Cold-Inhibited Phloem Translocation in Sugar Beet

Experiments were undertaken on sugar beet plants, reduced to one source leaf and two sink leaves, to observe the translocation response at two sinks when a slow- or quick-cooling treatment was applied to the source leaf petiole. Our results indicated that with slow-cooling treatments, both step increases and step decreases could be found in the rate of translocation to the two monitored sinks. The responses of the two sink leaves were not necessarily identical, and three classes of response were observed: (I) both sinks showed translocation increases or decreases at the same time, (2) one sink showed a translocation increase, while the other showed a concurrent decrease, or (3) one sink exhibited a translocation rate change (increase or decrease), while the other remained unchanged. Our data suggest that these translocation changes were the result of a repartitioning in the amount of flow between the two sink leaves and the unmonitored hypocotyl/crown region of the simplified plant system. We suggest that the cooling treatments induced the transmission of a rapid, physical signal along the phloem pathway to the crown region of the plant, which in turn caused alterations in the pathway and redirected the assimilate flow amongst the three terminal sinks. In addition, our results indicated that pathway repartitioning could also account for a part of the quick-cooling induced translocation recovery response.