Introduction
Transport in the phloem of plants is a critical biological process that involves the movement of organic solutes, primarily sucrose, from regions of production or storage (sources) to regions of growth or storage (sinks). This process is known as translocation. Understanding the mechanisms behind this transport is essential for comprehending how plants distribute nutrients and maintain their physiological functions.
9.2.1 Translocation in Plants
Definition and Importance
Translocation refers to the movement of organic solutes within the phloem tissue. The liquid being transported, known as phloem sap, primarily consists of sugars in the form of sucrose, along with water and other dissolved substances such as amino acids. These dissolved substances are collectively referred to as assimilates.
Sources and Sinks
Sources
Sources are regions where organic solutes are produced or stored before being transported:
- Mature green leaves and green stems: Photosynthesis in these regions produces glucose, which is converted into sucrose.
- Storage organs (e.g., tubers and tap roots): These release stored substances at the beginning of a growth period.
- Food stores in germinating seeds: These provide nutrients for the developing plant.
Sinks
Sinks are regions where organic compounds are required for growth or storage:
- Meristems: Actively dividing regions.
- Growing roots: Actively taking up mineral ions.
- Young leaves in buds: Require nutrients for development.
- Developing seeds, fruits, or storage organs: Sites of nutrient storage.
Sources can become sinks and vice versa, depending on the time of year and the processes occurring within the plant.
9.2.2 Sucrose Loading
Pathways for Sucrose Transport
Sucrose is transported from sources to sinks through phloem sieve tubes via two main pathways:
Apoplast Pathway
- Through the cell wall spaces: This is an active process involving ion pumps.
- Mechanism:
- Companion cells pump hydrogen ions ($H^+$) out of their cytoplasm into their cell walls using proton pumps, an ATP-dependent process.
- The high concentration of $H^+$ ions in the cell wall space causes them to move back into the cytoplasm via a co-transporter protein, which simultaneously carries sucrose molecules into the companion cell against the sucrose concentration gradient.
- Sucrose then diffuses into the sieve tubes through plasmodesmata.
This process occurs in specialized companion cells called transfer cells, which have a folded cell surface membrane to increase surface area and many mitochondria to provide energy for proton pumps.
Symplast Pathway
- Through the cytoplasm and plasmodesmata: This is a passive process where sucrose molecules move by diffusion.
- Mechanism:
- Sucrose travels from the cytoplasm of one cell to the next through plasmodesmata, maintaining a continuous flow.
