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As a Master Gardener, you will be asked all kinds of questions about plants. To effectively answer them, you need
to have a basic understanding of the physiological processes that control plant growth and development. This unit
discusses basic plant physiology which means how plants do what they do! Plants are more similar to us in more
ways than you think, but they are different, too. Plants can do things than you and I will never be able to do, like
make food and oxygen from sunlight and water. But because of their ability to perform such functions, plants
suffer from certain restrictions. Unlike us, plants have to endure storms, frost, drought and heat, and survive
through it all because they cannot move. What makes plants similar yet so different than us? In the next two
hours, we will use your existing knowledge of plant structure and we will tie it in with how plants function. And at
the end, we promise you, you’ll never look at a plant the same way!
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Life on our planet depends on the flow of energy from the sun and plants are an integral part of this cycle. Without
plants and products they produce in their growth, we would not be here.
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Let’s take a brief look at what plants do for us. Plants are responsible for generation of oxygen that we breathe. The
photo on the right shows an oxygen bar, where customers pay $1/minute to breathe 97% O 2. Oxygen is not only
important for us for breathing, but also for atmospheric gas regulation.
Oxygen gas in the atmosphere is converted to ozone in the stratosphere, as it absorbs ultraviolet light. Ozone then
absorbs more ultraviolet light and is eventually converted back into oxygen gas. This process blocks up to 99% of
all the harmful UVB rays that would otherwise sterilize the surface of the Earth, making life impossible.
Plants use carbon dioxide to build sugar in the process of photosynthesis.
Besides being toing toxic to humans at elevated levels, CO2 is a “greenhouse gas,” meaning that it absorbs infrared
radiation from earth, keeping that energy in the atmosphere longer and contributing to global warming.
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Plants give us simple sugars and their polymer carbohydrate, starch. The cereal that you’re eating for breakfast is
made of grains packed with nutritious starches, vitamins and minerals. Even if you are on a low-carb diet, you can
still thank plants for all the leafy greens you’re (hopefully) eating. And then there’s the fact that all the meat you
eat was part of an animal that probably ate plants to live. When eating beans, you’re benefitting from the proteins
that are packed in the legume seed, and when you’re munching on nuts, you’re consuming healthy oils and fats. So
almost all our food comes from plants.
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Plants also produce fibers such as cotton which your jeans are made of. Paper: Average paper use per person in
North America is almost 5 lbs per day. Fibers such as nylon and rayon are processed from wood fibers.
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And of course, very important are the medicinal compounds produced by plants. Noteworthy are taxol from the
bark of the pacific yew tree, a source of the first anti-cancer drug. Foxglove produces digitalis, which treats heart
disease. Rosy periwinkle not only brightens the landscape but also produces compounds used to treat two cancers,
juvenile leukemia and Hodgkin’s disease.
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Plants produce spices, which are important flavoring agents in food, also oils used in the food industry. Plants give
us essential amino acids, there are 8 amino acids that we need in our cells, but we don’t have the ability to produce
them ourselves. We can most easily get them from plants. To get the complete set of essential amino acids, a
combination of legumes and cereals is best.
Fossil fuels like coal, crude oil, and natural gas are the products of plants that died a long time ago.
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Every living organism performs a myriad of chemical reactions, collectively known as metabolism. Plant
metabolism is the multitude of interrelated biochemical reactions that maintain plant life. Tens of thousands of
different organic compounds have been discovered in plants. These include compounds involved in the metabolic
pathways that assimilate nutrients from the environment, in energy metabolism, and in biosynthetic pathways that
produce the basic components of the plant cell: proteins, membranes, and cell walls.
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The plant is a factory fueled by sunlight. The primary metabolic processes are photosynthesis, respiration and
mineral assimilation. Primary metabolism refers to the processes and molecules absolutely essential for plant
existence such as energy, genetic material, proteins and components of cell membranes. These products come from
three processes, photosynthesis, which produces carbohydrates; respiration, which releases energy stored in
carbohydrates and lipids, and mineral assimilation from soil.
These three processes occur in various plant organs, tissues, and organelles. In addition to primary metabolism,
plants perform a wide variety of processes, collectively called secondary metabolism. Secondary metabolites, unlike
proteins, lipids, and nucleic acids, are not essential for plant growth. Instead, they perform a variety of very
important functions, such as protection from herbivores, pests, and pathogens.
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All of these processes occur within the plant body, in various organs and tissues. These organs and tissues have
developed adaptations to maximize the efficiency of their primary metabolic function. For example, most plant
leaves are flat and green to maximize light interception by specialized molecules, because that is what drives
photosynthesis, the primary function of leaves. Roots branch out and spread in all directions in the soil to
maximize water and mineral absorption, because that is their primary function. Stems are centrally located major
conduit to link leaves and roots and transport products of photosynthesis from the leaves to the roots, and water
and minerals from the roots to the leaves.
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Have you ever taken a stroll in the park or enjoyed a lazy afternoon in a mountain meadow and wondered what all
the flowers and trees are made of? From the tiniest duckweed to the stateliest redwood tree, all plants are built the
same way, from small individual cells which are organized in tissues and organs.
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To get an idea of what a typical plant cell is like and what it can do, think of a large factory, that manufactures thousands of
different and complex products from raw materials – water, air and soil. The factory uses sunlight instead of electricity or oil
as an energy source. Now, mentally squeeze this factory into a box, each side approx. 1/2000 of an inch. That is the plant
cell.
The living part of the cell is called the protoplasm, which consists of nucleus, the center of inheritance and cell control, and
the cytoplasm, a soft, jelly-like material in which most of the cell’s metabolism takes place. The cytoplasm is enclosed in a
sac, the plasma membrane. This, like other membranes in a cell, is composed of protein and fatty substances and has the
ability to control the passage of water, foods and selected minerals across the boundary that it defines. Suspended in the
semi-liquid cytoplasm are numerous small organelles, which specialize in separate functions. Some of these are very similar
to organelles found in our human cells, like mitochondria which specialize in energy production. Other organelles are unique
to plants, such as chloroplasts, which perform photosynthesis. The vacuole is another organelle that is unique to plants. The
vacuole can take as much as 90% of the space in mature cells and is responsible for storage of water and maintenance of cell
turgidity.
Another cell feature that sets plant cells away from human cells is the cell wall. The cell wall is the outer boundary of the cell.
It is made of cellulose arranged in long fiber-like strands. The cell wall imparts rigidity and mechanical strength to the plant
cell.
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Cells are formed in the growing points of the plant, in areas called meristems. These areas are found at the tips
and at the base of internodes, and at the root tips. This photo shows a view of the growing point of Chandelier
plant. The younger leaves surrounding the meristem are light-green in color.
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Meristems also are found in woody branches – they are the terminal buds, or end buds and the axillary, or lateral
buds. When the buds break in spring, the meristem starts growing and new shoots grow from these buds. When
you prune, you remove part of the stem which has end and lateral buds.
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The Meristem regions are places where “undifferentiated” cells divide. These cells become part of leaves, stems,
flowers, roots, and associated tissues. As the cell moves ‘away’ from the meristem and becomes part of a
tissue and organ, it becomes specialized. For example, leaf mesophyll cells acquire a columnar shape and
develop functional green chloroplasts to perform photosynthesis.
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Apical meristem at the stem tip is responsible for the stem’s growth in length.
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Growth of the root results from cell divisions in its apical meristem.
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A lateral meristem is a cylinder of cells, dividing both inward and outward to thicken the stem and root.
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Photographs of plant parts as seen through a microscope are called micrographs. There are two types of cuts (or
sections) that microscopists make, there’s the cross section where the cut is made transversely to the long axis of
the plant, or longitudinal, where the plant is cut along the long axis of the plant.
When the growing point is cut longitudinally, this is what we see with a microscope. The colors are blue and red
because of the special stains that were used to make cells and tissues visible.
Apical meristem is found at the apex, and has a dome shape, seen in red color. This is a region of actively dividing
cells. Believe it or not, all plant cells, tissues and organs come from this tiny place! Young leaves are seen as small
‘horns’ on the side of the apical meristem, a pair of older leaves are found on the outside of these. Small buds are
also colored red because similarly to the apical meristem, they also are made of actively dividing cells.
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On the other end of the plant, we find the root tip, which also has a meristem. Notice that the apical root
meristem, like the shoot apical meristem, is dome-shaped. Unlike the shoot meristem, root apical meristem is
covered by a root cap, a region of cells which protects the dividing cells from abrasion with the soil particles. Every
root on a plant has an apical meristem, as many as thousands root growing points.
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The 4 basic parts: leaves, stems, roots, and flowers are divided into vegetative (leaves, stems, and roots) and
reproductive, flowers. The vegetative parts provide nourishment for the reproductive plant parts. Recall that each
plant organ is specialized to carry out a specific function. The overarching goal of the plant is to reproduce itself
and to ensure survival of its progeny. To this end, leaves, stems and roots work together to feed flowers, fruits,
and seeds.
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The specialization is carried out from the plant organs to the three tissue systems that they are composed of:
dermal, vascular, and ground tissue systems. Each system is continuous throughout the plant body.
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The dermal tissue, or epidermis, is generally a single layer of tightly packed cells that covers and protects all young
parts of the plant. The epidermis has other specialized characteristics consistent with the function of the organ it
covers. For example, the roots hairs are extensions of epidermal cells near the tips of the roots. The epidermis of
leaves and most stems secretes a waxy coating, the cuticle, that helps the aerial parts of the plant retain water.
Refer to this and the next slide for descriptions of the three tissue systems and their location within the plant
organs.
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Vascular tissue consists of xylem and phloem. These two types of vessels run side-by-side, extending from roots to
leaves. They are responsible for transporting water, minerals, food and other organic materials between the roots,
stems and leaves of the plant. Xylem and phloem form vascular bundles with each other, which means that
together they are responsible for the efficient transportation of food, nutrients, minerals, and water in the plant,
and hence the survival of the plant. Ground tissue is packing and supporting tissue. It is involved in food
manufacture and storage.
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The external leaf structure includes a petiole, or stalk, which attaches the leaf to the stem, and blade, the wide, flat
leaf surface. Within the blade is a system of veins, which contain the vascular, or conductive tissues.
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The internal leaf structure includes epidermis, both on the upper and lower leaf surface. The epidermis protects
the inside of the leaf from loss of too much water, or desiccation. The inside of the leaf is made of two layers of
mesophyll cells; the layer closest to the leaf surface, called palisade parenchyma, is made of columnar cells, while
the layer closer to the lower surface, called spongy parenchyma, is made of irregularly-shaped cells, interspersed
with intercellular spaces. Both palisade and spongy parenchyma are ground tissue. The vascular bundles are the
veins seen from the outside. In cross section, they reveal two tissues, xylem, which conducts water and minerals,
and phloem, which conducts sugars.
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Now let us delve deeper into the plant structure and peer into the innermost workings of the green factory. Why is
it important for the plant to be green?
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Recall that the plant is a factory fueled by light. You may recall studying the electromagnetic spectrum in high
school biology class. Light energy from the sun is emitted in various wavelengths, ranging from the very short and
penetrating cosmic rays to gamma rays, x-rays and ultraviolet rays to the very long radio waves. It is only a narrow
band of this electromagnetic spectrum that is visible light. If this visible light is passed through a prism, we find it
is broken out into a series of colored wavelengths, ranging from violet on one end to infra-red on the other. Plants
actually use a very narrow part of this spectrum, which happens to coincide with the colors our eyes can
distinguish.
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Plants actually use a very narrow part of this spectrum. Chlorophyll is the pigment responsible for the green color
of plants. It is the chlorophyll which absorbs light primarily in two regions of the visible spectrum - the red area
and the blue area, and reflects in the green area. The chlorophyll molecule absorbs the particles of light called
photons; in the process the chlorophyll molecule become excited and thus elevated to a higher energy state. This is
basically the capture of the energy of the sun. This energy will be used in the photosynthetic process.
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Plants respond to two aspects of light: quantity and quality. In the first, light is a direct source of energy – more
photons, more photosynthesis. For growth and developmental processes, light is utilized as a developmental
trigger and therefore is needed in much smaller quantity. Here, what is important for the plant is not how many
photons, but what wavelength these photons have. Plants have many molecules which act as light-sensors, or
photoreceptors. Besides chlorophyll, there are phytochromes which sense and respond to the wavelength of light.
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The chlorophyll molecules are found inside tiny organelles called chloroplasts. In effect, the color of a leaf is the
combined appearance of millions of chloroplasts discernible only on the level of the microscope. Let us look more
closely at the internal structure of a chloroplast, the food depot of the plant cell. The chloroplasts are enclosed in a
membrane, the chloroplast envelope. Inside the chloroplast, we find grana (pronounced ‘gra-na’, sing. granum)
which is a stack of thylakoid (pronounced ‘tai-la-koid’ membranes. The grana hold chlorophyll molecules in
position. Grana thylakoids are stacked like coins and separate stacks are connected through flat sheets of stroma
thylakoids. The grana are floating in watery matrix, called stroma. After carbohydrates are produced in the process
of photosynthesis, the excess sugars are converted to starch, the main food reserve. Starch is depicted here as
white cottony-looking balls. Any plant part which contains chloroplasts, is capable of photosynthesis.
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The process by which plants produce food is photosynthesis. Photo means ‘light’ and synthesis means ‘to make’. In
photosynthesis, carbon dioxide and water, are converted to carbohydrate and oxygen using energy from the sun.
Notice something interesting here – oxygen is a by-product of the process. Thanks to plants, we have oxygen to
breathe!
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This is a summary diagram of all the starting molecules (water, carbon dioxide, and oxygen) and the end product,
glucose. The active components are sunlight and chlorophyll in the leaf.
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The products of photosynthesis, sugars, are stored as glucose polymers, cellulose and starch. Polymers are
molecules made from repeating units.
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Cellulose is a structural carbohydrate, a glucose polymer. Glucose molecules link to one other to form long
chains…
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And build up trees! The stiff cell wall is composed of cellulose. Imagine billions of stiff cells, connected to one
other, spreading upward and outward. This is how trees are able to form wood, creating a firm architecture of
trunks and branches.
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Starch is a storage carbohydrate, another very important glucose polymer. The glucose molecules are linked
differently compared to cellulose and form a chain that can be twisted and wrapped in packages, the starch grains,
shown in the photo. The chains are so tight, that they have crystalline property of bending light. Next time when
you’re chewing on a French fry, think of the starch, because the potato is loaded with those grains.
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This is a photomicrograph of a wheat grain showing cells, packed with starch grains (green ovals and circles). You
can imagine how much starch is packed in that seed and how hard the plant had to work to put it there!
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Starch is power! What do we mean by that? In the process of respiration, chemical-bond energy in sugars is
converted to energy, which can then be used to drive plant’s metabolic reactions. For the plant, starch is kind of
like money in the bank. Along with the energy, starch generates other carbon molecules, which are used for plant
growth.
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This is what it looks like to break down a starch molecule: glucose are separated from the chain at branch points,
and then individual glucose molecules are further broken down to water and carbon dioxide, releasing the energy in
the chemical bonds.
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The vascular system distributes carbohydrates from leaves, called source, to other organs, called sinks. Fruits, and
in particular seeds, are an important sink. The tissue responsible for this transport is the phloem. Recall what we
said about the ultimate goal of the plant, ensuring survival of the species by spreading its seeds. Well, the plant
needs to work hard indeed to produce many, often thousands of viable seeds. That’s why seeds are sinks for
carbohydrates.
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Respiration also produces building blocks for lipids and nucleic acids. Lipids and proteins make up cellular
membranes. Membranes enclose all plant organelles, including the plasma membrane, which separates the interior
of the cell from the outside environment. Enzymes which facilitate many biological processes are protein
molecules. Nucleic acids store genetic information. All of these polymers play an integral part in plant growth and
development.
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For plants photosynthesis and respiration can be viewed (albeit not scientifically), as yin and yang – two
interrelated parts of a whole. Plants need to make food to not only survive but also grow. Hence, they need to
store products of photosynthesis in various tissues. This will help the plant survive the winter cold.
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Trees, being woody plants, use carbohydrates such as cellulose to increase in diameter (girth) and height. They also
store reserves in the wood. The xylem is the heartwood (sapwood) or internal part of the stem. It is generated to
the inside of the cambium, the area of the stem undergoing rapid cell division and growth. As the plant ages and
stems become woody, the xylem often consists of dead cells that simply serve as the aqueducts for transporting
water and nutrients upward. On the outside of the cambium is the phloem which transports the food substances
throughout the plant. As the plant ages, the phloem becomes the bark. When a tree is girdled and bark is removed
to the cambium, the phloem tissue is removed. This prevents the movement of food substances downward in the
plant to the root. The tree continues to absorb water and nutrients to sustain the top, but because food can no
longer reach the roots, the roots gradually deplete their food reserves, waste away and shut down. Then the tree
dies, sometimes several months after the injury occurred.
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Going back to our discussion of light, we can ask the question – how much light does the plant need to grow? On a
bright sunny day, light intensity outdoors is in the range of 10,000 foot candles. On a cloudy day, light intensity
may be between 500 to 2,000 foot candles. A conference room may be lit with 20 to 30 foot candles of light energy
while stores may have 30 to 100 foot candles of light energy. For maximum photosynthetic rate, most plant
species need between 1,200 and 2,000 foot-candles. Interesting enough, giving a plant higher than 2,000 ft-c at any
given time will not result in higher rate of photosynthesis. This is because there is a finite number of chlorophyll
molecules which are available to do work. Once the chlorophyll molecule has absorbed a photon of light, it must
rapidly dissipate that extra energy and pass it to other molecules, or it will get damaged. In fact, scientists measure
that excess radiation, given in the form of fluorescence, to assess rate of photosynthesis.
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Plants vary greatly in their light requirement, which is related to their native habitats. Tall trees which reach above
the canopy layer are used to high levels of light, while understory plants are used to the lowest light levels. For
tropical house plants, such as Weeping Fig, Ferns, most require between 50 and 1,000 ft. candles of light for
optimum growth.
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Excessive light overloads the photosynthetic apparatus and causes chlorophyll bleaching. This is seen as sun scald
on the Bird Nest Fern on the left and as bleached foliage on the Aglaonema on the right. This is the reason why it is
not recommended after winter passes to take house plants from the house and put them in full sun on the porch.
They need to be acclimated first.
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On the other hand, if light is not sufficient for optimum growth, the following symptoms will develop. You can
recognize a plant grown in too little light by its elongated, spindly, weaker stems, few leaves, and limited or no
flowering. Under such conditions, it really is best to choose plants such Impatiens, Caladium, as well as variety of
low-light-adapted tropical plants that are typically grown as interiorscape plants. They will perform as annuals.
Perennial Hosta would be another good choice.
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An interesting response observed when plants are exposed to bright light coming from one direction only. The
result is flower stems and blooms that grow toward the direction of the light. This is called ‘phototropism’, or
growth toward light.
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We mentioned the term acclimatization. It is the gradual process by which a plant adapts to a new environment.
This usually takes between 6 and 15 weeks, depending on the species. This applies to greenhouse plants,
houseplants, landscape plants, and aquarium plants!
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The other important way plants respond to light, is as a developmental trigger. This response is easily observed
when seeds are germinated in darkness and later grown under very low insufficient light. Such seedlings are
referred to as ‘etiolated’, their young stems long and slender, unable to support their own weight, and their young
leaves yellow, because chloroplasts develop in light.
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Photomorphogenesis is light-mediated development. The photomorphogenesis of plants is often studied by using
tightly frequency-controlled light sources to grow the plants. The process is observed during seedling
establishment, as a light-grown seedling displays dramatic differences in morphology and physiology from one
grown in the dark. When a seed germinates underneath the ground, it first adopts the dark-grown program, and
then switches to the light-grown program as soon as it encounters light. This critical transition in a plant’s life is
called de-etiolation. It has been well known that phytochromes, an ancient family of red and far-red
photoreceptors, are the prominent photoreceptors mediating de-etiolation.
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It has been well known that phytochromes, a family of red and far-red photoreceptors, are the prominent
photoreceptors mediating de-etiolation. Their name refers to the area of the light spectrum where they show
maximum absorption, red is between 655 and 665 nanometers, and far-red is between 725 and 735 nm. The first
graph shows the spectral energy distribution of incident solar radiation on a clear day under the three conditions:
Upper curve is midday, Middle curve is midday sun filtered through a canopy of mustard seedlings, Lower curve is
dusk. Note the small change in R/FR ratio (655-665 nm / 725-735 nm) between midday and dusk compared to the
much larger change caused by light absorption by leaf tissue.
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Photomorphogenetic response is also seen when plants shade each other, whether in a natural situation, such as a
tree shade canopy, or in cultivated environment, such as corn field. Both light filtered through leaves and reflected
off nearby stems or leaves will be enriched in far-red light relative to direct sunlight. Phytochrome helps plants
avoid over-shading.
Plants are able to sense shade and avoid it by elongating their stems and reaching toward higher light.
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Phytochrome also controls a plants response called PHOTOPERIODISM. This light effect is a response to the
duration of timing of day and night. Photoperiodic plants actually measure the duration of darkness. This has had
a pronounced effect on the floriculture industry, because by regulating photoperiod, growers can make plants
flower at specific times, such as holidays. You will be asked “how to make amaryllis /paperwhites /Easter lilies/
Christmas cactus/ etc. bloom again?” The answer is in understanding photoperiodic flowering response.
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Plants could now be categorized into three photoperiodic groups: Short day plants, long day plants and day neutral
plants.
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Short day plants initiate floral buds when the day length is shorter than some critical minimum and the night
length is longer than a critical night length. The night length is most critical. Plants must be provided a period of
uninterrupted darkness that exceeds the critical minimum in order to initiate floral organs. Some plants known to
be short day or long night plants include poinsettias, chrysanthemums, aster, goldenrod, ragweed, Christmas
cactus and spider plant.
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Other flowering plants, classified as long-day plants, require a night period of uninterrupted darkness shorter than
some critical minimum. Examples are hollyhock, radish, beet, spinach, iris and red clover.
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This illustrates how flower growers manipulate flowering in poinsettia plants. To lengthen the night, poinsettia
plants are covered with a blackout shade cloth, which is applied in late afternoon and removed in the morning (5
pm to 8 am). This ensures longer than critical dark period.
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Let us turn our attention to another important environmental factor, temperature. Just like any living biological
entity, plants are affected by temperature. Temperature is an important environmental factor in plant growth and
development. It governs the rate of photosynthesis (food production) and respiration (food utilization). Depending
on its origin, each plant species (e.g. tropical vs. temperate), has temperature range where it grows optimally. In
general, this growth rate is fast in the middle, and slow at the low and high temperature end. As you can see from
this graph, photosynthesis does not work well below the point of freezing. As temperature increases, so does the
photosynthetic rate, until it reaches a plateau about 26 degrees Celsius. From this point on, photosynthesis drops
rapidly at temperatures higher than 35 degrees Celsius.
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Recall that plant metabolic process depend on the action of enzymes, which are complex protein molecules. It is
these enzymes which have a narrow range for optimal activity. This is a diagram of Rubisco, the most abundant
protein within plants. It carries out the transformation of carbon dioxide into oxygen in the photosynthetic
process. The chart shows that Rubisco works best at 37 degrees Celsius. Notice also that the rate of
photosynthesis drops because the Rubisco is destroyed at high temperatures. This is the reason why plants cannot
grow well when stressed with high heat.
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Each plant has a different optimal growth temperature and a different rate of response or tolerance to changes in
temperatures outside the optimal range. Tomato plants grow fastest at 26 degrees Celsius, while cucumber likes it
hotter!
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When higher summer temperatures, especially night temperatures, are encountered, plant respiration increases,
resulting in in loss of carbohydrate reserves. This is the reason why fruits grown in a climate where night
temperatures are cooler, taste sweeter than when that same fruit is grown in climate where night temperatures are
higher; oranges grown in Florida are not as sweet as oranges grown in California. Same applies to ornamental
plants – flowering is reduced when plants experience high night temperature. This, in addition to high humidity,
makes summer conditions in the Southeast particularly challenging for growing ornamental plants.
In addition, if the high heat is accompanied by lack of water, it may cause overheating. As temperature inside the
plant cell raises higher than optimal, enzymes become inhibited and growth processes stop.
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This graph shows optimal growth temperatures in three species, primula, fern, and lantana. Primula is a coldseason crop, so it grows fastest when temperature is around 15 degrees Celsius, or about 60 degrees Fahrenheit.
Fern likes it warmer, at about 75 degrees Fahrenheit, while lantana, a warm-season crop around 86 degrees
Fahrenheit. Greenhouse growers are well-aware of these crops physiological preferences, so they don’t even start
growing lantana, until the temperatures in the spring have warmed up. They can finish lantana in one month, if
they plant in early April, compared to starting it in February. The crop just sits on the bench and does not grow!
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Temperature influences fall color and leaf fall, and also controls plant hardiness. The latter is the plant’s ability to
withstand the average minimum temperature of a region. Although winter hardiness is genetically determined, it is
influenced by the duration of cool temperatures. Cool temperatures also acclimate plants and prepare them for
winter dormancy. Many woody plants, for example, need 2 to 4 weeks of cool temperatures to achieve maximum
hardiness.
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Freezing injury (also frost injury) occurs at or below the freezing point (32 0F). Frost damage occurs during a
radiation freeze, while freeze damage happens during an advection freeze. In both situations, ice crystals form in
plant tissues, dehydrating cells and disrupting plant membranes. Advective freeze occurs when an air mass which
temperature is below freezing moves into an area and displaces warmer air, causing the temperature of plants to
become low enough so ice crystals form inside their cells. Radiation freeze occurs on clear nights in absence of
wind when plants radiate more heat to the atmosphere than they receive. A temperature inversion is created where
cold air close to the ground is trapped by warmer air above it. When the air temperature at plant level is near or
below freezing, the temperature of the plant becomes colder than the air temperature. If the plant becomes
sufficiently cold, the water in their cells freezes, causing physical damage as well as dehydration damage.
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Hardy plants have an ability to concentrate their cytoplasm with electrolytes, such as sugars and ions. These
compounds have the ability to attract water molecules. Even though rapid temperature drop causes water between
the cells to crystallize, the condensed cytoplasm is able to keep water from leaving the cell, thus keeping the plant
cell alive.
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When moderate, prolonged temperature drop occurs, the ice crystals between the cells exert more attractive force
on water molecules. But because of the concentrated cytoplasm, hardy plants are able to keep enough liquid water
inside their cells, and to keep ice crystals from forming inside the cell. Even though this causes dehydration of the
cytoplasm and slowing of most growth, the plants survive.
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Chilling injury occurs above the freezing point (32 0F). Injured foliage appears purple or reddish, sometimes wilted.
Chilling injury can be obvious or invisible. Chilling can delay crop blooming, cause direct damage or reduce plant
vigor. Chilling injury happens often with tropical and subtropical plants grown in most of the U.S., but can happen
with native, temperate forest plants as well, depending on critical temperatures, the duration of low temperature,
temperature changes, age, hydration status of plants, and time of year. The top photo shows chill damage on
Indian Hawthorn. Mature foliage turns brown to dark brown or nearly black. This damage can occur throughout
winter. A more severe chill damage on Gingko is manifested by wilted foliage. This damage is common in late
spring when plants have come out of winter dormancy and have started developing young foliage.
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Plants of tropical and subtropical origin are most susceptible. These photos show a progression of chill damage on
Princess Flower, Tibouchina, manifested by red lower foliage. The damage is fairly mild, although the affected
leaves will defoliate. This type of damage is common in early fall.
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Now let’s move on to a discussion of another major environmental factor that influences plant physiology and
plant growth - WATER. Water has many functions in the plant. It is the substance in which most chemical
reactions take place. It is responsible for the translocation of nutrients and food substances within the plant, and
it helps cool the plant via transpiration. Just as we perspire, plants transpire moisture through their leaf surfaces.
Moisture cools the plant as it evaporates from the leaf surfaces.
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Recall the enzymes which moderate plant metabolic reactions – they need to be in hydrated state in order to
function properly.
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This diagram is a summary of water movement through the plant. Water from the soil is absorbed by root hair,
entering the root cortex and the xylem. Xylem transports the water upward to the leaves. Inside the leaf, water
moves from cells, evaporating from cell wall surfaces to intercellular spaces, where it turns into water vapor. Water
vapor diffuses through stomata, leaving the plant and entering the surrounding air. As it moves through the plant,
water participates in all metabolic processes.
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Transpiration, or the leaf water pump, is the driving force behind water movement through the plant. The air
outside the leaf is much dryer than the air between the leaf parenchyma cells. The greater the difference in
humidity levels, the faster evaporation can occur. Transpiration is the heat-driven, humidity gradient-enhanced,
diffusion of water from plant leaves. In detail, the water moves from the soil upward through the stem, enters the
leaf and exits via the stomata. Water also carries with it essential mineral nutrients such as nitrogen and
phosphorous (components of proteins and nucleic acids). Water transport is driven by a gradient between the
concentration of water in the soil and the concentration of water in the atmosphere. A column of water is drawn
up by the xylem by the evaporation in the leaves. As it reaches the leaf, water from the xylem in the stem enters the
xylem in the leaf vascular bundles. From there the water molecules move to the spongy parenchyma cells and
evaporate into the intercellular spaces between the spongy parenchyma cells. Air movement carries water vapor
away and maintains the gradient between soil and atmosphere.
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The stoma (plural stomata), are pores in the leaf surface. Stomata allow the plant to exchange gases (oxygen and
carbon dioxide, and transpire, give off moisture (water vapor). This is the mechanism of plant thermoregulation.
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Stomata regulate water loss. A pair of guard cells control the pore’s opening. Water status in cells signals guard
cells to open or close. When the plant is under water stress, a signal is sent from the roots to the guard cells to
close, thus preventing transpiration and water loss. The presence or absence of light also regulates the stomatal
response. Recall that photosynthesis occurs in light – in the morning, a photoreceptor molecule acts as sensor,
which sends a signal to the guard cells to open, therefore allowing carbon dioxide to enter the leaf. At night, the
opposite process occurs.
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An interesting phenomenon occurs when the air is still. The boundary is a thin air layer adjacent to the leaf on
both sides.
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When this air is still, that is, there is no wind, this air layer offers a high resistance to transpiration. Water loss is
reduced. However, if temperatures are too high, these trapped water molecules causing overheating.
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Wind disturbs the air boundary layer, offering low resistance. Water molecules are increasingly driven by the large
humidity gradient, and water loss is increased. This is the reason why plants become quickly dehydrated in high
winds.
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Plants which evolved in desert areas, with low humidity, and desiccating winds, develop a thick cuticle, which
prevents water loss through the leaf surface. The cuticle is waxy outer layer of the epidermal cells. On some
plants, such as the Aucuba, the thick cuticle gives a shiny appearance to the foliage.
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Air movement across the leaf surface intensifies the water loss gradient. As more water is lost, more nutrients
from the soil solution are drawn up with the water entering the roots. Therefore, air movement drives nutrient
uptake and distribution in the plant.
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Let’s look more closely at the root’s internal structure. It is similar to stems. Older roots have xylem, phloem and
cambium. Vascular tissues, embedded in the cortex, are in the center. In fine roots, closer to the tips are root
hairs, extension of epidermal cells. They serve as the interface between the root and the soil. The water moves
from the root hairs and through the cortex to reach the vascular tissues. In old woody plants, the xylem is simply
dead cells called vessel elements that serve as the duct system through which the water moves. As water moves
inward, it transports with it nutrient elements that are involved in food production and in all sorts of chemical
reactions in the plant.
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The root hairs are very important for the absorption of water and mineral nutrients but they are very fragile. Once
damaged, the root hair is forever lost. New epidermal cells with new root hairs must take its place.
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A thin layer of water coating the root hairs must always be present to ensure constant contact between the plant
and the soil particles/soil solution.
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What happens when plants wilt? The top photo shows epidermal cells from petals of a red flower. The red color is
due to a pigment, anthocyanin, dissolved in the vacuole. Notice that in the normal cells the vacuoles take up the
entire space between cell wall, this is because they are turgid. Under drought, the vacuole contracts and becomes
flaccid, pulling away from the cell wall. When water becomes available, the vacuole gets refilled and the plant is
turgid once again.
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When the drought persist, the plant cell structure, which uses water to maintain cell turgidity, eventually collapses
irreversibly. This point of no return is called the “permanent wilting point” or PWP. Plants cannot recover from
that.
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In addition to water, roots need oxygen to respire and make energy. If the air spaces (pores) within the soil stay
filled with water, and do not allow gasses to flow through the soil, the roots will die. Notice that air should take 20
to 30% of the pore space in ideal soil. Roots will not grow into oxygen-depleted soil. This is the reason why house
plants grown in pots should not be overwatered. Same applies to plants grown outside – roots have a hard time
growing in compacted, heavy clays, because they become easily depleted of air.
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The last environmental influence on plant physiology and plant growth and development that we will discuss is the
nutrient content of the soil. Along with water, plant roots absorb mineral nutrients, such as nitrogen, phosphorus,
potassium, calcium, magnesium, sulfur, and many others. These mineral nutrients enter metabolic processes
which incorporate them into organic molecules, such as pigments, enzyme cofactors, amino acids, lipids, nucleic
acids, etc. This process is called mineral assimilation. The nutrients come from three sources - soil clay particles,
soil organic matter, and added fertilizer (liquid or solid). Once in the soil solution, nutrients are absorbed by the
plant root and transported to various cells throughout the plant body.
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There are sixteen elements known to be essential for plant growth. These include the atmospheric elements carbon, hydrogen and oxygen; the primary plant nutrients - nitrogen, phosphorous and potassium; the secondary
plant nutrients - calcium, magnesium and sulfur; and the minor or micro-nutrients required in small quantities but
still essential. These include iron, copper, zinc, boron, manganese, molybdenum and boron. All of these sixteen
nutrients are essential for some life process in the plant, and no other nutrient element can substitute for them.
Each plays a unique role in the life processes of the plant.
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Nitrogen, for instance, is an essential component of proteins, amino acids, chlorophyll and many organic acids.
Phosphorous is heavily involved in the energy producing compounds in the plant and it plays an important role in
nucleic acid manufacture. Sulfur is a component of many proteins and other organic compounds.
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This ornamental cabbage is showing typical symptoms of nitrogen deficiency, lower leaf yellowing and leaf drop.
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Certain crops, such as New Guinea Impatiens, are sensitive to too high nitrogen. In this case, the upper leaves of
the plant roll up.
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Without calcium the plant would not be able to stand upright. Calcium is a key player in calcium pectate formation
which makes cell walls rigid and strong. Iron is a component of many enzymes that drive chemical reactions, and
magnesium is a component of the chlorophyll molecule. Without magnesium, there would be no chlorophyll to
drive photosynthesis. Each of the trace elements also serves some essential function in various chemical reactions
within the plant. When one of the trace elements is deficient, the plant often becomes anemic looking.
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This is the chemical structure of the chlorophyll molecule. Note the key role magnesium plays in the formation of
this molecule. Nitrogen, carbon, hydrogen and oxygen are also key ingredients. Much of the hydrogen and oxygen
comes from water, and much of the carbon comes from atmospheric CO2.
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Symptoms of Mg deficiency in Gerber Daisy, lower leaves with interveinal chlorosis.
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Enzymes need co-factors (metal ion activator) in order to function properly. The diagram shows a
carboxypeptidase enzyme with its cofactor, zinc. Trace or minor elements are essential in many chemical
reactions.
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Soil pH affects nutrient uptake. Too high or too low of a soil pH may affect nutrient uptake, stress the plant, and
reduce the plant’s chances of surviving more than one year. Most plants prefer a pH between 5.2 and 6.7, which is
to say, slightly acidic. This ‘goldilocks’ range assures that two important micronutrients iron (Fe) and manganese
(Mn) are being absorbed by the plant roots in optimal concentrations. If pH is below 5, both of these elements can
interfere with the uptake of other nutrients, while if the pH is over 7, Fe and Mn become deficient. Symptoms
include yellow (chlorotic) tips and young foliage. The shadowed box shows pH range in which mineral nutrient
uptake is optimal.
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Geranium with chlorotic lower leaves which quickly turn necrotic: cause is Mn and Fe toxicity due to low pH.
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Petunia showing interveinal chlorosis in upper leaves caused by Fe deficiency due to high soil pH.
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When we fertilize we have to be aware of the following. Fertilizers are salts - apply too much and you’ll burn the
roots! Remember that once the root hairs are damaged, they will not recover.
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In addition, water leaves the leaves via stomates, but the mineral elements cannot escape. The effect is most
pronounced at leaf margins where air movement causes the greatest evaporation. This Fall Mum leaf is showing
too high nutrient concentration which has caused marginal leaf burn.
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One has to be very careful about fertilizing seedlings and young plants; they need a quarter to one-half of the dose
of nutrients, normally given to mature plants. And this applies to liquid feed fertilizer, organic fertilizer, and slowrelease fertilizer.
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Let us look at all the information we talked about and sum it up. There is a law in ecology, known as the Law of the
Minimum. It states that plant growth progresses to the limit imposed by the factor in least relative supply. What
this means is that if any environmental factor, such as light, water, or mineral nutrient, is deficient, it does not
matter that all other factors are optimal – the plant will grow only as much as the level of the limiting factor allows.
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