for 67% of the total genotypic variation. Using a diallel cross, including reciprocals, of six wheat genotypes, Moffatt et al. (1990) studied the genetic control of high temperature tolerance based on chlorophyll fluorescence. Their analysis revealed significant GCA and maternal effects.This suggests the potential for exploiting additive genetic effects of acquired high temperature tolerance.
High temperature disrupts water, ion, and organic solute movement across plant membranes, which interferes with photosynthesis and respiration (Christiansen, 1978). Damage to membranes may be assayed by the membrane thermal stability (MTS), which measures electrolyte leakage from leaves subjected to elevated temperatures (Sullivan, 1972). Although MTS is positively associated with yield performance under heat-stressed conditions (Reynolds et al., 1994), the genetic control of MTS in wheat has not been thoroughly studied. This study determined the genetic control of MTS, by estimating general combining ability (GCA), specific combining ability (SCA) and reciprocal (maternal and non-maternal) effects.
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MATERIALS AND METHODS
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Six wheat cultivars ‘TAM 107’, ‘TAM 108’, ‘Arlin’, ‘Kauz’, ‘Glennson 82’, and ‘Siete Cerros’, were crossed in all combinations, including reciprocals to form a full diallel population. TAM 107, TAM 108, and Arlin are winter wheat cultivars originating in the Great Plains of the United States while Kauz, Glennson 82, and Siete Cerros are CIMMYT-Mexican wheats grown in many hot environments in the world. The F1 seeds were germinated, and 12 d old seedlings were assayed for membrane thermal stability.
Membrane Thermal Stability Assay
Seedlings were grown on moistened, folded germination paper at 20 to 22°C. A dilute solution of Teracoat fungicide (0.15% w:v) was applied to the seedlings daily as needed. The germination paper containing the seedlings was transferred after 4 d to a greenhouse with a 16-h photoperiod, a light intensity of 200 uE m-2 s-1, and at about 17°C temperature regime day and night. Acclimation was started when the first leaf was about 8 to 10 cm long (about 8 to 10 d after germination). The seedlings were transferred to a water bath maintained at 39°C for 48 h, with their roots immersed in water (about 1 cm), and covered with transparent plastic to ensure adequate light interception. Each sample consisted of 10 leaf segments, 7 cm long, rinsed twice in deionized water, and placed in 16x150 mm test tubes with 10 mL deionized water. Tubes were submerged to a depth equal to the height of water in the tubes (about 7 cm) in a water bath at 49°C for 30 min. After the treatment period, the tubes were held overnight at room temperature. Conductance was measured with an electrical conductivity meter (Electroanalyzer 4400, Markson Science, Inc., Del Mar, CA) after calibration with a standardized KCl solution. Test tubes were then autoclaved for 10 min at 120°C and 0.10 Mpa, and conductance was measured again. Membrane thermal stability was expressed in percentage units as the reciprocal of relative leakage:
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where T1 is the conductivity reading after heat treatment, and T2 is the conductivity reading after autoclaving.
Statistical Procedures
The data were analyzed as a complete diallel design, including reciprocals, with three replications using Griffing (1956) Method 3, model 1 (fixed). A SAS program (DIALLEL-SAS) was used for analyzing the combining ability of the 30 F1 crosses according to Zhang and Kang (1997). The program also allowed the partition of reciprocal effects of MTS into maternal (general specific) and non-maternal (specific reciprocal) components.
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RESULTS AND DISCUSSION
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Among the 30 crosses, MTS values ranged from a high of 58.5% for the cross Kauz/Glennson 82 to a low of 26.8% for the cross Arlin/Siete Cerros (Table 1). Genotypic variation among the 30 crosses was significant for MTS (Table 2). The genotypic variation was further partitioned into general combining ability (GCA), the average performance of a parent in hybrid combinations, and specific combining ability (SCA), the hybrid deviation from the averaged GCA effects of two parents. General combining ability accounted for 43% of the total genotypic variability (P < 0.001), whereas specific combining ability accounted for only 19% of the total genotypic variability (P < 0.05). Acquired thermal tolerance, as measured by MTS for this genetic sample, appears to be conditioned primarily by additive gene action. Other researchers also demonstrated the importance of additive gene effects in acquired thermal tolerance. Porter et al. (1995) concluded that only the GCA effects for TTC-measured thermal tolerance were significant, accounting for 67% of the total genotypic variability. Moffatt et al. (1990) showed significant GCA and maternal effects for heat tolerance as measured by chlorophyll fluorescence.
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Table 1. Acquired thermal tolerance values of a 6 x 6 F1 diallel wheat experiment as measured by membrane thermal stability (MTS).
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Progeny resulting from crossing parents with positive GCA effects should have high MTS levels. By contrast, progeny resulting from crossing parents with negative GCA effects should have low MTS levels. Kauz and TAM 107 had positive and significant GCA effects, indicating that they contribute to high levels of heat tolerance as measured by MTS (Table 3). Arlin had a negative and highly significant GCA effect, indicating that it contributes to heat susceptibility as measured by MTS. Glennson 82, Siete Cerros, and TAM 108 had intermediate GCA effects (Table 3).
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Table 3. Estimates of general combining ability (gi) and maternal (mi) effects for membrane thermal stability (MTS) of six wheat genotypes.
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Reciprocal effects accounted for 38% of the total genotypic variability (Table 2). The reciprocal effects ranged from -7.18 in Arlin/Siete Cerros to 8.55 in Siete Cerros/Glennson 82 crosses (Table 4). When reciprocal effects were further partitioned, only maternal effects were found to be significant (P < 0.001) (Table 2). Maternal effects accounted for 74% while the non-maternal effects accounted for 26% of the total reciprocal variability. In other species, maternal effects are due to cytoplasmic differences ascribed to the DNA of replicating organelles or to differences in the maternal environment provided to the developing embryo (Borges, 1987). Non-maternal effects are due to interaction between nuclear and cytoplasmic factors in the crosses (Borges, 1987). These results agree with the Porter et al. (1995) finding of maternal effects accounting for 67% of the variation among reciprocals. Kauz had both positive and highly significant GCA and maternal effects (Table 3). TAM 107 had positive but non-significant maternal effects. Arlin, on the other hand, had both negative and highly significant GCA and maternal effects (Table 3). Crosses that included Kauz (especially as a female parent) generally had high mean MTS values while those that had Arlin (especially as a female parent) generally had low mean MTS values (Table 1). SCA effects varied from -7.33 in TAM 108/Kauz to 5.22 in TAM 108/Arlin crosses (Table 4). These results indicate that heat tolerance based on MTS measurements can be enhanced by utilizing the genetic variability existing within the wheat germplasm evaluated in this study.
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Table 4. Estimates of reciprocal (rij) and specific combining ability (Sij) effects for membrane thermal stability of 15 crosses from six wheat cultivars.
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NOTES
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Part of a dissertation submitted by A.M.H. Ibrahim in partial fulfillment of the requirements for a Ph.D. degree in plant breeding and genetics.
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REFERENCES
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ABSTRACT
INTRODUCTION
