Phloem is a specialized vascular tissue found in plants responsible for the transportation of sugars, hormones, and other essential nutrients throughout the organism. Along with xylem, phloem is part of the two transport systems in plants that allow for long-distance movement of fluids and dissolved substances in a unidirectional manner.
The transport system is critical in helping plants meet their metabolic demands, allowing for the synthesis and utilization of glucose during photosynthesis. Phloem is the primary conduit for the movement of the photosynthates, i.e., nutrients produced in the leaves’ mesophyll, throughout the plant.
Phloem tissues consist of living cells known as sieve elements, which are arranged in long tubes that run longitudinally through the plant. The cells are connected to each other with perforated plates known as sieve plates. Companion cells, which are non-conducting but metabolically active cells, are located adjacent to the sieve elements and help in the loading and unloading processes of the transport system.
The pressure flow model, proposed by Ernst Munch in 1930, explains how the phloem functions. In this model, the sieve tube elements at the source, usually the leaves, load the photosynthates into the phloem via the companion cells, which require energy in the form of ATP. This loading creates a high concentration of solutes within the phloem, requiring a lowered water potential, creating a pressure gradient that drives the movement of the solutes from the source to the sink throughout the plant.
The sink in this model includes the regions of the plant that are in demand of the nutrients, such as the roots, fruits, and meristems. The unloading of the phloem in the sink is achieved through the use of enzymes that break down the solutes, reducing their concentration. This process increases the water potential, creating a gradient that allows for the movement of water out of the sieve tubes and into the adjacent cells, allowing for the distribution of nutrients throughout the plant.
In conclusion, the phloem is a critical component of plant life and plays a vital role in the transportation of nutrients and metabolic products throughout the plant. Understanding the functioning and regulation of the phloem is essential in agriculture, horticulture, and forestry and helps with the production and breeding of crops, management of plant diseases, and general advancements in plant sciences.