Flowering in Pisum: the Effect of Age on the Gene Sn and the Site of Action of Gene Hr

Late cultivars of peas behave as quantitative long day plants.The reason that they flower between nodes 20 and 35 under an8 h photoperiod is shown to be because the leaves and maturestem produce a more promotory ratio of the flowering hormonesas they age. Later formed leaves may also start with a slightlymore promotory ratio than the leaves produced at a lower node.The gene Sn controls the production of a flower inhibitor andit is suggested that the activity of this gene in a leaf isgradually reduced as the leaf ages. From grafting experiments,the site of action of the gene Hr is shown to be in the leavesor mature stem and not at the shoot apex. This supports a previoussuggestion that the gene Hr is a specific inhibitor of the ageingresponse of gene Sn. Gene Hr is shown to cause a substantial delay in the floweringnode of decotyledonized plants of genotype If e sn hr undershort day conditions, suggesting that Hr has little effect inthe cotyledons. It is argued that the gene sn is a leaky mutantand that gene Hr does not control a photoperiod response inits own right but has its effect through the Sn locus. From a comparison of intact plants and self-grafts of the lategenotype If e Sn hr it is shown that under the conditions usedphysiological age may be of more importance than chronologicalage in determining flowering in peas. Reasons for the smalleffect of defoliation treatments on flowering are discussedas well as possible reasons for the promotory effect of decotyledonizationon the flowering node of late lines. Pisum sativum L, flowering, ageing, genetic control