Measurement of Cytoplasmic pH by the DMO Technique in Hydrodictyon africanum

The 5, 5-dimethyl-[2-¹⁴C]oxazolidine-2, 4-dione (DMO) distributiontechnique for the measurement of intracellular pH has been appliedto giant cells of Hydrodictyon africanum. Significant metabolism of DMO was found in this alga; the free[DMO + DMO–] in subcellular samples is thus derived fromthe total label in cells equilibrated in [¹⁴C]DMO solutionsby measuring and subtracting the label in metabolic productsof DMO. A further problem arises from the observation that theDMO concentration in the vacuolar sap is always lower than thatpredicted by the transmembrane equilibration of undissociatedDMO from the bathing medium. This is interpreted in terms ofa finite permeability to the anion DMO–. Since the effectof P_DMO– on the DMO distribution is much smaller at thetonoplast (where the transmembrane electrical potential differenceis small) than at the plasmalemma, the values of cytoplasmicpH are computed assuming equilibration of undissociated DMOacross the tonoplast. At an external pH of 7.0 the cytoplasmic pH is about 7.4; decreaseor increase of external pH by 1 unit causes a decrease and anincrease in cytoplasmic p11 respectively of about 0.2 pH units.Determinations of _vo at pH 6, 7, and 8, together with an assumedconstant value of _cv, permit calculations of µ_H+ at theplasmalemma and tonoplast. The values are relatively independentof external pH in the range pH 6–8 at 21–25 and12–14 kJ mol^–1 respectively. The significance ofthese results for the regulation of intracellular pH, and forthe regulation and energising of the fluxes of ions, is discussed.     

A Transient Stimulation of Net Photosynthesis of Rye Leaves by {alpha}-Hydroxy-2-pyridinemethanesulphonic Acid ({alpha}-HPMS) due to Inhibition of Photorespiratory CO2 Release

The effects of -hydroxy-2-pyridinemethanesulphonic acid (-HPMS)upon net photosynthesis (P_n, the CO₂ compensation point (),post-lower illumination burst of CO₂ (PLIB) and post-lower temperatureburst of CO₂ (PLTB) in detached rye (Secale cereale L.) leaveswere investigated. At low concentrations ( 0.5 mol m^–3),-HPMS initially stimulated P_n and decreased the magnitude ofboth PLIB and PLTB. The decreased at all concentrations of-HPMS (0.05–5.0 mol m^–3. The effects of -HPMS onP_n and were time-dependent and, after a few minutes, the P_nwas inhibited while values increased considerably. At a higherconcentration (5.0 mol m ^–3), the transient effects of-HPMS were shorter () or not observed at all (P_n. Both PLIBand PLTB, when expressed in relation to P_n, increased at higherlevels of this compound. Similar data with respect to the effectsof -HPMS on PLIB and PLTB were found for leaves of dandelion(Taraxacum officinale L.). The results suggest that -HPMS may stimulate P_n by inhibitingphotorespiration, as originally suggested by Zelitch (1966),but only at low concentrations and over a short time span. Thedecrease of PLIB and PLTB values at low -HPMS levels is consistentwith these processes being a residual activity of the glycolatepathway. Key words: CO₂ compensation point, -hydroxy-2-pyridinemethanesulphonic acid, photorespiration, photosynthesis     

A comparison of the impacts of various nitrogen sources on acid-base balance in C3 Triticum aestivum L. and C4 Zea mays L. plants

This work aimed to study the impacts of acquisition and assimilationof various nitrogen sources, i.e. NO₃^–, NH₄⁺ or NH₄NO₃,in combination with gaseous NH₃ on plant growth and acid-basebalance in higher plants. Plants of C₃ Triticum aestivum L.and C₄ Zea mays L. grown with shoots in ambient air in hydroponicculture solutions with 2 mol m^–3 of nitrogen source asNO₃^–, NH₄⁺ or NH₄NO₃ for 21 d and 18 d, respectively,had their shoots exposed either to 320 µg m^–3 NH₃or to ambient air for 7 d. Variations in plant growth (leaves,stubble and roots), and OH^– and H⁺ extrusions as wellas the relative increases in nitrogen, carbon and carboxylatewere determined. These data were computed as H⁺/N, H⁺/C, (C-A)/N,and (C-A)/C to analyse influences of different nitrogen sourceson acid-base balance in C₃ Triticum aestivum and C₄ Zea maysplants. Root growth in dry weight gain was significantly reduced bytreatment with 320 µg m^–3 NH₃ in Triticum aestivumand Zea mays growing with different N-forms, whereas leaf growthwas not significantly affected by NH₃. In comparison with C₃Triticum aestivum, non-fumigated C₄ Zea mays had low ratiosof OH^–/N in NO₃^–3-grown plants and of H⁺/N in NH₄⁺- and NH ₄NO₃-grown plants. Utilization of NH₃ from the atmospherereduced both the OH^–N ratios in NO₃^– -grown plantsand the H⁺/N ratio in NH₄⁺ - and NH₄NO₃ -grown plants of bothspecies. Furthermore, Zea mays had higher ratios of (C-A)/Nin NH₄⁺ - and NH₄NO₃-grown plants than Triticum aestivum. Thismeans that C₄ Zea mays had synthesized more organic anion perunit increase in organic N than C₃ Triticum aestivum plants.Within both species, different nitrogen sources altered theratios of (C-A)/N in the order: NH₄NO₃>NH₄⁺>NO₃^–.Fumigation with NH₃ increased organic acid synthesis in NO₃^–- and NH₄⁺ - grown plants of Triticum aestivum, whereas it decreasedorganic acid synthesis in Zea mays plants under the same conditions.Furthermore, these differences in acid-base regulation betweenC₃ Triticum aestivum and C₄ Zea mays plants growing with differentnitrogen sources are discussed. Key words: Acid-base balance, ammonia, ammonium, nitrate, ammonium nitrate, C₃ Triticum aestivum L., C₄ Zea mays L.     

Cl- Fluxes during Osmoregulation in Chara

Measurements of Cl^– influx in cells of Chara corallinashow that control of this flux contributes to the ability ofthis cell to regulate its osmotic pressure. Transcellular osmosiswas used to generate cell fragments with abnormally high ₁,(H-cells), and with abnormally low ₁, (L-cells). Plasmalemmainflux (_oc) was very high in L-cells, and markedly reduced inH-cells. Influx was not affected by the presence of sucrosein the pond water and the consequent reduction in turgor. InH-cells the chloride flux from cytoplasm to vacuole (_cv) wasalso strongly inhibited. It is suggested that control of Cl^–fluxes at both plasmalemma and tonoplast is involved in osmoregulationin these cells. Key words: Chara corallina, osmoregulation, Cl^– flux     

Analysis of Pressure-Volume Curves of Leaves of Rosa hybrida cv. Sonia

By analysing the relationship between inverse water potential(^–1), and relative water content (RWC) measured on leavesof roses (Rosa hybrida cv. Sonia), grown soilless, it was foundthat a non-linear (NL) model was better suited than a linearmodel to reproduce values observed in the non-turgid region.To explain this apparent curvature, it is assumed that a reductionof the non-osmotic water fraction (A_p) takes place when decreases.Osmotic potentials () measured on fresh and frozen leaf discstend to support this hypothesis. A method for exploiting PVcurves, which takes into account the variation of A_p, is described.It delivers values for the turgor pressure (_p), the relativeosmotic water content, and the mean bulk volumetric elasticitycoefficient, lower than those given by the linear model. Onthe other hand, it gives higher estimates for A_p and for . Whenapplying the traditional model to obtain estimates for waterrelations characteristics of rose leaves, and comparing resultsfrom two distinct salinity treatments (electrical conductivitiesof 1·8 mS cm^–1 and 3·8 mS cm^–1, respectively),one deduces a significant reduction of at turgor-loss in thehigh salinity treatment. The NL method is, in addition, ablesimultaneously to reveal a reduction of and a significant increasein _p at RWC=100% this proves that soilless–grown roseplants are able to osmoregulate when subjected to a constantand relatively high degree of salinity. Key words: Apoplastic water, non-linear regression, pressure-volume curves, tissue-water relations     

Phosphate and Membrane Electropotentials in Trifolium repens L.

In Trifolium repens L. there were immediate transient depolarizationsof the membrane electropotential (E_vo) when KH₂PO₄ was addedto phosphate-free media, but these were of the same magnitudeas the controls (K²SO⁴ and KCI). Furthermore, the extents ofdepolarization were the same as the expected effect of the addedK⁺ calculated using the Goldman equation. There was no significantdepolarization on adding H₃PO₄ to buffered media. Consequently,there was no evidence for a depolarization caused by phosphate.This result provides evidence that the H⁺–H₂PO₄ symportin roots of T. repens operates with a stoichiometry of 1: 1. In a group of control plants ( + P plants) and a group whichwere stressed by reducing the supply of phosphate (– Pplants), the – P plants had lower values for E_vo than+P plants (– 118 mV and – 130 mV, respectively).The absence of phosphate from the measurement media also reducedE_vo (mean effect = 9 mV). A significant difference in E_vo between– P and + P plants persisted when phosphate was addedto – P plants. The electropotential difference acrossthe tonoplast (E_vo) in – P plants became more positivewith time. Key words: White clover, membrane transport, roots, tonoplast, symport     

Abscisic Acid Accumulation and Water Relations of Four Cultivars of Phaseolus vulgaris L. under Drought

Larqué-Saavedra, A., Rodriguez, M. T., Trejo, C. andNava, T. 1985. Abscisic acid accumulation and water relationsof four cultivars of Phaseolus vulgaris L. under drought.—J.exp. Bot 36: 1787–1792. Plants of four cultivars of Phaseolus vulgaris L. differingin drought resistance were grown in pots under greenhouse conditionsand prior to flowering water was withheld from the pots untilthe mid-day transpiration rate reached values below 1.0 µgH₂O cm^–2 s^–1 (designated the ‘drought’stage). At this point leaves were harvested on 3 or 4 occasionsover 24 h to determine the abscisic acid (ABA) concentration,total water potential (), solute potential (₁) and turgor potential(_p). Results showed that values of , ₁, and _p differed between cultivarswhen they reached the ‘drought’ stage. The stomatalsensitivity to changes in and _p, was as follows: Michoacán12A3 > Negro 150 Cacahuate 72 > Flor de Mayo. These datacorrelated well with the pattern of drought resistance reportedfor the cultivars. ABA accumulation at the ‘drought’ stage differedbetween cultivars at each sampling time, but overall differencesin ABA level between cultivars were not significant. ABA levelsdid not, therefore, correlate with the drought resistance propertiesreported for the cultivars. Results are discussed in relationto and hour of the day when bean samples were taken for ABAanalysis. Key words: Phaseolus vulgaris L., drought resistance, abscisic acid     

Potassium Transport Across the Membranes of Chara: II.42K FLUXES AND THE ELECTRICAL CURRENT AS A FUNCTION OF MEMBRANE VOLTAGE

Smith, J. R. 1987. Potassium transport across the membranesof Chara. II. ⁴²K fluxes and the electrical current as a functionof membrane voltage.—J. exp. Bot. 38: 752–777. The current required to clamp the trans-membrane voltage ofinternodal cells of Chara australis at different levels wasmeasured simultaneously with either the ⁴²K influx or efflux.Examination of the voltage-dependence of the ratio of the electricalcurrent to the unidirectional tracer fluxes yielded no evidenceof any amplification of the electrical driving force on theK⁺ ions. There was thus no evidence for the interaction of K⁺ions with themselves or any other species during their passageacross the membrane. These measurements allow the determinationof , the fraction of the electrical current carried by K⁺ ions.When the external [K⁺] = 10 mol m^–3, the average valueof was 0?85 for V_m > –125 mV and 07?5 for V_m <–150 mV. When the external [K⁺] = 0?1 mol m^–3, was 0?6 for V_m < –80 mV and 0?1 for V_m > –250mV. It was also found that the conductance associated with K⁺transport was inhibited by hyperpolarization. Key words: Potassium, conductance, flux-ratio     

Water Relations of Chromium VI Treated Bush Bean Plants (Phaseolus vulgaris L. cv. Contender) under both Normal and Water Stress Conditions

The effect of Chromium VI on leaf water potential (^w), solutepotential (^a), turgor potential (^p) and relative water content(RWC) of primary and first trifoliatc leaves of Phaseolus vulgarisL. was studied under normal growth conditions and during anartificially induced water stress period in order to establishthe possible influence of this heavy metal on the water stressresistance of plants. Plants were grown on perlite with nutrientsolution containing 0, 1•0, 2•5, 5•0 or 10•0µg cm^–3 Cr as Na₂Cr₂O₇.2H₂O. The effect of Cr onwater relations was highly concentration dependent, and primaryand first trifoliate leaves were affected differently. The growthreducing concentrations of Cr (2•5, 5•0 and 10•0µg cm^–3) generally decreased _s and _w and increased_p in primary leaves. The 1•0 µg cm^–3 Cr treatmentdid not affect growth, but altered water relations substantially:in primary leaves _w and _p were increased and _s decreased, whilein trifoliate leaves the effect was the opposite. All Cr treatedplants resisted water stress for longer than control plants.The higher water stress resistance may be due to the lower _sand to the increased cell wall elasticity observed in Cr VItreated plants. Key words: Phaseolus vulgaris, Chromium VI, water stress, Richter plot     

Effect of Low Relative Humidity on {delta}13C Value in Two C3 Grasses and in Panicum milioides, a C3-C4 Intermediate Species

When grown under conditions of low relative humidity, the C₃–C₄intermediate Panicum milioides, as well as the C₃ grasses Triticumaestivum and Poa pratense, exhibited ¹³C values which were upto 2–7%o less negative than the ¹³C values of the correspondingplants grown at high relative humidity. At both humidity levels,there was no evidence of a substantial contribution of phosphoenolpyruvatecarboxylase to carbon gain in Panicum milioides     

The Relationship Between Transpiration Rate, Water Potential, and Resistances to Water Movement in Sunflower (Helianthus annuus L.)

The effects of transpiration rate on the vertical gradientsof leaf and stem xylem water potential ( and ) were examinedusing hydroponic sunflower plants. Transpiration was variedby stepwise alterations of environmental conditions. The gradientsof and were relatively small (2.3 and 0.8 x 10⁵ Pa m^–1)when transpiration rates approached zero, but increased sharplyto 5.4 and 2.3 x 10⁵ Pa m^–1 as transpiration increased.However, the gradients were independent of transpiration ratesabove 0.4 g dm^–2 h^–1 owing to variability of theplant resistance. The gradients of _I were usually less thanhalf those of _I. ₁ in individual leaves remained constant over a wide range oftranspiration rates (0.4—2.4 g dm^–2 h^–1) andeach leaf possessed a characteristic plateau value related toits elevation. _I responded similarly but was approximately 2.0x 10⁵ Pa higher than _I at the same elevation. Identical resultswere obtained regardless of the procedure employed to vary transpiration. The drop in water potential between stem and leaf implies thatthe leaf resistance is appreciable. This was confirmed usingrapidly transpiring excised leaves freely supplied with water._I increased by 2.0–2.5 x 10⁵ Pa following removal of theroot resistance but remained 2 x 10⁵ Pa lower than similar excisedleaves in darkness. Furthermore, ^I in excised leaves remainedconstant over a wide range of transporting rates, demonstratingthat the leaf resistance is also variable. The results are discussed in relation to previous reports.     

Evaluation of the 15N Natural Abundance Method and Xylem Sap Analysis for Assessing N2 Fixation of Understorey Legumes in Jarrah (Eucalyptus marginata Donn ex Sm.) Forest in S.W. Australia

Hansen, A. P. and Pate, J. S. 1987. Evaluation of the ¹⁵N naturalabundance method and xylem sap analysis for assessing N₂ fixationof understorey legumes in jarrah (Eucalyptus marginata Donnex Sm.) forest in S.W. Australia.—J. exp. Bot 38: 1446–1458. Nodulated seedlings of Acacia pulchella, A. alata and A. extensawere grown in glasshouse sand culture under a range of levels(0–16 mol m³) of nitrate, supplied as ¹⁵NO₃, or as unenrichedlaboratory grade nitrate (¹⁵N value 5·5%_o). Nitrate at8·0 mol m ³ or above was highly inhibitory to growthof all species. Using ¹⁵N dilution analysis of the ¹⁵N enrichedcultures to measure symbiotic dependency, it was shown that¹⁵N values of the parallel unenriched cultures increased innear linear fashion from close to zero in fully symbiotic plantsto values close to that of the supplied NO₃ in plants experiencingnitrate levels (4·0 mol m³ or above) inhibiting N₂ fixationby over 90%. Xylem sap analyses (0·4 mol m³ NO₃ treatments)showed asparagine as the major nitrogenous solute, relativelylittle spill-over of free nitrate, and no evidence of majorshifts in balance of amino compounds with increasing dependenceon nitrate. This essentially invalidated use of the techniqueas a field assay for N₂ fixation by the species. ¹⁵N values for total N of soil sampled at 64 widely distributedsites in jarrah forest ranged from – 2·15 to +5·4(mean +2·1). Comparable values for soil mineral N (NH⁺₄and NO₃) were +0·3 to + 14·2 (mean +5·1).¹⁵N values of the total plant N of the legumes and of non-N₂-fixingreference species were also highly variable between sites, withlittle evidence of reference plant N accurately reflecting the¹⁵N abundance of soil nitrogen, or of visibly well nodulatedlegume components showing consistently lower ¹⁵N values thantheir companion reference plants. At one site it was possibleto compare ¹⁵N values of first season seedling legumes withpreviously published estimates of their progressive N₂ fixationusing C₂H₂ reduction assays. It was concluded that heterogeneity in ¹⁵N discrimination ofsoil within the ecosystem precluded effective use of the ¹⁵Nnatural abundance technique for assessing legume N₂ fixation. Key words: Acacia spp., ¹⁵N natural abundance,, xylem sap analysis,, nitrogen fixation.     

Corrigendum

Light response curves for (•) gross ¹⁶O₂ evolution, and() CO² uptake in 210 mmol mol^–1 O₂ with 900–1000µbar CO₂ or () in air by leaves of Hirschfeldia incana.The difference between (•) and () or () was quantitativelyequivalent to the measured ¹⁸O₂ uptake. The areas under thecurves are labelled to identify regions of assimilatory andnon-assimilatory electron flow redrawn from data of Canvin etal. (1980). It should be noted that the data and the labelling of the figureaxes are correct as printed.     

Physiological Responses to Drought of Lolium perenne L.: Measurement of, and Genetic Variation in, Water Potential, Solute Potential, Elasticity and Cell Hydration

Thomas, H. 1987. Physiological responses to drought of Loliumperenne L.: Measurement of, and genetic variation in, waterpotential, solute potential, elasticity and cell hydration.—J.exp. Bot. 38: 115–125. Clonally-replicated genotypes of Loiium perenne L. were grownin a controlled environment. Leaf water potential (_w) osmoticpotential (_s), turgor potential (_p = _w – _s), elasticity(E), leaf hydration (g water per g dry matter, H) and numberof green leaves per tiller (N_GL) were measured before and duringa 42 d drought treatment. A simplified method of estimating E (at _w < 1?0 MPa) usingonly six measurements was developed to permit a measurementrate of 8 leaves per hour. Measurement errors in all characterswere 3% or less. During drought, _w and _s (at _w = 0?5 MPa) decreased significantly,_p and E increased significantly, and H decreased slightly. Plantsize during drought was negatively correlated with _s, and ^Hand positively correlated with _p, osmotic adjustment, E andN_GL. Measurements made on the genotypes before draughting didnot give a reliable indication of their physiological conditionafter adaptation to drought. Genetically controlled variation (‘broad sense heritability’)of drought-adapted plants for E was 15%, _w 23%, _s, 34%, _p, 35%,H 34% and N_GL 64%. The possibilities for, and effectivenessof, divergent selection of genotypes with high and low expressionof the characters are discussed. Key words: Water relations, Lolium, genetic variation     

Effects of Priming and Endosperm Integrity on Seed Germination Rates of Tomato Genotypes: II. GERMINATION AT REDUCED WATER POTENTIAL

Seed germination rates (GR =inverse of time to germination)are sensitive to genetic, environmental, and physiological factors.We have compared the GR of tomato (Lycopersicon esculentum Mill.)seeds of cultivar T5 to those of rapidly germinating L. esculentumgenotypes PI 341988 and PI 120256 over a range of water potential(). The influence of seed priming treatments and removal ofthe endosperm/testa cap enclosing the radicle tip on germinationat reduced were also assessed. Germination time-courses atdifferent 's were analysed according to a model that identifieda base, or minimum, allowing germination of a specific percentage(g) of the seed population (_b(g)), and a ‘hydrotime constant’(_H) indicating the rate of progress toward germination per MPa.h.The distribution of _b(g) determined by probit analysis was characterizedby a mean base (_b) and the standard deviation in _b among seeds(_b). The three derived parameters, _b, _b) and _H, were sufficientto predict the time-courses of germination of intact seeds atany . A normalized time-scale for comparing germination responsesto reduced is introduced. The time to germination at any (t_g())can be normalized to be equivalent to that observed in water(t_g(0)) according to the equation t_g(0)=[l–(/_b(g))]t_g().PI 341988 seeds were more tolerant of reduced and had a morerapid GR than T5 seeds due to both a lower _b and a smaller _H.The rapid germination of PI 120256, on the other hand, couldbe attributed entirely to a smaller _H. Seed priming (6 d in–1.2 MPa polyethylene glycol 8000 solution at 20 ?C followedby drying) increased GR at all >_b(g), but did not lower theminimum allowing germination; i.e. priming reduced _H withoutlowering _b. Removing the endosperm/testa cap (cut seeds) markedlyincreased GR and lowered the mean required to inhibit germinationby 0.7 to 0.9 MPa. However, this resulted primarily from downwardadjustment in _b during the incubation of cut seeds at low inthe test solutions. The difference in _b between intact and cutseeds incubated at high was much less (0.l MPa), indicatingthat at the time of radicle protrusion, the endosperm had weakenedto the point where it constituted only a small mechanical barrier.In the intact seed, endosperm weakening and the downward adjustmentin embryo _b ceased at < –0.6 MPa, while the reductionin _H associated with priming proceeded down to at least –1.2MPa. Based on these data and on the pressure required to pushthe embryos from the seeds at various times after imbibition,it appears that the primary effect of priming was to shortenthe time required for final endosperm weakening to occur. However,as priming increased GR even in cut seeds, priming effects onthe embryo may control the rate of endosperm weakening. Key words: tomato, Lycopersicon esculentum Mill., water potential, germination rate, seed priming, genetic variation     

Adaptation of the Euryhaline Charophyte Lamprothamnium papulosum to Brackish and Freshwater: Turgor Pressure and Vacuolar Solute Concentrations During Steady-State Culture and After Hypo-osmotic Treatment

The euryhaline charophyte Lamprothamnium papulosum (Wallr.)J. Gr. was adapted to media with decreasing salinities rangingfrom 550 to 0 mosmol kg^–1. Vegetative plants grown inmedia with osmotic pressures (⁰) in the range of 550 to 130mosmol kg^–1 maintained a constant turgor pressure () at309 + 7 mosmol kg^–1. The ions K+, Na+ and Cl–, werethe predominant solutes in the vacuole. Changes in their concentrationsaccount for the variation in internal osmotic pressure (¹) with,⁰. The divalent ions Mg²⁺, Ca²⁺ and were also present in significant amounts, but their concentrationsdid not alter with changes in, ⁰. In cells subjected to hypo-osmotic shock the regulation of was incomplete. The turgor pressure increased from 302 to 383mosmol kg^–1. The first rapid response to the sudden decreasein ⁰ was a loss of K⁺ and Cl^–. In contrast to the decreasein ionic concentrations an accumulation of sucrose occurredwhich could account for the increase of . The increase in sucroseconcentration started 24 to 48 h after the downshock and reachedits highest value after 3 to 4 weeks. The sucrose concentrationin the vacuole was up to 320 mol m^–3. During this timethe ionic content continued to decrease but did not counterbalancethe sucrose concentration sufficiently to regain the original. High sucrose levels accompanied by an enhanced were also observedduring the period of fructification (sexual reproduction: formationof antheridia and oogonia) in Lamprothamnium kept under conditionsof constant salinity. It is concluded that high sucrose content and elevated arecharacteristic of sexual reproduction in this charophyte. Lamprothamniumis able to tolerate different during various developmentalstages (e.g. vegetative and reproductive phases). Key words: Lamprothamnium papulosum, sucrose, turgor pressure     

Investigations of the {delta}13C Pattern in Leaves of Fagus sylvatica L.

The natural abundance of ¹³C in different parts of beech (Fagussylvatica L.) leaves was analysed. Values for leaf ribs wereconsistently higher than those for intercostal tissue. Similardifferences occur between petiole and stem, with petiole beingless negative. The pattern of results is the same, independentof the position in the tree. However, the absolute values differby up to 6%. from the bottom to the top of the tree. Valuesof ¹³C are in the range of – 29 to – 32%. for thelower leaf strata; while values between – 24 and –26%. have been measured for the top of the tree. Absolute ¹³Cvalues of the whole tissue and cellulose differ by about 2%.,but relative ¹³C trends are almost identical. However, ¹³C trendsare not identical for different leaf parts. A comparison ofcellulose and whole tissue ¹³C data makes it unlikely that the¹³C variations are primarily due to different compositions ofchemical compounds. No fractionation seems to exist betweenleaf and wood cellulose. Tissue from different areas of a leafrevealed identical carbon isotope compositions. Key words: Carbon isotope ratio, Fagus sylvatica L., beech leaves     

Stomatal Behaviour and Water Relations of Chilled Phaseolus vulgaris L. and Pisum sativum L.

Leaf diffusion resistance interpreted as stomatal resistance,leaf water potential (_w), solute potential (_s) and leaf turgorpotential (_p) of the chilling sensitive species Phaseolus vulgariswere determined during chilling at 4 °C in the light. Bothchill-hardened and non-hardened plants were used. For comparison,the chilling resistant species Pisum sativum was also used. The results for chilled P. sativum were similar to those obtainedfor chill-hardened P. vulgaris plants receiving a chilling treatment.In both cases a reduction in stomatal aperture and the maintenanceof a positive leaf turgor were the responses to chilling. Leavesof chilled but non-hardened P. vulgaris plants were found tomaintain open stomata throughout the chilling treatment despitea severe wilt developing after 7 h at 4 °C. This was incontrast to the chill-resistant P. sativum. which showed a rapidclosing and subsequent re-opening of the stomata to a new reducedaperture. During the first 12 h of chilling _wof P. vulgaris leaves changedfrom –0.47 MPa to –1.24 MPa. On more prolonged chilling_w tended to return to pre-chilling values. In addition. _p decreasedfrom 0.42 MPa to zero after only 9 h of chilling, and remainedat this value for the remainder of the chilling period, _s, changedrapidly from –0.89 MPa to –1.35 MPa in the first7.5 h, and after 9 h. _w and _s, were equal, i.e. zero _p. In contrast,the chilling resistant plant P. sativum maintained a positive_p throughout the chilling period, and there was little differencebetween values of _w, and _s in control and chilled leaves. Key words: Chilling, Stomata, ater relations, Phaseolus vulgaris, Pisum sativum     

An Assessment of Gibberellin Structure-activity Relationships

Information on the biological activities of gibberellins (GAs)in the barley aleurone, Tangin-bozu dwarf rice, dwarf pea, lettucehypocotyl and cucumber hypocotyl bioassays is reviewed and discussedin the context of GA structure-activity relationships. The barley aleurone bioassay exhibits a limited response toGAs and it is suggested that this may be because the aleuronecells are able to carry out few GA interconversions. Consequentlyactivity is determined by the degree of compatibility betweenthe GAs and a receptor site. In this assay high biological activityis associated with GAs having a 3ß-hydroxy--lactonestructure. This activity is substantially enhanced by the additionalpresence of a 13-hydroxyl group. The substitution of a -lactoneor a -lactol for a -lactone results in reduced activity while3ß,13-dihydroxy GAs with either 20-carboxyl or 20-methylfunctions are completely inactive. The Tanginbozu dwarf ricebioassay responds to many more GAs than the barley aleuronesystem possibly because the rice seedlings can carry out extensiveGA interconversions. Under these circumstances GAs that areinactive per se can be metabolically converted to active forms.There is no interaction between the 3ß- and 13-hydroxyfunctions of GA molecules in the rice assay. Activity appearsto be determined by the degree oxidation of the C-20 group.The order of activity is usually -lactone > -lactol >-lactone > methyl > carboxyl. It is suggested this mayindicate that in rice seedlings C₂₀-GAs are converted to C₁₉-GAsvia a Baeyer-Villiger type oxidation. Activity in the dwarfpea bioassay is dependent upon GAs possessing both 3ß-and 13-hydroxyl groups and is again related to the state ofoxidation at the C-20 locus. In the lettuce bioassay activityis restricted to GAs with a -lactone function. In some instancesa -lactone, but not a -lactol, can substitute effectively. Thismay imply that the applied C₂₀-GAs are not converted to C₁₉-GAsand that the response to the -lactone results from the six-memberedring mimicking the -lactone at the receptor site. Only GAs havinga 19,10 or a 19,20 lactonic bridge show substantial activityin the cucumber bioassay. The additional presence of eithera 12- or a 13-hydroxyl group severely reduces activity.     

Effects of soil moisture deficits on the water relations of bambara groundnut (Vigna subterranea L. Verdc.)

The components of leaf water potential (_l) and relative watercontent (RWC) were measured for stands of bambara groundnut(Vigna subterranea) exposed to three soil moisture regimes incontrolled-environment glasshouses at the Tropical Crops ResearchUnit, Sutton Bonington Campus. Treatments ranged from fullyirrigated (wet) to no irrigation from 35 days after sowing (DAS)(dry). RWC values varied between 92–96% for the wet treatment,but declined from 93% to 83% in the dry treatment as the seasonprogressed. _l at midday decreased in both the wet and dry treatments,but the seasonal decline was more pronounced in the latter:seasonal minimum values were –1.19 and –2.08 MPa,respectively. Plants in the wet treatment maintained turgor(_p) at about 0.5 MPa throughout the season, whereas values inthe dry treatment approached zero towards the end of the season.There was a linear relationship between _p and _l9 with _p approachingzero at a _l of –2.0 MPa. Mean daily leaf conductance wasconsistently higher in the wet treatment (0.46–0.79 cm^-1)than in the intermediate and dry treatments (0.13–0.48cm s^-1 Conductances in the intermediate and dry treatments weresimilar, and the lower evapotranspirational water losses inthe latter were attributable to its consistently lower leafarea indices (L): L at final harvest was 3.3, 3.3 and 1.9 forthe wet, intermediate and dry treatments. Bambara groundnutwas apparently able to maintain turgor through a combinationof osmotic adjustment, reductions in leaf area index and effectivestomatal regulation of water loss. Key words: Vigna subterranea, water relations, soil moisture