Variation in traits associated with chilling tolerance during emergence in cowpea germplasm

Warm-season annuals are sensitive to chilling temperatures. For cowpea, chilling tolerance during emergence has been associated with two independent seed traits: a specific dehydrin protein and slow electrolyte leakage in chilling conditions as a measure of membrane integrity. We screened F-2 and backcross populations for the presence of the dehydrin protein in seed using immunoblot analysis. Segregation of the dehydrin protein indicated a single dominant gene controls its expression. We screened four sets of cowpea accessions with diverse origins for the two traits that had been associated with chilling tolerance and the presence of hard seed that did not readily imbibe water. The hard seed trait also may enhance chilling tolerance since slower imbibition of water at low temperature can reduce chilling injury. We tested the hypothesis that the extent of these three chilling-tolerance traits may vary for germplasm populations of subtropical and tropical origins because tropical environments exhibit little chilling. The dehydrin protein was present in many landraces (59% out of 140) that evolved in subtropical zones around the Mediterranean sea where soils often are cool during emergence. However, the dehydrin protein also was present in most of the tropical accessions (88% out of 59), which suggests that it may have an additional adaptive role during seed maturation and/or germination since chilling tolerance would not be of significance in the tropics. Only three out of 61 US cultivars, which had been developed in subtropical zones where soils can be cool, contained the dehydrin protein. Only four out of 40 lines in a population of tropical origin that had been selected for chilling tolerance during vegetative and reproductive growth contained the dehydrin protein in their seed. Genotypes were identified that had similar slow electrolyte leakage from seeds at low temperature as the chilling tolerant line 'UCR 1393-2-11', but none of these accessions also contained the dehydrin protein as does 'UCR 1393-2-11'. Frequency of hard seeds was high among Mediterranean landraces (48%) and US cultivars (21%) but low among tropical accessions (5%). We describe genotypes with traits associated with chilling tolerance during seedling emergence that could be useful in cowpea breeding. (C) 2002 Elsevier Science B.V. All rights reserved.