In the United States, watermelon fruit quality is based on absence of external defects (odd shapes, sunburn, injury) and internal quality of high soluble solids (SS) (greater than 10%), full pink to red color flesh, and crisp, non-mealy texture (U.S. Department of Agriculture, 2006). Watermelon fails to increase in sugars after being removed from the vine so it must be harvested near full ripeness (Rushing et al., 2001). Watermelon fruit have few external indicators of ripeness. Unlike tomatoes, there is no color break visible on watermelon rind, and the plethora of rind patterns and colors makes ripeness difficult to predict among genotypes and cultivars. Like tomatoes, using days after anthesis is an indicator, but not absolute predictor, of fruit maturity (Kano et al., 2008; Young et al., 1993). A number of subjective systems have been used by growers, including ground spot yellowness, senescent tendril next to the fruit pedicel, change in fruit wax (loss of shine), and thumping (dull sound when fruit are rapped with the knuckles) (Rushing et al., 2001). None of these harvest cues apply to all genotypes, because smaller types tend to sound different from larger types, and senescent tendrils may yield overripe watermelons for some. Introduction of near infrared spectroscopy (NIR) has been used in Japan and holds promise for screening watermelons after harvest, using SS content as the primary indicator of commercial acceptance (Jha and Mutsuoka, 2004). In the field, however, a relatively primitive system is used in the United States to select fruit for harvest. In this system, a cutter goes through the field and selects fruit considered to be ripe, which are then cut from the vine to be picked up by the harvest crew. The selection process is based partly on the attributes mentioned previously, by cutting fruit thought to be ripe to control internal color and sweetness, and by experience. There is a great need to find maturity indicators that can be integrated as technical aids during harvest for rapid, accurate selection of watermelon with acceptable SS content and color. Acoustic applications have been tried, but like with NIR, success is highly dependent on the cultivar and type (round versus oblong, seeded versus seedless) (Diezma-Iglesias et al., 2004; Stone et al., 1996).
Studies done on seeded, large (greater than 10 kg) watermelons indicate that vine tendril proximal to the stem-end attachment may be the most useful indicator of maturity at harvest. A green non-wilted tendril indicates that maturity has not been attained (Mizuno and Pratt, 1973), whereas a wilted but not fully senescent tendril may indicate commercial maturity has been reached (Suslow, 2002). In other cultivars, tendril senescence may not provide a sharp delineation in maturity at harvest (Corey and Schlimme, 1988). Ground spot, the portion of the rind in contact with the soil, changes color over time and may be a useful indicator of watermelon maturity (Corey and Schlimme, 1988; Nip et al., 1968). Surface color expressed in terms of Hunter a* or b* coordinates have similarly been used to measure ground spot color. Hunter a* coordinates measure variations in redness/greenness color and, therefore, chlorophyll metabolism. As chlorophyll content in watermelons decreases, carotenoids and anthocyanins (measured by Hunter b* coordinates or changes in yellowness/blueness) become more prominent (Goldschmidt, 2001). Notwithstanding, many cultivars have been developed with an array of colors and rind patterns, making ground spot color difficult to evaluate subjectively (Corey and Schlimme, 1988). Still other studies determined that rind gloss measurements calculated from Hunter L values could lead to a nondestructive technique for determining watermelon maturity; however, rind gloss is cultivar-dependent as a result of differences in the quantity and structure of surface waxes (Corey and Schlimme, 1988). Finally, earlier studies suggest that fruit weight and diameter could potentially serve as indices of maturity; however, fruit size can be greatly influenced by factors such as plant density (Corey and Schlimme, 1988; Hassell et al., 2009; Nip et al., 1968). During the latter half of fruit development, sucrose accumulates (Motomura et al., 1989) as the number of enlarged cells increases (Kano, 2004). As internal cells enlarge, there is an increase in fruit weight and diameter. Sugars and organic acids accumulate in the heart and blossom end of watermelons as they advance toward maturity (Chisholm and Picha, 1986). The ratio of percent soluble solids and percent total acid (SS:TA) was correlated with watermelon quality (Elmstrom and Davis, 1981).
Mini-watermelons are usually seedless and are defined as weighing 2 to 4 kg. Often, these watermelons have firm flesh, high SS, and high lycopene content (Perkins-Veazie et al., 2006). Although mini-watermelons have increased in popularity since their introduction in 2003 (Walter, 2009), little is known about the maturation process and external physical changes in the fruit offer few clues about its state of maturity. No studies have been done to determine the best external predictors of the internal quality in mini-watermelons. The objective of this study was to ascertain whether external indicators of maturity such as tendril senescence, ground spot color, watermelon circumference, and/or weight are reliable predictors of maturity in mini-watermelons. Internal maturity will be determined by measuring soluble solids, SS:TA ratio, or pH.
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