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Food/organic material conduction in plants mainly occurs by phloem. (Proved by Girdling experiment).
Food conduction occurs in between source and sink.
Source is net exporter while sink is net importer.
Generally green photosynthetic plant parts acts as source like leaves while non photosynthetic parts like root, shoot, fruits acts as sink.
The transfer of food depends on requirement and seasonal activities. For ex in germinating potato tuber tuber acts as source and developing buds acts as sink, similarly in early spring roots acts as source and developing buds as sink.
Food conduction may be in any required direction unlike the water conduction which is a unidirectional process.
Translocation of food mainly occurs in the form of sucrose or it is non-reducing sugar and chemically inert in its pathway of conduction.
Pressure flow/mass flow hypothesis of food/ sucrose translocation-Given by E. Munch (1930).
This is the most accepted theory of food conduction in plants.
According to it food translocation occurs in between source and sink in order of turgor pressure gradient i.e. high TP to low TP.
Phloem Loading/sucrose loading at source: It is an active process helped by carrier molecules. At source due to phloem loading concentration of seive cells increase, results in increase in osmotic pressure and water will moves from nearby xylem into seive cells results in increase in Turgor pressure (T.P) and increase in water potential {ψw). It establish a higher TP. at source and in seive tubes. Sucrose moves from source in seive tubes towards sink from high TP (High ψw to towards the low TP / low ψw.
Phloem unloading/sucrose unloading at sink: It is an active process helped by carrier molecules. At sink sucrose is unloaded results in decrease in osmotic pressure (O.P), it results in exit of water into near by xylem leads to decrease in turgor pressure (T.P) and water potential (ψw) of phloem. In sink cells the unloaded sucrose is either changed into starch (as starch not change O.P) or consumed, to maintain low O.P and continuous unloading.
So the process of sucrose loading at source and unloading at sink continues. This turgor pressure difference will be maintained and water will continue to move in at source and out at sink.
This mechanism was experimentally demonstrated by Bimodel expo of Munch.
According to evidences of modem research phloem conduction is an active process and it required metabolic energy in phloem cells.
Temperature: Optimum temperature for translocation ranges between 20-30°C. The rate of translocation increases with the increase of temperature upto an upper limit and then starts declining. At low temperature, the rate of translocation decreases.
Light: Hartt and his coworkers (1964) proposed that the movement of assimilates of a leaf can depend upon radiant energy with Increase in light intensity more food starts being. translocated to roots than to shoots. At lower intensity the growth of root and shoot is inhibited thereby the rate of translocation also decreases.
Hormones: Cytokinins have a pronounced effect on the translocation of water soluble nitrogen compounds.
Oxygen: Oxygen is necessary during transfer of food from mesophyll cells into phloem which is called as phloem loading.
Minerals: Boron is highly essential for translocation of sugar. Phosphorus also helps in translocation of solutes.
Water: Translocation of photosynthates out of the leaves is highly sensitive to the amount of water in the plant cells.
Metabolic inhibitors: The metabolic inhibitors which inhibit the process of respiration (e.g., iodoacetate, HCN, carbon monoxide" etc.) adversely affect the process of translocation because phloem loading and unloading require ATP.
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