Flowering inPisum: the Effect of Genotype, Plant Age, Photoperiod, and Number of Inductive Cycles

Abstract

The genotypes If e Sn hr, Lf e Sn hr, and If e Sn Hr require fewer inductive cycles as they age. It is suggested that this results from a decrease in the activity of the Sn gene in the leaves as they age, resulting in a higher ratio of promoter to inhibitor. Gene Lf does not affect the rate of this aging but it does increase the number of inductive cycles required for flower induction over the first 5 weeks of growth. The gene Hr has no effect until week 4 but thereafter causes a reduction in the effect of age on the Sn gene. The genotype If e Sn Hr can be induced by a single inductive cycle (32 h of light) for a relatively long period. The length of dark period required for the expression of the Sn gene is shown to be less than 4 h providing a relatively long photoperiod precedes the dark period. It appears that no per manent induction of tissue by photoperiods favourable to flowering occurs in peas. The critical photoperiod for plants of genotype if e Sn Hr is shown to be between 12 and 14 h at l7·5 °C and the usefulness of the term ‘critical photoperiod’ is discussed with respect to quantitative long-day plants.