What types of radiation are you attempting to harden the plants against? As from my basic understanding melanin is good for absorbing UV radiation but not anything of greater energy like the cosmic rays and high energy protons that are the real menace in space.
What types of radiation are you attempting to harder the plants against? As from my basic understanding melanin is good for absorbing UV radiation but not anything of greater energy like the cosmic rays and high energy protons that are the real menace in space.