A painting based on nature like flowers, plants, trees, botany, animals water, fire, environment etc. Nature, in the broadest sense, is equivalent to the natural world, physical universe, material world or material universe. "Nature" refers to the phenomena of the physical world, and also to life in general.
That gives us a lot to work with! Nature is both all around us and deep within us. We are inseparable from nature - our bodies, lives and minds depend on the air we breathe and the food we eat. The earth sustains our very life force. Without the earth - without nature - what would we be?
Painting based on nature can take many forms and serve many purposes. Because "nature" is such an immense topic that encompasses so many things, I can only provide a partial list of the various subtopics that may appear in nature-related artwork:
flowers, plants, trees, botany, animals, cells, anatomy, bodily systems, weather, geology, matter, energy, fossils, any of the natural sciences, water, fire, environment, conservation, natural history, processes, evolution, birth, growth, aging, decay, change…. 
And of course within each subtopic, there are further subtopics, and on and on... So if you want to create a work of art based on the theme of "nature", you've really got a whole planet full of ideas to work from!
