Hot weather can substantially reduce pod production by cowpea [Vigna unguiculata (L.) Walp.]. The objectives were to characterize and classify cowpea based on their responses to heat during reproductive development. Diverse cowpea accessions were grown over six summers in Imperial Valley, CA under hot temperatures (mean daily max/min 41/24 °C) and long days (14 h, 51 min decreasing to 12 h, 18 min). Selected genotypes were grown at Riverside, CA, under more optimal temperatures (35/17 °C) and the same daylengths. At Imperial Valley, genotypic variation was observed for duration to first macroscopic floral bud and extent of floral bud abortion, peduncle elongation, flower production, and podding. At Riverside, similar genotypic variation occurred for days to first floral bud, but subsequent reproductive development was normal. Controlled environment studies in growth chambers showed that the detrimental effects induced in sensitive genotypes by high temperature were enhanced by long photoperiods. Based on these differing reproductive responses, the accessions were classified into eight groups. Heat‐tolerant accessions in Group I exhibited normal peduncle elongation, early flowering, and produced many flowers and pods. Accessions in Groups II and III performed similarly but set few pods (Group II) or no pods (Group III). Accessions in Groups IV through VII differed in duration to first floral bud, with Group VII being the latest and exhibiting substantial bud abortion, suppressed peduncle elongation, and no flowers. Group VIII accessions produced no visible floral buds. Cowpea developed in different regions and for specific seasons within a region fell into discrete classification groups, indicating that the system has a genetic and ecological basis. This classification system is useful for characterizing the types of heat responses present in cowpea genotypes and for selecting appropriate combinations of parents for breeding heat‐tolerant cultivars for different production zones and seasons.
Studies on the inheritance pattern of bacterial leaf spot (BLS), yellow mosaic (YM) and Cercospora leaf spot (CLS) reactions in crosses of BLS and YM resistant/tolerant but CLS susceptible x CLS resistant but BLS and YM susceptible parents indicated that resistances to BLS and CLS were governed by single dominant genes, whereas YM tolerance was a monogenic recessive character . The studies also indicated that these three genes were inherited independently . The simple inheritance pattern and independent assortment of the genes governing resistance/tolerance to these diseases suggest that the usual breeding methods will be adequate to develop multi-disease resistant mungbean cultivars .
High temperatures in tropical and subtropical zones often have detrimental effects on plants. Plants in these zones experience differences in daylength that could influence sensitivity to heat. Contrasting genotypes of cowpea, Vigna unguiculata (L.) Walp., were grown under fluorescent plus incandescent (F) or metal halide plus incandescent (MH) lamps at different daylengths (11, 12, 13, 14, or 16 h) to determine whether the sensitivity of floral development to high night temperatures is influenced by light quality and photoperiod. Floral bud development was suppressed in heat‐sensitive genotype (CB5) at a 14‐h photoperiod under MH, while a 16‐h photoperiod was required to elicit a similar response under F. Spectral analysis showed five times more ultraviolet‐A light (UV‐A) (315‐400 rim) in MH than in F, but F supplemented with UV‐A light did not elicit the same suppression of floral bud development as MH. The reproductive response to long days with hot nights (30 °C) was closer to that of field‐grown plants under MH than under F. Percent pod set of two sensitive genotypes (CB5 and 7964) subjected to high temperatures (33/30 °C day/night) was higher (23 and 19%) at an ll‐h photoperiod than at a 14‐h photoperiod (5%) under F. No pod set occurred in an ll‐h photoperiod with red light (R) during the night. Substantial pod set (41%) was observed a 14‐h photoperiod was followed by far‐red light (FR). The effect was reversed when FR was immediately followed by R. Apparently, pod set at high nightemperatures in heat‐sensitive, day‐neutral cowpea is dependent on photoperiod through a mechanism involving phytochrome.
Cowpea [Vigna unguiculata (L.) Walp.] accession TVu 4552 useful in breeding programs as a donor of heat tolerance during flowering. Unfortunately, seed coats of TVu 4552 and some other cowpea strains (e.g. 'TVx 3236') develop a pronounced brown discoloration when the plants are grown under hot air temperatures. The browning is intraceilular and confined to the seed coat. It does not influence germination, but it makes the grain less desirable to consumers. Inheritance of browning was studied in crosses of TVu 4552 with 'California Blackeye #5' (white seed coat, blackeye), 'Bambey 21' (white seed coat, no eye), and PI 204647 (kaiser brown seed coat, no eye) whose seed coat colors were not affected by high air temperatures. Segregation for seed coat browning was determined by observing the frequency of plants that produced at least some seeds with some brown discoloration, and plants for which all seeds had normal seed coat color. Analysis of F~, F2, F3, and backcross progenies showed that the seed coat browning reaction is dominant to normal seed coat color and governed by a single nuclear gene. It is suggested that this gene be designated as Hbs. No linkage was observed between Hbs-controlled seed coat browning and the gene governing heat tolerance during floral bud development in TVu 4552. Similarly, no apparent association was observed between the development of normal brown seed coat pigmentation in PI 204647 and heat-induced brown seed coat discoloration controlled by Hbs.
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