Short communication. A steep Ca2[IMAGE]-dependence of a K[IMAGE] channel in a unicellular green alga

An increase of cytosolic Ca² in the unicellular green alga Eremosphaera viridis activities Ca²-dependent K channels causing a hyperpolarization of the plasma membrane. Data from parallel calcium, and potential measurements were combined with I/V relationships. This yielded a steep Ca²-dependence of K channels with a co-operativity of 4 and an affinity of 300 nM.Key words: Eremosphaera viridis, plasma membrane, Ca²-dependent K channel, co-operative binding.      

The Investigation of Stomatal Ionic Relations using Guard Cell Protoplasts: II. OSMOTIC RELATIONS OF GUARD CELL PROTOPLASTS IN SHORT AND LONG-TERM INCUBATION

Clint, G. M. 1985. The investigation of stomatal ionic relationsusing guard cell protoplasts. II. Osmotic relations of guardcell protoplasts in short and long-term incubation.—J.exp. Bot 36: 1739–1748 Measurements were made of the volume changes exhibited by isolatedguard cell protoplasts (GCPs) of Commelina communis L, whenexposed to a range of concentrations of external osmotica Inshort-term incubation, GCPs behaved as osmometers and showedrapid volume changes in response to changing external osmoticpressure (₀). In long-term incubation, GCPs prepared and incubatedwith added external KCl showed further slow changes in volume,in a manner suggesting that regulation of volume occurred. Protoplastsprepared and incubated without added external KCl had smallervolumes for a given value of ₀, and their ability to regulatevolume in long-term incubation was reduced or absent. Treatment with fusicoccin caused an increase in both the volumeand the K⁺ content of GCPs. The increase in volume continuingafter the increase in K⁺ content had ceased, in a manner similarto that observed in walled guard cells in epidermal strips. Key words: Guard cell protoplasts, volume regulation, Commelina communis     

Natural abundance of 15N in amino acids and polyamines from leguminous nodules: unique 15N enrichment in homospermidine

The natural ¹⁵N abundance (¹⁵N value) in acetylpropyl derivatives of amino acids and in ethyloxycarbonyl derivatives of polyamines was determined using a gas chromatography/combustion/mass spectrometer-(GC/C/MS). ¹⁵N value determined for 12 amino acids and five polyamines by GC/C/MS were identical to those obtained by a direct combustion method using an automatic nitrogen and carbon analysis (ANCA) mass spectrometer, the difference being less than 1.0% in most cases. The GC/C/MS method was used to analyse ¹⁵N values in the amino acids and polyamines from root nodules of pea and faba bean and from stem nodules of Sesbania rostrata. The analysis of ¹⁵N values revealed that homospermidine had high ¹⁵N values, as much as +40%, while the amino acids investigated had ¹⁵N values between -3 and +6%, putrescine between +2 and +8%, cadaverine between +1 and +7%, spermidine between -2 and +4%, and spermine between 0 and +6%. The mechanism of ¹⁵N enrichment in homospermidine is discussed.     

Guard Cells Extrude Protons Prior to Stomatal Opening--A Study using Fluorescence Microscopy and pH Micro-electrodes

A method for the demonstration of pH changes in the apoplastis described. The fluorescent pH indicator pnmulin was usedto follow pH changes in the epidermis of leaves of Commelinacommunis during stomatal movements. Previously darkened leavesexposed to light showed quenching of fluorescence in the apoplastsurrounding theuard cells up to 20 min before the stomata opened.This indicated that proton efflux by the guard cells precededstomatal opening. This result was substantiated by apoplasticpH measurements using pH micro-electrodes. Acidification ofthe apoplast spread outwards from the guard cells to the surroundingsubsidiary and epidermal cells. This phenomenon persisted forsome time after subsequent stomatal closure, supporting thehypothesis that closure is brought about by a process otherthan the cessation of proton pumping. Key words: Commelina communis, stomata, proton pumping     

The effect of rhizosphere dissolved inorganic carbon on gas exchange characteristics and growth rates of tomato seedlings

The possibility that an enhanced supply of dissolved inorganic carbon (DIC=CO2+HCO3^-) to the root solution could increase the growth of Lycopersicon esculentum (L.) Mill. cv. F144 was investigated under both saline and non-saline root medium conditions. Tomato seedlings were grown in hydroponic culture with and without NaCl and the root solution was aerated with CO2 concentrations in the range between 0 and 5000 mol mol^-1. The biomass of both control and salinity-stressed plants grown at high temperatures (daily maximum of 37C) and an irradiance of 1500 mol m^-2 s^-1 was increased by up to 200% by enriched rhizosphere DIC. The growth rates of plants grown with irradiances of less than 100 mol m^-2 s^-1 were increased by elevated rhizosphere DIC concentrations only when grown at high shoot temperatures (35C) or with salinity 28°C). At high light intensities, the photosynthetic rate, the CO2 and light-saturated photosynthetic rate (jmax) and the stomatal conductance of plants grown at high light intensity were lower in plants supplied with enriched compared to ambient DIC. This was interpreted as 'down-regulation' of the photosynthetic system in plants supplied with elevated DIC. Labelled organic carbon in the xylem sap derived from root DI¹⁴C incorporation was found to be sufficient to deliver carbon to the shoot at rates equivalent to 1% and 10% of the photosynthetic rate of the plants supplied with ambient- and enriched-DIC, respectively. It was concluded that organic carbon derived from DIC incorporation and translocated in the xylem from the root to the shoot may provide a source of carbon for the shoots, especially under conditions where low stomatal conductance may be advantageous, such as salinity stress, high shoot temperatures and high light intensities.     

Correlation of decreased calcium contents with proline accumulation in the marine green macroalga Ulva fasciata exposed to elevated NaCl contents in seawater

The involvement of Na⁺, K⁺, Cl^- or Ca²⁺ in the regulation of salinity stress-induced proline accumulation via the inhibition of the activity of proline dehydrogenase (PDH; EC 1.4.3.1), a catabolic enzyme of proline, was investigated in the marine green macroalga Ulva fasciata Delile. After 6 h of exposure to elevated artificial seawater (ASW) salinity, adjusted either by increasing the NaCl content in 30 ASW (a change in ion ratio) or by concentrating ASW (a constant ion ratio), the contents of Na⁺, K⁺ and Cl^- linearly accumulated with increasing salinity from 30-90 (parts per thousand); the accumulation pattern of each ion was similar between the two treatments. An increase in NaCl content in ASW induced proline accumulation, but decreased both the PDH activity and the total water-soluble Ca²⁺ contents, while concentrated ASW had no effect. As compared to a constant value at 30, both the contents of total and water-soluble CA²⁺ and the activity of PDH decreased 1 h after exposure to 60 (adjusted by increasing NaCl content in 30 ASW) and concomitantly the content of seawater Ca²⁺ increased, while proline accumulated after 3 h. The addition of 15 mM ethylene glycol-bis-(2-aminoethyl ether) N,N,N-tetraacetic acid (EGTA) in 60 ASW (adjusted by increasing the NaCl content in 30 ASW) enhanced both the proline accumulation and the decrease in the content of total and water-soluble cellular Ca²⁺ and the activity of PDH; the effects of EGTA were reversed by 10 mM CaSO4. These results indicate that a loss of cellular Ca²⁺ is associated with the NaCl induction of proline accumulation via an inhibition of PDH activity in U. fasciata.     

Gene note. Characterization of a tomato karyopherin {alpha} that interacts with the Tomato Yellow Leaf Curl Virus (TYLCV) capsid protein

A karyopherin (LeKAP1) cDNA was isolated from tomato plants. The deduced LeKAP1 protein sequence of 527 amino acids showed similarity to other plant karyopherin proteins. When LeKAP1 was expressed in a yeast two-hybrid system together with the gene coding for the capsid protein (CP) of the tomato yellow curl leaf virus (TYLCV), it interacted directly with CP. Thus, LeKAP1 may be involved in the nuclear import of TYLCV CP and, potentially, the TYLCV genomes during viral infection of the host tomato cells.     

Effects of Photoperiodic Induction and Debudding in Xanthium pennsylvanicum and of Partial Defoliation in Phaseolus vulgaris on Rates of Net Photosynthesis and Stomatal Conductances

Xanthium pennsylvanicum plants received four treatments in thefactorial experiment (a) debudded v. not debudded, and (b) longdays . photoinductive short days. Rates of net photosynthesis,carbon dioxide compensation points (), and stomatal conductanceswere assessed after 8 days and before leaf growth or stomatalsize were appreciably affected. Leaf size, stomatal frequencies,and lengths of guard cells were estimated at this time and again22 flays after treatment. Debudding alone slightly Increased stomatal conductance; inductionalone had a similar but larger effect. Debudding and inductiontogether caused a more than additive increase in net photosynthesisat 8 days, with marked decreases in . At 22 days this combinationcaused more than additive increases in leaf size and guard-celllengths while stomatal frequencies had decreased. Induction alone directly increased stomatal conductance andthis may be responsible for the increase in photosynthesis;but debudding alone may directly affect photosynthesis by increasingthe supply of cytokinins to the leaves. The positive interactionof these factors in photosynthesis could not be explained interms of stomatal conductance and a synergism between cytokininsand a photoperiodically induced hormone is suggested. In Phaseolus vulgaris plants, 4 days after partial defoliation,stomatal conductances and rates of net photosynthesis increasedgreatly in the remaining leaflets.     

Overexpression of phosphoenolpyruvate carboxylase from Corynebacterium glutamicum lowers the CO2 compensation point (*) and enhances dark and light respiration in transgenic potato

The effect of decreased or increased phosphoenolpyruvate carboxylase (PEPC) activity on the CO2 compensation point, respiration in the light or dark as well as the partitioning of carbon into starch, soluble sugars, organic acids, and amino acids was investigated using transgenic potato plants. Engineered PEPC activity ranged form 0.5-fold wild-type level in antisense plants to 5-fold wild-type levels in lines overexpressing the ccpc gene of Corynebacterium glutamicum encoding for a PEPC not modulated by protein phosphorylation. The CO2 compensation point determined according to Brooks and Farquhar (1985) was lower in PEPC overexpressors (32 l l^-1 CO2) compared to control potato lines (38 l l^-1 CO2), but was increased in antisense PEPC plants (42 l^-1 CO2). 3-fold overexpression of PEPC gave a minimum CO2 compensation point of 32 l l^-1 CO2. Increased PEPC activity resulted in enhanced respiration in the light and dark. Altered PEPC activity had no effect on the pattern of ¹⁴CO2 incorporation into leaf discs in the light. ¹⁴C pulse-chase experiments in the dark, demonstrated that substantially more total label was lost in the leaf discs from PEPC overexpressors. Metabolite levels were determined in 21 PEPC overexpressing lines after 8 h in the light. A 5-fold increase in PEPC over the wild-type increased malate (61%), starch (75%) and significantly increased sucrose contents 9150%). Total amino acid contents were only marginally increased. From gas exchange characteristics and labeling experiments it was concluded that PEP carboxylation, followed by an increased rate of respiratory CO2 release, might work as a HCO3^-/CO2 pump. This might result in elevated CO2/O2 ratios in the mesophyll, concomitant with a more favoured carboxylation/oxygenation ratio of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco).     

High-light damage in air-dry thalli of the old forest lichen Lobaria pulmonaria-interactions of irradiance, exposure duration and high temperature

High-light damage in air-dry thalli of Lobaria pulmonaria were measured in the laboratory as reductions in maximal PSII efficiency (Fv/Fm) after a 48 h recovery in a hydrated state at low light to account for permanent damage. Thalli treated with the lowest light dose (90 mol photons m^-2) recovered normal Fv/Fm-values with increasing irradiances (400-700 nm) up to 1000 mol photons m^-2 s^-1. Doubling this dose lowered the threshold level for damage from 1000 to 320 mol photons m^-2 s^-1, and reduced Fv/Fm at 1000 mol photons m^-2 s^-1 by more than 50%. A second doubling of the dose to 360 mol photons m^-2 caused damage at 200 mol photons m^-2 s^-1, and a nearly complete cessation of PSII efficiency occurred at 1000 mol photons m^-2 s^-1. No reciprocity of irradiance and duration of illumination for PSII function was found. The measured time-dependent decrease in Fv/Fm was remarkably similar for the naturally coupled, but artificially separated, light and temperature factors. Therefore, the damage of high light on desiccated L. pulmonaria seemed to be an additive effect of high irradiance and high temperatures. Air-dry thalli were highly heat susceptible, being affected already at temperatures around 40C. Logging operations in forests are likely to raise the solar radiation at remaining lichen sites to destructive levels.Keywords: Lichens, high-light damage, heat stress, poikilohydric organisms, reciprocity.      

Stomatal Response to Water Stress and its Relationship to Bulk Leaf Water Status and Osmotic Adjustment in Pearl Millet (Pennisetum americanum [L.] Leeke)

The water potential () at which stomata completed closure (_8Lmin)was determined for pearl millet (Pennisetum americanum [L.]Leeke) at two growth stages by monitoring changes in leaf conductance(g_L) and following shoot detachment. Leaf water status wasevaluated concurrently using a pressure-volume (P-V) technique. In a pot experiment with young vegetative plants, _8Lmin closelyapproximated to the estimated at zero turgor (_u) both for controland for drought-conditioned plants which had osmotically adjusted.However, for penultimate leaves of field-grown flowering plants,_8Lmin was found to be 0.61 (irrigated plants) and 0.87 (droughtedplants) MPa below _u. In drought-stressed field-grown plants,osmotic adjustment (characterized by a decrease in solute (osmotic)potential (_s ) at both full hydration and zero turgor) was insufficientto maintain a positive bulk leaf turgor potential (_p) once had declined to below about -1.5 MPa. It is suggested that localizedadjustment by the stomatal complex in response to environmentaldifferences, leaf ageing and/or ontogenetic change, is responsiblefor the uncoupling of stomatal from bulk leaf water status. Key words: Stomata, Water stress, Pennisetum americanum     

Reduced aleurone {alpha}-amylase production in aged wheat seeds is accompanied by lower levels of high-pI [IMAGE]-amylase transcripts and reduced response to gibberellic acid

With the aim of determining the level at which ageing exerts its effect on the expression of -amylase, GA3 regulation of -amylase production was studied in isolated aleurone layers from aged wheat seeds. GA3-induced -amylase activity was lower in the tissue from aged seeds than in controls. However, the proportion of ³⁵S-methionine incorporated into -amylase was higher in the aged than in control tissue. The pattern of -amylase isoforms was resolved by isoelectric focusing and showed that two isogroups were present with the activity of the high-pI isogroup being higher in the control than in the aged lot. These apparently contradictory results may be explained in terms of differences in isozyme expression. Studies on the expression of -amylase genes indicated a reduction in the level of high-pI mRNA in aged tissue. Dose-response curves showed lower GA3-responsiveness of aleurone layers from aged seeds as compared to the controls. From these results, it is proposed that the diminished capacity of -amylase production in aleurone from aged seeds is apparently due to a decrease in the expression of the high-pI -amylase genes, and this reduction is associated with a decrease in the response to GA3.Key words: Seed ageing, wheat aleurone, gibberellic acid, -amylase isozymes, gene expression.      

Physiological and morphological responses to elevated CO2 and a soil moisture deficit of temperate pasture species growing in an established plant community

Periods of limited soil water availability are a feature of many temperate pasture systems and these have the potential to modify pasture plant and community responses to elevated atmospheric CO2. Using large pasture turves, previously exposed to elevated CO2 concentrations of 350 or 700 mol mol^-1 for 324 d under well-watered conditions the morphological and physiological responses of pasture species growing at these CO2 concentrations were compared when subjected to a soil moisture deficit-and to recovery from the deficit-with those that continued to be well watered.Net leaf photosynthesis of Trifolium repens (C3 legume), Plantago lanceolata (C3) and Paspalum dilatatum (C4) was increased by exposure to elevated CO2, but there was no consistent effect of CO2 on stomatal conductance. At low soil moistures, net photosynthesis declined and stomatal conductance increased in these three species. There was a strong CO2 x water interaction in respect of net photosynthesis; in Trifolium repens, for example, elevated CO2 increased net photosynthesis by approximately 50% under well-watered conditions and this increased to over 300% when soil moisture levels reached their minimum values. Similar values were recorded for both Paspalum dilatatum and Plantago lanceolata. Potential water use efficiency (net photosynthesis/stomatal conductance) was increased by both exposure to elevated CO2 and drought.Leaf water status was measured in three species: Trifolium repens, Paspalum dilatatum and Holcus lanatus (C3). Total leaf water potential (t) and osmotic potential () were decreased by drought, but CO2 concentration had no consistent effect. t and were highest in the C4 species Paspalum dilatatum and lowest in the legume Trifolium repens.In the wet turves, rates of leaf extension of the C3 grasses Holcus lanatus and Lolium perenne at elevated CO2 were frequently higher than those at ambient CO2, but there was no effect of CO2 concentration on the rate recorded in the C4 grass Paspalum dilatatum or the rate of leaf appearance in the legume Trifolium repens. Drought reduced leaf extension rate irrespective of CO2 in all species, but in Holcus lanatus the reduction was less severe at elevated CO2. Immediately after the dry turves were rewatered the leaf extension rate on tillers of Holcus lanatus and Lolium perenne were higher than on tillers in the wet turves, but only at ambient CO2. Consequently, despite the greater leaf extension rate during the soil moisture deficit at elevated CO2, because of the overcompensation after rewatering at ambient CO2, total leaf extension over both the drying and rewetting period did not differ between CO2 concentrations for these C3 grass species. Further investigation of this difference in response between CO2 treatments is warranted given the frequent drying and wetting cycles experienced by many temperate grasslands.     

Quantitative effects of the genes Lf, Sn, E, and Hr on time to flowering in pea (Pisum sativum L.)

Flowering time in pea (Pisum sativum L.) is determined by genetically controlled responses to photoperiod and temperature. To investigate these responses, 11 lines homozygous for the flowering genes Lf, Sn, E, and Hr were grown under contrasting semi-controlled photothermal environments and the durations (d) from sowing to first flower (f) were recorded. The effects of the four genes were quantified using a two-plane photothermal model which linearly relates the rate of progress from sowing to flowering (1/f) with the mean pre-flowering values of temperature (T) and/or photo-period (P), based on 1/fa + bT (when P is longer than the critical photoperiod, Pc) and 1/fa + bT + cP (when P<Pc). The main effect of Lf alleles was on temperature sensitivity (b) when P>Pc, which increased in the sequence Lf^d<Lf< lf<lf^a. Gene Hr, when together with Sn, increased photoperiod sensitivity (c) and reduced the intercept (a) when P<Pc. Allele sn determined a single plane response to temperature alone (i.e. a day-neutral response). Gene E, when present with lf Sn, increased 1/f in both the thermal (P<Pc) and photothermal (PPc) domains, mainly by increasing a and b, respectively. Variations in the coefficients of the thermal and photothermal responses determined that the critical photoperiod varied with temperature in all photoperiod-sensitive genotypes. A common base temperature of 0.2C was determined amongst Day-Neutral Class genotypes (sn) and thermal time from sowing to flowering increased in the sequence lf^a<lf< <:f<Lf^d. Intra-Class variations attributed to the Lf alleles were also detected in the Late (Sn hr) and Late High Response (Sn Hr) Classes. The linear photothermal model provided a sound basis for studying the quantitative effects of flowering genes in pea.     

Mastoparan analogues stimulate phospholipase C- and phospholipase D-activity in Chlamydomonas: a comparative study

The G-protein activator mastoparan and its analogues are becoming popular tools for studying signalling in plants. Therefore the abilities of mastoparan, mas7, mas8, and mas17 to activate phospholipase C (PLC), PLD and to induce the deflagellation response in Chlamydomonas moewusii Gerloff were compared. The aim was to test whether their relative potencies in a plant system resemble those reported for bovine brain Go and Gi, as is generally assumed, and to determine at which concentrations cells become permeabilized, a known effect of higher concentrations. The concentrations at which 50% deflagellation was induced, were 2.0 M mastoparan, 3.0 M mas8, 3.6 M mas7, and 5.8 M mas17. Similar activities were found for the production of phosphatidic acid, which is the result of the combined activities of PLD and PLC (together with diacylglycerol kinase). PLD activity alone was measured in vivo by its ability to phosphatidylate n-butanol. Surprisingly, the concentrations that stimulated maximum activity were about 10-fold lower (1 M) than those that stimulated maximum PLC activity (10 M). Mas17 was an exception with both maxima above 10 M. All the compounds except mas17 permeabilized C. moewusii cells. The concentrations at which 50% of the cells were permeabilized to Evan's blue were 7.4 M mas8, 16.0 M mas7 and 22.4 M mastoparan. In conclusion, only mastoparan itself and the least active analogue mas17 induced maximum deflagellation, PLC and PLD activities without permeabilizing the cells.Keywords: Chlamydomonas, deflagellation, mastoparan, phospholipases C and D, phospholipid metabolism      

Optimization theory of stomatal behaviour II. Stomatal responses of several tree species of north Australia to changes in light, soil and atomospheric water content and temperature

In a companion paper several methods of calculating the marginal unit water cost of plant carbon gain (E/A) were tested to determine whether stomata were behaving optimally in relation to regulating leaf gas exchange. In this paper one method is applied to several tropical tree species when leaf-to-air vapour pressure difference (D), photosynthetic photon flux density, leaf temperature, and atmospheric soil water availability were manipulated. The response of leaves that had expanded during the dry season were also compared to that of leaves that had expanded in the wet season. Few differences in absolute value of E/A, or the form of the relationship, were observed between species or between seasons. In the majority of species, E/A increased significantly as either leaf-to-air vapour pressure difference increased, at a leaf temperature of either 33C or 38C, or as in photosynthetic photon flux density increased. In contrast, as leaf temperature increased at constant D, E/A was generally constant. As pre-dawn water potential declined, E/A declined. The relationship between E/A and D did not differ whether internal or ambient carbon dioxide concentration were kept constant. It is concluded that stomata are only behaving optimally over a very small range of D. If a larger range of D is used, to incorporate values that more closely reflect those experienced by tropical trees in a savanna environment optimization is incomplete.Key words: Stomatal optimization theory, marginal unit water cost.      

Sexual hybrids of Tanacetum: biochemical, cytological and pharmacological characterization

Novel sexual hybrids have been produced between the medicinally-important species T. parthenium (L.) Schultz-Bip. (feverfew) and T. vulgare (L.) Schultz-Bip. (tansy). Morphologically, the F1 hybrids were more closely aligned to feverfew than to tansy, although notable differences were observed in floral and leaf morphologies of the hybrid plants compared to both parental species. Ultrastructurally, the lower epidermal leaf surfaces of the F1 hybrids displayed characteristics from both parents, with glandular trichome morphology and density similar to that of feverfew, but non-glandular trichome density comparable to that of tansy. Diploid F1 (2n2x18) hybrids and their parental progenitors were analysed biochemically, using chromatographic techniques. The bioactive germacranolide, parthenolide, was present in high concentrations [1.720.16% dry leaf weight (mean s.d., n5)] in leaf extracts from feverfew, but to a much lesser extent in both tansy and the F1 hybrids (<0.03% and <0.01% dry leaf weight, respectively). Whilst secondary metabolite accumulation in the leaves of the F1 hybrids was largely additive compared to the parental species, novel compounds were also detected in the F1 hybrids by HPLC, GC and TLC, indicating the expression of new metabolic pathways as a result of sexual hybridization. Pharmacologically, leaf extracts from the F1 hybrids inhibited human polymorphonuclear leucocyte (PMNL) activity in vitro, despite containing only trace amounts of parthenolide, the principal bioactive moiety from feverfew. This, in conjunction with the isolation of a fraction from the crude F1 leaf extract displaying significant (>5%) inhibition of PMNL activity, provides further evidence that parthenolide is not the sole determinant of pharmacological activity in the genus Tanacetum.Keywords: Feverfew, tansy, sexual hybridization, parthenolide, novel compounds.      

The role of solute accumulation, osmotic adjustment and changes in cell wall elasticity in drought tolerance in Ziziphus mauritiana (Lamk.)

Ber (Ziziphus mauritiana Lamk.) is a major fruit tree crop of the north-west Indian arid zone. In a study of the physiological basis of drought tolerance in this species, two glasshouse experiments were conducted in which trees were droughted during single stress-cycles. In the first experiment, during a 13 d drying cycle, pre-dawn leaf water (leaf) and osmotic () potentials in droughted trees declined from -0.5 and -1.4 MPa to -1.7 and -2.2 MPa, respectively, for a decrease in relative water content () of 14%. During drought stress, changes in sugar metabolism were associated with significant increases in concentrations of hexose sugars (3.8-fold), cyclitol (scyllo-inositol; 1.5-fold), and proline (35-fold; expressed per unit dry weight), suggesting that altered solute partitioning may be an important factor in drought tolerance of Ziziphus. On rewatering pre-dawn leaf and recovered fully, but remained depressed by 0.4 MPa relative to control values, indicating that solute concentration per unit water content had changed during the drought cycle.Evidence for osmotic adjustment was provided from a second study during which a gradual drought was imposed. Pressure-volume analysis revealed a 0.7 MPa reduction in osmotic potential at full turgor, with leaf at turgor loss depressed by 1 MPa in drought-stressed leaves. Coupled with osmotic adjustment, during gradual drought, was a 65% increase in bulk tissue elastic modulus (wall rigidity) which resulted in turgor loss at the same in both stressed and unstressed leaves. The possible ecological significance of maintenance of turgor potential and cell volume at low water potentials for drought tolerance in Ziziphus is discussed.Keywords: Ziziphus mauritiana, drought, solute accumulation, osmotic adjustment, proline.      

Cell Potentials and Turgor Pressures in Epidermal Cells of Tradescantia and Commelina

Cell membrane potentials have been measured both in epidermalstrips and intact leaf sections of Tradescantia virginiana andCommelina communis, and in epidermal cells over green and overalbino mesophyll cells of T. albiflora var. albovittata. Membranepotentials (_cell) in strips were considerably lower than thosein intact sections and were insensitive to light and to theabsence or presence of calcium. Their response to external cationlevels was indifferent to ionic species. However, in intactleaf sections incubated with calcium present, membrane potentialsresponded to K⁺ levels but not to Na⁺. were more negative thancells in epidermal strips, and responded to changes in illumination. Long-term recordings of _cell and vacuolar K⁺ levels in T. virginianaduring stomatal closure suggest that the fluctuations of _cellwere unrelated to K⁺ movement (which we could not detect) andthus probably to stomatal movement as well. Turgor pressures measured in epidermal cells of intact leafsections of T. virginiana were found to be of the same magnitudeas those previously reported for epidermal strips. It is concludedthat epidermal cells maintain their solute contents during strippingwithout the involvement of an electrophysiological transportsystem. With the possible exception of lateral subsidiary cells,there was no evidence suggesting that ordinary epidermal cellsare capable of osmotic adjustment even when additional KCI wassupplied in the osmoticum. Absolute turgor levels in intactleaf sections kept at constant external KCI were unrelated tosteady state _cell.