Responses in Flowering Plants (chpt 36)

External factors of growth

1. Temperature

   • When air temp is below 5°C, enzyme activity is low, slowing rate of chemical reactions

   • Temp enzymes work best at → optimum temp (25°C in plants, 40°C in animals)

   • Some plants respond to intense heat by releasing heat shock proteins which protect other vital proteins from denaturing

2. Day Length

   • As days get longer, plants know it is spring, get ready to grow/reproduce, and vice versa when getting colder; lose leaves, fruit ripen, seeds drop

   • Not as useful to plants in equatorial regions, not much variation

3. Light Intensity

   • More light → grow better as increases rate of photosynthesis until limiting factor (eg. CO₂ concentration/temp)

4. CO₂ Concentration

   • Affects rate of photosynthesis → increased artificially in greenhouses to increase photosynthesis rate

5. Water

   • If not enough water, wilts and stomata close to conserve water (grass roll up into tubes to shelter stomata from wind)

   • Plants communicate to neighbouring plants if short of water, causing others to close stomata

6. Gravity

   • Ensure growing up/down of shoots/roots, detected by dense components of cytoplasm → statoliths (settle to bottom of cells)

   • Statoliths promote redistribution of auxin in root to grow down and cause opposite reaction in stems

Internal factors of growth

1. Growth regulators

   • aka plant hormones. Compounds eg. auxins, gibberellins, cytokinins, ethylene, etc.

2. Injury and disease

   • Cause changes in plant and cause chemical attack using group of chemicals → phytoalexins

Tropism

A growth response of a plant to a stimulus

Types of tropisms

1. Phototropism

   • Growth response to light (increases photosynthesis)

   • Stems grow towards light (positive phototropism), roots away from light (negative phototropism)

   • Main regulator → auxin called indole acetic acid (IAA)

2. Geotropism

   • Growth response to gravity (grow in direction of light and water)

   • Stems grow away from gravity (negatively geotropic) and roots grow towards gravity (positively geotropic)

3. Thigmotropism

   • Growth response to touch

   • Modified end of pea leaves → tendrils. Grow around anything they touch, giving support to weak stems (allows them to grow bigger)

   • Cells that touch support grow less causing it to wrap

   • All plants respond like this → grow better the less they are touched

4. Hydrotropism

   • Growth response to water

   • Roots grow towards (positively hydrotropic), therefore can obtain more water and minerals

5. Chemotropism

   • Growth response to varying concentration of chemicals in environment

   • Eg. growth of pollen tube. Finds way to ovary by chemotropism. Specific pollen respond correctly to chemical(s)

Growth Regulator

Substance that controls growth of a plant

Promoters

Growth regulators that increase growth, eg. auxins like IAA (also can be inhibitor)

Inhibitors

Growth regulators that inhibit growth, eg. abscisic acid

How are growth regulators similar to hormones

• Chemical in nature

• Transported through vascular system ie blood in animals and phloem/xylem in plants

• Slow-acting and tend to have long-lasting effect

• Produced in one place and have effect in another place

Auxins

• Can be both synthetic and natural. Most common natural is indole acetic acid (IAA) produced in apical meristems in young tissue and diffuses down stem

• Elongates stems, grows roots and shoots, develops fruit

• Travels away from shoot tip → polar

• Cell elongation occurs when auxins cause hydrogen bonds between cellulose fibres to loosen and cause cells to absorb more water, causing cell to swell and loosen bonds

Auxins in phototropism

• Darwin found phototropism is controlled by coleoptile (shoot), and that curving happens further down stem

• Peter Boysen-Jensen placed layer of gelatine between cut tip and coleoptile, stopping cells being in contact but allowed chemicals to diffuse → signal passed as normal. Layer of impervious substance (mica) didn’t allow signal past → no bending

• When plant exposed to unilateral light (one side) causes more auxin to diffuse down shaded side

• Due to exposure of higher concentration of auxin, cells on shaded side elongate more, resulting in tip growing towards light (not bending)

Apical dominance of auxins

• Auxin produced by meristem at tip of stem and transported down stem, decreasing in concentration as it moves

• High auxin concentration near growing tip inhibits growth of lateral buds, keeping them dormant

• As auxin concentration continues to drop as goes down stem, reaches concentration where no longer inhibits lateral buds, begin to develop into side shoots

• If apical bud removed, IAA production stops, and all lateral buds develop making plant bushy

Effect of concentration of auxins

• Auxin concentration that promotes growth in one part can inhibit growth in another part

• Roots are more sensitive than shoots, shown in graph

Use of Plant Growth Regulators

1. Auxins

   • Used in rooting powders, especially synthetic eg. IBA. Cut end dipped in powder before insertion into growing medium; promotes growth of adventitious roots

   • Synthetic auxins used as selective herbicides also to remove dicots, eg. daisies, dandelions, trees. Monocots are resistant (ie grasses)

2. Cytokinins

   • Used in micropropogation to promote cell differentiation, eg. xylem and phloem, works together w IAA and gibberellins

   • Prolongs shelf life of cut flowers and vegetables

   • Used to break dormancy in buds and seeds, triggering leaf and growth in spring

3. Ethylene

   • Ethene (C₂H₄), used to ripen fruit. Produced by burning paraffin

   • As fruit rots, also releases ethylene, encouraging other fruit nearby to ripen

   • Triggers trees to drop leaves

4. Gibberellins

   • Cause stem lengthening. Mobilise stored food in germinating seeds and trigger digestion and absorption of food by embryo

   • Used to break dormancy in buds and seeds dormancy in spring

5. Abscisic Acid

   • Triggers bud and seed dormancy in autumn

   • Inhibits cell growth and premature seed germination in late winter/early spring

   • Sprayed on nursery plants before transportation, as when plants dormant are more resistant to damage

Adaptations that protect plants

• Hawthorn: sharp throns discourage animals from eating it

• Chillies: oils cause burning sensation in the mouth of deter browsers. Tend to become less hot when ripe and red

• Nettles: stings containing formic acid (methanoic acid) discourages animals from eating or touching them. When touching, brittle pointed sting ppierces skin and injects acid resulting in welts

• Gorse: hard stiff pointed leaves which discourage animals from eating it. When leaves die and dry out, become like tinder → spread fire and produce intense heat

Investigate effect of IAA on plant tissue

Serial Dilution

1. Make stock solution of 1ppm auxin

2. Put 10cm³ of this solution into test tube and 9cm³ distilled water into 5 other test tubes

3. Remove 1cm³ from stock solution (1ppm) and add to first test tube containing only water. Stopper and mix well

4. Remove 1cm³ from tube and add to next test tube of distilled water, stopper and mix well

5. Repeat until all test tubes used

Experiment Steps

1. 7 sterile petri dishes and place 10 cress seeds on acetate grid in each petri dish

2. Place filter paper and enough cotton wool on top of seeds to hold them in position

3. Pour different concentration of IAA solution into each dish, ie 10⁻¹, 10⁻², 10⁻³, etc. Add water as control

4. Label each dish, then cover and seal

5. Stand dishes on edge so roots grow down and shoots up

6. Incubate at room temp for 3 days

7. Measure length of all roots and shoots in each dish and calculate average length of each

Results

• Control had little growth

• Low concentrations of IAA have longer roots than shoots

• High concentrations of IAA have longer shoots than roots

Conclusions

• Low concentrations of IAA promote root growth

• high concentrations of IAA promote shoot growth