Monocots Better Adapted to Water?

I’ve had an interest in aquatic and wetland plants for a while and more recently have been actively researching and looking for aquatic plants. I have noticed that a lot of them tend to be monocots and I was wondering if anyone knew whether or not they are better adapted for aquatic conditions and why?

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welcome to the community.

are you asking whether monocots do better in water than land? or are you asking whether monocots do better than other plants in water?

maybe it doesn’t matter… i’m not sure either is true.

That’s an interesting idea! Apparently there’s something called the Acoranan hypothesis, which suggests that because Acorus, a genus of freshwater plants, is the oldest surviving line of monocots, then ancestral monocots would also have been aquatic.

In this 2004 paper Mark Chase points out that one basal lineage alone is poor evidence. “However, the combination of the two basalmost nodes (Acorus and Alismatales) with predominantly aquatic (both submerged and emergent) taxa does support the hypothesis that monocots were primitively aquatic or at least associated with wet habitats.”

He also identifies a reason that these basal lineages are well-suited to growing in water. “Aquatic angiosperms, such as Nymphaeales, also have scattered vascular bundles (atactosteles), vessels in the vascular tissue are also absent from aquatic taxa, and this syndrome is present in both Acorales and Alismatales.”

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This is a very interesting question. It is true that all aquatic plants are adaptable and that most aquatic plants are monocots, but of course, that doesn’t really answer your question.

I do a lot of work with pondweeds (Potamogetonaceae), which are probably the most diverse aquatics on earth. They are highly adaptable in a couple of ways: 1.) They exhibit widespread phenotypic plasticity to adapt their morphology to do their best to thrive even in less-than-optimum conditions. They can do this by increasing leaf surface area to adapt to low light conditions, increased absorption of nutrients from either the substrate or water column, modifying their reproductive capability to match their environment, et cetera. 2.) They adapt their genetic make-up maximizing hybridization and polyploidy. All pondweeds have a base chromosome count of either n=13 or n=14. The linear-leaved species tend to be diploid (2n=26 or 28), while the broadleaf species are nearly all tetraploid (4n=52), with some higher. Again, monocots tend to more often exhibit phenotypic plasticity, hybridization and polyploidy than dicots, but there is no definitive correlation that suggests that monocots are inherently more adaptable.

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I’m asking if monocots are better suited for aquatic environments than dicots and have an easier time adapting to them.

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do you know roughly how many aquatic plants are monocots vs how many are dicots vs how may are more basal angiosperms vs how many are other kinds of plants?

I’ll have to give this paper a read thank you!

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Wow, that is a very difficult question. Of the primary aquatic species that I follow in Vermont, about two-thirds of both total species and total observations are monocots (41 monocot species, 904 observations; 19 eudicot species, 447 observations). Of course, those numbers could change significantly in other parts of the world. The primary basal angiosperms in North America (at least) are in the order Nymphaeales, - the water lilies. In Vermont, we have seven species of water lilies, with 903 observations. If you add primarily wetland angiosperms to the data set, you include the massive glumiflorae species (Grasses, sedges, rushes and cattails), those ratios would be even higher.

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that’s why i was curious about your earlier claim that “most aquatic plants are monocots”.

i don’t have exact figures, but suppose there are 80,000 monocot species, and half of them are Asparagales. i’m just going to assume most Asparagles are not aquatic. so then the other half of monocots – let’s say they all are aquatic to some degree. that makes 40,000 aquatic monocots.

now let’s suppose there are 200,000 eudicot species. let’s carve out 30,000 Asteraceae species because most of those are not going to be aquatic. is it not possible that 25% of these remaining 170,000 eudicot species are aquatic?

(I haven’t met that word, I was using Poales - which of course includes grass)

Fynbos probably has a slightly higher proportion of bulbs that grow in water. Kniphofia, Wachendorfia thyrsiflora, Witsenia
Then dicots - swamp daisy Osmitopsis astericoides, protea Mimetes, fountain bush (pea) Psoralea, some Erica.

California’s vernal ponds?

Enough exceptions to break the rule, and keep you focused on what is that.

“The order Alismatales includes core Alismatales and two other families, Araceae and Tofieldiaceae. Core Alismatales consists of 12 families and 56 genera. Species of Alismatales are wetland or aquatic herbs, most of which have a completely submerged seedling phase. All marine angiosperms and most water-pollinated angiosperms are confined to this order.” https://onlinelibrary.wiley.com/doi/full/10.1111/jse.12182#:~:text=In%20monocots%20and%20eudicots%2C%20there,S1).

And Araceae is also known for having aquatic members – to name just a few, Orontium aquaticum, Pistia stratiotes, and all of the Lemnoidea. At the opposite extreme, there are also desert Araceae.

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hmmm. the article you linked is useful, but i’m not entirely sure why you quoted the particular passage about Alismatales specifically, since there are aquatic angiosperms that are neither marine nor water-pollinated.

i think the more relevant passages are these:

We found that aquatic angiosperms could be divided into two different categories: the four aquatic orders and the aquatic taxa in terrestrial orders. Aquatic lineages evolved early in the radiation of angiosperms, both in the orders Nymphaeales and Ceratophyllales and among basal monocots (Acorales and Alismatales)

We annotated the aquatic taxa on the tree of vascular plants (Fig. S1, red=aquatic families, blue=terrestrial families with aquatic members)

Aquatic plants are phylogenetically well dispersed across the angiosperms, with at least 50 independent origins, although they comprise less than 2% of the angiosperm species (Cook, 1990; Les et al., 1997).

looking at the families they’ve highlighted in their figure S1, i don’t disagree with the items they’ve highlighted in red and blue, but i wonder why some additional families aren’t classified as blue (terrestrial families that include aquatic members)? for example, i would think of some magrove trees in Rhizophoraceae as living in water and so would fit the authors’ definition of aquatic (“Aquatic plants are plants that have adapted to live in aquatic environments”), but the family is not highlighted blue.

but if go with the authors’ classifications, then their <2% of angiosperm species should be less than 5000 or 6000 species, and given that Alismatales are roughly 4500 species, then that does imply that a vast majority of aquatic plants according to these authors are Alismatales, and so a vast majority of aquatic plants would be monocots.

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I think you’re perhaps a bit fixated on the numbers, but I am happy that you are satisfied that there are more aquatic monocots than eudicots.

We still haven’t really defined what an aquatic plant is, or how we would quantify them for our question. I think that there are three primary problems that an angiosperm must overcome to live in an aquatic environment: 1.) Gas exchange/respiration; 2.) Nutrition and 3.) Reproduction. Water lilies (Nymphaceae) developed unique mechanisms for transferring CO2 from stomata in their floating leaves to their rhizomes across several meters of stem. Most aquatics need to be able to obtain nutrients from both the substrate and/or directly from the water column (the latter requiring thin submerged leaves without cuticles to absorb nutrients directly). Reproduction is a severe challenge. Various aquatics use entomophilic, anemophilic or hydrophilic sexual reproduction, nearly always combined with an asexual backup plan, using rhizomes, fragmentation and turions to reproduce.

So there are various degrees of “aquaticness” to further define wetland, emergent, floating and submerged plants.

I think the radiation diagram in the paper shows this best:
image

From a phylogenetic point-of-view, the basal angiosperms (technically, this paper is outdated and Ceratophyllales are no longer considered to be basal angiosperms but rather a sister clade to the eudicots) and basal monocots dominated the early aquatic environment. Indeed, one of our local lakes (an ancient oxbow lake) still has about 90% of total aquatic plant biomass wrapped up in one water lily (Nymphaea odorata ssp. tuberosa) and one hornwort (Ceratophyllum demersum). The remainder are all submerged rooted monocots (Najas, Potamogeton, Vallisneria, Zosterella, Elodea). In fact, the only submerged eudicots are bladderworts (Utricularia vulgaris).

So we know that basal monocots and basal angiosperms tend to outnumber eudicots in both the number of aquatic species and total biomass, but the best explanation of this has more to do with how long they have been adapted to aquatic environments than whether or not they are, for some reason, better adapted because they are monocots. Phylogenetics is still an emerging field, and it’s difficult to say how eudicots came into the aquatic picture in the first place. As mentioned earlier, the Ceratophyllales are now believed to be a sister clade to eudicots, but there is no evidence that any current eudicots, including Utricularia spp. (in the Lentibulaceae) and Myriophyllum spp. (in the Saxifragales) emerged from the Ceratophyllales, despite their general morphological similaries. Rather these species appear to have emerged from terrestrial plant lines, unlike their monocot relatives.

So in summary, we cannot point to any specific characteristics that appear to favor monocots over dicots, other than the former appear to have been at this game much longer than the latter. Indeed, the aforementioned Utricularia spp. and Myriophyllum spp. appear to be very successful aquatics, which in many places are filling in niches in existing environments and often supplanting monocot species with which they are in competition, especially with regards to low-light conditions and extracting nutrients directly from the water column.

In a few hundred millennia, they may completely dominate aquatic environments.

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well, i’m satisfied that there are more aquatic monocots than eudicots, according to the classification used by the authors of that article. as i noted before, there are plenty of plant families that i think of having aquatic members that the authors did not classify as aquatic. but i assume that’s just based on a difference in the definition of aquatic, as you note:

it’s more clear to me that the total biomass of aquatic plants would more likely lean monocot than eudicot, just because of how much area seagrass meadows cover, and I think every plant in such an ecosystem that could be classified as one or another would be a monocot.

since i’m trying to understand a claim that there are more aquatic monocots than aquatic dicots, i think it’s reasonable to actually try to quantify species numbers. even if the thought process is that monocots have just had more time in the water, i come from the perspective that there are just that many more (2-3x) eudicot species than monocot species. so i’d be surprised if their greater total species numbers for eudicots didn’t result in a ton of aquatic eudicot species, at least by some definitions of “aquatic”.

whatever that ratio is (based on whatever your definition is of aquatic) i think answers the original question, if you test by looking in bodies of water around the world and just count the species you find.

if you interpret the question as asking whether, in the long term, it’s easier for new aquatic species to arise from existing monocots or from existing dicots, then that’s a much harder question to answer because there’s a lot more uncertainty when you’re trying to predict the future.

in this context, i think the most relevant nugget from that article is that:

that suggests to me that there are lots ways to adapt to an aquatic lifestyle. so in this context, i think what you said here is reasonable way to look at the question from the evolutionary angle:

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Here is another paper relating to this subject that folks may find interesting: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4272260/

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