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     

Comparison of the Effects of Root Temperature on Nitrate and Ammonium Nutrition of Oilseed Rape (Brassica napusL.) in Flowing Solution Culture: II. CATION-ANION BALANCE

Macduff, J. H., Hopper, M. J., Wild, A. and Trim, F. E. 1987.Comparison of the effects of root temperature on nitrate andammonium nutrition of oilseed rape (Brassica napusL.) in flowingsolution culture. II. Cation-anion balance.—J. exp. Bot.38: 1589-1602. The effects of root temperature and form of N nutrition (NH4or NOJ) on the mineral composition of the plant, the balanceof inorganic cation-anion uptake and on the apparent net effluxof H ⁺/OH^–ions from the roots were studied with 49-d-oldoilseed rape (Brassica napusL. cv. Bien venu) in flowing solutionculture. Plants were pre-treated for 14 d at a root temperatureof 5 °C prior to constant root temperatures of 3, 7, 11or 17°C for 14 d, with a common shoot temperature of 20/15°Cday/night. Nitrogen was supplied as NH⁺₄4 or NO^–₃ at 10mmol m³. Values of Q₁₀ (7-17°C) for mean unit absorptionrates of all the major nutrient ions (K⁺ , Mg⁺⁺ , NH⁺₄, SO₄,H₂PO₄, NO₃), except Ca⁺⁺, were > 2.0 over the first 5 d oftreatment but thereafter were < 1.5; the apparent effectof temperature on uptake rates diminished with time. Under NH⁺₄nutrition, inorganic cation uptake (Mg^{+ +} + K⁺+Ca^{+ +} +NH⁺₄)exceeded inorganic anion uptake (SO4 ^–₄+ H₂PO₄) over 14d at all temperatures, with the proportion of cation uptakeas NH4 remaining constant (0.67-0-68) irrespective of root temperature.The net efflux of H ⁺ from the roots approximately balancedNH⁺₄ uptake (1:1) over 14 d at each temperature and also balancedthe difference between the total uptake of inorganic cationsand inorganic anions. Under NO₃ nutrition, the sum of the netefflux of OH and the change in the carboxylate contents of plantsover 14 d approximately balanced the sum of NO₃ and SO ^–₄reduced in the plant. The majority of the negative charge associatedwith the reduction of NO₃ and SO ^–₄ was apparently effluxedas OH, but this fraction was lower at low root temperatures.The results are discussed in terms of mechanisms that have beenproposed to regulate the internal pH of plants. Key words: Brassica napus, oilseed rape, root temperature, cation-anion balance, H⁺/OH efflux.     

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.     

The Composition of Phloem Exudate and Xylem Sap from Broccoli (Brassica oleracea var. italica) Supplied with NH4+, NO""3 or NH4NO3

Shelp, B. J. 1987. The composition of phloem exudate and xylemsap from broccoli (Brassica oleracea var. italica) suppliedwith NH⁺₄, NO₃ or NH₄NO₃.—J. exp. Bot. 38: 1619–1636. The detailed composition of xylem sap and exudate from stemincisions of attached inflorescences of broccoli (Brassica oleraceavar. italica) was compared in plants supplied with NH⁺₄, NO₃or NH₄NO₃. A phloem origin for the exudate was suggested fromthe high levels of sugars (71–133 mg cm^-3), amino acids(8·1-26·7 mg cm³) and K. (2·3–3·8mg cm³), the low levels of NO₃ and Ca, the high C: N (w/w) ratios(8·3–33), and the alkaline pH (7·2–7·3).In contrast, the xylem sap was mildly acidic (pH 5·6–6·0),and possessed lower levels of all organic and inorganic solutesbut NO₃ and Ca, and lower ratios of K: Ca, Mg: Ca and C: N (0·6–4·4). Glutamine was the predominant o-phthalaldehyde-reactive aminocompound in both transport fluids with the next most abundantamino acids dependent on sap type and N-form. Together witharginine, -aminobutyric acid, which was found only in the xylemstream, was enhanced by NH⁺₄compared to NO₃ -nutrition suggestingthat glutamate metabolism was stimulated in the roots. Underlimiting N the amino acid concentrations in the transport fluidswere greater with NH⁺₄ than with NO₃. NO₃ reduction occurredin both the root and shoot with the latter site predominatingover the entire N range (0-300 mol m³). Even though the compositionof nitrogenous solutes in the xylem was dependent on cultivarand N source, the composition of the phloem streams supplyingthe developing inflorescence was relatively unaffected. The data on the element composition of organs and phloem sapare interpreted to suggest that, in spite of the restrictedmobility of some elements such as B and Mn, a significant proportionof their total supply to developing sinks is carried in thephloem stream. Key words: Transport fluid composition, plant nutrition, phloem mobility.     

Effects and Interactions of Ozone and the Anti-ozonant EDU at Different Stages of Radish {Raphanus sativus L.) Development

The use of the anti-ozonant ethylenediurea (EDU) to assess croplosses due to atmospheric ozone (O₃), is controversial becauseof incomplete knowledge on the optimal dosage, the extent ofprotection provided and potential effects on plant performanceof EDU per se. In a greenhouse experiment, radish (Raphanussativus L.) cv. ‘Cherry Belle’ plants were treatedwith EDU as a soil drench (150 mg1¹, 60 ml plant¹) at threestages of development: early, maximum and late hypocotyl thickening.Two days later the plants were exposed to O₃, mimicking an ambientpattern (70 nl 1¹ O₃, 7 h d¹, 5 d, and up to 120 nl 1¹ O₃ on2 d), or remained in filtered air. Assessment of leaf injury showed complete protection from ozone-inducedleaf damage at all stages of growth due to EDU. EDU did causeslight leaf margin necrosis and hyponastic leaf deformationirrespective of the atmospheric treatment, but biomass and relativegrowth rate (RGR) of the shoot and leaf number duration werenot altered and leaf area duration was increased when comparedto control plants. EDU did not affect dry weight accumulationnor RGR of the hypocotyl. Ozone-induced reductions in growthof below-ground organs (hypocotyl and root), which were mostprominent in the intermediate and early stage of growth, werelargely suppressed by the EDU-treatment. Decreases of hypocotylbiomass due to O₃ were transient and significant increases inRGR of the hypocotyl during the post-exposure period indicateda substantial capability of compensation during the period ofrecovery. Key words: Radish (Raphanus sativus), ozone, anti-ozonant, ethylenediurea (EDU), development, phytotoxicity     

Carbon Isotope Discrimination and the CO2 Accumulating Mechanism in Chlorella emersonii

The relationship between the CO₂ accumulating mechanism andcarbon isotope discrimination has been investigated in the unicellulargreen alga Chlorella emersonii. Growth of Chlorella at highC0₂ levels (5%) which repress the activity of the CO₂ accumulatingmechanism results in more negative ¹³C values. The data presentedin this paper suggest that it is possible to induce the accumulatingmechanism by nitrogen limitation as well as by carbon limitation.Activity of the accumulating mechanism, irrespective of whetherit is induced by C or by N limitation, is accompanied by ¹³Cvalues considerably less negative than those of cells whichdo not possess such a mechanism. It is suggested that the CO₂accumulating mechanism results in essentially a closed systemin which the inherent isotope discrimination by RuBP carboxylaseis not expressed.     

The Effect of Hydroxymethanesulphonate on Photosynthesis in Chlorella pyrenoidosa

Photosynthesis under conditions known to favour glycollate excretionby algae did not result in glycollate excretion in a strainof Chlorella pyrenoidosa unless an inhibitor of glycollate oxidase,-hydroxypyridin-2yl-methane sulphonate (-HPMS), was present.This inhibitor increased the total amount of glycollate presentin the supernatant from the cells during photosynthetic carbondioxide fixation and gave accumulation of ¹⁴C in glycollateduring ¹⁴CO₂ fixation under conditions favouring glycollatesynthesis. At pH 8.3 -HPMS did not stimulate photosynthetic¹⁴CO₂ fixation in C. pyrenoidosa as occurs with some algae.Photoassimilation of acetate was inhibited by -HPMS, and thiswas shown to result from acetyl-CoA synthetase inhibition by-HPMS.     

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.     

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     

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     

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.     

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     

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     

Plant Characteristics and Nutrient Composition and Mobility of Broccoli (Brassica oleracea var. italica) Supplied with NH4+, NO3"" or NH4NO3

Shelp, B. J. 1987. Plant characteristics and nutrient compositionand mobility of broccoli (Brassica oleracea var. italica) suppliedwith NH⁺₄, NO₃ or NH₄NO₃.—J. exp. Bot. 38: 1603–1618. The effects of varying NH⁺₄, NO₃ or NH₄NO₃ concentration onthe final plant characteristics, element composition, and accumulationof NO₃-N, NH⁺₄-N and organic-N were evaluated in broccoli (Brassicaoleracea var. italicacv. Futura and Premium Crop) plants culturedin vermiculite under greenhouse conditions supplemented withlight. NH⁺₄-grown plants were stunted and exhibited signs ofmarginal necrosis on the old leaves, accompanied by an accumulationof NH4. The tissue levels of N, P, Mn, Cu, Zn and B were generallyincreased by NH⁺₄ versus NO₃ nutrition whereas the reverse wastrue for Ca; Mg and K were only slightly affected, if at all.These results are attributed to: changes in element availabilityresulting from reduced rhizosphere pH due to NH⁺₄uptake ratherthan NO ₃uptake; competition of Ca uptake by NH⁺₄; and dilutionof N by increased vegetative growth with NO₃-nutrition. Theelement concentrations of N, P or K were similar in all tissueswhereas Ca, B and Mn were markedly less in the florets and youngleaves compared to mature leaves; this supports literature indicatingthat the former elements are phloem-mobile whereas the latterare not. Assuming that the nutrient supply for mature leavesis delivered principally via the xylem stream, the data suggestthat nutrients for developing leaves and florets are suppliedpredominantly in the phloem. If so, under our experimental conditions.Zn and Cu were also readily mobile in the phloem whereas Mgmovement was restricted. NH₄⁺ versus NO₄⁺ J nutrition alteredthe distribution of these elements. The two broccoli cultivarstested under the greenhouse environment differed in NH⁺₄ toleranceand in the distribution of K and Cu suggesting there was a geneticbasis for cultivar variation in mineral acquisition and redistribution. Key words: Plant nutrition, phloem mobility, elemental composition.     

Alternative Methods of Analysing Water Potential Isotherms: Some Cautions and Clarifications: I. THE IMPACT OF NON-IDEALITY AND OF SOME EXPERIMENTAL ERRORS

In recent years alternative ways have been proposed to transformmeasurements of leaf water potential, , and relative water content,R^*, in order to derive values of osmotic pressure at full turgidityin leaves and shoots, ^o(when 0). Two types of transformationsare usually considered: 1/ versus R^* and versus 1/R^*, and linearregression is used to fit the data in the region where turgoris thought to be zero. It appears that when ^o is estimated bylinear extrapolation of 1/Psi; versus R^* then apoplastic watermight not influence the accuracy of ^o but when the versus \/R^*transformation is used apoplastic water causes an underestimateof ^o. We examine the accuracy of the estimate of ^o obtainedfrom the two transformations when there are random errors in, systematic errors in , and when the osmotic solutions arenon-ideal. The 1/ versus R^* transformation generally producesthe best estimate of ⁰ by linear extrapolation.     

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     

Stomatal Control of Photosynthesis of Eucalyptus globulus Labill. Trees under Field Conditions in Portugal

Pereira, J. S., Tenhunen, J. D. and Lange, O. L. 1987. Stomatalcontrol of photosynthesis of Eucalyptus globulus Labill. treesunder field conditions in Portugal.—J. exp. Bot. 38: 1678–1688. Stomatal behaviour of adult leaves of Eucalyptus globulus treeswas studied under field conditions in Portugal. In the absenceof severe plant water stress stomata were open when the summedtotal of photosynthetically active photon flux density incidenton both leaf surfaces was above 100 µmol m²s¹ and leafconductance to water vapour reached 245 mmol m ² s¹ on a total(both epidermes) leaf area basis. The stomata of both leaf epidermesresponded similarly to changes in solar radiation and waterstress. Water stress resulted in decreasing daily maxima inleaf conductance as predawn leaf water potential decreased.Maximal leaf conductance decreased to less than 50 mmol m ²s ¹ when predawn leaf water potential decreased below —1·0MPa. At similar values of predawn leaf water potential stomatawere more closed as the leaf to air water vapour partial pressuredifference increased. The effect of increasing air dryness onstomata was greatest at high predawn leaf water potential. Dailymaxima in photosynthetic rates and in leaf conductance werelinearly related to one another in spring and summer. Both decreasedwith increase in leaf water stress. In autumn and winter, increasesin leaf conductance occurring under natural conditions duringthe course of the day were not necessarily accompanied by increasesin net photosynthesis. Stomata were more closed in the afternoonthan in the morning at the same rates of net photosynthesis,temperature or leaf to air water vapour partial pressure difference. Key words: Eucalyptus globulus,, photosynthesis, stomata, water stress.     

The Derivation of the Cell Wall Elasticity Function from the Cell Turgor Potential

An equation is derived expressing average turgor pressure ofa leaf (_p) as a function of relative water content (RWC). Basedon this derivation, the relationships of the bulk elastic modulus(_v) and both RWC and _p, are formulated and discussed. The bulkelastic modulus (_v) becomes zero for _p = 0, that is at the turgorloss point for the leaf. At full water saturation the valueof ev is proportional to the water saturation turgor potential_p(max). The factor relating _P and _v (structure coefficient ,Burstrom, Uhrstr?m and Olausson, 1970) changes only very littlefor values of _p, which are not too close to zero. An exampleis given for the calculation from experimental data of the turgorpressure function, the structure coefficient function, and the_v function. Key words: Cell wall, Turgor pressure, Bulk elastic modulus     

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     

Characterization of Spring Wheat Genotypes Differing in Drought-Induced Abscisic Acid Accumulation: II. LEAF WATER RELATIONS

Aspects of the water relations of spring wheat (Triticum aestivumL.) are described for cultivars Highbury (low ABA) and TW269/9(high ABA), and low and high ABA accumulating F⁶selections derivedfrom a cross between them. In a pot experiment, pressure-volume (P-V) curves were constructedfor main stem leaf four (MSL4) of well-watered plants of Highburyand TW269/9. Estimates of solute potential (²) from these curveswere similar for the two cultivars, but varied with the timeof sampling and the time allowed for hydration in dim light. In a field experiment with four low and four high ABA F⁶lines,P-V curves for flag leaves from both droughted and irrigatedplants gave at both zero turgor (^p) and zero water potential(¹) which differed with degree of stress, sampling time andgenotype. ¹was strongly dependent on the initial_L of the leafand was reduced on average by c. 0.4 MPa per MPa decline ininitial L.5, was lower (more negative) by c. 0.1-MPa in theafternoon than in the morning. Overall, was also 0.1 MPa lowerin low ABA lines than in high ABA lines. In another field experiment, flag leaves of five low and fivehigh ABA F⁶lines were sampled over a 4 week period from droughtedplots and _L and 5, measured (the latter by osmometry with expressedsap). For these leaves 5, at zero p or zero _L was consistentlylower by 0.3–0.5 MPa than estimates of 5, from the P-Vcurves with flag leaves. However, data for the low ABA lineswere again lower (by c. 0.1 MPa) than those for high ABA lines. The consequences of these differences in 1 are discussed inrelation to the stimulation of ABA accumulation in low and highABA selections. Key words: Water potential, Solute potential, P-V curves, Wheat (Triticum aestivum), Drought stress