Biological Foundations 112, Lecture 19


Plant Hormones II and Environmental Clues

Chapter 35, pages 757-768


I.   Plant Hormones Continued

     A.   Gibberellins

            1.   Discovery

                  a.   By the Japanese in the 1920s

                  b.   Young rice seedlings grow extremely tall, fall over, and die

                  c.   Caused by a fungus in the genus Gibberella

                        (1)  Produced gibberellin or gibberellic acid (GA)

                        (2)  Gibberellins/GA can cause the same symptoms without the fungus

                  d.   First natural gibberellins were found in bean seeds in 1960

            2.   Chemical structure

                  a.   Complex, composed of five rings

                  b.   Over 70 naturally-occurring gibberellins

                  c.   Some are active and some are not

            3.   Produced in the root and stem apical meristems, young leaves, and embryos of  
                  seeds

            4.   Functions

                  a.   Cell Elongation and cell division

                        (1)  Can cause dwarf mutants of peas and corn to grow to normal heights

                        (2)  Causes bolting

                        (3)  Mechanism of cell elongation is different with GA than with IAA

                  b.   Reproductive functions

                        (1)  Stimulates flowering in long-day plants

                        (2)  Substitutes for cold requirements in biennials

                        (3)  Affects development of fruits (used commercially on some varieties of                                                grapes for larger berries)

                  c.   Germination

                        (1)  Embryo produces GA that triggers physiological responses for germination

                        (2)  Substitutes for light or cold requirement for seed germination

                        (3)  Produced by the aleurone layer of barley seeds

                               (a)  Stimulates scutellum to produce digestive enzymes (alpha-amylase) for
                                     changing endosperm into sugar

                               (b)  Digested food from the endosperm is absorbed by the scutellum

                               (c)  Stimulates mRNA coding for alpha-amylase

                  d.   Seems to have little effect on root growth

                  e.   Fruit Growth

                        (1)  Dual role by auxin and gibberellin

                        (2)  Some species require both auxin and gibberellin for fruit set

                        (3)  Spraying Thompson Seedless grapes by these two hormones causes the
                              grapes to grow larger and farther apart

                  f.    Breaking of Dormancy

                        (1)  Acts antagonistically to abscisic acid

                        (2)  Acts in seed germination and apical meristems

     B.   Abscisic Acid

            1.   Discovery

                  a.   1963 simultaneously by two different research teams

                        (1)  One team was working on bud dormancy in woody plants

                        (2)  One team was working on the abscission of cotton fruits

                  b.   Molecules that did both were found to be identical

                  c.   Called abscisic acid (ABA)

                  d.   Unfortunate because this hormone deals with dormancy much more than with
                        abscission

            2.   Chemistry

                  a.   Single compound

                  b.   Six-carbon ring

            3.   Production of leaf, rootcap, and stem

            4.   Transported in the vascular tissue

                  a.   High levels in seeds and fruits

                  b.   Not clear whether it is produced in the seeds and fruits or transported there

            5.   Functions

                  a.   Apical Bud

                        (1)  Slow growth

                        (2)  Directs the development of bud scales

                  b.   Inhibits cell division in the vascular cambium

                  c.   Seed dormancy

                        (1)  Germination occurs when ABA is removed or inactivated

                        (2)  Other seeds need a light treatment to trigger degradation of abscisic acid

                  d.   Stress hormone

                        (1)  Water stress

                               (a)  Best known in water stress of plant

                               (b)  High level of ABA triggers the outflow of potassium ions from the
                                     guard cells

                               (c)  Induces water to flow out of the guard cell and turgor pressure to be
                                     reduced in the guard cell

                        (2)  Winter dormancy (another type of stress) in woody plants

                               (a)   Causes growth to cease

                               (b)   Causes bud scales to grow over buds

                        (3)  Dormant seeds contain high levels of ABA so as to prevent germination in
                              the winter

                  e.   Gibberellin can reverse the effects of ABA

                        (1)  In winter the amount of ABA decreases by gradual breakdown  while                                              gibberellin increases

                        (2)  Cytokinins can also do this

     C.   Ethylene

            1.   Discovery

                  a.   Effects noted in the 1800s

                        (1)  Coal gas used in street lights caused plants growing in planters to alter their
                               growth pattern:

                               (a)  Trees shed leaves sooner

                               (b)  Flowers faded and quickly dropped their petals

                               (c)  Newly sprouted seedlings grew horizontal rather than vertical

                        (2)  1901 it was determined that it was the ethylene in the coal gas that caused
                               this

                        (3)  1934 ethylene was first found in plants

                  b.   Chemistry

                        (1)  A gas

                        (2)  Colorless and smells like ether

                  c.   Produced in nodes of stems, ripening fruits, and in senescing tissues like leaves

            2.   Functions

                  a.   Senescence in Plants

                        (1)  Process toward the death of the plant

                        (2)  Orderly process in tracheids, cork cells, leaves, etc.

                        (3)  Best understood with fruit ripening and leaf abscission

                  b.   Ripening of fruits

                        (1)  Changes in fruits that ethylene triggers

                               (a)  Starch and acids are converted to sugar

                               (b)  Fruit cell walls broken down making the fruit softer

                               (c)  Certain esters (flavors) synthesized

                        (2)  Domino effect

                               (a)  Ripening fruit produces ethylene

                               (b)  The ethylene produced by the first fruit triggers ripening process in other
                                      fruits

                               (c)  Ethylene produced by one fruit (apple) cause other fruit to ripen also

                               (d)  To keep fruit, like apples, from ripening in long term storage, the                                                       ethylene must be constantly removed

                        (3)  Used commercially

                               (a)  Bananas picked green

                               (b)  Ethylene is used to evenly ripen the bananas when they are at market
                                      ready to be sold

                  c.   Causes leaf abscission

                        (1)  Acts in an antagonistic way with auxin

                        (2)  As autumn approaches the amount of auxin decreases

                        (3)  As a result several changes occur in the abscission zone

                        (4)  Abscission zone cells produce ethylene

                        (5)  Complicated process

                               (a)  Recycling of nutrients to stem

                               (b)  Chlorophyll production stops

II.  Environmental Clues

     A.   Photoperiodism

            1.   Definition:  response of plants to relative lengths of daylight and darkness

            2.   Short-day plants

                  a.   Plants respond to environmental clues for flowering to days that are getting
                        shorter

                  b.   Plants flower when the length of night is equal to or greater than some critical  
                        minimum

                  c.   Chrysanthemum and poinsettias bloom in late summer or fall

            3.   Long-day plants

                  a.   Plants respond to environmental clues for flowering to days that are getting
                        longer

                  b.   Plants flower when the length of night is equal to some critical period or less

                  c.   Clover, black-eyed susan, and lettuce bloom in the late spring or summer

            4.   Day-neutral plants

                  a.   The length of day is not important for blooming

                  b.   These will generally bloom anytime of the year

                  c.   An environmental stimulus other than day length will stimulate these plants to
                        flower

                  d.   Tomatoes, dandelions, string beans, and pansies will bloom any time of the year

     B.   Photoreceptors - pigments that receive the light stimulus and transmit it to the various
                                        parts of the plant

     C.   Phytochrome

            1.   Photoreceptor for photoperiodism and other responses

            2.   Blue-green protenaceous pigment

            3.   Structure

                  a.   Exists in two forms:

                        (1)  Inactive form

                               (a)  Absorbs red light

                               (b)  Is symbolised as Pred or Pr

                               (c)  Upon absorbing red light it immediately turns in to the active form

                               (d)  Pr is continuously being synthesized and slowly destroyed by enzymes

                        (2)  Active form

                               (a)  Absorbs far-red light

                               (b)  Is symbolised as Pfar-red or Pfr

                               (c)  Upon absorbing far-red light, it immediately turns in to the
                                     active form

                               (d)  Most rapidly destroyed by enzymes of the two forms of phytochrome

                               (e)  The only way it is made is by red light shining on Pred

                  b.   With constant full spectrum sunlight, the two forms reach an equilibrium during
                        the day

     D.   Phytochrome and Photoperiodism

            1.   Short-day plants

                  a.   Active form (Pfr) inhibits flowering

                  b.   For flowering, all the Pfr must disappear and be gone for some minimal amount  
                        of time, depending upon the species

                  c.   Thus, these plants need a long enough night for this to happen

                  d.   Thus, NO Pfr for a long enough period of time will induce flowering

                  e.  A burst of red light for as short as 10 seconds during the night will inhibit
                       flowering

                  f.   The above effect is reversable with a brief flash of far-red light after the red flash

           2.   Long-day plants

                 a.   Active form (Pfr) induces flowering

                 b.   For flowering Pfr must be present to induce flowering

                 c.   Thus, the night must be short enough to have some Pfr remain

     E.   Other roles of phytochrome

            1.   Needed for inducing seed germination in some species of plants (lettuce)

            2.   Sleep movements of leaves

            3.   Shoot dormancy

            4.   Leaf abscission

            5.   Pigment formation in flowers, fruits and leaves

     F.   Temperature and Vernalization

            1.   Vernalization

                  a.  The natural promotion of flowering by a cold period

                  b.  These types of plants live in temperate climates where there is a cold enough
                       winter to destroy the flowers if flowering occurs during the winter

                  c.   Thus, these plants use a cold period of several weeks to indicate that the winter
                        is over and it is safe to flower

                  d.   Examples:

                        (1)  Biennials - carrots

                        (2)  Winter annuals - wheat

                        (3)  Perennials - peach trees in the Central Valley of California

            2.   Stratification

                   a.   The artificial promotion of seed germination by a cold treatment

                   b.   Process:

                         (1)  A layer of cheese cloth is wet and laid out in a refrigerator

                         (2)  Seeds are sprinkled over the cheese cloth

                         (3)  Another layer of wet cheese cloth is laid down over the seeds

                         (4)  More seeds are sprinkled on this layer of cheese cloth

                         (5)  This is continued for as many layers as is necessary

                         (6)  These layers of seeds and cheese cloth are left in the refrigerator for two or
                                more months

                   c.   Why?  

                         (1)  The wet cheese cloth hydrates the seeds

                         (2)  The cold period is measured by the hydrated seed

                         (3)  When enough time in this cold environment has past, it is now safe for the
                                seed to germinate because all the killing frosts are past

                          (4)  This also is a phenomena of temperate seeds


PUC Home Page | Gilbert Muth Home Page | Botany Syllabus Home Page

E-mail Gilbert Muth gmuth@puc.edu