Challenges and Opportunities for Enhancing Sustainable ... - IITA

Cowpea genetics and breeding
Table 1. Grain yields of six pairs of heat-tolerant and heat-susceptible cowpea lines
grown with complete irrigation at three locations in the subtropical zone of California,
USA, over 2 years with high levels of solar radiation and optimal management.
Riverside with
Coachella
early sowing Shafter valley
1995 & 1996 1995 1996 1995 & 1996
kg/ha
Heat-tolerant lines 3086 3357 2492 894
Heat-susceptible lines 2976 3310 2098 340
Significance NS NS ** ****
Daily minimum
air temperature o C † 14.6 16.4 17.4 23.7
** and **** are significant at the 0.01 and 0.0001 levels whereas NS is not significant at the 5% level.
†
Average for the 3-week period beginning one week prior to the start of flowering.
Source: From Tables 2 and 3 in Ismail and Hall 1998.
the heat-susceptible lines in the hotter environments, but similar grain yields in the cooler
environments. One of these heat-tolerant lines has been released as California Blackeye No.
27 (CB27) for use as a dry grain cultivar in California (Ehlers et al. 2000). Heat-tolerant
lines were much shorter compared with heat-susceptible lines and this effect was more
pronounced in hotter environments (Ismail and Hall 1998). In California, the semi-dwarf
cultivar CB27 has a greater yield advantage over current standard height cultivars, such
as CB5, when grown on rows 51 to 76 cm apart rather than the wide rows (about 102 cm
apart) that are used by some growers (Ismail and Hall 2000). Row spacing in this study
was 10 cm between plants in the row.
In the relatively high night temperatures experienced in tropical zones, the heat-tolerant
lines developed in California experience even more dwarfing than in subtropical zones.
We have studied whether the heat-tolerance genes shown to be effective in the subtropical
zone of California are also effective in tropical zones of West Africa where there is substantial
cowpea production. Daily minimum air temperatures were substantially higher in
the savanna and Sahelian zones (Table 2) than the threshold of 17 o C for causing damage
to flower development and pod set of the heat-susceptible lines indicated by the studies
of Ismail and Hall (1998) in Table 1. In all of the trials in West Africa, however, there was
no significant difference in grain yield between the averages of the six heat-tolerant and
six heat-susceptible lines (Table 2).
The contrasting performance of the 2 sets of lines may be explained by the longer
day lengths experienced by plants in California compared with West Africa. Controlledenvironment
studies had shown that high night temperatures could be more damaging to
cowpea in long-day than in short-day conditions (Mutters et al. 1989). Studies in which
the heat tolerance of contrasting cowpea lines were evaluated in greenhouses with high
night temperature and either long or short days (Ehlers and Hall 1998) have provided
some explanations for the contrasting performance of the lines in different field conditions.
A subset of the data is presented in Table 3. Heat-tolerant California lines had greater
grain yields than heat-susceptible California lines in hot long-day greenhouse conditions
(Table 3) as had been observed in hot long-day field conditions in Shafter in 1996 and
in the Coachella Valley in both 1995 and 1996 (Table 1). In contrast, in hot, short-day
greenhouse conditions, heat-tolerant and heat-susceptible California lines had similar
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