Origin and Delivery - Transport in Phloem
We have stated above that transport of photosynthates starts from leaves and ends up in one or the other sink tissue. Though this is true, however, it is necessary to define the concept of sinks and sources at any given point of time. For this consider an experiment on the plant Saxifraga. This plant spreads by giving out long offshoots with a bunch of leaves and a potential root system at the end. If the latter comes in contact with moist soil, it develops into a self-sufficient shoot-root system. However, so long this has not happened, the long link provides water and minerals taken up by the root of the parent plant to the juvenile bunch of leaves. The phloem transport system initially provides the buds of the distant shoot with the necessary nutrition.
Figure: a) Saxifraga plant with offshoot, b) Supply of food by the offspring, c) Supply of food by the parent plant.
When fully grown, the new cluster of leaves becomes excess producers of photosynthates and contributes their output to the parent plant. If we shut off the light falling on the parent plant or the off-shoot over a substantial period of time, so that either of them is incapable of photosynthesis, which way will the phloem translocate the food? We are sure that you have already arrived at the correct answer. It is observed that translocation is from the system in which photosynthesis takes place to that in which it is prevented. Thus, the two systems of leaves can be sink at one time and source at another. The direction of phloem transport in the above case is, therefore, dependent on the relative production of photosynthates in the two systems of leaves. Similar situations among typical sinks also exist. In tropics when the new leaves in deciduous plants emerge in the spring on a denuded tree, they need the supply of nutrients. What could be the possible source of nutrients in the absence of pre-existing mature leaves? Naturally from the stored stock of metabolites in sink tissues. In spring, both the xylem and the phloem receive metabolites from all over the plant body and deliver them to the buds.