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

Abstract

Late cultivars of peas behave as quantitative long day plants. The reason that they flower between nodes 20 and 35 under an 8 h photoperiod is shown to be because the leaves and mature stem produce a more promotory ratio of the flowering hormones as they age. Later formed leaves may also start with a slightly more promotory ratio than the leaves produced at a lower node. The gene Sn controls the production of a flower inhibitor and it is suggested that the activity of this gene in a leaf is gradually reduced as the leaf ages. From grafting experiments, the site of action of the gene Hr is shown to be in the leaves or mature stem and not at the shoot apex. This supports a previous suggestion that the gene Hr is a specific inhibitor of the ageing response of gene Sn. Gene Hr is shown to cause a substantial delay in the flowering node of decotyledonized plants of genotype If e snhr under short day conditions, suggesting that Hr has little effect in the cotyledons. It is argued that the gene sn is a leaky mutant and that gene Hr does not control a photoperiod response in its own right but has its effect through the Sn locus. From a comparison of intact plants and self-grafts of the late genotype If e Snhr it is shown that under the conditions used physiological age may be of more importance than chronological age in determining flowering in peas. Reasons for the small effect of defoliation treatments on flowering are discussed as well as possible reasons for the promotory effect of decotyledonization on the flowering node of late lines.