36 Homeostasis and Adaptation

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

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.

Tropical Forest Plant

Rain is channelled by funnel shaped leaves

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.

Dry Desert Plant Ocean Margin Plant

leaves modified into spines or hairs to reduce water loss

~ Shallow, but extensive

fibrous root system

Surface area reduced by producing a squat, rounded plant shape

Mangrove trees take in brackish water, excreting the salt through glands in the leaves

Seaweeds growing in the intertidal zone tolerate exposure to the drying air every 12 hrs

Stem becomes the major photosynthetic organ, plus a reservoir for water storage

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.

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.

Methods of ~afer Conservation in Various Plant 'Species .

Adaptation for Water Conservation Effect of Adaptation Example

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 Prickly pear (Opuntia), Nerium sp. which water loss can occur.

Stomata sunken in pits, grooves, or depressions Moist air is trapped close to the area of Sunken stomata: Pinus sp., Hakea sp.; l eaf surface covered with fine hairs water loss, reducing the diffusion gradient Hairy leaves: l amb's ear; l eaf rosettes: Massing of leaves into a rosette at ground level and therefore the rate of water loss. Dandelion (Taraxacum), daisy Stomata closed during the light, open at night Carbon dioxide is fixed during the night, CAM plants e.g. American aloe,

water loss during the day is minimised. pineapple, Kalanchoe, Yucca

leaves reduced to scales, stem photosynthetic Reduction in surface area from which leaf scales: Broom (Cytisus); Rolled leaf: leaves curled, rolled, or folded when flaccid transpiration can occur. Marram grass (Ammophila), Erica sp. Fleshy or succulent stems When readily available, water is stored in Fleshy stems: Opuntia, Candle plant Fleshy or succulent leaves the tissues for times of low availability. (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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Homeostasis and Adaptation

Adaptations in Halophytes and Drought Tolerant Plants

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.

l eaf hairs

TS of Marram grass leaf ~

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.

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.

37

1. Define the term xeromorphic: ---------------------------------

2. Describe three xeromorphic adaptations of plants:

(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-dwell ing adaptations: ____________ _

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38 Homeostasis and Adaptation

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.

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.

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.

1-- Cytokinins move up i the plant to the \ shoot and leaves

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.

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.

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.

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.

1. Describe one commercial application of a plant hormone (name the hormone in your answer): Hormone: ____________________________ _

Application: -------------------------------------

2. Explain the role of auxin (IAA) in the following plant growth processes:

(a) Apical dominance: ------------------------------------------------------------------(b) Stem growth:

3. Explain why pruning (removing the central leader) induces bushy growth in plants: _ _______________ ______ _

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Homeostasis and Adaptation 39

Tropisms 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.

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).

Hydrotropism Growth response to water. Roots are mainly influenced by gravity but will also grow towards water and are said to be positively hydrotropic.

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

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: ____________________________________________________ __

(b) Distinguish between a tropism and a nastic response: --------------- -----------

2. Explain the adaptive value of the following tropisms:

~)Pos~vegeotrop~minroo~: ______ __________________________ __

(b) Positive phototropism in coleoptiles: --------------------- ---------

(c) Positive th igmotropism in weak stemmed plants, such as vines:---------------------

(d) Positive chemotropism in pollen grains:---------------- ------------(e) Negative geotropism in shoots: _ ________ ___________ ________ __ _

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40 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 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.

(a) Name the hormone that regulates this growth response:

(b) Give the full name of this growth response:

(c) State how the cells behave to cause this change in shoot direction at:

Point A:------------------

Point 8:------------------

(d) State which side (A or B) would have the highest concentration of hormone:

(e) Draw a diagram of the cells as they appear across the stem from point A to 8 (in the rectangle on the right).

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:

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: __________________ _

PlantS: _________ _________ _

understood. Auxins increase cell elongation only over a certain 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).

Directional Sunlight

~ Draw your cells here:

Directional Sunlight

~

Shoot grows in the direction of sunlight

B

Growing ___ .J.-_., shoot

of plant

-Tin foil cap

A B

Growing shoot ---+-- 0

~ ~ l ~ ~ e> .S c: 0 jjj

The auxin concentrations that enhance stem growth ---4 inhibit the growth of roots

1Q- 5 10--3 1Q- 1 101 Increasing concentration of auxin mg 1- 1 (log10 scale)

103

Auxin Concentration and Root Growth 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 surface 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.

1. Explain the mechanism proposed for the role of auxin in the geotropic response in:

(a) Shoots (stems):------------------------- --------

(b) Roots: _________________________________ _

2. (a) From the graph above, state the 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: -------------------------------- -------Photocopying Prohibited t4 Biozone Internat ional 2000

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