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Light and 02 are essential for a rapid bleaching of the pigment system of broadleaf bean by paraquat (1,1-dimethyl-4,4-dipyridylium dichloride). This bleaching does not appear to be directly related to physiological activity but to the destruction of a protective system which normallY prevents photooxidation. Light, but not 02., is also essential for the changes in membrane permeability brought about by paraquat in mesquite (Prosopis glandulosa), honeysuckle (Lonicera saponica), and broadleaf bean (Phaseolus vulgaris). Changes in permeability are also temperature dependent. Light is not essential for paraquat's effect on root elongation in mesquite.

The buoyant density of ribosomal DNA is similar in species with or without satellite DNA, and in all species examined was distinguishable from that of the satellite DNA. In melon tis- Because there is little cytological evidence for extrachromosomal DNA replication during plant development and no evidence for massive rDNA amplification (1), the observed correlation between satellite DNA and rDNA in plants requires further examination. The similarity in buoyant density between satellite and rDNA may be fortuitous, for rDNA has a similar density in species containing no satellite DNA (9, 16). Furthermore, a high percentage hybridization with rRNA does not necessarily imply a high redundancy of the rRNA gene, inasmuch as determination of this latter value depends on the size of the genome. In fact, high numbers of rRNA genes are found in species with low percentage hybridization but with large genomes (10, 18). The relationship between satellite DNA, defined as a minor component on neutral CsCl centrifugation, and rDNA has therefore been examined in terms of their respective buoyant densities in a larger range of plant species, and considered in the light of both percentage hybridization and the level of rRNA gene redundancy. MATERIALS AND METHODSDNA was prepared from total tissue by homogenization in a detergent mix followed by chloroform deproteinization. Further purification involved digestion with RNase and pronase, recovery by high speed centrifugation, and purification on a CsCl equilibrium density gradient (16). All DNA samples were monitored by Model E CsCl analytical centrifugation (19).Explants of artichoke (Helianthus tuberosus) were cultured in medium containing 3P orthophosphate or 3H uridine, and seedings of swisschard (Beta vulgaris var. cicla), pea (Pisum sativum), onion (Alliumn cepa), wheat (Triticum aestivum), flax (Linuin usitatissimnumn), maize (Zea inays) and Norway spruce (Picea abies) were grown in water culture in the presence of 32p orthophosphate for 5 to 10 days. Total nucleic acid was prepared and then fractionated by gel electrophoresis, and the cytoplasmic rRNAs, 1.3 x 106 and 0.7 X 106 daltons, were eluted and recovered by high speed centrifugation (16

1. Possible mechanisms regulating the activities of three enzymes involved in nitrate assimilation, nitrate reductase, nitrite reductase and glutamate dehydrogenase, were studied in radish cotyledons. 2. Nitrate-reductase and nitrite-reductase activities are low in nitrogen-deficient cotyledons, and are induced by their substrates. 3. Glutamate dehydrogenase is present regardless of the nitrogen status, and the enzyme can be increased only slightly by long-term growth on ammonia. 4. Although nitrate is the best inducer of nitrate reductase, lower levels of induction are also obtained with nitrite and ammonia. The experiments did not distinguish between direct or indirect induction by these two molecules. 5. Nitrite reductase is induced by nitrite and only indirectly by nitrate. 6. The induction of both nitrate reductase and nitrite reductase is prevented by the inhibitors actinomycin D, puromycin and cycloheximide, indicating a requirement for the synthesis of RNA and protein. 7. The decay of nitrate reductase, determined after inhibition of protein synthesis, is slower than the synthesis of the enzyme. Nitrite reductase is much more stable than nitrate reductase. 8. The synthesis of nitrate reductase is not repressed by ammonia, but is repressed by growth on a nitrite medium. 9. There is no inhibition of nitrate reductase, nitrite reductase or glutamate dehydrogenase by the normal end products of assimilation, but cyanate is a fairly specific inhibitor of nitrate reductase.

Instability of chloroplast rRNA has been observed with essentially all chloroplast RNA preparations. This paper describes experiments that show that, under normal conditions of preparation and fractionation, only the heavy chloroplast component (mol.wt. 1.1x10(6)) is unstable, the light chloroplast rRNA (mol.wt. 0.56x10(6)) and the cytoplasmic rRNA species (mol.wt. 1.3x10(6) and 0.70x10(6)) being stable. The stability of the 1.1x10(6)-mol. wt. molecule varies with different plant species, as also does the size and the number of fragments produced. Cleavages in three particular regions of the molecule are very frequent within the range of tissues studied. The 1.1x10(6)-mol.wt. rRNA is, however, stabilized by the presence of Mg(2+) during the preparation and fractionation of the RNA.