Translocation of the Floral Stimulus

Leaving aside for the moment the nature of the floral stimulus, it is possible to ask how it is transmitted from an induced leaf to the apex. Treatments which restrict phloem transport, such as localised heat, cold treatments, removing a ring of tissue external to the xylem, or narcotic treatments with chemicals such as chloroform, prevent the movement of the floral stimulus. Most experiments have been carried out with SDP (chrysanthemum, Perilla, Kalanchoe, Xanthium) but at least one LDP, Hyoscyamus, has been shown to behave in the same way (Lang, 1965). From this and similar evidence it has generally been concluded that the floral stimulus moves in the phloem with assimilates. However, all of these treatments would be expected to limit any form of symplastic movement and indicate only that the stimulus does not move in the xylem with the transpiration stream. Perhaps the best evidence for transport in the phloem is that grafted donor leaves or shoots can cause flowering only when a functional phloem connection has been established. This was first demonstrated for Perilla (Zeevaart, 1958). Other workers have also found that stimulus transport across a graft required tissue union, although phloem connection was not strictly shown to be the limiting factor.

Assuming that the floral stimulus moves primarily in the phloem, is it co-transported with the mass flow of assimilates? In some cases, good correlations have been reported between the translocation of assimilates and the movement of floral stimulus. For example, a high flowering response in Perilla was associated with the presence in the bud of a large amount of 14C label from an induced leaf, while low flowering was associated with label from a non-induced leaf (Chailakhyan and Butenko, 1957). In contrast, movement against the expected flow of assimilate has been demonstrated in young, predominantly-importing leaves of Lolium, which were highly active in causing flowering (Evans and Wardlaw, 1966). This experiment is not conclusive, however, since 5-7% of the labelled carbon was exported and the floral stimulus could have accompanied this fraction (Evans and King, 1985). However, induced leaves of Pharbitis held in darkness exported floral stimulus at a rate comparable with that from leaves in the light, although no concurrent movement of labelled assimilate was detected (King et al„ 1968).

Simultaneous studies of the rates of assimilate movement and the movement of the floral stimulus have been carried out in only a few cases. Needless to say, one of the problems here is the accuracy with which the rate of movement of an unknown substance(s) can be determined. Another complication arises in the interpretation of experiments in which leaf excision has been carried out; while this has been necessary in order to estimate the rate of movement from induced leaf to responding apex, removing a leaf must have affected the supply of substances other than the floral stimulus (Vince-Prue and Gressel, 1985). In Pharbitis (Fig. 6.7), the calculated rates for the movement of floral stimulus (240-370 mm h-1) and labelled assimilates (330370 mm h-1) through the stem were similar, indicating that both were moving by mass flow in the phloem. In contrast, experiments with the LDP Lolium indicated that the transport rate through the leaf blade was only 10-24 mm h-1 for the floral stimulus, compared with approximately 1000 mm h"1 for sucrose (Evans and Ward-law, 1966). Based on these apparent differences in the rate of transport it was suggested that the stimulus in the LDP Lolium might be different from that in the SDP Pharbitis and transported via a different mechanism. However, a detailed analysis of 14C profiles in Lolium revealed that, although the major component moved at 400-840 mm h-1, there was a slower component which moved along the leaf blade at 10 mm h-1 and which might be associated with the movement of the floral stimulus (Evans and King, 1985).

Based on the rather limited evidence presently available, it appears that the floral stimulus moves from the induced leaf to the apex by a symplastic route which is the phloem in most, but perhaps not all instances. The transport mechanism has not yet been established. Movement together with assimilates has been demonstrated in some

FIG. 6.7. Velocities of translocation in mature Pharbitis plants. Assimilate movement was established bv cutting 2 cm long segments between the donor leaf and receptor bud at various times after exposure to C02. Floral stimulus movement was estimated by removing the donor leaf at various times; the excisions were made below the petiole of the donor leaf, or about 25 or 40 cm down the stem as shown. In this way it was possible to estimate the velocity of movement specifically through the stem. All other leaves were removed prior to the experiment. After Vince-Prue and Gressel, 1985 (data of King et al., 1968).

Average rate 30 36 cm / h

FIG. 6.7. Velocities of translocation in mature Pharbitis plants. Assimilate movement was established bv cutting 2 cm long segments between the donor leaf and receptor bud at various times after exposure to C02. Floral stimulus movement was estimated by removing the donor leaf at various times; the excisions were made below the petiole of the donor leaf, or about 25 or 40 cm down the stem as shown. In this way it was possible to estimate the velocity of movement specifically through the stem. All other leaves were removed prior to the experiment. After Vince-Prue and Gressel, 1985 (data of King et al., 1968).

cases; however, movement against the main assimilate flow can also occur and so a co-transport mechanism with sugars seems unlikely.

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