Hii Aditi
As every topic has its own weightage and in competition level you just can't go with important topics only,you have to go through each and every topic.Plant physiology is a very important topic itself so you can't skip any topic from it but in general photosynthesis , transport in plants and  respiration in plants are most important chapters out of them because in them you need to understand the concept so make sure you go through these chapters multiple times.
To score good,you need to go through your NCERT textbooks thoroughly as most of the questions are asked from it so make sure you go through your NCERT textbooks thoroughly after that you can opt any reference book for solving MCQ'S like trueman of biology, MTG,Allen module etc.i hope this information helps you.
Thank you