Imbibed nonviable lettuce ( Lactuca sativa L.) seeds have been shown to have lower density than imbibed control seeds. The purpose of this study was to investigate density differences associated with seed death. The relationship between endosperm integrity and the volume, density, and leakage of imbibed control and heat-killed ‘Montello’ lettuce seeds was studied. After an 8-hr soak, heat-killed seeds imbibed 23% more water than control seeds. The percentage of heat-killed seeds with density of 1.08 g·cm -3 was 2%, compared to 90% for the control. Mean electrical conductivity of the steep water was similar for heat-killed and control seeds. Seeds were punctured to rupture the endosperm layer surrounding the embryo. Puncturing the heat-killed seeds decreased total water uptake, as measured by decreased swelling, and increased density compared to intact heat-killed seeds. Leachate from punctured heat-killed seed had a 41% higher mean conductivity than that from punctured control seed. These data suggest that the undamaged endosperm restricted leakage of electrolytes from the embryo to the soak water. We speculate that the endosperm caused osmotically active solutes to accumulate in the extra-embryonic fluid of heat-killed seeds. This accumulation of solutes decreases the water potential inside the embryonic pouch, resulting in a greater uptake of water from the environment. The additional water uptake by heat-killed seeds would increase seed swelling and decrease seed density relative to control seeds.
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Hill et al. (1989) studied this question.