11.3 - Resources for fruit growth

As fruit grow, proportions of cell wall, carbohydrate, organic acid, lipid, phospholipid and volatile (aroma) compounds change dramatically; and within each of those groups there are changes in the proportion of individual group members. Of these, by far the most important in practical terms is carbohydrate economy. Two sets of issues are at stake: (1) rate of growth, attainment of maturity and final fruit size, and (2) aroma, flavour and texture in ripe fruit. Both carry commercial implications.

Enlarging fruit require carbohydrate to sustain cell division, enlargement and tissue specialisation. Only in later stages are carbohydrates typically retained as either starch or soluble sugars. Soluble carbohydrate is mainly imported as photoassimilate, with only a minor contribution from local CO2 fixation, and reassimilation of respiratory CO2.

During peak fruit expansion, usually early summer, there is an intense flow of photoassimilate from mature leaves (sources) into rapidly enlarging fruit (sinks). Sugars generated by photosynthesis, along with amino acids and phosphate within the plant’s vascular network, move via the phloem into enlarging fruit.

11.3.1 - Photoassimilate distribution

Sources of photoassimilate can be identified by providing individual leaves with 14CO2 and following the pattern of labelled material into neighbouring organs (Figure 11.7). Leaves typically begin to show a net export of photoassimilate at about 50–60% of full size. In kiwifruit, leaves 49% expanded failed to export the radiolabelled products of 14CO2 photosynthesis, whereas those 64% expanded transported labelled photoassimilate into younger leaves.

Fig 11.07.jpg

Figure 11.7 Photoassimilate moves from mature leaves of peach (a) and apricot (b) into the pericarp of maturing fruit nearby. 14CO2 was administered for about 1 h to source leaves (boxed area top left side in (a) and (b)), and movement of 14C-labelled photoassimilates over the subsequent 24 h was traced by autoradiography of harvested material (right side a and b). Intense labelling of source leaves indicates a high level of residual activity, but strong incorporation of 14C-labelled photoassimilates into the pericarp of adjacent fruit is also evident. Endocarp tissues had hardened and failed to import current photoassimilate, although seeds developing inside the endocarp did become labelled. (Based on Kriedemann (1968) Aust. J. Agric. Res. 19, 775-780)

Distribution patterns of 14C-labelled products relate to developmental morphology of fruiting shoots. Typically, source leaves are nearby on the same lateral branch, both above and below the fruit. In apple, fruiting spurs may develop primary leaves (emerging soon after budburst), then spur leaves (in a rosette at the base of the flower), then bourse leaves (growing on spur bourse shoots). Each in turn provides assimilate for the next phase of leaf growth (primary → spur → bourse); then as leaf expansion ceases, all provide assimilate to the developing fruit. Leaves on adjacent extension shoots can provide some photoassimilate to fruit, but if indeterminate growth continues the furthermost leaves become progressively less important as suppliers, and more significant as competitors. If the normal suppliers are removed, carbohydrate can come from longer distances, sometimes from leaves more than a metre away.

The relative strength of source and sink is a major factor for distribution patterns, but transport options are dictated by vascular connections. During plant growth, development occurs in an orderly and patterned manner, creating separate files of leaves. This pattern (phyllotaxis) is accompanied by a matching pattern of vascular connections. Photoassimilate tends to move along a pathway of least resistance, following these direct vascular connections where they exist, hence distribution patterns generally follow phyllotaxis.

This importance of phyllotaxis in carbohydrate allocation to the fruit is well shown in kiwifruit, where specific leaf–fruit connections exist. Patterns of assimilate distribution from leaf to fruit have been studied by taking a number of matched lateral fruiting branches of kiwifruit vines, then supplying 14CO2 to one leaf on each, at various nodal positions along the stem, from node 1 (base) to node 10 (tip). Each lateral also had one fruit each on nodes 1 and 2, while the remaining nodes had leaves only. Distribution of 14C-labelled photosynthate was allowed to proceed for 6 d, and the total radioactivity in each leaf and fruit on the lateral was then measured. Specifically, node 1 fruit received assimilate from their own subtending leaf (node 1 leaf) and from leaves on nodes 6 and 9. Node 2 fruit was supplied by its subtending leaf and leaves on nodes 7 and 10. Assimilate from remaining leaves was distributed generally within the main body of the plant. However, if the apex of the lateral was removed to stop extension growth, fruit then drew assimilate from all leaves. By implication, a drastic change in source–sink relationships can override restriction on carbon transport imposed by vascular patterns in intact plants.

11.3.2 - Composition of photoassimilates

Radiolabelling of photoassimilates has also been used to identify which compounds are transported into storage organs. Analyses of phloem tissues and phloem sap show that in most plants carbohydrate enters fruits primarily as sucrose. However, other soluble carbohydrates can predominate in some plants of commercial importance (Table 11.1).

In the woody Rosaceae (apple, pear, stonefruit), the sugar alcohol sorbitol is the major photosynthetic product at 60–85% of transported carbon, the remainder being mainly sucrose. Regardless of transport form, photoassimilate arriving in fruits is rapidly converted to the storage products characteristic of the fruit in question (principally starch, glucose, fructose and sucrose). Thus the identity of labelled sugars in fruit often differs markedly from the form transported. For example, sorbitol concentration is high during early development of apple fruit and more or less reflects the composition of photoassimilate in transit. By maturity, sorbitol content will typically decline to below 5% of the total soluble carbohydrate.

If sorbitol reaching fruit is not fully metabolised, apoplastic accumulation results and pericarp tissues become glassy in a disorder called ‘watercore’ (see below; Figure 11.23). This is a common problem with some apple cultivars such as Fuji. Sugar transport and accumulation can thus have economic importance — both in terms of desired taste characteristics and postharvest fruit quality.

In kiwifruit, the polyol myo-inositol may comprise up to 35% of soluble carbohydrate in developing fruit, and up to 20% in leaves. As yet, we do not know whether inositol, like sorbitol, is transported in the phloem, or whether there may be physiological disorders caused by inadequate metabolism of sorbitol within fruit. Such findings challenge our common perception of sucrose as the universal transport carbohydrate in economic crops, and suggest that we still have a lot to learn about the control of carbohydrate metabolism.

11.3.3 - Fruit composition and sensory attributes

Carbon transport and subsequent metabolism in developing fruit cannot be viewed in isolation, particularly when aspects of fruit quality, such as taste and flavour, are directly dependent on such processes. In particular, sugar–acid balance and contents are primary determinants of the taste attributes of fruit, and so are of major significance for consumers. Too much acid and the fruit is tart and unpalatable; too little and the fruit is insipid and bland. In horticultural terms, acid levels are often expressed as titratable acidity (TA), and this is used as one indicator of taste. Another indicator used is the refractive index of the expressed sap (recorded as °Brix). This is a measure of the soluble solids concentration (SSC %) of expressed juice and represents the sum of organic acid, salts and sugar contents. Several organic acids may be present, but certain ones are characteristic of particular species or cultivars. For example, malic acid predominates in pipfruit (pomefruit), citric acid is dominant in citrus, while tartaric acid is dominant in grape. In kiwifruit, malic, citric and quinic acids are the major ones, and in total may exceed 1.5% of the fresh weight.

Acids are not transported into fruit via phloem connections, but are synthesised in situ. Part of the acid component comes from metabolism of the sugar imported through the phloem, but part can be synthesised by local fixation. In citrus, dark fixation of CO2 by mature fruit makes a meagre contribution to acid balance, but inter-conversion of imported carbon is of more consequence. In that case, citrate synthase and subsequent enzymes in the citric acid cycle appear to determine whether imported carbon (as sucrose) is transformed into other sugars or is metabolised further to organic acids.

Starch–sugar balance is a major factor in consumer perceptions of fruit quality. In many fruit, including apple, banana and kiwifruit, starch accumulates throughout development, being laid down as granules in plastids. In kiwifruit, starch may reach 50% of the total dry matter towards the end of fruit growth (at about 15 weeks after pollination). As fruit approach maturity (17–20 weeks after pollination), there is a rapid onset of starch hydrolysis. Starch content at the onset of this conversion is not enough to account for all the sugar present in ripe fruit, and this implies that maturing fruit continue importing sugar up to harvest. Continuing import of 14C-labelled photoassimilate into maturing peach and apricot fruit confirms that pattern (Figure 11.7).

The dynamic between starch breakdown and soluble sugar increase can be a critical index of fruit maturity. ‘Hayward’ kiwifruit, for example, are judged to be mature enough to be harvested and to ripen properly if their soluble solids levels reach a specific target value of 6.2%. Starch pattern tests are used as maturity indices for some apple cultivars. For kiwifruit, the starch content of fruit at harvest may vary according to season, growing system, and cultivar. Starch content is strongly linked with sugar content of ripe fruit and hence with consumer perceptions of fruit taste. Starch content of fruit at harvest (commonly estimated from fruit dry matter content) is used commercially as a proxy for potential fruit taste.

As an additional factor in their dietary appeal, fruit are rich sources of vitamins, particularly vitamin C (L-ascorbic acid). Moreover, vitamin C can be a major metabolite (greater than 2 g kg–1 fresh weight) in fruit such as acerola, rosehip, quandong, kiwifruit, citrus, blackcurrant and guava, and has strong anti-oxidant properties. This may account for a notable absence of browning in kiwifruit and citrus when sliced (in conjunction with relatively low levels of polyphenols and polyphenol oxidase in those tissues). Vitamin C levels increase in the fruit during early growth, and tend to be stable through to maturity.

A number of other important vitamins have fruit or seeds as their major sources in the human diet. The B group vitamins such as B1, B2, pantothenic acid and biotin are present in both fruit and seeds, while B3 and B6 are particularly abundant in seeds. The vitamin A precursor β-carotene is found in useful quantities in some fruit, for example peach, apricot, melon and cherry.

Phenolics such as anthocyanins and tannins are also important in fruit and are responsible for much of the visual appeal of intact fruit (e.g. tamarillo), exposed flesh (e.g. cherry) or extracted juice (e.g. guava). They also contribute to flavour characteristics, adding a slight and pleasing astringency (as with the dessert apple) or a more aggressive one (as with cider apple and green banana).

Tannins in persimmon fruit are a special feature of that fruit and provide an interesting example of the potential dominance of a single quality characteristic in determining how a given fruit is used. The first cultivars of persimmon originating in China were markedly astringent, having high soluble tannin levels that made the fruit inedible until the tannins became condensed during the softening stages of ripening and early senescence. These original cultivars were therefore not eaten until the fruit flesh had become a glutinous paste. Later selection in Japan produced non-astringent cultivars such as ‘Fuyu’ that lose their astringency during the later stages of maturation, so that they can be eaten in a firm crisp state more typical of a fruit like apple. In persimmon, water-soluble tannins are compartmented in specific tannin cells of the mesocarp tissue. Tannin accumulation ceases with cell growth, and in non-astringent cultivars astringency declines both through soluble tannin dilution and through polymerisation, where soluble tannins are condensed into an insoluble form.

11.3.4 - Mineral nutrients

Fig 11.08.png

Figure 11.8. In kiwifruit, as in most fruit, accumulation of calcium is confined to early stages of development that coincide with cell division. By contrast, phosphorus and potassium move into fruit over the whole growing season and are able to enter via either the xylem or phloem. Magnesium import is meagre but progressive. Note the expanded scale for potassium. Fruit content of nutrient ions at maturity would be as follows: phosphorus 1600, calcium 600, magnesium 400 and potassium 10 000 μmol per fruit. (Based on Ferguson (1980) N. Z. J. Agric. Res. 23, 349-353)

Just as fruit require an inward flow of carbohydrate and water to provide for seed growth and pericarp expansion, so mineral nutrients are also supplied. As a rule, concentrations of the major mineral nutrients in fruit are lower than in other organs such as leaves, and the patterns of phosphorus, potassium, calcium, magnesium and nitrogen accumulation usually differ.

Mineral nutrients move into the fruit most rapidly during the early stages of development (Figure 11.8) at a time when xylem water flow dominates. As fruit approach maturity, surface to volume ratio declines, the skin becomes less permeable to water loss, and large amounts of photoassimilate are imported via phloem connections. As a result, a significant part of the water reaching fruit now enters through the phloem and is accompanied by photoassimilate. Mobile ions such as K+ and HPO42– are loaded into the leaf veins along with the photoassimilate, travel in the phloem and so reach fruit over the whole growing season. In contrast, less mobile nutrients such as Ca2+ fail to reach fruit during later stages, so that Ca2+ concentration remains steady or even declines slightly (Figure 11.8).

Nutrient deficiencies in fruit are relatively uncommon, except for those associated with calcium. Calcium deficiencies are expressed in the form of blossom-end rot in tomato, and bitter pit plus lenticel blotch in apple fruit. These apple disorders tend to be expressed during postharvest storage, but symptom expression is somehow related to the previous ripening environment. These disorders show up as a pitting of flesh and skin, reducing fruit value or even rendering those commodities unmarketable. Such commercial penalties have resulted in development of preharvest sprays and postharvest dips of calcium salts that diminish bitter-pit incidence in harvested fruit. Where there is little or no calcium recycling via phloem, calcium needs to be applied directly to fruit to have a beneficial effect.