Flowering Plant Physiology Lecture 10 PDF

BIOL 2370 Flowering Plant Physiology LECTURE 10 Photoassimilate translocation  Learning objectives ❑Describe the structure of phloem tissue ❑ Explain the process of translocation in plants ❑ Discuss the pressure-flow hypothesis  Translocation ◼...

BIOL 2370 Flowering Plant Physiology LECTURE 10 Photoassimilate translocation  Learning objectives ❑Describe the structure of phloem tissue ❑ Explain the process of translocation in plants ❑ Discuss the pressure-flow hypothesis  Translocation ◼ Translocation of photosynthetic products (photo assimilates) is facilitated by phloem elements. ◼ Storage or photosynthesizing organs (exporters) are known as source. E.g., Leaves ◼ Actively metabolizing organs or the ones which store carbohydrates are importers and known as sinks. E.g., roots, tubers, fruits, stems)  Translocation ◼ Source: Any exporting region that produces photosynthate above and beyond that of its own needs ◼ Sink: Any organ that does not produce photosynthate or enough to meets its own needs  Experimental Evidence Girdling Girdling experiments, where a strip of bark is removed from around a tree trunk, block phloem transport. This causes photosynthates to accumulate and swell above the girdle while decreasing below it, demonstrating the direction of translocation. 1st done in 1686 then 1927.  Experimental Evidence ❑ Analysis of sap from aphid style High turgor pressure and wound reactions complicate phloem sap collection, leading to contamination and dilution. A more reliable method uses aphid stylets, which act as natural syringes. Aphids are anesthetized and their stylets are cut with a laser, allowing phloem sap to ooze out and be collected for analysis. This method provides relatively pure sap  Phloem cell types ❑ Parenchyma – store and release food molecules ❑ Fibres – Protect and strengthen the tissue ❑ Sieve tubes – conduct sugars and organic materials ❑ Companion cells - regulate activity of the adjacent sieve element and to take part in loading and unloading sugar into the sieve element. Phloem cell types  Phloem cell types  Mechanism of Photoassimilate Translocation ❑ Loading (active) - movement of photosynthetic products into the sieve elements of mature leaves ❑ Transport (passive) ❑ Unloading (passive + active) - The movement of photosynthates from the sieve elements to neighbouring cells  Active Transport Across Membrane ❑ Loading via a sucrose/proton symporter ❑ Against a [sucrose] gradient ❑ Greater [sucrose] in phloem than in mesophyll cells  Translocation mechanism ❑ Translocation occurs in phloem tissue ❑ Munch’s Pressure-Flow model ❑ Suggests that movement in the phloem is due to mass flow along a turgor pressure gradient ❑ Assimilates enter the phloem sieve tubes by active transport (phloem loading) at the Source ❑ Assimilates are removed from the phloem sieve tubes (unloading) at the Sink ❑ Translocation is linked to water flow in xylem  Pressure flow hypothesis ◼ Accumulation of sugars in the sieve elements → low (negative) solute potential → steep drop in ψ ◼ Waters enters the sieve elements and causes the turgor pressure to increase. Increase causes the bulk flow of phloem from source to sink ◼ At sink phloem unloading → ↓ [sugar] in sieve elements → higher (less negative) solute potential in the sieve elements of sink tissues. ◼ As ψ of the phloem rises > xylem, water leaves the phloem in response to the water potential gradient, causing a decrease in turgor pressure in the sieve elements of the sink. Pressure flow hypothesis  Pressure flow hypothesis  Assimilate Partitioning ◼ Differential distribution of Assimilates/photosynthates to multiple sinks within the plant ◼ Sinks compete with each other for assimilates ◼ Sink demand can affect source supply ◼ Partitioning depends on: ◼ Nature of vascular connections ◼ Proximity of sink to the source ◼ Sink strength (sink size ×sink activity) ◼ Affected by hormones & environment  Consider… ◼ During vegetative growth, roots, stems, and leaves are competitive sinks for assimilate. ◼ The investment of assimilate into greater leaf area development results in greater light interception. However, the leaves also require water and nutrients, so investment in root growth is necessary. ◼ Some crop plants, such as most grasses, have essentially no stem growth during vegetative development and favor partitioning to leaves and roots. ◼ Some meristems are in more favorable positions to intercept assimilate. E.g., the intercalary meristems of leaves are in a better position to intercept translocated assimilate than are the peripheral root and shoot meristems. Partitioning of Assimilate in Plants | Crop Plants | Botany by B Menakhi  Summary ◼ Plant organ systems: root, shoot ◼ Cells form tissues and tissues form organs ◼ Plant cells consist of cell wall + protoplasm ◼ Cytoplasm consists of cytosol + organelles ◼ Major tissues: ground, dermal, vascular and meristematic ◼ Tissues can function for support, storage, protection, transport and cell division