Anthony E. Hall

COWPEA (VIGNA UNGUICULATA L. WALP.)

Abstract Cowpea (Vigna unguiculata L. Walp.) is a widely adapted, stress tolerant grain legume, vegetable, and fodder crop grown on about 7 million ha in warm to hot regions of Africa, Asia, and the Americas. This review focuses on major breeding achievements, current objectives, and future opportunities for cowpea improvement. Early maturing cultivars have been developed with regionally acceptable grain quality and resistance to some important diseases and pests including bacterial blight (Xanthomonas campestris), cowpea aphid-borne mosaic virus (CABMV), cowpea aphid (Aphis craccivora), cowpea curculio (Chalcodermus aeneus), root-knot nematodes (Meloidogyne incognita and M. javanica), cowpea weevil (Callosobruchus maculatus) and the parasitic weeds Striga gesnerioides and Alectra vogelii. earliness is important in Africa and other regions because early cultivars can escape drought and some insect infestations, can provide the first food and marketable product available from the current growing season, and can be grown in a diverse array of cropping systems. New early maturing cultivars with indeterminate growth habits have been very effective in the extremely dry and hot environment of the Sahel. Heat tolerant breeding lines have been developed which have markedly higher pod set than most cultivars under high night temperature conditions. Development of cultivars with multiple resistances to biotic and abiotic stresses is an important current breeding objective. Earliness, delayed leaf senescence, and indeterminate growth habit are characteristics which are being combined to improve drought adaptation. In the future, high levels of resistance to very important insect pests such as flower thrips (Megalurothrips sjostedti), maruca pod borer (Maruca testulalis), lygus (Lygus hesperus), and pod bugs (Clavigralla tomentosicollis and others) need to identified. Genes from wild cowpeas or related Vigna species or genetic engineering may be necessary to develop cultivars with high levels of resistance to several of the major insect pests.

A genetic linkage map of cowpea (Vigna unguiculata ) developed from a cross between two inbred, domesticated lines

Abstract We have constructed a genetic linkage map within the cultivated gene pool of cowpea (2n=2x=22) from an F8 recombinant inbred population (94 individuals) derived from a cross between the inbreds IT84S-2049 and 524B. These breeding lines, developed in Nigeria and California, show contrasting reactions against several pests and diseases and differ in several morphological traits. Parental lines were screened with 332 random RAPD decamers, 74 RFLP probes (bean, cowpea and mung bean genomic DNA clones), and 17 AFLP primer combinations. RAPD primers were twice as efficient as AFLP primers and RFLP probes in detecting polymorphisms in this cross. The map consists of 181 loci, comprising 133 RAPDs, 19 RFLPs, 25 AFLPs, three morphological/classical markers, and a biochemical marker (dehydrin). These markers identified 12 linkage groups spanning 972 cM with an average distance of 6.4 cM between markers. Linkage groups ranged from 3 to 257 cM in length and included from 2 to 41 markers, respectively. A gene for earliness was mapped on linkage group 2. Seed weight showed a significant association with a RAPD marker on linkage group 5. This map should facilitate the identification of markers that “tag” genes for pest and disease resistance and other traits in the cultivated gene pool of cowpea.

Development of cowpea cultivars and germplasm by the Bean/Cowpea CRSP

This paper reviews accomplishments in cowpea cultivar and germplasm development by the Bean/Cowpea Collaborative Research Support Program (CRSP) which was funded by the United States Agency for International Development for a period of about 20 years. Drought-adapted, pest and disease resistant cultivars ‘Mouride’, ‘Melakh’ and ‘Ein El Gazal’ were developed for rainfed production in the tropical Sahelian zone of Africa. Cultivars ‘CRSP Niebe’ and ‘Lori Niebe’ which have seed and pod resistance to cowpea weevil and some disease resistance were developed for rainfed production in the tropical Savanna zone of West Africa. Cultivar ‘California Blackeye No. 27’ was developed for irrigated production in subtropical California, USA and is a semidwarf with heat tolerance and broad-based resistance to root-knot nematodes and Fusarium wilt. Various cultivars with persistent-green seed color including ‘Bettergreen’ and ‘Charleston Greenpack’ were bred for use in the food freezing industry in the USA. Germplasms were developed with unique traits including: snap-type pods, green manure/cover crop capabilities, heat tolerance during reproductive development, chilling tolerance during emergence, delayed leaf senescence as a mechanism of adaptation to mid-season drought and high grain yields, differences in stable carbon isotope discrimination, harvest index, rooting and plant water- and nutrient-relations traits, broad-based resistance to root-knot nematodes and Fusarium wilt, and resistance to flower thrips, cowpea aphid, lygus bug and cowpea weevil, and various quality traits including all-white and sweet grain. These germplasms provide a valuable resource for breeding additional cowpea cultivars for Africa and the USA.

Reproductive responses of cowpea (Vigna unguiculata (L.) Walp.) to heat stress. II. Responses to night air temperature

The influence of high night temperature and drought on the abscission of reproductive organs were studied with cowpea (Vigna unguiculata (L.) Walp. cv. ‘California Blackeye No. 5’) in controlled environments. Floral development of cowpeas at 33/22°C day/night temperature appeared to be normal. However, at 33/30°C day/night temperature all flowers abscissed within 48 h after anthesis due to male sterility resulting from abnormal pollen development and anther indehlscience. By moving plants between growth chambers with high and moderate night temperatures, it was demonstrated that the stage of floral development most sensitive to high night temperature occurred 5–7 days before anthesis. This corresponds with the stage just after the release of the tetrads from the microscope mother cell sac. Plants subjected to drought were smaller, and at the lower night temperature drought resulted in less seed production but this was due to fewer reproductive nodes and the percent abscission of reproductive organs was not influenced by drought. Observations were made on the effects of high temperatures on reproductive growth of six cowpea strains grown under field conditions. Complete abscission of flowers was observed during hot weather under field conditions followed by pod set approximately 6 days after decrease in night temperature. These data are consistent with the observations on cowpeas in growth chambers which demonstrated that high night temperatures can cause male sterility and complete abscission of flowers.

Heat tolerance of contrasting cowpea lines in short and long days

High-temperature damage to reproductive processes of cowpea (Vigna unguiculata L. Walp.) occurs when minimum night air temperatures are greater than 20°C. The extent of damage is strongly influenced by photoperiod. Genes conferring heat tolerance under hot long-day environments have been identified in cowpea, and breeding lines with this trait have been developed. Genetic variability within cowpea germplasm for heat tolerance has not been assessed in short days. The objectives of this study were to determine if the heat-tolerant accessions and breeding lines developed from them under hot long-day conditions also exhibit heat tolerance under hot short-day conditions. In addition, we evaluated the heat tolerance of 48 lines that do not flower in hot long-day conditions because of sensitivity to heat or photoperiod. Three short-day experiments and one long-day experiment were conducted in greenhouses with average daily minimum and maximum air temperatures of 27 and 36°C, respectively. Simultaneously, moderate-temperature short-day experiments were conducted in nearby greenhouses with average daily minimum and maximum air temperatures of 18 and 32°C, respectively. Days to first flower, pods per peduncle, total number of pods, and grain yield were determined in all experiments. Individual seed weight and seeds per pod were determined for two of the three hot short-day and moderate-temperature short-day experiments. Heat-tolerant lines that had been selected under hot long-day conditions had larger than average grain yield (i.e. heat tolerance) under hot short days. Several lines bred for high grain yield under hot tropical conditions had heat tolerance under short days but not under long days, whereas many cultivars developed under cooler tropical conditions did not exhibit heat tolerance. In hot short-day environments, grain yield was positively correlated with pods per peduncle and number of pods per plant, indicating the importance of these traits for conferring heat tolerance in these lines. Most lines experienced substantial heat-induced reductions in number of seeds per pod, even though there were no strong indications of carbohydrate source limitations. Two of the heat-tolerant lines, B89-600 and TN88-63, did not exhibit heat-induced reductions in number of seeds per pod in hot short days, however, and represent additional sources of heat tolerance that would further enhance grain yield in hot production zones.

Responses of cowpea (Vigna unguiculata (L.) Walp.) in the field to high night air temperature during flowering. II. Plant responses

Nielsen, C.L. and Hall, A.E., 1985. Responses of cowpea (Vigna unguiculata (L.) Walp.) in the field to high night air temperature during flowering. II. Plant responses. Field Crops Res., 10: 181--196. The agronomic responses of a heat-sensitive (CB5) and two heat-tolerant (TVu4552 and Prima) cowpea strains to different levels of night air temperature during flowering were evaluated. An experimental field system at Riverside, California, was used to subject large plots of the contrasting genotypes to different controlled levels of warmed air at night. Differential thermostats were used to raise air temperatures in nighttime enclosures to simulate differential tropical and subtropical locations where cowpea are commercially grown. In 1982, CB5 and TVu4552 were subjected to five treatments in which average daily minimum temperature (Train) was 15.0, 16.5, 20.0, 23.0, and 26.5°C for the 4-week period of flowering and pod-fill. In 1983, CB5 and Prima were subjected to four treatments in which Tmin was 16.5, 22.0, 24.0, and 25.5°C for the 2-week period when most flowering occurred. During the day, plots were not enclosed and average daily maximum temperature during the treatment period was 33.5°C in 1982 and 34.5°C in 1983. In both years, percent flower abscission of CB5 was significantly and substantially increased by all higher night air temperature treatments. Percent flower abscission of TVu4552 and Prima was only significantly increased when mean Tmi n was 24.0°C or higher. TVu4552 and Prima also exhibited lower absolute levels of flower abscission, more pods per plant, and smaller relative decreases in pods per plant than CB5 with higher night air temperatures during flowering. Increase in flower abscission was the primary factor responsible for reductions in the number of pods per plant in all three genotypes. Reduced pot setting ability was the major cause of reductions in grain yield under higher night air temperatures, while reductions in seed weight and the number of seeds per pod were small. It should be noted that during 1983, plants were only subjected to higher night temperatures during the 2 weeks of flowering and plants experienced the same environmental conditions during a major part of the period when seeds were growing. TVu4552 and Prima exhibited less reduction in seed yield from warm air at night in relation to ambient controls, than did CB5 which was strongly affected by high night air temperature during flowering.

Influences of early and late nitrogen fertilization on yield and nitrogen fixation of cowpea under well-watered and dry field conditions

Abstract Influences of nitrogen fertilization on grain yield and nitrogen fixation of cowpea [Vigna unguiculata (L.) Walp. cv. ‘California Blackeye No. 5’] were studied under well-watered and dry field conditions. The soil was an Arlington fine sandy loam (coarse-loamy, mixed thermic Haplic Durixeralf). Nitrogen fertilization treatments involved applications to the soil either at sowing or early flowering, and foliar applications during pod filling, of 60 kg N/ha. Similar relative yield responses to fertilizer treatments were observed with cowpea subjected to both well-watered conditions and late drought. Nitrogen application at sowing had little effect on grain yield or N fixation from 45 days to the end of the season, whereas later applications to the soil or foliage substantially increased grain yield (by 300–600 kg/ha). Early-flowering soil application increased the numbers of branches and pods/m2 and reduced N fixation, whereas foliar fertilization delayed leaf senescence and increased seed weight. Applying N fertilizer both to the soil at early flowering (60 kg/ha) and to foliage (60 kg/ha) reduced N fixation, and increased both number of pods/m2 and seed weight. However, grain yield was increased only to the same extent as when fertilizer was applied to either the soil or the foliage alone. Folian, which contained N, P, K, S, and Fe, was a more effective foliar fertilizer than urea alone, although N was probably the major nutrient responsible for the increases in grain yield. Higher concentrations of Folian (33 g N/L) caused leaf burn, and did not increase yield as much as lower concentrations (22 g N/L and 11 g N/L), which did not cause leaf burn and increased yield significantly. Drought reduced both N fixation and the extent to which plants depended on fixed N for their total N supplies.

Global agricultural intensification during climate change: a role for genomics

Summary Agriculture is now facing the ‘perfect storm’ of climate change, increasing costs of fertilizer and rising food demands from a larger and wealthier human population. These factors point to a global food deficit unless the efficiency and resilience of crop production is increased. The intensification of agriculture has focused on improving production under optimized conditions, with significant agronomic inputs. Furthermore, the intensive cultivation of a limited number of crops has drastically narrowed the number of plant species humans rely on. A new agricultural paradigm is required, reducing dependence on high inputs and increasing crop diversity, yield stability and environmental resilience. Genomics offers unprecedented opportunities to increase crop yield, quality and stability of production through advanced breeding strategies, enhancing the resilience of major crops to climate variability, and increasing the productivity and range of minor crops to diversify the food supply. Here we review the state of the art of genomic‐assisted breeding for the most important staples that feed the world, and how to use and adapt such genomic tools to accelerate development of both major and minor crops with desired traits that enhance adaptation to, or mitigate the effects of climate change.

Effects of moisture regime and phosphorus on mycorrhizal infection, nutrient uptake, and growth of cowpeas (Vigna unguiculata (L.) Walp.)

Abstract Mycorrhizal infection, nutrient uptake, and growth of cowpea (Vigna unguiculata (L.) Walp.) were investigated in a greenhouse under dry and well-watered conditions of soil moisture (plants were irrigated when the matric potential of the soil, as measured by tensiometers, was within −60 to −65 kPa and −20 to −25 kPa, respectively. Substantial mycorrhizal infection occurred with low soil phosphorus concentrations (6 ppm P) under both well-watered and dry soil conditions. With high soil phosphorus concentrations (56 ppm P) mycorrhizal infection was not suppressed in dry conditions, as it was for well-watered plants; even though the concentrations of phosphorus were equally high in the plants grown under dry and well-watered conditions. Total dry matter and pod yield were significantly higher in mycorrhizal compared with non-mycorrhizal plants under limiting conditions of soil moisture and phosphorus. When soil phosphorus concentration was high, the presence of mycorrhizae had no influence on plant dry matter production and pod yield under both dry and well-watered conditions. The concentrations of phosphorus, zinc and copper were significantly higher in mycorrhizal plants under dry, low soil phosphorus conditions. The improved growth of plants due to the presence of mycorrhizae, under limiting conditions of soil moisture and phosphorus, was probably due to improved phosphorus nutrition.

Flowering of contrasting cowpea (Vigna unguiculata (L.) walp.) genotypes under different temperatures and photoperiods

Abstract The practical objective of this research was to develop a field screening technique that reliably detects genotypes which flower early and are insensitive to photoperiod (day-neutral). In pursuing this objective, the number of days to the first appearance of floral buds and open flowers was studied in two field environments and in controlled environments with eight contrasting cowpea (Vigna unguiculata (L.) Walp.) genotypes. The summer field environments had decreasing long days (15-13 h) and either high (Riverside, CA) or extremely high (Imperial Valley, CA) temperatures. The controlled environments included two daylengths (short, 11.5 h, or long, 15 h) and different day/night temperatures (30/20°C, 30/27°C, 30°C for 15 h/27°C, 36°C for 11.5 h/27°C) in eight different combinations. The flowering responses observed in the controlled environments were consistent with the responses observed in the field environments. Environmental control of flowering of four of the genotypes consisted of a strong photoperiod × temperature interaction. The number of days to first flower was not influenced by daylength at moderate temperatures but was delayed by long days at high temperatures, and with the combination of long days and very hot environments flowers were not produced. When flowering was delayed, the flowers first appeared on later nodes. Delays in flowering were mainly due to delays in first appearance of floral buds, except at extremely high temperatures where bud development was inhibited as well. Extending the daylength with low intensity light did not influence the number of days to first appearance of floral buds but it slowed down bud development. This indicated that bud initiation and bud development may be regulated differently by environment and that regulation of bud development may involve a classical photoperiod response. The other four genotypes were day-neutral and days to first flower exhibited an optimum response to temperature, with earliest flowering occurring at intermediate temperatures. For the same genotypes in different environments, floral buds first appeared at the same node and the first flowers were produced on the following node. A screening procedure is proposed for detecting cowpea genotypes that are early and day-neutral. This simply consists of screening under hot, long-day field conditions, such as occur in Imperial Valley, California, in the summer. Cowpea genotypes that flower early in this environment would probably flower early, compared with other cowpeas, in most cowpea production environments in the world.