F RUIT D EVELOPMENT

DM (%)

Core 7% 24-25%

Inner Pericarp 36% 17-18%

Outer Pericarp 57% 15-16%

The starch accumulation pattern in the entire fruit reflects changes in outer pericarp, whereas the starch concentration follow a distribution similar to the DM concentration one. Fructose and glucose concentrations increase at harvest time in all tissues, whereas sucrose general level does not change in pericarp tissues, but in the core. Acid content is lower in the core.

Quinic acid concentration increases from the core to the outer pericarp, whereas citric acid is higher in the inner pericarp (MacRae et al., 1989). All these proportion needs to be considered at the time of sampling, if a representative sample of the fruit wants to be collected.

1.2 F

D

The term fruit development is used to refer to the series of processes from the initiation of growth to death (Watada et al., 1984). The chronological development of the fruit from flowering to maturity and senescence involves a sequence of physical and biochemical changes, both macro- and micro-levels. The molecular, cellular and physiological mechanisms involved in fruit development (Bollard, 1970; Coombe, 1976; Gillaspy et al., 1993), the regulation of assimilation supply to fruit (Ho, 1992; Marshall and Grace, 1992), and the role of endogenous auxins (Miller, 1990) in developing fruit are well documented.

The entire period during which these developmental changes occur can be broadly classified into four stages: growth, maturation, ripening, and senescence (Fig. 1-2).

DEVELOPMENT

GROWTH

MATURATION

PHYSIOLOGICAL MATURITY

RIPENING

SENESCENCE

DEATH

INITIATION

1.2.1 G

All living organism are capable of ‘growth’ in the sense of change in size, change in form and change in number, given suitable conditions. These three processes together form an important part of the phenomena of life itself (Hunt, 1982). Hunt (1979) define ‘growth’ as

‘all the irreversible changes with time, mainly in size (however measured), often in form, and occasionally in number that occur in living organisms’.

As part of a living organism, fruit are able to grow. The growth starts at the onset of fruit development, and consists in increase in size, change in shape, and mobilization reserves from other parts of the plant. Fruit growth is stimulated by pollination and commonly involves the enlargement of the ovary, the enlargement of the receptacle, or both. The ovary grows rapidly soon after the pollination, and this is the beginning of fruit development. Fruit growth is accompanied by significant changes in cellular structures, which results in observed changes in fruit size and shape. The fruit growth pattern is often characterized by an initial period of rapid cell division and development of cell walls, followed by a long period of slow cell expansion (Opara, 2000). The duration of cell division and its contribute to whole fruit growth vary considerably among species (Ho, 1992).

1.2.1.1 GROWTH ANALYSIS

Growth analysis is a helpful instruments to data interpretation, considering that when fruit growth is analyzed plotting primary data (fresh weight, fruit length, etc…) against time very little of the first phase of development is revealed (Hunt, 1990). Primary data analysis is therefore necessary to provide a detailed insight on fruit growth dynamics (Opara, 2000).

The classical approach is the easiest and feasible to apply. Absolute growth rate and relative growth rate are then described.

Absolute growth rate (AGR) is a simple derived index of fruit growth, representing the rate of a change in size per unit time. In whole fruit studies primary data generally considered are fruit diameters, fruit fresh weight or fruit dry weight. The generic formula used to calculate AGR is:

Eq. 1-1

where X is the primary datum and t is the time (Opara, 2000).

Unless a functional study is performed, generally a mean AGR (MAGR) is calculated over the interval of time tn and tn-1:

Eq. 1-2

where Xn is the primary datum at tn time and Xn is the primary datum at tn-1 time (Opara, 2000).

In Fig. 1-3 AGR of ‘Hayward’ and ‘Hort16A’ kiwifruit is reported from Minchin et al.

(2003). The functional study of fruit growth allowed to get the AGR as first derivative, resulting in a smooth curve. This requires a relevant number of sampling dates.

Fig. 1-3: AGR in ‘Hayward’ and ‘Hort16A’ kiwifruit calculated as the first derivative of the fresh weight growth curve (Minchin et al., 2003).

AGR is an useful index to compare bodies of like data, such as comparison of fruit growth under different management conditions, whereas to compare the overall of unlike systems relative growth rate (RGR) is required (Opara, 2000). Therefore, AGRs do not account for the initial difference in size, which is a limiting factor of potential growth (Opara, 2000).

RGR (Eq. 1-3) expresses growth as the rate of increment in size per unit of initial size per unit of time (Opara, 2000). This index is suitable for quantitative analysis of fruit growth. It takes account of original size at the onset of growth measurement, and allow more equal comparisons than AGR (Hunt, 1990).

Eq. 1-3

As for AGR, unless a functional study is performed, a mean relative growth rate (MRGR) is used, and the logarithmic transformation of primary data (X) allow an homogenization of the variability (Eq. 1-4) (Opara, 2000).

Eq. 1-4

AGR is usually measured as size per time [size • time^-1], whereas RGR is usually measured as size per size per time [size • size^-1 • time^-1] (Hunt, 1979).

An example of plots of AGR and RGR against time are reported in Fig. 1-4. They refers to dry weight accumulation, with a sigmoid pattern. AGR is initially increasing to a maximum

Fig. 1-4: Changes in (A) AGR and (B) RGR of dry weight (Opara, 2000).

1.2.1.2 FRUIT GROWTH IN KIWIFRUIT

There is a linear increase in total dry weight content of kiwifruit from fruit set to harvest (Clark and Smith, 1988; Hopping, 1976b; Richardson et al., 1997). In contrast, the increase in fruit fresh weight follows a two or three phase curve. There has been some debate whether kiwifruit berries grown with a single sigmoid growth pattern (Walton and De Jong, 1990), a double sigmoid growth pattern (Hopping, 1976b) or triple sigmoid growth pattern (Pratt and Reid, 1974). These opinion are based on measurements of length and diameter of ‘Monty’

kiwifruit, fresh weight of ‘Hayward’ kiwifruit, and measurements of length and diameter of

‘Bruno’ kiwifruit respectively.

Hopping (1976b) divided the double sigmoid growth curve in three stages:

• Stage I (0-58 days after flowering): it is a period of rapid growth and weight gain due initially to cell division in all three different tissue, followed by cell enlargement;

• Stage II (58-76 days after flowering): it is a period of reduced growth and weight gain due to a slowing of cell enlargement in both the inner pericarp and central core;

• Stage III (76-160 days after flowering): it is a second period of growth and weight gain due to cell enlargement of the inner pericarp and central core. Fruit growth can still be detected at harvest as both weight and volume increment.

Walton and De Jong (1990) sustain that the growth of kiwifruit berries may be best described by a single sigmoid growth curve and any variations from this are due to sampling error and/or cultural conditions.

Fig. 1-5: Double sigmoid growth curve of developing kiwifruit observed by Hopping (1976b).

More recent study allow to divide the fruit growth according to the start of net starch accumulation (Fig. 1-6). According to Hopping (1976b), fruit growth is rapid in the first 50 days after anthesis, because of the cell division phase. Then a longer period (50–120 days) follows when starch accumulates in fruit and growth slows. In the final maturation period of 30–60 days fruit grow and accumulate starch very slowly or not at all, as seeds mature and the fruit starts to ripen (Richardson et al., 2004). The relative timing of each phase appears to be strongly affected by growing environments (Richardson et al., 1997; Tombesi et al., 1994; Walton and De Jong, 1990). By commercial harvest, when fruit fresh weights typically change little (Currie et al., 1999), both the total dry weight, and the DM concentration of the fruit, may still be increasing appreciably (Snelgar et al., 2005).

Fig. 1-6: Fruit growth division in three phases as reported in Richardson et al. (2004).

1.2.2 M

The maturation concept has got different meaning according to the context. In physiology, maturation is the process associated with completing natural growth and development and the attainment of full size. At physiological maturity, a fruit will continue ontogeny even if detached (Watada et al., 1984).

In horticulture, maturity is the stage of development when the plant or part of the plant possesses the prerequisites for utilization by consumers (Watada et al., 1984). According to the intended use, fruit may be horticulturally mature in the early stage, mid-stage, or late stage of development.

In postharvest technology, maturation is commonly defined as “that stage at which a commodity has reached a sufficient stage of development that after harvesting and postharvest handling, its quality will be at least the minimum acceptable to the ultimate consumer” (Reid, 1992).

1.2.3 R

The term ripening refers to the processes that qualitatively transform the mature fruit as it reaches the end of its growth period (Leopold, 1964). As reviewed by Opara (2000), the changes in ripening fruit are well documented in literature and generally include tissue softening, with the associated change in coloration and flavour, hydrolytic changes, which usually result in the rise of soluble sugar concentration, increased permeance of the cuticle to gases, respiratory climacteric and flavour production. Ripening is a precisely regulated developmental program (Brady, 1987). Based on the occurrence of a respiratory climacteric, the ripening biochemical pathways can be classified as climacteric or non-climacteric.

Ripening of fleshy fruit terminates with senescence and decay of the tissue.

1.2.4 S

Senescence is used collectively to refer to the degradative changes that naturally lead to death whole plants or organs. The period of senescence is characterized by depressed growth rate, and termination of growth sets the stage for senescence. During this stage the deterioration of fruit structural integrity occur. These changes in fruit manifests as loss of firmness, colour degradation and increased skin membrane permeability (Leopold, 1964).

During fruit growth and development, senescence marks the period during which fruit has lost the power of growth (Opara, 2000).