Photoperiodic effects in Vigna unguiculata (L.) Walp.

Lengthening of daylight with fluorescent light (intensity 6000 erg/sec/cm ²) from 6 to 24 h for 15 days increased stem length, internode length, number of leaves and leaf size in 16 varieties of cowpea. Four varietal types of growth habit could be distinguished. Experiments on vegetative growth were mainly with varieties whose growth was inhibited in short days (SD) before the onset of flowering (not reversed by subsequent long days (LD)), whereas in LD growth continued for at least 6 to 7 months. The SD stimulus was perceived by the leaves and its effect increased with number of leaves. Preceding LD treatment decreased or nullified sensitivity to SD. The inhibitive effect of SD on stem elongation could partly be overcome by gibberellic acid or dimmer light. In SD a diurnal change of temperature from 30°C during day to 20° at night (less at 15°) favoured stem elongation. It was suggested that in SD growth inhibitors and in LD growth promotors were produced.

The cowpea flowered only in SD, except the variety Early Red which showed an ambi-photoperiodic reaction. Photoperiods from 8 to 11 h proved to be optimum to initiate inflorescence. Photoperiodic influences on development and flowering of the inflorescences were also investigated, as well as those of position and removal of leaves. The practical value of the results for growing cowpeas was assessed.