How plants warm up in the Himalayas — a hard bract to follow!

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Rheum nobile in the Himalyas.  Photo: Bill Baker, Royal Botanic Gardens, Kew

Rheum nobile is a weird plant.   It grows in the alpine regions of the Eastern Himalayas and is known as ‘Sikkim Rhubarb’.   The striking white area above the basal leaves is made up of overlapping bracts, which are modified leaves generally attached to reproductive structures.  Sometimes the bracts have no photosynthetic capabilities, as characterized by the white color in the photo.  We have all seen bracts at Christmas —  the red structures that make up a Poinsettia’s ‘flower’.  The actual flowers are the small modules in the middle of the bracts.

Botanists discovered that the bracts of R. nobile contain special chemicals called flavonoids that effectively block some of the high UV waves found at high altitude.  Kind of like ‘built-in’ sunscreen!  Recently, Song et al (2013) found that the bracts are also have other functions – for the experiment they removed bracts from some plants and left some other plants intact.  Their most important finding is that plants with bracts had more pollinators and matured more seed than those without bracts.

Why?  Well, it seems that bracts can act to heat up the reproductive parts of the plant.  Higher temperatures increase the metabolism of the flowers and fruit, possibly speeding up development of the seeds.  In addition, insects seem to be attracted to warmer reproductive structures, as has been found in other alpine plants like some species of buttercup.  When it’s this high and cold, a plant really needs a thermal blanket!

Spores and Slug Poop

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                                                                                    Photo from Wikimedia Commons

Many of you know that animals eat fruit and then deposit the seeds elsewhere in their feces.  Think of birds eating those wild grapes in your front yard, or bears eating berries in your front yard if you live in Alaska.  Scientists would consider the relationship between the animal and the plant a mutualism , where both partners benefit from the interaction.

Some plants, and all fungi, reproduce without seeds.  For example, ferns reproduce by producing spores that germinate to form gametophytes.  The gametophytes then produce – guess what? – gametes (sperm and eggs).  Plants like liverworts also reproduce through spores.  Now, spores can sometimes be transported by wind or water, but there is little evidence that they can be transported by animals.  (This type of transport, FYI, has the horrible name endozoochory.)

This is where the featured study comes to the rescue.  Steffen Boch and friends recently published an article in the journal Oecologia that investigated whether slugs were able to ingest and pass viable fern and liverwort spores.  They fed fertile parts of ferns and liverworts to three different species of slug and then collected their fecal pellets (poo).  They found that around 50% of the pellets contained spores that actually germinated, meaning that slugs, in theory, could serve to help propagate plants.  So, there you go…slugs, poop, and liverworts…an awesome combination!

 

Nectar robbers and nectar guides

 

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Nectar is often a reward for pollinators in many plants.  I usually envision a pollinator entering the flower from the open corolla, but there are other ways that pollinators, especially bees, can access nectar.  Bumblebees can also chew holes near the base of the petals to lap up nectar without ever touching the pollen on a plant.  This shortcut to the nectar is termed nectar robbing’.   This is good for the bee, but bad for the plant because no pollen is transported.

Plants can actually direct bees to nectar using the ‘proper’ path through the open flower.  This is done with nectar guides, which are colored areas or areas of high UV-contrast that lead to the center of the flower.  In a recent paper, Anne Leonard and colleagues tested the idea that these guides could decrease the amount of nectar robbing.   They did so by constructing artificial flowers, some of which had nectar guides and others that had no guides.  Nectar was available to bumblebee foragers at both the top and the side of the flowers.  The researchers found that flowers with nectar guides had more legitimate visits by bees and fewer robbers than flowers without guides.  Also, the time it took foraging bees to find the center of the flowers was much faster with the guides than without the guides.  So, it appears that nectar guides might help turn potential nectar robbers into good pollinators!

Crops may benefit from being close to natural areas

Well, I’m back after a long semester teaching about plant ecology and evolution!.  I’d like to share with you an article I just read from Ecology Letters, authored  by Lucas Garibaldi and colleagues.

Because effective pollination is so important to crop output and many natural areas have high pollinator abundance and richness, people have wondered if declines in natural areas may have negative effects on agricultural areas. These negative effects may range from decreases in pollinator richness to decreases in pollinator stability over time and space.  Pollinator stability is generally thought to be a good thing because it provides for consistent and predictable pollinator service for crops.  Unfortunately, there is not a lot of information about pollinator stability out there.  This is where this study comes in.

The main question of this study was whether proximity to natural areas A) had an effect on pollinator richness or visitation rate or B) had any effect on the stability of pollinator richness or visitation rate.  The authors (there are many!) collected many studies that individually looked at pollination service and distance to natural areas and analyzed them all together, which is termed a meta-analysis.

The upshot is that the scientists found that both pollinator richness and visitation to crop plants declined with increasing distance from natural areas.   Stability in pollinator richness and visitation also declined with distance from natural areas.  These results held even when honeybees, which are introduced pollinators from Europe, were excluded from the meta-analysis, suggesting that the benefits on crop plants were due to native bees! So, just ‘bee’ aware out there that pollinators originating from natural areas could be helping put fruits and vegetables on your table….have a good holiday season!

Fieldwork in Arizona: Searching for the lost pollinators!

A female honeybee approaching a Datura flower

Well, I’ve been in the field with my student assistant Skyler Jordan (DU ’13) for about two weeks now and we are finding some interesting things.

Datura bud, about to open in the evening

Our aim on this mission was to first discover if honeybees were visiting Datura wrightii flowers (see picture), and if they were collecting pollen, collecting nectar, and/or pollinating the plant. This is interesting because most folks think that Datura is mostly hawkmoth-pollinated, as a flower blooms at sunset, emits a strong sweet odor, and then shrivels up in the heat of the next day.

So far, we have found that the majority of visits to D. wrightii are by honeybees at this point in the season (this probably changes as time goes by), and that the bees are carrying major loads of D. wrightii pollen, both on their abdomens and also in their pollen baskets. So far so good, but we still don’t know if they are actually bringing pollen to the female part of the plant, the stigma. To do that, we are going to let single bees visit flowers to see if that one visit is sufficient to set seed. Wish us luck! More updates from the field in later posts…

What inbreeding does to flowers

Datura wrightii flower

Hi all, it has been some time since I have posted a story, I apologize.  I have been traveling the country, visiting awesome plants and ending up in Tucson, AZ.  I’m studying  Datura wrightii flowers (seen at left) and their pollinators and herbivores with Dr. Judie Bronstein at the University of Arizona.

In any case, I last threatened to discuss the implications of having sex with yourself.  Now, the answer may be intuitive.  For example, humans normally do not wish to mate with close relatives because a) it seems abhorrent to us, and b) it probably seems abhorrent because the children of these relationships often have severe physical or mental problems.

Why does this happen?  Let’s say there is a gene mutation out there that causes severe physical problems, and let’s say it is recessive, or that it is masked by the normal version of the gene when they are together in a cell (called the heterozygous state).  Now this normally doesn’t pose a problem because of the masking effect.  If, however, two people who are both heterozygous (likely if they are related to each other) mate, there is a 25% chance, on average, that their offspring will have both forms of the mutant gene (called the homozygous recessive state).  Then, there is no masking at all,  and the mutation causes the negative changes in that offspring.

The same thing happens in plants, except that they can actually have sex with themselves, and don’t need another separate plant.  So, inbreeding in plants can cause serious problems in the offspring.  ‘Like what?’ you might ask?  Well, read on!

In a seminal work using our old friend Mimulus guttatus, Ivey and Carr (2005) manipulated plants so that one set was produced from inbreeding parents and one set from non-inbreeding, or outcrossing parents.  They then measured important floral traits like the diameter of the petals (together they are called the corolla), the length of the petals, and the distance between the stigma (the female bit) and the anthers (the male bits).  They also counted how many flowers were produced and seed set.  Finally, they counted how many pollinators visited the plants and how long each pollinator took to forage on the flower.

Figure showing effect of inbreeding on seed set

Not surprisingly, inbreeding significantly decreased corolla diameter, corolla length, the number of flowers produced, and the number of seeds produced (see figure above).  This might be expected if mutant genes are being expressed in the homozygous state through inbreeding.  Perhaps through the effects on corolla size, inbred plants also received fewer initial pollinator visits

The implications of these results are that once inbreeding starts, plants may get fewer pollinator visits and thus may start to self-pollinate, leading to even more inbreeding.  One could imagine that this cycle could continue until the population goes extinct or the population evolves to be completely self-pollinating, which we sometimes see in nature.  It might be more likely that a few outcross pollen grains landing and fertilizing eggs in the population bring enough genetic diversity to avoid such  cycles.  A little genetic diversity can go a long way!

With that, I’ll sign off — I hope everyone has a bloomin’ Fourth of July!

Reference:

Ivey, C. T. and D. E. Carr.  2005. Effects of herbivory and inbreeding on the pollinators and mating system of Mimulus guttatus (Phrymaceae).American Journal of Botany 92: 1641-1649

Invasive plants can alter pollination of native plants

Foxglove, Digitalis purpurea

Exotic plants are species that those that have been moved, usually by humans, from their native habitat to another area where they do not occur naturally.  Invasive exotic plants are those that significantly impact the ecosystem in the introduced area.  These types of plants spell big trouble for many areas in the United States, and some are even known to the general public like kudzu and purple loosestrife.

We have known for decades that invasives can negatively impact natural species by competing for resources such as soil nutrients and light.  Unfortunately, we do not have a good grasp on how invasive might affect the pollination of naturally-occurring species.  There are two main hypotheses about the effects.  First, invasives that occur in high densities may actually attract lots of pollinators to other plants in a given area, thus benefitting the co-occurring natives (the ‘magnet species effect’), or the invasives could ‘steal’ pollinators that would normally visit the native species.  Few studies have tested these hypotheses over multiple populations, which brings me to this week’s topic experiment.

Dietzsch et al. (2011) were interested in how the invasive plant Rhododendron ponticum affected pollination of the native Digitalis purpurea, or foxglove, in Ireland.   As an aside, Foxglove is famous because it produces digoxin (digitalis), which is used to treat some heart disorders.   It is worth noting that foxglove can be found in the U.S. too, but here it is considered an exotic because it is native to Europe.  The authors surveyed six areas that contained foxglove, but differed in the amount of invasive Rhododendron present.  At each site, they     counted how many times pollinators visited patches of foxglove, how much foxglove pollen was found on foxglove stigmas (the female part), and how many seeds, on average, were produced per fruit.  Simply put, if the ‘magnet species effect’ was occurring, then pollinator visitation, the amount of Rhododendron pollen found on Rhododendron stigmas, and Rhododendron seed set would increase with higher amounts of Rhododendron, whereas you would expect the opposite relationship if Rhododendron was taking pollinators from foxglove.

The researchers found that as the amount of Rhododendron increased, pollination visitation rates to foxglove decreased, thus supporting the idea that the invasive plant was stealing pollinators from the native plant.  They found that conspecific pollen (pollen from foxglove) found on foxglove stigmas also decreased with increasing amounts of Rhododendron,  supporting the hypothesis that invasives were stealing pollinators from foxglove.   Interestingly, the authors did not find that seed or fruit set declined with increasing amounts of Rhododendron, even though pollination did decline.  One explanation for this puzzle is that foxglove can produce seeds from self pollen that is deposited by a visitor, even if the visitor just bumbles around the flower one time without depositing any pollen from another plant.  So, like Mimulus guttatus from last week, foxglove may be able to assure some sort of reproduction by self-pollination.  Again, thatleads us to the problem of inbreeding depression, which is where I will take us next week…  Until then, take a close look around you and enjoy the plants!

Reference:

Deitzsch A.C., Stanley D.A., and J. C. Stout.   2011.  Relative abundance of an invasive alien plat affects native pollination processes.  Oecologia, published online April 12 2011