The Effect of 2,4‐Dinitrophenol on Translocation in the Phloem

The effect of 2,4‐dinitrophenol (DNP) on sucrose‐ 14 C transport in Soya seedlings has been analysed. The aim was to distinguish between an effect of the inhibitor on sugar movement within the phloem sieve tubes themselves, and on the prior steps of uptake and secretion of sugar into the conducting cells. DNP drastically inhibited sucrose‐ 14 C transport if it was applied to the 14 C‐treated leaf immediately before, or during, 14 C supply. Transport was also strongly inhibited if DNP was applied along the translocation path while the 14 C‐treated leaflet was still in position on the plant. When, bowever, DNP was applied through the cut petioles of the primary leaves after removal of the 14 C‐treated terminal leaflet of the first trifoliate leaf, no inbibition was observed. On the contrary, transport appeared to have been promoted: significantly more 14 C disappeared from the upper regions of DNP‐treated plants as compared with controls, while in the lower plant parts more 14 C accumulated. Different rates of synthesis of sucrose‐ 14 C into non‐alcohol‐soluble compounds could not account for this result. A similar stimulatory effect was observed when DNP was applied to the cut petiole of a primary leaf opposite that treated with 14 C. Several indications were obtained that 14 C which has reached the lower parts of the plant may circulate upwards again through the phloem within about 15 minutes. When sucrose‐ 14 C was introduced into the roots via the xylem, both DNP treatment and prior steam girdling resulted in the apparent accumulation of 14 C in the lower plant parts. the results would be compatible with DNP inhibition of upwards movement in the phloem. DNP might also have affected sugar uptake processes in cells neighbouring the translocation path. It is concluded that the inhibitory effect of DNP on downwards phloem transport reported by earlier workers was probably due to an effect on uptake and/or secretion into the sieve tubes, not to an effect on the conducting cells themselves. Modern theories for phloem transport are discussed in the light of these findings.