Mass Flow Hypothesis Ernst Munch, a German plant physiologist, proposed this theory in 1930. This theory gives the most accurate explanation of mass flow in the phloem. According to this theory, the ions are absorbed by the roots along with the mass flow of water under the influence of transpiration because rapidly transpiring plants absorb more minerals from the soil and the rate of mineral absorption increases with increase in the rate of transpiration. It is a non-selective process.
Only inorganic nutrients are transported by xylem, while only organic elements are transported by phloem. The phloem tissue in plants is responsible for the transport of sugars or glucose from the area where it is prepared (the leaf which is considered as a source) to other parts of the plant where it is needed or stored (considered as a sink).
Translocation is the bulk transport of substances through the conducting or vascular tissues of plants.
The mechanism of translocation in plants is explained by The Pressure Flow or Mass Flow Hypothesis:
Pressure flow hypothesis is used to explain the translocation of sugars from source to sink and it is the most accepted mechanism. When glucose is produced at the source i.e., in leaves (by photosynthesis), it is converted into a sucrose (which is a disaccharide). This sugar is then carried into companion cells and ultimately into living phloem sieve tube cells in the form of sucrose by active mode of transportation. The phloem at the source becomes hypertonic as a result of this loading technique.
Osmosis transports water from the adjacent xylem to the phloem. Phloem sap will move to lower pressure areas when osmotic pressure builds up. At the sink, the osmotic pressure must be lowered. Once more, the active transport will be required to get sucrose from the phloem sap to the cells that will utilize it to convert it to starch, cellulose or energy. The osmotic pressure drops when sugars are removed, and water travels outside the phloem. Sieve tube cells, create elongated columns with pores in their wall end. These are called sieve plates and make up phloem tissue. Continuous filaments are formed when cytoplasmic strands pass through the holes in the sieve plates. Pressure flow begins when hydrostatic pressure of phloem in the sieve tube rises, and sap travels across the phloem. Meanwhile, sugars that enter the phloem are moved out actively and ejected as complex carbohydrates at the sink. The loss of solute causes a high-water potential in the phloem, which causes water to pass out and finally returns to the xylem.
The tissues through which food is delivered were identified using a simple experiment called girdling. In this experiment, a ring of bark can be gently removed from the trunk of a tree up to the depth of the phloem layer. After a few weeks, the area of the bark above the ring on the stem swells due to a lack of downward food circulation. Because, in most of the plants, phloem tubes lie outside the xylem, a tree or other plant can be killed by removing the bark in a ring around the trunk or stem. This demonstrates that the tissue responsible for food translocation in phloem, and that food is transported in just one direction, that is, towards the roots.
This theory has the following drawbacks:
- People believe that metabolic processes are responsible for mechanisms like translocation in plants and not hydrostatic pressure.
- Mass flow is primarily a passive process in which companion cells support sieve tube vessels. This implies that the hypothesis ignores the living character of phloem.
- As this is a non-selective process, it cannot explain the absorption of selective minerals from the soil.
- The hypothesis also claims that the rate of material transport is uniform, which has been shown false. Amino acids and other nutrients, such as carbohydrates, move at different rates throughout.
- The Munch mass flow hypothesis only accounts for pressure flow that moves in one direction. It doesn’t explain the bilateral movement phenomenon, in which materials move in opposite directions at the same time.
- In rapidly transpiring barley plants, absorption of Ca++ and Mg++ ions is more but absorption of K+ and Na+ ions is very low. The mass flow theory fails to explain the exact reason for this difference in absorption between the divalent and monovalent cations.