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VCE Biology Unit 2Area of Study 01
Adaptations of OrganismsChapter 14
Plant tropisms and hormonal control
Chapter 14.2 Responding to the environment
Plants do not have highly specialised receptors. They respond to external stimuli
Chapter 14.2 Responding to the environment
Physical factors, such as direction and wavelength of light, photoperiod (day/light length) gravity, temperature and touch
Chapter 14.2 Responding to the environment
Chemical factors, such as H2O, CO2, and specific chemicals (e.g. ethylene gas [C2H4] which ripens fruit)
Chapter 14.2 Responding to the environment
Directionality is an important aspect of some plant sensing and responding.e.g. light and gravity
Chapter 14.2 Responding to the environment
Sensing lightPhototropism is a response to blue light (350-500 nm [nano is 1×10-9]). The pigment carotene (orange-yellow colour) is likely to be involved.
Chapter 14.2 Responding to the environment
Sensing lightOther growth responses are stimulated by orange-red light (620-670 nm) and reversed by far-red light (710-750 nm).
Chapter 14.2 Responding to the environment
Sensing lightThese responses include germination of seeds, expansion of leaves plus induction and breaking of dormancy
Chapter 14.2 Responding to the environment
Sensing lightFlowering in response to photoperiod and elongation of stems are stimulated by far-red light and inhibited by orange-red light.
Chapter 14.2 Responding to the environment
Photoperiod and flowering• Flowering in most plants is unaffected by
photoperiod.• Other plants are grouped as ‘short-day’ or ‘long-
day’ plants Actually they are stimulated by the length of night, not day.
Chapter 14.2 Responding to the environment
Photoperiod and flowering• Short-day plants require long nights to trigger flowering.
Flower in late summer, autumn or winter• Long-day plants will flower if nights are short or if
continuously exposed to light. Flower in late spring and early summer
• Neutral plants are unaffected by day length for flowering
Chapter 14.2 Responding to the environment
Photoperiod and floweringThe receptor is a leaf-pigment that responds to red light.The response is slow, taking hours or days
Chapter 14.2 Responding to the environment
Sensing gravity• Shoots are negatively geotropic• Roots are positively geotropic: upper side out
grows lower side • Process not fully understood, but amylopasts
(starch containing organelles) are appear tro be important
Chapter 14.2 Responding to the environment
Sensing touch (Thigmotropism)• Growth response that occurs as a result of contact with
the surface• Coiling and bending of tendrils around objects (pea
plants and vines)• Coiling growth of vines twining their way up a tree trunk
towards light is a result of thigmotropism and phototropism
Chapter 14.2 Responding to the environment
Movements and plant rhythmsSleep movements – where plants close their flower petals or fold their leaves. Controlled by turgid pressure.Solar tracking – leaves twist on their petioles (stalk) in response to light. Controlled by turgid pressure.
Chapter 14.2 Responding to the environment
Movements and plant rhythmsNodding and contraction movements – pea seedlings nod side to side. Controlled by ethylene which causes thickening and bend of shoot. Help shoot push through soilLily bulbs have contractile roots. Temperature fluctuations at soil surface stimulate contraction. Young lily bulbs formed from seed burrow into ground.
Chapter 14.2 Responding to the environment
Movements and plant rhythmsRapid response – some carnivorous plants (e.g. Venus flytrap, Trigger plant have a ‘column’ where both male anthers and female stigma are located. When triggered the column is flung against the bee
Chapter 14.2 Responding to the environment
TemperatureEffect of temperature is related to the general effect of temperature on enzymes. Plants will not usually grow below 0-5°C, optimal temperature is 28-35°C and will not grow above 45-52°C.
Chapter 14.2 Responding to the environment
TemperatureSome plants grow best when the temperature varies between day and night
Chapter 14.2 Responding to the environment
TemperatureSeed dormancy – as the seed matures the development of the embryo is arrested and the seed enter a dormant stage. The seed remains dormant until the conditions are right for successful germination.
Chapter 14.2 Responding to the environment
TemperatureSeed dormancy – many seeds require cold for a couple of months, seeds of some desert-adapted species require certain amounts of water (i.e. leached). Leaching removes the inhibitory substance (abscisic acid).
Chapter 14.2 Responding to the environment
TemperatureBud dormancy – some plants in temperate regions become dormant prior to winter. The trees lose chlorophyll from their leaves, withdraw nutrients to the roots and stems, then drop their leaves and the buds become dormant. Thought to be due to abscisic acid produced in leaves.
Chapter 14.2 Responding to the environment
TemperatureVernalisation – period of winter cold that stimulates flowering. E.g. snow gums require a cold spell to break seed dormancy to allow seedlings to grow in spring
Chapter 14.2 Responding to the environment
TemperatureVernalisation – other plants respond to cold after their first year of growth. They flower in the second year. Known as biennials
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