There seems to be two different trends in Indian arguments in favour of plants.
1. On the one hand, some schools considered plants as simple living beings and, hence, respected them. For these schools, it is not uncontroversial that plants have a sort of basic awareness and that they take part to the cycle of karman. Jainism is the more consistent in arguing for the living status of plants. Early Buddhism may have included plants among living beings, but it is not clear whether only because of a general costume (and later Buddhism explicitly considers plants as just 'things'). 'Orthodox' schools of the so-called Hinduism are, again divided into two: epics and literature seem not only to argue for the sentience of plants, but to take it as self-evident, whereas philosophical schools usually deny it and interpret these passages as metaphorical.
2. On the other hand, plants are not only regarded as either insentient things or as simple living beings: in many cases we witness instances of plants being regarded as noble living beings, to be honoured and respected. This may have to do with the idea that plants are inhabited by divinities, but I sense that this is only a later, rationale, explanation of a primordial respect towards plants. Mahendra Kumar Mishra kindly made me aware of a paper of him about tribal ideas about nature, which by and large go in this direction. I am not daring to conclude that the 'Hindu' respect for plants as noble beings derives from a tribal concept of plants (nor do I believe in a 'uniform' tribal worldview, unchanged throughout centuries). I would rather propose that a 'lateral' (=non mainstream) notion of plants as noble beings has been preserved in some tribal milieus. Whatever the case, the appraisal of plants, even within those who acknowledged them as living beings, is highly differentiated.