Delayed-Leaf-Senescence and Heat-Tolerance Traits Mainly Are Independently Expressed in Cowpea Abdelbagi M. Ismail, Anthony E. Hall,* and Jeffrey D. Ehlers ABSTRACT root rot in Phaseolus vulgaris L. (Burke and Miller, 1983) that involve similar or the same causal organisms. Reproductive activity in cowpea [Vigna unguiculata (L.) Walp.] Cowpea breeding lines with resistance to premature is characterized by two separate flushes of pod production. Grain yield from the second flush strongly depends on extent of plant death death have been discovered and described as having the due to soil pathogens after the first flush is completed. A delayed- delayed-leaf-senescence (DLS) trait (Gwathmey et al., leaf-senescence (DLS) trait enhances the extent of plant survival but 1992a). Two DLS lines exhibited 53 to 98% survival may reduce first-flush grain yield. Heat tolerance during reproductive after producing the first flush of pods compared with development can increase first-flush grain yield and has been incor- 15 to 28% survival for two non-DLS lines (Gwathmey porated into cowpea but may reduce plant survival after the first et al., 1992a). The DLS lines produced greater total flush is completed. Interactive effects when combining DLS and heat- (first-flush plus second-flush) yields with a tendency for tolerance traits were evaluated. A cross was made between a DLS smaller first-flush yields than the senescent lines heat-susceptible line and a non-DLS heat-tolerant line. Evaluation (Gwathmey et al., 1992a). The latter result might be and selection for these traits were carried out over several genera- due to the increased partitioning of carbohydrates to tions and a set of 40 lines with 10 each of the four combinations of DLS 1/2 heat tolerance and non-DLS 1/2 heat tolerance was devel- stems and roots in DLS lines (Gwathmey et al., 1992b), oped. These lines were evaluated in contrasting field environments. which makes less carbohydrate available to pods. Sur- Under senescence-inducing conditions, the DLS trait greatly increased vival of plants and expression of DLS may be associated plant survival and individual seed size, and it only caused a small with maintenance of high carbohydrate levels in roots. reduction in first-flush grain yield that would have been off-set several Overall this indicates the DLS trait may reduce produc- fold by an enhanced second-flush grain yield. The heat-tolerance trait tivity in short-season conditions, even though it en- increased first-flush grain yield in very hot environments and only hances productivity when conditions permit the harvest- slightly enhanced the tendency for premature plant death in non-DLS ing of two flushes of pods. The evidence, however, is lines with no effect on lines having the DLS trait. The DLS and heat- not conclusive in that the breeding lines that were com- tolerance traits can be effectively incorporated into cowpea and would pared by Gwathmey et al. (1992a) differ in many traits. have beneficial effects on grain yield in specific circumstances with only small detrimental interactive effects. The DLS trait has been observed in soybean lines and also was associated with a decrease in grain yield (Phil- lips et al., 1984). Many cowpea cultivars are susceptible to high night C owpea (blackeye dry bean) growers in the south- temperatures during reproductive development (Patel ern San Joaquin Valley of California often manage and Hall, 1990; Ehlers and Hall, 1996). California culti- the crop to accumulate two flushes of pods before cut- vars can exhibit a 13.5% decrease in grain yield per ting (Hall and Frate, 1996) because the second flush degree centigrade increase in average daily minimum can produce up to 2000 kg ha 21 which can result in total night temperature above 16.58C for the 3-wk period grain yields of about 5000 kg ha 21 (Gwathmey et al., starting 1 wk prior to the first appearance of flowers 1992a; Ismail and Hall, 1998). Unfortunately, in fields (Ismail and Hall, 1998). Heat-induced reductions in where cowpea have been grown for several years, many grain yield of cowpea under field conditions mainly are plants often die after producing the first flush of pods due to reductions in pod set and harvest index (Nielsen (Gwathmey et al., 1992a), even with alternate-year rota- and Hall, 1985; Ismail and Hall, 1998). Damage during tions. This premature death causes substantial reduc- reproductive development can occur at two distinct tions in total grain yield in that second-flush yield is stages: floral bud development and anther development. proportional to the number of plants surviving to pro- Following initiation, floral bud development and flower duce the second flush (Ismail and Hall, 1998). The causal production are suppressed by a combination of high organism for the premature death of cowpea probably night temperatures and long photoperiods (Dow el- is Fusarium solani (Mart.) Sacc. f. sp. phaseoli (Burk.) madina and Hall, 1986; Patel and Hall, 1990). High night Synd. & Hans., type A (Erwin et al., 1991). Conse- temperature during anther development does not affect quently, this cowpea disease may have some similarities flower production but can impair pod set by causing with Sudden Death Syndrome in soybean [Glycine max anther indehiscence and low pollen viability (Warrag (L.) Merr.] (Rupe, 1989; Njiti et al., 1997) and Fusarium and Hall, 1984; Ahmed et al., 1992). Through selection in field and greenhouse environ- Dep. of Botany and Plant Sciences, Univ. of California, Riverside, ments with high night temperatures (Hall, 1992), cowpea CA 92521-0124. Research partially supported by USDA NRICGP lines (Hall, 1993) and a cultivar (Ehlers et al., 2000) Award No. 98-35100-6129 and USAID Grant No. DAN-G-SS-86- 00008-00 to AEH. The opinions and recommendations are those of were bred with heat tolerance during reproductive de- the authors and not necessarily those of USAID. Received 18 Oct. 1999. *Corresponding author (anthony.hall@ucr.edu). Abbreviations: DLS, delayed-leaf-senescence; T max and T min , maximum and minimum daily air temperature. Published in Crop Sci. 40:1049–1055 (2000). 1049