PLANT PROCESSES
CHAPTER 15
TRANSPORTING NUTRIENTS
Plants rely on columns of water in its vascular tissues to transport nutrients.
Plants have two types of vascular tissue: xylem and phloem.
Xylem carries primarily water and dissolved minerals from the soil.
Phloem carries sugars from leaves to storage organs or areas of growth.
While the fluids in xylem and phloem are different from one another, they are both often called sap.
TRANSPIRATION
The process by which water is carried through plants from roots to leaves and is released through the stomata of the leaves.
The roots absorb water and dissolved minerals from the soil through active transport and osmosis.
The root pressure caused by the incoming water will push the water and dissolved minerals up the xylem.
Water’s property of adhesion- an attraction between
molecules of different substances, also helps it to rise in the xylem. This is called capillary action.
TRANSPIRATION
Root pressure and capillary action can only move water a short distance. If they were the only forces moving the
xylem sap, trees would be much shorter.
These forces move water up the xylem by pushing from the bottom.
Most of the force that moves water up the xylem operates by pulling water from the top.
This is due to a property of water called cohesion- the attraction of water molecules for each other.
COHESION-TENSION THEORY
As transpiration occurs, water molecules in the leaves’
spongy mesophyll exit the leaf as water vapor through the stomata.
More water moves from the xylem into the spongy mesophyll.
As these molecules move out of the xylem, cohesion pulls on the water molecules in the xylem, causing all the water to move up the entire length of the xylem.
THE ROLE OF WATER IN PLANTS
As water moves up the xylem, it accomplishes several purposes:
Some water enters the cells’ central vacuoles, where it maintains turgor pressure, which gives a plant rigidity.
Some water is used along with carbon dioxide in photosynthesis to make sugar for the plant.
The moving water also carries necessary minerals such as nitrates and phosphates from the soil throughout the plant.
As much as 99% of the water is simply released into the atmosphere as water vapor.
SUGAR MOVEMENT IN PLANTS
The sugar that the leaves make needs to reach every part of the plant for nourishment.
Sugars move in the phloem rather than xylem since
phloem sap can move upward or downward in the plant.
Cells requiring large amounts of sugars are called sinks
Rapidly dividing meristematic tissue or storage structures such as roots or fruits.
Cells that make the sugars are called sources and are found in the leaves and sometimes herbaceous stems.
PRESSURE-FLOW HYPOTHESIS
Source cells use active transport to move sugars into the water-filled phloem.
The higher concentration of sugar in the phloem causes water to move osmotically into the phloem from nearby xylem cells.
This influx of water creates an area of high pressure near
the source.
At the same time, the sink cells remove sugar molecules from the phloem by active transport.
The water in that part of the phloem is directed back into xylem cells, creating an area of low
pressure near the sink.
This pressure gradient causes the water to flow from the source to the sink, carrying the sugar molecules along with it.
Since the flow of phloem sap seems to be driven by
a difference in pressures, it is able to move in any
direction toward a sink.
PRESSURE-FLOW HYPOTHESIS
PLANT HORMONES
Plant hormones are messenger molecules that are
typically produced by one tissue to produce a response in other tissues.
Many plant hormones can be grouped into one of five major categories:
1. Abscisic acid
2. Auxins
3. Cytokinins
4. Ethylene
5. Gibberellins
Plant hormones very rarely work alone. They can work together or against each other.
ABSCISIC ACID
Inhibits other hormones and slows cell growth and division.
Causes dormancy in buds and
seeds.
AUXINS
Produced by the apical meristem.
Promote cell elongation in the shoot.
Suppress cell division in the shoot.
CYTOKININS
Promote cell division.
Promote lateral bud development.
ETHYLENE
A gas that causes fruit to ripen.
Causes trees to form abscission layers
between stems and leaf
petioles.
GIBBERELLINS
Produced in the roots.
Encourage stem elongation along with cell division in
shoots and leaves.
NASTIC MOVEMENT
Movement of a plant not based on the direction of the stimulus.
Based on changes in turgor pressure.
Examples:
leaflet folding of “sensitive plant”
Venus flytrap closing
TROPISMS
A plant responds differently to the same stimulus depending on the direction of the stimulus.
Positive tropism- toward the stimulus
Negative tropism- away from the stimulus
Four main types of tropisms:
Phototropism- light Gravitropism- gravity Thigmotropism- touch Hydrotropism- water
PHOTOTROPISM
Plant stems exhibit positive phototropism by growing toward the light.
Light causes growth inhibitors to concentrate on the light side of the plant.
Therefore, the auxin has a greater effect on the dark side of the stem.
The lengthening of the dark side of the stem causes the plant to bend toward the light.
Roots exhibit negative phototropism.
GRAVITROPISM
A germinating seed produces a radicle and a plumule.
The radicle is the future root of the plant.
The plumule is the future stem of the plant.
The radicle must grow downward and the plumule must grow
upward or the young plant will die.
The radicle grows toward the pull of gravity, exhibiting positive gravitropism.
The plumule grows against the pull of gravity, exhibiting negative gravitropism.
THIGMOTROPISM
Some plants, primarily climbing plants such as vines, exhibit thigmotropism.
Plants such as the grape have specialized stems called tendrils.
When a tendril touches an object, the cells on the side opposite the object elongate so that the
tendril begins to bend around the object, providing support for the entire plant.
Some vines, such as morning glories, don’t have tendrils but their entire stems demonstrate
thigmotropism by wrapping around other objects.
PHOTOPERIODISM
The response of a plant to changes in the duration and intensity of light exposure.
It is the length of the nights and not the days that controls photoperiodism.
Photoperiodism is controlled by a group of chemicals called phytochromes.
PHOTOPERIODISM
Short-day plants- won’t flower until nighttime is longer than a critical length of time.
Long-day plants- won’t flower until nighttime is shorter than a critical length of time.
Plants that bloom without regard to day length are known as day-neutral plants.
ASEXUAL PLANT REPRODUCTION
In addition to sexual
reproduction through the uniting of pollen and egg in the plant’s flowers, some plants also reproduce
asexually.
Asexual reproduction in plants is also called
vegetative propagation.
ASEXUAL PLANT REPRODUCTION
Vegetative propagation produces a clone, an organism that is genetically identical to the parent plant.
Many plants reproduce asexually using specialized
underground storage structures such as bulbs, tubers, or corms.
Plants can also reproduce asexually using stolons, roots and leaves.
BULBS
TUBERS
CORMS
STOLONS
Specialized stem structures, alsocalled runners, that form new plants that take root some distance from the parent plant.
ROOTS
Many trees, like aspens, use their roots to send up new shoots. As a result, whole groves of trees may be clones of the original tree connected to each other through a vast underground root system.
LEAVES
New plants rarely grow from leaves. However, the leaves of a few plants, such as theAfrican violet, will produce new plants when they are placed in soil. Dipping the leaf in rooting hormone greatly increases the leaf’s chances of forming a new plant.
PEOPLE AND PLANTS
Plants fulfill many human needs:
Shelter
Clothing
Fuel
Food
GROWING PLANTS FOR FOOD
Most plants that we eat typically reproduce through sexual
reproduction.
Sexual reproduction can create a problem for farmers since it results in a mixing of alleles.
Farmers who plant seeds hoping to grow a whole crop like the parent plant may end up with plants that are very different from each other and the parent plant.
SOLUTION: ASEXUAL REPRODUCTION
Some crops, such as potatoes, are raised entirely by asexual reproduction.
Asexual reproduction is natural to potatoes, but some other crops pose more of a challenge.
Since many fruit trees must be cross-pollinated by a tree of another variety, a fruit tree of the parent’s variety
cannot be grown from a seed.
Instead, when an orchardist wants to plant a new apple tree, he uses a technique called grafting.
GRAFTING
A branch called a scion is cut from a tree and
inserted into a rooted
sapling, called the stock.
LAYERING
Bending a branch so that a part of it can be buried to produce new plants.
TISSUE CULTURE
PROBLEMS WITH VEGETATIVE PROPAGATION
Human intervention has produced plant varieties that do not produce
seeds and can’t reproduce (bananas
and seedless watermelons)
PROBLEMS WITH
VEGETATIVE PROPAGATION
Produces a large group of plants with identical
genotypes, limiting genetic diversity.
Because of limited genetic
diversity, these plants are very
susceptible to diseases.
PLANTS IN THE ENVIRONMENT
Plants play a major role in the water cycle.
Help filter water before it enters aquifers.
Reduce local temperatures.
Control erosion, holding the soil in place.
Use carbon dioxide and release oxygen during
photosynthesis to replenish the atmosphere.
