CALLOSE DEPOSITION IS RESPONSIBLE FOR APOPLASTIC SEMIPERMEABILITY OF THE ENDOSPERM ENVELOPE OF MUSKMELON SEEDS

Semipermeable cell waits or apoplastic ''membranes'' have been hypothe sized to be present in various plant tissues. Although often associate d with suberized or lignified walls, the wall component that confers o smotic semipermeability;is not known. In muskmelon (Cucumis melo L.) s eeds, a thin, membranous endosperm completely encloses the embryo, cre ating a semipermeable apoplastic envelope. When dead muskmelon seeds a re allowed to imbibe, solutes leaking from the embryo are retained wit hin the envelope, resulting in osmotic water uptake and swelling calle d osmotic distention (OD). The endosperm envelope of muskmelon seeds s tained with aniline blue, which is specific for callose (beta-1,3-gluc an). Outside of the aniline-blue-stained layer was a Sudan III- and IV -staining (lipid-containing) layer. In young developing seeds 25 d aft er anthesis (DAA) that did not exhibit OD, the lipid layer was already present but callose had not been deposited. At 35 DAA, callose was de tected as distinct Vesicles or globules in the endosperm envelope. A t hick callose layer was evident at 40 DAA, coinciding with development of the capacity for OD. Removal of the outer lipid layer by brief chlo roform treatment resulted in more rapid water uptake by both viable an d nonviable (boiled) seeds, but did not affect semipermeability of the endosperm envelope. The aniline-blue-staining layer was digested by b eta-1,3-glucanase, and these envelopes lost OD. Thus, apoplastic semip ermeability of the muskmelon endosperm envelope is dependent on the de position of a thick callose-containing layer outside of the endosperm cell walls.