Deterination of Effective Partition Coefficients of Alkali Chlorides by Measurements of Concentration Potential across the Nitella flexilis (L.) Ag. Isolated Cell Wall

We report a study in which we estimate by means of non-equilibriumprocesses the value of an effective partition coefficient, x,a parameter that we use to take into account the non-ideal behaviourof the electrolyte of the external medium in the cell wall space.This parameter is defined by the ratio of the salt partitioncoefficient to the activity of the non-diffusing charges. Tothat purpose, concentration potential in NaCl, LiCI and CuCl₂solutions was measured across an isolated cell wall of Nitellaflexilis at different pH. No significant difference appearedbetween the potential values measured in the presence of NaClor LiCl (pH 5.5, 4.0 and 3.0). At pH 3.0, in CuCl₂ solutions,the Nitella wall behaved as an uncharged membrane. Two methods—high concentrations and least-squares approximation-wereused to calculate from these results the value of the effectivepartition coefficient. Both gave similar values of x At a givenpH, the values of x are independent of the concentration ofthe external medium over a large range of concentration ratios.The activity of the exchange sites was estimated by the experimentallydetermined fraction of absorbed monovalent cations. The ionicconcentrations in the wall space were calculated at pH 5.5 and3.0 by introducing the values of x in the relations which correlatethem to the ionic concentrations in the wall for that givenvalue of the activity of the carboxylate sites. From their values,it appears that the negative fixed charges of the wall werefunctionally shaded by the condensation of a part of the adsorbedmonovalent cations. The activity of the former was, therefore,reduced to a value considerably lower than that estimated bythe cationic exchange capacity during equilibrium processes. Key words: Nitella flexilis (L.) Ag., cell wall, concentration potential, partition coefficient, ion condensation     

Tentoxin Suppresses Stomatal Opening by Inhibiting Photophosphorylation

Tentoxin and, to a lesser extent, dihydrotentoxin (both at 10mmol m^–3) reduce stomatal opening in epidermal stripsof Commelina communis in the light but not in darkness. Thiseffect was significantly greater in normal air than in CO₂-freeair. Fusicoccin overcame the tentoxin effect. However, tentoxindid not inhibit stomatal opening in the light in epidermal stripsof Paphiopedilum harrisianum, a species which lacks guard cellchloroplasts. It is concluded that tentoxin exerts its actionon stomata not by an ionophorous effect in the plasmalemma ofguard cells but by the inhibition of photophosphorylation intheir chloroplasts. The effects of DCMU and tentoxin on guardcells are discussed in terms of their effects on chloroplastsand the extent to which energy is supplied from this organelleduring stomatal opening in the light. The results indicate thatneither photophosphorylation nor non-cyclic electron transportin guard cell chloroplasts are essential for stomatal opening. Key words: Commelina, epidermal strips, Paphiopedilum, photophosphorylation, stomata, tentoxin