The heavy metal zinc was administered to barley seedlings by increasing its concentration in the hydroponic medium. The most dramatic effect was a severe inhibition of root elongation with little effect on root biomass production. The growth of primary leaves was little affected although the zinc content of the primary leaves increased several‐fold. A detailed compartment analysis was performed for 10‐d‐old barley primary leaves. Under low zinc nutrition (2mmol m −3), highest zinc contents were observed in the cytoplasm of mesophyll protoplasts. At inhibitory zinc concentrations in the hydroponic medium (400 μmol m −3), zinc levels dramatically and preferentially increased in the apoplastic space. Elevated zinc levels were also observed in the epidermal cells, and to a lesser extent, in mesophyll vacuoles. The cytoplasmic content of mesophyll protoplasts was unchanged, indicating perfect zinc homeostasis within the leaf. In order to understand the transport mechanisms underlying the steady‐state distribution profile, we used 65Zn to conduct uptake experiments with leaves whose lower epidermis had been stripped. The leaves were placed on zinc solutions of varying concentrations containing 65Zn for 5 min to 6 h. After the incubation, the leaves were fractionated into mesophyll and epidermis protoplasts and residue, the latter mainly representing cell wall. Adsorption of Zn to the extracellular matrix was 100 times faster than Zn uptake into the cells. By far the largest portion taken up into the mesophyll protoplasts rapidly appeared in the vacuolar compartment. These results demonstrate the importance of compartmentation and transport as homeostatic mechanisms within the leaves to handle high, possibly toxic, zinc levels in the shoot.

SUMMARY Barley was grown at inhibitory concentrations of cadmium, molybdenum, nickel and zinc. Primary leaves were analyzed for cellular and sub cellular compartmentation of the heavy metals. Epidermis and mesophyll protoplasts, mesophyll vacuoles and chloroplasts were isolated and apoplasmic washing fluid prepared, and the heavy metal contents of the various fractions determined. Efflux experiments showed that heavy metals were not lost from the preparations within the time span of the experiment. The different heavy metals were subjected to distinct distribution mechanisms within the leaves: (1) On a relative basis, the order of preferred epidermal accumulation was Cd = Zn > Mo > Ni (P < 0.01). (2) Within the mesophyll, Mo showed the highest degree of vacuolar compartmentation, whereas Ni was compartmentalized into the cytoplasm including chloroplasts to almost 80%. (3) The low degree of vacuolar compartmentation was correlated with the development of damage in the leaves, as visualized by chlorosis and decreased quantum yield efficiency. (4) Damage was inversely correlated with apoplasmic compartmentation. (5) Interestingly, sulfhydryl contents of stressed leaves were neither positively nor negatively correlated with toxicity of the heavy metals: maximum induction was seen in the presence of Cd, followed by Zn, and no changes under Ni and Mo‐stress. Increases in leaf SH‐contents were small as compared with induction in the roots.

The immediate effects of short exposures to high concentrations of different air pollutants (20 min SO2, 2 h O3, and 4 h NO2, 5 ppm each) on chlorophyll fluorescence and P700 absorbance changes at 830 nm of intact spinach leaves were investigated. Three different types of fluorescence measurements were used: Fluorescence rise kinetics in saturating light, fast fluorescence induction kinetics (Kautsky-effect), and slow induction kinetics with repetitive application of saturation pulses (saturation pulse method).The results show that the various air pollutants caused rather different damage in the photosynthetic apparatus of the leaves: 1. SO 2: The main effect is due to the acidifying action, weakening the PS II donor side (suppression of I1-I2-P phase in fluorescence) and inhibiting Calvin cycle activation (no relaxation of membrane energization). 2. O 3: Ozone has apparently no specific point of attack due to its high reactivity. It obviously reacts with all cell membranes, but primarily with the plasma membrane which it first passes on the way into the leaf. 3. NO 2: NO2 produces HNO3 and HNO2, when dissolved in the leaf water. The nitrite reductase, however, is highly effective, so that (in the light) nearly all nitrite is reduced. By the reduction of nitrite to ammonia, OH(-) is produced preventing net acidification. Obviously, the electron transport rates, which are possible with nitrite as acceptor are very high, being comparable to those observed with the well-known Hill reagent methylviologen, as revealed by P700 measurements in saturating light. Such high reactivities with NO2 (-) must prevent assimilatory electron flow.

The electrophysiological membrane parameters of the unicellular green alga Eremosphaera viridis were determined using an improved computer-supported single-microelectrode technique. These cells developed an average membrane potential of-150 mV in the light and a specific resistance of 1 Ω m(2) with an external potassium concentration of 1.1 mM and pH 5.5. In the dark, many cells showed a less polarized potential of 30-40 mV and a smaller membrane resistance. At potassium concentrations in the external medium higher than 1 mM, the membrane potential strongly depends on the external potassium content apart from a small electrogenic component. At concentrations lower than 1 mM K(+), a dependence of the membrane potential upon external potassium concentrations could not be verified. Inserting the internal ion activities in the Goldmann equation shows that, in this range, the proton conductance seems to be predominant over the potassium conductance. Transient changes in the membrane potential and in the membrane resistance were observed after switching off the light, after addition of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea or N,N'-dicyclohexylcarbodiimide, after a sudden decrease in temperature, and after current pulses. These changes resemble the action potentials (AP) found in other plant cells (Chara, Acetabularia). On average, the AP has a delay period of 5.1 s and a duration of 43.8 s showing a sudden decrease and a slower regeneration. The voltage peak during an AP followed exactly the Nernst potential of potassium over a range of external potassium concentrations from 5 μM to 0.2 M. This is true for depolarization or hyperpolarization, depending on the external K(+)-concentration. Tetraethylammonium-hydrogensulphate, a rather specific inhibitor of K(+) channels in nervous cells, suppressed the AP. The correlation of the appearance of the AP with a short-term opening of potassium channels in the membrane of Eremosphaera is discussed.

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