Reproductive development in Pisum : the role of genes Sn and Lf

The influence of the flowering genes Sn/sn and Lfd/Lf/lf/lfa on postinitiation reproductive events in Pisum was examined using pure lines and two specially selected near-isogenic lines in the case of the Sn/sn difference and pure lines and segregating progenies in the case of the Lf series. From a comparison of lines 299+ (Sn, early photoperiodic) and 299- (sn, early day neutral) in short day conditions, it was found that gene Sn slowed the rate of flower bud, pod and seed development, prolonged flower life-span and increased the number of pods per raceme. Sn also promoted vegetative growth by delaying the onset of apical arrest and increasing the tendency to produce lateral branches. The delay in growth of the proximal pod at the first reproductive node remained even when the distal flower was excised at this and all subsequent nodes. The delay was therefore not a consequence of raceme ontogenesis. In contrast, removal of the apical bud immediately above the node bearing the first pod partly eliminated the effect of the Sn/sn difference on pod growth. By comparing genetically male sterile with fertile plants in a segregating progeny, it was found that flower lifespan was substantially extended in the absence of fertilisation. However, when the growth of Sn and sn pollen tubes was examined in vivo with the aid of fluorescence microscopy, no difference in the rate of growth could be demonstrated for the two genotypes. Hence, the increased life-span in Sn flowers does not appear to be a consequence of slower pollen tube growth. Since short days increased the interval between flower initiation and open flower in Sn plants but not sn plants, this interval is neither independent of photoperiod nor genotype in Pisum. The effect of the Lf series was examined on a day neutral background. The rate of flower bud development and pod growth was slightly promoted as the sequence lfa , lf, Lf and Lfd was ascended, i.e., as the time to flower initiation was delayed, and the promotion in reproductive growth appears to be a consequence of the delay in time to initiation. It is suggested that the underlying metabolism is, at first, strongly geared toward vegetative growth but that it becomes more favourable to reproductive growth with the passage of time. In contrast, Sn appears to interfere with the metabolic pathway, probably by delaying the time at which autonomous changes in the underlying metabolism occur. Sn may act by controlling assimilate partitioning within the shoot system. One of the crosses used to examine the effect of an Lfd/lf segregation also permitted the effect of segregation for the flowering gene pair Hr/hr to be studied on an Lfd sn background for the first time. Although many of the Lfd segregates initiated their first flower at node 20 or higher, no significant effect of segregation for Hr/hr could be detected. It seems therefore that sn remains epistatic to Hr in these circumstances.