Apical senescence in genotypes If E Sn hr and If E Sn Hr is shown to exhibit a true photoperiod response. Partial far-red reversibility of a red night break occurs, implicating phytochrome in the response. Although the flowering node of these genotypes is independent of photoperiod several measures of flower development also exhibit true photoperiod responses. The results can be interpreted on the basis of the control of the activity of the Sn gene by light, this gene having pleiotropic effects on flower initiation, flower development, fruit set and senescence. This gene therefore acts as a major controller of reproductive development in peas and suggests that senescence cannot be studied without regard to flower development in the above genotypes.