Seed development is normally divided into three stages: morphogenesis, reserve accumulation, and maturation drying (Kermode et al., 1985). At the morphogenetic stage, the seeds do not possess germination ability, which is gradually obtained with the accumulation of reserve. Seed vigor gradually increases after physiological maturity, whereas seed desiccation is not only drying of the seed but also an active stage of preparing for germination (Angelovic et al., 2010). Numerous studies have confirmed that the seed maturity has a strong influence on its vigor (Hay et al., 1997; Jalink et al., 1998). Physiological maturity of the seed (Ellis and Pieta Filho, 1992), and maximum accumulation of dry seed weight represent maximum seed yield in many crops and have also been proposed to represent maximum seed vigor.
An image analyzing system for seed vigor was first used by McCormac et al. (1990). They captured a seedling image of lettuce with a video camera and then measured the root length on the image by hand, which was a revolutionary attempt to determine seed vigor. Currently, there are various image analysis methods for seed testing, such as vigor tests (Howarth and Stanwood, 1993), purity analysis (Chtioui et al., 1997), or physical characterization of seeds (Anouar et al., 2001; Kruse, 2000). Sako et al. (2001, 2004) developed a computer-controlled system for seedling image analysis and seed vigor testing that integrates image acquisition and processing technology. However, the evaluation of seed maturity using fruit images is rarely employed and is a more valuable approach for seed production.
Capsule color has been used for evaluation of seed maturity in tobacco for a long time. Accordingly, based on color, capsule goes through three developing stages: green ripening, yellow ripening, and brown ripening. Nevertheless, this is a subjective approach, and it is difficult to ensure that the harvested seeds are of the same maturity. Therefore, an objective testing method on seed maturity that can be completed in the field is of utmost importance. Previously, we demonstrated that CIELAB color space is a quantitative method for capsule color measurement, which could be used to imitate the seed development and predict seed vigor in tobacco (Li et al., 2015a). In addition, CIELAB color space is a useful tool for analyzing the dryness of the capsules and determining harvest time (Li et al., 2015b). However, misinterpretations may occur because CIELAB color space is only a single color indicator. Therefore, the objective of this study was to develop a new method that could integrate multiple capsule traits for assessing capsule development and prediction of seed maturity in tobacco. This approach has two aspects: First, it is necessary to establish whether the 3D phenotypic platform can differentiate capsule maturity through quantitative evaluation of traits, such as capsule color, shape, density, water content, and temperature sensitivity. Second, if difference in those parameters among physiological developmental capsules is observed, can this be clearly identified through mechanical sensing?
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