I am trying to imagine a terraformed solar system in the late 3rd millennium to early 4th millennium AD. As a result, I am curious if plants on this Mercury, which would have all the oxygen, CO2, soil nutrients, and water they need to survive the sunlight intensity on Mercury. Also, this Mercury has had its day shortened to 20 hours. MANDATORY EDIT: This Mercury has a magnetosphere that is strong enough to block a similar percentage of UVB and A that Earth's does, or what I mean is, only an Earth-like amount of radiation reaches its surface. It has a surface pressure of 96,879 millibars.
It seems unlikely. Ultraviolet will basically kill anything we know of at that intensity. We use it to purify water as even microscopic life can't survive it. It definitely damages plants even on Earth as anyone involved with hydroponics can attest, too much will kill them in short order as it messes with their chemistry in a myriad of ways.
Our plants have evolved for the amount they receive on Earth, too little or too much is bad for them.
One of the main things plants get from UV light is that it stresses them, and therefore causes important protective mechanisms to combat the stress. These things are great for the plant, kind of like stressing muscles to make them grow for body builders. Overdoing it will cause irreversible damage.
Frame Challenge: Why expose them to full direct sunlight?
If you have the technological capacity and resources needed to Terraform Mercury you must by default also have the capacity to basically just 'green house' it. The gravity on Mercury is only about .4 of G BTW so it would have a thinner atmosphere than Earth by default and the intense radiation would strip that away very quickly.
Better to invest all your efforts in constructing a planet spanning network of giant arcologies (well as much of it as you want to anyway). You still end up a habitual planet without the hassle of dealing with the issues I raised above. Hundreds of thousands of square kilometers of the surface covered in sealed domes and towers (or any other shape you want) each stretching kilometers upwards into the sky and/or downwards into the crust. All of them with their own controlled ecosystems with Earth like air pressure, oxygen levels and weather as well as all the other comforts of home. Think rivers and seas, forests, hills, valleys and even mountains if you want. Just like the kind of thing you might see on larger space habitats only on a far grander scale. The planet would sparkle and glitter in the harsh sunlight when seen from space.
Rainforest trees often grow high, wide leaf canopies to gather as much sunlight s they can in the jungle, simultaneously blocking most light from reaching the forest floor. Additionally, in real life, there are many plants that grow branches or leaves that die, but remain attached to the plant. This happens with plants that live in dry areas such as deserts, so it isn't a far stretch to imagine plants on a brightly lit planet doing the same. Your plants could combine the traits above to quickly grow a dense canopy after being planted, which then shrivels and dries up. then the plants could grow at a more reasonable pace under the shelter of the canopy, like grass sprouting up under a tree in dry areas. To accomplish the canopy growth, the plants could grow from large seeds that contain plenty of stored nutrients so they don't need the sun until after they have assembled their canopy. the disadvantage of this method would be that the canopy would be dry tinder on a baking planet. it would probably be wise for the plants to fabricate some type of flame-retardant chemical on the canopy leaves.
There's another option: the plants hide from the sun. again, this is similar to what many real plants do, like morning glories, which open their flowers in the morning and close them later, or sunflowers, which move their flowerheads throughout the day to follow the sun during the day. your plants could have tough leaves that shield them during the day, then blossom at dusk for the plants to grow. They could even employ a hibernative state like some plants and animals, where they practically dry up in the day, perhaps storing all their water in their roots, then when the worst of the heat is gone, they could rehydrate themselves.
Third option: If your planet follows a normal spin pattern, there is probably somewhere like our poles that gets less light than most of the planet for at least part of the year, meaning that plants without to many special adaptations could survive there, especially if they have shelter from rocks, mountains, or other natural formations.
Fourth option: Water plants. Plants that live in swamp-like or even fully submerged environments. Apologies for the length of this answer, your question really got me thinking. Thank you!
"... Also, this Mercury has had its day shortened to 20 hours..."
If your future-science civilization has the technological power to alter the very spin of an entire planet... then surely they have the ability to build a sunshield or two.