Table of Contents

 

 

 

 

Topics                                                                                                             Page

 

Plant Anatomy and Morphology                                                                             2

 

Hormones and Elicitor Molelules                                                                         15

 

The Genetic Basis of Life                                                                                     24

 

Genetically Modified Organisms (GMOs) or Transgenic Crops                        30

 

Seed Germination, Dormancy and Priming                                                          33

 

Growth Kinetics                                                                                                     38

 

Source Sink Relations                                                                                           44

 

Senescence and Post Harvest Storage                                                                 50

Plant Anatomy and Morphology

 

A horticulturist who does not know the basic anatomy of plants is like is like a nurse that does not know basic human anatomy.  It could turn out to be down right uncomfortable where he/she sticks that thermometer!  So we are going to take a tour of plant structure. A working knowledge of plant anatomy is absolutely essential in:

§         plant propagation: grafting, budding, division, cuttings, layering, tissue culture

§         pruning

§         making crosses in plant breeding

§         diagnosing plant disorders

 

Anatomy is very simply.  Anatomists simply look at the outside and inside of plants and when they see distinctive structures they give them a name.  At the whole plant level, plants are divided into four organs: The root, stem and leaf are vegetative organs, and the flower, and resultant fruit, is a reproductive organ.

Plant Organs

§         root

§         stem

§         leaf

§         flower

 

Each organ is composed of three tissue systems:

Tissue Systems

§         dermal tissue system

§         vascular tissue system

§         ground or fundamental tissue system

 

Each tissue system is composed of distinctive tissues (epidermis, periderm, xylem, phloem, cortex, pith and mesophyll), and tissues are in-turn composed of cells (parenchyma, collenchyma, sclerenchyma, and specialized cells such as trichomes, vessels, companion cells, laticifers, etc.).

 

Plants produce all these structures by growing from discrete clusters of dividing cells called meristems.  Herbaceous tissue is growth in length from:  1) apical meristems, which occur at the end of every shoot and root, and 2) intercalary meristem at the base of grass leaves.  Woody tissue is due to growth in diameter from:  1) vascular cambium, which produce secondary xylem (wood) and phloem, and 2) phellogen, which produces the periderm (bark).

 

Virtually all of the crops we grow in horticulture are monocots (linear leaves, ex. grasses, corn, dracaena, and palm), dicots (broad-leaved plants, ex. oak, lettuce, apple) or gymnosperms (leaves as needles and scales, ex. pine, juniper).  The internal anatomy of monocots, dicots and gymnosperms are sometimes similar and sometimes different.  Different types of plants are not like animals - all the tissues and organs are not always in the same location.  Thus, one must know the basic anatomical similarities and differences of each, or else you are not going to know where to insert that thermometer - ouch!

Organs and Tissue Systems

 

Plants are composed of 3 vegetative organs and 1 reproductive organ.

Three tissue systems comprise each organ and are contiguous between each of the four organs.

 

 

 

 

How Do Plants Grow?

 

Meristems and Growth

 

Primary Growth -       growth in length that gives rise to primary (herbaceous) tissues called the primary plant body.

 

      2 -Types

      apical meristem or apex - the growing points located at the tips of stems and roots

      intercalary meristem -          the growth region at the base of grass leaves which causes
                                                     leaves to elongate.

 

Secondary Growth -         growth in width or diameter which gives rise to secondary (woody
                                          or corky) tissues called the secondary plant body.

 

      lateral meristem - meristematic regions along the sides of stems and roots.

 

      2 Types

      vascular cambium or cambium -  gives rise to secondary xylem (wood) on the
                                                                        inside and phloem on the outside.

 

      cork cambium or phellogen -             gives rise to the periderm (bark).

 

Stem Anatomy

Herbaceous Dicot or Gymnosperm - Primary Growth

 

 

(Fig. 16.1 from Esau 1960)

Stem Anatomy

Woody Dicot or Gymnosperm - Secondary Growth

 

 

 

 

 

 

(Plate 28 from Esau 1965)

Stem Anatomy

Herbaceous Monocot - Primary Growth

 

 

(Plate 58 from Esau 1965, Fig. 17.8 from Esau 1960)
Root Anatomy

Herbaceous Dicot, Gymnosperm or Monocot - Primary Growth

 

 

(Plate 84 & 86 from Esau 1965)
Root Anatomy

Woody Dicot or Gymnosperm - Secondary Growth

 

A woody dicot or gymnosperm root in secondary growth looks very similar to a stem in secondary growth.  The tissue is more porous and less dense, and the periderm is thinner.  Rings of xylem growth may not be as distinctive as occurs in stems.  This is because roots of temperate plants do not posses a distinctive “rest” or “physiological dormancy” period during the winter as do buds and shoots.  Root growth may occur whenever the soil moisture, fertility and temperature are favorable.

 

 

(Fig.15.4 from Esau 1960)

Leaf Anatomy

 

Dicot

(Plate 73 from Esau 1965)

 

Monocot

(Similar to dicot, except no palisade, mesophyll is all spongy parenchyma)

(Fig. 19.6 from Esau 1960)

Leaf Anatomy

 

Gymnosperm

(Plate 78 from Esau 1965)

SUMMARY OF ANATOMY – VEGETATIVE STRUCTURES

		MONOCOT	DICOT	GYMNOSPERM
STEM	PRIMARY (herbaceous) GROWTH
	SECONDARY (woody) GROWTH	none
ROOT	PRIMARY (herbaceous) GROWTH
	SECONDARY (woody) GROWTH	none
LEAF	PRIMARY (herbaceous) GROWTH
	SECONDARY (woody) GROWTH	none	none	none

FLOWER STRUCTURE

 

FRUIT STRUCTURE

Example of a dry fruit                                                Example of a fleshy fruit

 

SEED STRUCTURE

 

Anatomical Structure and Function

 

"Structure and function" is a term used when the anatomy of a plant part explains how it functions. Structure and function brings anatomy to the real world, and it is what makes anatomy exciting.  We are going to take a close look at one of the most important structure function relationships in plants - translocation.  The tissues responsible for long distance translocation in plants are xylem and phloem.

 

Xylem is composed dead, hollow cells with perforated walls.  The xylem cells are called vessel elements or tracheids.  .  They are connected end to end and clustered side by side.  They are like a cluster of leaky pipes with holes on all sides.   If you took sewer drain field pipe and connected them end to end, and bundled many of them together side by side, you would have a perfect model of xylem.  Xylem only flows up.  All xylem is dead and the water is "passively" pulled up stems by transpiration of water from the leaves.  It is like sucking water up a straw.  In young tissue, these bundles of xylem cells occur inside the vascular bundles, which are the stringy tissue in herbaceous tissue (ex. veins in leaves).  In woody plants, xylem is the wood.  The sapwood is functional because the hollow xylem cells are open and water easily flows up the tubes.  All the water flows up the sapwood.  The heartwood is old clogged xylem, and does not translocate water, and thus is not functional.  The heartwood is clogged with resins and tannins and this makes the heartwood both waterproof and prevents it from rotting.  

 

 

 

Phloem is composed of specialized cells that remain alive and "actively" translocate solutes (salts, sugars, metabolites, hormones, etc.) around plants.  The phloem tissue is very concentrated in sugars, amino acids, and many nutrients.  It is the phloem that sucking insect, such as aphids, puncture in order to feed on the sugar and nutrients...  This is similar to a mosquito piercing your veins and arteries as a food source.

 

Phloem flows both up and down and all around.  It is commonly stated that phloem flows down, but this is wrong.  Phloem flows to where it is needed.  Phloem flows from sources to sinks, which will be discussed next.

 

HORMONES AND ELICITOR MOLECULES

 

Hormone -                  an endogenous or naturally-occurring compound that is produced or synthesized in one part of the plant and causes a change in physiology, growth or development in another part of the plant; usually present in very small quantities.

 

Elicitor Molecule -     a compound which, when introduced in small concentrations to a living cell system, initiates or improves the biosynthesis of specific compounds; a compound with hormone-like activity.

 

Growth Substance -    all naturally-occurring or synthetically produced compounds that affect the physiology, growth and development of plants.

 

References

Moore, T.C.  1979. Biochemistry and Physiology of Plant Hormones. Springer-Verlag, NY.

 

 

Plant Hormones and Elicitor Molecules

Classically, plants have been known to contain five hormones, which are auxin, cytokinin, gibberellic acid, ethylene and abscisic acid.  Recently, other endogenous compounds have been shown to elicit hormone-like reactions, which are brassinosteroids, jasmonic acid, salicylic acid and polyamines.  Some do not elevate these to the status of one of the five classical hormones, so often they are called elicitor molecules.

 

1)   Auxin

 

2)   Cytokinin

 

3)   Gibberellic Acid

 

4)   Ethylene

 

5)   Abscisic Acid

 

6)   Brassinosteroid

 

7)   Jasmonic Acid

 

8)   Salicylic Acid

 

9)   Polyamines

AUXIN

 

Naturally-Occurring	Synthetic	Structure	Site of Production
indoleacetic acid (IAA)	indolebutyric acid (IBA) naphthaleneacetic acid (NAA) 2,4-dichlorophenoxy-acetic acid (2,4-D)		shoot tips, embryos

 

SYNTHESIS

 

tryptophan ® indoleacetic acid

 

 

TRANSPORT

 

·        3:1 basipetal transport

·        primarily in phloem parenchyma

 

EFFECTS

 

1)   Cell elongation - causes acid induced cell wall growth

 

2)   Cell division - stimulates

 

3)   Tropism - response of plants to environmental or physical stimuli.

      a)      phototropism - response to light

      b)      geotropism - response to gravity

      c)      thigmotropism - response to touch

 

4)   Apical dominance - determined by correlative inhibition of apical bud, partly due to auxin produced

5)   Sprout Inhibitors – retard basal branching.

 

6)   Branch angle - causes wide branch angles

 

7)   Fruit set - low concentrations stimulate

 

8)   Fruit or flower thinning - high concentrations cause

 

9)   Herbicides - 2,4-D at high concentrations

 

10) Adventitious root formation -         a) stem and leaf cuttings

                                                               b) tissue culture

CYTOKININ

 

Naturally-Occurring	Synthetic	Structure	Site of Production
zeatin kinetin (not in plants)	benzyladenine (BA) pyranylbenzyladenine (PBA)		root tips, embryos

 

 

SYNTHESIS

 

adenine ® zeatin

 

 

TRANSPORT

 

·        xylem transported, found in root exudates

·        primarily acropetal, but not necessarily polar

 

 

EFFECTS

 

1)   Cell division - stimulates cell division; named after cytokinesis

 

2)   Nutrient mobilization - nutrients transported towards high cytokinin concentration.

 

3)   Apical dominance - high cytokinin/low auxin may overcome apical dominance

 

4)   Chlorophyll breakdown - decreases chlorophyll breakdown

 

5)   Leaf Aging or abscission - may delay

 

6)   Seed germination - may overcome dormancy or stimulate germination

 

7)   Adventitious shoot formation -           a) leaf and root cuttings

                                                                    b) tissue culture

 

8)   Root growth - may be inhibitory to root growth

GIBBERELLIC ACID  (GA)

 

Naturally-Occurring	Synthetic	Structure	Site of Production
over 50 (named by consecutive numbers)	none		shoot tips, root tips, embryos

 

SYNTHESIS (see next page)

mevalonate ® farnesyl pyrophosphate ®

 

® geranylgeranyl pyrophosphate ® copalyl pyrophosphate® kaurene ® GA

 

growth retardants -   chemicals that block synthesis of GA;  most block the ring closure steps between geranylgeranyl pryophosphate ® copalyl pyrophosphate ® kaurene.

 

TRANSPORT

 

·        no polarity

·        in phloem or xylem

 

EFFECTS

 

1)   Protein synthesis - triggers de novo synthesis of some proteins, ex. a-amylase.

 

2)   Cell elongation - primary stimulus for cell elongation

 

3)   Rosette or dwarf plants - lack of endogenous GA often contributes to decreased height.

 

4)   Height control

·        GA used to increase height

·        growth retardants used to decrease height

5)   Flowering - may cause bolting in biennials

6)   Fruit size - increases size of seedless grapes

 

7)   Bud dormancy - may overcome and substitute for cold treatment

 

8)   Seed germination - may increase or speed up

 

9)   Sex expression - favors staminate flower formation on monoecious plants

Biosynthetic Pathway of Gibberellic Acid

(from Moore, 1979)

 

 

 

 

 

 

 

 

 

 

 

Mode of Action of Growth Retardants

 

·        block ring closure between geranylgeranyl pyrophosphate and copalyl pyrophosphate

·        block ring closure between copalyl pyrophosphate and kaurene

ETHYLENE

 

Naturally-Occurring	Synthetic	Structure	Site of Production
ethylene	ethephon or ethrel (release ethylene inside plant)		ripening fruits, aging flowers, germinating seeds, wounded tissue

 

 

SYNTHESIS

 

methionine ® s-adenosylmethionine ® 1-aminocyclopropane-1-carboxylic acid ® ethylene

                                 (SAM)                                                 (ACC)

 

 

ETHYLENE  INHIBITORS

 

ethylene inhibitors - chemicals that inhibit the synthesis or action of ethylene

 

Synthesis Inhibitors (block synthesis of SAM ® ACC)

·        AVG - aminoethoxyvinyl glycine

·        MVG - methoxyvinyl glycine

·        AOA - aminoacetic acid

 

Action Blockers (ethylene ® block action)

·        STS - silver thiosulfate

·        CO₂ - carbon dioxide

·        Ni - nickel

·        Co – cobalt

·        MCP – 1-mehtylcyclopropane

o          it is a gas that can saturate the receptor sites, and block action for several days

o          EthylBloc – commercial compound

 

 

TRANSPORT

 

·        diffusion as a gas throughout plant (in and out)

EFFECTS

 

1)      Auxin transport - alters basipetal transport

 

2)      Membrane permeability - increases

 

3)      Respiration - increases

 

4)      Cell elongation – decreases

 

5)      Aerenchyma formation – induces aerenchyma formation under anaerobic or hypoxic conditions (i.e. under low oxygen or flooded conditions)

 

6)      Fruit ripening - stimulates in many fruits, ex. banana

 

7)      Flowering - triggers flowering in some bromeliads, ex. pineapple

 

8)      Flower fading - increases

 

9)      Flower longevity - causes senescence (death) of cut flowers

 

10)    Fruit color - decreases green, increases other colors

 

11)    Seed germination - increases in some seeds

 

12)    Leaf abscission (leaf drop) - causes in some plants

 

13)    Leaf epinasty (curling and contortion or leaves) - causes in some plants

 

14)    Sex expression - favors pistillate flower formation on monoecious plants

 

ABSCISIC ACID (ABA)

 

Naturally-Occurring	Synthetic	Structure	Site of Production
abscisic acid	none		plastids, especially chloroplast

 

 

Historically also called:

 

abscisin - because early investigators found caused leaf abscission

 

dormin - because early investigators found caused dormancy

 

 

 

SYNTHESIS

 

mevalonate ® farnesyl pyrophosphate ® ABA

 

 

 

EFFECTS

 

1)   Dormancy - causes bud or seed dormancy

 

2)   Leaf abscission (leaf drop) - may cause in some plants

 

3)   Stoma - causes stomata to close (a response to drought stress)

ELICITOR MOLECULES

 

 

Brassinosteroid

Effects:

• pollen tube growth
• stem elongation
• unrolling/bending grass leaves
• orientation of cellulose microfibrils
• enhanced ethylene production

                            

 

Jasmonic Acid

Effects:

·        defense mechanisms, promotes antifungal proteins

·        growth inhibitor

·        inhibit seed and pollen germination

·        promotes curling of tendrils

·        induces fruit ripening

 

 

Salicylic Acid

Effects:

• blocks ethylene synthesis
• induces flowering in some long day plants
• induces thermogenesis in voodoo lily
• defense mechanisms, promotes antifungal proteins

 

 

Polyamines

Effects:

• elicit cell division, tuber formation, root initiation, embryogenesis, flower development and fruit ripening
• may not have a truly hormonal role; rather participate in key metabolic pathways essential for cellular functioning.

 

THE GENETIC BASIS OF LIFE (From http://generalhorticulture.tamu.edu)

Analogy

DEFINITIONS DNA (deoxyribonucleic acid)- a double helix chain of sugar-phosphates (deoxyribo            sugar-phosphates) connected by nucleic acids (adenine, thymine, guanine, cytosine). RNA (ribonucleic acid) - a single stranded chain of sugar-phosphates (ribo sugar-phosphates)            containing nucleic acids (adenine, uracil, guanine, cytosine). nucleic Acids - organic acids that form the base pairs of DNA and single-bases of  RNA.   Base Pairing of Nucleic Acids between the double strands of  DNA     A- T  (adenine-thymine)     G - C (guanine-cytosine)   Base Pairing of Nucleic Acids between DNA strands and RNA strands     A - U  (adenine-uracil)     G - C (guanine-cytosine) gene - a length of DNA that codes for the production of a protein or protein subunit.          - also codes for active RNAs (such as tRNA). protein - a polymer or chain of amino acids. enzyme - a protein that acts as a metabolic catalyst.

 

 

Using restriction enzymes for Mapping or Finger Printing

When DNA from the same source is digested with a particular restriction enzyme it will always give a set of the same sized fragments. For example if lambda bacteriophage DNA is cut with EcoR1 we know that it will give six fragments of the sizes:  21.23, 7.42, 5.8, 5.65, 4.87, 3.53 kbp. This is because, mutations apart, the phage sequence will always be the same, and so EcoR1 cutting sites will always be present in the same places. The fragments can be separated and their sizes determined by agarose gel electrophoresis.

We can use the positions of restriction enzyme sites as convenient markers along DNA sequences. The map obtained can be used for DNA identification and to plan DNA manipulations.

Finger Printing

Gel Showing Banding from use of Different Restriction Enzymes

 

Genetically Modified Organisms (GMO) or Transgenic Crops

(From: http://www.colostate.edu/programs/lifesciences/TransgenicCrops/index.html)

Authors:  Pat Byrne, Sarah Ward, Judy Harrington, Lacy Fuller (Web Master)

 

Crops and acreage

 

Widely Used GMOs

Herbicide Tolerance  Herbicide tolerant crops resolve many of those problems because they include transgenes providing tolerance to the herbicides Roundup® (chemical name: glyphosate) or Liberty® (glufosinate). These herbicides are broad-spectrum, meaning that they kill nearly all kinds of plants except those that have the tolerance gene. Thus, a farmer can apply a single herbicide to his fields of herbicide tolerant crops, and he can use Roundup and Liberty effectively at most crop growth stages as needed.

 

Bt Insect-Resistant Crops

"Bt" is short for Bacillus thuringiensis, a soil bacterium whose spores contain a crystalline (Cry) protein. In the insect gut, the protein breaks down to release a toxin, known as a delta-endotoxin. This toxin binds to and creates pores in the intestinal lining, resulting in ion imbalance, paralysis of the digestive system, and after a few days, insect death.

Bt insect-resistant crops currently on the market include

·         Corn: primarily for control of European corn borer, but also corn earworm and Southwestern corn borer. Cotton: for control of tobacco budworm and cotton bollworm

·         Potato: for control of Colorado potato beetle. Bt potato has been discontinued as a commercial product.

 

Papaya ringspot virus

Papaya is a tropical fruit rich in Vitamins A and C, but susceptible to a number of serious pests and diseases. The transgenic variety UH Rainbow, resistant to the papaya ringspot virus, is currently in production in Hawaii.

 

 

Risks And Concerns

(http://www.colostate.edu/programs/lifesciences/TransgenicCrops/risks.html)

The introduction of transgenic crops and foods into the existing food production system has generated a number of questions about possible negative consequences. People with concerns about this technology have reacted in many ways, from participating in letter-writing campaigns to demonstrating in the streets to vandalizing institutions where transgenic research is being conducted. What are the main concerns? What scientific support is there for these concerns?

• damage to human health
  allergenicity
  horizontal transfer and antibiotic resistance
  eating foreign DNA
  cauliflower mosaic virus promoter
  changed nutrient levels
• damage to the natural environment
  Monarch butterfly
  crop-to-weed gene flow
  antibiotic resistance
  leakage of GM proteins into soil
  reductions in pesticide spraying: are they real?
• disruption of current practices of farming and food production in developed countries
  crop-to-crop gene flow
• disruption of traditional practices and economies in less developed countries.

Seed

Germination, Dormancy and Priming

 

Terminology

 

pollination - deposition of pollen on the stigma of the pistil.

 

fertilization - the union of male and female gamete (nuclei, 1N) to produce zygote (2N).

 

double fertilization       - in higher plants only (angiosperms)

                                      - union of 1 1N male gamete with 1 1N female gamete (the egg) to
                                         produce a 2N zygote; and union of 1 1N male gamete with 2 1N
                                         polar nuclei to produce a 3N endosperm.

 

apomixis -     development of an embryo without fertilization; hence, it is not true sexual
                       propagation even though it produces a seed.

 

parthenocarpy - development of fruit without seeds.

 

vivipary - germination of seeds inside the fruit while still attached to the parent plant.

 

 

 

Seed Dormancy Terminology

Primary Dormancy
Old Term	New Term	Definition
Quiescence	Ecodormancy	Dormancy imposed by an external unfavorable environmental factor or external structure.   Example: too dry, external hard seed coat
Correlative Inhibition	Paradormancy	Dormancy imposed by physiological factor external to the embryo.   Example: inhibitors in testa or pericarp
Rest or Physiological Dormancy	Endodormancy	Dormancy imposed by a physiological factor internal to the embryo.   Example: embryo rest
Secondary Dormancy
Photodormancy		Dormancy due to lack of light (red) in light requiring seeds.

 

STAGES OF SEED GERMINATION

 

1st Stage

 

a)   imbibition - initial absorption of water to hydrate seed

 

b)   activation of metabolism - increased respiration and protein synthesis

 

2nd Stage

 

a)   digestion of stored food - for example, starch to sugars in cotyledon or endosperm

 

b)   translocation to embryo

 

3rd Stage

 

a)   cell division and continued growth and development of seedling

 

 

SEED DORMANCY

 

      CAUSED BY                              TYPE DORMANCY        HOW OVERCOME?                     

 

1)   Dry Seeds:

 

      dehydration of seed                               quiescence                 sow in moist environment

 

2)   Seed Coat Dormancy or Hardseededness:

 

      hard seed coat impermeable                  quiescence                 scarification - physical or
      to water and gases                                                                  chemical abrasion of seed coat.

 

3)   Embryo Rest:

 

      low growth promoters                                 rest                       stratification - cold (35-40 ^oF
      and/or high growth                             (or physiological             moist storage for 4-12  weeks.
      inhibitors in embryo                               dormancy)

 

4)   Double Dormancy:

 

      hard seed coat plus                                quiescence                 scarification then stratification
      embryo rest                                          and rest (or
                                                         physiological dormancy)

 

5)   Chemical Inhibitors:

 

      inhibitors in pericarp (fruit                       correlative                  I)    remove fleshy pericarp (fruit
      wall) or testa (seed coat)                         inhibition                         wall) or testa (seed coat).
                                                                                                     2)   leach in running water if
                                                                                                           pericarp or testa is dry.

6)   Immature Embryo:

 

      underdeveloped or                              developmental               I)    after ripening - store for 4-6    rudimentary embryo      dormancy                                                                                weeks under ambient
                                                                                                           conditions

                                                                                                     2)   warm stratification - warm                                                                                                moist storage.

                                                                                                     3)   embryo culture

 

7)   Light Requirement

 

      phytochrome in Pr form                          secondary                  I)    expose to any white light

                                                                    dormancy                  2)   expose to red light

                                                                                                     3)   sow shallow or on surface

Seed Treatments to Enhance Germination

 

Seed Priming

Seed priming is a seed treatment  that allows imbibition and activation of the initial metabolic events associated with seed germination, but prevents radicle emergence and growth.  Obviously, seeds are tolerant of desiccation, and even though during seed priming imbibition allows water uptake, the tolerance to desiccation is not lost.  Thus, the seed can be dried again and stored.  If the seeds are primed too long, desiccation tolerance will be lost, and the seeds may loose viability upon re-drying.  The secret to successful seed priming is to stop the priming treatment at just the right time to allow re-drying.

 

 

The advantage of primed seed is that when the primed seeds are planted their germination is faster and more uniform.

Types of seed priming

 

1)   Osmopriming (osmoconditioning):

This is the most common technique used. The seeds are soaked in an osmotic solution to allow imbibition and metabolic activation, but the osmotic conditions do not allow expansion and growth of cells.  Osmotica used are:   mannitol, polyethyleneglycol (PEG) or salts such as KCl.

2)   Hydropriming:

Imbibition is obtained by partially hydrated the seeds  using a limited amount of water by exposing them to a limited amount of water, using very humid air or exposing them for a short time in warm water.

 

3)   Matrix priming:

A solid, insoluble matrix is used to obtain a water solution with low water potential.  The matrix potential keeps the water potential low.  Vermiculite, diatomaceous earth or cross-linked highly water-absorbent polymers are used.

 

 

 

Hormone Treatments to Enhance Germination

Seeds of some species are very difficult and slow to germinate due to primary and secondary dormancy, the need for after ripening periods, immature embryos, etc.  Many of the seeds respond to hormones to increase the speed of germination, uniformity of germination and/or percent germination.  For example, the seeds of many tropical foliage plants are difficult to geminate, but respond to hormonal treatments.  Hormones can also be added to seed priming treatments.

 

1)   cytokinin – 100 to 200 mg/liter for a 12-24 hour soak.

 

2)   gibberellic acid – 200 to 1,000 mg/liter fro 12-24 hour soak

 

GROWTH KINETICS

 

Growth - an irreversible increase in size, mass or number.

 

Many growth phenomena in nature exhibit a logarithmic or exponential increase.  The size, mass or number increases by a constant, similar to simple compound interest.  The principal (current size, mass or number) times the interest rate (growth rate) yields the interest (growth increase for that day).  The interest is added to the principal, to yield a new principal.  The new principal times the interest rate yields and even higher interest for the next day, which again is added back to the principal.  So growth occurs at a compounded rate (logarithmic or exponential growth).

 

Absolute Growth Rate (AGR)

If you plot growth (size, mass or number) versus time, a constantly increasing growth curve is obtained.  If you calculate the slope between any two times, you get the absolute growth rate, which is the change in actual growth over time.  You get a different slope, hence different AGR for each pair of times chosen to calculate the slope. (Fig. 2.23A, Wareing and Philips 1981)

 

 

Relative Growth Rate (RGR)

If you plot the logarithm of growth (size, mass or number) versus time, a linear line is obtained.  If you calculate the slope of the line, you get the relative growth rate, which is the change in relative growth over time.  Since the line is linear, you get the same RGR, regardless of which time interval chosen to calculate the slope. (Fig. 2.23A, Wareing and Philips 1981).

 

GROWTH KINETICS- con't

 

Sigmoidal Growth Curve

Exponential growth can never be sustained indefinitely.  Eventually, substrates are depleted, the population exceeds the area available, tissues or individuals begin to die, etc., which decreases the growth rate.  Growth may still increase, but at a reduced rate (ex. if crowding causes shading), it may reach a steady state (everything is in equilibrium, for example in a population), or growth may begin to decrease (ex. due to death or senescence of individuals or plant parts).  If you plot long term growth versus time you get the classical sigmoidal growth curve.  If you plot the logarithm of the sigmoidal growth curve, you get a linear line during the exponential phase, after which the curve decreases over time. (Fig. 2.24, Wareing and Philips 1981)

 

 

Changes In Growth Rates Over Time

If you calculate the absolute growth rate (AGR) over increments of time, then plot AGR versus the time interval, you get a bell-shaped curve, i.e. the AGR changes constantly with time. If you calculate the relative growth rate (RGR) over increments of time, then plot RGR versus the time interval, you get a straight-line region during the logarithmic phase followed by a decreasing RGR.  The RGR is constant during the logarithmic phase. (Fig. 2.27, Wareing and Philips 1981).

 

MATHEMATICAL MODELS OF GROWTH

 

Linear Model – Used During Logarithmic or Exponential Phase

 

ln n = ln n_o + (slope) (time)

where    n = number, size (height, leaf area), or mass (dry weight, fresh weight) at any time > 0.

              n_o = number, size (height, leaf area,), or mass (dry weight, fresh weight) at time = 0.

              slope = rate of growth

 

Or more commonly expressed as the slope equation

y = a + bx

 

y = intercept + (slope) (x)

 

Absolute Growth Rate (AGR)

AGR       =             dn

                             dt

 

             =           n₂ - n₁                    yields average slope over that time interval

                           t₂ - t₁

 

Relative Growth Rate (RGR)

RGR       =          dn  _· 1

                          dt     n

 

             =          ln n₂ - ln n₁                  yields constant slope during logarithmic phase

                              t₂ - t₁

QUANTITATIVE MEASUREMENTS OF GROWTH

 

Leaf Area Ratio (LAR)

 

a)   over life of crop         LAR     =                    final leaf area                     =         LA

                                                                     final plant dry weight                        W

 

b)   over any time             LAR     =                      leaf area₂  -  leaf area₁                     =     LA₂ -LA₁

      interval                                               plant dry weight₂  -  plant dry weight₁ W₂ - W₁

 

                                                            ;  units = cm² g^-1   or   cm²/g

 

LAR is an indication of the efficiency of a given leaf area to produce a given plant size.

 

 

Net Assimilation Rate (NAR)

 

NAR  =  RGR        =       1        _·  RGR

               LAR              LAR

 

                              =           1                  _·    ln W₂ - ln W₁

                                        LA₂ - LA₁                            t₂ - t₁

                                        W₂ - W₁

 

                              =            W₂ - W_{1       ·}    ln W₂ - ln W_{1   ;}        units = g cm^-2 day^-1   or    g/cm²/day

                                          LA₂ - LA₁                    t₂ - t₁

 

NAR measures the accumulation of plant dry weight per unit leaf area per unit time.

It is a measure of efficiency of production.

 

 

Leaf Area Index (LAI)

 

LAI      =     leaf area       =    LA      ;  units = cm²_leaf cm^-2_soil   or   cm²_leaf/cm²_soil

                    soil area               A

 

Measures the fraction of crop cover.

LAI is near 0 at planting, and is usually 2-3 at full canopy coverage

 

 

Crop Growth Rate (CGR)

 

CGR  =  NAR  ·  LAI     ;     units = g cm^-2_soil day^-1   or   g/cm²_soil/day

 

CGR measures the efficiency of production of a total field of plants over a given soil area.

APPLICATION OF QUANTITATIVE MEASUREMENTS OF GROWTH

 

Efficiency of Different Species of Plants

The following table gives the net assimilation rates (NAR) of various species.  The higher the NAR the more efficient the species, which usually translates into higher growth rates.  (from Table 3.10, Larcher 1980)

 

	Net Assimilation Rate (mg dry matter per dm2 leaf area per day)
Plant Type	Average Over Growing Season	During Main Growth Phase
C4 Grasses	>200	400-800
Herbaceous C3 Plants Grasses Dicots	50-150 50-100	70-200 100-600
Woody Dicots Topical and Sub-Tropical Deciduous Temperate Trees Conifers Ericaceous Shrubs	10-20 10-15 3-10 5-10	30-50 30-100 10-50 15
CAM Plants	2-4	10

 

 

 

Efficiency of Sun versus Shade Plants

The following table gives the net assimilation rates (NAR), leaf area ratio (LAR), and relative growth rate (RGR) of shade versus sun plants at both high and low light intensities.  (from Table 3.1, Leopold and Kreidmann1975).

 

Note:  At low light intensities, the sun plant has 6-fold decrease in NAR and tries to compensate by increasing its LAR (i.e. produces about 2-fold more and/or larger leaves), but the RGR still decreases dramatically.  At low light intensities, NAR of the shade plant only decreases 3-fold, and increases its LAR 2.4 fold, both of which help maintain a higher RGR; in other words the shade plants have adapted themselves to the lower light intensity.

 

	NAR		LAR	RGR
% Daylight	mg/cm2/ wk	%	cm2/g	g/g wk	%
Sun Plant - Sunflower
100% 24% 12%	8.0 2.9 1.3	100 36 17	82 140 170	0.66 0.42 0.23	100 64 35
Shade Plant - Impatiens
100% 24% 12%	6.1 3.3 2.0	100 54 33	132 239 315	0.80 0.78 0.63	100 98 79

 

APPLICATION OF QUANTITATIVE MEASUREMENTS OF GROWTH - con't

 

Effect of Leaf Area Index (LAI) on

Net Assimilation Rate (NAR) and Crop Growth Rate (CGR)

Note that as the LAI increases (due to greater canopy coverage of soil), the NAR (productivity of each plant) decreases (probably due to increased plant-plant shading), but the CGR (productivity of the entire crop over a given area of soil) increases.  Thus, the best LAI is somewhere around 4. 

(from Fig. 3.64, Larcher 1980)

 

 

 

Use of Quantitative Growth Measurements to Explain

Other Growth Phenomena

Increasing ambient carbon dioxide increases photosynthesis, which in turn increases growth.  In tomato and bean, increasing carbon dioxide increases both total plant growth, as measured by increased RGR, and the efficiency of growth, as measured by increased NAR.  This increased growth efficiency allows the plant to have a smaller shoot system (decreased LAR), which is the source, while still enhancing the size of the root system (see increased root/shoot ratio), which is a sink (from Table 3-2, Leopold and Kriedemann 1975).

 

	Tomato		Bean
	300 ppm CO2	1,000 ppm CO2	300 ppm CO2	1,000 ppm CO2
RGR   (mg g-1 d-1)	222	254	122	172
NAR   (mg dm-2 d-1)	71	89	46	80
LAR   (dm2 g-1)	3.0	2.8	3.2	2.7
root/shoot ratio	0.19	0.21	0.18	0.25

 

Source Sink Relations

 

PHLOEM AND XYLEM TRANSLOCATION

(Figure 3.9 and Table 3.8 from Marshner 1986, Summary from Bidwell 1974)

Fig 3 9  Long-distance transport in xylem (X) and phloem (P) in a stem with a connected leaf, and xylem-to-phloem transfer mediated by a transfer cell (T).

 

Table 3.8. Solutes in the Phloem and Xylem Exudates of tobacco.

⁴ND, Not detectable; NA, data not available., 'Milligrams per milliliter.

 

summary.  The general conclusions about the pathways and tissues of translocation:

1.                          Salts and inorganic substances move upward in the xylem.

2.                          Salts and inorganic substances move downward in the phloem.

3.                          Organic substances move up and down in the phloem.

4.                          Organic nitrogen may move up in the xylem (trees) or phloem (herbaceous plants).

5.                          Organic compounds like sugar may be present in the xylem sap in large concentrations during the spring when sap rises in trees before the leaves emerge.

6.                          Lateral translocation of solutes from one tissue to another occurs, presumably by normal mechanisms of transfer (osmosis, active transport, and so on).

7.                          Exceptions to these generalizations are known to occur.

CARBON MOBILIZATION

Redistribution Between Sources and Sinks

(Fig. 10.19 from Taiz and Zeiger 1998, Fig. 3.61 from Larcher 1980)

 

Figure 10.19   Autoradiographs of a leaf of summer squash (Cucurbita pepo), showing the transition of the leaf from sink to source status. In each case, the leaf imported ¹⁴C from the source leaf on the plant for 2 hours. Label is visible as black accumulations. (A) The entire leaf is a sink, importing sugar from the source leaf. (B-D) The base is still a sink. As the tip of the leaf loses the ability to unload and stops importing sugar, as shown by the loss of black accumulations in B through D, it gains the ability to load and to export sugar. (From Turgeon and Webb 1973, courtesy of R. Turgeon.)

 

 

Fig. 3.61. Variations in starch deposition by trees throughout the year. Maximal accumulation of starch is indicated by black, large amounts by cross-hatching, and small amounts by stippling; in the parts left white, starch is present in traces or not at all. Fagus sylvatica (Central Europe): 1, just before leaf emergence in the spring; 2, during leaf unfolding; 3, midsummer; 4, just before abscission in the autumn; 5, conversion of starch to soluble carbohydrates at low temperatures during winter. After Fischer (1891), Gaumann (1935), and K. Kober (unpubl.). Abies veitchii (Japan): 1, during growth of new shoots in spring; 2, late summer; 3, during winter frost. After Kimura (1969). Olea europaea (Northern Italy): 1, during shooting and flowering in spring; 2, during a dry period in midsummer; 3, in winter after the end of the rainy season. After Thomaser (1975). For the storage dynamics of Atlantic dwarf shrubs see Stewart and Bannister (1973), and Grace and Woolhouse (1973); of chaparral species, Mooney and Hays (1973); of mountain plants, Larcher (1977) and Zachhuber and Larcher (1978)

NUTRIENT MOBILITY

 

Redistribution Between Sources and Sinks

(Fig. 13-12 from Bidwell 1974, Table 3.9 from Marschner 1986)

 

Figure 13-12 (opposite). A sequence of six autoradiograms showing the fate of an aliquot of ³⁵S absorbed as ³⁵S0₄ during a 1-hr absorption period. The plants, after the hour in the nutrient solution containing the tracer, were removed to a normal (nonradioactive) solution where they remained for the following periods: A, 0 hr; B, 6 hr; C, 1 2 hr; D, 24 hr; E, 48 hr; and F, 96 hr. Most of the ³⁵S, which moved directly into the mature leaves, was withdrawn within 1 2-24 hr. It moved predomi­nantly into younger leaves near the stem apex, where it remained. [From 0. Biddulph: Plant Physio/. 33:295 (1958). Used with permission. Photograph courtesy Dr. Biddulph.]

 

Table 3.9. Mobility of Mineral Elements in Phloem

From Bukovac and Wittwer (1957).

DIAGNOSING NUTRIENT DEFICIENCIES

Based on Nutrient Mobility

(from Vetanovetz 1996)

 

Mobile Nutrients – deficiencies typically appear on older growth first.

 

Immobile nutrients – deficiencies typically appear on newer growth and shoot tips first

 

MONOCARPIC SENESCENCE

Changing Sources and Sinks During Vegetative and Reproductive Growth

(Fig. 1 from Egli and Leggert 1973, Fig. 3 from Harper 1971)

 

Fig. 1.   Dry matter accumulation patterns for Kent and D66-5566, 1971.

 

Fig. 3. Seasonal uptake and accumulation of N, P, K, Ca, and Mg by soybeans at weekly intervals¹ from field hydroponic gravel culture systems.

EPISODIC GROWTH OF TEMPERATE WOODY PLANTS

Cycling Between Shoot and Root Growth and Implications on Fertilizer Timing

(Fig. 2 on growth from Mertens and Wright 1978, Fig. 2 on uptake from Hershey and Paul 1983, Table 1 from Gilliam and Wright 1978)

 

Fig. 2. Root and shoot growth rates of 'Helleri' holly grown at 150 ppm N applied as 20N-8.7P-16.5K soluble fertilizer.

Fig. 2. Uptake rates for K+ and Mg2+ for a single plant of Euonymus japonica (plant 5). Bars indicate periods of shoot elongation

 

 

Table 1. Effect of the time and no. of weekly fertilizer applications during 1st growth flush on tissue N accumulation and sub­sequent shoot and root dry wt of 'Helleri' holly.

 

Senescence and Post-Harvest Storage

 

MONOCARPIC SENESCENCE

Monocarpic senescence literally means “flower once then die”.  During the reproductive phase, the “sink” demand of the developing flowers, fruit then seed can drain the vegetative “sources” to the point that senescence occurs.

 

Fig. 1.   Dry matter accumulation patterns for Kent and D66-5566, 1971. (from Egli and Leggert 1973)

 

Fig. 3. Seasonal uptake and accumulation of N, P, K, Ca, and Mg by soybeans at weekly intervals¹ from field hydroponic gravel culture systems.(from Harper 1971)

RESPIRATION AND SENESCENCE

 

All living organisms must conduct respiration in every living cell and at all times.  Sometimes respiration is very fast, for example if the organ if actively growing, and sometimes it barely perceptible, for example if the organ is dormant.  Respiration breaks down glucose and uses the energy that was in the carbon-carbon bond to make metabolic energy (mainly a compound called adenosine triphosphate or ATP).  Carbon dioxide is given off as a by-product.  If there is no oxygen around, then only partial respiration occurs in the form of anaerobic fermentation.  This produces ethanol as a by-product and is the basis of wine making and all fermentation (yogurt, cheese, etc.).

 

One process involving respiration that is particularly important to horticulturist is ripening of fruit.  In climacteric fruit, the respiration rises very rapidly during ripening, then decreases as the fruit senesces.  If you prevent or decrease the rise of respiration, then you can prolong post-harvest storage life.  Ethylene is what causes the increase in respiration, so decreasing ethylene is also a strategy used to increase post-harvest storage life.

 

What are other ways to decrease respiration and prolong the storage life of fruit and vegetable produce or cut flowers?  Look at the equation for respiration.  We can make the reaction go slower by either decreasing things on the left side of the arrow or increasing things on the right side of the arrow.  Practically, we can decrease respiration by either increasing carbon dioxide or decreasing oxygen.  You want to increase carbon dioxide to about 2-5% (up from about 350 ppm in the ambient air) and/or decrease oxygen to about 3% (down from 21% in the ambient air).  You never want to decrease oxygen to near zero, because anaerobic fermentation would occur and anaerobic bacteria might start growing.

 

Of course the easiest way to decrease respiration is to decrease temperature.  You may not have thought about it, but the refrigerator in your house is nothing more than a respiration inhibitor chamber.

 

All of the above is the basis of controlled-atmosphere storage.

 

If in addition to the above, if you store produce or flowers under a light vacuum, you will pull the ethylene out of the inside of the plant and the atmosphere around the plant.  This will dramatically decrease respiration.  This is called hypobaric storage.

 

Summary, we can decrease respiration by doing the following:

§         decrease temperature

§         decrease oxygen

§         decrease pressure, e.g. light vacuum

§         decrease ethylene

§         increase carbon dioxide

NET CHEMICAL EQUATION FOR RESPIRATION

 

 

 

 

 

BIOCHEMICAL REACTIONS OF RESPIRATION

 

Ethylene, Respiration and Senescence Relations

 

 

1)      Climacteric Fruit –    a fruit  where ethylene triggers an increase in respiration and the ripening process..

 

Climacteric Fruit Ripening and Climacteric Rise

 

Climacteric Fruit		Non-Climacteric Fruit
apple apricot avocado banana cantaloupe fig guava ? mango	olive pawpaw peach pear plum persimmon tomato	bell pepper blueberry cherry grape pineapple strawberry citrus watermelon

 

2)      Non-Climacteric Fruit color – used to cause degreening of citrus

 

3)      Flower Senescence

 

a)    Flower fading – flower fade after pollination

 

b)   Flower longevity - causes senescence (death) of cut flowers

 

4)      Leaf Senescence

 

a)    Leaf  epinasty (curling and contortion or leaves) - causes in some plants

 

b)   Leaf abscission (leaf drop) - causes in some plants

 

 

Manipulating Ethylene, Respiration and Senescence

 

1)   Ethylene

 

Biosynthetic Pathway of Ethylene Synthesis

methionine ® s-adenosylmethionine ® 1-aminocyclopropane-1-carboxylic acid ® ethylene

                           (SAM)                                                 (ACC)

 

Ethylene inhibitors - chemicals that inhibit the synthesis or action of ethylene

 

Ethylene Synthesis Inhibitors (block synthesis of SAM ® ACC)

·        AVG - aminoethoxyvinyl glycine

·        MVG - methoxyvinyl glycine

·        AOA - aminoacetic acid

 

Ethylene Action Blockers (ethylene ® block action)

·        STS - silver thiosulfate

·        CO₂ - carbon dioxide

·        Ni - nickel

·        Co – cobalt

·        MCP – 1-mehtylcyclopropane

o          it is a gas that can saturate the receptor sites, and block action for several days

o          EthylBloc – commercial compound

 

2)   Temperature

• respiration decreases when temperature decreases.
• respiration ceases at about freezing temperatures (32 ^oF)
• increasing temperature increases respiration, until temperature gets too high, then respiration decreases when tissue deteriorates

 

3)   Oxygen

• respiration decreases when oxygen decreases
• under very low to no oxygen, anaerobic respiration occurs.

 

 

4)   Carbon Dioxide

• respiration decreases when carbon dioxide increases

Post-Harvest Storage to Extend Shelf Life

 

1)         Refrigeration

 

• decrease temperature – mid-30’s ^oF

 

 

2)   Controlled Atmosphere Storage – CA Storage

 

How Apples are Packaged and Stored Apples are stored in cold storage warehouses. Inside a regular warehouse, apples can be stored for about 5 months because it is cooled to 30-32 degrees Fahrenheit. Inside a special controlled atmosphere warehouse, apples can be stored for almost 12 months because the temperature, humidity, oxygen and carbon dioxide are constantly monitored and controlled to prevent the fruit from ripening too quickly. (from Dole: www.dole5aday.com/ReferenceCenter/ Encyclopedia/Apples/apple_transported2.jsp

 

• low temperature - mid-30’s ^oF

·        high CO_{2 -} approx. 2-5%)

• low 0₂ - approx. 3%)
• high humidity  -approx. 90%)
• ethylene removed -scrubbed

 

 

3)   Hypobaric Storage        low pressure storage, i.e. a light vacuum.

 

Hypobaric Storage Shipping Container

(from http://www.refrigeratedvehicles.com/)

 

·        same as above, plus

·        low pressure

o       decreases 0₂

o       _{decreases ethylene}

 

4)   Modified Atmosphere Packaging – MAP

 

 

MAP broccoli

(from http://www.packagingdigest.com/articles/200203/32.php)

 

MAP uses selectively permeable bags and the fruit or vegetable’s own respiration to maintain an increased level of carbon dioxide and decreased level of oxygen, but avoiding low enough oxygen to avoid anaerobic respiration.

 

Fruits and vegetables continue to respire after harvest.  If you seal them in a plastic bag, the produce will deplete the atmosphere in the bag of oxygen and will cause the produce to undergo anaerobic respiration.  This will causes ethanol and off-flavors to form and may allow anaerobic bacteria to grow and cause spoilage.

 

In MAP, the produce is place in a selectively permeable bag that allows oxygen, carbon dioxide and ethylene to diffuse in and out so equilibrium is set-up between the inside of the bag to the outside of the bag.  The goal is to use a bag that allows some oxygen to diffuse in to avoid anaerobic fermentation, but allow excessive carbon dioxide and ethylene to escape..

 

Permeability of Various Films

Film	Thickness (micron)	Permeability (l/m2/d/atm)
		O2	CO2
polyvenylchloride	14-18	20	120
ethylenevenlyacetate	10-25	32	134
low density polyethylene	25-50	6	20
p0lystyrene	50	4	13

 

The bag must be designed for each fruit and vegetable.  Produce with very high rates of respiration require a bag that allows more oxygen to diffuse in to avoid anaerobic respiration.

Respiration Rates of Vegetables

 

 

Two Types of MAP

 

1)  Passive MAP

 

The produce is put in a bag.  If the permeability of the bag is properly matched with the respiration of the produce, the ideal atmosphere will evolve inside the sealed bag.  Absorbers may be added to scavenge ethylene. 

 

2)  Active MAP

 

The produce is put in a bag, and the air in the bag is replaced with air that has the proper mixture of oxygen and carbon dioxide.  Absorbers may be added to scavenge ethylene. 

Maximum Storage Time with Various Storage Methods

(from http://atlasuhv.com/products/hypobaric_storage/hypobaric_storage.php

 

Commodity	Maximum Storage Time (days)			Hypobaric Benefit Factor
	Standard Refrigeration	Control Atmosphere	Hypobaric Advanced Atmosphere
spinach	14-Oct	slight benefit	50	5 x
avocado (Lula)	30	42-60	>102	3.5 x
banana	14-21	42-56	150	11 x
cherry (sweet)	14-21	28-35	56-70	4 x
lime (Persian)	14-28	juice loss, peel thickens	90+	6.5 x
mango (Fla. varieties)	14-21	little or no benefit	>50	3.5 x
papaya (Solo)	12	12+ (slight benefit)	28	2.3 x
pear (Bartlett)	60	100	200	3.3 x
strawberry	7	7+ (off-flavor)	21	3 x
asparagus	14-21	slight benefit - off odors	28-42	2 x
cucumber	14-Sep	14+ (slight benefit)	49	3.5 x
green pepper	14-21	no benefit	50	3.5 x
mushroom	5	6	21	4.2 x
apples (various)	200	300	300+	1.5 x
carnation (flower)	21-42	no benefit	140	6.6 x
protea (flower)	<7	no benefit	30+	4.2 x
rose (flower)	14-Jul	no benefit	42	6 x

The above data from S.P. Burg in Postharvest Physiology and Hypobaric Storage of Fresh Produce, CABI Publishing, 2004, ISBN 0 85199 801 1