From Involvement of receptor potentials and action potentials in mechano-perception in plants Australian Journal of Plant Physiology (2001) volume 28, page 567-576:
Plants are always exposed to various external stimuli, such
as light, gravity, chemicals, temperature and mechanical
stress. The sensitivity of plants to these stimuli is as high as
that of animals, or sometimes higher (Shropshire 1979).
Compared with animals, plants are quiet and do not show
dynamic responses to these stimuli, except for some special
plants, such as Mimosa and some carnivorous plants.
However, all are capable of perceiving external stimuli and
show responses, although these responses may not sometimes
be recognised by us.
A brief investigation was made into whether the apparatus
developed for characean cells could be applied to higher
plants. A seedling of broccoli (Brassica oleracea var.
bolrytis) was mounted on the apparatus shown in Fig. 6.
Upon dropping the glass tubing onto the hypocotyl, a
significant change in the electrical potential was induced, the
amplitude of which increased with increase in H. (Fig. 10).
Thus, this apparatus developed for internodal cells of
Characeae is also useful for analysis of electrical responses
of higher plants to mechanical stimuli.
Caption of Fig. 10:
Electrical response of hypocotyl of broccoli to mechanical
stimulation. A seedling was mounted in the apparatus shown in Fig. 6.
The hypocotyl was stimulated at pool B by dropping a piece of glass
tubing (1.3 g). Numbers below the records represent the height (cm)
from which the glass tubing was dropped.
According to The dignity of plants Plant Signaling & Behavior (2009) Volume 4, pages 78-79:
We do not know if plants are capable of subjective sensation. There
is no scientific proof that plants feel pain. But it is also quite clear
that we cannot simply rule this out. There is circumstantial evidence
for this, although not a complete chain of evidence. However, claims
that plants have no subjective sensations are as speculative as the
opposite. We simply do not know.