Why are plants not sentient beings? Why in Buddhism plants are not recognized as sentient beings?

Can it be immoral to kill a plant?  Can a plant be reincarnated as an animal?  Is a plant alive?

Edit: I read somewhere a theory according to which Buddhism did not consider plants as sentient beings, part of saṃsāra, because although they are alive they do not have the 5 khandhas and especially they do not have consciousness (viññāṇa) although they have perception (saññā).  A sunflower perceives the sun and reacts by turning towards it but it is not conscious of its perception, it has no metacognition.  I found this idea interesting but I don't know how true it is since Buddhism seems to define consciousness more as what allows discrimination than as what allows metacognition.
Can it be immoral to kill a plant?

It's contrary to the Vinaya for monks -- I don't know whether you consider that as implying that it's "immoral".


  DESTROYING VEGETATION  
  
  The common belief at the time of the Buddha was that plants (and even soil) were 'one-facultied life.' Today we have ecologically 'green' beliefs that are often equivalent — at least they seem to lead to much the same attitudes.[41] (In Thailand, forest monks are well known as the best protectors of the jungle.)
  
  The eleventh Confession offence concerns destroying plant life. It originated because a bhikkhu harmed 'one-facultied life' by cutting down trees. He continued to cut down a tree even when the tree-deva asked him to stop, so she went and complained to the Buddha. This led to lay criticism of such behavior and a rule was set down:
  
  
    "Intentionally damaging or destroying a living plant is [an offence of Confession.]"(Summary Paac. 11; See BMC p.294)
  
  
  Therefore destroying a living plant — for instance, felling a tree, uprooting a flower, burning grass — is a Confession offence; as is picking fruit from a tree, a flower from a bush, etc. It is an offence of wrong-doing (dukka.ta) to damage or destroy fertile seeds or pips, or viable seedlings. (See Kappiya).
  
  Bhikkhus who live in tropical forest monasteries constantly have to protect both the jungle and themselves. When paths are overgrown, snakes and other dangerous 'creepy-crawlies' can be trodden on — and bite back! There also may be a need for firebreaks. One way that forest monks cope with this is a daily routine of sweeping the paths. However they are not allowed to dig or clear the land.
  
  The tenth Confession offence arose when bhikkhus dug the ground and got others to dig, and the local people criticized them because they considered the earth to be 'one-facultied life.' The rule is phrased like this:
  
  
    "Should any bhikkhu dig soil or have it dug, it is [an offence of Confession.]" (Paac. 10; BMC p.292)
  
  
  Digging, breaking the surface of the earth, lighting a fire on it, pounding a stake into it are all disallowed. (If such 'earth' is more gravel or sand than 'soil' — and has no living creatures in it — it may then be dug.)
  
  It is, however, allowable for monks to hint to laypeople or novices about what needs doing as long as the words or gestures fall short of a command. When bhikkhus need paths to be cleared, necessary work done on the ground, firebreaks made, etc., any lay attendant wanting to help should listen out for hints and indications: 'A post hole dug over there would be useful'; 'make this ground allowable,' etc. What is needed can then be clarified.
  
  One practical and long term effect of these rules is that they have steered bhikkhus away from involvement in agriculture and land ownership. Such a development would also have isolated bhikkhus from the lay community because they would no longer have needed to depend on alms food.


See also Harmlessness to non-sentient nature and can consciousness (pure awareness) exist?



An Introduction to Buddhist Ethics: Foundations, Values and Issues says on page 175,



So "one-facultied" means "possessing the sense of touch". I guess that might imply non-sentient if it's incapable of thought -- or, it says, it's a "borderline" case.

It also says that people perceive them as alive and that it's therefore not seen as being harmless to kill them.

Whether they are actually sentient isn't stated e.g. by the Buddha, nor in the Abhidhamma.