Published in Crop Sci 39:1762-1768 (1999)
© 1999 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Crop Science 39:1762-1768 (1999)
© 1999 Crop Science Society of America
CROP PHYSIOLOGY & METABOLISM
Reproductive-Stage Heat Tolerance, Leaf Membrane Thermostability and Plant Morphology in Cowpea
Abdelbagi M. Ismaila and
Anthony E. Halla
a Dep. of Botany and Plant Sciences, Univ. of California, Riverside, CA 92521-0124 USA
anthony.hall{at}ucr.edu
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ABSTRACT
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High night temperatures during reproductive development can reduce yields of cowpea [Vigna unguiculata (L.) Walp.]. Screening whole plants for degree of flowering and pod set in hot environments has been effective in breeding for heat tolerance, but suitable screening environments often are not available. An indirect screening technique was evaluated involving relative electrolyte leakage from leaf tissue sampled at the end of the dark period with incubation at temperatures that are hot for night-time conditions. This technique was tested with four different pairs of cowpea lines with similar genetic backgrounds but different heat sensitivities during reproductive development. Plants were grown in growth chambers at optimal temperatures. Additional tests were conducted with plants grown in moderately hot and extremely hot field environments. Three sets of three genotypes were used that are either heat susceptible during both early flowering and pod set or heat tolerant during early flowering and heat susceptible during pod set or heat tolerant during both early flowering and pod set. Similar genotypic differences in electrolyte leakage were obtained from plants grown in the growth chamber or field environments. Genotypes with heat tolerance during flowering and pod set had less leaf electrolyte leakage than either genotypes with heat susceptibility during flowering and pod set or genotypes having heat tolerance only during early flowering. Leaf electrolyte leakage, as a measure of leaf membrane thermostability, may provide an efficient indirect screening technique for reproductive-stage heat-tolerance genes that can be used with plants grown in a range of field nursery environments.
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INTRODUCTION
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HIGH NIGHT TEMPERATURES during the growing season can have detrimental effects on reproductive development and yield of several crops (Hall, 1992). Many cowpea cultivars are susceptible to high temperatures during reproductive development (Patel and Hall, 1990) and can exhibit 13.5% reduction in first-flush grain yield per degree centigrade increase in daily minimum night temperature above 16.5°C (Ismail and Hall, 1998). Heat-induced reduction in grain yield of cowpea under field conditions is mainly due to reductions in pod set and harvest index (Nielsen and Hall, 1985; Ismail and Hall, 1998).
Damage during reproductive development occurs mainly during two distinct stages: early flowering and pod set (Warrag and Hall, 1984; Mutters et al., 1989b; Ahmed et al., 1992; Ahmed and Hall, 1993). Following initiation, floral bud development is suppressed by a combination of high night temperature and long photoperiods (Dow el-madina and Hall, 1986; Patel and Hall, 1990). Two or more weeks of consecutive or interrupted high night temperature during the first 4 wk after germination can cause complete suppression of floral buds and prevent flowering (Ahmed and Hall, 1993). Inheritance of tolerance to heat-induced floral bud suppression is consistent with the effect of a single recessive nuclear gene (Hall, 1993).
High night temperature during a later stage of flower development can impair pod set by causing anther indehiscence and low pollen viability (Warrag and Hall, 1984). Flowers are susceptible to heat 9 to 7 d before anthesis, and the injury involves premature degeneration of the tapetum and lack of endothecial development (Ahmed et al., 1992). Damage to pod set is caused mainly by high night temperature between midnight and dawn (Mutters and Hall, 1992). Heat injury during flower development was associated with decreased proline accumulation in pollen and greater accumulation of proline in the anther wall (Mutters et al., 1989a). Inheritance of tolerance to heat-induced low pod set is consistent with the effect of a single dominant nuclear gene (Marfo and Hall, 1992).
Conventional breeding has been used to incorporate heat tolerance during reproductive development into adapted germplasm in an effort to minimize heat-induced reduction in yield. Progress has been achieved with whole plant screening approaches in a few crop species including tomato (Lycopersicon esculentum Mill.) (Stevens, 1979; Villareal and Lai, 1979), cotton (Gossypium barbadense L.) (Feaster and Turcotte, 1985), common bean (Phaseolus vulgaris L.) (Dickson and Petzoldt, 1989), and cowpea (Hall, 1992, 1993). In cowpea, breeding for heat tolerance has been achieved by screening for flower production and pod set in extremely hot field nurseries and greenhouses with high night temperatures. Effective screening environments are not available in many parts of the world or are available only in the summer. Greenhouse screening is expensive on a unit plant basis. A rapid yet effective procedure for screening for heat tolerance that can be used during the early vegetative stage with plants grown in any reasonable environment would allow screening of multiple generations within each year and accelerate breeding programs.
Cellular membrane thermostability, measured as the conductivity of electrolytes leaking from leaf disks at high temperature, has been suggested as a screening technique for heat tolerance in plants (Sullivan, 1972). This technique is based on the observation that when leaf tissue is injured by exposure to high temperatures, cellular membrane permeability is increased and electrolytes diffuse out of the cell into the bathing solution. The amount of electrolytes leaking from injured cells can be evaluated by measuring the electrical conductivity of the solution. Several studies suggested the effectiveness of this technique in detecting genetic variability in heat tolerance in warm season crops such as soybean [Glycine max (L.) Merr.] (Martineau et al., 1979), sorghum (Sorghum bicolor L. Moench) (Sullivan and Ross, 1979), and melon (Cucumis melo L.) (Lester, 1985) as well as in cool season crops such as wheat (Triticum aestivum L.) (Blum and Ebercon, 1981; Saadalla et al., 1990) and Kentucky bluegrass (Poa pratensis L.) (Marcum, 1998). This technique is simple, quicker, and less expensive than the whole plant screen. Potentially, the technique could be used with early vegetative-stage leaf tissue from plants grown in field nursery environments used by breeders for selecting plants and advancing generations. However, it is not known whether differences in leaf electrolyte leakage occur in cowpea and are associated with differences in heat tolerance during reproductive development.
Cowpea provides a model genetic system for testing whether differences in leaf electrolyte leakage reflect differences in heat tolerance during reproductive development. Pairs of lines are available that have similar genetic backgrounds and either have heat tolerance during flowering and pod set or are heat susceptible during these two stages (Ismail and Hall, 1998). In addition, cowpea lines are available that have heat tolerance during early flowering but are heat susceptible during pod set (Ehlers and Hall, 1996). Some studies have indicated that detection of genotypic differences in heat tolerance based on leaf electrolyte leakage may be more effective with plants already subjected to moderately hot temperatures, referred to as high-temperature-acclimation potential (Li et al., 1991). However, we decided to study leaves from plants that had not been subjected to heat treatments for the following reasons. Taking leaf samples directly from field-grown plants would enable us to screen in a more efficient manner because heat acclimation would require putting plants in a controlled environment facility, which would reduce the number of plants that could be processed. Also, we thought it might be important to sample tissue at the time when cowpea is most sensitive to heat-induced disruption of floral development, which is the late night (Hall, 1992).
In this study, we used cowpea genotypes with different levels of heat tolerance to determine whether the major genes controlling heat tolerance during early flowering and pod set also influence electrolyte leakage of leaf tissue and whether the effects depend on the environment in which the plants are grown. The overall objective was to develop a more efficient indirect method for screening cowpea plants for heat tolerance during the reproductive stage. In addition, this study enabled us to determine whether the dwarfing that has been associated with heat tolerance during reproductive development in cowpea (Ismail and Hall, 1998) is a pleiotropic effect of the gene that confers heat tolerance during early flowering.
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Materials and methods
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Growth Chamber Studies
Experiments were conducted in reach-in growth chambers with 3.7-L plastic pots containing UC mix no. III (Matkin and Chandler, 1957). Both the pots and the soil were steam sterilized before use. In each experiment, a different pair of cowpea lines was used consisting of one heat-tolerant and one heat-susceptible line having similar genetic backgrounds (Ismail and Hall, 1998). Day/night temperatures of the air were maintained at 33/20°C, which is about optimal for the growth and development of cowpea in growth chambers, and relative humidity was 60 ± 10%. The day/night photoperiod was 14/10 h and a combination of 12 clear 400-W metal halide lamps and ten 150-W tungsten lamps provided a photon flux density of about 500 µmol photon m-2 s-1 over 400- to 700-nm wavelength at 50 cm below the source which was close to the top of the plants on the dates of sampling. Five seeds of each line were sown in each pot and thinned later to one seedling per pot and there were five pots per line. Four different pairs of cowpea lines were studied in different experiments.
Leaf disks 13 mm in diameter were harvested during the period when seedlings were 3 to 5 wk old. Seven leaf disks were collected from each plant at the end of the dark period from new leaves that had recently become fully expanded and pooled to provide one sample per plant and five replicate samples. Leaf disks were then rinsed three times with deionized water to remove electrolytes from injured cells at the cut edge and any surface-adhering electrolytes, and incubated in 20 mL of deionized water at different temperatures. Electrical conductivity of the solution was measured every 6 h in the first experiment to determine the optimum incubation time and once after 24 h in subsequent experiments with a dip cell (Model 4063, Control Company, Friendswood, TX). Solutions containing leaf disks were then boiled for 45 min and incubated for 24 h at 25°C, after which conductivity was measured again. Relative electrolyte leakage was calculated as percentage of maximum leakage after boiling.
Field Studies
Three groups, each comprising three cowpea lines, were used in this study with individuals within each group that are genetically related but differ in heat tolerance during reproductive development. The first group consisted of `H36', `UCD7964', and `CB5', the second group consisted of `H8-9', `UCD8517', and `CB3', and the last group consisted of `DLS99', `DLS21', and `DLS127'. Each group included a completely heat-tolerant line, a completely heat-susceptible line, and a third line that is heat-tolerant during floral bud development but susceptible to heat injury during pod set (Table 1)
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Table 1 Electrolyte leakage from leaf disks of cowpea lines grown at Riverside and Coachella Valley during the summer of 1998 as a percentage of electrolyte leakage from boiled leaf disks. Leaf disks were collected during the last hour of darkness, 38 d after sowing at Riverside and 44 d after sowing at Coachella Valley, and incubated at 25°C for 24 h
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During the summer of 1998, these lines were sown at two Agricultural Experiment Stations of the Univ. of California with contrasting thermal environments. Riverside, CA, was expected to be moderately hot, whereas Oasis, Coachella Valley, CA, was expected to be extremely hot. The thermal environment at Riverside is representative of commercial production areas of cowpea in California (Hall and Frate, 1996), whereas the Coachella Valley environment is too hot for commercial production of cowpea and has been used for growing heat-screening nurseries where selections were made to incorporate heat tolerance (Hall, 1992, 1993). For both experiments, the same standard cultural practices and measurements were used unless stated otherwise. Weather data were collected from weather stations located at each experimental site.
Riverside Experiment
Seeds were sown on 26 June 1998 with a four-row planter. Seeds were inoculated with rhizobia (EL type, Nitragin Co., Milwaukee, WI) and sown into ridges with 76 cm between rows and 10 cm between seeds within rows. Individual plots of each of the nine lines consisted of four rows, 6 m long arranged in a randomized complete block design with four replications. Irrigation water was applied immediately after sowing by furrow irrigation and then continued as needed, about weekly, to maintain well-watered conditions. Nutrient deficiencies were not detected visually and standard agronomic practices were used to manage weeds and pests.
Leaf disks were harvested during the last hour of the night, 38 d after sowing. One leaf disk 13 mm in diameter was collected from each of 12 plants in the center two rows of each four-row plot and pooled to provide a sample for each of the four replicate plots. Leaf disks were washed and then incubated in a water bath at 25°C for 24 h. Electrolyte leakage was measured after 24 h and after boiling, following the same procedure used for plants grown under growth chamber conditions. The two central rows were harvested 77 d after sowing, at which time the first flush of pods was mature and dry. A random sample of 10 plants was used to determine pod set, plant height, number of main stem internodes, and harvest index. Pod set was estimated on the basis of the average number of pods per peduncle on the first five reproductive nodes. Harvest index was determined as the ratio of grain weight to total shoot biomass after drying the samples at 65°C for 7 d. The remainder of the two central rows were allowed to dry in the field and then threshed. Subsamples of seeds were dried at 105°C for 48 h to determine their moisture content by weight. Grain yield was determined on the basis of all plants that were harvested from the two middle rows of each plot and is reported on a 100 g kg-1 seed moisture content by weight basis.
Coachella Valley Experiment
Seeds were sown on 22 June 1998 with a two-row planter. Drip irrigation was provided immediately following sowing and then on a daily basis. Leaf disks were harvested during the last hour of the night, 44 d after sowing as described for the Riverside experiment. Electrolyte leakage measurements were made as described for the Riverside experiment. Measurements of reproductive and growth attributes and grain yield were made as in the Riverside experiment but at 67 d after sowing.
Statistical Analysis
Statistical analysis was performed for each character studied on the basis of a completely randomized design with five replications for the growth chamber studies and a randomized complete block design with four replications for the field experiments. Means and LSD (after a significant F-test) or standard error values were calculated. Associations among characters were examined by simple correlation analysis.
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Results and discussion
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Association of Reproductive-Stage Heat Tolerance with Electrolyte Leakage
Growth Chamber Studies
In the first experiment, two cowpea lines with similar genetic background but with differences in heat tolerance during reproductive development (Ismail and Hall, 1998) and in chilling tolerance during seedling emergence (Ismail et al., 1997) were used. The cowpea line 1393-2-1 is tolerant to high night temperature during reproductive development but sensitive to chilling soil temperature during seedling emergence, whereas 1393-2-11 is sensitive to high night temperature during reproductive development and tolerant to chilling soil temperature during seedling emergence. The goal of this experiment was to test whether genetic differences in heat tolerance during reproductive development can be detected at the seedling stage by measuring electrolyte leakage from leaf tissue. Different temperatures and incubation periods were used to establish the temperature and incubation intervals during which maximal separation between lines can be achieved.
For both lines, leakage of electrolytes from leaf disks was slower when leaf disks were incubated at cooler temperatures and became progressively faster at warmer incubation temperatures. Significant differences between lines were observed at 10°C and also at 30°C with no differences at 20 and 40°C (Fig. 1)
. At 30°C, electrolyte leakage was significantly slower from leaf disks collected from the heat-tolerant line compared to the heat-susceptible one. The difference between the two lines was statistically significant 6 h after incubation and became more apparent with prolonging the incubation period up to 30 h. For this reason an incubation period of 24 h was used in subsequent experiments. Also note that at 10°C, the opposite response occurred, electrolyte leakage from leaf disks was significantly slower from line 1393-2-11. This result is consistent with the chilling tolerance of this line during seedling emergence and the slower electrolyte leakage from seed of this line under chilling conditions (Ismail et al., 1997).
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Fig. 1 Electrolyte leakage from leaf disks of cowpea lines 1393-2-1 and 1393-2-11 incubated at different temperatures for different periods as a percentage of electrolyte leakage after boiling the leaf disks. Standard errors are indicated by vertical bars
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Four experiments were then conducted in growth chambers. In each experiment, a different pair of cowpea lines was used with individuals that differ in heat tolerance during reproductive development but with similar genetic backgrounds (Ismail and Hall, 1998). In all experiments, leaf disks were incubated at different temperatures for 24 h at which time measurements were made of electrolyte leakage. The relation between percentage electrolyte leakage from leaf disks and treatment temperature was sigmoidal (Fig. 2)
. Similar sigmoidal response curves were found in other crops such as sorghum (Sullivan, 1972), soybean (Martineau et al., 1979), melon (Lester, 1985), citrus (Citrus spp.) (Ahrens and Ingram, 1988), and Kentucky bluegrass (Marcum, 1998). Maximal separation between individuals within each pair was in the temperature range of 25 to 30°C in the first three experiments (Fig. 2a, b, and c), and 30 to 35°C in the fourth experiment (Fig. 2d). In these temperature ranges, electrolyte leakage from leaf tissues collected from the heat-tolerant lines was significantly slower than that from the heat-susceptible sister lines (Fig. 2). Night temperatures in the range of 25 to 30°C are known to induce substantial damage to heat-susceptible cowpea cultivars during two distinct stages of reproductive development: floral bud development and anther development (Warrag and Hall, 1984; Mutters et al., 1989b; Ahmed et al., 1992; Ahmed and Hall, 1993). The sensitivity of anther development to late-night but not early-night temperatures of 30°C has been proposed to be caused by a specific heat-sensitive process that is under circadian control and only occurs in the late night (Mutters and Hall, 1992). The fast rates of electrolyte leakage from leaves, harvested in the late night, at only moderate temperatures of 25 to 30°C (Fig. 2), also may be caused by a process under circadian control.
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Fig. 2 Electrolyte leakage from leaf disks of four pairs of cowpea lines after incubation for 24 h at different temperatures as a percentage of electrolyte leakage after boiling the leaf disks. Lines with closed symbols have heat tolerance during reproductive development. Standard errors are indicated by vertical bars
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These findings confirm our results from the first experiment and suggest that slow electrolyte leakage from leaf tissue at temperature that is hot for night-time conditions may be used to detect genes for tolerance to hot night-time temperatures during reproductive development. Electrolyte leakage from leaf disks will be easier to measure than reproductive development. If screening for electrolyte leakage could be conducted at different times within the year, it would be a very efficient screening technique but it was not known whether the relative electrolyte leakage response is consistent for plants grown in different field environments.
Studies under Field Conditions
There was substantial difference in shelter air temperature between the two field environments. Mean daily Tmax/Tmin for the first 60 d after sowing were 36/17°C for Riverside and 41/25°C for Coachella Valley.
Electrolyte Leakage from Leaf Tissue
Despite the variation in air temperatures, similar results for electrolyte leakage were obtained from leaf disks harvested from plants grown in the two locations (Table 1). Relative electrolyte leakage from leaf tissue collected from lines that are heat tolerant during both early flowering and pod set was significantly slower than that from lines that are heat susceptible (by 38 and 43 percentage points at Riverside and by 31 and 29 percentage points at Coachella Valley compared with lines that are heat susceptible at pod set or lines that are heat susceptible at both early flowering and pod set, respectively). Percentage electrolyte leakage from the two latter groups was similar at both locations. These findings are consistent with our results under growth chamber conditions. Also, they demonstrate that it may be possible to screen for heat tolerance during reproductive development on the basis of electrolyte leakage from leaf tissue obtained from field nurseries used by breeders to select plants and advance generations, such as the one at Riverside which is similar to commercial production environments. These findings indicated that either both the heat-tolerance gene for early flowering (Hall, 1993) and the gene for pod set (Marfo and Hall, 1992) are required for slow electrolyte leakage or that only the heat-tolerance gene that is effective during pod set is required for slow electrolyte leakage. The potential value of this screening technique is that it may be effective in many parts of the world and seasons where very hot field screening environments are not available. Also, the technique is easy and the screening can be done within 5 wk from sowing, permitting selection of potential parents for making crosses.
Association of Reproductive-Stage Heat Tolerance with Yield and Yield Attributes
Significant differences in grain yield were observed among the three heat-tolerance and heat-susceptibile groups of cowpea lines in the extremely hot environment of Coachella Valley with no differences within each group (Table 2)
. Cowpea lines that are heat tolerant at both early flowering and pod set produced the highest grain yield, whereas lines that are susceptible at both stages produced the lowest grain yield, with the ones that are heat tolerant during early flowering but heat susceptible during pod set being intermediate. Compared with heat-susceptible lines, grain yield increased 4- and 6-fold for lines with heat tolerance during early flowering and lines with heat tolerance during both early flowering and pod set, respectively. Similar effects of heat-tolerance on harvest index and pods per peduncle were also apparent (Table 2). Harvest index increased by 4- and 9-fold and pods per peduncle increased by 3- and 7-fold for lines with heat tolerance during early flowering, and lines with heat tolerance during both early flowering and pod set, respectively, compared with heat-susceptible lines (Table 2). These results suggested that the heat-tolerance genes, that are effective at early flowering and pod set, contribute equally to final grain yield through their equal effects on pod set and harvest index. Our previous studies with pairs of cowpea lines that are either heat tolerant or heat susceptible during both stages of reproductive development, but with similar genetic backgrounds, indicated that heat tolerance genes significantly increase grain yield in hot environments mainly due to increases in pod set and harvest index (Ismail and Hall, 1998).
In the cooler environment at Riverside, heat tolerance during either early flowering or during both early flowering and pod set had no effect on grain yield (Table 3)
. Pods per peduncle were significantly greater for lines that are heat tolerant during both early flowering and pod set and individual seed weight and seeds per pod were similar for the three groups. This suggested that heat-tolerance genes at early flowering and pod set have no effect on yield and harvest index under the cooler conditions of Riverside, which is consistent with the findings of Ismail and Hall (1998).
Correlation coefficients for associations of genotypic mean values of percentage electrolyte leakage from leaf disks with grain yield, pods per peduncle, and harvest index were negative and highly significant at Coachella Valley but only significant for pods per peduncle at Riverside (Table 4)
. The strong associations between relative electrolyte leakage and reproductive attributes that are known to be injured by heat under hot conditions indicate the potential usefulness of this technique as a screening procedure to select for heat tolerance.
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Table 4 Correlation coefficients for the association of electrolyte leakage with yield, harvest index, pods per peduncle, and internode length at Riverside (Riv.) and Coachella Valley (Coach.). Values in parentheses are P-values
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Association of Reproductive-Stage Heat Tolerance with Vegetative Growth
Under the extremely hot conditions at Coachella Valley, cowpea lines that are heat tolerant during both early flowering and pod set were significantly shorter than lines that are heat susceptible during both early flowering and pod set (Table 5)
. However, for lines that are heat tolerant during early flowering but heat susceptible during pod set, one line (UCD7964) was shorter and similar in height to the lines that are heat tolerant during both stages, whereas the other two lines (UCD8517 and DLS21) were taller and similar to the lines that are susceptible to heat during both stages (Table 5). This indicates that the dwarfing effect associated with heat-tolerance genes reported by Ismail and Hall (1998) is not due to a pleiotropic effect of the heat-tolerance gene controlling early flowering and may not be due to the effect of the gene controlling heat tolerance during pod set but may rather be due to a gene that is closely linked with the gene conferring heat-tolerance during early flowering. The importance of this observation is it indicates that it may be possible to breed cowpea lines that are heat tolerant during reproductive development but not dwarfed when grown under hot conditions. However, it is possible that the heat-tolerance gene that enhances pod set may have a pleiotropic effect that results in dwarfing through the diversion of photosynthate to reproductive organs and associated hormonal effects. Effects of heat-tolerance genes in decreasing the number of internodes and internode length were similar to the effects observed in our previous studies (Ismail and Hall, 1998).
Under the more optimal temperatures at Riverside, cowpea lines that are either heat tolerant during early flowering and heat susceptible during pod set or heat susceptible during both stages were taller, on average, than lines that are heat tolerant during both stages (Table 6)
. But this effect was mainly because of two lines, CB5 and DLS21, that are significantly taller than the lines that are heat tolerant during both stages of development. The taller lines had longer internodes. These effects are consistent with the observations of Ismail and Hall (1998) that the threshold temperature above which heat-tolerance genes are associated with dwarfing is lower than the threshold temperature for effects on grain yield. The association between percentage electrolyte leakage and internode length was positive at both locations but only significant at Coachella Valley (Table 4).
This study has developed an indirect technique that may be effective for detecting genes that confer heat tolerance during reproductive development in cowpea. This technique is based on measurement of electrolyte leakage from leaf tissue harvested at the end of the dark period and after incubation at temperatures that are hot for night-time conditions for 24 h. Consistent genotypic differences in electrolyte leakage were observed with plants grown in growth chambers and two contrasting field environments. This suggests that the technique may be effective with leaf tissue obtained from conventional field nurseries used in breeding programs for selecting plants and advancing generations.
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NOTES
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Research partially supported by USDA NRICGP Award No. 98-35100-6129 and USAID Grant No. DAN-G-SS-86-00008-00 to AEH. The opinions and recommendations are those of the authors and not necessarily those of USAID.
Received for publication February 15, 1999.
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A. M. Ismail, A. E. Hall, and J. D. Ehlers
Delayed-Leaf-Senescence and Heat-Tolerance Traits Mainly Are Independently Expressed in Cowpea
Crop Sci.,
July 1, 2000;
40(4):
1049 - 1055.
[Abstract]
[Full Text]
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TOP
ABSTRACT
INTRODUCTION
