activity of the participating enzymes, i.e., regulated both by gene expression and the
transcription level [68]. However, diffusion of largely hydrophobic VOCs at
published rates would require extremely high concentrations which are potentially
harmful, and it has therefore been proposed that plant VOCs are actively emitted
using transmembrane structures [67] which are yet to be identified. Either way,
emission of plant VOCs is a controlled process which is adjusted to the developmental stage of a particular tissue or even on smaller scales such as the circadian
rhythm [68, 69].
2.2.1 The Ripening Process
The chemicals described above were documented in the scent of wild and cultivated
ripe fruits. However, the data available per species is almost exclusively based on
snapshots – records taken in a single moment, often in an unstandardized moment in
the fruit’s maturation process. However, fruit scent is not static. Many – but notably
not all – fruits change their scent qualitatively and quantitatively once ripe [14, 37,
39, 40, 51]. In cultivated species, which have probably been artificially selected to
increase their aroma, the amount of scent emitted by ripe fruits increases by a factor
of up to 30 [14, 70]. In wild fruits the median increase in 19 species specializing on
seed dispersal by primates was found to be 2.3 [51]. Notably, the same study found
no increase in the amount of scent emitted in fruits consumed by sympatric birddispersed species, highlighting the not surprising similarity between human artificial
selection and natural selection by our closest living relatives. An increase in the
amount of scent emitted by ripe lemur-consumed fruits was found in other sites in
Madagascar [71] and in two fig species from Panama [37]. Fruit scent also changes
qualitatively upon ripeness, with increased emission of compounds that were present
in low proportions or fully absent in unripe fruits [14, 37, 39, 40, 51].
Studies conducted in the wild [37, 39, 40, 51, 71] have aimed to record dichotomous behavior among animals while interacting with ripe and unripe fruits.
This choice led in most cases to a comparison of two snapshots of fruit scent –
one before the onset of ripening and the other around its peak. Thus, studies of wild
species are not informative with regard to the process of change in fruit scent.
A single exception is a study by Sánchez et al. [72], who reported a decrease in
the amounts of ethanol and acetaldehyde in rusty figs (Ficus rubiginosa).
In contrast, several studies on cultivated fruits have tracked the changes in aroma
compounds, along with changes in fruit quality and seed development [14, 73]. With
regard to the ripening process, fruits can be roughly divided into two groups:
climacteric and non-climacteric. Climacteric fruits are those whose final stage of
development is characterized by increased respiration and ethylene production, and
they tend to exhibit a rapid ripening process, while non-climacteric fruits mature
more gradually [73, 74]. In climacteric cultivated species, the shift to ripe fruit scent
is rapid. For example, in peaches, the major shift in fruit scent occurs abruptly, and
once the seeds approach their final weight, as emission of three GLVs characteristic
to unripe fruits decreases, while the emission of compounds typical to ripe fruits
increases [75]. Similarly, another study of apricot and plum X apricot hybrids
compared the volatile profiles of fruits in three late developmental stages: mature
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transcription level [68]. However, diffusion of largely hydrophobic VOCs at
published rates would require extremely high concentrations which are potentially
harmful, and it has therefore been proposed that plant VOCs are actively emitted
using transmembrane structures [67] which are yet to be identified. Either way,
emission of plant VOCs is a controlled process which is adjusted to the developmental stage of a particular tissue or even on smaller scales such as the circadian
rhythm [68, 69].
2.2.1 The Ripening Process
The chemicals described above were documented in the scent of wild and cultivated
ripe fruits. However, the data available per species is almost exclusively based on
snapshots – records taken in a single moment, often in an unstandardized moment in
the fruit’s maturation process. However, fruit scent is not static. Many – but notably
not all – fruits change their scent qualitatively and quantitatively once ripe [14, 37,
39, 40, 51]. In cultivated species, which have probably been artificially selected to
increase their aroma, the amount of scent emitted by ripe fruits increases by a factor
of up to 30 [14, 70]. In wild fruits the median increase in 19 species specializing on
seed dispersal by primates was found to be 2.3 [51]. Notably, the same study found
no increase in the amount of scent emitted in fruits consumed by sympatric birddispersed species, highlighting the not surprising similarity between human artificial
selection and natural selection by our closest living relatives. An increase in the
amount of scent emitted by ripe lemur-consumed fruits was found in other sites in
Madagascar [71] and in two fig species from Panama [37]. Fruit scent also changes
qualitatively upon ripeness, with increased emission of compounds that were present
in low proportions or fully absent in unripe fruits [14, 37, 39, 40, 51].
Studies conducted in the wild [37, 39, 40, 51, 71] have aimed to record dichotomous behavior among animals while interacting with ripe and unripe fruits.
This choice led in most cases to a comparison of two snapshots of fruit scent –
one before the onset of ripening and the other around its peak. Thus, studies of wild
species are not informative with regard to the process of change in fruit scent.
A single exception is a study by Sánchez et al. [72], who reported a decrease in
the amounts of ethanol and acetaldehyde in rusty figs (Ficus rubiginosa).
In contrast, several studies on cultivated fruits have tracked the changes in aroma
compounds, along with changes in fruit quality and seed development [14, 73]. With
regard to the ripening process, fruits can be roughly divided into two groups:
climacteric and non-climacteric. Climacteric fruits are those whose final stage of
development is characterized by increased respiration and ethylene production, and
they tend to exhibit a rapid ripening process, while non-climacteric fruits mature
more gradually [73, 74]. In climacteric cultivated species, the shift to ripe fruit scent
is rapid. For example, in peaches, the major shift in fruit scent occurs abruptly, and
once the seeds approach their final weight, as emission of three GLVs characteristic
to unripe fruits decreases, while the emission of compounds typical to ripe fruits
increases [75]. Similarly, another study of apricot and plum X apricot hybrids
compared the volatile profiles of fruits in three late developmental stages: mature
408
O. Nevo and M. Ayasse