When Sunflowers Follow the Sun

Tropisms are widely studied biological phenomena that involve the growth of an organism in response to environmental stimuli. Phototropism is the growth and development of plants in response to light. Heliotropism, a specific form of phototropism, describes growth in response to the sun. Discussions of heliotropism frequently include sunflowers and their ability to “track the sun.” This conjures up images of a field of sunflowers in full bloom following the sun across the sky. However cool this might sound, it simply doesn’t happen. Young sunflowers, before they bloom, track the sun. At maturity and in bloom, the plants hold still.

What is happening in these plants is still pretty cool though, and a report published in an August 2016 issue of Science sheds some light on the heliotropic movements of young sunflowers. They begin the morning facing east. As the sun progresses across the sky, the plants follow, ending the evening facing west. Over night, they reorient themselves to face east again. As they reach maturity, this movement slows, and most of the flowers bloom facing east. Over a series of experiments, researchers were able to determine the cellular and genetic mechanisms involved in this spectacular instance of solar tracking.

Helianthus annuus (common sunflower) is a native of North America, sharing this distinction with dozens of other members of this recognizable genus. It is commonly cultivated for its edible seeds (and the oil produced from them) as well as for its ornamental value. It is a highly variable species and hybridizes readily. Wild populations often cross with cultivated ones, and in many instances the common sunflower is considered a pesky weed. Whether crop, wildflower, or weed, its phototropic movements are easy to detect, making it an excellent subject of study.

Researchers began by tying plants to stakes so that they couldn’t move. Other plants were grown in pots and turned to face west in the morning. The growth of these plants was significantly stunted compared to plants that were not manipulated in these ways, suggesting that solar tracking promotes growth.

The researchers wondered if a circadian system was involved in the movements, and so they took sunflowers that had been growing in pots in a field and placed them indoors beneath a fixed overhead light source. For several days, the plants continued their east to west and back again movements. Over time, the movements became less detectable. This and other experiments led the researchers to conclude that a “circadian clock guides solar tracking in sunflowers.”

Another series of experiments helped the researchers determine what was happening at a cellular level that was causing the eastern side of the stem to grow during the day and the western side to grow during the night. Gene expression and growth hormone levels differed on either side of the stem depending on what time of day it was. In an online article published by University of California Berkeley, Andy Fell summarizes the findings: “[T]here appear to be two growth mechanisms at work in the sunflower stem. The first sets a basic rate of growth for the plant, based on available light. The second, controlled by the circadian clock and influenced by the direction of light, causes the stem to grow more on one side than another, and therefore sway east to west during the day.”

The researchers observed that as the plants reach maturity, they move towards the west less and less. This results in most of the flowers opening in an eastward facing direction. This led them to ask if this behavior offers any sort of ecological advantage. Because flowers are warmer when they are facing the sun, they wondered if they might see an increase in pollinator visits during morning hours on flowers facing east versus those facing west. Indeed, they did: “pollinators visited east-facing heads fivefold more often than west-facing heads.” When west-facing flowers where warmed with a heater in the morning, they received more pollinator visits than west-facing flowers that were not artificially warmed, “albeit [still] fewer than east-facing flowers.” However, increased pollinator visits may be only part of the story, so further investigations are necessary.

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I’m writing a book about weeds, and you can help. For more information, check out my Weeds Poll.

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When Alien Plants Invade – The Four Stages of Invasion, part two

In a review published in New Phytologist (2007), Kathleen Theoharides and Jeffrey Dukes examine four stages of invasion as they relate to alien (i.e. introduced or non-indigenous) plant species. In part one we discussed transport and colonization, in which species must survive being transported long distances and then take root and reach maturity in an unfamiliar location. Alien plant species don’t become invasive until they have reached the last two stages: establishment and landscape spread. Removal of the species upon reaching these stages is no easy task. Luckily, introduced species have a few barriers to overcome before this point.

An established population is one that is self-sustaining and expanding. Environmental conditions may be a limiting factor, as they were during colonization, but the main constraints at this stage are “biotic filters.” Theoharides and Dukes define these as “barriers to invasion created by the actions or presence of living organisms.” Through competition for various resources, as well as herbivory and disease, neighboring organisms affect the survival, growth, and reproduction of introduced plant species. Thus, “traits that enhance competitive performance, reduce niche overlap between [introduced species] and natives, or increase enemy resistance may be most important during establishment… Other advantageous traits include secondary chemical compounds that deter herbivores, ‘novel weapons’ such as root exudates that negatively impact other plants, fast growth, and high fecundity.”

Plants compete for light, moisture, and soil nutrients, as well as for pollinators and seed dispersers. Competition inhibits the establishment of invaders when neighboring plants consume available resources more efficiently. Introduced plants risk being outcompeted by plants that are of the same functional type (plants that are “morphologically, phenologically, and physiologically similar”). They also risk competition by a single dominant species (or group of similar species) or by “an assemblage of species with different traits.” As a general rule, plant communities with greater diversity are more resistant to invasion.

“In forests of the northeastern USA, Alliaria petiolata, an herbaceous mustard species, contains a type of phytotoxic glucosinolate that appears to disrupt the mutualism between arbuscular mycorrhizal fungi and hardwood canopy trees.” – – Theoharides and Dukes (2007) [photo credit: eol.org]

Two hypotheses postulate the success of some plant invaders in establishing themselves: the enemy release hypothesis and the evolution of increased competitive ability hypothesis. In the first hypothesis, plant species – having been removed from their native habitat – are freed from their natural enemies and are thus able to allocate more resources to growth and reproduction. The second hypothesis states that, in light of “reduced enemy pressure,” introduced species quickly evolve to allocate resources “from enemy defense to faster growth.” Escape from herbivory and diseases, however, is likely not the only factor in the success of invaders, and much still depends on the competitiveness of the plant and the availability of key resources.

After introduced plants become established, a lag phase generally occurs before landscape spread. This can be a result of a lack of genetic variation, a dearth of suitable habitat, unfavorable environmental conditions, or some combination of the three. New introductions may occur, and the population may continue to adapt and expand. Suitable habitat may be made available, and environmental conditions may shift. In time, landscape spread becomes a possibility.

Landscape spread occurs when multiple populations of a species are connected via long-distance dispersal. At this “metacommunity scale,” populations of an introduced plant species interact across a large area, with each population in a different stage of colonization and establishment. This means that transport, colonization, and establishment are all at play during the landscape spread stage.

Abutilon theophrasti (velvetleaf) was originally introduced before 1700 in the USA. This species has only recently become an aggressive invader as a result of the evolution of different life-history strategies based on the nature of competition in its new environment.” — Theoharides and Dukes (2007) [photo credit: wikimedia commons]

Dispersal ability and habitat connectivity are key factors in determining the success of an introduced plant species during landscape spread. Long-distance dispersal can occur via wind, water, or animals. Species that depend on animals to spread their seeds rely on specific animals to be present. The seeds of Prunus serotina (black cherry), for example, are dispersed by birds. So, landscape spread is reliant on birds and “roosting trees” where the birds can perch and defacate the seeds. In many cases, “humans also play a large role in intraregional dispersal.”

Habitats vary across the landscape due to a combination of numerous geological and biological processes. The disturbance regime – “the frequency, spatial extent, severity, and intensity of killing events over time” – also helps determine landscape patterns. Natural disturbances, such as fire, weather, and natural disasters, are differentiated from disturbances caused by human activity. Large scale development and disturbance of natural areas by humans disrupts the natural disturbance regime and alters historical landscape patterns. As the authors write, “alterations of the disturbance regime that increase resource availability or change landscape patterns can promote non-indigenous plant species spread by creating favorable patches for colonization and establishment.”

Fragmented landscapes consisting of small patches of natural areas dispersed among large areas of human development are particularly prone to invasion by introduced plant species for many reasons, including increased influx of propagules and a high degree of edge effects (habitat edges have environmental conditions that are generally more prone to invasion than habitat interiors).

Habitat patches can be connected via corridors. It is through these corridors that dispersal can occur between populations in a metacommunity. Corridors connect populations of both introduced and native plant species. However, “native plants often require wide undisturbed corridors of intact habitat, while [introduced plant species] may disperse best through strips of human-disturbed habitat or ‘disturbance corridors.'” The environmental conditions in disturbance corridors and the presence of dispersal agents (including humans and domesticated animals) help facilitate the connectivity of populations of introduced plant species and promote the colonization and establishment of new populations.

In their abstract, Theoharides and Dukes write, “both research and management programs may benefit from employing multiscale and stage approaches to studying and controlling invasion.” With their conclusion they provide a list of potential management strategies for each stage, and they advise employing “natural filters in order to prevent invasion succees.” Examples include reducing habitat fragmentation and edge effects, promoting intact native communities, reducing human disturbances, promoting natural disturbance regimes, and minimizing disturbance corridors.

More Posts about Invasive Species:

Introducing Invasive Species

The terms “invasive” or “invasive species” get thrown around a lot. They are frequently used to describe anything that is “misbehaving,” or acting in a way that doesn’t fit our idealized vision for how a landscape should look and function. Oftentimes a species that is introduced (by humans) or is not native to an area automatically gets labeled invasive, even if it isn’t acting aggressively or having any sort of dramatic impact on the ecosystem. It is an alien species in an alien environment; it has invaded, therefore it is invasive.

image credit: cartoon movement

image credit: cartoon movement

Determining what is actually invasive in what location and at what time is much more complex than that. We do our best to understand the natural features and functions of ecosystems, and we single out any species, whether introduced or not, that is acting to upset things. That species is considered invasive and, if the goal is to restore the natural balance, it must be controlled. To what degree a species should be controlled depends on the degree that it is upsetting things. Ultimately, it comes down to human judgement. Hopefully that judgement is based on the best available evidence, but that isn’t always the case.

But we are getting ahead of ourselves. What I mostly want to accomplish with this post is to introduce the concept of invasive species and point you to a selection of resources to learn more about them. I defined invasive species in a post I wrote back in August 2015, so I will repeat myself here:

“Invasive species” is often used inappropriately to refer to any species that is found outside of its historic native range (i.e. the area in which it evolved to its present form). More appropriate terms for such species are “introduced,” “alien,” “exotic,” “non-native,” and “non-indigenous.” The legal definition of an invasive species (according to the US government) is “an alien species that does or is likely to cause economic or environmental harm or harm to human health.” Even though this definition specifically refers to “alien species,” it is possible for native species to behave invasively.

These terms refer not just to plants but to all living organisms. The term “noxious weed,” on the other hand, is specific to plants. A noxious weed is a plant species that has been designated by a Federal, State, or county government as “injurious to public health, agriculture, recreation, wildlife, or property.” A “weed” is simply a plant that, from a human perspective, is growing in the wrong place, and any plant at any point could be determined to be a weed if a human says so.

Invasive species are easily one of the most popular ecological and environmental topics, and resources about them abound – some more credible than others. Here is a list of places to start:

That should get you started. There are, of course, numerous books on the subject, as well as a number of peer-reviewed journals dedicated to biological invasions. You should also be aware that IUCN maintains a list of the Top 100 World’s Worst Invasive Species and that there is a National Invasive Species Awareness Week, which is quickly approaching. This episode of Native Plant Podcast with Jamie Reaser (executive director of National Invasive Species Council) offers an informative discussion about invasive species, and a search for “invasive species” on You Tube brings up dozens of results including this brief, animated video:

 

I want to believe that we are doing the right thing when we make concerted efforts to remove invasive species and restore natural areas, but I’m skeptical. The reason why I have chosen to spend an indefinite amount of time exploring the topic of invasive species is because I truly want us to get it right. Yet I don’t even know that there is a “right.” It seems to me that there are endless trajectories – each one of them addressing different objectives and producing different outcomes. In a way we are playing God, regardless of which approach we take. We are making decisions for nature as if we know what’s best for it or that there even is a “best.”

Humans have had major impacts on virtually every square inch of the planet and have been placing our fingerprints on every ecosystem we touch since long before we became the humans we are today, and so it is difficult for me to envision a planet sans humans. It is also difficult for me to buy into the idea that our planet should look as though humans haven’t touched it (i.e. pristine). Because we have been touching it – for hundreds of thousands of years. Efforts to rewind time to before introductions occurred or to hold an ecosystem in stasis, securing life for only those species that “belong” there, seem noble yet fanciful at best and misguided, arrogant, and fruitless at worst.

To be the best conservationists we can be, we probably need to find a middle ground regarding invasive species – not a deter and eliminate at all costs approach, but also not a complete surrender/all are welcome and all can stay stance. Somewhere in between seems reasonable, acknowledging that the strategy taken will be different every time based on the location, the species in question, and our objectives. Of course, none of my beliefs or opinions on this topic (or any topic for that matter) are fully formed. I am trying to do my best to maintain an open mind, seeking out the best information available and following the evidence where it takes me. A topic as complex as invasion biology, however, is never going to be easy to finalize one’s opinions on, and so this journey will be boundless. I hope you will join me.

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Last but not least, here are two articles that discuss updating our approach to dealing with invasive species:

 

Confidential Carnivore

This is a guest post. Words and images by Jeremiah Sandler

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If you live in North America or Europe, chances are you have seen Dipsacus fullonum, commonly called teasel.  Its tall (up to 2 meters), spiky flower stalks with large purple flowers are easy to spot in low-lands, ditches, or along highways.  Since this prolific seeder’s introduction to North America from Europe, it has steadily increased its habitat to occupy nearly each region of the United States. Of course, like all plants, teasel has its preferences and is more frequent in some areas than in others.

dipsacus fullonum_jeremiah sandler

Teasel is an unassuming, herbaceous biennial.  It takes two years to complete its life cycle: First-year growth is spent as a basal rosette, and second-year growth is devoted to flowering.  Standard biennial, right?  As of 2011, an experiment was conducted on this plant that changed the way we see teasel, and possibly all other similar plants.

“Here we report on evidence for reproductive benefits from carnivory in a plant showing none of the ecological or life history traits of standard carnivorous species.” -Excerpt from the report titled Carnivory in the Teasel Dipsacus fullonum — The Effect of Experimental Feeding on Growth and Seed Set by Peter J.A. Shaw and Kyle Shackleton.

We all have favorite carnivorous plants, Venus flytraps, pitcher plants, sundews, etc.. Their showy traps and various means of attracting insects are all marvels of evolution in the plant kingdom.  These insectivorous plants evolved these means of nutrient acquisition in an answer to the lack of nutrients in their environment’s soil.  In some of these plants, there is a direct relationship between number of insects consumed and the size of the entire plant. In others, there is no such relationship.

The unassuming, biennial teasel can now join the ranks of carnivore, or protocarnivore.  It didn’t evolve in bogs or swamps where soil nutrients are depleted.  It has no relationship to the standard carnivorous species. It doesn’t have any flashy traps. In fact, it has no obvious traits which suggest it can gain nutrients from insects. Teasel’s carnivorous habits can be likened somewhat to the carnivorous habits of bromeliads; water gathered in their leaves traps insects.

In Shaw and Shackleton’s experiment (done in two field populations), maggots were placed in water gathered in the center of some first-year rosettes of teasel.  Other rosettes in the same population were left alone as controls.  Not surprisingly, the teasels which were ‘fed’ larvae did not change in overall size.  The size of the overwintering rosette did not offer any predictability towards the size of flower shoots for the coming year. However, something strange did happen:

“…addition of dead dipteran larvae to leaf bases caused a 30% increase in seed set and the seed mass:biomass ratio.”…“These results provide the first empirical evidence for Dipsacus displaying one of the principal criteria for carnivory”

Teasel has some physiology to absorb nutrients from other macroorganisms despite teasel evolving in an entirely different setting than typical carnivorous plants.  Teasel’s already proficient reproductive capacity is enhanced by using insects as a form of nutrients in a controlled setting.  

Many exciting questions have been raised by this experiment. How has this absorption mechanism come about, without the obvious use of lures or other structures to attract insects? And how does teasel maximize upon its own morphology in the wild, if at all?  What would the results be if these experiments were recreated on other similar species?

There are studies being conducted all the time that further the boundaries of what we know about these stationary organisms. There are new discoveries waiting just around the corner. Carnivory in plants is amazing because it transcends common notions about plants; especially in the case of the unassuming teasel.

Selected Resources:

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Jeremiah Sandler lives in southeast Michigan where he works in the plant health care industry. He has a degree in horticultural sciences and is an ISA certified arborist. He is interested in all things plant related and plans to own a horticulture business where he can share his passion with others. Follow Jeremiah on Instagram: @j.deepsea

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Would you like to write a guest post? Or contribute to Awkward Botany in some other way? Find out how.

Horticulture Students Wanted

“Horticulture is under siege.” At least that’s the claim made in a letter and action plan penned by the top administrators of six prominent horticulture institutions based in North America. In their letter addressed to “Colleague[s] in Horticulture,” they claim that among the general public there is a “lack of horticulture awareness and poor perception of horticulture careers”. This has lead to low enrollment in high school and college horticulture programs and a dearth of qualified, young horticulturists entering the work force. Because the youth of today “appear to have little or no awareness of the importance and value of horticulture,” they are not choosing to pursue “interesting, challenging, and impactful careers” in the field.

In order to address this issue, this team of horticulture professionals has developed a plan “to increase public awareness of the positive attributes of horticulture.” Plants are essential for life on earth; humans could not exist here without them. It is the field of horticulture that supplies humanity with much of the food that it consumes, including fruits, vegetables, nuts, and herbs. Horticulture also fills our landscapes with plants that provide the backdrop to our daily lives, transforming otherwise drab and harsh urban areas into lush green spaces. And speaking of “green,” horticulture is helping us save our planet. Through teaming up with engineers and other professionals, horticulturists are helping to develop solutions to issues like climate change, water quality, storm water runoff, energy production, and biodiversity loss. Innovative and emerging strategies such as green roofs, wildlife gardens, carbon sequestration, biofuels, and sustainable agriculture require horticulture expertise in order to succeed.

These are just some of the benefits of horticulture that the authors of this plan hope to share with the general public in an effort to change public perception and attract young recruits. If they don’t succeed, the consequences may be dire – or at least that’s how they make it sound. An article on philly.com regarding the recent letter put it this way: “if something isn’t done soon…horticulture could become a lost art and a forgotten science.”

Yeah, it’s a bit dramatic sounding. It’s hard for me to believe that the situation is really that desperate. However, what I will say is that a career in horticulture is not for everyone. It certainly isn’t for anyone who dreams of being rich and/or famous one day. That’s probably not going to happen. People who choose a career in this field do so because they have a passion for plants, a love of beautiful, inviting landscapes, and perhaps a proclivity for fresh, homegrown fruits and vegetables. A career in horticulture is not glamorous by any means, but it is highly rewarding – at least from my perspective. So sure, youngsters should consider it…but they should also consider themselves warned.

And now it’s time for show and tell. I graduated with a degree in horticulture at a four year university in the intermountain northwest. After that, I ventured off to the Midwest to pursue a graduate degree researching green roof technology. Perhaps the following pictorial of some of my adventures will inspire a few of you young folks to consider a similar path. Either that or there is always that liberal arts degree you’ve been dreaming of…

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As an undergraduate, I helped manage a student-run organic farm

community garden plot

I had a community garden plot overlooking the rolling hills of the Palouse

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I took a jet boat trip up the Snake River to help prune an abandoned apple orchard

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Then I went to Illinois to study green roof technology as a graduate student

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I presented my research findings at a big conference in Philadelphia

And so can you…or something like it. Comment below if you would like to put in your plug (or caveat) for pursuing a career in horticulture. The world needs you.