Red and blue light-stimulated proton efflux by epidermal leaf cells of the Argenteum mutant of Pisum sativum

Light stimulates leaf expansion in dicotyledons by increasingapoplastic acidification, cell wall loosening and solute accumulationfor turgor maintenance. Red and blue light enhance growth viadifferent photo-systems, but the cellular location and modesof action of these systems is not known. Here, the effect of red and blue light was studied on transportprocesses in epidermal cells of expanding leaves of the Argenteummutant of Pisum satlvum. Both red and blue light caused extraceiiuiaracidification by isolated epidermal tissue, which was stimulatedby extracellular K⁺ and inhibited by DCCD at 0.1 mol m^–3.Acidification induced by red compared with blue light showeddifferent saturating kinetics in fluence rate-response curves.Under near saturating light conditions the effects of red andblue light were additive. The red light-induced acidificationwas inhibited by far-red light while the blue light-inducedacidification was not. Light caused a hyperpoianzation of themembrane potential in epidermal strips, and stimulated ⁸⁶Rb⁺uptake by epidermal protoplasts. These results show that phytochromeand an additional blue light-photoreceptor function in isolatedepidermal cells to promote proton efflux, hyperpolarization,and cation uptake. Key words: Pisum sativum, light-induced acidification, ion transport, epidermis, photoreceptor     

Red light and auxin effects on rubidium uptake by oat coleoptile and pea epicotyl segments

Apical segments of etiolated oat (Avena sativa L. cv. Victory) coleoptiles showed enhanced uptake of [⁸⁶Rb⁺] when tested 30 minutes after a 5-minute red irradiation. The response was partly reversible by far red light. Uptake was sensitive to carbonyl cyanide m-chlorophenyl hydrazone, but not to isotonic mannitol. Indoleacetic acid (10⁻⁷ molar) caused a very pronounced and rapid stimulation of uptake. Basal coleoptile segments also exhibited a red light-enhanced uptake, but not an effect of red light on changes in the pH of the medium. The [⁸⁶Rb⁺] uptake of third internode segments from etiolated peas (Pisum sativum L. cv. Alaska) was not affected by either red light or auxin. This tissue also showed no red light effect on acidification of the medium. It is concluded that alteration of [⁸⁶Rb⁺] flux is not a general feature of phytochrome action.     

Role of K+ in leaf growth: K+ uptake is required for light-stimulated H+ efflux but not solute accumulation

The stimulation of dicotyledonous leaf growth by light depends on increased H⁺ efflux, to acidify and loosen the cell walls, and is enhanced by K⁺ uptake. The role of K⁺ is generally considered to be osmotic for turgor maintenance. In coleoptiles, auxin‐induced cell elongation and wall acidification depend on K⁺ uptake through tetraethylammonium (TEA)‐sensitive channels (Claussen et al., Planta 201, 227–234, 1997), and auxin stimulates the expression of inward‐rectifying K⁺ channels ( Philippar et al. 1999) . The role of K⁺ in growing, leaf mesophyll cells has been investigated in the present study by measuring the consequences of blocking K⁺ uptake on several growth‐related processes, including solute accumulation, apoplast acidification, and membrane polarization. The results show that light‐stimulated growth and wall acidification of young tobacco leaves is dependent on K⁺ uptake. Light‐stimulated growth is enhanced three‐fold over dark levels with increasing external K⁺, and this effect is blocked by the K⁺ channel blockers, TEA, Ba⁺⁺ and Cs⁺. Incubation in 10 mm TEA reduced light‐stimulated growth and K⁺ uptake by 85%, and completely inhibited light‐stimulated wall acidification and membrane polarization. Although K⁺ uptake is significantly reduced in the presence of TEA, solute accumulation is increased. We suggest that the primary role of K⁺ in light‐stimulated leaf growth is to provide electrical counterbalance to H⁺ efflux, rather than to contribute to solute accumulation and turgor maintenance.     

Potassium and Phosphate Uptake in Corn Roots: Further Evidence for an Electrogenic H/K Exchanger and an OH/Pi Antiporter

Evidence is presented that K⁺ uptake in corn root segments is coupled to an electrogenic H⁺/K⁺ -exchanging plasmalemma ATPase while phosphate uptake is coupled to an OH⁻/Pi antiporter. The plasmalemma ATPase inhibitor, diethylstilbestrol, or the stimulator, fusicoccin, altered K⁺ uptake directly and phosphate uptake indirectly. On the other hand, mersalyl, an OH⁻/Pi antiporter inhibitor, inhibited phosphate uptake instantly but only slightly affected K⁺ uptake. Collapse of the proton gradient across the membrane by (p-trifluoromethoxy) carbonyl cyanide phenylhydrazone resulted in immediate inhibition of K⁺ uptake but only later inhibited phosphate uptake. Changing the pH of the absorption solution had opposite effects on K⁺ and phosphate uptake. In addition, a 4-hour washing of corn root tissue induced a 5-fold increase in the rate of K⁺ uptake with little or no lag, but only a 2- to 3-fold increase in phosphate uptake with a 30- to 45-minute lag. Collectively these differences strongly support the coupling of an electrogenic H⁺/K⁺ -exchanging ATPase to an OH⁻/Pi antiporter in corn root tissue.     

Action Spectra for Guard Cell Rb Uptake and Stomatal Opening in Vivia faba

Abaxial epidermal strips, containing guard cells as the only viable cells, were prepared from leaves of Vicia faba following a period in darkness, and floated, under CO₂-free air, on 2 mm RbCl + 0.1 mm CaCl₂ labeled with ⁸⁶Rb⁺. Under white light (high pressure mercury vapor lamp), stomatal opening in these strips approached its maximum at less than 0.02 calorie per square centimeter per minute. Under light of different wavelengths, 20 nanometers apart, and at a low quantum flux density of 7 × 10¹⁴ quanta per square centimeter per second, Rb⁺ uptake and stomatal opening were activated only in the blue and long ultraviolet regions, with a peak at 420 to 460 nanometers. The action spectrum suggests that the underlying process is not photosynthesis. At higher quantum flux density (38 × 10¹⁴ quanta per square centimeter per second), uptake and opening also responded to red (600-680 nanometers) and somewhat to green light, with a minimum at 540 to 560 nanometers, indicating a possible involvement of the photosynthetic process. This light-induced opening appeared not to be mediated by a lowering of CO₂ concentration, since CO₂-free air was used in all treatments and controls. Stomatal opening paralleled Rb⁺ uptake in all cases. This constitutes further evidence for the potassium transport hypothesis of stomatal movement.     

Light-Dependent Changes of the Cytoplasmic H and Cl Activity in the Green Alga Eremosphaera viridis

Ion-sensitive microelectrodes were used to measure Cl⁻ and H⁺ activities in the cytoplasm of the unicellular green alga Eremosphaera viridis de Bary. In the light, cytoplasmic Cl⁻ activity was 2.2 millimolar at most and cytoplasmic H⁺ activity was about 5.4·10⁻⁸ molar (pH 7.3). Darkening resulted in a permanent increase of the Cl⁻ activity to 3.2 millimolar and in a transient acidification, which was compensated within 3 to 5 minutes. Switching light on again decreased the Cl⁻ activity to the light level (2.2 millimolar). Simultaneously, a transient alkalization of the cytoplasm was observed. The transient character of the light-dependent pH changes was probably caused by pH-stat mechanisms, whereas the light-dependent Cl⁻ activity changes were compensated to a much smaller degree. Studies with different inhibitors (3-(3,4-dichlorophenyl)-1, 1-dimethylurea, piretanide, venturicidin) indicated a direct relation between the light-driven H⁺ flow across the thylakoid membrane and the observed light-dependent Cl⁻ and H⁺ activity changes in the cytoplasm. It is suggested that light-driven H⁺ flux across the thylakoid membrane was in part electrically compensated by a parallel Cl⁻ flux. The resulting Cl⁻ and H⁺ activity changes in the stroma were compensated by Cl⁻ and H⁺ fluxes across the chloroplast envelope giving rise to the observed Cl⁻ and H⁺ activity changes in the cytoplasm.     

Photoacclimation in the Red Alga Porphyridium cruentum: Changes in Photosynthetic Enzymes, Electron Carriers, and Light-Saturated Rate of Photosynthesis as a Function of Irradiance and Spectral Quality

Acclimation of the photosynthetic apparatus to changes in the light environment was studied in the unicellular red alga Porphyridium cruentum (American Type Culture Collection No. 50161). Absolute or relative amounts of four photosynthetic enzymes and electron carriers were measured, and the data were compared with earlier observations on light-harvesting components (F.X. Cunningham, Jr., R.J. Dennenberg, L. Mustárdy, P.A. Jursinic, E. Gantt [1989] Plant Physiol 91: 1179-1187; F.X. Cunningham, Jr., R.J. Dennenberg, P.A. Jursinic, E. Gantt [1990] Plant Physiol 93: 888-895) and with measurements of photosynthetic capacity. P_max, the light-saturated rate of photosynthesis on a chlorophyll (Chl) basis, increased more than 4-fold with increase in growth irradiance from 6 to 280 μeinsteins·m⁻²·s⁻¹. Amounts of ferredoxin-NADP⁺ reductase, ribulose-1,5-bisphosphate carboxylase, and cytochrome f increased in parallel with P_max, whereas numbers of the light-harvesting complexes (photosystem [PS] I, PSII, and phycobilisomes) changed little, and ATP synthase increased 7-fold relative to Chl. The calculated minimal turnover time for PSII under the highest irradiance, 5 ms, was thus about 4-fold faster than that calculated for cultures grown under the lowest irradiance (19 ms). A change in the spectral composition of the growth light (irradiance kept constant at 15 μeinsteins·m⁻²·s⁻¹) from green (absorbed predominantly by the phycobilisome antenna of PSII) to red (absorbed primarily by the Chl antenna of PSI) had little effect on the amounts of ribulose-1,5-bisphosphate carboxylase, ATP synthase, and phycobilisomes on a Chl, protein, or thylakoid area basis. However, the number of PSI centers declined by 40%, cytochrome f increased by 40%, and both PSII and ferredoxin-NADP⁺ reductase increased approximately 3-fold on a thylakoid area basis. The substantial increase in ferredoxin-NADP⁺ reductase under PSI light is inconsistent with a PSI-mediated reduction of NADP as the sole function of this enzyme. Our results demonstrate a high degree of plasticity in content and composition of thylakoid membranes of P. cruentum.     

THE INVOLVEMENT OF A PLASMA MEMBRANE H+‐ATPase IN THE BLUE‐LIGHT ENHANCEMENT OF PHOTOSYNTHESIS IN LAMINARIA DIGITATA (PHAEOPHYTA)1

Many brown algae, including the kelp Laminaria digitata (Huds.) Lamour., exhibit enhanced photosynthesis when they are given a small amount of blue‐light in addition to a background of saturating red light. This blue light effect is correlated with an increased uptake of carbon. In the present study, we tested the hypothesis that blue light acts by increasing the activity of a plasma membrane H ⁺ ‐ATPase, thereby promoting an active carbon uptake across the plasma membrane. Photosynthetic carbon uptake was studied in pH‐drift experiments under illumination with red and blue light and using different inhibitors. Vanadate, an inhibitor of plasma membrane H ⁺ ‐ATPases, had a minor inhibitory effect on carbon uptake rates under saturating red light conditions, but inhibited the blue‐light enhancement by approximately 60%. An inhibitor of external carbonic anhydrase, acetazolamide, decreased the carbon uptake in both red light and in red plus blue light by 48% and 68%, respectively. These results suggest that photosynthetic carbon uptake depends on an external carbonic anhydrase under both red and red plus blue light conditions, and that blue light induces an increased activity of a P‐type H ⁺ ‐ATPase in the plasma membrane. The proton buffer Tris, which has a buffering capacity similar to vanadate in seawater, had no inhibitory effect on carbon uptake rates neither in red light nor in red plus blue light, showing that the inhibitory effect of vanadate is not caused by its effect as a buffer. The blue‐light enhancement was also abolished by a protein kinase inhibitor (H‐7), suggesting that the transduction of the blue‐light signal involves a protein kinase, which activates the plasma membrane H ⁺ ‐ATPase by phosphorylation.     

Redox Processes in the Blue Light Response of Guard Cell Protoplasts of Commelina communis L

Guard cell protoplasts from Commelina communis L. illuminated with red light responded to a blue light pulse by an H⁺ extrusion which lasted for about 10 minutes. This proton extrusion was accompanied by an O₂ uptake with a 4H⁺ to O₂ ratio. The response to blue light was nil in darkness without a preillumination period of red light and increased with the duration of the red light illumination until about 40 minutes. However, acidification in response to a pulse of blue light was obtained in darkness when external NADH (1 millimolar) was added to the incubation medium, suggesting that redox equivalents necessary for the expression of the response to blue light in darkness may be supplied via red light. In accordance with this hypothesis, the photosystem II inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (10 micromolar) decreased the acidification in response to blue light more efficiently when it was added before red light illumination than before the blue light pulse. In the presence of hexacyanoferrate, the acidification in response to a blue light pulse was partly inhibited (53% of control), suggesting a competition for reducing power between ferricyanide reduction and the response to blue light.     

Nature of the light-induced h efflux and na uptake in cyanobacteria

We investigated the nature of the light-induced, sodium-dependent acidification of the medium and the uptake of sodium by Synechococcus. The rate of acidification (net H⁺ efflux) was strongly and specifically stimulated by sodium. The rates of acidification and sodium uptake were strongly affected by the pH of the medium; the optimal pH for both processes being in the alkaline pH range. Net proton efflux was severely inhibited by inhibitors of adenosine triphosphatase activity, energy transfer, and photosynthetic electron transport, but was not affected by the presence of inorganic carbon (C_i). Light and C_i stimulated the uptake of sodium, but the stimulation by C_i was observed only when C_i was present at the time sodium was provided. Amiloride, a potent inhibitor of Na⁺/H⁺ antiport and Na⁺ channels, stimulated the rate of acidification but inhibited the rate of sodium uptake. It is suggested that acidification might stem from the activity of a light dependent proton excreting adenosine triphosphatase, while sodium transport seems to be mediated by both Na⁺/H⁺ antiport and Na⁺ uniport.     

Responses of Avena coleoptiles to suboptimal fusicoccin: kinetics and comparisons with indoleacetic Acid

Proton excretion induced by optimal concentrations of indoleacetic acid (IAA) and fusicoccin (FC) differs not only in maximum rate of acidification but also in the lag before onset of H⁺ excretion and in sensitivity to cycloheximide. Because these differences might simply be a consequence of the difference in rate of proton excretion, FC and IAA have now been compared using oat coleoptiles (cv. Victory) under conditions where the rates of acidification are more similar, i.e. suboptimal FC versus optimal IAA. As the concentration of FC is reduced, the rate of H⁺ excretion decreases, the final equilibrium pH increases, and the lag before detectable acidification increases up to 7-fold. This enhanced lag period is not primarily a consequence of wall buffering, inasmuch as it persists when a low concentration of FC is added to sections which were already excreting H⁺ in response to IAA. An extended lag also occurs, upon reduction of FC levels, in the hyperpolarization of the membrane potential, before enhancement of O₂ uptake and before the increased rate of Rb⁺ uptake. The presence or absence of a lag is not a distinguishing feature between FC and IAA actions on H⁺ excretion and cannot be used to discriminate between their sites of action. In contrast, the insensitivity of FC-induced H⁺ excretion to cycloheximide, as compared with the nearly complete inhibition of this auxin effect by cycloheximide, persists even at dilute concentrations of FC. This seems to be a basic difference in H⁺ excretion by IAA and FC.     

Chlorophyll is not the primary photoreceptor for the stimulation of P-type H+ pump and growth in variegated leaves of Coleus×hybridus

There has been persisting controversy over the role of photosynthesis in the stimulation of the plasma membrane H⁺-ATPase and growth of dicotyledonous leaves by light. To investigate this, we compared the effects of light on growth, H⁺ net efflux and membrane potential (V_m) of strips which contained either only chlorophyll-free (white) mesophyll cells or chlorophyll-containing (green) cells cut from variegated Coleus leaves. White mesophyll cells responded to white, blue and red light with a hyperpolarization of V_m, an acidification of the apoplast and a promotion of growth, all of which began after a lag of 2–7 min. In contrast, green mesophyll cells showed a biphasic light response in which the hyperpolarization and the acidification were preceded by a rapid depolarization of V_m and an alkalinization of the apoplast. Nevertheless, green and white tissues showed comparable growth promotions in response to light. The light response of the leaf mesophyll is a composite of two separate photosystems. The initial depolarization and alkalinization are mediated by photosynthesis and blocked by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. The slower hyperpolarization, acidification and growth response, on the other hand, are clearly in response to light absorption by pigments other than chlorophyll. Received: 11 February 2000 / Accepted: 2 May 2000     

Phytochromabhängiger Ionentransport in Erbsensämlingen

Summary Five to 6 day old dark-adapted dwarf and tall pea seedlings grown in water culture were illuminated for ten minutes with red light and/or ten minutes with far-red light, and 90 to 170 minutes later their roots were immersed in a 0.2 mM K⁺ solution containing labeled ⁸⁶Rb⁺. After two hours uptake the fresh-weights and radioactivities of the shoot organs were determined. It was found that red light inhibits K⁺uptake into internodes and promotes uptake into the plumula. The red-light effect on K⁺transport precedes the red-light induced growth inhibition of internodes and growth promotion of leaves and is abolished by far-red light given immediately after red. The red-light effect on K⁺transport is independent of the concentration of K⁺ given to the roots in the range between 0.2 to 125 mM.     

Light acclimation during and after leaf expansion in soybean

Soybean plants (Glycine max var. Ransom) were grown at light intensities of 850 and 250 μeinsteins m⁻² sec⁻¹ of photosynthetically active radiation. A group of plants was shifted from each environment into the other environment 24 hours before the beginning of the experiment. Net photosynthetic rates and stomatal conductances were measured at 2,000 and 100 μeinsteins m⁻² sec⁻¹ photosynthetically active radiation on the 1st, 2nd, and 5th days of the experiment to determine the time course of photosynthetic light adaptation. The following factors were also measured: dark respiration, leaf water potential, leaf thickness, internal surface area per external surface area, chlorophyll content, photosynthetic unit size and number, specific leaf weight, and activities of malate dehydrogenase, and glycolate oxidase. Comparisons were made with plants maintained in either 850 or 250 μeinsteins m⁻² sec⁻¹ environments. Changes in photosynthesis, stomatal conductance, leaf anatomy, leaf water potential, photosynthetic unit size, and glycolate oxidase activity occurred upon altering the light environment, and were complete within 1 day, whereas chlorophyll content, numbers of photosynthetic units, specific leaf weight, and malate dehydrogenase activity showed slower changes. Differences in photosynthetic rates at high light were largely accounted for by internal surface area differences with low environmental light associated with low internal area and low photosynthetic rate. An exception to this was the fact that plants grown at 250 μeinsteins m⁻² sec⁻¹ then switched to 850 μeinsteins m⁻² sec⁻¹ showed lower photosynthesis at high light than any other treatment. This was associated with higher glycolate oxidase and malate dehydrogenase activity. Photosynthesis at low light was higher in plants kept at or switched to the lower light environment. This increased rate was associated with larger photosynthetic unit size, and lower dark respiration and malate dehydrogenase activity. Both anatomical and physiological changes with environmental light occurred even after leaf expansion was complete and both were important in determining photosynthetic response to light.     

The effect of red and far-red light on proton secretion from mesophyll-cell protoplasts of Vicia faba L.

The influence of red and far-red irradiation on the transport of H⁺ and ⁸⁶Rb⁺ through the plasmalemma was studied using parenchymal protoplasts isolated from Vicia faba leaves. The results indicate that red light stimulates H⁺ secretion and the uptake of ⁸⁶Rb⁺. Moreover, it has been demonstrated that far-red irradiation acts antagonistically with respect to red light in both these processes.     

Evidence for Na-Independent HCO(3) Uptake by the Cyanobacterium Synechococcus leopoliensis

At low levels of dissolved inorganic carbon (DIC) and alkaline pH the rate of photosynthesis by air-grown cells of Synechococcus leopoliensis (UTEX 625) was enhanced 7- to 10-fold by 20 millimolar Na⁺. The rate of photosynthesis greatly exceeded the CO₂ supply rate and indicated that HCO₃⁻ was taken up by a Na⁺-dependent mechanism. In contrast, photosynthesis by Synechococcus grown in standing culture proceeded rapidly in the absence of Na⁺ and exceeded the CO₂ supply rate by 8 to 45 times. The apparent photosynthetic affinity (K_½) for DIC was high (6-40 micromolar) and was not markedly affected by Na⁺ concentration, whereas with air-grown cells K_½ (DIC) decreased by more than an order of magnitude in the presence of Na⁺. Lithium, which inhibited Na⁺-dependent HCO₃⁻ uptake in air-grown cells, had little effect on Na⁺-independent HCO₃⁻ uptake by standing culture cells. A component of total HCO₃⁻ uptake in standing culture cells was also Na⁺-dependent with a K_½ (Na⁺) of 4.8 millimolar and was inhibited by lithium. Analysis of ¹⁴C-fixation during isotopic disequilibrium indicated that standing culture cells also possessed a Na⁺-independent CO₂ transport system. The conversion from Na⁺-independent to Na⁺-dependent HCO₃⁻ uptake was readily accomplished by transferring cells grown in standing to growth in cultures bubbled with air. These results demonstrated that the conditions experienced during growth influenced the mode by which Ssynechococcus acquired HCO₃⁻ for subsequent photosynthetic fixation.     

Extra- and Intracellular pH and Membrane Potential Changes Induced by K, Cl, H(2)PO(4), and NO(3) Uptake and Fusicoccin in Root Hairs of Limnobium stoloniferum

Short-term ion uptake into roots of Limnobium stoloniferum was followed extracellularly with ion selective macroelectrodes. Cytosolic or vacuolar pH, together with the electrical membrane potential, was recorded with microelectrodes both located in the same young root hair. At the onset of chloride, phosphate, and nitrate uptake the membrane potential transiently decreased by 50 to 100 millivolts. During Cl⁻ and H₂PO₄⁻ uptake cytosolic pH decreased by 0.2 to 0.3 pH units. Nitrate induced cytosolic alkalinization by 0.19 pH units, indicating rapid reduction. The extracellular medium alkalinized when anion uptake exceeded K⁺ uptake. During fusicoccin-dependent plasmalemma hyperpolarization, extracellular and cytosolic pH remained rather constant. Upon K⁺ absorption, FC intensified extracellular acidification and intracellular alkalinization (from 0.31 to 0.4 pH units). In the presence of Cl⁻ FC induced intracellular acidification. Since H⁺ fluxes per se do not change the pH, recorded pH changes only result from fluxes of the stronger ions. The extra- and intracellular pH changes, together with membrane depolarization, exclude mechanisms as K⁺/A⁻ symport or HCO₃⁻/A⁻ antiport for anion uptake. Though not suitable to reveal the actual H⁺/A⁻ stoichiometry, the results are consistent with an H⁺/A⁻ cotransport mechanism.     

Cell growth and ouabain-sensitive Rb uptake and (Na + K)-ATPase activity in 3T3 and SV40 transformed 3T3 fibroblasts

The uptake of ouabain-sensitive ⁸⁶Rb⁺ uptake measured at 5 min and the uptake measured at 60 min was 4.5- and 2.7-fold greater respectively for SV40 transformed 3T3 cells compared to 3T3 cells during the late log phase of growth. This uptake, however, varied markedly with cell growth. Ouabain-sensitive ⁸⁶Rb⁺ uptake was found to be a sensitive indicator of protein synthesis as measured by total protein content. Cessation of cell growth as measured by total protein content was associated with a decline in ouabain-sensitive ⁸⁶Rb⁺ uptake in both cell types. This increased ouabain-sensitive cation transport was reflected in increased levels of (Na⁺ + K⁺)-ATPase activity for SV40 3T3 cells, which showed a 2.5-fold increase V but the same K_rmm as 3T3 cells.These results are compared with the results of related work. Possible mechanisms for these effects are discussed and how changes in cation transport might be related to alterations in cell growth.     

Prediction of growth rate at different light levels from measured photosynthesis and respiration rates

Light integrators with a linear response are not suitable for measuring the light climates of plants because plants are not linear integrators. It should be possible to make a quantitative allowance for this nonlinearity by using the CO₂ uptake curve of the plant. To test this, we have subjected white clover plants to different levels of constant light, comparing the rate of increase of total dry matter with the net rate of uptake of CO₂ per day. Temperature, humidity, daylength and nutrient supply were kept constant. The growth rate calculated from CO₂ uptake agreed well with the observed rate over the light levels tested (3.7-88 w·m⁻², 0.4-0.7 micron: 1 w·m⁻² = 10³ erg · sec⁻¹ cm⁻²). All plants put on weight over the few days of the experiment, even those placed at light levels below their compensation point. The plants adapted their respiration rates to be a constant proportion of their growth rates. Most of the adaptation occurred within 24 hours of the light change. The adaptation of respiration has implications for models of light/growth relations in plant communities, almost all of which assume that respiration is proportional to leaf area and independent of growth rate or light level. The only model which does not is that of de Wit, and this gave good agreement with our results.     

Rapid auxin- and fusicoccin-enhanced Rb+ uptake and malate synthesis in Avena coleoptile sections

The short-term effects of auxin (indole-3-acetic acid) and fusicoccin (FC) on Rb⁺ uptake and malate accumulation in Avena sativa L. coleoptile sections have been investigated. FC stimulates ⁸⁶Rb⁺ uptake within 1 min while auxin-enhanced uptake begins after a 15–20-min lag period. Auxin has little or no effect on ⁸⁶Rb⁺ uptake at external pHs of 6.0 or less, but substantial auxin effects can be observed in the range of pH 6.5 to 7.5. Competition studies indicate that the uptake mechanism is specific for Rb⁺ and K⁺. After 3 h of auxin treatment the total amount of malate in the coleoptile sections is doubled compared to control sections. FC causes a doubling of malate levels within 60 min of treatment. Auxin-induced malate accumulation exhibits a sensitivity to inhibitors and pH which is similar to that observed for the H⁺-extrusion and Rb⁺-uptake responses. Both auxin- and FC-enhanced malate accumulation are stimulated by monovalent cations but this effect is not specific for K⁺.Abbreviations FC fusicoccin - IAA indole-3-acetic acid