CALCIUM DISTRIBUTION IN FERTILIZED AND UNFERTILIZED OVULES AND EMBRYOSACS OF NICOTIANA-TABACUM-L

Potassium antimonate was used to localize Ca2+ in tobacco ovules from 0 to 7 d after anthesis in pollinated and emasculated flowers. Antimon ate binds ''loosely bound'' Ca2+ into calcium antimonate; less-soluble forms are unavailable and free calcium usually escapes. Ovules are im mature at anthesis. Abundant calcium precipitates in nucellar cells su rrounding the micropylar canal. A difference between calcium in the tw o synergids emerges at 1 d, which is enhanced in pollinated flowers. T he future receptive synergid accumulates more precipitates in the nucl eus, cytoplasm and cell walls. After fertilization, micropyle precipit ates diminish, and the ovule is unreceptive to further tube entry. In emasculated flowers 6 d after anthesis, ovular precipitates essentiall y disappear; however, flowers pollinated at 4-5 d and collected 2 d la ter largely restore their prior concentration of precipitates. Ovular precipitates occur initially in the nucellus, then the embryo sac, and finally the synergid and micropylar filiform apparatus. Possibility, calcium is released from the embryo sac, although no structural eviden ce of exudate formation was observed. Calcium precipitates in the ovul e correlate with the ability of the ovule to be fertilized, suggesting that successful pollen tube entry and later development may require c alcium of the class precipitated by antimonate.