Picture11.12.png

Figure 11.12. Ethylene evolution and respiration (measured as CO₂ production) undergo a rapid increase then decline as fruit ripen and soften. In tomato (A), the peak of ethylene production and respiration occur relatively early in ripening, shortly after the first visible sign of red coloration on the outside of the fruit (known as the breaker stage). Abbreviations of ripening stages: IG, immature green; MG, mature green; BR, breaker; PK, pink; LR, light red; RR, red ripe; OR, over ripe. In kiwifruit (B) that were harvested and stored at 20°C, the peak of ethylene evolution occurs very late, when substantial softening has already occurred and the fruit are almost at the eating ripe stage. The earlier peak in respiration may have been caused by harvest. The two studies use different units so absolute values cannot be compared between the species. (Data taken from Maclachlan and Brady (1994) Plant Physiol. 105, 965-974; Rothan and Nicolas (1989) HortScience 24, 340-342; Taglienti et al. (2009) Food Chem. 114, 1583-1589)

Ethylene production is closely associated with fruit ripening in many species, and is the plant hormone that regulates and coordinates the different aspects of the ripening process; colour development, aroma production and texture are all under the control of ethylene (Klee and Giovannoni 2011). Typically, fruit will generate barely detectable amounts of ethylene until ripening when there is a burst of production (Figure 11.12). Historically fruit have been categorised into two classes of behaviour with respect to ethylene physiology and respiratory pattern (Table 11.2). In the first type, as fruit progress towards edibility the respiratory rate increases followed by a decline as fruit senesce. This is known as the climacteric rise. Pear, banana and avocado (Figure 11.13) show an especially strong respiratory rise. Ethylene production also increases sharply to a maximum at this time, and then declines before fruit rots intervene and lead to a renewed output. The major rise in ethylene production may take place before, just after or close to the respiratory peak. Such fruit are classed as ‘climacteric’, with apple, avocado, banana, fig, mango, papaya, passionfruit, pear and tomato being classic examples. As with the respiratory rise, the levels of ethylene produced vary widely between species. Climacteric fruit ripen after harvest, and need not remain on the tree or vine. A second category of fruit, exemplified by blueberry, cherry, citrus, cucumber, grape, pineapple and strawberry (Table 11.2) do not show such sharp changes. Respiration rate either remains almost unchanged or shows a steady decline until senescence intervenes, with little or no increase in ethylene production; these are called ‘non-climacteric’ fruit, and fruit ripen only if they remain attached to the parent plant.

Munns_PlantsInAction_Tab11.2.jpg.png

While all fruit were once classified under this either/or nomenclature, more recent work has shown the distinction to be less clear-cut (Paul et al. 2012). In some species, notably pepper and melon, different cultivars or genotypes exhibit characteristics typical of climacteric or non-climacteric behaviour. It has been shown that in climacteric fruit some ripening changes occur independently of ethylene, and that some non-climacteric fruit have ethylene-requiring changes during ripening. With the development of more sensitive ethylene measuring devices, many non-climacteric fruit appear to show an increase in ethylene evolution at previously undetectable levels upon ripening. The existence of ethylene-dependent and ethylene-independent pathways in both climacteric and non-climacteric species (Barry and Giovannoni 2007) suggests that regulation by ethylene is ubiquitous, and that climacteric and non-climacteric behaviour are more accurately envisaged as the extremes of a continuum of responses with the acquisition of sensitivity to ethylene playing an important role (Johnston et al. 2009). This sensitivity model is supported by the observation that unripe climacteric and non-climacteric fruit both increase their respiration rate when exposed to exogenous ethylene (Paul et al. 2012).

With other fruit, such as kiwifruit, a hybrid ripening pattern is seen, with most of the ripening changes occurring in the absence of any detectable rise in ethylene and CO₂ production; a climacteric response occurs only towards the end of ripening. Exposure to exogenous ethylene promotes ripening of kiwifruit, but if exposure to ethylene is insufficient, or fruit are too immature, then removal of ethylene results in non-climacteric behaviour. Ethylene as a ripening trigger is used commercially with banana, avocado and early-season kiwifruit to ensure that fruit are at optimum ripeness when eaten. Conversely, if kiwifruit are to be stored for a long time, then ambient ethylene must be removed (usually by scrubbing this gas from coolstore environments).

Fig 11.13.png

Figure 11.13. Respiratory output of CO₂ can undergo dramatic change as fruits ripen. Early research on apple and pear led to a classic model of a postharvest climacteric rise associated with ripening and linked in time with ethylene production. Studies with tropical fruits such as avocado and banana then revealed characteristic 'waveforms' of even wider amplitude. (Based on Biale (1950) Annu. Rev. Plant Physiol. 1, 183-206)

Ethylene metabolism has been a main focus for biochemical research into fruit ripening (see Feature essay 11.1). The pathway of biosynthesis is as follows (Figure 11.14): methionine (a sulphur-containing amino acid also important in protein synthesis) is converted to SAM (S-adenosylmethionine) through the action of SAM synthase; SAM is converted to ACC (1-aminocyclopropane-1-carboxylic acid) through the action of ACC synthase (ACS); ACC is converted to ethylene through the action of ACC oxidase (ACO).

11.5.6 drawing_0.png

Figure 11.14. The pathway of ethylene biosynthesis in plants. The two genes controlling the committed steps to ethylene biosynthesis, ACC synthase (ACS) and ACC oxidase (ACO) are highly transcriptionally regulated. In one series of experiments, the biochemical precursor of ethylene, ACC, was depleted by expression of a bacterial ACC deaminase transgene, but this reaction does not normally occur in plants. 

In fruit with a climacteric behaviour, ethylene biosynthesis occurs at very low and basal levels during fruit development prior to ripening. This ethylene production is auto-inhibitory, and has been termed System 1 ethylene. At the initiation of ripening there is a change in the regulation of ethylene biosynthesis, which become auto-stimulatory and involves the induction of specific ripening-related ACO and ACS genes different from those that are responsible for System 1 ethylene (Barry and Giovannoni 2007). Production of ethylene greatly increases due to the autocatalytic regulation, and is known as System 2 ethylene. At the point at which the fruit becomes competent to ripen, there is a transition from System 1 to System 2 ethylene that may be regulated by developmental genes such as RIN (Barry et al. 2000).

Transgenic studies in a number of fruit types have yielded much information about steps in the control of ripening. Tomato fruit with reduced levels of the ripening associated ACC oxidase or ACC synthase, or with depletion of ACC levels using a bacterial ACC deaminase, developed and grew normally but ripening was delayed or almost completely prevented, depending on whether the fruit were attached to the plant or detached (Barry and Giovannoni 2007). Gassing of the ethylene-suppressed fruit with exogenous ethylene caused ripening to resume. Work on melon and apple found that the effects of the suppression of ethylene biosynthesis depended on the species and the extent of suppression achieved. Colour development, fruit softening, the accumulation of sugars and organic acids, and the production of aroma volatiles could in some cases be separated. For example, in antisense ACO melon fruit, degreening of the rind, softening and the accumulation of organic acids were sensitive to different levels of ethylene, and flesh pigmentation was ethylene independent. Such experiments show that ethylene does not control all the processes of ripening as once believed, and that additional regulation by other hormones and developmentally controlled factors occurs (see Section 11.5.2). Ripening is a series of parallel processes involving both ethylene-independent and ethylene-dependent pathways, the latter requiring different sensitivities to ethylene to proceed. This leads to a model whereby ethylene acts as a modulator to coordinate ripening in a developmentally choreographed pattern (Johnston et al. 2009).

The other important factor in the regulation of fruit ripening is the way in which plants perceive ethylene and the signal transduction pathway that leads to the ethylene response (see Chapter 9). In summary, ethylene is perceived by receptors that are negative regulators of the signalling pathway. In the absence of ethylene the receptors actively supress ethylene responses, but when these receptors bind ethylene they undergo a conformational change, leading to removal of the suppression and this allows de-repression of the signalling pathway. The signal is transduced through a MAP kinase pathway that ultimately leads to the stabilisation of a class of EIN3 (ETHYLENE INSENSITIVE 3) transcription factors. The EIN3 name originated from the ethylene insensitive phenotype observed in Arabidopsis mutants. The stabilisation of EIN3 leads to an increase in the transcription of genes associated with each ripening trait.

Ethylene receptors are multi-gene families (six genes in tomato) encoding two types of closely related proteins, one subfamily with a histidine kinase domain and the other subfamily with a serine/threonine kinase function. In Arabidopsis the receptors appear to act redundantly since removal of any one by mutation does not cause complete insensitivity to ethylene. However, this is not the case in tomato, where suppression of either of two receptor genes caused an early-ripening phenotype (Kevany et al. 2007). The importance of receptors in tomato fruit ripening has also been shown by the semi-dominant mutant Never-ripe (Nr), the fruit of which are unable to ripen. This was found to be due to a mutation in the ETR3 receptor, making the fruit impaired in its ability to perceive ethylene. Antisense inhibition of this mutant gene restored normal ripening to the Nr mutant (Hackett et al. 2000).

In tomato the turnover of receptors (degradation of existing receptor proteins and the synthesis of new ones) controls the timing of ripening (Kevany et al. 2007). During ripening some ethylene receptors increase in transcription and it appears that receptor expression is used to restore the ability to respond to ethylene, implying that there is a corresponding loss of receptor protein. This suggests a model in which during climacteric fruit ripening there is an increase in receptor turnover, allowing the fruit the ability to rapidly turn off ripening if ethylene is removed from the system. This is observed in tomato, apple and kiwifruit suppressed in ACO expression, which require continuous exposure to exogenous ethylene for ripening. It is also the basis for the temporary inhibition of ripening obtained using 1-methylcyclopropene (1-MCP), a compound that binds irreversibly to the existing ethylene receptors and prevents the physiological action of ethylene (Sisler and Serek 1997) (see section 11.6.4).

Ethylene is not the only regulator of fruit ripening. A cold treatment can trigger ripening in detached apple and kiwifruit, acting either independently of ethylene or by increasing sensitivity to existing very low levels of ethylene (Tacken et al. 2010; Mworia et al. 2012). Other hormones also appear to play important roles; particularly, declining levels of auxin and increasing levels of abscisic acid may control the onset of ripening in non-climacteric species such as grape and strawberry. Abscisic acid may also play a role in controlling the onset of ripening of climacteric species. Uncovering the interaction between auxin, abscisic acid and ethylene in ripening regulation is an emerging area of research (McAtee et al. 2013).

While most ripening-associated traits appear to be regulated by hormonal changes, there are also a number of genes that control the developmental switch to ripening (Klee and Giovannoni 2011). Some of these were identified in tomato by the study of ripening mutants that arose spontaneously during commercial tomato production. The ripening-inhibitor (rin), Colorless non-ripening (Cnr) and non-ripening (nor) mutants all have fruit that fail to ripen, though with different characteristics. Although these fruit do not produce elevated levels of ethylene and will not ripen in response to exogenous ethylene, they are not completely insensitive and some ethylene-responsive genes (but not the whole ripening process) can be induced by ethylene treatment. The products of these three genes are transcription factors and are thought to be key developmental genes that control ripening progression, apparently acting upstream of the ethylene production pathway.

RIN encodes a MADS-box gene that clusters in the SEPALLATA clade. CNR encodes a SQUAMOSA promoter binding protein. Both proteins are necessary for the induction of ripening-related increases in respiration and ethylene biosynthesis, although since they are important in the ripening of both climacteric and non-climacteric fruit they appear to be more global regulators of ripening with some functions that are ethylene independent. Transgenic tomato fruit that had been suppressed in the ethylene signalling pathway and treated with 1-MCP showed an ethylene-independent increase in the expression of ripening-related ACS genes and ethylene production (Yokotani et al. 2009). This is apparently controlled by developmental factors, and would be sufficient to induce the autocatalytic increase in ACS expression and ethylene production typical of tomato ripening.

Fig 11.14.jpg

Figure 11.15. Anatomical features such as cell size, wall thickness and the distribution of intercellular gas spaces greatly influence our perception of fruit texture and eating pleasure. Cross-sections of ripe apple flesh (top pair) and ripe kiwifruit flesh (bottom pair) at low (x120) and high (x310) magnification. The apple tissue (top left) shows cells having densely staining thin walls. Tissue from kiwifruit (bottom left) shows cells with thick, swollen and weakly staining walls. The two right-hand figures give views of individual cells, corresponding to the tissue views. (Photomicrographs courtesy I.C. Hallett, E.A. MacRae and T.F. Wegrzyn)

During fruit ripening, softening and textural changes (including the development of juiciness) are components of the suite of modifications that make ripe fruit attractive to animals that might disperse the seeds. The texture of ripe fruit differs drastically between species, with crisp, hard apple and deformably soft avocado representing the extremes. The characteristic textures of different fruit and their manner of softening can be linked both with anatomical features and with changes that occur to the cell wall during ripening (Figure 11.15). Some fruit that are picked while hard, such as kiwifruit and tomato, will subsequently soften markedly as a result of extensive modifications to the cell wall structure that include substantial swelling. Other fruit, such as apple or watermelon, remain crisp and soften only slightly. Their thin cell walls remain relatively unaltered. Both types of softening occur in the pear family: Asian pear (Nashi) shows a crisp apple-like texture, whereas many European pears soften to give ripe fruit with a melting texture. Interspecific crosses between the two types show that texture is heritable (Harker et al. 1997).

Many textural characteristics relate to the fate of fruit flesh when it is fractured and crushed in the mouth. Contributing factors include cell size, cell adhesion, turgor and packing, wall thickness, wall composition and the reaction of cells to shearing stress as they are chewed (Harker et al. 1997). For example, a ripe apple has large (0.1–0.3 mm diameter), turgid, thin-walled cells that are loosely packed (airspace c. 20% of fruit volume). When that flesh is chewed, cells fracture and release their sugary contents as free juice. In contrast, ripe kiwifruit has minimal airspace (c. 2% of fruit volume) and cell walls are thick and hydrophilic (Figure 11.15). Such cells tend to pull apart when the flesh is chewed, resulting in a paste moistened by liquid held in cell walls or released by damaged cells. Avocado also has cells with walls that are thick and soft and which tend to pull apart, but also has a high proportion of oil that gives the pulp an oily quality in the mouth.

Picture11.16.png

Figure 11.16. Schematic representation of postharvest ripening in kiwifruit, showing the timing of key physiological events. At harvest, fruit do not produce ethylene but are highly sensitive to exogenous ethylene. Softening is initiated (phase 1) and becomes rapid (phase 2). Relatively late in softening, compared with other fruit species, endogenous autocatalytic ethylene production begins, aroma volatiles are produced and fruit become soft enough to eat (phase 3). If fruit progress to the over-ripe stage (phase 4), they become unacceptably soft and exhibit ‘off-flavour’ notes. (Figure reproduced from Atkinson et al. (2011) J. Exp. Bot. 62, 3821-3835, with permission from the Society for Experimental Biology.)

Softening in kiwifruit occurs over a period of weeks, and can be divided into a number of phases (Figure 11.16). Modification of the cell wall plays an important part in determining fruit texture and ripening characteristics. The plant primary cell wall consists of a network of strong, rigid cellulose microfibrils held together by a complex matrix of polysaccharides consisting of two types: the hemicelluloses (composed mainly of neutral sugars) and the pectins (rich in galacturonic acid), together with smaller amounts of structural proteins. The outer part of each cell wall, which abuts and provides attachment to neighbouring cells, is composed mainly of pectins and is called the middle lamella. Fruit softening involves alterations to various pectin and hemicellulose polysaccharide wall components, and changes to the bonding between some polymers. Wall modification has been the subject of much research worldwide, mostly using tomato as a model, but also using other fruit in the search for common themes (Brummell 2006). Chemical analyses of cell wall components in a range of species, notably kiwifruit and tomato, show some consistent changes during the early stages of ripening. In kiwifruit, these include:

1. Solubilisation of pectin (but without further degradation);
2. The cell wall swells and shows an increased affinity for water (become more hydrophilic);
3. Loss of galactose from pectins (especially of a galactan that is tightly associated with the cellulose microfibrils);
4. De-esterification of some pectins.

These changes continue once kiwifruit have begun rapidly softening to ripeness (phase 2 in Figure 11.16). Phase 2 softening is associated with a further increase in pectin solubilisation, loss of galactan and arabinan side chains from pectic polymers, and more cell wall swelling. As softening progresses into phase 3 two more important changes begin, both of which appear to be regulated by ethylene:

5. Depolymerisation (a reduction in size) of the hemicellulosic polysaccharide xyloglucan, which is associated with a reduction in cell wall strength;
6. Depolymerisation of pectin, which is associated with dissolution of the middle lamella and reduced intercellular adhesion.

These six changes have been observed in a wide range of fruit types, although the extent and relative timing varies somewhat between species. Such observations indicate that pectin solubilisation and cell wall swelling are important events in the control of softening in kiwifruit and probably most other species with a melting texture. Cell wall modification is much less extensive in fruit with a crisp, fracturable texture such as apple and capsicum pepper. As fruit become fully ripe, dissolution of the middle lamella means that it eventually virtually disappears as a visible structure under the microscope. Dissolution of the middle lamella results in a great reduction in intercellular adhesion, and cells now have fewer regions of attachment to each other and become more rounded in appearance as they pull away from neighbouring cells. The primary walls are also weakened by the various changes that have occurred, and cells easily rupture when bitten or chewed, releasing the cell contents as juice.

Fig 11.22.jpg

Figure 11.17. Genetic manipulation can have a profound effect on ripening. Normal tomatoes (right) and Flavr Savr^TM tomatoes (left) were picked when both were nearly ripe (light red) and held at room temperature for four weeks. By this time normal fruit had softened and rotted but Flavr Savr^TM fruit was still firm and edible. This genetically modified fruit was deficient in polygalacturonase and had a much better shelf life, as well as improved flavour and handling qualities. Scale bar = 2 cm. (Photograph courtesy Robert Lamberts)

Many of the cell wall-modifying enzymes produced during ripening are regulated by ethylene. Polygalacturonase (an enzyme that depolymerises pectins) increases de novo 10–50-fold in tomato fruit during ripening (Sitrit and Bennett 1998). Understandably, this enzyme was originally accorded a major role in the ripening process. However, studies with transgenic tomato fruit in which polygalacturonase was suppressed found only a small reduction in softening during ripening, although there was a very substantial increase in the storage life of the fruit (Figure 11.17). Because transgenic fruits retained firmness for longer, they were left on vines longer, resulting in more carbohydrate accumulation prior to harvest. Moreover, fruit could be harvested partially coloured rather than mature green, thereby allowing ripening processes to progress more naturally and yielding fruit with better flavour and appearance. To move from experimental results to public availability, the fruit had to go through a series of tests and be de-regulated. Following USDA approval, a transgenic cultivar producing fruit with >99% reduction in polygalacturonase activity was named Flavr Savr^TM by Calgene, and was released for marketing in the USA under the brand identity of McGregor. Tomato paste with increased viscosity derived from similar transgenic fruit was successfully marketed in the UK for several years.

The effects of reduced polygalacturonase activity on firmness and shelf life were probably largely due to decreased degradation of the middle lamellae, and thus the maintenance of intercellular connections and fruit integrity.

Although this work was originally interpreted as suggesting a very minor role for polygalacturonase in fruit softening, the relatively small reduction in firmness observed may have been due to two factors. Firstly, tomato has atypically high levels of polygalacturonase enzyme (far more than in other species), and secondly silencing of the polygalacturonase gene was incomplete, meaning that in this species even 0.5% of wild-type activity was still substantial. Subsequently, silencing of polygalacturonase was found to partially but significantly reduce fruit softening in strawberry and apple, thus confirming that pectin depolymerisation is one part of the softening process (Quesada et al. 2009; Atkinson et al. 2012). What these studies have also clearly demonstrated is that softening is not controlled by a single cell wall-modifying enzyme. Rather, many different enzymes are involved, with enzymes such as polygalacturonase, pectate lyase, expansin, β-galactosidase and pectin methylesterase making specific contributions to the softening process (Brummell and Harpster 2001; Brummell 2006). It is the action of these many enzymes working together that brings about the wall swelling, reduced wall strength and weakened middle lamellae that result in the final softening and textural characteristics of the ripe fruit. Indeed, the actions of the various enzymes may be interdependent. For example, polygalacturonase requires the prior de-methylesterification of pectin by pectin methylesterase to make the substrate susceptible (Brummell and Harpster 2001), and the action of expansin to increase the accessibility of enzyme to substrate in the cell wall (Brummell et al. 1999).

In addition to cell wall disassembly, work in several species has shown that a decrease in cellular turgor accompanies fruit ripening and is an important component of softening (Shackel et al. 1991). This is caused partly by internal water movements resulting from the movement of solutes from symplast to apoplast, and partly to the loss of water from the fruit. In tomato, analysis of the non-softening DFD mutant attributed its enhanced postharvest firmness to very low water loss from the fruit and therefore to cellular turgor being maintained at higher levels than in wild-type (Saladié et al. 2007).

Picture11.18.png

Figure 11.18. Colour diversity in ripe kiwifruit and apple is determined by the presence or absence of chlorophyll, carotenoid and anthocyanin compounds in different fruit tissues. (Photographs courtesy Plant & Food Research)

Colour

During ripening many fruit change colour. Their bright colour, which evolved to attract dispersal agents such as birds, browsing animals and primates, has now become a particularly important visible indicator of maturity and ripeness. Bananas, berryfruit and stonefruit provide good examples where colour is a prime indicator of ripeness. Novel colours are also used to market new varieties to consumers, e.g. in kiwifruit, green-fleshed Actinidia deliciosa ‘Hayward’ vs. yellow-fleshed A. chinensis ‘Hort16A’ (Figure 11.18).

By analogy with senescence in most green tissues such as leaves, colour change in fruit typically involves chlorophyll loss and an increase in production of yellow, orange, red or purple pigments. In green-fleshed ‘Hayward’ kiwifruit chlorophyll is retained in the flesh of ripe fruit, whilst in yellow-fleshed ‘Hort16A’ chlorophyll is degraded during ripening by catabolic enzymes in the chlorophyll degradation pathway. This suggests that in ‘Hayward’ fruit chloroplasts are not converted to chromoplasts as is typical for ripening fruit.

The gold, orange and red colours of many fruit such as tomato and citrus are formed by enzymes in the carotenoid biosynthetic pathway (Tanaka et al. 2008). Carotenoids are divided into two classes: the hydrocarbon carotenes, e.g. lycopene (red) and b-carotene (orange); or the oxygen-containing xanthophylls, e.g. lutein (yellow). Besides providing highly attractive pigmentation, carotenoids protect the plant’s photosynthetic apparatus from excessive light energy and are essential requirements for human and animal nutrition.

Other red and purple pigments of the type seen in grapes and boysenberries are anthocyanins, which are products of the phenylpropanoid pathway. Anthocyanin pigments are water-soluble, synthesised in the cytosol and localised in vacuoles. Their basic ring structure can be modified by hydroxylation, methylation or glycosylation and their specific colour is modified by pH, metal ions and co-pigments to produce the subtlety of colours seen in nature. Like carotenoids, anthocyanins have many human health benefits and are widely used as natural food colourants (Tanaka et al. 2008).

The accumulation of anthocyanins is regulated by transcription factors of two classes (R2R3 MYB and basic helix loop helix), regulatory proteins that co-ordinate gene expression of the whole phenylpropanoid pathway. In fruit, this regulation system has been well characterised in grape and apple. In white berry grapes, VvMYBA2 is inactivated by mutations in the coding region and VvMYBA1 has a retrotransposon in the promoter and is not transcribed (Kobayashi et al. 2004; Walker et al. 2007). In apple fruit, a mini-satellite repeat structure in the promoter region of the MYB10 gene up-regulated the expression of this regulatory gene, which increased the level of anthocyanin throughout the plant producing a fruit with striking red colour throughout the flesh (Espley et al. 2009).

Flavour and aroma

Flavour is the most important factor determining if consumers will repurchase a particular fruit. Therefore, all varieties are produced and stored to maintain the very best flavour and aroma properties. Two main factors determine a fruit’s characteristic flavour – the correct sugar/acid balance and the production of aroma volatile compounds. These volatile compounds can include a mixture of volatile acids, aldehydes, alcohols, esters, terpenoids and aromatics.

Human taste sensations and experiences play an important part in characterising volatile compounds in fruit and wine, so a vocabulary has been developed to describe their sensory nature. The terms used relate a particular flavour sensation to that of a widely available standard, and have led to terms like ‘woody, ‘grassy’, ‘floral’, ‘spicy’ and ‘citrus’ (Figure 11.19).

Picture11.19.png

Figure 11.19. A flavour wheel that is used to systematically categorise and define sensory characteristics of wine. (Courtesy F.R. Harker)

With advanced GC-MS techniques many ripe fruit can be shown to contain >100 volatile compounds that contribute to their flavour and aroma. However, the absolute concentration of a volatile compound itself does not determine how important it is to perceived flavour and aroma. Sometimes compounds found at very low levels, e.g. parts per billion, are required to give a fruit its characteristic aroma. Compounds are given odour activity values (compound concentration divided by the minimum concentration that can be detected by the human nose) to show their importance to aroma.

Sometimes, one or two key volatile compounds can be regarded as characteristic for fruit of a given species or cultivar and are used in synthetic mixtures to represent that commodity. Specific volatiles are especially important in wine grapes where an individual volatile can become the dominant characteristic used in marketing a specific wine type. Examples include the ‘grassy’ character of methoxypyrazine in Sauvignon Blanc, the ‘richness’ of b-damascenone in red wine, or the ‘foxy’ character of methyl anthranilate produced by Vitis labruscana. Other examples of important volatiles in fruit include: raspberry – raspberry ketone; ‘Hort16A’ kiwifruit – ethyl butanoate and 1,8-cineole; ‘Hayward’ kiwifruit – (E)-2-hexenal and hexanal; apple — 2-methyl butyl acetate and a-farnesene, and strawberry — furaneol.

Our understanding of how flavour compounds are synthesised has rapidly advanced in the last 20 years. Aldehydes, acids and esters are derived from fatty acids and branched chain amino acids. The first committed step in straight-chain ester production is performed by lipoxygenases that produce 13-hydroperoxide linoleic acid from linoleic acid. These compounds are converted by cytochrome p450 lyases to aldehydes such as hex-3-enal and hex-2-enal. Alcohol dehydrogenases (ADHs) can then transform aldehydes to the corresponding alcohols, which contribute ‘green’ aromas. Alcohol acceptor substrates are then esterified with coenzyme acid donors by alcohol acyl transferases (AATs) to form esters, which generally contribute ‘fruity’ and ‘sweet’ characteristics. In apple, the enzyme MpAAT1 can produce a range of esters in ripe fruit and is up-regulated by ethylene during ripening (Souleyre et al. 2005). Branched chain esters are produced from isoleucine by branch chain aminotransferases and then pyruvate decarboxylase to produce aldehydes. These aldehydes are then available for ADHs and AATs to form branched chain alcohols and esters, respectively.

Sesquiterpenes and monoterpenes also contribute to fruit flavour and aroma profiles, often by adding ‘floral’ or ‘spicy’ top notes. In apple, the most important ripe-fruit terpene is a-farnesene produced by the sesquiterpene synthase AFS1. ‘Hort16A’ kiwifruit produce 1,8-cineole and A. arguta (baby kiwifruit) produce a-terpinolene, which add spicy/minty notes to these fruit. Terpenoid compounds are produced by terpene synthase enzymes, using farnesyl diphosphate (FPP) and geranyl diphosphate (GPP) as substrates. FPP and GPP are produced in plants by the action of short chain prenyltransferases in two compartmentally separated pathways (Lichtenthaler et al. 1997). In the plastid, the MEP (2-C-methyl-D-erythritol 4-phosphate) pathway leads to the production of GPP, while in the cytoplasm the mevalonate pathway provides precursors for FPP formation. The primary terpenoid skeletons can subsequently be modified further, e.g. by oxidation, hydroxylation, glycosylation or methylation by a range of other enzymes to increase terpenoid diversity.