Flowering in Pisum: The influence of photoperiod and vernalising temperatures on the expression of genes Lf and Sn

Abstract

Summary The node of first initiated flower in the early pea genotype lf e sn hr is little affected by photoperiod and vernalisation. Gene Sn confers a marked ability to respond to both photoperiod and vernalisation, the flowering delay in short days being reduced by vernalisation and increased by Lf. Gene Lf does not by itself confer a photoperiod response but it does confer a potential to respond to vernalisation, the magnitude of the response being greatest in those plants having the latest background of Lf modifiers. Genotypes lf e sn hr and lf e Sn hr flowered at a similar early node if the plants were exposed to continuous light from the start of germination, but lf e Sn hr was delayed by 5 nodes (to node 15) if the cotyledons were kept in the dark. lf e sn hr scions grown in short days were also delayed to node 15 when grafted to lf e Sn hr stocks whose cotyledons were held in the dark. These results suggest that Sn activity is totally suppressed by continuous light and that the flower inhibitor formed by the Sn gene in the dark is not destroyed by long days. They also show that the cotyledons of genotype lf e Sn hr can respond to photoperiod like foliage leaves from the time Sn activity is possible and that Sn activity in the cotyledons only delays flowering to node 15, delays beyond node 15 in lf shoots being due to Sn activity in the shoot itself. Genotype lf e Sn hr showed no vernalisation response when Sn activity was precluded by light, and continued cold temperatures at night (2–4° C) prevented a photoperiod response in vernalised plants, possibly by retarding the formation of inhibitor. The same direct effect on Sn activity may also account for the vernalisation response in lf e Sn hr but a second effect of vernalisation was not ruled out. In the case of Lf and its modifiers vernalisation possibly acts by altering the sensitivity of the apex to the flowering hormones.