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I trying to describe colonization of an Earth Like planet orbiting a red dwarf in its habitable zone.

  • It is NOT tidally locked with the star
  • Similar to Earth though somewhat larger
  • Magnetic field is strong enough to protect the surface from the flares
  • Its year lasts 20 Earth days
  • Has large tidally locked moon that orbits it on a distance of 100,000 km
  • Obliquity of 90 degrees

What kind of crops should be able to grow on the planet surface to provide food for the colonists?

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    $\begingroup$ "20 days" relative to your planet or to Earth? $\endgroup$ – Zxyrra Nov 18 '16 at 23:05
  • $\begingroup$ You effectively have a 20 day diurnal cycle, right? I would think the ten days of darkness and ten days of light cycles would be pretty hard on the crops. $\endgroup$ – Samuel Nov 18 '16 at 23:57
  • $\begingroup$ @Zxyrra Earth days $\endgroup$ – Soba Nov 18 '16 at 23:59
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    $\begingroup$ @Soba And it likely will be, eventually, without the 90 degree obliquity. But it appears your story isn't about life evolving on this planet, it's about life visiting this planet. If you don't require a timeline that allows intelligent life to form on this planet, then I think you can know the planet will eventually suffer that fate in a million years, but not be currently be tidally locked. But that's irrelevant to your question, I was just clarifying the setup. $\endgroup$ – Samuel Nov 19 '16 at 0:12
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    $\begingroup$ @JDługosz That's why I have large satellite to keep the planet from wobbling. Mercury type resonance is not good enough, if forced I would move my setting but so far my locking prevention seems plausible. $\endgroup$ – Soba Nov 20 '16 at 15:47
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Ability to photosynthesize

Here is a graph of blackbody emission spectra:

enter image description here

The Sun is the 5777K line, while a red dwarf would be more like the 3000K line. The good news is that the spectrum is reasonably similar to that of earth. The chorophyll ab complex in plants has good absorption in both the 400-500nm (purple-blue) and 600-700nm (red-infrared) range. In the case of the red dwarf, that red-infrared absorption will be good. The blue absorption is useful in plants because of scattering in the blue sky makes relatively more blue light available in shade; I don't know about your sky composition, but assuming it is nitrogen-oxygen, there will be significantly less blue light to scatter, and your plants will do relatively more poorly in the shade.

There is some research on growing plants under red lights in greenhouses. Some of the things of note are that far-red radiation is good at stimulating flowering of long-day plants otherwise kept in darkness. That means, your plants will likely recognize that it is 'daylight' with red-infrared heavy sunlight. Also of interest, plants grown under red lights tend to have elongated appearances, growing taller with long thin leaves.

Additionally, some cyanobacteria have different accessory pigments that assist chlorophyll in absorbing other ligth wavelengths. Generally, these are evolved for blue light absorption in deep-water, but there are some pychobillin pigments that have good absorption in the 650nm range.

Ability to tolerate long daylight hours

You mention 90% obliquity, and there is some discussion in the comments about whether that is required or not. I will address the situation of 24 hour light during the growing season. There are many plants on earth already optimized for daylight lengths between 12-20 horus. Obviously, 24 hour dark is a non-starter, but day lengths down to as low as 6 hours can be easily accomodated for plants adapted for shade.

This paper tested four plants under 24-hr photoperiod; lettuce improved yield at 24 hours; cucumbers did well when young but needed 4 hours of darkness as the plant matured; chili's tolerated 24-hr light; and tomatoes did poorly. I found evidence in other papers for clover, chickpeas, oats, and barley all doing well with long (20+ hour) photoperiods.

Generally, I would assume plants that are grown near the arctic circle, especially northern Europe, would be better adapted for 24-hour light. In Iceland, these include potatoes, turnips, cabbage, and kale, and that is just below the arctic circle.

Additionally, plants use a circadian rhythm to time how to open and close their stomatae. When their timing is good, their productivity and yield goes up. So your plants will need to 'learn' when to open and close their stomatae for maximum efficiency in permanant daylight. This is just a matter of plant breeding and selecting the best specimins. Although productivity may be low at first, a few dedicated years by some agronomists should yield appropriate 24-hour strains for those crops which are suitable for permanant light.

Conclusion

A red dwarf has a similar enough spectra that plants would survive, and the presence of far-red light will help the plants manage their flowering cycles. Plants will need to adjust to unusual photoperiods, so there will be some years of low yield while better varietals are bred on-world.

While not all plants will thrive in 24-hour light conditions, there is a good variety that will. Most earth crops will do well with 12-20 hour photoperiods, and shady crops (mostly vegetables; brassicas, leafy greens, many root vegetables) will survive in lower light levels.

There should be a wide variety of plants usable, so long as atmospheric and soil conditions permit.

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