Tobacco plants (Nicotiana rustica L.) pre-exposed to leaf dehydration, mineral deprivation, salination, or B033-toxicity exhibited increased resistance to subzero temperature and to reduced oxygen in the root medium. The stressed plants all showed an elevated content of leaf abscisic acid. Upon transfer of mineral deprived and salinated plants to prestress conditions, a decline in leaf abscisic acid content to prestress levels took place together with a loss of the increased resistance to subzero temperature and to deprivation of root oxygen. Treatment with abscisic acid by direct application to the leaves or by addition to the root medium improved leaf resistance to subzero temperature and to deprivation of root oxygen. A common hormone-regulation mechanism involving abscisic acid is suggested for this phenomenon of "cross-adaptation" by which a given stress confers increased resistance to other, apparently unrelated stresses.The concept of cross-adaptation (2) holds that exposure of an organism to a given adverse environment modifies its response to other adverse factors. Examples of cross-adaptation covering the entire biological spectrum have been reported (1,2,4,7). The phenomenon is widely recognized in plants. Levitt (5) Response to this stress was determined by measuring leaf WSD 4 hr after imposing the lack in root aeration.Recovery treatment was imposed by transferring the plants from the stressing environment back to aerated half-Hoagland solution used in the prestress period and as the control.
Methylammonium was taken up rapidly by illuminated cells of Anacystis nidulans R-2, leading to internal concentrations of 1.3 0.1 mM within 1 min, and a gradient of up to 200 between the cells and medium. Accumulation of 14CH3NH3' required at least 5 mM NaCl, but the uptake rate was independent of medium pH between 6.5 and 9. The kinetics of uptake could be resolved into an initial fast phase lasting less than 1 min (approximate Ki,, 7.2 ,uM; Vmax, 12.5 nmol minmg of protein-l at 15°C). A second, slower phase associated with product formation was eliminated by preincubation with methionine sulfoximine, a specific inhibitor of glutamine synthetase; the rapid phase was unaffected by this treatment. Ammonium ions competed with 4CH3NH3' for entry, and addition of 5 ,iM NH4' or 100 ,uM CH3NH3+ released 14CH3NH3+ accumulated during the rapid phase of entry. Small additions of NH4+ made at the same time as additions of 14CH3NH3+ delayed the start of radioactivity uptake by a time which corresponded accurately with the period needed for the complete removal of the added NH4+. The effects of inhibitors on accumulation and carbocyanine dye fluorescence suggest that ATP-dependent membrane potential was needed to drive 14CH3NH3+ transport. Spheroplasts were as active as whole cells in accumulating NH4' and 14CH3NH3', indicating that soluble
S. 1997. Isolation and characterization of plasma membranes from cyanobacteria. -Pliysiol. Plant. 99: 495-504.The two-phase partition system in comparison to the traditional methods used thus far (density gradients) for the isolation of tlie plasma membrane from cyanobacteria is described. The advantages of the two-phase system are: A short-time preparation of 3-4 h compared to 16-48 h required for the density gradient method; a purer fraction, resulting from separation according to membrane surface charge and hydrophobicity, not specific density; and, ease of scaling-up for obtaining large quantities. Also, the different biological activities attributed to this membrane to date are summarized. Findings on the typical plasma membrane ATPase (P-type ATPase) as well as the nutrient transporters and the corresponding proteins are included. As for the electron transport chain, one may conclude that this membrace contains a complete system (similar to that of the mitochondrion), portraying apparently F-type (F9F1) ATPase activity.
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