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CROP PHYSIOLOGY & METABOLISM

Delayed-Leaf-Senescence and Heat-Tolerance Traits Mainly Are Independently Expressed in Cowpea

Dep. of Botany and Plant Sciences, Univ. of California, Riverside, CA 92521-0124 USA

anthony.hall{at}ucr.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Reproductive activity in cowpea [Vigna unguiculata (L.) Walp.] is characterized by two separate flushes of pod production. Grain yield from the second flush strongly depends on extent of plant death due to soil pathogens after the first flush is completed. A delayed-leaf-senescence (DLS) trait enhances the extent of plant survival but may reduce first-flush grain yield. Heat tolerance during reproductive development can increase first-flush grain yield and has been incorporated into cowpea but may reduce plant survival after the first flush is completed. Interactive effects when combining DLS and heat-tolerance traits were evaluated. A cross was made between a DLS heat-susceptible line and a non-DLS heat-tolerant line. Evaluation and selection for these traits were carried out over several generations and a set of 40 lines with 10 each of the four combinations of DLS +/- heat tolerance and non-DLS +/- heat tolerance was developed. These lines were evaluated in contrasting field environments. Under senescence-inducing conditions, the DLS trait greatly increased plant survival and individual seed size, and it only caused a small reduction in first-flush grain yield that would have been off-set several fold by an enhanced second-flush grain yield. The heat-tolerance trait increased first-flush grain yield in very hot environments and only slightly enhanced the tendency for premature plant death in non-DLS lines with no effect on lines having the DLS trait. The DLS and heat-tolerance traits can be effectively incorporated into cowpea and would have beneficial effects on grain yield in specific circumstances with only small detrimental interactive effects.

Abbreviations: DLS, delayed-leaf-senescence • T_max and T_min, maximum and minimum daily air temperature

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
COWPEA (BLACKEYE DRY BEAN) GROWERS in the southern San Joaquin Valley of California often manage the crop to accumulate two flushes of pods before cutting (Hall and Frate, 1996) because the second flush can produce up to 2000 kg ha^-1 which can result in total grain yields of about 5000 kg ha^-1 (Gwathmey et al., 1992a; Ismail and Hall, 1998). Unfortunately, in fields where cowpea have been grown for several years, many plants often die after producing the first flush of pods (Gwathmey et al., 1992a), even with alternate-year rotations. This premature death causes substantial reductions in total grain yield in that second-flush yield is proportional to the number of plants surviving to produce the second flush (Ismail and Hall, 1998). The causal organism for the premature death of cowpea probably is Fusarium solani (Mart.) Sacc. f. sp. phaseoli (Burk.) Synd. & Hans., type A (Erwin et al., 1991). Consequently, this cowpea disease may have some similarities with Sudden Death Syndrome in soybean [Glycine max (L.) Merr.] (Rupe, 1989; Njiti et al., 1997) and Fusarium root rot in Phaseolus vulgaris L. (Burke and Miller, 1983) that involve similar or the same causal organisms.

Cowpea breeding lines with resistance to premature death have been discovered and described as having the delayed-leaf-senescence (DLS) trait (Gwathmey et al., 1992a). Two DLS lines exhibited 53 to 98% survival after producing the first flush of pods compared with 15 to 28% survival for two non-DLS lines (Gwathmey et al., 1992a). The DLS lines produced greater total (first-flush plus second-flush) yields with a tendency for smaller first-flush yields than the senescent lines (Gwathmey et al., 1992a). The latter result might be due to the increased partitioning of carbohydrates to stems and roots in DLS lines (Gwathmey et al., 1992b), which makes less carbohydrate available to pods. Survival of plants and expression of DLS may be associated with maintenance of high carbohydrate levels in roots. Overall this indicates the DLS trait may reduce productivity in short-season conditions, even though it enhances productivity when conditions permit the harvesting of two flushes of pods. The evidence, however, is not conclusive in that the breeding lines that were compared by Gwathmey et al. (1992a) differ in many traits. The DLS trait has been observed in soybean lines and also was associated with a decrease in grain yield (Phillips et al., 1984).

Many cowpea cultivars are susceptible to high night temperatures during reproductive development (Patel and Hall, 1990; Ehlers and Hall, 1996). California cultivars can exhibit a 13.5% decrease in grain yield per degree centigrade increase in average daily minimum night temperature above 16.5°C for the 3-wk period starting 1 wk prior to the first appearance of flowers (Ismail and Hall, 1998). Heat-induced reductions in grain yield of cowpea under field conditions mainly are due to reductions in pod set and harvest index (Nielsen and Hall, 1985; Ismail and Hall, 1998). Damage during reproductive development can occur at two distinct stages: floral bud development and anther development. Following initiation, floral bud development and flower production are suppressed by a combination of high night temperatures and long photoperiods (Dow el-madina and Hall, 1986; Patel and Hall, 1990). High night temperature during anther development does not affect flower production but can impair pod set by causing anther indehiscence and low pollen viability (Warrag and Hall, 1984; Ahmed et al., 1992).

Through selection in field and greenhouse environments with high night temperatures (Hall, 1992), cowpea lines (Hall, 1993) and a cultivar (Ehlers et al., 2000) were bred with heat tolerance during reproductive development. Six pairs of cowpea lines with and without heat tolerance but having similar genetic backgrounds were evaluated in eight field environments with contrasting temperature regimes (Ismail and Hall, 1998). The heat-tolerance trait was found to increase first-flush grain yields when minimum night temperatures were above 17.8°C for the 3-wk period starting 1 wk prior to the appearance of first flowers by increasing pod set and enhancing overall partitioning of carbohydrates into grains. In addition, the heat-tolerance trait was associated with a tendency for fewer plants surviving after producing the first flush of pods (Ismail and Hall, 1998). This may be explained by inadequate partitioning of carbohydrates to stems and roots in plants with heat tolerance that are producing and filling many pods.

In this study, we evaluated the interaction of the heat-tolerance and DLS traits using sets of cowpea lines developed by crossing a heat-susceptible DLS parent with a heat-tolerant non-DLS parent. We tested whether the DLS trait reduces first-flush grain yields and reduces the beneficial influence of the heat-tolerance trait. We also tested whether the heat-tolerance trait enhances senescence after the first flush of pods is produced and reduces expression of the DLS trait. This information will show whether the heat-tolerance and DLS traits can be combined in an effective manner.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
A cross was made in 1992 between cowpea breeding lines 8517 and H8-9. Line 8517 was developed at University of California, Davis, and has the DLS trait (Gwathmey et al. 1992a) but is susceptible to heat injury during pod set. Line H8-9 was developed at the University of California, Riverside, and has heat-tolerance during both floral bud development and pod set (Ehlers and Hall, 1996) but lacks the DLS trait. Both lines have similar earliness and seed size and are well adapted to the field environments used in these studies. A set of 91 F₂-derived F₄ families was produced with propagation of plants in a greenhouse at Riverside and in a field at Coachella Valley, CA, from 1992 to 1994, by a single-plant-decent procedure. During the summer of 1994 these families were grown at Riverside in single-row unreplicated plots in a field where premature plant death had occurred in previous years. After the first flush of pods was produced, two plants, one living (DLS) and one dead (senescent), were selected within each family. The objective was to produce pairs with and without DLS from the same families but, subsequently, we found the within family selection was not very effective and we emphasized selection between families. The 182 individual plants were advanced one generation in a greenhouse at Riverside with sowing on 2 Feb. 1995, and F₆ seeds were harvested from single plants and used for subsequent selection and evaluation. The two parental lines, 8517 and H8-9, were included as checks in all subsequent evaluation and selection experiments. In all field experiments, seeds were inoculated with rhizobia (EL type, Nitragin Co., Milwaukee, WI).

Selection of Lines for the DLS Trait in 1995
The first rigorous evaluation for the DLS trait was made during the summer of 1995, when the 182 F₆ families were evaluated for DLS at the Agricultural Experiment Station of the Univ. of California at Riverside. The F₆ families were sown on 28 June 1995 in a field where substantial premature plant death had occurred in previous years. Single-row plots with two replications were used. Individual rows were 6 m long with about 10 cm between seeds. Irrigation water was applied immediately after sowing using furrow irrigation and then on about a weekly basis to maintain well-watered conditions. Individual rows were scored visually for DLS toward the end of the first flush using a scale of zero (all plants had died) to 9 (all plants were living). Families that did not set a substantial number of pods were eliminated because our criteria for selecting DLS types is to select families that have both DLS and substantial first-flush pod load because plants with few pods exhibit a different type of DLS with limited agronomic value. Additional families were eliminated that were either prostrate or began flowering earlier or later than the parents. Families (138) were selected that had very high or very low DLS values. Seeds were harvested from single living plants from each DLS family and single dead plants from each non-DLS family.

Evaluation and Selection of Lines for Heat-Tolerance and DLS Traits in 1996
A subset of F₇ seeds from single plants harvested from 138 F₆ families in 1995 was sown in a hot greenhouse in pots with two replicate pots per family. Three seeds were sown per pot and thinned later to one seedling per pot. Sowing was on 22 June 1996 in 3.7-L plastic pots filled with sterilized UC mix no. III (Matkin and Chandler, 1957) and the daily maximum–minimum air temperatures of the greenhouse were 34/30°C. Grain weight and number of seeds per plant, number of pods per peduncle on the first five reproductive nodes, pods per plant, and individual seed weight were measured.

A second subset of F₇ seeds was sown on 20 June 1996 in a field at Riverside where substantial premature plant death had occurred in previous years. Single-row plots with two replications were planted, managed as in 1995, and evaluated for DLS through visual scoring at the completion of the first flush as in 1995. Seeds were harvested from single living plants in DLS families and single dead plants in senescent families from a total of 101 F₇ families that had a date of first flowering similar to the parents, semi-erect growth habit and high pod set.

Evaluation and Selection of Lines for the DLS Trait in 1997
The F₈ seeds from the 101 families that were harvested in 1996 were grown in a trial at Riverside. Lines were grown in single-row plots with two replications. Sowing was on 25 June 1997 in dry ridges with irrigation following immediately after sowing and management was as in 1995. Individual lines were scored visually for DLS at the completion of the first flush as in 1995. Eighty lines were selected and harvested in bulk, 40 of these lines exhibited strong DLS and 40 were strongly senescent. Individual rows were harvested manually, left in the field to dry, and then threshed. Grain yield and individual seed weight were measured.

Evaluation of Lines with DLS and Heat-tolerance Traits at Riverside and Shafter in 1998
A set of 40 lines with 10 each that have either DLS + heat tolerance or DLS + heat susceptibility or non-DLS + heat tolerance or non-DLS + heat susceptibility was selected from the 80 lines that were harvested in 1997. These selections were based on high and low heat-tolerance scores for both extent of flowering and pod set made in the hot greenhouse and consistently high and low DLS scores over years. The F₉ lines were sown at two Agricultural Experiment Stations of the Univ. of California with contrasting soil environments in the summer of 1998. Riverside was expected to be hot during the growing season due to late sowing and a field was used where strong expression of the DLS/senescence trait occurred in previous years. An early sowing date was used at Shafter, CA, to try to provide similar hot temperatures as in the Riverside experiment. A field was used at Shafter where strong expression of the DLS–senescence trait was not expected to occur. For both experiments, the same standard cultural practices and measurements were made unless stated otherwise. Weather data were collected from stations located at each experimental site.

At Riverside, seeds were sown on 26 June 1998 with a four-row planter. Seeds were sown into ridges with 76 cm between rows and 10 cm between seeds within rows. Each of the 40 lines was grown in individual plots consisting of four 7.5-m long rows. Irrigation water was applied immediately after sowing with furrow irrigation and then about weekly to maintain well-watered conditions. Nutrient deficiencies were not detected visually and standard agronomic practices were used to manage weeds and pests.

Plant survival was determined after the first flush of pods became mature on the basis of the percentage of living plants in one of the two central rows. The two central rows were cut 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.

At Shafter, seeds were sown on 22 May 1998 on preirrigated ridges. Cultural practices and measurements were the same as at Riverside experiment except that plants were cut 90 d after sowing, at which time the first flush of pods was mature and dry.

Statistical Analysis
Analysis of variance was performed for each character in each experiment on the basis of a randomized complete block design for 1996 and 1997 experiments and a completely randomized design in 1998 experiments, considering the 10 genetic lines in each class as replications, with evaluations of main effects due to DLS and heat tolerance and their interactions. Means and coefficients of variation were calculated. Since some of the variation described by the coefficient of variation in the 1998 experiments is genetic, regression and correlation analyses were used to assess associations between some traits. Associations between family-mean values of DLS scores for F₆, F₇ and F₈ generations were examined by correlation analysis.

	Results

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Selection for Heat Tolerance in a Greenhouse
The objectives of the hot greenhouse experiment were to select lines with and without heat tolerance and to make a preliminary assessment of whether there is an interaction between the genes conferring heat tolerance and DLS. The DLS–senescence traits were not obvious in this environment possibly because the plants were grown in a sterilized medium in pots. Of the 138 lines that were screened, data are presented for the 40 lines that subsequently were used in the field experiments in 1998. All selected lines exhibited vigorous vegetative growth. Mean grain yield of the heat-tolerant lines was substantially greater than that of the heat-susceptible ones with no interaction with the DLS classification (Table 1) . The greater yield of the heat-tolerant lines was associated with levels of pod set typical of plants in optimal environments. The heat susceptible lines exhibited substantial floral bud suppression and close to zero pod set and grain yield under these conditions. A significant interaction of DLS classification and heat tolerance was observed for yield components. The heat-tolerant non-DLS lines had higher pod set, and more pods and seed per plant than the heat-tolerant DLS lines indicating that incorporation of the DLS genes may have lowered the first-flush yield potential of these lines. Individual seed weights were 223 mg seed^-1 and 208 mg seed^-1 for the heat-tolerant DLS lines and the heat-tolerant non-DLS lines, respectively. The greater individual seed weight of the heat-tolerant DLS lines partially compensated for the potential loss in yield because of their lower pod set compared with the heat-tolerant non-DLS lines.

	Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Incorporation of the DLS trait into cowpea resulted in a substantial increase in plant survival under field conditions where the majority of plants from non-DLS lines died after producing the first flush of pods (Table 3). The correlations reported by Ismail and Hall (1998) predict that the increase in plant survival from 27 to 99% could increase second-flush pod yield by 1560 kg ha^-1. The DLS trait was associated with increases in individual seed weight by 6 to 15% (Tables 3 and 2, respectively). Presumably, the DLS (stay-green) trait results in maintenance of photosynthetic activity and thereby, enhances grain filling. An alternative possibility, that the gene(s) conferring DLS is linked to a gene that independently enhances individual seed weight is not consistent with the observation that DLS lines had similar individual seed weight as non-DLS lines in an environment where there was little premature plant senescence (Table 5).

The DLS trait was associated with a reduction in first-flush grain yield of 330 to 441 kg ha^-1 (Tables 5 and 3, respectively). Gwathmey et al. (1992a) reported average first-flush grain yield over 3 yr of 2528 kg ha^-1 for two DLS lines compared with 2865 kg ha^-1 for two non-DLS lines, a difference of 337 kg ha^-1. The reduction in first-flush grain yield of cowpea lines with the DLS trait under senescence-inducing conditions might be due to diversion of assimilates for maintenance of root and stem function and leaf area. Large amounts of non-structural carbohydrates are known to accumulate in the base of the stems of the DLS lines (Gwathmey et al., 1992a, b). The observed reductions in first-flush grain yield of DLS lines of up to 441 kg ha^-1 would be more than compensated for in environments with senescence-inducing soil conditions and a long growing season because of the higher plant survival of the DLS lines which enable them to produce a substantial second flush of pods with up to 2000 kg ha^-1 greater yield (Ismail and Hall, 1998). Gwathmey et al. (1992a) reported that second-flush grain yield of the DLS lines can contribute 30 to 41% of their overall yield in a full season compared to only 5 to 14% for non-DLS lines. In soybean the DLS phenotype also was found to be associated with a decrease in grain yield (Phillips et al., 1984). Increased seed size associated with the DLS trait may partially compensate for the potential loss in first-flush yield and could result in improved grain quality since larger seed are preferred in some markets for blackeye dry beans. In the field environment at Shafter where plant survival of all lines was high, only the heat-tolerant non-DLS lines had average plant survival below 97%. The lower (84%) survival of these lines may have resulted from a diversion of carbohydrates away from roots associated with their very high pod set and grain yield.

The causal organism for the premature death of cowpea probably is Fusarium solani (Erwin et al., 1991). Survival of plants and expression of the DLS trait may be associated with maintenance of high carbohydrate levels in stems and roots (Gwathmey et al., 1992b). It is also possible that high carbohydrate levels in stems and roots enhance resistance to F. solani. Sorghum [Sorghum bicolor (L.) Moench] hybrids with DLS also accumulate more carbohydrates in stems during grain maturation than senescent hybrids (McBee et al., 1983) and have enhanced resistance to charcoal rot [Macrophomina phaseoli (Tassi) Goid] (Duncan et al., 1981).

The high correlations of the DLS and plant survival scores between F₆ and F₉ generations shows the ease with which this trait can be incorporated into elite breeding lines provided a suitable screening environment is available. We have found selection for DLS on a family basis beginning in the F₄ generation can be effective. In choosing families, it is important to select ones that have both DLS and a substantial first-flush pod load because plants with few pods exhibit a different type of DLS with limited agronomic value. In sunflower (Helianthus annuus L.), studies on inheritance of the stay-green trait using two crosses indicated that additive effects were the main source of genetic variation and the authors concluded that selection for this trait could be made in early-generation segregating populations (Cukadar-Olmedo and Miller, 1997). In contrast, soybean studies using different crosses indicated that expression of the DLS phenotype was influenced by either one or two major genes or controlled in a quantitative manner and is strongly susceptible to environmental influence (Pierce et al., 1984). Expression of the DLS trait in cowpea, however, has been consistent for the parental line 8517 and most DLS breeding lines in different fields in California and Senegal where cowpea has been grown for several years (Hall et al., 1997).

Phenotypic variation of the heat-tolerance trait was well expressed at Shafter in 1998 (Table 5) and in a hot greenhouse in 1996 (Table 1) and partially expressed at Riverside in 1997 (Table 2). In these environments, heat tolerance increased grain yield because of increases in harvest index, pod set, and individual seed weight. Our previous studies with pairs of cowpea lines that are either heat tolerant or heat susceptible 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 with no consistent differences in individual seed weight (Ismail and Hall, 1998). However, in the current studies, individual seed weight of the heat-tolerant lines was consistently higher than that of the heat-susceptible lines in all of the field experiments (Tables 2, 3, and 5). It is possible that, in this cross, the gene conferring heat tolerance at pod set is linked to an allele that independently enhances individual seed weight since an association between heat tolerance and individual seed weight was not observed by Ismail and Hall (1998) who used different genetic lines. Heat-tolerant lines had shorter stems because of fewer and shorter internodes than heat-susceptible lines (Table 6 and 7). This is in agreement with our previous studies with different sets of heat-tolerant and heat-susceptible lines (Ismail and Hall, 1998, 1999) indicating that heat tolerance and dwarfing may be pleiotropic. At Shafter, heat-susceptible lines set their first pods at significantly higher internodes than heat-tolerant lines (Table 7) possibly due to high-temperature-induced suppression of early floral buds under these conditions (Dow el-madina and Hall, 1986; Warrag and Hall, 1984; Ismail and Hall, 1998, 1999).

In the field environments, the only significant interaction between the DLS and heat-tolerance traits was a tendency for heat-tolerance to be associated with enhanced premature death in the non-DLS lines but not in lines with the DLS trait (Table 5). The heat-tolerant non-DLS lines also had the highest pod set (Table 5) which could have resulted in less partitioning of carbohydrates to roots causing greater root senescence and premature plant death.

The DLS and heat-tolerance traits that were incorporated into cowpea only had small interactive effects. Both traits exerted substantial beneficial effects on grain yield in specific circumstances. Under senescence-inducing conditions, the DLS trait greatly increases plant survival after the first flush of pods is produced and may also increase individual seed size. The increased survival will result in substantial grain yield from a second flush of pods under long-season conditions. The DLS trait also causes a small decrease in first-flush grain yield which will result in reduced yield in short-season conditions. The heat-tolerance trait increases first-flush grain yield in very hot environments and only slightly enhances the tendency for premature plant death in non-DLS lines with no effect on lines having the DLS trait. Breeders should consider incorporating DLS and heat tolerance into cowpea cultivars grown for long-season (double flush) production and only heat tolerance for short-season (single flush) production.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
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 October 18, 1999.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 

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