Florida citrus production is ranked number one in the United States, accounting for 63% of the 371,700-ha production area. California, Texas, and Arizona account for 32.5%, 3.3%, and 1.6%, respectively (USDA, 2011). The Florida citrus processing industry faces daunting challenges as a result of citrus Huanglongbing (Candidatus Liberibacter asiaticus) (Morgan et al., 2009) and rising costs of hand-harvesting combined with decreasing availability of labor (Trimmer, 2012). Harvesting accounts for ≈50% of the total citrus production costs (Muraro, 2009; Whitney et al., 1996). Strong competition for laborers among different industries resulted in reduction of labor availability from 2002 to 2006 (Roka, 2004; Roka et al., 2000). Advantages of mechanical harvesting include reduced harvesting costs by 20 to 40 cents per box, which reduces the dependence on seasonal laborers thereby making the harvesting procedure relatively easier and cost-effective.
In the past 50 years, there has been commercial and research interest in use and improvement of mechanical harvesting to offset the costs associated with hand-harvesting (Ebel et al., 2012). Although mechanical harvesting of citrus was introduced to Florida in the mid-1950s (Futch et al., 2009), it has not been widely accepted in commercial orchards as a result of: 1) loss of leaves and twigs and scuffing of the bark on trunk and branches; 2) limb breakage and removal of flowers and young green fruit; and 3) exposure of shallow roots at the soil surface (Li et al., 2005). The primary concern of most Florida citrus growers was the effect of mechanical harvesting on long-term citrus tree health and productivity. However, long-term studies have demonstrated little effect on fruit yield by the mechanical harvesting method compared with hand-harvesting (Hedden et al., 1988). Studies have also indicated that well-managed, healthy citrus trees can sustain up to 25% defoliation without reducing canopy growth, fruit yield, and fruit quality (Yuan et al., 2005), in part because citrus can compensate for leaf loss by increasing photosynthesis of remaining leaves (Syvertsen, 1994). However, studies on the effect of water management associated with mechanical harvesting have not been conducted.
Citrus trees require a properly designed, operated, and maintained water management system to maximize yield and fruit quality (Zekri et al., 2009). Insufficient water supply results primarily in reduced growth, young fruit drop, and low sugar accumulation and reduced quality of the mature fruits (Enciso et al., 2008). Optimal irrigation also contributes to the efficiency of fertilizer programs resulting in increased yields and better tolerance to pests and stresses (Zekri et al., 2009). Lack of irrigation leads to reduced vegetative growth, limited number of new fruit-bearing branches, underdeveloped roots and leaves, which affect the number and size of the fruit and accentuates alternate bearing, i.e., high production 1 year followed by lower production the next year (Enciso et al., 2008).
Previous experiments on the effect of mechanical harvesting with trunk shakers (Burns et al., 2006; Li et al., 2005; Li and Syversten, 2005) and canopy shakers (Ebel et al., 2012) found little effect on citrus tree health that reduced long-term yield when they were conducted on well-watered trees with high canopy densities. Burns et al. (2006) did find that trees treated with an abscission agent [5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP)] and harvested with a trunk shaker at low speed and hand-harvesting did not reduce yield the next year. However, the use of CMNP and trunk shakers at high speed was found to reduce following-year yields in the same study. To evaluate the effect of short- and long-term water drought stress on mechanical harvesting, a 3-year field study was conducted to test the hypothesis that no significant injury by mechanical harvesting occurs to healthy, well-watered citrus trees that reduces long-term productivity. The objective of the current field study was to determine the effect of short-term drought stress before and/or after harvesting on: 1) fruit detachment force required to remove fruit at harvest; 2) tissue loss and canopy density after harvest; 3) stem water potential before and after harvest; 4) changes in water use; and 5) fruit yield associated with harvest method.
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