Spring.wmf (18300 bytes) Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

The Plant Way of Life or, On Being A Plant

(or Plants are smarter than you think!)


I. What is a plant?
 

    Recall - by most definitions, a plant:

  • is multicellular;
  • is non-motile
  • has eukaryotic cells
  • has cell walls composed of cellulose
  • is a photosynthetic autotrophic; and
  • exhibits alternation of generations - has a distinctive diploid (sporophyte) and haploid (gametophyte) phase.

    Examples include the angiosperms (flowering plants), gymnosperms (cone-bearing plants), ferns, and bryophytes (mosses & liverworts). Recent classification systems suggest that these organisms, in addition to the red algae and green algae, should be classified in the Plant Kingdom (Plantae).

II. What is the single most important characteristic that distinguishes plants from other organisms? 
    Autotrophism!  Yup, that's my guess, too. We should recognize that a systematist (someone who studies classification systems) familiar with the most recent notions of classification might disagree since members of a "new" kingdom, Chromista, are also photosynthetic autotrophs. Nevertheless, both of these groups are closely related so we can still safely agree that autotrophism is important to the plant way of life.    

A. Take-Home-Lesson 1: An autotroph makes its own food (energy-rich organic compounds) from simple, inorganic materials in the environment. Plants use light as their energy source, hence they are photosynthetic (vs. chemo-synthetic for certain bacteria). The general equation for photosynthesis is:

CO2 + H2O + light → (CH2O)n +O2

        In contrast, animals are heterotrophic, meaning that they must obtain their food (pre-fabricated organic compounds) from the environment. They cannot manufacture their own food. Examples of heterotrophs include mycotrophs (plants that obtain their nutrient source from a fungus like Indian pipes (Monotropa), decomposers (fungi, bacteria), carnivores, and herbivores.  Some parasitic plants (holoparasites like dodder (Cuscuta) and dwarf mistletoe that lack chlorophyll are obligate heterotrophs that can only obtain their nutrients from another plant.  Others parasites, like mistletoe (Phoradendron) and Indian paintbrush are green and can make their own organic compounds but obtain water and minerals from a host plant (Hershey).  Finally, some plants, like the carnivorous species, feed both autotrophically and heterotrophically.

B. Take-Home-Lesson 2: The autotrophic mode of nutrition evolved early in the evolution of life, ca. 3 billion years ago. This event set in motion the evolutionary events that culminated in modern plants. Therefore, modern plant characteristics can be explained as a direct or indirect consequence of the autotrophic mode of nutrition.  Plants colonized land about 440 million years ago.  The transition from water to land required the evolution of (in approximate sequence):  cuticle (to resist drying out), stomata (gas exchange), and vascular tissue (for water/nutrient transport).  

III. Consequences of autotrophic nutrition 
    Plants required specialized structures adapted for the autotrophic mode of nutrition. Specialization occurs at all levels of biological organization (e.g., organ, tissue, cell, organelle). Specific problems, and their solutions, related to autotrophic nutrition are:

Problem: Photosynthesis is a complicated biochemical process. 
    In order for photosynthesis to function properly and efficiently, it was necessary to separate it from other reactions that occur in the cell. Thus, the evolution of a specialized organelle for this process - the chloroplasts. Even within the chloroplast specialization was required. Recall from intro bio that there are three major areas in a chloroplast -  the stroma, inner membrane, and inter-membrane space. Each of these three regions is important for the functioning of photosynthesis. Electron transfer reactions require the highly ordered environment provided by the inner membrane. The Calvin cycle (light-independent reactions) are aqueous biochemical reactions which occur in the stroma and the inter-membrane space is needed to generate the pH gradient across the membrane that is important for photophosphorylation (ATP production).

Problem: Photosynthesis requires efficient light harvesting.
    Leaves are perfect solar collectors. These organs are broad and flat to allow for efficient light harvest. The leaves are broad to maximize surface area for light harvest and they are thin since light cannot penetrate too deeply into the leaf (the amount of light decreases exponentially with distance). As an aside, although the majority of light is absorbed near the leaf surface, in some situations plant tissues act like fiber optic cables that can funnel some light deeply into the plant body (Briggs et al).  The window plant in the Namib desert funnels light through translucent cell into the photosynthetic tissue that is buried in the soil (Attenborough)

    Even within the thin leaf, most chloroplasts are found in the upper layer of cells, the palisade layer, which is the tissue layer just beneath the upper epidermis. This makes "sense" since these cells will be receiving the greatest amount of light of any region in the leaf. Thus, this is an example of specialization at the tissue level.

Problem: Photosynthesis requires an apparatus for gas exchange.
    Leaves double as a means to exchange photosynthetic gases (take up carbon dioxide and get rid of oxygen) with the environment. Leaves have pores in the surface (stomata) that regulate the entry/exit of gases and prevent the loss of excessive water.

    The spongy layer of the leaf acts like a "lung" increasing the internal surface area and provides for more rapid diffusion within the leaf. Note again that leaves are thin - this avoids the need for lungs or other type of pump to move gases. Since diffusion rates are inversely related to distance, diffusion can account for gas movements into/out of a leaf.  As a consequence, no cell if more than 2 or 3 cells from the air.  An added advantage of having large leaves for light harvest is that they provide lots of surface area for absorption of carbon dioxide.

    Again, note the specialization of the leaf at the organ, tissue, and cellular levels for gas exchange.

Problem: Thin leaves, required for light absorption and gas exchange, need support.
    This problem was solved by the evolution of the cell wall which provided for the support of thin structures without the need (or potential) for significant numbers of internal support structures.  Leaves also have some internal "struts" (in other words, veins).

Problem: Photosynthesis requires a water supply.
    With the exception of the algae and aquatic plants, plants obtain their water through the roots from soil. Essentially the roots "mine" the soil for water. Thus, photosynthesis and the transition to a terrestrial environment necessitated the evolution of a root system to obtain water (specialization at the organ level). And, it required the evolution of specialized transport tissue (xylem) to move the water from the roots to the leaves.

Problem: Photosynthesis requires a mechanism to transport end products throughout the plant.
    Once carbohydrate is produced during photosynthesis there must be a mechanism to transport it to other locations throughout the plant. The evolution of vascular tissue, specifically phloem, permitted movement of photosynthate from leaves to roots, fruits and other tissues where required.

IV. Consequences of autotrophic nutrition - Motility is no longer required; Or possible.
    One of the main reasons for motility is to obtain food. Since the nutrients required by plants are "omnipotent" there was never an evolutionary pressure for "motility."  Let’s quickly compare the nutrients used by plants and animals:

Table 1: Comparison of Plant & Animal Nutrition

Nutrient

Plant

Animal

form of uptake inorganic (CO2, water, ions) organic (proteins, carbohydrates, fats)
concentration dilute (i.e., CO2 = 0.03%) concentrated
distribution omnipotent localized

Conclusion: plants must be adapted for harvesting dilute nutrients that occur everywhere, whereas animals are adapted for searching out and trapping widely dispersed, concentrated packets of food.

Supportive Evidence: if this is true, then we hypothesize that animals with a nutrient source like a plant should have similar features to a plant. Check out corals, sea fans, and hydra. These are all non-motile animals that occur in aquatic environments which enables them to "feed like a plant" - food is essentially brought to them via water currents. Thus, they never had any pressure for motility and they have very similar lifestyles/forms as plants.

    In addition, note that motility is really not possible for terrestrial plants. Once plants evolved roots it precluded movement. These evolutionary "choices" are closely connected.

However, being stationary has its own problems/consequences.

V. Consequences of a Stationary Lifestyle - The need to exploit a limited volume of the environment for resources.
    The problem: a fixed (stationary) organism must be able to continually obtain nutrients without using them up. Plants face the additional problem that their nutrients are "dilute."  Thus, plants must be designed for collecting dilute nutrients in the environment. Plants have several solutions to this "problem":

A. Plants are dendritic
    In other words, the basic shape of the plant body is dendritic - which means "tree-like" or "filamentous". The advantage of this shape is that it provides a large surface-to-volume (s/v) ratio which enables a plant to exploit a large area of the environment. In contrast, animals are more compact (spherical) to minimize their s/v ratio. Among other things, this is an advantage for motility. Surface-to-volume ratios are very important in many areas of biology.  In class we will investigate surface/volume ratios in more detail.

B. Plants have indeterminate growth.  
    Process by which a plant continues to grow and get larger throughout its life cycle. The advantage of this is that it allows the plant, especially roots, to grow into new areas. In contrast, determinate growth is where an organism or part reaches a certain size and then stops growing. This is characteristic of animals and some plant parts (e.g., leaves, fruits).

C. Plants have an architectural design.  
    In other words, the plant body is constructed like a building - modular (Silverton & Gordon). It is built of a limited number of units, each of which is relatively independent of the others and that are united into a single structure. Thus, just like a building is made of rooms, the leaves, stems and roots of a plant are analogous to a rooms in the building. Each room is somewhat independent, yet they all function together to make an integrated whole. You can seal off a room in a building, or remove a leaf or fruit, with little harm to the overall integrity of the structure. This is critical for plants to be able to add or remove parts (leaves, stems, flowers, fruits) as necessary. One conclusion is that because of their indeterminate growth and architectural design, plants are not limited by sizeThis gives plants the ability to colonize and exploit new areas for resources.

    In contrast, an animal has a mechanical design. In other words, animals are built more like a machine, made of numerous, different parts that function together. The parts are highly integrated. Parts cannot be added or removed without reducing the efficiency of the operation of the whole. Animals are limited by size.

    As a consequence, plants are not a static shape - plants constantly change shape by adding/loosing parts - by accumulating modular units. Animals don’t change shape - they remain the same general shape throughout their life.  Thus, growth in plants occurs by the addition of new "units" not enlargement.

D. Plants have a well developed ability to reproduce asexually
    This can be viewed as a quick and energetically inexpensive way to expand the influence of the parent into a new location. One testable prediction from this hypothesis is that plants under nutrient stress should increase their rate of asexual reproduction (see foraging data below).

E. Plants (may) exhibit heterophylly. 
    Heterophylly refers to leaves with different shapes. For example,  the aerial leaves of aquatic plants are entire but the submerged leaves are dissected. Sun leaves tend to be smaller and thicker than shade leaves. Dandelions are toothier when grown in a carbon dioxide enriched (700 vs. 350 ppm) environment.  The leaves of the vine Monstera are more or less heart-shaped and pressed to the trunk as the vine climbs into the tropical forest canopy.  Once in the canopy, the leaves take on the mature form with slits and holes. 

F.  Leaves are arranged to minimize overlapping.
     Phyllotaxy is the fancy term for leaf arrangement.  Interestingly, phyllotaxy patterns have always been shown to be related to Fibonacci number series (1, 1, 2, 3, 5, 8, 13, 21....etc).  For more on phyllaxy, visit this web site.

G. Plants can forage
    The growth patterns of plants, especially vines and plants with stolons (runners), are similar to the foraging tactics of animals. As an example, rhizomes of Hydrocotyle veer from patches of grass to avoid competition.  A brief overview of the anatomy of a clonal plant like Glechoma hederacea (ground ivy): parent plant, stolon (internode), ramet (individual of a clone).

    For a Case Study on Foraging, click here.

    Thus, plant growth is essentially analogous to animal behavior. One of the first to express this idea was Arber (1950; The Natural Philosophy of Plant Form. Cambridge).  She said, "Among plants, form may be held to include something corresponding to behavior in the zoological field...for most though but not for all plants the only available forms of action are either growth, or ascending of parts, both of which involve a change in the size and form of the organism."

VI. Consequences of a Stationary Lifestyle - Positioning in the environment.
   
Problem: a non-motile organism is unable to move to a more favorable location to carry out its vital functions. Thus, plants have at least three major problems to contend with:

A. Environmental Positioning/Location - or, Getting Started in the Right Spot.
    Obviously a motile organism can move to a favorable location, but a plant is stuck once the seed germinates. For most plants getting started in the right place is a matter of luck. Thus, it is no surprise that plants exhibit a Type III survivorship curve (produce lots of offspring, few survive, no parental care of offspring - think oak tree and acorns). However, there are a few "tricks" that plants use to help increase the odds that seeds will germinate in a favorable environment:

  1. Light - some seeds, like certain varieties of lettuce, require light for germination. This is a mechanism to insure that they germinate on the soil surface. It's no surprise that a garden develops a healthy crop of weeds after the soil is turned - it brings light-sensitive seeds to the surface. Light sensitive seeds are usually small and without much stored food. Thus, it is important that they begin to photosynthesize soon after germination.
  2. Ethylene - some seeds require ethylene to germinate. This naturally occurring plant hormone is produced by plants and soil microbes. Once the ethylene concentration reaches a critical level it induces the seeds to germinate. This happens if the seed is buried. These seeds are usually larger than light sensitive ones. One advantage of being buried is that the seeds will be more likely to be in a moist, humid environment.  e.g., witchweed (Striga)
  3. Specialized Dispersal Mechanisms - some plants have specialized mechanisms for dispersal that will increase the odds of the seed getting into the proper place.  For example, mistletoes have sticky seeds that often stick to the beak of a hungry bird.  The bird will try to rub it off on a branch where the seed will adhere and germinate.

B. Axis orientation. 
    Once a seed germinates in a favorable environment it must determine which way is up/down to insure that the roots grow down and shoots up. Thus, gravitropism is a very important physiological response characteristic of all plants.  

C. Fine Tuning.  
    Even non-motile organisms need to "fine-tune" their position in the environment. Thus plants have a variety of mechanisms that enable them to optimize their position in the environment including:

  1. Phototropism - grow toward light, maximize light reception.  Although plants are typically positively phototropic, some show negative phototropism.  For example, the tendrils of a tropical vine (Bignonia capreolata) grow away from the light and ivy stems bend away from the light (hence the reason they tend to come into a window) but the leaves grow towards the light.  
  2. Skototropism - growth of vines (e.g., Monstera) toward a darkened region of the environment. Mechanism by which some tropical vines find a support to grow up (Ray, 1975).  Video clip from Private Lives of Plants series;
  3. Thigmomorphogenesis - response to touch in which the plant is shorter with thicker stems - prevents plants from getting too spindly and reduces risk of breaking in wind;
  4. Solar tracking (i.e., flowers follow the movement of the sun) - keeps pollen dry, maximize photosynthesis;
  5. Leaf mosaics - pattern of leaves which minimizes overlapping such as ivy on a building (Oxlade, 1998);
  6. Etiolation - the response of plants to growth in the dark or with reduce light. Etiolated plants are typically yellow, have elongated internodes (stems) with unfolded leaves and the stems are thinner. These features can be considered ways of conserving energy until conditions improve (i.e., light).
  7. Habitat selection - one example of habitat selection is shown by rhizomatous plants like western ragweed (Ambrosia psilostachya) that preferentially colonize non-saline soil (Salzman, 1985).  In this case, the growth rate is higher in saline soil than non-saline - "moving away from the salt" increasing the likelihood of finding new habitat.  Stilt palms grow to avoid competition and the roots of many plants grow to avoid one another.  Dodder, a parasite, actually "chooses" its host when presented with several potential species.  Roots track humidity and mineral gradients and can change branching patterns in response.
  8. Climbing plants hold their leaves at right angles from the support stem to better intercept light (Dicks 2000)
     

VII. Consequences of a Stationary Lifestyle - the need to sense & respond to the environment
   
Problem: plants, like other organisms, must be able to respond to changes in their environment.  Plants respond to their environment in a variety of ways.  

  1. Plants typically respond by various growth movements to unpredictable, usually short-term environmental fluctuations, like changes in temperature or light.  For example, some flowers like crocus are temperature sensitive and open when it is warm and close when it is cooler.  The flowers of an alpine species (Gentiana algida) close up before a thunderstorm.  The flowers actually respond to the decreased temperature associated with the storm front and close to avoid pollen being washed out of the flower by the rain.  
  2. Plant form responds to the environment.  Light is one of the most important environmental cues for plant development.   This phenomenon is called photomorphogenesis and one classic example is etiolation (discussed above).  Another example: increased levels of carbon dioxide in the air has lead to a decrease in the number of stomata on leaf surfaces and have similarly been shown to increase the toothiness of dandelions.  The leaves of plants in the rainforest usually have an elongated tip (drip tip) to help funnel water off the leaf surface to minimize the growth of fungi and other leaf epiphytes that might cause disease or block sunlight.  Leaf margins are also a response to the environment.  For example, entire leaf margins are correlated with warmer temperatures in tropical forests.  We also discussed sun and shade leaves earlier.
  3. Plants usually respond to predictable, often longer-term environmental changes, like seasonal changes, by growth and developmental changes because of the constraints of their architectural design.  Since these changes take time, a plant must "know" or "predict" when the environment will change and prepare for the change. Indeterminate growth is important here since it provides plants with the ability to change developmentally through the life cycle. Some examples of this phenomenon include: preparing for winter (by forming buds in summer) and photoperiodism (timing flowering so the appropriate pollinator is available and that the seeds have enough time to develop before winter); circadian rhythms (various types in response to day/night), and nyctinasty (sleep movements).   In contrast, animals typically respond to their environment behaviorally (by movement). Since they are motile, they can "move" to a favorable position. In fact, because they are motile they need a nervous system to respond to the environment. Plants don’t need a nervous system since they are constrained to respond to the environment by growth/developmental changes; hence they never had a pressure to evolve a nervous system.


VIII. Consequences of a Stationary Lifestyle - need to protect themselves from physical and biological dangers in the environment.
    Problem: a non-motile organism cannot flee when conditions get tough. It must "fight" it out. Both the physical environment and biological environment threatens the well being of plants.

A. Physical dangers - wind, water (flood), drought, cold (winter) are among the physical dangers that a plant faces. In general, plants cope with these (at least the predictable ones like winter and drought during summer) by dormancy, senescence, and even death.  The evergreen and deciduous lifestyle are in part a response to adverse conditions.  Evergreens are much better able to tolerate cold, dry conditions.  They also do better in poor soil because they don't loose as many leaves.  Plants also respond to enviromental challenges morphologically - for example, xeric plants reduce their S/V to minimize water loss.  Arctic or montane herbs are small and hug the ground.

B. Biological dangers - predators (=herbivores) and competitors (=other plants). Plants have evolved:

  1. Anatomical weapons (thorns, hairs, thick cuticle).  Some plants are even "smart" enough to stop producing defenses when they are out of range of a herbivore.  For example, the upper parts of Acacia trees above giraffe height produce few thorns.  Similarly, holly leaves have few prickles above herbivore (e.g., deer) height; 
  2. Chemical weapons - produce toxic, unpalatable chemicals. These can be inducible (produced in response to attack) or constitutive (always present) (Karban & Myers. 1989. Ann Rev Systemat. Ecol 20:331); Allelopathy is chemical warfare between plants.  Phytoalexins are chemicals produced by plants to resist microbial infection.    
  3. Mimicry - "tricking" predators. For example, lithops in S. African deserts look like pebbles - are stone mimics.  NZ mistletoe leaves look just like the host tree leaves to avoid being eaten which is important because they contain even higher amounts of nitrogen.  Alseuosmia is a non-toxic New Zealand plant that looks very similar to Wintera pseudocolorata (a toxic species).  When young, NZ lancewood looks very unappetizing - much like a woody umbrella that is folded up.  However, when it gets about 15 feet tall, above predator (e.g., moa) height, it branches out and has a more "traditional" appearance.
  4. Armed Forces - some plants, like wild tobacco, when attacked by herbivores release volatile chemicals that summon predatory insects to the damaged plants.  These insects in turn, kill the herbivores.

    Overall, volatile chemicals play a very important role in plant defense.  These chemicals, released when the plant is attacked by an herbivore serve as signals that:  (a) warn other plants that danger is imminent.  For example, tobacco eaten by herbivores produces salicylate (similar to aspirin) that stimulates its own defense response and is converted into methylsalicyate which is volatile.  This compound travels to other plants to induce their defense response; (b) tell other herbivores that it is being attacked and that they should look for another food source unless they want to battle it out (compete) with another herbivore; (c) alert predator insects that their are tasty herbivores in the area (see #4); (d) tell herbivores that the chemical defense system of the plant is ready for them and that should go pick on someone else; and (e) the volatiles themselves help to repel the attack.

    Plants are more sensitive to being handling than perhaps we give them credit.  Recent studies have shown that simply stroking the leaf of a plant just once will affect herbivory - some plants suffer greater damage while others less damage (Sci News 159: 119, 2001).
 

IX. Consequences of a Stationary Lifestyle - reproduction
    Problem: A non-motile organism cannot seek a mate (for gamete transfer) or easily disperse offspring. Plants solve the gamete transfer problem by relying on various pollination vectors. Fruits/seed dispersal mechanisms help disperse offspring.  Check out the Private Lives of Plants video, Volume 1 (dispersal) and Volume 3 (pollination) for some great examples.  These document some absolutely fascinating stories about plants and their pollination and dispersal vectors.  To add a recent story, the fragrance of a particular orchid changes after pollination.  Prior to pollination the orchid released a fragrance that was an "aphrodisiac" for the male but once the flower was pollinated it produced a fragrance to repel him.  And, when a few fruits of Hamelia patens, a neo-tropical tree, are removed it stimulates the rest of the fruit to ripen - it essentially tells them that a dispersal agent is in the area.
 

X. Additional Consequences: The consequences of the cell wall
    Problem
: As we mentioned, plants evolved cell walls: (a) as a means of support; (b) to prevent the protoplast from bursting in a hypotonic medium; and (c) some speculate that walls may even be a way to dispose of excess carbon.  In any event, by surrounding their cells with a rigid box, this imposed certain limitations. These include:

  1. Growth is restricted to meristematic regions (primary meristems - responsible for growth in length such as apical meristems at root and shoot tips; and secondary meristems - responsible for growth in girth, like vascular cambium). In contrast growth in animals occurs throughout the body.
  2. Since plants are built from rigid structures, morphogenesis occurs by way of cell addition, not cell movement (as is found in development of animals)
  3. Plants respond to their environment developmentally rather than behaviorally
  4. Plants exhibit indeterminate growth (vs. determinate for animals)
  5. Plants grow by the progressive accumulation of similar units (i.e., architectural design); whereas animals have a fixed shape that enlarges (from Adrian Bell, 1986)
  6. Since each cell is walled off from neighboring cells, plants need an effective means of cell - to -cell communication. Plants accomplish this through: plasmodesmata - cytoplasmic connections; hormonal regulation and some electrical signals.

XI.  Plants are Smarter than you Think!
     For many years, I have joked that "plants are smarter than you think."  Hopefully, our discussion of the plant way of life has led you to agree.  Although my comment was somewhat tongue-in-cheek, there have been recent discussions about the intelligence of plants.  Historically, plants have not been considered to be "intelligent" because this concept has been associated with movement.  Thus, animals are smart, plants are not.  But, if you agree with Anthony Trewavas (2002) who first argued in Nature that if "intelligence is defined as adaptively variable behaviour during the life of the individual, then, in plants, behavioural plasticity is where intelligence should be apparent."  The examples cited in this document and lots of others from Trewavas, convince me that Trewavas is correct - plants are intelligent.  For more reading, Trewavas has expanded his Nature article into a longer review which is excellent and available on-line.  David Hershey agrees, and has also addressed this issue in an eloquent essay, "Plants are indeed, intelligent," that was submitted to the American Biology Teacher.  Both of these articles contain lots of examples why plants are so "cool." 

    One caveat, don't associate the concept of intelligence with the notion of intelligent design, which is an idea that suggests God must have created the earth and its inhabitants because everything is too perfect to have happened via the somewhat random or chancy process of evolution.
 

XII. Plant Blindness/Animal Chauvinism/Plant Neglect/Plant Prejudice
    Hopefully, I've made a good case for why it's important to study plants and why are they are "cool."  Which brings up a question - why don't more people like or want to study plants?  Why is plant physiology normally a small class?  Evidence shows that even grade school kids prefer zoological topics to botanical ones.  But why?

    Wandersee and Schussler (1999) argue that the main problem is that by human nature we are "blind" to plants due to limitations in our visual perception.  Others, like Hoekstra (2000) and Hershey (2002) argue that a more critical concern is that plants are neglected in the curriculum and that botanical concepts are often either not taught, or not presented well.   Whatever the reason, this semester we are going to be Plant Chauvinists practicing Animal Neglect, seeing clearly the beauty and excitement in the study of plant physiology. 
 

References:

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Last updated:  01/29/2009     Copyright  by SG Saupe