Fruit size and quality

Fruit size can be controlled by a number of agronomic methods (see Hepton, Chapter 6, this volume). Fruit size is set by plant size at forcing, as plant size influences the number

Fig. 10.2. Variation in sugar and acid percentage of mature fruit harvested at different times of the year (from Sideris and Krauss, 1934).

Month

Fig. 10.2. Variation in sugar and acid percentage of mature fruit harvested at different times of the year (from Sideris and Krauss, 1934).

Fig. 10.3. The percentage of tasters who ranked fruit as sweet or acid against an index of sweetness (TSS/% citric acid) for a range of pineapple fruit (redrawn from Sideris and Krauss, 1934).

Sweetness index (TSS/acidity)

Fig. 10.3. The percentage of tasters who ranked fruit as sweet or acid against an index of sweetness (TSS/% citric acid) for a range of pineapple fruit (redrawn from Sideris and Krauss, 1934).

of florets (eyes) per fruit developed (see Hepton, Chapter 6, this volume). Plant size can be represented as plant weight, 'D'-leaf weight or the number of leaves (van Overbeek, 1946; Py et al, 1987; Malezieux, 1993). The larger the stump, the greater the number of florets possible. As a rule of thumb for 'Smooth Cayenne', each 1 kg of plant weight at forcing gives 1 kg of har-vestable fruit. Cultivars will influence the relationship (Fig. 10.4) between plant weight at forcing and harvestable fruit weight (Chan and Lee, 2000). Some adjustment can be made in final fruit weight by increasing individual eye weight, initial weight of planting material, plant density and fertilization, especially nitrogen. However, too much nitrogen fertilization can lead to an increase in the number of fruit with double crowns and percentage of plants with collar of slips. Fruit size shows an inverse relationship to planting density, with about 45.4 g reduction in fruit weight for each 400 plants ha-1 increase above 7000 plants ha-1 (Sanford, 1963). Each 5 cm increase in spacing between 20 and 40 cm increases the fruit weight by about 5% and reduces the number of slips on the plant. The nearer that forcing is done to the natural differentiation, the larger the plant and the larger the fruit produced. Thus smaller fruit can be obtained by forcing earlier.

In the 'Smooth Cayenne' cultivar, nitrogen fertilizer tends to decrease fruit acidity while increasing fruit weight, fruit translucency in winter fruit and susceptibility of harvested fruit to chilling injury. Fruit chilling-injury intensity is reduced by potassium fertilization, especially when supplied as the chloride (Teisson et al., 1979). Magnesium fertilization does not significantly influence fruit acidity, while high magnesium application led to a significant decline in fruit TSS (Sanford, 1963). Potassium fertilization has no effect on TSS or slightly increases it. Other nutrients - calcium, phosphorus and micronutrients - have little effect on fruit quality and chilling sensitivity.

A temperature increase from 25 to 27°C over 5 months before harvest is related to a decrease in acidity from 12 to 7 mEq 100 ml-1 of juice (Huet, 1958a). Sunlight can also cause a similar decline in acidity (Lacoeuilhe, 1978). An inverse relationship is found between evapotranspiration in the 1-2 weeks before harvest and malic acid content (Gortner, 1963), while citric acid levels are little affected and more related to fruit development. There is a lag of about 1 week

'A04-16'

/ y = 0.62 + 0.49x

'Gandul'

y = 0.6637 + 0.2381*

'A25-34'

Plant weight (kg)

Plant weight (kg)

Fig. 10.4. Interaction between cultivars and plant weight and fruit weight at harvest (from Chan and Lee, 2000). Early-fruiting F1 hybrids ('Spanish' X 'Smooth Cayenne') were compared with 'Smooth Cayenne' ('Gandul').

between the increase in evaporation (sunlight) and the decrease in malic acid content.

In addition to the decrease in fruit weight with increasing planting density, there is also a decline in translucency, which may be related to light competition and fruit photo-synthate supply (Chen and Paull, 2000). Planting density has little effect on fruit TSS or acidity, though they tended to be slightly higher (Sanford, 1963). Fruit esters do tend to decrease in fruit grown at higher planting densities. Though probably unrelated to density, ratoon crops tend to have smaller fruit with higher sugars, less acid and more flavour.

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