Discovery, Symbol, Properties,
Uses, & Facts
Plants
Any of the eukaryotic organisms that are photosynthetic and with a rigid cell wall.
Plant Definition
(botany) Any of the eukaryotic organisms of the biological kingdom Plantae, characterized by being photosynthetic and having a rigid cell wall. Etymology: from Latin planta (“sprout, shoot, cutting”).
Plant Characteristics
A plant refers to any of the eukaryotes that belong to the biological kingdom Plantae. Plants, in the strictest sense, are embryophytes that include vascular plants, liverworts, hornworts, and mosses. Some references that are less strict considered green algae as plants. The green algae are comprised of unicellular and multicellular species that have chloroplasts and cell wall. The fundamental characteristics listed below focus on the embryophytes. They are as follows:
• Plants are autotrophs. They make their own food through photosynthesis. They are capable of capturing energy via the green pigment (chlorophyll) inside the chloroplast, and using carbon dioxide and water to produce sugars as food and oxygen as by-product. As autotrophs, plants are often placed at the start of the food chain. They are labeled as producers. They serve as food to other organisms, including animals. Animals, in contrast, are heterotrophs and they need to consume other organisms for sustenance. Some animals (particularly, herbivores) depend exclusively on plants while others eat only meat or a mix of animal or plant material. Since plants are capable of making their own food, they do not feed on animals to grow and survive. The exception is a group of carnivorous plants (e.g. Venus flytrap) that catch and feed on animal prey, especially when conditions are less favourable for photosynthesis.
• Plants are eukaryotes. Similar to animals, plants have distinct, membrane-bound nucleus inside the cell. The nucleus is an organelle that contains chromosomes that bear genes. Other organelles suspended in the cytoplasm of a plant cell are Golgi apparatus, endoplasmic reticulum, lysosomes, peroxisomes, and plastids.
• Plants have plastids. The presence of plastids inside a eukaryotic cell is an indication that it is more likely a plant rather than an animal. There are different types of plastids. Chloroplasts are plastids containing chlorophyll (green pigments) and involved in photosynthesis. Chromoplasts contain pigments apart from green and involved in the synthesis and storage of pigments. The chlorophyll systems absorb light energy at particular wavelengths of the electromagnetic spectrum. The pigments are also responsible for the coloration of plant structures (e.g. green leaves, red flowers, yellow fruits). Leucoplasts (e.g. amyloplasts, elaioplasts, proteinoplasts) are non-pigmented plastids. Their function is primarily for food storage. Plants store food in the form of sugar, e.g. starch.
• Plants have a large vacuole inside the cell. This cytoplasmic structure is involved in the regulation of turgor pressure.
• Plants have rigid cell walls apart from the plasma membrane. The cell wall confers added structural support to a plant cell. Plants may not have a skeletal system as that in animals but their cell wall is comprised primarily of cellulosic material that aids in providing structural support.
• Plants have a distinctive cell division where a cell plate (phragmoplast) separates daughter cells.
• Plants are not as motile as animals. They do not have the capacity to move from one location to another at will. As such, they have to deal with harsh conditions, such as heat. One of the ways they are able to withstand heat is through their cell walls that prevent their body from drying up. In spite of this, plants do still exhibit movement but in another form. For example, nastic movement is exemplified by the folding of the leaflets of the plant Mimosa pudica when touched and the closing of the leaf of the Venus flytrap when capturing prey. Some plants (e.g. Betula pendula – silver birch) would even droop their branches and leaves at night as if they are “sleeping”. Another form of plant movement is tropism. Tropism, though, is more of growth response to a stimulus rather than a movement. For instance, plants tend to grow towards the source of light (phototropism).
• Plants have plasmodesmata. While animals have cell junctions that hold cells in an animal tissue, plants have plasmodesmata that act as if like cell junctions between plant cells. The cell wall forms these cytoplasmic bridges between adjacent cells. These “bridges” facilitate communication between cells and allow the circulation of fluid, thereby help maintain the tonicity of plant cells.
• Plants are multicellular, being made up of many cells organized into tissues and organs that perform a specific function as a unit. Plant organs are specialized for anchorage, support, and photosynthesis (e.g. roots, stems, leaves, etc.)
• Plants are capable of unlimited growth through meristematic tissues. The tissue is comprised of indeterminate, actively dividing cells that give rise to differentiated tissues such as epidermis, trichomes, phellem, and vascular tissues.
• Plants lack sense organs but they can perceive their surroundings albeit differently. Plants can “see”, “hear”, and “smell” despite the lack of eyes, ears, and nose. They seem to “feel” and respond in ways not as obvious as in animals. Plants may not have a nervous system as that of animals but they apparently have a system of their own based on how they respond to their surroundings. Arabidopsis, for example, despite lacking eyes, possesses photoreceptor (at least 11 types) that help the plant detect light.1 In another example, herbivory could instigate the release of certain chemicals on the affected plant part.2 Plants have also been observed to release defense chemicals that deter herbivores. Tomatoes were observed to release volatile signals to warn nearby plants of an impending attack of herbivores.3
• Plants reproduce by asexual and sexual means. Asexual reproduction in plants is carried out by budding, fragmentation, fission, spore formation, vegetative propagation, apomixis, etc. Sexual reproduction involves male and female gametes that fuse at fertilization. In general, the plant life cycle incorporates alternation of generations, i.e. the alternating phases of sporophyte and gametophyte.
• Plants “breathe”. Through stomata, carbon dioxide from the atmosphere enters the plant cell. Through photosynthesis, carbon dioxide is converted into oxygen, which the plant releases as a metabolic by-product into the atmosphere through the stomata.
• Plants may not have other well-defined biological systems but they produce chemicals involved in plant defense and immune functions and plant hormones that act as signaling molecules.
Plant Body
Embryophytes, in general, have two major organ systems: (1) shoot system and (2) root system. The shoot system includes body parts that are located in the upper portion of the plant whereas the root system consists of body parts found in the lower portion. The shoot system may include plant organs such as stems, branches, leaves, flowers, and fruits. They are often found above the ground. The root system includes roots, tubers, and rhizomes. They are often found underground.
The tissues of plants are:
• Embryonic or meristematic tissues – plant tissues made up of undifferentiated and mitotically active cells. Examples are apical meristem and cambium
• Permanent tissues – plant tissues that consist of differentiated cells. The permanent tissues may be further classified into fundamental (e.g. parenchyma, collenchyma, sclerenchyma) and complex (e.g. phloem and xylem tissues)
• Reproductive tissues – plant tissues that are involved in reproduction. Example is the sporogenous tissues
The cells of the plants are eukaryotic, i.e. with well-defined nucleus. The nucleus contains the chromosomes that bear genes. Apart from the nucleus, the other organelles are endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, and plastids. The plastids may be classified based on the pigments: chloroplasts (with chlorophyll, green pigment), chromoplasts (with pigments apart from green), and leucoplasts (colorless plastids). The large structure inside the plant cell is the vacuole. It is responsible for the regulation of turgor pressure.
The plasma membrane surrounds the cytoplasm where these organelles are suspended. Apart from the plasma membrane, the cell has an additional layer called the cell wall. The cell wall, though, is not exclusive to embryophytes. Other organisms such as fungi, algae, and certain bacteria have cell walls. The cell wall of embryophytes is made up of primary and secondary cell walls. A primary cell wall contains cellulose, hemicelluloses, and pectin. A secondary cell wall is a thicker layer. It is rich in lignin that strengthens and waterproofs the wall. The cell wall has many important roles and one of them is to help resist osmotic pressure.
When a plant cell is placed in a hypertonic solution, water moves into the cell and causes the cell to swell. The presence of the cell wall prevents the cell from bursting during excessive osmosis. Conversely, when a plant cell is placed in a hypotonic solution, water diffuses out of the cell, and turgor pressure is lost causing the cell to become flaccid. Further loss of water will result in plasmolysis, and finally to cytorrhysis, the complete collapse of the cell wall.
Apart from osmoregulation, the fundamental physiological processes that plants carry out include photosynthesis, respiration, transpiration, tropisms, nastic movements, photoperiodism, circadian rhythms, seed germination, and dormancy.
Plant Genomics
Plant Life cycle
The life cycle of plants is comprised of two generations: gametophyte generation and sporophyte generation. The alternating phase of diploid and haploid forms is called alternation of generations. This is also observed in certain algae such as Archaeplastida and Heterokontophyta. In algae with alternation of generations, the sporophyte and the gametophyte are independent organisms.
In embryophytes, the gametophyte generation is one in which the phase begins with a spore that is haploid (n). The spore undergoes series of mitotic divisions to give rise to a gametophyte. A gametophyte is a haploid multicellular plant form. It would have only one set of chromosomes. The gametophyte phase is the sexual phase in the life cycle and therefore the plant would develop sex organs that produce gametes, which are also haploid. The gametes that participate in fertilization would later enter the sporophyte generation characterized by the plant form that is diploid following the union of gametes.
In tracheophytes (vascular plants), the sporophyte is in a multicellular form and the dominant phase. Thus, the sporophyte comprises the main plant that we see. Conversely, in the bryophytes (e.g. mosses and liverworts), the gametophyte is the dominant, and therefore is the main plant we perceive.
In general, the life stages of tracheophytes start from a seed that develops into a scion when conditions are conducive for growth. The scion grows by producing leaves and growing stems and branches. It develops into an adult plant that eventually produces flowers. The flowers bear sex cells such as sperm cells in pollen grain and ova in the ovules of the ovary. The union of the sex cells results in a zygote contained inside the seed. Monoecious plants bear both sex cells whereas dioecious plants bear only one type of sex cell.
Plants can also reproduce asexually. They do so by not involving the gametes. By asexual reproduction, new plants arise by means of budding, fragmentation, fission, spore formation, vegetative propagation, and apomixis.
Plant senescence refers to the aging process of plants. For instance, the yellowing of leaves occurs as a result of chlorophyll degradation, thus leaving only the carotenoids, during leaf senescence. However, some plants may continue to form new leaves, such as in deciduous plants.
Plant Ecology
Since plants are capable of photosynthesis, they do not need to hunt or feed on animals for food (with the exception of carnivorous plants). They can manufacture their own food by utilizing energy from light, carbon dioxide from the atmosphere, and water molecules. Nevertheless, one of the sources of carbon dioxide is the waste that animals breathe out during respiration. In return, they give off oxygen as a waste product of photosynthesis. Oxygen is crucial to the survival of aerobic organisms, including animals.
Plants derive other vital nutrients from the minerals dissolved in the soil. They absorb them via their roots. Some of the macronutrients they derive from the soil are calcium, magnesium, nitrogen, phosphorus, potassium, and sulfur. As for micronutrients, plants absorb boron, chloride, copper, iron, manganese, and molybdenum. Thus, dead parts of, or entire, plant leads to their decomposition and the return of essential minerals and compounds to the Earth.
Because of their sense of independence, they are often placed at the start of a food chain. They are the major producers in an ecosystem. Thus, the extinction of plant species can cause a major impact on an ecosystem. The International Union for Conservation of Nature (IUCN)’s Red List of Threatened Species, a system of assessing the conservation status of species worldwide, utilized a system of labeling species based on extinction risk. Accordingly, species may be categorized as: “data deficient”, “least concern”, “near-threatened”, “vulnerable”, “endangered”, “critically endangered”, “regionally extinct”, “extinct in the wild”, and “extinct”. In 2016, IUCN reported 2,493 plants were critically endangered whereas 3,654 plants are endangered.5
Plants interact with other organisms and form symbiosis. Examples are as follows:
• mutualism – e.g. plants providing nectar for honeybees while honeybees help spread plant’s pollen grains
• predation – e.g. carnivorous plants that capture insects and small animals
• competition – e.g. plants that compete with other plants for habitat in terms of available space and nutrients
• commensalism – e.g. plant’ fruits that stick to animal fur for free transport
• parasitism – e.g. parasitic plants that derive nutrients from their host, such as Cuscuta (dodder) that attaches on, and produces haustoria that absorb nutrients from, an acacia tree
In 2011, the Census of Marine Life estimated that there could be around 8.7 million eukaryote species on Earth, and of this figure, about 298,000 was predicted to be the total number of plant species. 215, 644 had already been described and cataloged .6
Plant Evolution
According to the endosymbiotic theory, organelles like plastids and mitochondria represent the formerly free-living prokaryotes. The chloroplasts seem to be related to the prokaryotic cyanobacteria. The basis is the structural similarity between cyanobacteria and chloroplasts. Furthermore, both of them have the same photosynthetic pigments and a single circular DNA molecule as the genome. Apparently, the endosymbiotic events led to the appearance of the first photosynthetic eukaryotes one billion years ago. Charophyta (a sub-group of green algae) is believed to be that which the embryophytes emerged from. The charophytes and the embryophytes share many similar traits, e.g. phragmoplast formation during mitosis.
A brief timeline of the evolution of embryophytes is shown below:
• Phanerozoic eon » Paleozoic era » Ordovician period: In the Ordovician period (485 million years to 440 million years ago), the first embyophytes (land plants) appeared.
• Phanerozoic eon » Paleozoic era » Devonian period: In the Devonian period (415 million years to 360 million years ago), primitive plants, trees, and shrub-like forests dominated the land and provided new habitats for terrestrial animals. The early seed fern Elkinsia evolved seeds, particularly in the late Devonian period.
• Phanerozoic eon » Mesozoic era: This era spanned from 252 million to 66 million years ago. In the Triassic (approximately 200 million years ago) the flowering plants appear.
• Phanerozoic eon » Cenozoic era: This era called the “new life” is the most recent geological era that spans from 66 million years ago to the present day. During this era, from around 40 million years ago, the grasses appeared. These plants and many other plant groups evolved a new mechanism of metabolism to survive the low CO2 and arid conditions of the tropics.
Recommended Source: Darwin Reviews – the Journal of Experimental Botany’s most prestigious review series and topics that are carefully chosen in the most progressive fields of research.
Plant Taxonomy
The initial definition of plants includes the green algae, fungi, and the embryophytes since they all have chloroplasts and cell wall. However, algae and fungi eventually were moved to their respective kingdoms.
In the narrowest sense, plants (i.e. Plantae sensu strictissimo) are those that basically are multicellular, with cell walls containing cellulose, and have chloroplasts for photosynthesis. In this case, the kingdom Plantae is comprised of embryophytes, such as vascular plants, liverworts, mosses, and other fossil plants that share the same features.
Plantae sensu stricto (“plants, in a narrow sense”) includes embryophytes and green algae (Chlorophyta and Charophyta). This is still a widely recognized definition of plants. They make up the clade, Viridiplantae (or Chlorobionta), which is commonly called the green plants. The different divisions of the kingdom Plantae sensu stricto are as follows:
• Chlorophyta
• Charophyta
• Marchantiophyta (liverworts)
• Anthocerotophyta (hornworts)
• Bryophyta (mosses)
• Lycopodiophyta (club mosses)
• Pteridophyta (ferns, whisk ferns, and horsetails)
• Cycadophyta (cycads)
• Ginkgophyta (ginkgo)
• Pinophyta (conifers)
• Gnetophyta (gnetophytes)
• Magnoliophyta (flowering plants)
Significance
Plants are essential to the lives of different organisms since they are the producers in the food chain. They store starch. They also serve as an important source of minerals and compounds.
Plants serve as habitats to certain organisms (e.g. insects and arboreal organisms). They are also the major source of oxygen that the aerobic animals need to live.
Certain plants have medicinal properties. Dandelion (Taraxacum officinale) as a mild laxative, plantain (Plantago major) leaves for reducing inflammation and pain, and burdock (Arctium minus) roots and leaves for alleviating eczema or cracked skin are just a few of the multifarious medicinal plants.
Humans use plants for the manufacture of diverse products such as essential oils, pigments, resins, tannins, alkaloids, amber, waxes, cosmetics, plastics, rubber, varnish, lubricants, inks, and so on.
Wood from plants is used in the construction of buildings, musical instruments, boats, and furniture. It is also used in making paper.
“If we can optimize plants’ natural ability to capture and store carbon, we can develop plants that not only have the potential to reduce carbon dioxide in the atmosphere but that can also help enrich soils and increase crop yields.”
Joanne Chory
Addressing Climate Change – With Plants
Global climate change is an existential threat to plants, animals and people. Too much atmospheric carbon is raising temperatures around the globe, generating deadly storms, catastrophic flooding and persistent droughts. This is not a problem for future generations to solve. We must address it now.
The Salk Institute’s Harnessing Plants Initiative (HPI) offers a bold, scalable solution that can be rapidly implemented. Plants are the original carbon scrubbers, removing CO2 from the atmosphere and storing it in their biomass. Unfortunately, this carbon storage is often temporary. When crops and other plants die and decompose, much of that carbon returns to the atmosphere.
HPI is an innovative approach that relies on Earth’s existing carbon storage mechanisms to help solve climate change. To keep more carbon in the ground, and store it in long-lasting roots, Salk scientists are developing a new generation of crop and wetland plants.
HPI is comprised of two programs: CRoPS (CO2 Removal on a Planetary Scale), which aims to develop crops called Salk Ideal Plants™ that can store more carbon in the ground for longer; and CPR (Coastal Plant Restoration), which is working toward genetically informed restoration and preservation of the world’s wetlands, which act as significant carbon sinks.
HPI Scientific Research Projects
CO2 Removal on a Planetary Scale
The CO2 Removal on a Planetary Scale (CRoPS) project is developing Salk Ideal Plants, which puts CO2 in the ground and keeps it there. The key is suberin, a plant tissue that loves carbon and is already found in roots. By increasing root mass, depth and suberin content, Salk researchers will transform wheat, rice, corn and other crops into carbon-storing machines. In addition, more ground carbon means farmers benefit from improved soil health.
Coastal Plant Restoration
What type of light is essential for healthy plants?
Companies that manufacture LED grow lights have a lot of different ideas about what types of light are needed during the growth process, and while we’ve looked at the idea of how different types of light affect plant growth elsewhere on this site I thought it would be a good idea to take a closer look at this topic. The sun itself doesn’t discriminate between different types of light – it simply emits its light and how living things on the earth process that light is entirely up to them. It turns out plants are quite particular in which parts of the light from the sun they use during the photosynthesis process and that’s what we plan on taking a look at here.
At the lower end of the visible light spectrum are lights in shades of blue; as you move towards longer wavelengths of light eventually you get to the red end of the spectrum. Most people working with indoor plant operations are more familiar with blue and red lights, but it’s a good idea to understand how the overall color spectrum affects plant growth. If we start at the blue end of the spectrum and move towards the red we’ll get a good idea of how light in different parts of the spectrum influence plant growth.
Blue light
Blue light is essential at the beginning of a plant’s growth cycle as this is the type of light that plants first absorb to help with chlorophyll production. Your plants need lots of blue light during the seeding process and right through the first part of their growth cycle to ensure healthy roots, strong stems, and healthy leaves as well. Without blue light your plants will never get out of the ground, so any lighting system that you put together should include a healthy dose of exposure to this type of light.
Purple light
As we move up the light spectrum you move into purple light, and while this may not be familiar to most of us it can certainly help your plants to grow. Purple light has a much shorter wavelength than red light and it can be just as effective in helping your plants through the vegetative growth process as blue light. This type of light is very energetic and that energy can be used by your plants, although it won’t be overly effective by itself.
Green light
While there has been some debate recently on the merits of green light during a plant’s growth cycle there’s a good reason plants normally have green leaves – that’s because they are least effective at absorbing this type of light. In general, plants use less of the green light they absorb than any other part of the spectrum and that’s why your plants appear green, but that doesn’t mean they don’t use any green light at all during the photosynthesis process. Some green light is retained by your plants for photosynthesis; leaving this part of the spectrum out altogether can negatively affect the growth of the plants.
Yellow or White light
Red light
Red light has longer wavelengths than blue light and is therefore a lot less energetic. It’s important that your plants are exposed to red light during the blooming or flowering stages, but this type of light is not essential during the vegetative stage of your plants growth – in fact if you were to use only red light during the initial stages of the plants growth cycle your results would not be very positive at all. It’s best to use red light towards the end of the growth cycle in combination with some blue light as well.
A full spectrum of light
Focused Intensity
If you expose your plants to higher concentrations of certain types of light at different ends of the growth process you’re likely to get the best results. As we’ve mentioned elsewhere on this site and touched on in this article as well, blue light is very effective during the vegetative stage of the plants growth cycle, but you should introduce more red light during the blooming stage.
Different light at different stages
Understanding how the color spectrum affects plant growth may seem a little overwhelming at first, but once you get the basics down it’s straightforward. Essentially, there are different types of light that are more important at different times during the plants growth cycle. In an indoor grow operation you can manipulate the type of light your plants are exposed to in order to optimize their growth; just don’t eliminate other light altogether at any stage and you should be just fine.
Superclean: light and quantum dots turn plants into hydrogen
Scientists have been sprouting new ways to cleanly produce hydrogen as a fuel source
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Hydrogen is often touted as a clean fuel source, as its use in cars only produces water vapor as a byproduct. The truth is though, that producing hydrogen in the first place can often be a process that relies on natural gas or other polluting chemicals that can damage the environment. Finding a way to produce hydrogen simply and cleanly would go a long way toward eventual use of the gas as a fuel source. And that’s exactly what researchers at the University of Cambridge (UC) have done, adding to a host of other green possibilities that have been proposed for creating the gas.
In the new Cambridge method, as with several other methods, the researchers used biomass as a starting point. In particular, they focused on lignocellulose, the support structure found in plants.
“Lignocellulose is nature’s equivalent to armoured concrete,” said Moritz Kuehnel, from UC’s department of chemistry and joint lead author on a new paper about the study. “It consists of strong, highly crystalline cellulose fibres, that are interwoven with lignin and hemicellulose which act as a glue. This rigid structure has evolved to give plants and trees mechanical stability and protect them from degradation, and makes chemical utilisation of lignocellulose so challenging.”
While lignocellulose can be converted into hydrogen, the researchers say that up to this point, the processes that do so rely on high heat, which means a good deal of energy needs to go into the task. Their new method relies simply on light along with a collection of nanoparticles.
The particles are actually very small semiconductors called cadmium sulfide quantum dots. First, they are suspended in alkaline water. Then, the biomass is added, the solution is beamed with light that mimics sunlight, and the dots go to work using the light to fuel a process in which they convert the biomass into hydrogen. The gas then rises out of the solution where it can be collected.
In the study, a variety of unprocessed biomass was successfully used including paper, leaves and pieces of wood.
“Our sunlight-powered technology is exciting as it enables the production of clean hydrogen from unprocessed biomass under ambient conditions,” said Erwin Reisner, the head of UC’s Christian Doppler Laboratory for Sustainable SynGas Chemistry where the research was carried out. “We see it as a new and viable alternative to high-temperature gasification and other renewable means of hydrogen production. Future development can be envisioned at any scale, from small scale devices for off-grid applications to industrial-scale plants, and we are currently exploring a range of potential commercial options.”
The work of the researchers, which has been published in the journal Nature Energy, can now be added to other less energy intensive ways of producing hydrogen including another sunlight-based process that uses grass to produce the gas; an enzyme-based process that breaks down parts of the corn plant to release hydrogen; and a solar-powered way to split the water molecule into its constituent parts thanks to the use of hematite.
‘We’ve had 5,000 years of farmers trying out different strategies for dealing with heat, drought and water scarcity. We need to begin to translate that.’
Crops are grown under a solar canopy that is key to an agrivoltaic project at Biosphere 2 in southern Arizona.
TUCSON — Indigenous peoples have known for millennia to plant under the shade of the mesquite and paloverde trees that mark the Sonoran Desert here, shielding their crops from the intense sun and reducing the amount of water needed.
The modern-day version of this can be seen in the Santa Catalina Mountains north of Tucson, where a canopy of elevated solar panels helps to protect rows of squash, tomatoes and onions. Even on a November afternoon, with the temperature climbing into the 80s, the air under the panels stays comfortably cool.
Such adaptation is central to the research underway at Biosphere 2, a unique center affiliated with the University of Arizona that’s part of a movement aimed at reimagining and remaking agriculture in a warming world. In the Southwest, projects are looking to plants and farming practices that Native Americans have long used as potential solutions to growing worries over future food supplies. At the same time, they are seeking to build energy resilience.
Learning from and incorporating Indigenous knowledge is important, believes Greg Barron-Gafford, a professor who studies the intersection of plant biology and environmental and human factors. But instead of relying on tree shade, “we’re underneath an energy producer that’s not competing for water.”
Vegetables such as squash, tomatoes and onions are being tracked in the agrivoltaic project at Biosphere 2. Planting crops under solar panels is a 21st-century version of farming techniques used by indigenous people in the Southwest, notes Greg Barron-Gafford, who is leading research at the facility north of Tucson. Dozens of solar panels rise skyward as part of the project.
On both sides of the Arizona border with Mexico, scientists are planting experimental gardens and pushing the potential of an “agrivoltaic” approach. Thirsty crops such as fruits, nuts and leafy greens — which require elaborate irrigation systems that have pulled vast quantities of water from underground aquifers and the Colorado and other rivers — are nowhere to be found.
“We’ve had 5,000 years of farmers trying out different strategies for dealing with heat, drought and water scarcity,” said Gary Nabhan, an ethnobotanist and agrarian activist who focuses on plants and cultures of the Southwest. Collectively, he added, “we need to begin to translate that.”
Some of the methods at Biosphere 2 — a facility marked by the largest closed ecological system in the world — are being applied in fishing villages on the parched Sonoran coast of Mexico. A multiyear effort there will help ensure water, energy and food sources for some 1,500 members of the Comcaac (or Seri) community.
Other researchers are creating a sustainability model for urban settings.
The University of Arizona’s Desert Laboratory on Tumamoc Hill will break ground next spring on Tumamoc Resilience Gardens, an initiative to be located at the base of a saguaro-studded hill within an 860-acre ecological preserve in the heart of Tucson. It will show how people can feed themselves in a much hotter, drier future.
The effect of light spectrum on plant development
Have you ever used grow lamps for your plants? If so, then you’ve probably been amazed by the effect of the light on the development of your plant. This article will tell you much more about the effects of light on plant development. As we will see, plant development really is something different from plant growth. We will explain the principles of light and its interaction with plants, and also give you some practical tips. Choosing the right lamp can make a huge difference to the quality and quantity of your crop.
By CANNA Research
Everyone knows that a plant needs light to grow by means of photosynthesis, a process that involves energy fixation and sugar production. But in addition to providing energy, light also plays a key role in many other plant processes, such as photomorphogenesis and photoperiodism. All these processes are influenced by the light spectrum, which is the distribution of the light across the electromagnetic spectrum. In order to explain a plant’s different responses to light, we first need to think about the phenomenon of light itself.
Principle of light and its spectrum
As you can see from figure 1, this is a very small range. In fact, it is less than 1 percent of the total spectrum. Photosynthetically active radiation (PAR), or photosynthetic photon flux density (PPFD), is the range of light that can be used by plants to photosynthesise. However, because the PPFD is a summation of all photons in the 400-700nm range, two very different spectral distributions can have the same PPFD. This means that there is no one-to-one relationship between PPFD and spectral distribution. It also means that when we compare light sources, we need to consider spectral distribution data as well as PPFD.
PPFD light is expressed as μmol/m2/s and tells us how many light photons will reach a predetermined surface area (m2) in a specified length of time (a second). To illustrate: most plants need a minimum of 30 – 50 μmol/m2/s PPFD to stay alive.
How a plant senses light
As well as providing the energy for photosynthesis, light also acts as an information source for plants. Different light spectra give the plant an indication of its environment and therefore how it should survive, and hopefully thrive and reproduce. In this sense, the composition of the light is as important as the total quantity of light used for photosynthesis. The light spectrum in the range of 300 to 800 nm causes a developmental response in the plant. Additionally, UV and infrared (IR) light are known to play a role in plant morphogenesis.
A plant gains information from the light that reaches it by means of special pigments, called photoreceptors. These photoreceptors are sensitive to different wavelengths of the light spectrum.
There are three groups of photoreceptors, see figure 2:
• Phototropins
• Cryptochromes
• Phytochromes
The first two photoreceptors – phototropins and cryptochromes – are active in the lower range of wavelengths (UV (A) and blue). These two receptors obviously have different functions. Phototropins are responsible for phototropism or plant movement, and the movement of chloroplasts inside the cell in response to the quantity of light. Phototropins are what cause stems to bend towards light and stomata to open.
Cryptochromes are pigments which sense the direction of the light. The inhibition of stem elongation is governed by cryptochromes as well as stomatal functioning, the synthesis of pigments and the tracking of the sun by the plants leaves. The other photoreceptors – phytochromes – are sensitive to red and far red light. There are two forms of phytochrome, Pfr and Pr, which interact. Phytochromes have the biggest influence on photomorphogenesis. Stem elongation, shade avoidance, chlorophyll synthesis and the flowering response are all functions typical controlled by phytochrome. See our article called ‘The effect of red and far-red light on flowering’, which will give you much more information about phytochromes.
Now that we have looked at the light spectrum and the photoreceptors responsible for plant development, we come to the next question: how can we apply this knowledge as a grower? What makes a good light spectrum for growing? In order to answer this question, we need to think about the plant’s response to different light spectra. Because these fall mainly under visible light, we can speak about ‘colors’, starting with the most important ones for plant development.
Blue light (400 – 500 nm)
A larger proportion of blue light has an inhibitory effect on cell elongation, which leads to shorter stems and thicker leaves. Conversely, a decrease in the amount of blue light will cause a larger leaf surface area and longer stems. Too little blue light will negatively affect the development of plants. Many plants need a minimum amount of blue light, which ranges from 5 to 30 μmol/m2/s for lettuce and peppers to 30 μmol/m2/s for soybean.
Interaction between red (600 – 700 nm) and far-red (700 – 800 nm) light
Because red and far-red light have a higher wavelength, they are less energetic than blue light. Combined with the profound influence of the red-induced phytochromes on plant morphogenesis, relatively more red and far-red light is needed for plants to develop.
The two forms of phytochrome, Pfr and Pr, play an important part in this process. Because red and far-red light are both present in sunlight, plants in nature will almost always contain both Pfr and Pr phytochromes. A plant senses its environment by the ratio between those two forms; this is called the phytochrome photostationary state (PSS).
The Pr phytochrome has a light absorption peak at a wavelength of 670 nm. When the Pr absorbs red light, it is converted to the Pfr form. The Pfr form acts the other way around – when it absorbs far red light at a peak of 730 nm, it converts into a Pr form. However, because Pfr molecules can also absorb red light, some of the Pfr molecules are converted back to Pr. Because of this phenomenon, there is not a linear relationship between PSS and the ratio of red to far red. For example, when the ratio of red to far red light exceeds two, there is barely any response in the PSS and thus plant development is not affected. It is therefore better to speak about PSS than the red to far red ratio of the light.
The amount of Pr and Pfr tells a plant which light it is receiving. When there is a lot of Pr present, this means that the plant is receiving more far red light than red light. When there is less red light, the opposite conversion (from Pr to Pfr) is hampered, meaning that there is relatively more Pr.
In environments in which many plants grow close together, all the red light from the sun is used for the photosynthesis process (between 400 and 700 nm) and much of the far red light is reflected by the plants (>700 nm). Most of the plants, especially those in the shade, will receive more far red than red light in this situation. As a consequence, Pr increases, and when this happens, the plant senses that it needs more light for photosynthesis and stem elongation is triggered (see figure 3). The result is taller plants with a bigger distance between the internodes and a thinner stem. This is a clear example of a shade avoidance response, where plants seek to capture, more light in order to survive.
Taller plants can absorb more red light which increases the quantity of Pfr forms. This will trigger greater branching, shorter distances between the internodes and less vertical growth in order to maximize the light absorption for the photosynthesis. As a result, plants expend less energy on growing as tall as possible and allocate more resources to producing seeds and expanding their root systems.
Influence of the light spectrum on flowering
Flowering is also influenced by the Pr and Pfr forms. The length of time for which Pfr is the predominant phytochrome is what causes the plant to flower. Basically, the levels of Pfr tell the plant how long the night is (photoperiodism). As the sun sets, the amount of far red light exceeds the amount of red light. During the darkness of the night, the Pfr forms are slowly converted back to Pr. A long night means that there is more time for this conversion to happen. Consequently at the end of the night period, the concentration of Pfr is low and this will trigger short-day plants to flower (see figure 4).
A low red to far-red ratio and consequently a limited amount of red light at the beginning of the night is thus very important for the flowering of short-day plants. Research conducted on three short-day plants – chrysanthemum, dahlia and African marigold – shows that when the night is interrupted by a red flash, causing a high red to far-red ratio, flowering decreased dramatically. It was also concluded that far red light alone does not regulate flowering. An equal or higher far-red light portion will improve flowering for short-day plants.
Limited effect of green light (500 – 600 nm) on plant development
It’s often assumed that only blue and red light help plants to grow and develop but that’s not completely true. Although much of the green light is reflected back off the plant’s surface (that’s why we humans see plants green), green light itself can also be beneficial for a plant. The combination of different light colors can lead to higher photosynthesis than the sum of its parts. Research conducted on lettuce also shows that plant growth and biomass increased when 24% green light was added to a red-blue LED, while maintaining equal PAR levels (150 μmol/m2/s) between the two objects. This indicates that even green light can have a positive influence on plant growth.
Ultraviolet light (300 – 400 nm)
Ultraviolet (UV) light has an effect on plants, too, causing compact growth with short internodes and small, thick leaves. However, too much UV light is harmful for plants, since it negatively affects the DNA and membranes of the plant. Photosynthesis can be hampered by too much UV light. Research shows that this happens at UV-values higher than 4 kJ/m2/day.
Conclusion
This brings us back to the general question of ‘what makes a good light spectrum for growing?’ It’s quite hard to give a general answer to this question, since it depends heavily on the type of plant and the requirements of cultivation. For a ‘normal’ plant development these specs are recommended:
• Most plants needs a minimal amount of 30 – 50 μmol/m2/s photosynthetic light to stay alive
• A minimum amount of blue light is required, which varies between 5 and 30 μmol/m2/s
• A somewhat larger portion of red and far-red light is required, compared to the blue light
• Balance between red and far-red light: preferably a red to far red light ratio of less than 2
• A limited amount of UV light, less than 4 kJ/m2/day
Also remember that:
• More blue light will lead to shorter stems and thicker leaves
• Too much far-red light or an unequal balance with the red light will result in elongated plants
• A low red to far-red ratio and consequently a limited amount of red light at the beginning of the night is important for the flowering of short-day plants
• Far red light alone does not regulate flowering
• Green light is beneficial for the photosynthesis, although it does not affect the flowering or plant development
The next step is to provide the best light spectrum for your situation. If sunlight is not sufficient, this can be done by selecting a good grow lamp. The emergence of light-emitting diodes (LEDs) in plant production, which you can read more about in our other article, makes it easier than ever before for growers to optimize the light spectrum.
Indoor Vertical Farms (Urban Habitat)
As indoor farms that don’t rely on soil become a growing strategy to tackle food insecurity, one academic has a vision to take vertical farming to new heights in China’s mega-cities: the farmscraper.
Carlo Ratti, an architect who runs MIT’s Senseable City Lab, is proposing a 51-story skyscraper for China’s technology hub of Shenzhen with a large-scale vertical hydroponic farm inside that can produce crops like salad greens, berries and tomatoes to feed up to 40,000 people per year.
The proposed tower, which would include other amenities like office space, a supermarket and a food court, is being shortlisted for Chinese hypermarket chain Wumart’s new headquarters.
It’s one of a number of ideas to expand vertical farms, as breakthroughs in hydroponic and aeroponic technology allow for these indoor facilities to produce crops with higher yields using less land and water. Vertical farms are intended to grow crops more efficiently — stacking them in trays or vertical planters in indoor climate-controlled conditions, and using algorithms and other technology to optimize light and growing conditions, often in urban environments.
In Britain, Shockingly Fresh expects to grow about 2 million heads of leafy greens a year on its first vertical farm. And AeroFarms in Newark, New Jersey — billed as the world’s largest vertical farm when it opened in in 2016 — grows about 2 million pounds of food each year on its 70,000-square-foot facility. For comparison, Ratti’s farmscraper is projected to produce about 600,000 pounds of food annually, mostly around the facade of the 715-foot-tall building (218 meters).
Other dense regions like Singapore and Abu Dhabi are making big investments in vertical farming as part of their goal to produce a larger share of food locally. In Jersey City, New Jersey, vertical farms are being constructed in public housing communities, with the aim of tackling food insecurity by merging technology, education and food access. Other types of greenhouses that are not technically vertical farms are also relying on technologies like LED lighting and robots to optimize growing — AppHarvest in Kentucky produces 45 million pounds of tomatoes per year in a facility that it says yields 30 times more per acre than open fields, with 90% less water.
In Shenzhen, Ratti says his proposal would take vertical farming “to the next level.” The proposed Jian Mu Tower would not only build taller, establishing what he says would be the world’s first farmscraper. It would also be a model for “how to integrate the natural world into building design,” by incorporating farming around the entire shell of a skyscraper where people are also working, shopping and eating. The greenery would sit in what’s known as a double-skin facade, with windows on both sides to allow natural sunlight to reach both the plants and the building interior. Ratti says this design — and the copious amounts of sunlight in Shenzhen — will enable the farm to be less reliant on artificial light and heating, which come with high energy use. But the farm is also intended to have benefits for the built environment: The heat that reflects off tall buildings can make a city hotter. Encasing a skyscraper with a farm is a good way to not just mitigate this effect and keep the building cooler without air conditioning, Ratti says, but also to produce food to feed the people in that building.
“Our point was, why don’t we try to harvest this energy from the sun on the facade of the skyscraper and turn it into a giant farm,” he said. “This would not have been possible a few years ago, but it’s possible today, thanks to advances in hydroponics and also robotics.”
key prong of the Shenzhen facility’s maintenance would be the use of an AI-supported “virtual agronomist” tasked with managing the farm’s day-to-day operations, including irrigation and nutritional conditions. Ratti says the difference with his newest proposal is scale. “What it really allows you to do is locally source, to produce food for tens of thousands of people just with a skyscraper,” he said.