Homeostasis and Adaptation
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Water Balance in Plants Without sufficient water plant cells will lose turgor and the plant tissue will wilt. If the plant passes its permanent wilting point the plant will die. Water is lost from the plant by transpiration: the loss of water vapour, primarily through the stomata. Water balance IS not a problem for aquatic plants. They simply allo.w water to flow 1n by osmosis until the cell wall stops further expans1on. Plants adapted to
Tropical Forest Plant
leaves modified into spines or hairs to reduce water loss
Ocean Margin Plant
Surface area reduced by producing a squat, rounded plant shape
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Mangrove trees take in brackish water, excreting the salt through glands in the leaves
Desert plants e.g. cacti, cope with low rainfall and high transpiration rates. Plants develop strategies to reduce water loss, store water, and access available water supplies.
TS of Marram grass leaf Ice plant (Carpobrotus): The leaves of many desert and beach dwelling plants are fleshy or succulent. The leaves are triangular in cross section and crammed with water storage cells. The water is stored after rain for use in dry periods. The shallow root system is able to take up water from the soil surface, taking advantage of any overnight condensation.
Stem becomes the major photosynthetic organ, plus a reservoir for water storage
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Marram grass (Ammophi/a): The long, wiry leaf blades of this beach grass are curled downwards with the stomata on the inside. This protects them against d rying out by providing a moist microclimate around the stomata. Plants adapted to high altitude often have similar adaptations.
land plants that colonise the shoreline (e.g. mangroves) must cope with high salt content in the water. Seaweeds below low tide do not have a water balance problem.
Grasses living in dry areas curl their Mosses are poor at obtaining and Hairs on leaves trap air close to the Excess water is forced from leaves leaves and have sunken stomata. storing water, restricting distribution. surface, reducing transpiration rate. (guttation) during high humidity.
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Effect of Adaptation
Ball cactus (De/osperma saturatum): In cacti, the leaves are modified into long, thin spines which project outward from the thick fleshy stem (see close-up above right). This reduces the surface area over which water loss can occur. The stem takes over the role of producing the food for the plant and also stores water during rainy periods for use during drought. As in succulents like ice plant, the root system in cacti is shallow to take advantage of surface water appearing as a result of overnight condensation.
1. Define the term xeromorphic: - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Methods of ~afer Conservation in Various Plant 'Species Adaptation for Water Conservation
l eaf hairs
Seaweeds growing in the intertidal zone tolerate exposure to the drying air every 12 hrs
Shallow, but extensive fibrous root system
Tropical plants live in areas of often high rainfall. There is also a corresponding high transpiration rate. Water availability is not a problem in this environment.
Adaptations in Halophytes and Drought Tolerant Plants
low water conditions are called xerophytes and they exhibit structural (xeromorphic) and physiological adaptations for water conservation. Some of these are outlined below. Halophytes (salt tolerant plants) and alpine species may also show xeromorphic features: an adaptation to the scarcity of physiologically available water and high transpirational losses in these environments.
Dry Desert Plant
Rain is channelled by funnel shaped leaves
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Homeostasis and Adaptation
Example
2. Describe three xeromorphic adaptations of plants: Thick, waxy cuticle to stems and leaves
Reduces water loss through the cuticle.
Pinus sp. ivy (Hedera), sea holly (Eryngium), prickly pear (Opuntia)
Reduced number of stomata
Reduces the number of pores through which water loss can occur.
Prickly pear (Opuntia), Nerium sp.
Stomata sunken in pits, grooves, or depressions l eaf surface covered with fine hairs Massing of leaves into a rosette at ground level
Moist air is trapped close to the area of water loss, reducing the diffusion gradient and therefore the rate of water loss.
Sunken stomata: Pinus sp., Hakea sp.; Hairy leaves: l amb's ear; l eaf rosettes: Dandelion (Taraxacum), daisy
Stomata closed during the light, open at night
Carbon dioxide is fixed during the night, water loss during the day is minimised.
CAM plants e.g. American aloe, pineapple, Kalanchoe, Yucca
leaves reduced to scales, stem photosynthetic leaves curled, rolled, or folded when flaccid
Reduction in surface area from which transpiration can occur.
leaf scales: Broom (Cytisus); Rolled leaf: Marram grass (Ammophila), Erica sp.
Fleshy or succulent stems Fleshy or succulent leaves
When readily available, water is stored in the tissues for times of low availability.
Fleshy stems: Opuntia, Candle plant (Kieinia); Fleshy leaves: Bryophyllum
Deep root system below the water table
Roots tap into the lower water table.
Acacias, oleander
Shallow root system absorbing surface moisture
Roots absorb overnight condensation.
Most cacti
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(a) _________________________________________________________________ (b) _ _ _ __ __ _ _ _ __
_ __ _ _ _ _ _ _ __ _ __ _ _ _ ___
(c) __________________________________________________________________
3. Describe a physiological mechanism by which plants can reduce water loss during the daylight hours:
4. Explain why creating a moist microenvironment around the areas of water loss reduces transpiration rate:
5. Explain why seashore plants (halophytes) exhibit many desert-dwelling adaptations: _ _ _ _ _ _ _ __ _ _ __
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Homeostasis and Adaptation
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Homeostasis and Adaptation
Tropisms
Plant Hormones Like animals, plants use hormones to regulate their growth and development. Plant hormones (phytohormones) are organ1c compounds produced in one part of the plant and transported to another part, where they produce a growth response. Hormones
are effective in very small amounts. There are five groups of phytohormones: auxins (e.g. IAA), gibberellins, cytokinins, ethene, and abscisic acid (ABA). Together they control the growth of the plant at various stages of development.
Young Leaves and Buds Auxin (IAA) is produced in the young leaves and buds. IAA is a strong promoter of growth in stem length and controls the differentiation of tissues. It is the hormone responsible for apical dominance: the growing leaves of the apical bud synthesise IAA at concentrations high enough to suppress the growth of lateral buds below.
Shoot growth Cytokinins promote cell division. They move from the roots to the leaves in the transpiration stream and, although they do not influence growth in the length of the stem, they keep the shoot and root growth in balance. Gibberellin promotes elongation in the region just below the shoot tip (subapical region). Fruit Ethene (ethylene) accumulates in mature fruit to induce ripening. Abscisic acid (ABA) is produced in ripe fruit, inducing fruit fall. Cytokinins made in the dividing cells of young fruit are essential for growth. Leaves Abscisic acid (ABA) is a growth inhibitor made in the leaf chloroplasts in response to water stress. It acts on the guard cells, causing stomatal closure and thereby reduces water loss.
1-- Cytokinins move up
Old Leaves Ethene and abscisic acid (ABA) are made in the old (senescent) leaves. Ethene promotes leaf fall through the development of a zone across the stem where the leaf will break off (the abscission zone). ABA promotes seed dormancy. Although it reaches high concentrations in senescent leaves, its exact role in leaf fall is unclear- it appears to promote abscission in only a few species. Together with IAA, gibberellins (produced in the chloroplasts, embryo, and young leaves), delay the onset of senescence and leaf fall.
E en though most plants are firmly rooted in the ground, they are c:pable of growth movements and responses to environmental timuli (cues). Some of these responses are slow and gradual ~hile others may be rapid and quite spectacular. Tropisms are lant growth responses to external stimuli, where the stimulus ~irection determines the direction of the growth response.
Geotropism Growth responses to the "-.:::,__~~~ 1 earth's gravitational pull. Stems and coleoptiles are negatively geotropic -they grow away from the direction of the earth's gravitational pull.
Tropisms may be positive or negative depending on whether the plant moves towards or away from the stimulus. The main stimuli that cause growth responses in plants are gravity, light, chemicals, and touch (pressure). Tropisms are distinguished from nastic responses by the directionality of the response. The direction of a nasty is independent of the stimulus direction.
Phototropism Growth reponses to light, particularly directional light. Coleoptiles, young stems, and some leaves are positively phototropic. The receptor for the phototropic response is probably a pigment or pigments in the light sensitive tissues. These are thought to trigger the redistribution of plant hormones (auxin and /or a growth inhibitor) in the region of cell elongation.
Thigmotropism Growth reponses to touch or a solid surface. Tendrils (modified leaves) have a coiling response stimulated by touch and are positively thigmotropic
Chemotropism A growth response to a chemical stimulus. Pollen tubes grow towards the chemical released by the ovule and away from the air (they are negatively aerotropic).
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the plant to the \ shoot and leaves
Cambium Activity Auxin and gibberellins promote cell enlargement and differentiation in the cambium, promoting the formation of secondary vascular tissues (secondary thickening).
Seeds Gibberellins are involved in breaking the dormancy of seeds and buds, and in mobilising food stores during seed germination. In some plants cytokinins are also involved in seed germination.
Roots In mature plants, cytokinins are synthesised in the root tips and travel to the shoots and leaves in the transpiration stream.
Root tip Auxin is synthesised in the meristematic tissues of the plant: especially the root and shoot tips, but also in the young leaves, flowers and fruits. From these areas it is transported to areas of growth in the plant.
Synthetic analogues of IAA Since the discovery of its chemical structure, many analogues of IAA have been produced for commercial use. As with IAA, these analogues are transported around the plant where they exert an effect on its growth and metabolism. IAA analogues are applied as growth promoters in rooting powders, and as inducers for fruit production . Some analogues (e.g. 2-4-5-T) even act as growth inhibitors and are used as selective herbicides.
Hydrotropism Growth response to water. Roots are mainly influenced by gravity but will also grow towards water and are said to be positively hydrotropic.
Roots are positively geotropic, and curve downward after emerging through the seed coat. The gravity-sensing mechanism is probably based on the presence of statoliths small clusters of unbound starch grains in cells. These change position in response to gravity, triggering the response through the action of hormones (auxin and ABA have been suggested).
1. (a) Briefly define the term tropism: _________________________________________ _____________ 1. Describe one commercial application of a plant hormone (name the hormone in you r answer): Hormone: _____________________________ Application: ------------------- -------- -- - - - - - - - -
(b) Distinguish between a tropism and a nastic response: - - - - - - - - - - -- - -- - - - - - - - - - - -
2 . Explain the adaptive value of the following tropisms: 2. Explain the role of auxin (IAA) in the following plant growth processes: (a) Apical dominance: ------------------------------------------------------------------
~)Pos~vegeotrop~minroo~:
______ _ _ _ _ _ _ _ __ __ _ _ _ __ __ _ _ _ _ _ _ __ __
(b) Positive phototropism in coleoptiles: - - - -- -- - - - - - - - - - - - - -- - - - - - - -- -
(b) Stem growth: (c) Positive th igmotropism in weak stemmed plants, such as v i n e s : - - - - - - - - - - - - - - - - - - - - -
3. Explain why pruning (removing the central leader) induces bushy growth in plants: _ _______________ _______
(d) Positive chemotropism in pollen grains:--- - -- - - - - - - - - -- - - - - - - - - - - - (e) Negative geotropism in shoots: _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ __ _ __
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Homeo!?ta!?i!? and Adaptation
Homeo!?ta!?i!? and Adaptation
Investigating Phototropism Phototropism in plants was linked to a growth promoting substance in the 1920s. A number of classic experiments, investigating phototropic responses in severed coleoptiles, gave evidence for the hypothesis that auxin was responsible for tropic responses in stems. Auxins promote cell elongation. Stem curvature in response to light can therefore result from the differential distribution of auxin either side of a stem. However, the mechanisms of hormone action in plants are still not well
Investigating Geotropism
understood. Auxins increase cell elongation only over a certai n concentration range. At certain levels, auxins stop inducing elongation and begin to inhibit it. There is some experimental evidence that contradicts the original auxin hypothesis and the early experiments have been criticised for oversimplifying the real situation. Outlined below are some experiments that investigate plant growth in response to light and the role of hormone(s) in controlling it (see also: Plant Hormones).
1. Directional Light: A pot plant is exposed to direct sunlight near a window and as it grows, the shoot tip turns in the direction of the sun. If the plant was rotated, it adjusted by growing towards the sun in the new direction.
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Although the response of shoots and roots to gravity is well known, the mechanism behind it is not at all well understood. The importance of auxin as a plant growth regulator, as well as its widespread occurrence in plants, led to it being proposed as the primary regulator in the geotropic response. The basis of auxin's proposed role in geotropism is outlined below. The mechanism is appealing in its simplicity but, as noted below, has been widely
criticised, and there is not a great deal of evidence to support it. Many of the early plant growth experiments (including those on phototropism) involved the use of coleoptiles. Their use has been criticised because the coleoptile (the sheath surrounding the young shoot of grasses) is a specialised and short-lived structure and is probably not representative of plant tissues generally.
The Role of Auxins in the Geotropic Response of Stems and Roots Directional Sunlight
Shoot grows in the direction of sunlight
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(a) Name the hormone that regulates this growth response:
Gravity
Gravity
) _.. /·-;
··~~:~
r----: .... ~···· B
(b) Give the full name of this growth response:
(.:::<;:::__ Growing _ _ _.J.-_., shoot of plant
(c) State how the cells behave to cause this change in shoot direction at: Point A : - - - - - - - - - - - - - - - - - Point 8 : - - - -- - - -- - - - - - - - - -
Draw your cells here:
(d) State which side (A or B) would have the highest concentration of hormone:
A horizontally placed root (radicle) tip grows downwards - this is positive geotropism.
Experiments on isolated shoot tips provide some evidence that geotropism (like phototropism) is due to different growth rates of upper and lower sides of the stem or root in response to the redistribution of auxin. In a horizontally placed shoot tip (see diagram, right), more auxin accumulates on the lower side than on the uppermost side. In stems, this causes elongation of the cells on the lower surface and the stem tip turns up. The root grows down because root elongation is inhibited by the high levels of auxin on the lower surface (see graph below).
A horizontally placed stem tip grows upwards -this is negative geotropism
Auxin moves to the lower side. The cells on the lower side elongate in response to auxin and the stem turns upwards.
Agar block 33% auxin Barrier ---iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii~•
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(e) Draw a diagram of the cells as they appear across the stem from point A to 8 (in the rectangle on the right).
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2. Light Excluded from Shoot Tip: With a tin foil cap placed over the top of the shoot tip, light is prevented from reaching it. When growing under these conditions, the direction of growth does not change towards the light source, but grows straight up. State what conclusion can you come to about the source and activity of the hormone that controls the growth response:
In a horizontally placed seedling, auxin moves to the lower side of the organ in both the stem and root. Whereas the stem tip grows upwards, the root tip responds by growing down. Root elongation is inhibited by the same level of auxin that stimulates stem growth (see graph left). The higher auxin levels on the lower su rface cause growth inhibition there. The most elongated cells are then on the upper surface and the root turns down. This simple auxin explanation for the geotropic response has been much criticised: the concentrations of auxins measured in the upper and lower surfaces of horizontal stems and roots are too small to account for the growth movements obseNed. Alternative explanations suggest that growth inhibitors are also somehow involved in the geotropic response.
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..
c:
Directional Sunlight
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Auxin Concentration and Root Growth
The auxin concentrations that enhance stem growth - - - 4 inhibit the growth of roots
.c:
·-
0
E 0
:; a:
- T i n foil cap
e> 0
~ ~l ~ ~
e> .S
A
c:
B
0
jjj
1Q- 5
10--3
1Q- 1
103
101
Increasing concentration of auxin mg 1- 1 (log 10 scale)
Growing shoot ---+--<~
of plant
1. Explain the mechanism proposed for the role of auxin in the geotropic response in: 3. Cutting into the Transport System: Two identical plants were placed side-by-side and subjected to the same directional light source. Razor blades were cut half-way into the stem , thereby interfering with the transport system of the stem. Plant A had the cut on the same side as the light source, while Plant 8 was cut on the shaded side. Predict the growth responses of: Plant A: _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ __
(a) Shoots ( s t e m s ) : - - - - - - - - - - -- -- -- - - - - - - - -Directional Sunlight
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(b) Roots: _ _ _ _ _ _ _ _ _ __ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
A PlantS: _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ __ Growing shoot of plant
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B
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Razor blade left in cut
2. (a) From the graph above, state t he auxin concentration at which root growth becomes inhibited: _ _ _ _ _ _ _ __ (b) State the response of stem at this concentration: _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ 3. Briefly state a reason why the geotropic response in stems or roots is important to the survival of a seedling: (a) Stems: ______________ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _____ _ ___ (b) Roots: - - - - - - - - - - - -- - - -- - - - - - - - - - - - - - --
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