Assuming that no other factors limit growth, the adequacy of the nutrient supply determines the plant growth rate, the plant mass at induction and ultimately the fruit mass at harvest. However, the plant-mass-fruit-mass relationship is by no means direct, as environment and the quality of forced induction are also important (see Bartholomew et al., Chapter 8, this volume). The literature seems to indicate that plants well supplied with nutrients at the time of induction are likely to have larger fruit than plants of the same mass that have less than optimum nutrition (Py et al., 1987). If nutrition is adequate at the time of induction, additional nutrients are typically not applied after that time, because nutrient absorption, except for potassium, ceases (Py et al., 1987). Fruit mass is well correlated with plant mass at induction, and fruit quality is primarily determined by environmental factors. Where nutrient supplies at the time of induction are inadequate, fruit mass may be increased and fruit quality can be affected by the application of fertilizers after induction has occurred. Since fruitlet numbers are fixed soon after induction, fruit mass is only increased to the extent that additional nutrients result in an increase in fruitlet size. The focus of the following discussion relates mainly to the effect of nutrition on fruit quality.
According to Py et al. (1987), N and K are the most important elements influencing fruit mass and quality in relation to each other and in relation to climate. It is not always possible to distinguish between the specific effect of N on fruit quality and its more general effect on overall plant growth and health. Hence, an increase in N increases the diameters of the core and the peduncle and the length of the peduncle. As a result, with excess N, there is an increased risk of lodging and fruit sunburn (Py et al., 1987). Nitrogen might also increase the number of double or multiple crowns (Gonzalez-Tejera and Gandia-Diaz, 1976) and increase the fragility and translucency of the flesh and the susceptibility to green ripe fruit (Lacoeuilhe, 1978). An increase in N resulted in a reduction in free acids (Marchal et al., 1970; Gonzalez-Tejera and Gandia-Diaz, 1976; Py et al., 1987), but may or may not reduce fruit total soluble solids (TSS). Cannon (1957) reported that fruit TSS decreased with increasing N when K was optimum but had no effect when K was suboptimum. As was noted above, fruit nitrate content increases with increasing N (Scott, 1993, 1994), especially if plants are fertilized with N at or after induction of flowering.
Phosphorus was reported to have a slight or no effect on fruit quality (Magistad and Linford, 1934b; Linford et al., 1935) and also to decrease TSS and acidity but to increase ascorbic acid content (Tay et al., 1968; Mustaffa, 1989). Because of the dominant role of P in growth, rather than in photosynthesis, as is the case for N and K, it seems likely that the effect of P on fruit quality will be small.
Essentially all studies of the effect of K on fruit quality show that increasing levels of K increased fruit acidity, TSS, ascorbic acid and flavours and aroma (Magistad and Linford, 1934a,b; Cannon, 1957; Su, 1958; Tay et al., 1968; Marchal et al., 1970; Gonzalez-Tejera and Gandia-Diaz, 1976; Py et al, 1987). There may be a decrease in the ratio TSS:titratable acidity, which is an important measure of eating quality (Smith, 1988), because acidity increases relatively more than TSS (Py et al., 1987). The overall response to increasing available K is completely consistent with the specific role of K in the opening and closing of plant stomata, which regulate gas exchange, including the absorption of CO2 during photosynthesis.
Calcium supply affected fruit aroma, possibly because higher levels of Ca could interfere with the absorption of K, so the effect is probably not specific. Higher levels of Ca in the fruit are also associated with reduced incidence of storage disorders (Wilson Wijeratnam et al., 1996; Selvarajah et al., 1998).
Magnesium had no effect on fruit quality on peat soil (Tay, 1974; Gonzalez-Tejera, 1975), while Py et al. (1987) stated that its effect on fruit aroma was comparable to that of K. Garcia (1983) found an increase in TSS in the fruit of 'Red Spanish' pineapple due to application of magnesium.
In the only study of the specific effect of iron on fruit quality found, Linford (1934) reported that improving iron nutrition increased fruit translucence.
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