A method is described for the rapid separation of cellular slime mold (Dictyostelium discoideum) cells into nuclear and cytoplasmic fractions . Sucrose density sedimentation profiles of radioactivity from cells that had been grown for long or short periods in the presence of uridine-3H indicate very low levels of cross-contamination between the fractions . The nuclear fraction contains few, if any, ribosomes . In exponentially growing cells, at least 80% of the ribosomes were associated in polysomal complexes. No loss of counts from prelabeled rRNA was observed during 2 generations (24 hr) of logarithmic growth and, within the polysomal complexes, the distributions of the preformed material and of rRNA synthesized during the 2 generations were identical . In stationary phase cells that had entered the developmental program leading to fruiting body construction, the rRNA turned over rapidly so that by the end of development at least 75% of the ribosomes fabricated during exponential growth had disappeared and had been replaced by new ones synthesized during the morphogenetic sequence . The preformed ribosomes disappeared preferentially from the monosomal contingent ; the newly synthesized ribosomes appeared exclusively in the polysomal contingent and did not appear as monosomes in appreciable numbers for at least 6 hr . The possible significance of this wholesale replacement of ribosomes is discussed .

Germination of Phacelia tanacetifolia Benth. (cv. Bleu Clair) seeds is accompanied in its early phases by the development of the K^ uptake capacity. Dormancy due to light and high temperatures and the negative effect on seed germination of a medium with low water potential inhibit the development of potassium uptake. Fusicoccin, which promotes seed germination under all examined conditions, accelerates also the appearance of the K"^ uptake capacity. The relationship between the development of K"^ uptake and seed germination in Phacelia tanacetifolia is discussed.

MATERIALS AND METHODSFe-efficient plants respond to iron stress both by morphological and physiological modifications. In roots of a Fe-efficient plant (Cucumis sativus L.) grown in the presence or in the absence of iron, the capacity to acidify the external medium, change in the transmembrane electrical potential, and the ATPase activity have been determined. Roots from plants grown in the absence of iron showed a great capacity to acidify the external medium, a higher transmembrane electrical potential difference (-145 millivolts, versus -105 millivolts), and a higher ATPase activity (+30%). The administration of Fe2, but not Fe3 , caused a block of the acidification capacity, a great decrease in the transmembrane electrical potential difference in root cells, and a large inhibition of the ATPase activity of isolated microsomal membrane vesicles. Growth of PlantsCucumber seeds (Cucumis sativus L. cv Marketer from F. lIi Ingegnoli, Milan) were sown in agriperlite, watered with CaSO4 (0.1 mM), allowed to germinate in the dark at 260C for 4,d, and then transferred to a nutrient solution that had the following composition (mM): 2 Ca(NO3)2, 0.75 K2SO4, 0.65 MgSO4,0.5 KH2PO4, lX 10-2 H3BO3, lxIO-'MnSO4,5x 0-4 CuSO4, 5x 10-4 ZnSO4, 5X i0-5 (NH4)6Mo7024 and 0.1 Fe-EDTA (if added); pH was adjusted to 6.0 to 6.2 with NaOH. Hydroponic cultures were maintained in a greenhouse and the growing solution was changed weekly. Plants became chlorotic after approximately 1 week of culture in -Fe, and in the second week no further deficiency symptoms were observed.

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