Some physical and chemical properties of the lettuce seed endosperm
Lettuce seeds (Lactuca sativa) are shown to be encased in an osmotically active cellulose membrane derived from the endosperm. The osmotic activity of this membrane is maintained in seeds treated with strong acid (4 M HCl), strong base (4.4 M NaOH), organic solvents, and protein solvents, indicating that there is no contribution of lipid, protein, or living cells to the osmotic properties of the membrane. Only cellulose solvents such as xanthate and cellulase were effective in disrupting the membrane.When dry seeds are treated with 50% solutions of methanol, ethanol, or acetone or with 10 mM HgCl2, they become osmotically distended seeds (ods) as a result of the accumulation of soluble molecules in the fluid layer (EEF) between the membrane and embryo. This process is temperature dependent and appears to require enzymatic activity.The molecular exclusion limit of the ods membrane was measured by osmotic equilibration techniques and was found to be about 300 daltons. This then explains previous observations on the impermeability of lettuce seeds to substances such as actinomycin D (mol. wt. 1255.5) during germination. Molecules of molecular weight greater than about 300 daltons will be excluded until the endospermal barrier is ruptured at about 10–12 h after germination onset.