Citrus HLB or greening is a devastating citrus disease associated with Candidatus Liberibacter asiaticus (CLas). HLB has been reported in 40 countries in Asia, Africa, Australia, South America, and North America and was originally discovered in southern China in 1919 (Bové, 2006). After being infected with CLas, citrus trees develop HLB symptoms including decline with typical symptoms such as reduced plant height, yellow shoots, blotchy mottled leaves, canopy thinning, premature abscission of fruits, and some out-of-season flushing and flowering (Bové, 2006; da Graça, 1991). HLB severely affected fruit are small, lopsided, and poorly colored, with the juice having a bitter or sour taste and off-flavors (Baldwin et al., 2010; Plotto et al., 2010). As HLB severity increases, preharvest fruit drop becomes an increasing problem, which has contributed to declining yields in Florida, especially in the last few years (Bassanezi et al., 2011; Gottwald, 2010; USDA NASS, 2015; Zhao et al., 2015).
Abscission is a naturally occurring process that enables plants to shed organs (Kostenyuk and Burns, 2004). Abscission is characterized by a series of physiological and biochemical events in the abscission zone that lead to cell wall breakdown in the few rows of cells on either side of the fracture line and ultimately to detachment of the organ (Burns et al., 2003; Roberts et al., 2002). However, very little data are available on the effect of HLB on FDF and abscission of citrus fruit. One of the objectives of this study was to determine the effect of HLB severity on FDF of ‘Hamlin’ and ‘Valencia’ sweet oranges (C. sinensis).
Another objective of this research was to determine how HLB severity affects orange peel and fruit physical and biomechanical properties, which may contribute to mechanical injury of fruit during harvest, and postharvest handling and processing, and may also influence juice extraction and resulting juice quality. Tissue morphology (cell size, cell arrangement, and cell wall structure etc.) and material composition of fruits have an effect on their mechanical properties. Puncture, compression, cutting, tensile, shear, and TPA tests are usually used to estimate the maturity or resistance of fruit to force (Singh and Reddy, 2006). It has been reported that the maximum force in a puncture test represents the resistance of the fruit peel, and the force-deformation ratio in a whole fruit compression test represents whole fruit resistance (Ortiz and Torregrosa, 2014).
McDonald and Shaw (1987) and Juste et al. (1988) found that puncture resistance of citrus peel decreased with fruit age. The force required to puncture or burst a fruit is directly related to the contact area. This is a function of two variables: the punch diameter used and the radius of curvature of the fruit. The fruit can withstand higher contact forces when subjected to larger punch diameter (Flood et al., 2006). ‘Dancy’ tangerines and ‘Navel’ and ‘Temple’ oranges showed lowest resistances to rupture (7.8, 20.6, and 92.1 N, respectively) and puncture force (0.5, 1.1, and 6.5 N, respectively); ‘Duncan’ grapefruit and ‘Valencia’ orange had medium resistance with 283.2 and 199.9 N in rupture force, respectively, and 8.5 and 11.8 N in puncture force, respectively; however, lemon and lime showed the highest resistance to rupture with 356.7 and 371.4 N, respectively, and to puncture with 30.4 and 39.2 N, respectively (Ahmed et al., 1973). Miller (1986) also showed significant differences in compression force and puncture parameters among varieties of citrus. No significant differences for harvest time (after reaching harvest maturity), or abscission chemical application was detected for burst, puncture, and peel tensile strength properties in three orange varieties (Churchill et al., 1980). Singh and Reddy (2006) studied postharvest physico–mechanical property changes of ‘Nagpur’ mandarins by performing peel tensile tests, peel cutting, fruit compression, puncture resistance, and fruit cutting tests under ambient and refrigerated storage conditions, and found that the peel tensile strength, modulus of elasticity, and cutting force decreased with storage period in both ambient and refrigerated conditions. In contrast to these studies, little is known about how a disease, such as HLB, affects peel and fruit mechanical properties in citrus, thus this study explores these effects in two major Florida juice orange varieties, ‘Hamlin’ and ‘Valencia’. The aim of this study was to assess the effect of HLB symptom severity on FDF and fruit mechanical properties in sweet oranges as indicators of potential HLB-influenced preharvest fruit drop and postharvest fruit damage and breakdown.
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