Which are the different substances excreted by plants why?

A very good resource for plant physiology and excretion is NCERT Biology Book which explains these concepts in simple terms for students.

Plants excrete oxygen during photosynthesis, carbon dioxide during respiration, water vapor during transpiration, and sometimes secondary metabolites like resins, gums, and latex. These processes help maintain balance and remove excess or harmful byproducts.

Some substances like alkaloids and tannins are excreted as defense mechanisms against herbivores. Others like resins and latex help seal wounds and protect against pathogens.