Why are plants not sentient beings?

It is not wrong to directly deprive plants of their life, in order to eat them, unlike animals, because they don't have all five aggregates. The first of the Five Precepts applies only to the taking of the life of sentient beings.

In my opinion, plants do not have mental formations and consciousness although they do have form, sensations and perception.

Without all five aggregates, plants are not sentient beings.

This can be traced to SN 5.10:

When the parts are assembled
we use the word ‘chariot’.
So too, when the aggregates are present
‘sentient being’ (satta) is the convention we use.

In the pali texts plants are considered living beings, are said to be born but are without that which is called mind, consciousness or intellect.

I guess they are classed as living beings because they are sustained by nutriment and are born from seed. They are becoming otherwise, are subject to old age, sickness and death.

They are composed of five elements; earth whatever density, water whatever liquidity, fire whatever heating fieryness, air whatever windy and space for whatever spatial.

Destroying plants is a minor offense in the Patimokkha, it isn't the same precepts as killing a human or animal.

As i understand it. A plant, unlike a cognizant being, is without elements which are derived from the essential elements, what is refered to as plant is then conceived & perceived as primary elements only, they do not serve as a ground for the element called mind, consciousness or intellect.

It is like a dead body, in that it is deprived of consciousness.

Plant is kind of like hair, it originates, grows and changes never serving as a base for hair-consciousness, unlike ie 'an eye' element, which in the Theravada Dhamma is a derived element also called 'a world' because it is that in the world which perceives and conceives the world with it's many elements derived and not derived.

What is that form which is the sphere of vision?

The eye, that is to say the sentient organ, derived from the four Great Phenomena, forming part of the nature of the self, invisible and reacting, (i.) depending on which eye, in consequence of some visible form, there has arisen, arises, will, or may arise

visual contact; … 177(ii.) and depending on which eye, in consequence of some visible form, there has arisen, arises, will, or may arise (born of that visual contact)

a feeling … [or iii.] a perception … [or iv.] thinking … [or v.] a visual cognition …

Difference is;

Thinking about mind we might see that the mind arises based on five senses or in and by itself. Thereforr it can be known that mind conceives and perceives visions, ideas or to perceives & conceives of and through other 5 doors:

Eyes are to perceive & conceive color&form through they eye door.

Ears are to perceive sound through ear door.

Nose is to perceive smell through the nose door.

Tongue is to perceive taste through the tongue door.

Body is to perceive bodily sensations though the body door.

That which is called mind, consciousness or intellect is to call/name/cognize/know/feel/perceive/conceive/objectify (what one discerns, that one cognizes, what one cognizes that one perceives, what one perceives that one feels, these quailities are conjoined) and to think about that which is called hair as hair, that which is called mind as mind.

Whenever contact occurs at the five sense bases therein contact at mind base can be delineated; contact at five is also mind made contact pertaining to sensuous sphere.

When an object enters ‘the door of the eye’ or any of the other 5 senses, it enters also the door of ideation; ‘the door of the eye’ is included as a delineation of the spheres of the mind-base, included in what is called ‘mind-base’; it is mind-made. (for reference; M. i. 295 (see Caroline A.F, Rhys Davis, first translation of Dhammasanghani Psychological ethics))

There should be a Sutta where Sariputta expresses it, i saw Buddhagosa reference it for sure but i don't recall which text it was. This shouldn't need substation anyway because it's derived from statements like 'all dhamma are mind made'.

Both the primary and the derived elements are perceived and conceived in dependence on derived elements.

That which perceives and conceives the world in the world, namely the mind, the eyes and other doors.

Hair is just hair, it is not a sense door neither are plants or parts of a plant.

World is conceived internally as internal elements, internally as external elements; externally as internal elements and externally as external elements.

Plants not having perception means that internal mind element is not generated internally.

Perception of a plant is thus only perceived & conceived externally, in dependence on 'internal mind element' which is generated internally or externally and which can be grasped with wrong view to be 'a self' or as 'belonging to a self' by this or that person.

Because they are not sentient.

Sentient: able to feel or perceive things.

There are too many possible, different interpretations of "rebirth" for there to be an authoritative answer on this. Secular Buddhism, the most modern iteration, says that genes and matter from one organism are eventually recycled into the species and ecosystem; as genes are passed to posterity and the body decays and is re-absorbed by other living organisms, one is physically "reborn" in that manner.

Other branches of Buddhism teach that "rebirth" happens day to day and moment by moment; one is re-born as an older version of one's past self, and hopefully as a wiser one, also.

Of course there are supernaturalist interpretations as well; that the soul is born into some other destination, as an angel or devil or animal of some sort. Ultimately, the teaching of "rebirth" is an all-inclusive one, and it expresses every possible relationship between cause and effect. One would have to be, literally, omniscient to fully understand rebirth -- how every cause is tied to every effect.

That is what makes the Buddha so special; in one moment he understood all of his own numerous past lives, and those of his ancestors; and how that made him who was was in his own present time. He was aware of karma of all other living beings as well; and that is why he, in his great compassion, taught them the dharma.

In short, while nobody, except for the Buddha, can possibly foresee how a plant could, over the course of many hundreds of millions of years, mutate into an animal or other lifeform -- they are a part of the cycle of samsara, and they do mutate, grow, and become "reborn" in some way. But of course, the Buddha never taught his disciples or lay followers to refrain from eating plants.

In Theravada (Pali), the 1st precept is about not destroying life that breathes (pāṇātipātī). The word 'pāṇā' means 'breath' and the word 'atipātī' means 'destroyer'. In other words, the term 'sentient being' is Mahayana. In Theravada, the Mahayana view of 'sentience', 'consciousness', 'perception', etc, is not relevant to the 1st precept.

I imagine, in the Buddha's time, before science, life that 'breathes' did not refer to plants. Plants cannot be seen to breath or exercise breath, such as via walking and moving.