Development in Cowpea ( Vigna unguiculata ). III. Effects of Temperature and Photoperiod on Time to Flowering in Photoperiod-sensitive Genotypes and Screening for Photothermal Responses

SUMMARY Seventeen photoperiod-sensitive genotypes of cowpea ( Vigna unguiculata ) were grown in approximately 30 photothermal environments in Nigeria. Photoperiods ranged from 10 to 16 h d −1 , mean temperatures from 19° to 30°C and times from sowing to flowering ( f ) from 32 to 140 d. Rate of progress towards flowering (1/ f ) was related to mean pre-flowering values of temperature and photoperiod using simple linear rate models comprising one, two or three planes (thermal, photothermal and insensitive). There were no significant differences (p > 0.25) among genotypes in response to temperature within the thermal plane and the common base temperature was estimated to be 7.6°C. Photoperiod-sensitivity varied by a factor of 15 among genotypes, and the critical and ceiling photoperiods varied from 12.2 to 13.4 and from 13.8 to more than 16 h d −1 at a mean temperature of 27°C, respectively. These simple models satisfactorily predicted f in an independent data set (R 2 = 0.62) for plants grown in the main cowpea growing seasons at latitudes between 7° and 13°N. The utility of photothermal models and methods to screen for photothermal responses are discussed.