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One of my worlds has a very sparse atmosphere with a pressure of about 0.17 bar, making it roughly one sixth as dense as our own. It is largely identical to our own in every other way, having the same temperature and mix of nitrogen, C02, and oxygen. The planet also has large oceans, just as on Earth.

What adaptations would help vascular plants survive and flourish in this atmosphere? Answers can be both macroscopic (e.g size, leaf shape) and microscopic (e.g stomata function, epidermal structure). Answers can extrapolate from real-world examples (likely from high altitude plants) or be newly synthesized.

I am looking for answers that generalize across vascular plants, but if you feel that to be too broad then you can focus on adaptations that would help the common oak (quercus robur) survive.

I am not particularly interested in methods of reproduction (e.g pollen dispersal) but moreso transpiration, gas exchange, things like that.

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    $\begingroup$ On Earth, the main problem which prevents plants from living at high altitudes is not the pressure: it is the cold. For example, to get a density of 0.2 kg/m³ on Earth we need to go to 15,000 meters above sea level, and the average temperature there is −56 °C (−69 °F). (On Earth, plants cannot grow above 6,500 meters above sea level.) $\endgroup$
    – AlexP
    Commented Jun 20, 2023 at 14:18
  • $\begingroup$ @AlexP has a good point, although I think your question can be answered despite it. Heat is transmitted through substance. Lower the amount of substance (aka, atmosphere), lower the amount of available heat to warm anything. While answers to this question will be interesting, I don't think you'll have a complete picture without dealing with temperature. Alex? Do you perceive those to be two questions, or are they too interrelated? $\endgroup$
    – JBH
    Commented Jun 20, 2023 at 14:44
  • $\begingroup$ @AlexP Please direct your attention to my first paragraph $\endgroup$
    – M S
    Commented Jun 20, 2023 at 14:51
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    $\begingroup$ For the record, I was just making a comment about the idea of basing the answer on high-altitude Earth plants. It is not hard to have an atmosphere with the required density and a resonable temperature, because air basically gets the temperature of the land or the sea; even with the amount of sunlight we get on Earth it would work just fine. My comment was intended to underline that here on Earth there is nothing similar with the required conditions. $\endgroup$
    – AlexP
    Commented Jun 20, 2023 at 15:27
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    $\begingroup$ Just to clarify, 15psi (Earth sea level) is 1 bar, or 100,000 pascals (Newton/meter^2). It is far more useful to define atmospheric conditions by describing the pressure, since things like "required to push oxygen into the human blood stream" is measured in partial pressures, also pascals. Density actually doesn't matter. You can replace all of the nitrogen in the air with helium and, as long as you have oxygen, you'll be fine, even though the air would be far less dense. $\endgroup$ Commented Jun 21, 2023 at 21:25

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How basal earth plants compare:

1/6th air pressure is pretty bad for earth plants. The highest altitude trees in the world is can survive at 17,000 ft, which puts their pressure at 0.52 atm. That's the limit for earth-based trees. The trees are called Polylepis tarapacana. They live in a dry desert climate. Access to more water would likely help them survive and grow.

Shrubs make it further, with a lovely looking sandwort called Arenaria bryophylla that can survive up to 20,000 ft at 0.46 atm on the slopes of Everest. Further than that, at least on earth, and most of the land is covered with snow.

So at least 0.46 atm is possible with earth plants. And if one plant can survive, more would be able to as well given enough time.

Challenges of low-pressure living:

  1. Lower moisture content in the air. Lose water through transpiration faster.
  2. Higher UV rays.
  3. Low C02 (unless the relative percentages of these are increased, Nitrogen decreased). I don't think lower O2 would be a problem for plants.
  4. As @laplap mentioned, the daytime-nightime temps would vary like a desert. High highs, low lows.

Here are some adaptations that help earth plants survive altitudes and lower water contents.:

  1. Oxygen Sensing
  2. Orchid adaptions to prevent water loss
  • Thicker cuticles on leaves to reduce water loss.
  • Ground bulbs that store water.
  1. Crassulacean acid metabolism (CAM), for CO2 uptake at night and reduced water lost during the day.
  2. Additional sinapoyl malate production to prevent UV damage.
  3. curling or direction-changing leaves. Allow plants to curl their leaves when they get too hot to avoid more sun exposure. If it freezes at night, curling leaves can help them survive de-thawing.

Edit: Water loss is likely the leading challenge in low-pressure environments. These researchers ran an experiment that tested common crops (radish, wheat, and lettuce) when exposed to 30 mins of less than 5 kpa atmospheric pressure. The temperature was maintained above freezing. The plants all survived after returning to regular pressures, and there was no difference in their growth rates afterwards. This suggests that earth plants can at least briefly survive very low pressures (much lower than 1/6th atm). The researchers noted that all the leaves had wilted during the 30 minutes, suggesting extreme water loss. This provides evidence that water loss in low-pressure environments is a major concern for terrestrial plants. The researchers speculate water loss would be the limiting factor for plant survival.

They also successfully grew those plants at 0.325685 atm, suggesting that even non-altitude specialized plants can survive at lower pressures than earth. Note that these plants were grown hydroponically, so water is not a concern. On open land, even rainy land, water will be much harder for plants to absorb.

In this study, researchers found that changing pressure didn't affect plant growth much, as long as CO2 concentration was above 0.07 kpa, lower pressures as low as 10 kpa. or 1/10th atm, didn't significantly affect plant growth. They grew Arabidopsis thaliana. The caveat to this study is they tested low pressures on already-develop plants, not a full life-cycle.

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The low density implies a low pressure, of about 17kPa. The main effect that I see is that the air is much drier, since the vapor pressure of water is about 3kPa at 25°C. In direct sunlight, the temperature can easily increase, and at about 60°C, the water will start to boil.

This very dry atmosphere would lead to usual adaptation for dry climate. and you can copy the plants in dryer climates.

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  • $\begingroup$ I may be wrong, but surely plant adaptations for dry climates are moreso due to the lack of soil moisture, rather than the lack of atmospheric moisture? $\endgroup$
    – M S
    Commented Jun 20, 2023 at 17:17
  • $\begingroup$ Further, would the decrease in difference between water's vapor pressure and the atmospheric pressure not if anything result in a relatively wetter atmosphere? $\endgroup$
    – M S
    Commented Jun 20, 2023 at 17:19
  • $\begingroup$ @MS No. Relative humidity is not relative to the rest of the atmosphere--it is only relative to the vapor pressure of water at a given temperature. If you keep the composition and temperature the same, but uniformly reduce the pressure, you necessarily reduce the humidity. $\endgroup$ Commented Jun 20, 2023 at 17:38
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    $\begingroup$ @MS en.wikipedia.org/wiki/Humidity#Relative_humidity High pressure weather systems in Earth's atmosphere are associated with low humidity because they tend to produce adiabatic heating which raises the vapor pressure without raising the actual mass of water in the air, and vice-versa for low pressure systems. That's completely different from uniformly reducing the pressure without altering temperature. $\endgroup$ Commented Jun 20, 2023 at 17:56
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    $\begingroup$ The chemical composition of the atmosphere has to be different, by quite a lot. If liquid water exists on the surface, then, the partial pressure of H2O in the atmosphere is 3kPa, at 25°C. This means that in average, about 1/5th of the atmosphere by mass is made of H2O, which is very important. It will change atmospheric chemistry and probably create strong winds and rains. $\endgroup$
    – LapLap
    Commented Jun 21, 2023 at 18:07
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Most of the gasses in the atmosphere are completely irrelevant to plants, except insofar as having more air mass provides for better thermal conductivity--which can be good or bad depending on whether the plant would currently prefer to be warmer or colder than it is. Yes, plants use oxygen, but not very much--1/6th of Earth's oxygen concentration would be more than enough. The two things you really need to care about, though, are CO2 and water. Since the temperature is specified to be the same as Earth, reducing water vapor concentrations to 1/6th means a severe drop in relative humidity--i.e., your atmosphere is dry, and plants will lose water much more easily. And if the atmosphere is consistently that dry, there must not be a lot of surface water to replenish it, so your plants will have trouble getting water from the ground as well, and can't just accept a high transpiration rate. In other words, they will need adaptations similar to Earthling desert plants to conserve water--small leaves, or no leaves, waxy cuticles, preferentially opening stomata for gas exchange only at night / when it's cold.

Currently, the concentration of CO2 in Earth's atmosphere is around 420ppm, or 0.042 atmospheres. That's higher than the recent past, but higher starting levels are better for us, so we'll go with it. That means your world will have a CO2 concentration of 0.007 atm.

The CO2 compensation point (i.e., the minimum concentration below which a plant cannot concentrate enough CO2 out of the air to perform photosynthesis faster than it performs respiration) for C3 plants is somewhere between 0.0025 and 0.005 atm--your world is still above that, so theoretically, standard C3 plants could survive with reduced growth rates, although going below 0.017 atm will cause most plants to struggle--the theoretical lower biochemical limits and what any given plant will actually be able to survive are rather different things. The CO2 compensation point for C4 plants, on the other hand, is 0.001atm--and C4 plants are in fact hypothesized to have evolved in response to dips in CO2 concentrations during previous ice ages.

So, your vascular plants will probably need to use CAM/C4 photosynthesis, doing gas exchange only at night at concentrating CO2 for later daytime use in light-dependent reactions.

Alternately, there are some aquatic plants which take in carbon in the form of bicarbonate rather than CO2 and chemosynthetic organisms which rely on consuming small organic molecules to build into larger organic molecules, rather than fixing CO2 at all. Thus, some of your vascular plants may get by without doing gas exchange to capture atmospheric CO2 at all, and instead rely on sucking carbon out of the soil. Given than even C4 and CAM plants are still much more comfortable at levels well above their lower biochemical compensation limit, this will probably a fairly widespread strategy.

EDIT: With the edit to the question, removing water vapor from the atmospheric downscaling, adaptations to dry desert conditions become irrelevant. So, you can forget about the CAM cycle--plants may still work to concentrate CO2 at night, because why not, but not exclusively so. Instead, you'd be looking at broad leaves for increased surface area, with lots of stomata doing gas exchange as much as possible. That will result in increased transpiration, leading to a faster water cycle and increased cloud formation over densely-vegetated areas. The requirements for more ground water are actually favorable if they can be met (actual desert areas on this world will still require desert adaptations, which are in conflict with optimal CO2-compensating adaptations), as it allows for a greater flow of dissolved carbon sources, and non-aquatic plants will probably develop specific relationships with soil fungi to provide carbonate in a convenient form (i.e., not paired with an excess of calcium ions that the plant would have to dispose of), and you would see increased plant growth in regions with large quantities of carbonate minerals, and established biomes where decaying organic carbon can be easily sourced from the soil for recycling.

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  • $\begingroup$ Why would the concentration of carbon dioxide be lower? The question says that the atmosphere is just like Earth's, except thinner, so that the concentration of carbon dioxide would be the same 0.04% as we have. Sure, the partial pressure of CO2 would be lower. (And for Earth plants, photosynthesis becomes extremely inefficient below 100 ppm CO2 at one atmosphere. While they can photosynthesize at lower concentrations of CO2, the rate of photosynthesis becomes too low to offset the respiration needs of the plant; in general, plants cannot reduce the concentration of CO2 below 100 ppm.) $\endgroup$
    – AlexP
    Commented Jun 20, 2023 at 19:38
  • $\begingroup$ This ("And if the atmosphere is consistently that dry, there must not be a lot of surface water to replenish it) seems moreso like working backwards (if the air is dry then surely the ground must be dry too) rather than a logical conclusion. If the planet has the same amount of water as on Earth (which I admittedly didn't specify but it can be assumed), then there is no reason for the planet to be more dry. $\endgroup$
    – M S
    Commented Jun 20, 2023 at 20:46
  • $\begingroup$ Otherwise, thank you for the educational answer $\endgroup$
    – M S
    Commented Jun 20, 2023 at 20:52
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    $\begingroup$ @MS Relative humidity is defined relative to the vapor pressure of water, not to the pressure of other atmospheric components, so both relative ans absolute humidity would go down, but relative humidity is the important measure--that's what controls the evaporation rate of water. If the planet has the same amount of surface water as Earth and the same temperature, then it is impossible to satisfy your condition of reducing the density of all atmospheric components by the same proportion to retain an identical composition; thus, the planet must be dry. $\endgroup$ Commented Jun 21, 2023 at 0:57
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    $\begingroup$ @MS The only factor determining how much water the air can "hold" is temperature. All the other gasses are irrelevant. So if you keep the temperature and surface water distribution the same as Earth, then you should expect humidity and rainfall patterns similar to Earth. The only effect of lowering the density of other gasses would be making it harder to support cloud droplets, so droplets would have a lower maximum size and rain would occur at slightly different times. $\endgroup$ Commented Jun 21, 2023 at 14:56
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First of all, the low pressure is a burden on the plants, since they cannot suck the water (in liquid form) up the capillaries indefinitely high because sucking water up a tube is in the best case scenario the same as the atmosphere pushing it up, while a vacuum in the upper part of the tube does not push in the opposite direction at all, so you are limited by the pressure of the atmosphere which in turn limits how far you can suck water up a tube, as the pressure of the water after reaching some height equals the pressure of the air pushing it up, so you cant suck the water up further, but rather just as high as the pressure allows.

On earth, the bound for sucking water up is about 10m.

On your planet it would be a sixth of that.

However, as you might have noticed, trees can grow up to 100m, so that vacuum suck explanation really is just a simplification leaving out other ways plants transport water, so your plants can simply use that other mechanisms too, there is no reason why they wouldn't work.

Also, there might even be much water on your planet without the system being obviously physically inconsistent, as long as your chosen pressure is less than the pressure the evaporated water would cause at your chosen temperature and gravitation, so the thin atmosphere may be less of a problem.

Furthermore, your plants may extract CO2 and possibly a little nitrogen from the air (plants only need very little CO2, so this is no problem) to create nitro-glycerine (yes the explosive) or something similar that actually can be converted to energy without external oxygen, which they then live off, so the lack of oxygen is no big deal.

(or they could simply have a slower metabolism, so the thin atmosphere of your planet suffices for them.)

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    $\begingroup$ The bound for sucking up water on Earth is considerably higher than 10 meters, because it is not limited by atmospheric pressure. Atmospheric pressure helps, but capillary action alone will raise water with no pressure differential at all, and cohesion allows plants to develop negative pressure in the water column to pull water well above the atmospheric pressure limit. $\endgroup$ Commented Jun 21, 2023 at 0:54
  • $\begingroup$ Im just talking about that negative pressure part here. If you take any hose (of considerable diameter) and pull it up higher than 10m the water wont go higher. Thats my point, and yes, as stated, thats not all, thats the point. $\endgroup$
    – KGM
    Commented Jun 21, 2023 at 2:05
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    $\begingroup$ You said "sucking water up a tube is in the best case scenario the same as the atmosphere pushing it up", but that's just not true. $\endgroup$ Commented Jun 21, 2023 at 2:37
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    $\begingroup$ Plants actually suck water up from their leaves. Transpiration creates negative pressure at the top, and the water is pulled up for most of the distance. So, yea, they are actually tubes with suction at the top. Molecular cohesion of water is strong enough to prevent bubbles from forming. This only works if there are no voids in the tube to start with. $\endgroup$ Commented Jun 21, 2023 at 21:36
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    $\begingroup$ "Suction is...the absence of force" Ok, you got me. It isn't actually suction, any more than you can call it suction when you pull a rope up. It's inter-molecular cohesion acting as a mediator between the suction over the massive area of the plant's pores, combining to haul the column of water up the great distance. They actually do measure it as negative pressure, which is more than the absence of force. Water can withstand a negative pressure up to around -3Mp before separating, and you only need about -1.5Mp to get to the top of a sequoia. $\endgroup$ Commented Jun 22, 2023 at 16:00

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