Transport system in a plant is concerned with the movement of materials from source to sinks. According to Lacher . W 1985, a source is a region of a plant, which is manufactures sugars during photosynthesis and supplies materials of any kind to the transport system (e. g leaves) and a sink as a region where the sugars and minerals are being removed or lost from the system to be used up or stored (for example the root and production of fruit). Transport of soluble products of photosynthesis is called translocation.
Translocation of soluble products of photosynthesis occurs in the part of the vascular tissue known as the phloem.
The phloem is the principle food conducting tissue associated with xylem in the vascular system. The basic components of phloem are sieve elements, companion cells and phloem parenchyma and phloem fibres. It is placed outside the cambium of the vascular bundle (www. /http://infotrac. thomsonlearning. com). It is well known that soluble products of photosynthesis formed in the photosynthetic tissue, enter the sieve tubes by active transport.
The movement of materials once they are in the sieve tubes is still under debate as to which and what is the driving force.
Although sugars and amino acids tend to move along concentration gradients the speed at which they travel is too fast to be explained simply by diffusion. There presently four proposals of translocation mechanisms; the Munch model, streaming model, electro-osmotic model and the peristaltic tubule model (Sperry J. S; 2003). In 1930 Ernst Munch proposed the Pressure Flow hypothesis also known as the Mass- flow hypothesis which states that a flow of solution in the sieve elements is driven by an osmotic pressure gradient that is generated between source and sink.
The pressure gradient is established as a consequence of the phloem loading at the source and phloem unloading at the sink (Knox et al, 2005). At the source region sugar is actively loaded into the sieve tube of the phloem. This results in an increase in solute concentration in the sieve tube, resulting in a low water potential being generated in the sieve tube. In the xylem there is a high water potential, water will move by osmosis from a region of high water potential to a region of low water potential in the sieve tube down a water potential gradient.
The driving force for water entry to the phloem is the osmotic gradient created by the accumulation of sugar. As water enters the sieve tubes and the phloem loading occurs the turgor pressure increases. The pressure that forces sucrose and other compounds through the sieve tubes comes from a gradient of hydrostatic (turgor) pressure which is near sources and low near the sinks. The pressure gradient along the sieve tube depends on the gradient created by the differences in sucrose concentration.
Unloading of solutes from the sieve tubes at the sink reign result in lower water potential, this causes the water to flow out of sieve tubes to neighboring cells, this results in a lower hydrostatic pressure relative to that of the sieve tubes in the source regions. Reduced pressure in the sink region encourages fluid flow from higher pressure regions. The loss of the concentration gradient is prevented by two mechanisms the continual pumping of sucrose at the source and removal of sucrose at the sink (Rost. T et al; 2006).
Hays S. M 1993 says; “that there is however controversy surrounding the Pressure flow hypothesis as to whether the hydraulic resistance of the sieve tube is sufficiently low that the turgor pressure in the phloem at the leaf created by osmotic entry of water there, can by itself drive translocation at the observed velocities. Water may be expected to exchange between the xylem and phloem of a vascular bundle all along its length, so that there will be at every point a local balancing between xylem and phloem water potentials. The continual water balancing between xylem and phloem will be taking place always. ”