Water movement has been considered a possible contribution
to fruit firmness, and aquaporin proteins may influence water
loss during ripening (Seymour et al., 2013). Aquaporins (AQPs)
are water channel proteins that allow rapid and selective transport
of water across membranes, which would presumably also
affect water movement out of the cell. Fruit softening could be
associated not only with cell wall disassembly, but also to loss of
cell turgor mediated by water flow through aquaporins (Alleva
et al., 2010). Aquaporins are also considered to be regulated by
ethylene during fruit ripening; for example, the stimulation of
AQP2 by ethylene in grape skin tissues reached 300% (Chervin
et al., 2008). Some aquaporins investigated in our study were
up-regulated in melatonin-treated fruit (Fig. 4B). This result
was consistent with the advancement of ethylene production,
implying that melatonin may have an effect on the ethylene
pathway to enhance the expression of aquaporin genes, leading
to increased water loss in melatonin-treated fruit.