Question

how do plants manage the waste materials they produce?

Answer 1

Plants don't have specialised organs for the removal of the waste materials, so they use various techniques for the removal of waste materials.
1. Excess water is removed through stomata by transpiration.
2. They combine the wastes with organic salts to get rid of wastes.
3. Stomata also serve as a excretory organ. During photosynthesis, oxygen is produced as a waste product. 

Answer 2

Unlike humans, plants do not have any specific system or organs to remove the waste materials from their body.

They usually use different techniques to remove waste material from their body. Some of them are listed below:

i) Excess amount of water is removed though stomata by transpiration  

ii) The oxygen produced during the photosynthesis process is removed from plants body using respiration.