The Floral Stimulus Is Transported via the Phloem

Once produced, the flowering stimulus appears to be transported to the meristem via the phloem, and it appears to be chemical rather than physical in nature. Treatments that block phloem transport, such as girdling or localized heat-killing (see Chapter 10), prevent movement of the floral signal.

It is possible to measure rates of movement of the flowering stimulus by removing a leaf at different times after induction, and comparing the time it takes for the signal to reach two buds located at different distances from the induced leaf. The rationale for this type of measurement is that a threshold amount of the signaling compound has reached the bud when flowering takes place, despite the removal of the leaf.

Studies using this method have shown that the rate of transport of the flowering signal is comparable to, or somewhat slower than, the rate of translocation of sugars in the phloem (see Chapter 10). For example, export of the floral stimulus from adult leaves of the SDP Chenopodium is complete within 22.5 hours from the beginning of the long night period. In the LDP Sinapis, movement of the floral stimulus out of the leaf is complete by as early as 16 hours after the start of the long-day treatment. These rates are consistent with a floral stimulus that moves in the phloem (Zeevaart 1976).

Because the floral stimulus is translocated along with sugars in the phloem, it is subject to source-sink relations. An induced leaf positioned close to the shoot apex is more likely to cause flowering than an induced leaf at the base of a stem, which normally feeds the roots. Similarly, non-induced leaves positioned between the induced leaf and the apical bud will tend to inhibit flowering by serving as the preferred source leaves for the bud, thus preventing the floral stimulus from the more distal induced leaf from reaching its target. This inhibition also explains why a minimum amount of photosynthesis is required by the induced leaf to drive translocation.

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