A stability analysis of time to flowering as a screen for responsiveness to temperature and photoperiod in cowpea (Vigna unguiculata)

Twenty-one genotypes of cowpea (Vigna unguiculata), comprising landraces and varieties, were grown in 22 photothermal environments in Nigeria and Niger, West Africa, and a stability analysis of days from sowing to flowering (f) was carried out. Cowpeas are rarely insensitive to photoperiod; they are typically quantitative short-day plants wherein f is delayed when photoperiod (P) is longer than the critical photoperiod (P-e). Therefore, in order to quantify genotypic variation in temperature sensitivity, genotype f was regressed against the mean trial f in circumstances where P < P-e (i.e. approximately less than or equal to 13 hd(-1)) and mean temperature (T) was between 19 degrees and 28 degrees C. Correspondingly, in order to assess genotypic variation in photoperiod sensitivity, trials where T was near optimal (25 degrees-28 degrees C) but where P ranged from 10-14.5 hd(-1) were used. These stability analyses detected no significant differences (P > 0.05) between genotypes in temperature sensitivity but revealed significant differences (P < 0.001) in photoperiod sensitivity. Regression coefficients from the stability analysis were strongly correlated (r = 0.94, 19df) with a photoperiod sensitivity constant, c', determined from a photothermal flowering model. A stability analysis of f from field trials can therefore identify and quantify genotypic variation in response to temperature and photoperiod in cowpea.