Silent Running with Bruce Dern:

[link to en.wikipedia.org]

Silent Running depicts a future in which all plant life on Earth has been made extinct. Only a few specimens have been preserved in enormous, greenhouse-like geodesic domes attached to a fleet of American Airlines space freighters, just outside the orbit of Saturn. Freeman Lowell (Bruce Dern), one of four crewmen aboard the Valley Forge, is the resident botanist and ecologist who preserves the forests for their eventual return to Earth and the reforestation of the planet. Lowell spends most of his time in the forests, cultivating the plant and animal life.[2]