Assessing genotypic variability of cowpea (Vigna unguiculata [L.] Walp.) to current and projected ultraviolet-B radiation

The current and projected terrestrial ultraviolet-B (UV-B) radiation affects growth and reproductive potential of many crops. Cowpea (Vigna unguiculata [L.] Walp.), mostly grown in tropical and sub-tropical regions may already be experiencing critical doses of UV-B radiation due to a thinner ozone column in those regions. Better understanding of genotypic variability to UV-B radiation is a prerequisite in developing genotypes tolerant to current and projected changes in UV-B radiation. An experiment was conducted in sunlit, controlled environment chambers to evaluate the sensitivity of cowpea genotypes to a range of UV-B radiation levels. Six cowpea genotypes [Prima, California Blackeye (CB)-5, CB-27, CB-46, Mississippi Pinkeye (MPE) and UCR-193], representing origin of different geographical locations, were grown at 30/22 °C day/night temperature from seeding to maturity. Four biologically effective ultraviolet-B radiation treatments of 0 (control), 5, 10, and 15 kJ m^-2 d^-1 were imposed from eight days after emergence to maturity. Significant genotypic variability was observed for UV-B responsiveness of eighteen plant attributes measured. The magnitude of the sensitivity to UV-B radiation also varied among cowpea genotypes. Plants from all genotypes grown in elevated UV-B radiation were significantly shorter in stem and flower lengths and exhibited lower seed yields compared to the plants grown under control conditions. Most of the vegetative parameters, in general, showed a positive response to UV-B, whereas the reproductive parameters exhibited a negative response showing the importance of reproductive characters in determining tolerance of cultivars to UV-B radiation. However, all cultivars, except MPE, behaved negatively to UV-B when a combined response index was derived across parameters and UV-B levels. Based on the combined total stress response index (C-TSRI) calculated as sum of individual vegetative, physiological and reproductive component responses over the UV-B treatments, the genotypes were classified as tolerant (MPE), intermediate (CB-5, CB-46 and UCR-193) and sensitive (CB-27 and Prima) to UV-B radiation. The differences in sensitivity among the cowpea genotypes emphasize the need for selecting or developing genotypes with tolerance to current and projected UV-B radiation.