Firmness is the main attribute that gives an indication of fruit texture and it is often used by producers to evaluate harvest date (Trillot and Tillard, 2002). This quality index can be influenced by many preharvest factors such as season, orchard location, nutrition, and exposure to sunlight, which are independent of the fruit maturity level (DeEll et al., 2001). Apple cultivar, rootstock, thinning timing, and the use of compounds that alter partitioning of photosynthates have been shown to have an impact on the levels of firmness measured at harvest (Harker et al., 1997; Johnson, 1994). Fruit calcium concentration also seems to be involved in the maintenance of membrane integrity linked to firmness loss rate (Conway and Sams, 1987; Marmo et al., 1985).
Fruit size and maturity at harvest influence the storage behavior of apples, including their rate of firmness loss (Saure, 1996; Tromp, 1997). In fact, fruit size is highly related to cell size and number with larger fruit containing less but larger cells per unit of volume than smaller fruit (Blanpied et al., 1978). Knowledge of interactions between cell water relations and resulting turgor and tensile properties of cell walls could lead to a better understanding of physiological basis of cell strength, which is an indicator of apple firmness.
Many studies on the distinction between apple quality levels at harvest highlighted the important influence of weather conditions during fruit development (Calderón Zavala et al., 2004; Chapon and Westercamp, 1996; Smock, 1953). Depending on the development stage, weather conditions may have multiple effects on final firmness that has been reported to increase and reach desired levels earlier when the apples are exposed to higher temperatures during the first 6 weeks of fruit growth (Warrington et al., 1999). Higher air temperature conditions after fruit set, when the maturation process begins, has also been highlighted as a factor leading to decreased firmness levels at harvest (Calderón Zavala et al., 2004; Kondo and Takahashi, 1987; Tromp, 1997; Warrington et al., 1999). Low photosynthetic photon flux, characteristic of shading, has been negatively related to apple firmness at harvest (Corelli Grappadelli, 2003). Water tends to condense when exposed to lower temperatures, reducing turgor pressure in apples and slowing the maturation process. Lower air temperature conditions at harvest would thereby influence fruit firmness by varying water status of apples and delaying harvest time (Johnston et al., 2002). A study on the effects of weather conditions on apples in Quebec (‘McIntosh’, ‘Cortland’, ‘Spartan’, and ‘Empire’) highlighted the negative effect of high relative humidity during the entire growing season and the positive effect of long periods without rain during the period of 46 d after fruit set until harvest on firmness at harvest time (G. Bourgeois, unpublished data).
The overall objective of this study was to explain the variations in ‘McIntosh’ firmness values at harvest over multiple years and sites in Quebec and Ontario, Canada. The purpose of this approach was first to highlight the periods of apple development sensitive to specific weather conditions in reducing firmness levels at harvest. Second, characterization of these preharvest weather conditions in specific parameters was done to determine their direct or indirect effects on apple firmness. For future studies, the results of these analyses could permit the development of a spatiotemporal forecasting model based on preharvest weather conditions. Predictions of texture levels, both at harvest and after removal from storage, could provide a helpful tool to apple producers in their marketing strategies.
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