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Peter J. Mes, B. Gregory Cobb, Douglas C. Scheuring, and J. Creighton Miller Jr.

The major markets for cowpea are seed sales and processing. During the storage period prior to sales, some seedcoats of `Mississippi Silver' darken from tan to varying shades of brown. This darkening is undesirable for the processing market, since it results in an unattractive product lacking uniformity. A series of studies was undertaken to investigate this problem. It was noted that darkening proceeded at a rapid rate when seed were left in the field after drying. Selection against darkening was unsuccessful, and attempts at extracting the colored compound with polar and nonpolar solvents were inconclusive. A method of screening for the darkening trait was devised. It was found that darkening could be induced by exposing seed to a high-oxygen environment, suggesting that oxidation is involved in the darkening process. In support of this, it was found that the lowest rate of darkening occurred when the seed were placed in an enriched nitrogen chamber after harvest.

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Stacie Grange, Daniel I. Leskovar, Leonard M. Pike, and B. Gregory Cobb

Triploid or seedless watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] cultivars often have erratic germination and low seedling vigor. The morphology of the seedcoat on two triploid cultivars—Tri X 313 and Tri X Sunrise—was examined by scanning electron microscopy (SEM) to identify structural differences compared to diploid seeds. Triploid seeds incubated with oxygen-enhanced treatments that included nicking, 1% hydrogen peroxide (H2O2), and 40% oxygen were investigated at low and high medium moisture levels. Triploid seed has a thicker seedcoat with a dense endotesta layer and a larger and highly variable air space surrounding the embryonic axis as compared with diploid seed. All cultivars rapidly imbibed water (≈50% of the original weight) during the first hour of imbibition, with a faster increase for triploids than for diploids. High moisture affected germination to a lesser extent in diploid than triploid seeds. Triploid germination under low medium moisture ranged from 96% to 76%, but was severely reduced to <27% under high medium moisture. Triploid seed germination was significantly improved at high moisture by H2O2 and by 40% oxygen. Triploid watermelon seed is very sensitive to submerged conditions, possibly due to a combination of physiological and morphological defects. The rapid imbibition and excess water collected in the seedcoat and air space surrounding the embryo, could reduce oxygen diffusion and impair metabolic pathways leading to normal germination and seedling development.