Genotypic comparisons of imbibition in chickpea (Cicer arietinum L.) and cowpea (Vigna unguiculata (L.) Walp.)
Examination of the imbibition characteristics of eighty chickpea genetic lines (53 Desi and 27 Kabuli types) and eleven cowpea cultivars revealed that in both species seeds with unpigmented testae imbibed more rapidly than the pigmented seeds. Rapid imbibition was associated with high solute leakage, indicative of possible imbibition damage. Further evidence of imbibition damage was revealed in cowpea where the rapidly imbibing unpigmented seeds had poor vital staining with tetrazolium chloride after 24h imbibition. In chickpea, all the genetic lines of white Kabuli seeds had similar rapid rates of imbibition. In contrast, the pigmented Desi genetic lines revealed a range of rates of water uptake although many imbibed more slowly than the Kabuli lines. The incidence of delayed imbibers, that is, seeds which failed to imbibe until after 8h in water, was the main reason for reduced rates of imbibition in Desi lines. The restriction to water uptake by the pigmented seeds was mainly influenced by the properties of the seed coat. In cowpea seeds, the permeability of the seed coat was the most important factor limiting water uptake whereas in chickpea the permeability of the seed coat as a whole and of the micropyle as well as the degree of adherence of the seed coat to the cotyledons were important in regulating the movement of water into the seed. Investigation of the rate of imbibition during the maturation of dwarf French bean, cowpea and chickpea seeds revealed that development of slow water uptake by coloured cultivars was clearly associated with the appearance of pigmentation. In chickpea, this also coincided with the development of the adherence of the seed coat to the cotyledons. In contrast, seeds from unpigmented cultivars imbibed rapidly at all stages of maturation. Examination of the seed coat anatomy of chickpea and cowpea indicated that the pigmented seeds tended to have shorter and narrower palisade cells and also shorter tracheid bars than the unpigmented seeds. Differences in water uptake by Kabuli and Desi seeds were not evident when seeds were stored at 100% relative humidity and 40oC for days. All seeds deteriorated at a similar rate. However, the importance of initial seed quality in influencing deterioration during storage was emphasised by the rapid loss of germination of the Kabuli seeds in which germination and vital staining revealed poor initial quality.