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Plant Structure and Function I - E col 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.

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the angiosperms flowering plants
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
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
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
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


- 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
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 plants12
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
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 xylem18
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
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


[CO2] ‘demand’



Stomatal Conductance

Hydraulic resistance

Soil Y

types of stomatal responses
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
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
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
Mechanisms of cavitation
  • Desiccation-induced – vulnerability to cavitation through air entry from pit membrane
    • size and number of pits becomes the important traits


Other mechanisms besides vessel diameter alone are important in determining drought stress tolerance

mechanisms of cavitation31
Mechanisms of cavitation
  • Freeze-thaw events – dissolved gases in sap are insoluble in ice and form bubbles under repeated low temperature conditions



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
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
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
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 phloem40
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 phloem41
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 phloem43
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 phloem44
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 phloem47
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 phloem48
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 phloem49
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 phloem50
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.
nutrient classification
Nutrient classification
  • Amount
    • Macronutrients: (H,C,O,N,K,Ca,Mg,P,S)
    • Micronutrients: (Cl,B,Fe,Mn,Zn,Cu,Mo)
  • Function
    • Constituents of organic material: (C,H,O,N,S)
    • Osmotic potential or contribute to enzyme structure/function: (K,Na,Mg,Ca,Mn,Cl)
    • Structural factors in methalloproteins: (Fe,Cu,Mo,Zn)
nutrient dynamics outline
Nutrient Dynamics (outline)
  • Nutrient availability
    • Sources of nutrients
    • Direct and indirect controls over sources
  • Nutrient Uptake
    • Plant and environmental interactions
  • Nutrient Return from the plant to the soil (cycling)
    • Ecological and environmental processes
nutrient sources for plants
Nutrient sources for plants
  • Mineral nutrients in the soil
    • 98% bound in organic matter (detritus), humus, and insoluble inorganic compounds or incorporated in minerals
    • 2% is absorbed on soil colloids
      • These are positively charged ions
    • 0.2% is dissolved in the soil water
      • Usually negatively charged, nitrates and phosphates
soil colloids
Soil Colloids
  • Ion exchangers
    • Exchange capacity depends upon surface area
      • Clay (montmorillonite) ~ 600 – 800 m2 g-1
      • Many humic substances ~ 700 m2 g-1
    • Retain charged substances (mainly cations, but to a lesser extent, anions)
soil colloids56
Soil Colloids
  • Adsorptive binding of nutrient ions result in:
    • Nutrients freed by weathering and decomposition are collected and protected from leaching
    • Concentration in soil solution remains low and constant
      • Removes a potential osmotic effect
    • Adsorbed nutrient ions are readily available to plants
nutrient uptake
Nutrient uptake
  • Conditions that affect nutrient content in the soil
    • Soil texture (clay content)
    • Soil organic matter content
    • Soil water content (precipitation)
    • Soil temperature
environments that tend to result in low nutrient contents
Environments that tend to result in low nutrient contents
  • Sandy soils – low clay content and thus inadequate exchange capacity
  • High rainfall – excessive leaching of nutrients
  • Low rainfall – inadequate soil moisture for organic matter decomposition
  • Cold soils – low decomposition; low root respiration and thus low nutrient uptake
  • Waterlogged soils – inadequate oxygen for root respiration and decomposition
ion uptake by roots
Ion uptake by roots
  • The rate at which nutrients are supplied to a plant depends on:
    • The concentration of diffusible minerals in the rooted soil strata
    • Ion-specific rates of diffusion and mass transport
      • Nitrate is fast and phosphate and potassium are slower (diffusivities)
  • Ions of nutrient salts are taken up by a purely passive process
    • Following the concentration and charge gradients between the soil solution and the interior of the root
mass flow versus diffusion nutrient delivery
Mass flow versus diffusion – nutrient delivery
  • Nutrient uptake is a function of BOTH plants and soils and includes two processes (1) Mass Flow and (2) Diffusion
  • Mass flow in soils is a rapid process, whereas diffusion is only measured in mm per day in soils
  • Where mass flow is insufficient to satisfy plant demand, ion concentrations at the root surface are reduced below that of the surrounding soil volume
  • Zones of depletion create concentration gradients that drives diffusional processes in the soil (as a function of soil water content)
nutrient uptake64
Nutrient Uptake
  • Absorption of nutrient ions from soil solution
    • NO3-, SO42-, Ca2+, Mg2+ (<1000 mg l-1)
    • K+ (<100 mg l-1)
    • PO42- (<1 mg l-1)
  • Exchange absorption of adsorbed nutrient ions
    • Release of H+ and HCO3- as dissociation products of the CO2 resulting from respiration
  • Mobilization of chemically bound nutrients
    • H+ ions and organic acids, nutrients fixed in chemical compounds are liberated and form chelated complexes
nitrogen acquisition
Nitrogen acquisition
  • Nitrogen is the mineral nutrient that plants require in the greatest quantity and that most frequently limits growth in both agricultural and natural systems
  • The carbon expended in acquiring nitrogen can make up a significant fraction of the total energy a plant consumes
  • Plants have developed several approaches to nitrogen acquisition, including:
    • Root absorption of inorganic ions ammonium and nitrate
    • Fixation of atmospheric nitrogen
    • Mycorrhizal associations
    • carnivory
nitrogen acquisition consists of
Nitrogen acquisition consists of:
  • Absorption – bringing N from the environment into the plant
  • Translocation – moving inorganic N within the plant
  • Assimilation – converting N from inorganic to organic forms
Carbon costs for N absorption include:
  • Growth and maintenance of absorbing organs (usually roots)
  • Transport of minerals against a concentration gradient
  • Assimilation of N in leaves
Variation in Acquisition – a cost / benefit function of availability
  • Variation in N acquisition – additional carbon costs for ‘other’ absorbing organs
      • Some plants have developed associations with bacterial symbionts that allow for the use of atmspheric nitrogen
      • These plants incur the expense of (1) constructing root nodules (locations of symbiosis) and (2) providing bacterial symbionts with carbon compounds
Variation in Acquisition – a cost / benefit function of availability
  • Variation in N acquisition – additional carbon costs for ‘other’ absorbing organs
      • Associations with fungi that allow greater soil exploration
      • Endomycorrhizae – fungus penetrates root tissue
      • Ectomycorrhizae – fungus forms a sheath over root
      • Effectively increases absorbing surface area
      • Costs (carbon compounds) can be extensive – 15% of total net primary production in a Fir species
Variation in Acquisition – a cost / benefit function of availability
  • Assimilation costs
      • Transfer N to host as amino acids
      • Conversions:
      • Ammonium to amino acid – 2 electrons and 1 ATP
      • Nitrate assimilation – 10 electrons and 1 ATP
      • N-fixation – 4-5 electrons and 7-10 ATP per nitrogen atom
    • Fraction of carbon budget spent on nitrogen acquisition (absorption, translocation, and assimilation)
      • 25-45% for ammonium
      • 20-50% for nitrate
      • 40-55% for N fixation
      • 25-50% for mycorrhizae
Variation in Acquisition – a cost / benefit function of availability
  • Advantages of each strategy shift with the availability of the different nitrogen forms
  • Advantages shift with the varying limitations by water, carbon and nitrogen
    • Gaseous N is always abundant but has a high carbon cost
    • In environments where N limits growth more than C or H2O, N fixation becomes advantageous (early successional sites)
Variation in Acquisition – a cost / benefit function of availability
  • Preference for ammonium versus nitrate
  • Substantial species-specific variation
    • Mixtures of ammonium and nitrate are requirements for many species – flexible N acquisition plans?
nitrogen allocation
Nitrogen allocation
  • 75% of leaf N is located within chloroplasts (most in PSN function)
  • Recall, leaf age / leaf thickness / photosynthetic capacity / leaf nitrogen relationships

Emergent patterns at ecosystem scale

Evergreen Forests

Productivity (g m-2 y-1)

Deciduous forests

N uptake (g m-2 y-1)

integrating nitrogen acquisition into a whole plant function perspective
Integrating nitrogen acquisition into a whole-plant function perspective
  • Processes / factors to consider
    • Water-use
    • Photosynthetic gas exchange
    • Root – shoot allocation
    • Reproduction
    • Stress tolerance
    • Competition
nutrient dynamics outline and big deals
Nutrient Dynamics (outline – and big deals)
  • Nutrient availability
    • Sources of nutrients
    • Direct and indirect controls over sources
  • Nutrient Uptake
    • Plant and environmental interactions
  • Nutrient Return from the plant to the soil (cycling)
    • Ecological and environmental processes
    • Complexity of cycling

Big Point –

Tight coupling of nutrient cycling in an ecosystem and the functional diversity of dominant plant species

resource flow and growth rate
Resource flow and growth rate
  • Inputs of resources govern growth potential (not necessarily growth rate)
  • Plants adjust allocation schedules to match resource supply rates (or loss rates) (e.g., adjustment of sources and sinks)
theory of allocation
Theory of allocation
  • Major assumption:
    • Finite supply of resources
    • Distributed among
      • Growth
      • Maintenance / defense
      • Reproduction
    • Key to linking life history theory and physiology (the basis for ecophysiology)

Relative conducting abilities of aboveground and belowground structures – surface area characteristics

Biomass is often used as a proxy of this allocation of energy to function (both surface exchange capacity and rates of exchange)

Compensatory changes in each exchange surface result in different patterns of growth

trade offs
  • Resources are allocated among COMPETING functions
    • Generates trade-offs
  • Not always true –
    • Photosynthetic fruit
    • Stems as biomechanical support
    • Vegetative reproduction
how do plants know that a neighbor is near
How do plants know that a neighbor is near?
  • Reduction in resource availability?
    • Reduction in PAR
    • Reduction in nutrients, etc.
  • Cryptochrome and phytochrome pigments
    • ‘perceive’ red / far-red ratios of radiation
  • Plants growing in closed-spaced rows or high densities receive lower red / far-red ratios than sparse populations
    • Red light is absorbed to a greater extent by plant tissue than far-red light
  • Reductions in R / fR promote stem growth (height)
    • Species specific
how do plants sense their neighbors
How do plants sense their neighbors?
  • Chemical signals
    • Jasmonate – herbivory induced and may influence neighbors (illicit a ‘defensive’ response)
    • Ethylene – often a senescence inducing ‘hormone’
how do plants sense their neighbors100
How do plants sense their neighbors?
  • Microclimate manipulation
    • Differential heat exchange
    • Eucalyptus seedlings surrounded by grass see a lower minimum air temperature inducing stress
  • Belowground interactions (black box!)
interactions among species
Interactions among species
  • From a physiological viewpoint ….to understand the mechanistic basis for patterns
  • Competition
    • Occurs between individuals using a common resource pool
      • similar to our understanding of allocation dynamics within a plant
theories of competitive mechanisms
Theories of competitive mechanisms
  • Phillip Grime (1977) – relative growth rate defines competitive potential
    • High RGR facilitates rapid growth and allows a species to dominate space and acquire resources
theories of competitive mechanisms103
Theories of competitive mechanisms
  • David Tilman (1988) – ability to tolerate the drawdown of resources to some critical level
    • Species that can reduce a critical resource to a level not tolerated by neighbors is competitively superior
    • Described by R*
      • R* is the level at which growth matches loss for any given species…it also varies by species
theories of competitive mechanisms104
Theories of competitive mechanisms
  • Tilman and Grime do not present competing hypotheses, but each has slightly different implications
  • Dependence on stable-state dynamics
    • Resource levels
    • Species composition
resource competition
Resource competition
  • Depletion of a shared limiting resource occurs by:
  • A species effectively removing the resource from the environment
  • A species tolerating relatively low resource environments

The physiological underpinnings of these two strategies are quite different but as a result of physiological trade-offs, these strategies may be highly correlated

trade offs106
  • Two major physiological trade-offs
  • Between rapid growth to maximize resource acquisition versus resource conservation through reductions in tissue turnover (recall Chapin N figure)
  • Between allocation to roots to acquire water and nutrients versus allocation to shoots to capture light

Because of these trade-offs, there are not “competitively superior species” for all environments

what keeps a species from dominating an environment
What keeps a species from dominating an environment
  • Some environments are dominated by a single species
  • Some environments have significant environmental heterogeneity that influence the costs / benefits of these trade-offs
    • This influences competitive ability