Resource Acquisition and Transport in Vascular Plants (Lecture 10)

Learning Objectives

• Resource acquisition: overview
• Shoot architecture & light capture
• Root architecture & acquisition of water, minerals
• Plant cell compartments involved in transport
• Transport continuums
• Short distance transport
• Long distance transport

1. Resource acquisition
Overview: Underground Plants

• Stone plants (Lithops) are adapted to life in the desert

–Two succulent leaf tips are exposed above ground;

the rest of the plant lives below ground

 

 

 

• The success of plants adaptation on land depends on

vtheir ability to gather and conserve resources from their environment

vResources must be shared throughout the structure of the multicellular body.

vThen the transportation of that materials is central to the integrated functioning of the whole plant

Adaptations for acquiring  resources were key steps in the evolution of vascular plants

• The algal ancestors of land plants absorbed water, minerals, and CO2 directly from the surrounding water
• Early nonvascular land plants lived in shallow water and had aerial shoots
• Natural selection favored taller plants with

*Flat appendages

*Multicellular branching roots

*Efficient transport system

• The evolution of xylem and phloem in land plants made possible the long-distance transport of water, minerals, and products of photosynthesis
• Xylem transports water and minerals from roots to shoots
• Phloem transports photosynthetic products from sources to sinks
• Adaptations in each species…………..enhancing photosynthesis and minimizing water loss

2. Shoot Architecture and Light Capture

• Stems serve as channels for water and nutrients and as supporting structures for leaves
• There is generally a positive correlation between water availability and leaf size on the shoot
• Light absorption by the plant can be affected by Phyllotaxy; the arrangement of leaves on a stem that is specific to each specie.
• Most angiosperms have alternate phyllotaxy with leaves arranged in a spiral
• The angle between leaves is 137.5° and likely minimizes shading of lower leaves

• Self-pruning usually occur in the lower leaves because of low light absorption. Self-pruning is the shedding of lower shaded leaves when they respire more than photosynthesize
• Light absorption is also affected by the leaf area index; the ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows

 

*Leaf orientation affects light absorption

• In low-light conditions, horizontal leaves capture more sunlight
• In sunny conditions, vertical leaves are less damaged by sun and allow light to reach lower leaves

*Shoot height and branching pattern also affect light capture

3. Root Architecture and Acquisition of Water and Minerals

• Soil is a resource mined by the root system
• Taproot systems anchor plants and are characteristic of gymnosperms and eudicots (largest group of flowering plants)
• Root growth can adjust to local conditions

–For example, roots branch more in a pocket of high nitrate than low nitrate

• Roots are less competitive with other roots from the same plant than with roots from different plants

• Roots and the hyphae of soil fungi form mutualistic associations called mycorrhizae
• Mutualisms with fungi helped plants colonize land
• Mycorrhizal fungi increase the surface area for absorbing water and minerals, especially phosphate

Plant Cell Compartments

• Transport is regulated by the compartmental structure of plant cells.
• The plasma membrane directly controls the traffic of molecules into and out of the protoplast.
• The plasma membrane is a barrier between two major compartments, the cell wall and the cytosol.
• The major compartment in most mature plant cells is the central vacuole, a large organelle that occupies as much as 90% or more of the protoplast’s volume.
• The vacuolar membrane = tonoplast – regulates transport between the cytosol and the vacuole.

4. Different mechanisms transport substances over short or long distances

• major pathways through plants

–The apoplast

–The symplast

–Transmembrane

*The apoplast consists of everything external to the plasma membrane

• It includes cell walls, extracellular spaces, and the interior of vessel elements and tracheids

*The symplast consists of the cytosol of the living cells in a plant.

A. Apoplastic path way (cell wall to cell wall)

• movement of water through intercellular spaces and cell wall
• Water and solute move without entering a cell
• involves the non-living vascular tissue
• due to capillary action
• blocked by the casparian strip at endodermis

*Impermeable band of suberin inside walls of endodermal cells

• regulates the quantity and type of minerals and ions reach the xylem

B. Symplastic path way

*the continuum of cytoplasm within a plant tissue

*formed by the plasmodesmata which pass through pores in the cell walls.

*responsible to transport water and minerals in order to reach the xylem.

*This path way involves the living part of the cell, water moves by osmosis

C. Transmembrane path way

Water sequentially moves from one cell to the next cell

by repeatedly crossing plasma membranes and cell walls.

Water and solute molecules can move

by any one of these routes or

a combination through switching from one to another.