What's the deal with nodding flowers?

While working in the garden the other day, I noticed that some of the nodding onion (Allium cernuum) we planted last year had finally come into bloom. I must have spent the good part of an hour watching bees pay a visit to their downward pointing flowers. I have seen a lot of onion species in bloom before, but this particular native is the only one that I know of personally that orients its flowers facing the ground. This got me to thinking about floral orientation. A lot of plants produce flowers that face the ground but many more do not. Why is there such variety among the orientation of flowers?

As always, I hit the literature. It turns out, many scientists have set out to investigate the function of floral orientation. What immediately stuck out to me is just how many different flowering plant lineages boast species whose flowers face down rather than out or up. I knew instantly that with so much variety in lineage, habitat, and pollination strategies, the answer wasn’t going to be simple or straight forward. Indeed, each investigation I read about seemed to end in a slightly different conclusion. Still, there were enough patterns among the results and conclusions to make some general statements about the subject.

The nodding flowers of the Michigan lily (Lilium michiganense)

We often find plants with downward facing flowers in harsh climates. Harsh can mean a lot of different things depending on the plant and region in question but take, for instance, the case of the genus Cremanthodium. This interesting group of asters resemble sunflowers in the basic appearance of their flowers but the plants themselves are vastly different in overall growth habit. Many hail from alpine environments in Asia and possess a short stature and flowers that face the ground instead of the sun. Research on the reproductive habits of these plants has revealed that the downward orientation of their flowers helps protect the sensitive reproductive parts from solar radiation and rain.

Growing at high elevations exposes these plants to lots of UV radiation and plenty of storms. If flowers were to orient towards the sky, rain could wash away pollen and UV radiation could really hinder successful reproduction. By facing the ground, the flowers are able to avoid these potentially harmful effects altogether. Similar results have been found for other members of the aster family in the genus Culcitium growing in alpine habitats in the Andes. Here again we see that downward pointing flowers help protect the sensitive reproductive parts from rain, snow, and too much sun.

The recently described Cremanthodium wumengshanicum growing at elevation in Yunnan, China. [SOURCE]

However, its not just the elements that have selected for downward pointing flowers. As you can probably imagine, pollinators also play a role in floral orientation. While watching bee visit our nodding onions, I noticed that they seem to be much better able to land on and collect pollen and nectar from downward pointing flowers than any of the flies I see attempting visits. Indeed, floral orientation can have a massive impact on what kinds of pollinators are able to effectively visit a flower.

A great example of this can be seen in members of the genus Zaluzianskya. Some species present their flowers horizontally or vertically while others present their flowers facing the ground. By comparing the visitors that frequent each species, researchers found that orientation matters. Upright or horizontally facing flowers were mostly visited by hawkmoths. Hawkmoths hover while they feed, which means they have a much harder time visiting downward facing flowers. By presenting their flowers in different orientations, the various species of Zaluzianskya ensure that only specific pollinators are able to access their rewards and thus achieve pollination. As such, upright, horizontal, and downward flowering species remain reproductively isolated from one another. Similar results have also been found in genera such as the afore mentioned Culcitium as well as some Commelina and Nicotiana.

Investigating pollinator visitation among different species of Zaluzianskya. [SOURCE]

I am sure many more examples exist out there but alas, I only have so much time to pursue my random curiosities these days. Nonetheless, what started as a fun observation in the garden turned into an entertaining dive into ideas that I had not given too much thought to before. What seems like a funny quirk of anatomy turns out to have massive implications for where plants are able to grow and how they are able to reproduce and all of these factors and more have shaped flowering plant evolution over time. Not bad for a few hours in the garden.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3] [4] [5] [6]

A Rare Case of Ant Pollination in Australia

Photo by Nicola Delnevo [SOURCE]

Ants have struck up a lot of interesting and important relationships with plants. They disperse seeds, protect plants from herbivores and disease, and can even help acquire nutrients. For all of the beneficial ways in which ants and plants interact, pollination rarely enters into the equation. More often than not, ants are actually detrimental to the sex lives of flowering plants. Such is not the case for a rare species of protea endemic to Western Australia called the smokebush (Conospermum undulatum).

The reason ants usually suck at pollination is thanks to a tiny organ called the metapleural gland. For many ant species, this gland secretes special antimicrobial fluids that the ants use to groom themselves. Because ants tend to live in high densities in close quarters, this antimicrobial fluid helps keep their little bodies clean of any pathogens that might threaten their existence. For as good as these fluids are for ants, they destroy pollen grains, rendering them useless for pollination.

Leioproctus conospermi. Photo by Sarah McCaffrey licensed under CC BY-ND 2.0.

As is so often the case in nature, there are always exceptions to the rule and it seems that one such exception is playing out in Western Australia. While investigating the reproductive ecology of the smokebush, researchers noted that ants were regular visitors to their small flowers. They knew that in drier climates, some ant species have evolved to produce considerably less antimicrobial fluids. The thought is that drier climates tend to harbor fewer microbial pathogens and thus ants don’t need to waste as much energy protecting themselves from such threats. If this was the case in Western Australia then it was entirely possible that ants could potentially serve as pollinators for this plant. Armed with this hypothesis, they decided to take a closer look.

It turns out that the floral morphology of the smokebush lends well to visiting ant anatomy. The tiny flowers produce a small amount of nectar at the base. As ants shove their heads down into the flower to get a drink, it triggers an explosive mechanism that causes the style the smack down onto the back of the ant. In doing so, it also mops up any pollen the ant may be carrying. At the same time, the anthers explosively dehisce, coating the visitor with a fresh dusting of pollen. During their observations, researchers noted that ants weren’t the only insects visiting smokebush blooms. They also noted lots of visitation from invasive honeybees (Apis mellifera) and a tiny native bee called Leioproctus conospermi.

(A) White flowers of Conospermum undulatum. (B) Floral details. (C–H) Insects visiting flowers of C. undulatum: (C) Leioproctus conospermi; (D) Camponotus molossus; (E) Camponotus terebrans; (F) Iridomyrmex purpureus; (G) Myrmecia infima; (H) Apis mellifera. [SOURCE]

After recording visits, researchers needed to know whether any of these floral visitors resulted in successful pollination. After all, just because something visits a flower doesn’t mean it has what it takes to get the job done for the plant. By looking at differences in seed set between ant and bee visitors, they were able to paint a fascinating picture of the pollination ecology of the rare smokebush.

It turns out that ants are indeed excellent pollinators of this shrub, contributing just as much to overall seed set as the tiny native Leioproctus conospermi. Alternatively, invasive honeybees barely functioned as pollinators at all. Their heads were too big to effectively trigger the pollination mechanism of the flowers but nonetheless were able to access the nectar within. As such, honeybees are considered nectar thieves for the smokebush, harming its overall reproductive effort rather than helping.

Amazingly, the effectiveness of ants as smokebush pollinators is not because they produce less antimicrobial fluids. In fact, these ants were fully capable of producing ample amounts of these pollen-killing substances. Instead, it appears that the plant itself has evolved to tolerate ant visitors. Smokebush pollen is resistant to the toxic effects of the metaplural gland fluids. With plenty of hungry ants always on the lookout for food, the smokebush has managed to tap in to an abundant and reliable vector for pollination. No doubt other examples exist, we simply have to go looking.

Photo Credits: [1] [2] [3]

Further Reading: [1]

Toxic Nectar

I was introduced to the concept of toxic nectar thanks to a species of shrub quite familiar to anyone who has spent time in the Appalachian Mountains. Locals will tell you to never place honeybee hives near a patch of rosebay (Rhododendron maximum) for fear of so-called "mad honey." Needless to say, the concept intrigued me.

A quick internet search revealed that this is not a new phenomenon either. Humans have known about toxic nectar for thousands of years. In fact, honey made from feeding bees on species like Rhododendron luteum and R. ponticum has been used more than once during times of war. Hives containing toxic honey would be placed along known routs of Roman soldiers and, after consuming the seemingly innocuous treat, the soldiers would collapse into a stupor only to be slaughtered by armies lying in wait.

Rhododendron luteum. Photo by Chrumps licensed under CC BY 3.0

The presence of toxic nectar seems quite confusing. The primary function of nectar is to serve as a reward for pollinators after all. Why on Earth would a plant pump potentially harmful substances into its flowers?

It is worth mentioning at this point that the Rhododendrons aren't alone. A multitude of plant species produce toxic nectar. The chemicals that make them toxic, though poorly understood, vary almost as much as the plants that make them. Although there have been repeated investigations into this phenomenon, the exact reason(s) remain elusive to this day. Still, research has drummed up some interesting data and many great hypotheses aimed at explaining the patterns.

Catalpa nectar has been shown to deter some ants and butterflies but not large bees. Photo by Le.Loup.Gris licensed under CC BY-SA 3.0

The earliest investigations into toxic nectar gave birth to the pollinator fidelity hypothesis. Researchers realized that meany bees appear to be less sensitive to alkaloids in nectar than are some Lepidopterans. This led to speculation that perhaps some plants pump toxic compounds into their nectar to deter inefficient pollinators, leading to more specialization among pollinating insects that can handle the toxins.

Another hypothesis is the nectar robber hypothesis. This hypothesis is quite similar to the pollinator fidelity hypothesis except that it extends to all organisms that could potentially rob nectar from a flower without providing any pollination services. As such, it is a matter of plant defense.

The nectar of Cyrilla racemiflora is thought to be toxic to some bees. Photo by Koala:Bear licensed under CC BY-SA 2.0

Others feel that toxic nectar may be less about pollinators or nectar robbers and more about microbial activity. Sugary nectar can be a breeding ground for microbes and it is possible that plants pump toxic compounds into their nectar to keep it "fresh." If this is the case, the antimicrobial benefits could outweigh the cost to pollinators that may be harmed or even deterred by the toxic compounds.

Finally, it could be that toxic nectar may have no benefit to the plant whatsoever. Perhaps toxic nectar is simply the result of selection for defense compounds elsewhere in the plant and therefore is expressed in the nectar as a result of pleiotropy. If this is the case then toxic nectar might not be under as strong selection pressures as is overall defense against herbivores. If so, the plants may not be able to control which compounds eventually end up in their nectar. Provided defense against herbivores outweighs any costs imposed by toxic nectar then plants may not have the ability to evolve away from such traits.

Where Spathodea campanulata is invasive, its nectar causes increased mortality in native bee hives. Photo by mauro halpern licensed under CC BY 2.0

So, where does the science land us with these hypotheses? Do the data support any of these theories? This is where things get cloudy. Despite plenty of interest, evidence in support of the various hypotheses is scant. Some experiments have shown that indeed, when given a choice, some bees prefer non-toxic to toxic nectar. Also, toxic nectar appears to dissuade some ants from visiting flowers, however, just as many experiments have demonstrated no discernible effect on bees or ants. What's more, at least one investigation found that the amount of toxic compounds within the nectar of certain species varies significantly from population to population. What this means for pollination is anyone's' guess.

It is worth noting that most of the pollination-related hypotheses about toxic nectar have been tested using honeybees. Because they are generalist pollinators, there could be something to be said about toxic nectar deterring generalist pollinators in favor of specialist pollinators. Still, these experiments have largely been done in regions where honeybees are not native and therefore do not represent natural conditions.

Simply put, it is still too early to say whether toxic nectar is adaptive or not. It could very well be that it does not impose enough of a negative effect on plant fitness to evolve away from. More work is certainly needed. So, if you are someone looking for an excellent thesis project, here is a great opportunity. In the mean time, do yourself a favor and don't eat any mad honey.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3] [4] [5] [6]

An Endangered Iris With An Intriguing Pollination Syndrome

The Golan iris (Iris hermona) is a member of the Oncocyclus section, an elite group of 32 Iris species native to the Fertile Crescent region of southwestern Asia. They are some of the showiest irises on the planet. Sadly, like many others in this section, the Golan iris is in real danger of going extinct.

The Golan iris has a rather limited distribution. Despite being named in honor of Mt. Hermon, it is restricted to the Golan Heights region of northern Israel and southwestern Syria. Part of the confusion stems from the fact that the Golan iris has suffered from a bit of taxonomic uncertainty ever since it was discovered. It is similar in appearance to both I. westii and I. bismarckiana with which it is frequently confused. In fact, some authors still consider I. hermona to be a variety of I. bismarckiana. This has led to some serious issues when trying to assess population numbers. Despite the confusion, there are some important anatomical differences between these plants, including the morphology of their rhizomes and the development of their leaves. Regardless, if these plants are in fact different species, it means their respective numbers in the wild decrease dramatically. 

Photo by Dr. Avishai Teicher Pikiwiki Israel licensed under CC BY 2.5

Like other members of the Oncocyclus group, the Golan iris exhibits an intriguing pollination syndrome with a group of bees in the genus Eucera. Their large, showy flowers may look like a boon for pollinators, however, close observation tells a different story. The Golan iris and its relatives receive surprisingly little attention from most of the potential pollinators in this region.

One reason for their lack of popularity has to do with the rewards (or lack thereof) they offer potential visitors. These irises produce no nectar and very little pollen. Because of this and their showy appearance, most pollinators quickly learn that these plants are not worth the effort. Instead, the only insects that ever pay these large blossoms any attention are male Eucerine bees. These bees aren't looking for food or fragrance, however. Instead, they are looking for a place to rest. 

A Eucerine bee visiting a nectar source. Photo by Gideon Pisanty (Gidip) גדעון פיזנטי • CC BY 3.0

The Oncocyclus irises cannot self pollinate, which makes studying potential pollinators a bit easier. During a 5 year period, researchers noted that male Eucerine bees were the only insects that regularly visited the flowers and only after their visits did the plants set seed. The bees would arrive at the flowers around dusk and poke around until they found one to their liking. At that point they would crawl down into the floral tube and would not leave again until morning. The anatomy of the flower is such that the bees inevitably contact stamen and stigma in the process. Their resting behavior is repeated night after night until the end of the flowering season and in this way pollination is achieved. Researchers now believe that the Golan iris and its relatives are pollinated solely by these sleeping male bees.

Sadly, the status of the Golan iris is rather bleak. As recent as the year 2000, there were an estimated 2,000 Golan irises in the wild. Today that number has been reduced to a meager 350 individuals. Though there is no single smoking gun to explain this precipitous decline, climate change, cattle grazing, poaching, and military activity have exacted a serious toll on this species. Plants are especially vulnerable during drought years. Individuals stressed by the lack of water succumb to increased pressure from insects and other pests. Vineyards have seen an uptick in Golan in recent years as well, gobbling up viable habitat in the process.

Photo by Dr. Avishai Teicher Pikiwiki Israel licensed under CC BY 2.5

It is extremely tragic to note that some of the largest remaining populations of Golan irises can be found growing in active mine fields. It would seem that one of the only safe places for these endangered plants to grow are places that are extremely lethal to humans. It would seem that our propensity for violent tribalism has unwittingly led to the preservation of this species for the time being.

At the very least, some work is being done not only to understand what these plants need in order to germinate and survive, but also assess the viability of relocated plants that are threatened by human development. Attempts at transplanting individuals in the past have been met with limited success but thankfully the Oncocyclus irises have caught the eye of bulb growers around the world. By sharing information on the needs of these plants in cultivation, growers can help expand on efforts to save species like the Golan iris.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3] [4]

California Bumblebee Decline Linked to Feral Honeybees

Photo by Alvesgaspar licensed under CC BY-SA 3.0

Worldwide, pollinators are having a rough go of it. Humans have altered the landscape to such a degree that many species simply can't keep up. The proverbial poster child for pollinator issues is the honeybee (Apis mellifera). As a result, countless native pollinators get the short shrift when it comes to media attention. This isn't good because outside of intense industrial agriculture, native pollinators make up the bulk of pollination services. Similarly, honeybee fandom often overshadows any potential negative effects these introduced insects might be having on native pollinators.

Long term scientific investigations are starting to paint a more nuanced picture of the impact introduced honeybees are having on native ecosystems. For instance, research based out of California is finding that honeybees are playing a big role in the decline of native bumblebee populations. What's more, these negative impacts are only made worse in the light of climate change.

Licensed under public domain

For over 15 years, ecologist Dr. Diane Thompson has been studying bumblebee populations in central California. At no point during those early years did any of the bumblebee species she focuses on show signs of decline. In fact, they were quite common. Then, around the year 2000, feral honeybees started to establish themselves in the area. Honeybee colonies were becoming more and more numerous each and every year and that is when she started noticing changes in bumblebee behavior and numbers.

You see, honeybees are extremely successful foragers. They are generalists, which means they can visit a wide variety of flower types. As a result, they are extremely good at competing for floral resources compared to native bumblebees. Her results show that increases in the number of honeybee colonies caused not only a reduction in foraging among the native bumblebees, they also caused a reduction in bumblebee colony success. The native bumblebees simply weren't raising as many young as they were before honeybees entered the system.

Decreased rainfall cause a decline in flower densities of Scrophularia californica, a key resource for native bumblebees in this system. Photo by USFWS - Pacific Region licensed under CC BY-NC 2.0

Climate change is only making things worse. As drought years become not only more severe but also more intense, the amount of flowers available during the growing season also declines. With fewer flowers on the landscape, bumblebees and honeybees are forced into closer proximity for foraging and the clear winner in most foraging disputes are the tenacious honeybees. As such, bumblebees are chased off the already diminishing floral displays. By 2014, Dr. Thompson had quantified a significant decline in native bumblebee populations as a result.

It would be all too convenient to say that this research represents an isolated case. It does not. More and more research is finding that honeybees frequently out-compete native pollinators for resources such as food and nesting sites. Such effects are especially pronounced in rapidly changing ecosystems. Although honeybees are here to stay, it is important that we realize the impacts that these feral insects are having on our native ecosystems and begin to better appreciate and facilitate the services provided by our native pollinators. 

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3] [4]

How Plants Influence Honeybee Caste System

Is has long been known that food fed to larval honeybees influences their development and therefore their place in the hive. Larvae fed a mixture of pollen and honey, often referred to as "bee bread," develop into sterile workers whereas larvae fed special secretions termed "royal jelly" from nurses within the colony will develop into queens. Despite this knowledge, the mechanisms underpinning such drastic developmental differences have remained a mystery... until now.

A team of researchers from Nanjing University in China have uncovered the secret to honeybee caste systems and it all comes down to the plants themselves. It all has to do with tiny molecules within plants called microRNA. In eukaryotic organsisms, microRNA plays a fundamental role in the regulation of gene expression. In plants, they have considerable effects on flower size and color. In doing so, they can make floral displays more attractive to busy honeybees.

As bees collect pollen and nectar, they pick up large quantities of these microRNA molecules. Back in the hive, these products are not distributed equally, which influences the amount of microRNA molecules that are fed to developing larvae. The team found that microRNA molecules are much more concentrated in bee bread than they are in royal jelly. Its this difference in concentrations that appears to be at the root of the caste system.

Larvae that were fed bee bread full of microRNA molecules developed smaller bodies and reduced, sterile ovaries. In other words, they developed into the worker class. Alternatively, larvae fed royal jelly, which has much lower concentrations of microRNA, developed along a more "normal" pathway, complete with functioning ovaries and a fuller body size; they developed into queens.

All of this hints at a deep co-evolutionary relationship. The fact that these microRNA molecules not only make plants more attractive to pollinators but also influence the caste system of these insects is quite remarkable. Additionally, this opens up new doors into understanding co-evolutionary dynamics. If horizontal transfer of regulatory molecules between two vastly different kingdoms of life can manifest in such important ecological relationships, there is no telling what more is awaiting discovery. 

Further Reading: [1]

Color Changing Asters

Fall is here and the asters are out in force. Their floral displays are some of the last we will see before the first fall frost takes its toll. Their beauty is something of legend and I could sit in a field and stare at them for hours. In doing so, an interesting pattern becomes apparent. Have you ever noticed that the disc flowers of the many aster species gradually turn from yellow to red? Whereas this certainly correlates with age, there must be some sort of evolutionary reason for this.

Indeed, there is. If you sat and watched as bees hurriedly dashed from plant to plant, you may notice that they seem to prefer flowers with yellow discs over those with red. The plot thickens. What about these different colored discs makes them more or less appealing to bees desperately in need of fuel? The answer is pollen.

A closer observation would reveal that yellow disks contain more pollen than those with red discs. Of course, this does relate to age. Flowers with red discs are older and have already had most of their pollen removed. In this way, the color change seems to be signaling that the older flowers are not worth visiting. Certainly the bees notice this. But why go through the trouble of keeping spent flowers? Why not speed up senescence and pour that extra energy into seed production?

Well, its all about cues. Bees being the epitome of search image foragers are more likely to visit plants with larger floral displays. By retaining these old, spent flowers, the asters are maintaining a larger sign post that ensures continued pollinator visitation and thus increases their chances of cross pollination. The bees simply learn over time to ignore the red disc flowers once they have landed. In this way, they maximize their benefit as well.

Further Reading: [1]