The response of cowpea (Vigna unguiculata L. Walp) seeds differing in seed coat colour to storage and reduced water availability during germination
These studies were conducted to determine the response of cowpea cultivars differing in seed coat colour to storage and reduced water availability during germination, and to relate these to the chemical composition of their seed coats. Seeds of five cowpea cultivars were stored under simulated tropical conditions (30°C/75.5% r.h.) for 6 months, subjected to accelerated ageing (40°C/100% r.h.) for 6 days or controlled deterioration (40°C/20% m.c.) for 4 days. Unpigmented seeds showed more rapid deterioration which was revealed by low germinations, high leachate conductivity and poor vital staining with 2, 3, 5 Triphenyl Tetrazolium Chloride (TTC). These differences were attributed to differences in moisture contents that occurred in storage as well as genotypic differences. Increased susceptibility to imbibition damage occurred during storage particularly in unpigmented seeds. The seed vigour parameters at the end of the stimulated tropical storage correlated well with accelerated ageing, suggesting the possible use of rapid ageing to predict the storage potential of cowpea seeds. Percentage radicle emergence was high in the pigmented cultivars germinated at -0.0015, -0.1 and -0.15 MPa and lower in unpigmented cultivars as matric potential was reduced. Imposition of reduced water availability (-0.15 Mpa) after 12, 24 and 48 hours germination at -0.0015 MPa led to slower rates of radicle emergence in all seeds transferred at 12 and 24 hours. In the unpigmented cultivars, these early transfers led to reduced radicle emergence after 7 days and low final normal germination after transfer to -0.0015 MPa for a further 7 days. This deleterious effect was revealed by poor vital staining of the embryonic axes. Protein synthesis of root tips after 24 hours -0.0015 MPa, 96 hours -0.15 MPa followed by a recovery period of 48 hours -0.0015 MPa was twice as much in the pigmented cultivar which explained its greater ability to recover from stress. Screening of a further ten cultivars for tolerance of reduced water availability revealed three groupings, (1) unpigmented cultivars that showed low final normal germination and rapid rates of water uptake in water, (2) smooth-coated pigmented cultivars that showed high final germination and slow rates of water uptake and (3) wrinkled -coated pigmented cultivars which showed intermediate germination and high rates of water uptake.