'Plant That Ate the South' Boosting Carbon Pollution

A plant called “the scourge of the South” has a new strike against it. Recent research shows that the impact of the invasive species in question, kudzu, is more troublesome than had been previously thought. When it takes over ecosystems, this invader causes soils to surrender their carbon and release it as greenhouse gas.

Alien invader
Kudzu is one of the most impressive invasive species in the world. Introduced to the US as a handful of plants in 1876, this invader now occupies over 3m hectares of land in the US, largely in the southeast of the country. It is estimated to be “consuming” land in the USA at a rate of 50,000 hectares per year to this day.


If anything could be said to grow like a weed, it is kudzu. It grows at an impressive rate of up to a metre every three days. The plant moves like a wave, smothering everything in its wake – trees, utility poles and even buildings.

It is a member of the legume family of plants – like beans – and grows in a vine-like manner, laying down roots whenever it comes into contact with the ground. Originally introduced as an ornamental plant and then for livestock feed and erosion control, it has since overrun entire ecosystems, destroying native long-needled pine forests, woodlots, and grasslands alike

In addition to the damage it inflicts by overwhelming other plants, kudzu has indirect effects as well. Most notably, it carries the “kudzu bug”. This foul-smelling insect is also an invasive species. Unfortunately, the kudzu bugs' taste extends beyond its namesake plant, and includes other legumes, such as beans grown for human consumption. This means kudzu’s impact is not only native ecosystems, but agricultural productivity as well.

Kudzu’s direct and indirect cost to the US economy is estimated to be in excess of US$500m annually. That cost may be set to increase. Rising temperatures and lengthened growing seasons in the northernmost front of the kudzu’s range are creating a welcoming environment for further invasion. Where it was once restricted to south-eastern states, Kudzu is now found in more northerly states, including New Jersey and Ohio.

New research suggests that kudzu’s negative impact may extend beyond that already documented. Its invasion may also be contributing to the rise in global greenhouse gases, by altering soil composition.

What lies beneath
Soil holds a phenomenal amount of carbon. In fact, there is more carbon stored in soil than in the atmosphere and in terrestrial plants, combined. Soil carbon comprises roots from plants, dead matter and waste from plants and animals, and a vast population of microbes. Together they are known as soil organic matter. Much of this comes from plants – mainly dead leaves – but also from dead roots, as well as stems, branches, and tree trunks that have fallen to the ground.

The carbon in the organic matter largely stays locked away in the soil, like an enormous reservoir. Over time, carbon is released as greenhouse gases – carbon dioxide and methane – when the matter is degraded by soil microbes. The extent to which carbon is determined by its susceptibility to microbial degradation.

The problem with kudzu is that it changes the rate at which carbon remains locked away in the soil. It changes the degradation rate of the organic matter.

n a paper published in the journal New Phytologist, plant ecologist Nishanth Tharayil and graduate student Mioko Tamura, of Clemson University, show that kudzu invasion results in an increase of carbon released from the soil organic matter into the atmosphere. Tharayil and Tamura investigated the impact of a kudzu invasion in native pine forests. They found that the invasion actually increased the amount of leaf material contributed to the soil, but, despite this, soil carbon decreased by nearly a third in those forests.

Tharayil and Tamura attribute the release of carbon from kudzu-invaded forests to the fact that kudzu adds material to the soil that is susceptible to degradation relative to that produced by pine. Simply put, kudzu leaves and stems are easy for microbes to degrade, pine needles and stems are not. This means that carbon is locked in with waste from pines; whereas, it gets released by kudzu.

When kudzu invades, its leaves, stems, and roots become the major plant contributors to the soil organic matter, replacing pines' contribution. This has a three-fold effect. First, over time, the hard-to-degrade pine matter decreases in abundance. Second, the easy-to-degrade kudzu matter actually encourages the degradation of the pine matter. That is, kudzu material “primes” the soil microbes to be more effective at degrading the plant material in the soil, including that previously contributed by pines. Finally, after invasion, the kudzu matter is simply more rapidly degraded itself. The net result of these three effects is that plant material is more rapidly degraded – it doesn’t persist like it did in the pine forests.

The south will rise again?
The impact of kudzu invasions on the release of former pine forests could be substantial. Tharayil has estimated that kudzu invasion might cause the release of 4.8 tonnes of carbon per year. This is the equivalent of the amount of carbon stored almost 5m hectares of forest, or the amount of carbon released by burning 2.3m tonnes of coal annually. That is approximately the same as the annual carbon footprint for a city of 1m in that part of the world.

The release of this amount of carbon into the atmosphere, as carbon dioxide, could itself contribute to global warming. This could create a snowball effect, as elevated temperature would enable kudzu to extend its range to more northern latitudes.

Not all news from Tharayil and Tamura is bad. They also looked at the impact of the invasion of another noxious weed, knotweed, on old fields. They found that knotweed, resulted in a net increase in carbon locked away in the soil. This is not to say that allowing knotweed to run rampant is the solution to kudzu’s carbon-releasing menace. Instead, the findings point to the fact that plant composition in different ecosystems could actually be managed to reinforce carbon retention in the soil, and prevent carbon release into the atmosphere.

In the meantime though, we are going to have to find a way to restrain the plant that ate the south, before it loads our skies with more carbon.

Dino-Killing Impact Remade Plant Kingdom, Too

In a matter of days, perhaps hours, a rare corpse flower will bloom in upstate New York. True to its name, the plant is expected to unleash a stench like rotting flesh.

Affectionately called "Wee Stinky," this corpse flower lives in a greenhouse at Cornell University in Ithaca, New York. Horticulturists at the school, who have been preparing for the plant to bloom for weeks, say it could open up any day now. Those curious can watch the rare spectacle online, mercifully, without the smell.
The species, also known as titan arum, is found in the rainforests of central Sumatra. The plant's bloom is just as short as it is pungent; corpse flowers only remain open for 24 to 48 hours before they wither away.

Wee Stinky had been dormant for more than two years, but last month, it became clear that the plant was ready to bloom again, according to Cornell's titan arum blog. The corpse flower started growing quickly. As of this morning, it measured more than 6 feet tall (1.8 meters). On Oct. 23, the plant wasn't even 2 feet tall (0.6 m).

It's hard to predict the exact day a titan arum will bloom, but Cornell's experts wrote that Wee Stinky's growth will slow and its outer layers will start to peel away right before it opens. To provide a sense of what this year's brief bloom might look like, Cornell has put together a time-lapse video of the first (and last) flowering of Wee Stinky. That bloom began on March 18, 2012, and lasted less than 48 hours. More than 10,000 visitors flocked to the greenhouse over five days to catch a glimpse (and perhaps a whiff) of the titan arum.
The open bloom may look like a single giant flower, but technically, it isn't. The plant's purple "petals" actually make up an outer skirt called a spathe, and the tubelike spike at its center is called a spadix. These structures have thousands of little flowers called an inflorescence.

Smelling like death actually helps this plant species survive. The fetid stench lures important pollinators like flesh-eating beetles and flies. The spadix heats up at the beginning of the bloom — becoming as warm as a human body — to help spread the odor.

Horticulturists at Cornell acquired a year-old seedling in 2002 that grew into Wee Stinky. A corpse flower may not bloom for the first time until it is about 10 years old. But after that, it could open up again every few years.

The bloom is a research opportunity for scientists at Cornell. Sensors on and above the blooming plant will collect data on the temperature and the volatiles that simulate the cues of a rotting corpse and attract pollinators.

Currently, Cornell's Kenneth Post Laboratory Greenhouses are open to the public from 9 a.m. to 4 p.m. EST, but visiting hours will be extended once the flower blooms.

Dino-Killing Impact Remade Plant Kingdom, Too

The killer meteorite that extinguished the dinosaurs also torched North America's forests and plants. The harsh conditions after the impact favored fast-growing flowering plants, nudging forests toward a new pecking order, a new study reports.

As a result, today's forests would baffle a Brachiosaurus. Most of the slow-growing trees and shrubs munched by dinosaurs are minor players in modern forests, because the plants couldn't adapt to post-impact climate swings, researchers report today (Sept. 16) in the journal PLOS Biology.

"When you look at forests around the world today, you don't see many forests dominated by evergreen flowering plants," lead study author Benjamin Blonder said in a statement. "Instead, they are dominated by deciduous species, plants that lose their leaves at some point during the year."

Dinosaurs stomped through forests ruled by evergreen angiosperms, which never drop leaves. Angiosperms are flowering plants, grasses and trees, excluding conifers like spruce and pine. The dinosaur-era angiosperms included ancient relatives of holly, rhododendrons and sandalwood. Other plants in the ancient forests included beeches, cycads, gingkoes, ferns and palm trees.

Fossil records show that angiosperms of all kinds thrived before a meteorite or asteroid crashed into Earth 66 million years ago. That stupendous blast charred vast woodlands that had grown from Canada to New Mexico. In North America, about 60 percent of plant species went extinct, according to earlier studies.

After the blaze, deciduous angiosperms, which drop their leaves seasonally, bounced back much better than the evergreens.

Blonder, an ecologist at the University of Arizona in Tucson, wanted to know why the deciduous angiosperms outcompeted their evergreen cousins during the cold, dark years after the impact (called an impact winter). The researchers pored through thousands of prehistoric leaves from Wyoming's Hell Creek Formation. The fossilized leaves spanned the impact, from the last 1.4 million years of the Cretaceous Period through the first 800,000 years of the Tertiary Period.

Based on their analysis, the researchers said the properties of the plant leaves likely helped them withstand the bleak climate. The impact winter pushed ecosystems toward plants with faster growing strategies, Blonder told Live Science in an email interview. "Leaves represent a drain on a plant's resources when photosynthesis can't occur. Thus, deciduous species should be favored over evergreen species," he said.

The researchers analyzed leaf mass per area, which indicates how much carbon a plant invests in growing a leaf. "[This] tells us whether the leaf was a chunky, expensive one to make for the plant, or whether it was a more flimsy, cheap one," Blonder said. The scientists also looked at leaf vein density, a measure of how fast a plant takes up carbon.

"Our study provides evidence of a dramatic shift from slow-growing plants to fast-growing species," Blonder said. "This tells us that the extinction was not random. And, potentially, this also tells us why we find that modern forests are generally deciduous and not evergreen."

How Plants Affect the Global Carbon Cycle

This ScienceLives article was provided to Live Science's Expert Voices: Op-Ed & Insights in partnership with the National Science Foundation.

The interplay of plant communities and the processes that influence their evolution fascinate Caroline Farrior, who, as a postdoctoral fellow at the National Institute for Mathematical and Biological Synthesis, builds mathematical models to better understand and predict plant behavior. Of particular interest to her is how wind storms, drought and other rare environmental disturbances affect forests. Farrior wants to know how these events shape plant communities and how plants respond in the absence of disturbances, when they have already expended effort to prepare for these rare events. With this understanding, Farrior aims to help ecologists make better predictions about how climate change will affect plant communities in the future.

The National Science Foundation: What is your field and why does it inspire you?

Caroline Farrior: I am a plant ecologist. I study how plants interact as individuals and as species. They have thousands of years of evolutionary history shaping their genetic makeup, yet any one individual finds itself only in a particular and new environment in competition with specific individuals and species. Thinking about the influence of the depth of time on the individual plants in communities in front of me is absolutely inspiring.
NSF: Please describe your current research.

C.F.: The effects of rare climatic events like drought and heavy winds have been historically difficult to study, precisely because of their rarity. These events nonetheless seem to play a fundamental role in shaping plant community composition and forest structure. Strategies that prepare plants for rare events can be costly in terms of growth and fecundity in the absence of the disturbance. I am currently developing the mathematical tools needed to examine the interactions between rare disturbances and competition among individuals, their influence on population dynamics, and ecosystem level properties.

NSF: What is the primary aim of your research? / What is your primary professional goal?

C.F.: Currently, our best estimates show that plants are taking up about one quarter of the carbon emitted by humans  into the atmosphere. Plants make their bodies out of carbon. When there is more carbon in the atmosphere they can take up more of it, scrubbing, slowing the rate of increasing atmospheric carbon dioxide. However, we are not sure whether we will be able to count on plants to continue to do this in the future. For instance, we are not sure whether plants will become limited by other essential resources or how the changes in carbon storage of plants may interact with other changes across the globe, including increasing temperatures, changing rainfall regimes, and more frequent extreme climatic events. My goal is to build an understanding of the role of plants in the global carbon cycle so that as scientists we may be able to predict the path of climate change accurately.

NSF: What is the biggest obstacle to achieving your objective(s)?

C.F.: Ecology is a young science. We are still working out many very basic components of plants. At the same time, because of today’s pressing environmental issues, we are asked to answer many high level questions and apply our knowledge to solve today’s problems. Many of these questions are not a natural next step from our established knowledge, but they still need attention. To make real progress, there is a delicate balance that must be struck between doing the needed fundamental research and the applied work that updates our understanding for policymakers.

NSF: How does your work benefit society?

C.F.: I work toward a better understanding of the role of plants in the global carbon cycle . With this understanding, as scientists, we will be able to more accurately predict the pace of climate change in the future. With better predictions of the rate of climate change, politicians are more likely to be able to write and pass effective legislation to mitigate climate change.

NSF: What do you like best about your work?

C.F.: I love the feeling of understanding something complex. When the pieces come together from field observations or experiments with a prediction from a model, understanding can suddenly become clear. The best results are those that seem so obvious and simple after discovering them.

NSF: What would your Tweet say about your work?

C.F.: Interestingly, trees engage in game theory, with their investment in fine roots in competition for water and nitrogen, and wood in competition for light.

NSF: What is the best professional advice you ever received?

C.F.: Work on questions that interest you! This advice can never be said enough. If you are genuinely interested in the science you do, everything comes easier and the work is more fun. Let your curiosity drive you!

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NSF: What exciting developments lie in the future for your field?

C.F.: Plant ecologists have studied the aboveground biomass of individuals and their species for over a century now. But what goes on below ground is still largely a mystery. With new and more cost-effective technology for studying the identity of specific root fragments and the community composition of microbial symbionts, we are beginning to see below ground of plants with much greater resolution.

NSF: What do you do when you’re not in the lab or out in the field?

C.F.: When I’m not working, I like to get outside and go exploring. I’ll go hiking and see some new waterfalls or mountain views. Or I’ll go kayaking and try to clear my head while paddling on a quiet lake.  

Marijuana May Trigger Allergies in Some People

Just like ragweed and birch trees, marijuana plants may trigger allergic reactions in some people, according to a new review of previous studies.

And because of the increasing use and cultivation of marijuana that has followed in the wake of legalization in some places, allergies to marijuana may be on the rise, experts say. People can be allergic to the plant's pollen, or to its smoke.

"Although still relatively uncommon, allergic disease associated with [marijuana] exposure and use has been reported with increased frequency," wrote the authors of the review, published March 3 in the journal Annals of Allergy, Asthma & Immunology.
n fact, allergies to marijuana have likely gone underreported, because of marijuana's illegal status, said Dr. Purvi Parikh, an immunologist of the Allergy & Asthma Network, a nonprofit organization that promotes allergy research and education.

"Now as the prevalence [of marijuana use] is increasing, and with the legalization in many states, it is going to become increasingly more common, and all these cases will surface that were not recognized before," Parikh said.

People who are allergic to the marijuana plant's pollen or smoke may get symptoms such as a runny nose, inflammation of the nasal passages, and coughing and sneezing, according to the review. Some people who have touched marijuana have developed hives, and itching and swelling around the eyes. There have also been reports of asthma triggered by exposure to its pollen, according to the review.

One patient who ate hemp seed-encrusted seafood experienced the severe allergic reaction called anaphylaxis, which affects the whole body and is potentially fatal, according to the review. (The man's doctors later excluded the seafood as the cause of his allergic reaction through testing.) In another case mentioned in the review, a person who used marijuana intravenously also experienced anaphylaxis.

Some people have experienced allergic reactions while handling marijuana at work, according to the review. A bird breeder developed allergy symptoms after feeding hemp seeds to his birds, and one unlucky medical marijuana grower, who previously was able to smoke pot recreationally, suddenly developed hives and itching after handling the plant.

For some marijuana users, it is not only the plant itself that may cause an allergic reaction. Pot can become very moldy when it is being stored, and people who are allergic to mold may have reactions, Parikh said.

Some people could even experience reactions to both the plant and mold, as many people with allergies are allergic to multiple substances, she said.

The review pointed out two studies conducted decades apart in Omaha, Nebraska, where the Cannabis plant grows widely and is cultivated commercially. In the studies, researchers looked at how common cannabis allergies were among people in the area. In the first study, published in 1940 in the Nebraska Medical Journal, researchers found that 22 percent of 119 patients with allergy symptoms were allergic to hemp pollen.

In the later study, published in 2000 in the journal Annals of Allergy, Asthma & Immunology, investigators found that 61 percent of 127 patients with allergies in Omaha were allergic to hemp, according to the report.

People who live in areas where large quantities of marijuana plants are grown may be particularly prone to experiencing allergic reactions to the pollen, Parikh said. "The quantity makes a big difference in the prevalence of allergies."

Smells Fishy: Putrid 'Corpse Flower' Blooms

He place smells like death, but that didn't keep crowds away from the UC Botanical Garden in Berkeley, California, this weekend.

More than 2,300 visitors queued up on Saturday (July 25) to meet Trudy, an enormous "corpse flower" that was in bloom. Corpse flowers (Amorphophallus titanum, which means "giant, misshapen penis") burst into enormous purple-and-yellow blooms only once every few years. But it's not the sight that attracts attention — it's the smell. These flowers get their name from their scent, which is reminiscent of rotting flesh.

"It's very difficult to describe the smell," Paul Licht, UC Botanical Garden director, said in a statement. "I've been saying for years that it smells like a large, dead mammal — a rat or a dog or a cow. Other people say it smells like dead fish."
The corpse flower, or Titan Arum, is native to Sumatra, Indonesia. The plants are rare, and they're threatened by rainforest destruction, according to the Biological Sciences Greenhouse at The Ohio State University.

Corpse-flower blooms are typically about 8 feet (2.4 meters) tall. Their stink is tailor-made to attract flies and other carrion-eaters that act as pollinators for the plant. Bizarrely, the yellow center of the bloom — called the spadix — actually heats up to help spread the smell. On the first night of blooming, the spadix warms to about 98 degrees Fahrenheit (36.6 degrees Celsius), according to the Biological Sciences Greenhouse.

Berkeley's Trudy is a bit of a pip-squeak, with a bloom only 4.5 feet (1.4 m) tall. Building such a large bloom takes a massive amount of energy, Licht said, which is why corpse plants muster up a flower only every few years. The blooms last mere days.

The rest of the plant's life cycle is fairly subtle. According to the Biological Sciences Greenhouse, the only visible part of a Titan Arum during the first year and a half of its life is a small leaf aboveground. The plant's belowground tuber grows during this time, after which the leaf dies and the plant goes dormant for up to six months. This cycle of growth and dormancy continues multiple times, with the leaf and tuber getting bigger each time. Finally, the plant will put out a bloom, sometimes up to a decade after it first sprouted.

It's very challenging to predict when a corpse flower will bloom, Licht said, adding that he has never been spot-on about when one of the UC Berkeley plants will flower. Part of the reason is that corpse flowers have not been studied much in the wild, he said.

"Sadly, we don't have very much information on these from the habitat of Sumatra," he said. "We don't know how many are left. We don't know how long they live. We don't know how old they have to be before they bloom. We don't know how often they bloom. We don't know what time of year they bloom."

As of Monday (July 27), Trudy (actually a male plant, according to the UC Botanical Garden) was wilting and no longer emitting its signature stink. Researchers at the Garden have been collecting pollen from the plant for study. Visitors can still see the wilting flower during normal operating hours.

'Corpse Flower' Blooms in Denver: Howch Live to Wat

DENVER – The first-ever bloom of a stinky "corpse flower" in the Rocky Mountain region is happening here today (Aug. 19).

The corpse flower, or titan arum, is famous for its rare-but-enormous blossoms. Blooms usually stand about 8 feet (2.4 meters) tall. They burst open on an unpredictable cycle that can stretch for more than a decade between flowers. The plant's scientific name, Amorphophallus titanum, means "giant, misshapen penis," hinting at the appearance of this bizarre plant.

But what really makes corpse flowers famous is their stench. The plants smell like rotting flesh, all the better to attract the carrion beetles and flies that carry the flowers' pollen. Each bloom lasts only about 48 hours, after which the plant goes dormant and may not bloom again for another seven to 10 years.

Science

Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe.[nb 2] In an older and closely related meaning, "science" also refers to this body of knowledge itself, of the type that can be rationally explained and reliably applied. Ever since classical antiquity, science as a type of knowledge has been closely linked to philosophy. In the West during the early modern period the words "science" and "philosophy of nature" were sometimes used interchangeably,[2]:p.3 and until the 19th century natural philosophy (which is today called "natural science") was considered a branch of philosophy.

In modern usage "science" most often refers to a way of pursuing knowledge, not only the knowledge itself. In the 17th and 18th centuries scientists increasingly sought to formulate knowledge in terms of laws of nature. Over the course of the 19th century, the word "science" became increasingly associated with the scientific method itself, as a disciplined way to study the natural world, including physics, chemistry, geology and biology. It is in the 19th century also that the term scientist began to be applied to those who sought knowledge and understanding of nature.

Modern science is typically subdivided into the natural sciences which study the material world, the social sciences which study people and societies, and the formal sciences like mathematics. The formal sciences are often excluded as they do not depend on empirical observations. Disciplines which use science like engineering and medicine may also be considered to be applied sciences.

What are the functions of roots in plants and how these help in the uptake of water and salts?

FUNTIONS:

Functions of roots in plants

Roots perform the following functions in plants:

         i.            These anchor the plants in soil.

       ii.            These absorb water and salts from soil.

      iii.            These provide conducting tissues for disturbing these substances to the tissues of the stem.

For better understanding of uptake of water and salts, the internal structure of root should be taken into account.

Anatomy of Root

The centre of the root in most of the cases is occupied by vascular tissue, The xylem composed of conducting elements, the Tracheids and vessels occupies the centre of the root is continuous with the xylem tissues in the stem. The phloem tissue is closely associated to the xylem tissue. The xylem and phloem elements are surrounded by layer of living cells,, the Pericycle. The vascular tissue and the epicycle form a tube of conducting cells called stele. Just outside the stele is a layer of cells called endodermis. This endodermis acts as watertight jacket around the conducting vascular elements because water with its dissolved substances cannot pass around the endodermal cells via their walls.

Outside the endodermis, several layer of large thin walled living cells with intercellular spaces among them are present. This is called as cortex. The air spaces form interconnected air channels necessary for internal aeration. The cell wall of cortical cells are highly permeable to water and its dissolved solutes. The cortex is surrounded by a layer of almost flattened cells. It is epidermis. Some epidermal cells develop long projections called root hairs that extend out among the soil particles around the root. The root hairs increase soil-root contact and enhance water absorption and the volume of soil penetrated.

Uptake of Water and Salt

Root hairs provide large surface area for absorption. The cytoplasm of the root hairs has higher concentration of salts than the soil water, so water moves by osmosis into the root hairs. Salts also enter root hairs by diffusion or active transport. After their entry into the root hairs, water and salts must move through the epidermis and cortex of the root and then into the xylem tissue in the centre of the root.

What are the Apoplast and Symplast water pathways?

There are two pathways through which water travels from the outside of the root to the inside. These pathways are as follows:

(i)                  Apoplast Pathway    (ii)   Symplast Pathway

• 1)      Apoplast pathway

Interconnected walls and water filled xylem elements should be considered a single system, which is called Apoplast. When water travels along cell walls and through intercellular spaces to reach the core of the root then we call this pathway as Apoplast pathway.

• 2)      Symplast Pathway

The rest of the plant living part (other than Apoplast) is termed as Symplast. In the Symplast pathway, water moves through Plasmodesmata.

 (rod like connections or bridges by which cytoplasm of the neighbouring cells is linked with each other).

What are different types of transpiration?

Types of Transpiration

There are three types of transpiration:

        i.            Stomatal Transpiration

Evaporation of water through stomata is called stomatal transpiration. More than 90% of water is lost through the stomata although stomata openings surface. Stomatal transpiration involves two processes:

• a)      Evaporation of water from cell wall surface bordering the inner cellular. Spaces, or air spaces of the mesophyll tissue.
• b)      Diffusion of the water vapours from the intercellular spaces into the atmosphere by way of the stomata.

      ii.            Cuticular Transpiration

The loss of water as a vapour, directly from the surface of leaves of leaves and herbaceous stems through the cuticle is called Cuticular Transpiration. Only a small fraction of water is lost by Cuticular Transpiration.

    iii.            Lenticular Transpiration

The loss of water through the lenticels in the bark is called Lenticular Transpiration. Lenticels are small openings present in the bark.

What is Transpiration? Why it is said be a necessary evil?

The loss of water by evaporation from a plant surface is called transpiration.

Over 90% of water escapes through the open stomata, while about 5 is lost directly from the epidermal cells. The combined area of stomatal pores is on average only 1-2% of the total leaf surface.

Transpiration rates are greatest when the leaf cells are fully turgid, stomata are open and relative humidity in the atmosphere is low.

Transpiration_Anessary Evil

Upward movement of water in plants is attributed to two processes:

         i.            Root pressure    (ii)   Transpiration

1.       Root Pressure

Root pressure is capable of moving water upward in a plant, but not in the quantity and to the heights necessary for most plants. So we are left with the hypothesis that water is pulled up through the plant body due to transpiration.

2.       Transpiration

Although water is used in the maintenance of turgidity and the possible translocation of dissolved minerals, water use in plants is inefficient and can endanger their survival. So water loss by transpiration becomes necessary because of these and some other reasons (cooling effect by evaporation) it is said that transpiration is a necessary evil.

The opening and closing of stomata regulates the transpiration. What is its mechanism?

Most plants keep their stomata open during the day and close them at night. The regulation of transpiration through stomata depends upon guard cells. Each stoma is surrounded by two guard cells, which are attached to each other at their ends. The inner concave sides of guard cells are thicker than the outer convex sides.

Mechanism

Initially, it was thought that concentration of glucose in guard cells is responsible for opening and closing stomata. When guard cells become turgid, their shapes are like two beans and stoma between them opens. When the guard cells loose water and become flaccid, their inner sides touch each other and the stoma closes.

Recently, it is has been revealed that opening and closing of stomata depends upon the movement of Potassium ions in and out of guard cells. The blue wavelengths of daylight cause the K⁺ to flow into the guard cells, from the surrounding epidermal cells. Water passively follows these ions into the guard cells. The guard cells become turgid and open. During the night time, the K⁺ flows back to the surrounding epidermal cells, which also lead to loss of water. Guard cells become flaccid and stomata close.

Define the term Water Potential

Water molecules posses’ kinetic energy, which means that in liquid or gaseous form thy move about rapidly and randomly from one place to another So greater the concentration of the water molecules in a system the greater in the total kinetic energy of water molecules. This is called water potential.

Water always moves from an area of higher potential to an area of lower water potential. The relationship between the concentration of solute and water potential. The relationship between the concentration of solute and water potential is inverse i.e. where there is a lot of solute the water potential is low.