Please, help me to answer this question

Hi friends,

I have not post for a long time because I did not find anything
interesting to contribute with, but I keep reading you. I belong to a
small group of Chilean Sci-Fi writers who is very interested in
science and tech, and that knows about this group. Unfortunatelly
most people of my generation, in my country, does not speak English,
so they ask me to be the interface with you.

We have a big question that might help us to write good tales.

Imagine that suddenly -because the action of some virus or whatever-
all vegetal life of the planet die.

Then question is. How long we will have oxigen to breath? What
would be the main ecological effects we would observe? would the
planet get hotter of cooler, becoming a new Venus or a new Mars?

We know that the planet current state is dependent on life, and that
a threat to plants would change the physical aspect of the planet.
How those changes would be and what secuence of events they will
follow?

Please, help us to get the information to answer those questions. And
any help in the right direction will be very appreciated.

Thank you,

Omar Vega,
Chilean Sci-Fi writer.