Plant Structure and Function I - Ecol 182 – 4-14-2005 Downloaded at 9:00 pm on 4-13
The Angiosperms: Flowering Plants • A number of synapomorphies, or shared derived traits, characterize the angiosperms: • They have double fertilization (upcoming figure). • They produce triploid endosperm. • Their ovules and seeds are enclosed in a carpel (modified leaf). • They have flowers (modified leaves). • They produce fruit (at minimum – mature ovary and seed). • Their xylem contains vessel elements (specialized H2O transport) and fibers (structural integrity). • Their phloem contains companion cells (assists with metabolic issues associated with transport).
Angiosperm vascular systems • Xylem in angiosperms consists of vessel elements in addition to tracheids • Vessel elements also conduct water and are formed from dead cells. • Vessel elements are generally larger in diameter than tracheids and are laid down end-to-end to form hollow tubes. • Sieve tube elements (Phloem) in Angiosperms are stacked, similar to xylem • Have adjacent companion cells that retain all organelles • Companion cells may regulate the performance of the sieve tube members through their effects on active transport of solutes
Angiosperms: Flowering Plants • Monocots - a single embryonic cotyledon (grasses, cattails, lilies, orchids, and palms) • Eudicots - two cotyledons, and include the majority of familiar seed plants • Additional clades - water lilies, star anise, and the magnoliid complex • Big question in plant evolution – what is the basal angiosperm?
Plant Structure and Function • Uptake and Movement of Water and Solutes • Transport of Water (& Minerals) in the Xylem • Transpiration and stomatal regulation of water-loss (use) • Translocation of Substances in the Phloem
General problem of water in plant function • Need for H2O for: • photosynthesis, • Solute transport, • temperature control, • internal pressure for growth • Plants obtain water and minerals from the soil via the roots • in turn roots extract carbohydrates & other important materials from the leaves. • Water enters the plant through osmosis • but the uptake of minerals requires transport proteins.
Plant function in the context of the soil-plant-atmosphere continuum Plants bridge the steep potential energy gradient between the soil and the air & use it as a mechanism for water and solute transport But…. - The soil is not an endless supply of water! • Compromises between biomechanics, size and growth rate • set the stage for catastrophic loss of water transport • Decreases in leaf water content result in stress that does • not allow for growth and may result in mortality
Uptake & Movement of Water & Solutes in Plants • Osmosis is the diffusion of water through a membrane – primary means of water transport in plants • Water movement across a membrane is a function of osmotic potential, or solute potential. • Potential refers to the potential energy contained in the system measured • Dissolved solutes effect the concentration of water (changing the potential energy). • Greater solute concentration results in a more negative solute potential and a greater the tendency of water to diffuse to the solution.
Uptake & Movement of Water & Solutes in Plants • Water potential is the tendency of a solution to take up water from pure water (Y). • Water potential of a system is the sum of the negative solute potential (ys) and the (usually positive) pressure potential (yp) [along with other potentials]. y = ys +yp • Each component is measured in megapascals (Mpa).
Figure 36.4 Apoplast and Symplast – routes of water movement from the soil into the plant
Transport of Water and Minerals in the Xylem • The adhesion-cohesion–tension theory of water movement: • Water vapor concentration is greater inside the leaf than outside, so water diffuses out through stomata • (this is transpiration). • Tension develops in the mesophyll drawing water from the xylem of the nearest vein into the apoplast surrounding the mesophyll cells • Removal of water from the veins establishes tension on the entire volume of water in the xylem, so the column is drawn up from the roots.
Figure 36.8 The Transpiration–Cohesion–Tension Mechanism • Hydrogen bonding – results in cohesion (sticking of molecules to one another). • The narrower the tube, the greater the tension the water column can stand. • Maintenance of the water column also occurs through adhesion of water molecules to the walls of the tube.
Transport of Water and Minerals in the Xylem • The key elements in water transport in xylem: • Transpiration • Tension • Adhesion / Cohesion • The adhesion–cohesion–tension mechanism does not require energy. • At each step, water moves passively toward a region with a more negative water potential. • Mineral ions in the xylem sap rise passively with the solution. • Transpiration also contributes to the plant’s temperature regulation, cooling plants in hot environments.
Short and long-term responses to water limitation When water is withheld – the pressure potential of the cells declines (hours to days) and rates of cell expansion are reduced (long-term). -Rates of photosynthesis declines (stomata close- short). -New leaves are smaller, with smaller cells (long). -Profound change in patterns of allocation (long).
Regulation of Transpiration by Stomata • Leaf and stem epidermis has a waxy cuticle that is impermeable to water, but also to CO2. • Stomata, or pores, in the epidermis allow CO2 to enter by diffusion. • Guard cells control the opening and closing of the stomata. • Most plants open their stomata only when the light is intense enough to maintain photosynthesis. • Stomata also close if too much water is being lost.
Figure 36.11 Stomata (Part 2) • Stomatal aperture is regulated by controlling K+ concentrations in the guard cells. • Blue light activates a proton pump to actively pump protons out of the guard cells. The proton gradient drives accumulation of K+ inside the cells. • Increasing K+ concentration makes the water potential of guard cells more negative, and water enters by osmosis. • The guard cells respond by changing their shape and allowing a gap to form between them. • Abscisic acid (a ‘stress’ hormone) can invoke this stomatal closure in addition to blue light • Changes in guard-cell photosynthesis can also invoke this stomatal response
Leaf temperature VPD [CO2] ‘demand’ PPFD Transpiration Stomatal Conductance Hydraulic resistance Soil Y
Types of stomatal responses • Isohydric species – control gas exchange such that daytime leaf water status is unaffected by soil water deficits. (Primarily responds to ABA) • Anisohydric species – exhibit decreases in leaf water potential proportional to changes in soil water potential. (responds to both ABA and Yleaf)
Conceptual understanding of stomatal function • Optimization theory (Cowan 1977) – stomata work to optimize or maximize water exchanges for carbon dioxide • Long-distance transport hypothesis – Tyree and Sperry – stomata regulate water loss to maintain long-distance water and nutrient transport • Operate to avoid of catastrophic xylem dysfunction (cavitation), that occurs through the development of excessive tension.
Cavitation or Embolism • Breakage of the xylem water column • Entry of air into the conduit • Primarily through the pit membrane • Large tensions in the xylem stream • Species and individuals differ in their vulnerability to cavitation – trade-offs produced relative to water flow rates
Mechanisms of cavitation • Desiccation-induced – vulnerability to cavitation through air entry from pit membrane • size and number of pits becomes the important traits EVEN THE WIDEST VESSELS IN RING-POROUS TREES ARE SUFFICIENTLY NARROW TO PREVENT BREAKING OF A WATER COLUMN….(Sperry 1995) Other mechanisms besides vessel diameter alone are important in determining drought stress tolerance
Mechanisms of cavitation • Freeze-thaw events – dissolved gases in sap are insoluble in ice and form bubbles under repeated low temperature conditions DIFFERENCES IN CONDUIT DIAMETER CAN AFFECT THE POTENTIAL FOR GAS EMBOLISMS FORMING FROM GAS MOVING OUT OF SOLUTION
Constraints on water transport when embolism occurs; differences in phenology and distribution Ring Porous Vessels confined to spring wood (uniform distribution) Oaks (Quercus) Emboli zed vessels cannot be re-filled; water transport is dependent upon new spring wood construction (which tent to have large vessels) Diffuse porous Vessels occur uniformly throughout the annular ring; re-filling can occur over the winter.
So…..water transport • Vessel diameter and pit membrane density • (why do desert species tend to have both reduced vessel diameter AND pit membrane density?) • Constraints from the interaction of water stress and temperature stress affect vulnerability to cavitation • Implications for plant functional strategies and controls over the distribution of plants
Think about this figure as a general example of how soils and plants interact in all different ecosystems
Safety Margins Mesic species with the ability to recover each night operate close to the xylem tensions that cause 100% cavitation Xeric species that do not have that opportunity to recover operate with a much larger safety margin
Mesic habitats Variation within and between species associated with variation in PSN capacity, leaf N content, leaf morphology/ontogeny Variation between species associated with adaptation to aridity Range of leaf conductance Arid habitats Photosynthetic capacity
Translocation of Substances in the Phloem • Sugars, amino acids, some minerals, and other solutes are transported in phloem and move from sources to sinks. • A source is an organ such as a mature leaf or a starch-storing root that produces more sugars than it requires. • A sink is an organ that consumes sugars, such as a root, flower, or developing fruit. • These solutes are transported in phloem, not xylem, as shown by Malpighi by girdling a tree.
Translocation of Substances in the Phloem • Translocation (movement of organic solutes) stops if the phloem is killed. • Translocation often proceeds in both directions— both up and down the stem simultaneously. • Translocation is inhibited by compounds that inhibit respiration and the production of ATP.
Translocation of Substances in the Phloem • Plant physiologists have used aphids to collect sieve tube sap from individual sieve tube elements. • An aphids inserts a specialized feeding tube, or stylet, into the stem until it reaches a sieve tube. • Sieve tube sap flows into the aphid. The aphid is then frozen and cut away from its stylet, which remains in the sieve tube. • Sap continues to flow out the sieve tube and can be collected and analyzed by the physiologist.
Translocation of Substances in the Phloem • There are two steps in translocation that require energy: • Loading is the active transport of sucrose and other solutes into the sieve tubes at a source. • Unloading is the active transport of solutes out of the sieve tubes at a sink.
Translocation of Substances in the Phloem • Sieve tube cells at the source have a greater sucrose concentration that surrounding cells, so water enters by osmosis. This causes greater pressure potential at the source, so that the sap moves by bulk flow towards the sink. • At the sink, sucrose is unloaded by active transport, maintaining the solute and water potential gradients. • This is called the pressure flow model.
Translocation of Substances in the Phloem • If the pressure flow model is valid, two requirements must be met: • The sieve plates must be unobstructed. • There must be effective methods for loading and unloading the solute molecules. • The first condition has been shown by microscopic study of phloem tissue. • Mechanisms for loading and unloading the solutes exist in all plants.
Translocation of Substances in the Phloem • Sugars and other solutes produced in the mesophyll cells leave the cells and enter the apoplast. • The solutes are then actively transported to companion cells and phloem tubes, thus reentering the symplast. • The passage of solutes to the apoplast and back to the symplast allows for selectivity of solutes to be transported.
Translocation of Substances in the Phloem • Secondary active transport loads the sucrose into companion cells and sieve tubes. • Sucrose is carried across the membrane by sucrose–proton symport. For this symport to work, the apoplast must have a high concentration of protons. • These protons are supplied by a primary active transport — the proton pump.
Translocation of Substances in the Phloem • Many substances move from cell to cell within the symplast through plasmodesmata. • The plasmodesmata participate in the loading and unloading of sieve tubes. • Solutes enter companion cells by active transport and move into the sieve tubes through plasmodesmata. • At sinks, plasmodesmata connect sieve tubes, companion cells, and the cells that will receive the solutes. Plasmodesmata in sink tissues are abundant and allow large molecules to pass.