Electromagnetic energy (light)
Mineral Elements
Carbon Dioxide

I.  Primary plant body fairly transparent to sunlight

    A.  Selective absorption of wavelengths associated with chlorophyll and accessory pigments

    B.  Pigments in epidermal cells may selectively absorb UV wavelengths, but see this New Zealand study

    C.  Primary cell wall network acts analogous to fiber optics to conduct light to the interior of the plant

II.  Two phases of matter involved in other absorptive phenomena

    A.  Gaseous

        1.  Diffusion

    B.  Liquid solutions

        1a.  Osmosis = diffusion across semi-permeable membranes 
        1b. Aquaporins   (Structure, Dynamics, Function   Plant Aquaporins)

        2.  Active transport = movement across semi-permeable membranes using energy

III.  Absorption of Soil Solutions via roots

    A.  Water and dissolved elements predominately in matrix phase of soil rather than capillary spaces in nonsaturated soils

    B.  Absorptive surfaces have to be in close proximity to this solution

        1.  Growth of roots

        2.  Growth of root hairs

                a.  Basal to zone of cell elongation to avoid being sheared off
                b.  Short lived, so zone migrates with root elongation
                c.  Grow primarily at apices which maximimizes penetration between soil particles
                d.  Greatly increases available surface area for absorption

        3.  In many plants, which lack root hairs, mycorrhizal fungi are associated with epidermal layers

                a.  The fungal hyphae are analogous to root hairs in these plants

    C.  Two potential pathways for diffusion of water and elements within the root

        1.  Apoplasm = Extracellular matrix continuum
        2.  Symplasm = Cytoplasm + plasmadesmata continuum
        3.  Plasmalemma separates these two continua

    D. Endodermis is critical to the absorptive function of roots
Cortex + Epidermis  Endodermis Xylem + Phloem + Pericycle
Fairly well aerated Suberized Casparian Strip of the Endodermis is an effective Diffusion Barrier in the Apoplasm Poorly aerated
High Metabolic Activity  Forces all material entering the inner part of the root to pass through the symplasm, providing a degree of regulation of solute uptake Low Metabolic Activity 
Capable of loading and retaining salts in symplasm Prevents leakage of salts back into outer part of root, facilitates development of hydrostatic root pressure Unable to retain salts in symplasm

III.  Absorption of Solutions (Rain and Dew) via Aerial organs

    A.  Some taxa (Epiphytic Orchids, Aroids, Bromiliads) lack subterranean roots

        1. Velamen

            a.  1-18 +- stratified subepidermal cell layers
            b.  Derived from periclinal divisions of the protoderm
            c.  Thick walled
            d.  Isodiametric or elongated in radial or longitudinal plane
            e.  Very small intercellular air spaces
            f.  Tissue often has spirally thickened fibers within it
            g.  Cells lack cytoplasm at maturity, taking on a silvery parchment appearance

        2.  Accumlates reservoir of solution via Capillary Action

        3.  Inner living cells absorb from this reservoir

    B.  Some taxa (Bryophytes, epiphytic Bromiliads) lack roots entirely

        1.  Specialized unicellular or multicellular trichomes that lack cutin absorb surface solutions

    C.  Bud scales associated with Pinus fascicles have little cutin and can absorb surface solutions

IV.  Absorption of gases in primary plant body

    A.  In leaves and primary stems gaseous exchange occurs primarily through pores surrounded by guard cells = stomata

        1.  In general total stomatal pore area is no more than 0.1%

        2.  No gaseous pumps so driving force for diffusion is chemical potential gradients

        3.  CO2 gradient is shallow

            a.  0.03% CO2 in air
            b.  Can't be less than 0.0% in leaf interior
            c.  Therefore maximum gradient = 0.03% / diffusion path length

        4.  H2O gradient is steep, ca 40 times that of CO2

            a.  1.25% H2O in air at 21oC and 50% Relative Humidity
            b.  ca. 2.5% H2O in saturated intercellular air spaces of leaves and stems
            c.  Therefore H2O gradient is ca. 1.25% / diffusion path length

        5.  Yet plants absorb CO2 at maximum rate and loose H2O at a minimum rate?

            a.  Experiments conducted on artificial membranes with pores

                1.  As pore size decreases, diffusion rate per unit area increases

                2.  Diffusion proportional to pore diameter rather than pore area

                3.  If pores are at least 10 diameters apart from one another, high efficiency is maintained

            b.  Diffusion Shell concept

                1.  Molecules dispersed in diffusion shells on opposite sides of the pores

                2.  Molecules acheive a momentarily high concentration in the pore opening

                3.  As long as diffusion shells do not overlap, high efficiency is maintained

                4.  Gas diffuses straight through pores and,

                5.  "spill" over pore edges

                6.  Gaseous diffusion linearly proportional to pore circumference (2 * PI *radius)

                7.  Gaseous diffusion exponentially proportional to pore area ( PI * radius^2)

                8.  In large pores, area is major factor

                9.  In smaller pores, circumference is relatively more important

            c.  For CO2, diffusion shells readily form on both sides of stomata, so uptake is very efficient

                1.  Rates are comparible to a nonexistant epidermal barrier!

            d.  For H2O, diffusion shells only form on outside of stomata

                1.  H2O is near saturation on interior, so diffusion shells can't form

    B.  Additional anatomical modifications on outer surface of plant

            1.  Increases the boundary layer of still air

            2.  Decreases diffusion gradient for H2O

            3.  Decrease rate of transpiration

            4.  Dense trichomes

            5.  Sunken stomata

            6.  Stomatal crypts

    C.  Various environmental factors influence stomatal action

        1.  Light

            a.  C3, C4 Plants stomata open

            b. CAM Plants stomata closed

        2.  Temperature

            a.  Up to 30oC stomatal pores increase

            b.  Above 30oC stomata usually close

            c.  Some plants, stomata don't open at low temperatures, even in strong light

        3.  [CO2]

            a.  [Low] promotes stomatal opening

            b. [High] stomata closed in light and dark conditions

        4.  [H2O] appears to be major controlling factor

            a.  [Low] stomata closed

            b.  [High] stomata open

            c.  [H2O] effects over ride other effects

    D.  Models for Stomatal Action

        1.  Classical Model

            a.  Light promotes photosynthesis
            b.  Decreases CO2 and Carbonic Acid (H2CO3)
            c.  pH increases
            d.  Starch hydrolyzed
            e.  Osmotic potential decreases
            f.   Osmotic pressure increases
            g.  Guard cell tuger pressure increases
            h.  Stomatal pores open
            i.  In the dark, no photosynthesis
            j.  Increases CO2 and Carbonic Acid
            k.  pH decreases
            l.  Starch formed from sugar
            m. Osmotic potential increases
            n.  Osmotic pressure decreases
            o.  Guard cell tuger pressure decreases
            p.  Stomatal pores close

        2.  Ion Pump Theory can explain functioning of CAM stomata

            a.  Osmolarity of guard cells regulated by K+ ion pumps activated by ATP
            b.  ATP generated by cyclic and noncyclic phosphorylation in photosynthesis
            c.  [ATP] high K+ pumps on
            d.  Osmotic potential decreases
            e.  Osmotic pressure increases
            f.  Guard cell tuger pressure increases
            g.  Stomatal pores open
            h.  In the dark, [ATP] decreases in C3 and C4, but increases in CAM
            i.  Low [ATP]  K+ pumps off
            j.  K+ diffuses out of guard cells
            k. Osmotic potential increases
            l.  Osmotic pressure decreases
            m.  Guard cell tuger pressure decreases
            n.  Stomatal pores close

V.  Gaseous absorption in secondary plant body

    A.  Lenticels

        1.  Various shapes, sizes, colors

        2.  Many loose cork cells with abundant intercellular air spaces

    B.  Lenticel phellogen

        1.  Laterally continuous with adjecent phellogen

        2.  Produces more cells than adjacent phellogen, so lenticels bulge both outward and inward

    C.  Development in species with initial phellogens originating in subepidermal layers

        1.  Tissue of substomatal chamber forms loose colorless mass of cells = complementary or filling cells

        2.  Deeper lying cells form the lenticel phellogen

        3.  Eventually the complementary cells rupture epidermis, die, blow away

        4.  Complementary cells replaced by loose cork cells produced by phellogen

     D.  Development in other species

        1. Not associated with substomatal chamber

        2.  Phellogen stops producing cork and begins making complementary cells

        3.  Eventually the complementary cells rupture surface, die, blow away

        4.  Complementary cells replaced by loose cork cells produced by phellogen

    E.  Characteristics of complementary cells

        1.  Gymnosperms

            a. Similar to cork cells but with thinner cell walls
            b.  More radially elongate
            c.  More intercellular air spaces

        2.  Three types in Dicots
Thin walled cells Thick walled cells Timing Examples
Suberized, abundant intercellular spaces Suberized, compact Annual alternation of cell types Magnoliaceae, Populus, Pyrus
Nonsuberized, abundant intercellular spaces Suberized, compact, "Closing Layer" Closing Layer produced at end of growing season Fraxinus, Quercus, Tilia
Wide, Nonsuberized, abundant intercellular spaces Narrow, Suberized, compact Multiple alternating layers produced during growing season Betula, Fagus, Prunus, Robinia