Irradiance and temperature

Mutual shading associated with higher planting densities decreased the proportion of translucent fruits and fruit specific gravity (Pinon, 1981; E. Malezieux, 1992, unpublished results). While shading materials or reflectant coatings can protect against fruit sunburn (Py et al., 1987; Swete Kelly and Bartholomew, 1993), complete shading can reduce TSS (Py et al., 1987), thus reducing fruit quality.

Irradiance or temperature or both are correlated with some compositional changes in the fruit. Relatively short-term effects have been observed with respect to malic and ascorbic acid. Over a 100-day sampling period that ended at maturity, the percentage of malate in fruit was inversely related to weekly pan evaporation for the week prior to sample collection. It was assumed that the changes in malic acid were associated with irradiance, since pan evaporation is highly correlated with irradiance (Gortner, 1963). Ascorbic acid content fluctuated in concert with daily irradiance, though the peak ascor-bate level lagged the peak irradiance by about 2 weeks (Singleton and Gortner, 1965). It is not known whether the effects of irradi-ance on ascorbate or malate impact fruit quality at maturity in any way.

Changes in irradiance can apparently affect fruit TSS and TA, but there have been few definitive studies, particularly since growth-regulator-induced flowering became a common practice. Sideris et al. (1936) reported that fruit specific gravity increased from 1.005 at 50% light to 1.034 at ambient levels. TSS in juice was unaffected by light level. Titratable acidity, expressed as citric acid, increased from 0.74 to 1.4% as light was decreased from ambient to 50% of ambient levels. Hamner and Nightingale (1946) reported that TSS was unaffected by 50% shade, but juice acidity as citrate increased from 0.79 for control fruit to 1.12% for fruit that ripened in 50% light. The increased acidity was attributed to the effect of shade on fruit temperature rather than available light (Hamner and Nightingale, 1946).

Fruit TSS in Côte d'Ivoire was relatively unaffected by seasonal changes in irradiance during the final 30 days of fruit development. Fruit TA varied relatively more than TSS and levels were highly negatively correlated with irradiance (Fig. 8.11; Malézieux and Lacoeuilhe, 1991). 'Smooth Cayenne' fruit grown in Thailand, where average temperatures are high, have such low acid levels that citric acid must be added to the fruit to permit low-temperature processing.

In Hawaii, fruit TA as citric acid in 'Smooth Cayenne' fruit varies from a low of about 0.85% in fruit harvested in summer to as much as 1.25% for fruit harvested in the winter. This seasonal variation in TA makes 'Smooth Cayenne' fruit less suitable for winter fresh-fruit production because TA reaches levels that lower fruit eating quality. Data from a study of the effects of elevation and season on growth, yield and fruit quality of 'Smooth Cayenne' pineapple showed that TA varied relatively more over the year than did TSS. TSS was not correlated with irradi-ance during the 30 days prior to harvest and was only poorly correlated with minimum temperature for the same period (R2 = 0.352, n = 12). Fruit TA was negatively correlated with irradiance and minimum temperature. Irradiance accounted for 52.5% of the variation (n = 12) in TA, while temperature accounted for a slightly greater 63.4%. The two weather variables together accounted for 70.9% of the variation in TA. Fruit TA increased with increasing plant density (Py et al., 1973; Chadha et al., 1974), presumably due to increased mutual shading at the higher planting densities.

In the absence of significant variation in environmental temperature, irradiance can clearly affect fruit TA. However, in the presence of temperature variation or where irradiance and temperature vary together, it is less clear which factor is more important.

Titratable acidity can also vary within a fruit in relation to exposure. Flesh TA under the skin was less in sunburned areas than in unaffected parts of the fruit (Teisson, 1979a). When only part of the fruit was painted black to reduce its albedo, TA was significantly less in the painted parts than in the remainder of the fruit (Teisson, 1979b).

A physiological disorder referred to as green-ripe fruit and characterized by yellow, translucent flesh and a green skin results from a disjunction between internal and external ripening processes (Py et al., 1987). The green-ripe disorder generally begins in the part of

Fig. 8.11. Effect of mean irradiance for the 2 weeks prior to harvest on the titratble acidity (TA) of 'Smooth Cayenne' fruit juice. Data are means of 30 fruits harvested from 120 plantings made monthly in Côte d'Ivoire during a 10-year period.

the fruit exposed to the sun and the expression of the phenomenon decreases with shading (Teisson, 1979a). In a Côte d'Ivoire experiment, the percentage of green-ripe fruits decreased from 10% with no shade to 7 and 4% for partially and completely shaded fruits, respectively (Teisson, 1979a). The physiological basis for the effects of irradiance or increased temperature associated with irradiance, or both, on green-ripe fruit is not known.

Black heart, an internal browning of fruit flesh, is a common problem when pineapples are moved from refrigerated storage to room temperature (see Paull and Chen, Chapter 10, this volume). However, 'Smooth Cayenne' fruit that mature in southern Queensland during the cold winter months of July and August can develop black heart before harvest. Work by Wassman and Scott in 1972 (D. Christensen, 1999, personal communication) established a clear link between plant density, plant shading and the low-temperature inducement of black heart in situ. In their study, black heart increased from about 14% at a planting density of 24,700 plants ha-1 to 75% at 74,100 plants ha-1 and severity was much higher at the higher planting densities. Black heart does not occur in the field in 'Smooth Cayenne' in Hawaii.

Fruit become more translucent as the fruit ripen and air cavities in the fruit flesh become filled with juice. Fruit translucence increases from the base of the fruit to the top and translucent fruit were considered superior to opaque fruit for canning. However, as more pineapple are sold in the fresh market, translucency has come to be viewed as a disorder associated with preharvest development, because translucent fruit are more fragile and more prone to leak during storage and shipping than are opaque fruit. The incidence of fruit translucency varies with the season, being more prevalent in the spring in Hawaii than at other times of the year. Fruit also tend to become excessively translucent in Australia in spring, especially if heavy rain near fruit maturity follows dry conditions. The cause of the variation in fruit translucency is not known and growers experiencing this problem are interested in identifying cultural practices that will reduce its incidence and severity.

As fruit approach maturity, fruit sunburn discolours the shell and the underlying flesh becomes translucent and fragile (Py et al., 1987) and, in extreme cases, the tissue desiccates and the injured portion of the fruit collapses. Ratoon fruit have a greater tendency to lodge than do fruit of the mother-plant crop, and lodging exposes the full length of the fruit cylinder to midday levels of irradi-ance (Swete Kelly and Bartholomew, 1993). This accounts for the observation that ratoon fruit are more susceptible to sunburn than plant-crop fruit. Sunburned fruit has reduced titratable acidity (Teisson, 1979b). In extreme cases, the exposed portions of both the shell and the underlying flesh are killed, the tissue dries out and the wound provides an entry point for disease organisms (Swete Kelly and Bartholomew, 1993). Fruit lying on their side have significantly higher temperatures than do upright fruit (Van Lelyveld, 1957; Gortner, 1960).

Gortner (1960) found that green-shelled fruits had a higher temperature than did yellow ones. Opaque and translucent fruits showed similar heating and cooling characteristics despite different internal air contents (Gortner, 1960). Spraying the fruit with water had a brief minor effect on the temperature beneath the shell but had no effect on the temperature at a 5 cm depth (Gortner, 1960). Shading the fruit with black, brown or white bags all reduced fruit temperature, relative to unshaded controls (Spiegelberg, 1960). Increasing the fruit albedo with white paint decreased fruit temperature while painting the fruit black had little effect on its temperature (Spiegelberg, 1960). By selection of the shading or coating material, a range in internal fruit temperature of more than 11 °C could be obtained (Spiegelberg, 1960).

No data show that plant population density directly affects fruit quality. However, fruit harvest is delayed and fruit weight is more variable when high plant population densities are used.

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