Response of Wheat Seedlings to Low O2 Concentrations in Nutrient Solution: II. K +/Na+ SELECTIVITY OF ROOT TISSUES9

This paper reports the effects of low O₂ concentration (0–01,0–055, and 0.115mol m^–3) in nutrient solutions onK⁺/Na⁺ selectivity of growing and mature root tissues of 6-to 8-d-old, intact, wheat (Triticum aestivum cv. Gamenya) seedlings. Increases in anaerobic catabolism and decreases in O₂ uptake,K⁺ uptake and K⁺/Na⁺ selectivity were all more pronounced and/oroccurred at higher external O₂ concentrations in the apex (0–2mm) than in the expanding tissues (2–4 mm); these growingtissues were, in turn, more affected than the expanded tissuesof the roots (4–12 mm). Selectivity for K⁺ over Na⁺ in roots and shoots was particularlysensitive to O₂ deficiency. For example, in apical tissues (0–2mm) K ⁺ /Na⁺ selectivity was already reduced at 0.115 mol m^–3O₂, yet at this O₂ concentration there was no effect on eithergrowth or (K⁺/Na⁺) uptake. Upon transfer from 0.01 to 0.26 mol m^–3 O₂, a detailedstudy of the 12 mm root tips showed that 70% of these tips regainedhigh (K⁺ + Na⁺) concentrations and K⁺/^Na+ ratios. In contrast,there was no recovery in the remaining 30% of the 12 mm roottips. Net K⁺ transport to the shoots during the period afterre-aeration was negative for the population as a whole. Theseverity of these effects supports the view that the root tipsand the stele were more susceptible to O2 deficiency than wasthe cortex of the fully-developed root tissues. Key words: Hypoxia, K⁺/Na⁺ selectivity, expanded and expanding tissues     

The Relationship Between Growth and Oxygen Uptake in Hypoxic Rice Seedlings

Atwell, B. J. and Green way, H. 1987. The relationship betweengrowth and oxygen uptake in hypoxic rice seedlings.—J.exp. Bot. 38: 454–465. Rice seedlings (Oryza saliva L.) were grown in the dark forup to 4 d in solutions containing various concentrations ofO₂. Compared with seedlings grown at 0·250 mol O₂ m^–3,the dry weight of the growing seedling was 14% lower at 0·110mol O₂ m^–3 and 60% lower at 0 mol O₂ m^–3. Decreasesin fresh weight were similar but not identical to decreasesin dry weight, possibly because leaf growth was suppressed evenabove 0·110 mol O₂ m^–3. Oxygen deficiency inhibitedroot growth more severely than coleoptile growth. Coleoptiles from seedlings grown in aerated solution were exposedto an atmosphere of pure N₂ for 30 min. Anoxia caused a declinein ATP content and energy charge, suggestive of decreased oxidativephosphorylation. It is not clear whether the decline in oxidativephosphorylation was solely responsible for impaired growth inhypoxia. In seedlings growing at O₂ concentrations less than 0·110mol O₂ m^–3, significant amounts of ethanol were synthesized.The rate of O₂ uptake decreased markedly below 0·06 molO₂ m^–3; this was presumably near the external O₂ concentrationat which oxidative phosphorylation became limited by the supplyof O₂. The stage of development of the seedlings appeared toinfluence O₂ uptake, possibly through changes in conductanceof the tissue to O₂. Uncouplers were used to confirm that thecritical O₂ concentration was dependent on O₂ diffusion ratherthan enzyme kinetics. Impaired growth above 0·110 molO₂ m^–3 may have been due to a decreased activity of oxygenasesof relatively low affinity for O₂, which in turn altered cellmetabolism. Key words: Growth, oxygen uptake, rice seedlings, hypoxia     

Radial Turgor Pressure Profiles in Growing and Mature Zones of Wheat Roots--A Modification of the Pressure Probe

A modification of the pressure probe is described which allowsaccurate routine recording of the turgor pressure of singlecells at measured depth within a tissue. Measurements of radial profiles of turgor pressure in wheatroots grown in some simple salt solutions (0.5 mol m^–3CaCl₂, 0.5 mol m^–3 CaCI₂ plus 10 mol m^–3 NaCl, and0.5 mol m^–3 CaCl₂ plus 10 mol m^–3 KCI), are described.Turgor pressure was constant (approximately, 0.65 MPa) alonga radius within the elongation zone irrespective of the natureof the bathing solution. In mature root tissue turgor pressurein the cortex was lower than that of the growing zone in alltreatments and the pressure of the stele was on average 0.22MPa higher than that of the cortex. Potassium in the mediumbathing the root increased the turgor pressure in mature root(both cortex and stele) relative to low salt and sodium treatments. The results are discussed in relation to both root growth andion accumulation. Key words: Pressure probe, wheat roots, salt solution     

C3 and CAM Photosynthetic Characteristics of the Submerged Aquatic Macrophyte Littorella uniflora: Regulation of Leaf Internal CO2 Supply in Response to Variation in Rooting Substrate Inorganic Carbon Concentration

The relationships between CO₂ concentrating mechanisms, photosyntheticefficiency and inorganic carbon supply have been investigatedfor the aquatic macrophyte Littorella uniflora. Plants wereobtained from Esthwaite Water or a local reservoir, with thelatter plants transplanted into a range of sediment types toalter CO₂ supply around the roots. Free CO₂ in sediment-interstitial-waterranged from 1–01 mol m^–3 (Esthwaite), 0.79 mol m^–3(peat), 0.32 mol m^–3 (silt) and 0–17 mol m^–3(sand), with plants maintained under PAR of 40 µmol m^–2s^–1. A comparison of gross morphology of plants maintained underthese conditions showed that the peat-grown plants with highsediment CO₂ had larger leaf fresh weight (0–69 g) andtotal surface area (223 cm² g^–1 fr. wt. including lacunalsurface area) than the sand-grown plants (0.21 g and 196 cm²g^–1 fr. wt. respectively). Root fresh weights were similarfor all treatments. In contrast, leaf internal CO₂ concentration[CO₂], was highest in the sand-grown plants (2–69 molm^–3, corresponding to 6.5% CO₂ in air) and lowest inthe Esthwaite plants (1–08 mol m^–3). Expressionof CAM in transplants was also greatest in the low CO₂ regime,with H⁺ (measured as dawn-dusk titratable acidity) of 50µmolg^– fr. wt., similar to Esthwaite plants in natural sediment.Assuming typical CAM stoichiometry, decarboxylation of malatecould account largely for the measured [CO₂]₁ and would makea major contribution to daytime CO₂ fixation in vivo. A range of leaf sections (0–2, 1–0, 5–0 and17–0 mm) was used to evaluate diffusion limitation andto select a suitable size for comparative studies of photosyntheticO₂ evolution. The longer leaf sections (17.0 mm), which weresealed and included the leaf tip, were diffusion-limited witha linear response to incremental addition of CO₂ and 1–0mol m^–3 exogenous CO₂ was required to saturate photosynthesis.Shorter leaf sections were less diffusion-limited, with thegreatest photosynthetic capacity (36 µmol O₂ g^–1 fr. wt. h^–1) obtainedfrom the 1.0 mm size and were not infiltrated by the incubatingmedium. Comparative studies with 1.0 mm sections from plants grown inthe different sediment types revealed that the photosyntheticcapacity of the sand-grown plants was greatest (45 µmolO₂ g^–1 fr. wt. h^–1) with a K_0.5 of 80 mmol m^–3.In terms of light response, saturation of photosynthesis intissue slices occurred at 850–1000 µmol m^–2s^–1 although light compensation points (6–11 µmolm^–2s^–1) and chlorophyll a: b ratios (1.3) were low.While CO₂ and PAR responses were obtained using varying numbersof sections with a constant fresh weight, the relationshipsbetween photosynthetic capacity and CO₂ supply or PAR were maintainedwhen the data were expressed on a chlorophyll basis. It is concludedthat under low PAR, CO₂ concentrating mechanisms interact inintact plants to maintain saturating CO₂ levels within leaflacunae, although the responses of the various components ofCO₂ supply to PAR require further investigation. Key words: Key words-Uttorella uniflora, internal CO₂ concentration, crassulacean acid metabolism, root inorganic carbon supply, CO₂ concentrating mechanism     

Influence of Elevated CO2 and Temperature on the Photosynthesis and Respiration of White Clover Dependent on N2 Fixation

Single clonal plants of white clover (Trifolium repens L) grownfrom explants in a Perlite rooting medium, and dependent fornitrogen on N₂ fixation in root nodules, were grown for severalweeks in controlled environments which provided two regimesof CO₂, and temperature 23/18 °C day/night temperaturesat 680 µmol mol^–1 CO₂, (C680), and 20/15 °Cday/night temperatures at 340 µmol mol^–1 CO₂ (C340)After 3–4 weeks of growth, when the plants were acclimatedto the environmental regimes, leaf and whole-plant photosynthesisand respiration were measured using conventional infra-red gasanalysis techniques Elevated CO₂ and temperature increased ratesof photosynthesis of young, fully expanded leaves at the growthirradiance by 17–29%, despite decreased stomatal conductancesand transpiration rates Water use efficiency (mol CO₂ mol H₂O^–1)was also significantly increased Plants acclimated to elevatedCO₂, and temperature exhibited rates of leaf photosynthesisvery similar to those of C340 leaves ‘instantaneously’exposed to the C680 regime However, leaves developed in theC680 regime photosynthesised less rapidly than C340 leaves whenboth were exposed to a normal CO₂, and temperature environmentIn measurements where irradiance was varied, the enhancementof photosynthesis in elevated CO₂ at 23 °C increased graduallyfrom approx 10 % at 100 µmol m^–1 s^–1 to >27 % at 1170 µmol m^–2 s^–1 In parallel, wateruse efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 In parallel,water use efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 to approx100 % at the highest irradiance Elevated CO₂, and temperatureincreased whole-plant photosynthesis by > 40 %, when expressedin terms of shoot surface area or shoot weight No effects ofelevated CO₂ and temperature on rate of tissue respiration,either during growth or measurement, were established for singleleaves or for whole plants Dependence on N₂, fixation in rootnodules appeared to have no detrimental effect on photosyntheticperformance in elevated CO₂, and temperature Trifolium repens, white clover, photosynthesis, respiration, elevated CO₂, elevated temperature, water use efficiency, N₂ fixation     

Elevated CO2 reduces O3 flux and O3-induced yield losses in soybeans: possible implications for elevated CO2 studies

Soybeans were grown for three seasons in open-top field chambersto determine (1) whether elevated CO₂ (360 versus 700 µmolmol^–1) alleviates some of the yield loss due to pollutantO₃, (2) whether the partial stomatal closure resulting fromchronic O₃ exposure (charcoal-filtered air versus 1.5 ambientconcentrations) is a cause or result of decreased photosynthesis,and (3) possible implications of CO₂/O₃ interactions to climatechange studies using elevated CO₂. Leaf conductance was reducedby elevated CO₂, regardless of O₃ level, or by exposure to O₃alone. As.a result of these effects on conductance, high CO₂reduced estimated midday O₃ flux into the leaf by an averageof 50% in charcoal-filtered air and 35% in the high O₃ treatment.However, while exposure to O₃ reduced seed yields by 41% atambient CO₂ levels, the yield reduction was completely amelioratedby elevated CO₂. The threshold midday O₃ flux for yield lossappears to be 20–30 nmol m^–2 s^–1 in this study.Although elevated CO₂ increased total biomass production, itdid not increase seed yields. A/C_i curves show a large reductionin the stomatal limitation to photosynthesis due to elevatedCO₂ but no effect of O₃. These data demonstrate that (1) reducedconductance due to O₃ is the result, and not the cause, of reducedphotosynthesis, (2) 700 µmol mol^–1 CO₂ can completelyameliorate yield losses due to O₃ within the limits of theseexperiments, and (3) some reports of increased yields underelevated CO₂ treatments may, at least in part, reflect the ameliorationof unrecognized suppression of yield by O₃ or other stresses. Key words: Stomatal limitation, elevated CO₂, O₃ flux, Glycine max, yield suppression     

Normalizing Development of Cultured Triticum aestivum L. Embryos: I. LOW OXYGEN TENSIONS AND EXOGENOUS ABA

Wheat (Triticum aestivum L.) embryos form in dynamically-regulatedovular environments. Our objectives were to improve developmentof cultured immature wheat embryos by simulating, in vitro,abscisic acid (ABA) levels and O₂ tensions as found in wheatovules during zygotic embryogenesis. We characterized from intactwheat kernels embryo respiration, embryo morphology and embryoand endosperm + ABA levels at 13, 19 and 25 d post-anthesis(DPA). Young (13 DPA) embryos were then excised and culturedin vitro, where they were exposed to 0·2 or 2·Ommol m^–3 ±ABA and 2.·1, 2·5 or 7·4mol m^–3 (6, 7 and 21%, respectively) gaseous O₂. At 6and 12 d in culture, + ABA levels, embryo respiration and embryomorphology were characterized by treatment. Thirteen-day-oldembryos from two different plant populations differed by 17-foldin initial ABA content. However, this difference did not affectprecocious germination in vitro, nor did it affect the amountof exogenous ABA required to reduce precocious germination by40%. In this respect, embryos from both populations were equallysensitive to exogenous ABA. Cavity sap O₂ levels (2·1to 2·5 mol m^–3) were much more effective in preventingprecocious germination of cultured embryos than were cavitysap levels of ABA (0·2 to 2·0 mmol m^–3).The combination of physiological levels of both ABA and O₂ largelynormalized DW accumulation and embryo morphology without alteringendogenous + ABA levels. Residual respiration of cultured embryoswas higher than that of embryos grown in situ, and was not influencedby the exogenous O₂ and ABA treatments Key words: Abscisic acid, embryo development, oxygen tensions, respiration, wheat     

Effects of Anoxia on Wheat Seedlings: I. INTERACTION BETWEEN ANOXIA AND OTHER ENVIRONMENTAL FACTORS

Anoxia was imposed on 4–6-d-old, intact wheat seedlings,after the roots had first been exposed for 1 d to O₂ concentrationsbetween 0·016 and 0·06 mol m^–3. Apices ofthe main axis of the seminal roots were considered to have toleratedanoxia if elongation occurred after return from anoxia to air,hereafter called ‘retention of elongation potential’.During anoxia, elongation potential was retained longer in rootsof intact seedlings than in 0–5 mm excised root tips suppliedwith 50 mol m^–3 glucose. In intact seedlings, elongation potential was retained longerat 15°C than at 25°C, and at pH 50 and 60 than at pH40. These differences between treatments were maintained inthe presence of exogenous glucose, and glucose supply prolongedthe retention of elongation potential in all anoxic treatments. Elongation potential was retained much longer at very low 02concentrations (0006 to 00l mol m^–3) than under anoxia;this was established at pH 40. Anoxia inhibited the transport of sugars from the shoots and/orendosperm to the root by 79-97%, as assessed from experimentswith roots of intact plants exposed to anoxia at pH 60 and 15°C. Overall, the results demonstrate: (i) that the occurrence ofadverse effects of anoxia during waterlogging in the field mayinteract with other environmental factors and (ii) that thereare pronounced difficulties integrating data on tolerance toanoxia obtained in different laboratories. Key words: Anoxia, wheat seedlings, pH, temperature     

Effect of elevated CO2 on the utilization of light energy in Nothofagus fusca and Pinus radiata

Red beech (Nothofagus fusca (Hook. F.) Oerst.; Fagaceae) andradiata pine (Pinus radiata D. Don; Pinaceae) were grown for16 months in large open-top chambers at ambient (37 Pa) andelevated (66 Pa) atmospheric partial pressure of CO₂, and incontrol plots (no chamber). Summer-time measurements showedthat photosynthetic capacity was similar at elevated CO₂ (lightand CO₂-saturated value of 17.2 µmol m^–2 s^–1for beech, 13.5 µmol m^–2 s^–1 for pine), plantsgrown at ambient CO₂ (beech 21.0 µmol^–2 s^–1,pine 14.9 µmol m^–2s^–1) or control plants grownwithout chambers (beech 23.2 µmol m^–2 s^–1,pine 12.9 µmol m^–2 s^–1). However, the higherCO₂ partial pressure had a direct effect on photosynthetic rate,such that under their respective growth conditions, photosynthesisfor the elevated CO₂ treatment (measured at 70 Pa CO₂ partialpressure: beech 14.1 µmol m^–2 s^–1 pine 10.3)was greater than in ambient (measured at 35 Pa CO₂: beech 9.7µmol m^–2 s^–1, pine 7.0 µmol m^–2s^–1) or control plants (beech 10.8 µmol m^–2s^–1, pine 7.2 µmol m^–2 s^–1). Measurementsof chlorophyll fluorescence revealed no evidence of photodamagein any treatment for either species. The quantity of the photoprotectivexanthophyll cycle pigments and their degree of de-epoxidationat midday did not differ among treatments for either species.The photochemical efficiency of photosystem II (yield) was lowerin control plants than in chamber-grown plants, and was higherin chamber plants at ambient than at elevated CO₂. These resultssuggest that at lower (ambient) CO₂ partial pressure, beechplants may have dissipated excess energy by a mechanism thatdoes not involve the xanthophyll cycle pigments. Key words: Carotenoids, chlorophyll fluorescence, photosynthesis, photoinhibition, photoprotection, xanthophyll cycle     

Effects of NO2 and O3 Alone or in Combination on Kidney Bean Plants (Phaseolus vulgaris L.): Growth, Partitioning of Assimilates and Root Activities

Ten-day old kidney bean plants (Phaseolus vulgaris L. cv. Shin-edogawa)were exposed to 2.0 and 4–0 parts 10^–6 NO2, and0.1, 0.2, and 0.4 parts 10^–6 O3 alone or in combinationfor 2, 4, and 7 d. The effects of these air pollutants wereexamined with respect to the growth, partitioning of assimilates,nitrogen uptake, soluble sugar content, and root respiration. Decreased dry matter production was significant with all treatmentsexcept 2.0 parts 10^–6 NO₂ and 0.1 parts 10^–6 O₃.Exposure to mixtures of the gases produced more severe suppressionof growth than exposure to the single gases. Root/shoot ratiowas significantly lowered at 7 d by the gas treatments otherthan 2.0 parts 10^–6 NO₂ and 0.1 parts 10^–6 O₃. Thetotal nitrogen content of plants was increased by all treatments;the higher percent of nitrogen found with O₃ exposure will resultfrom the growth retardation which increases the concentrationof nitrogen in the plants because the absorption of nitrogenby roots was unaffected. The combination of O₃ with NO₂ significantlydecreased the assimilation of NO₂ by the plants. The concentration of soluble sugars in roots was decreased bythe gas treatments. There was a strong positive correlationbetween soluble sugar content and dry weight of the roots harvestedat 7 d. Root respiration was relatively unchanged until 5 dand then decreased significantly at 7 d by 2.0 parts 10^–6NO₂ and 0–2 parts 10^–6 O₃. Retarded growth of theroots and the decreased root respiration may be due to diminishedtranslocation of sugars from leaves to roots caused by exposureto air pollutants. The uptake of soil nitrogen was not closelyrelated with root respiration in the case of O₃ exposure. Key words: NO₂, O₃, Phaseolus vulgaris, Growth, Sugars, Root respiration     

The Combined Effects of Salinity and Root Anoxia on Growth and Net Na+ and K+-accumulation in Zea mays Grown in Solution Culture

The effects of excess salinity and oxygen deficiency on growthand solute relations in Zea mays L. cv. Pioneer 3906 were examinedin greenhouse experiments. The roots of plants 14 d old growingin nutrient solution containing additions of NaCl in the range1.0–200 mol m^–3 were either exposed to a severedeficiency of O₂ by bubbling with nitrogen gas (N₂ treatment),or maintained with a supply of air (controls), for a periodof 1–7 d. The threshold NaCl concentration resulting inappreciable inhibition of leaf extension, and shoot f. wt gainin controls was between 10 and 25 mol m^–3. At 25 mol m^–3NaCl the ratio of Na+/K+ transported to shoots was about 20times greater than in plants in 1.0 mol m^–3 NaCl. Theeffect of addition of NaCl to the nutrient solution was to enhanceNa+ movement but simultaneously depress the rate of K+ transportto shoots (per g f. wt roots). Interactions between NaCl levels and aeration treatment wereshown by analyses of variance to be statistically significantfor leaf extension, shoot and root f. wt gains, Na+ and K+ concentrationsin shoots and roots. When roots were N₂-treated, shoot and rootgrowth were depressed, the effect of aeration treatment beinggreatest at NaCl concentrations of 50 mol m^–3 or less.Additionally, N₂-treatment greatly accelerated Na- transportto shoots while depressing K+ transport still further, so thatat 10 mol m^–3 NaCl the ratio Na+/K+ acquired by the shootswas 230 times greater than in controls. Over the concentrationrange 1.0 to 50 mol m^–3 NaCl, the ratio Na+/K+ transportedto shoots by anoxic roots increased by a factor of 860. Mechanisms controlling changes in solute flux to the shoot,and the significance in relation to plant tolerance of excesssalts or oxygen deficiency are discussed. Anaerobic, corn, flooding, maize, oxygen-deficiency, salinity     

Modification of the CO2 Responses of Maize Stomata by Abscisic Acid and by Naturally-Occurring and Synthetic Cytokinins

Fragments of maize leaves were incubated at controlled temperatureand irradiance either on distilled water or on one of threeconcentrations of cytokinin (10^–1, 10^–2 and 10^–3mol m^–3). The effects of zeatin or kinetin on stomatalaperture were determined by stripping abaxial epidermis fromthe fragments after incubation and immediately measuring stomatalapertures under the microscope. At each cytokinin concentrationleaf pieces were incubated at 5 or 350 µmol mol^–1CO₂ with or without ABA (10^–1 mol m^–3). At 5.0 µmolmol^–1 CO₂ increasing the concentrations of zeatin hada negligible effect upon stomatal aperture. When air containing350 umol mol^–1 CO₂ was bubbled through the incubationsolutions, apertures of stomata incubated on water were morethan halved. Increasing cytokinin concentrations reduced theeffect of CO₂ on stomata and incubation on 10^–1 mol m^–3zeatin completely removed any CO₂ response. The addition ofABA restored the effect of CO₂, even at the highest cytokininconcentration. Key words: Maize, CO₂ response, ABA, Cytokinins     

Hypoxia Induces Membrane Depolarization and Potassium Loss from Wheat Roots but does not Increase their Permeability to Sorbitol

This paper deals with the responses of roots of wheat {Triticumaestivum L.) to hypoxia with special emphasis on the effectsof severe O2 deficiency on membrane integrity, loss of K+ fromthe root and root membrane potentials. Seminal and crown roots of 26-d-old plants exposed to severehypoxia (0.003 mol O₂ m^–3) for 3 h or 10 d prior to excisionand subsequently exposed to hypoxic solutions, had slightlylower rates of sorbitol influx and a slightly smaller apparentfree space than roots in aerated solutions. These results indicatethat neither a few hours nor a 10-d exposure to hypoxia hadadverse effects on the membrane integrity of the bulk of thecells in the roots. However, both 6-d-old seedlings and 26-d-oldplants lost K⁺ from the roots following their transfer fromaerated to hypoxic nutrient solutions. In the 26-d-old plants,which were of high nutritional status, there was a net K⁺ effluxfrom the roots to the external solution. In contrast, with the6-d-old seedlings, which were of low nutritional status, thedecrease in the K⁺ content of the roots was smaller than thenet K⁺ uptake to the shoots. Exposure of excised roots to 0.008 mol O₂ m^–3caused arapid and reversible membrane depolarization from –120to ––80 mV. These data and the magnitude of thenet effluxes strongly suggest that K⁺ losses during the earlystages of hypoxia are due to membrane depolarization ratherthan to increases in the permeability of membranes to K ⁺. Key words: Hypoxia, membrane integrity, membrane potentials, seminal and crown roots     

N2 Fixation and Nitrate Uptake by White Clover Swards in Response to Root Temperature in Flowing Solution Culture

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown as simulated swards for 71 d in flowing nutrientsolutions with roots at 11 C and shoots at 20/15 C, day/night,under natural illumination. Root temperatures were then changedto 3, 5, 7, 11, 13, 17 or 25 C and the total N₂, fixation over21 d was measured in the absence of a supply mineral N. Alltreatments were subsequently supplied with 10 mmol m^–2NO₂ ^– in the flowing solutions for 14 d, and the relativeuptake of N by N₂, fixation and NO₃ ^– uptake was compared.Net uptake of K⁺ was measured on a daily basis. Root temperature had little effect on root d. wt over the 35-dexperimental period, but shoot d. wt increased by a factor of3.5 between 3 and 25 C, with the sharpest increase occurringat 7–11 C. Shoot: root d. wt ratios increased from 25to 68 with increasing temperature at 7–25 C. N₂-fixationper plant (in the absence of NO₂ ^–) increased with roottemperature at 3–13C, but showed little change above13 C. The ratios of N₂ fixation: NO₂ ^– uptake over 14d (mol N: mol N) were 0.47–0.77 at 3–7 C, 092–154at 11–17 C, and 046 at 25 C, reflecting the dominanceof NO₃ ^– uptake over N₂ fixation at extremes of high andlow root temperature. The total uptake of N varied only slightlyat 11–25 –C (095–110 mmol N plant^–1),the decline in N₂ fixation as root temperature increased above11 C was compensated for by the increase in NO^– ₃ uptake.The % N in shoot dry matter declined with decreasing root temperature,from 32% at 13 C to 15% at 3 C. In contrast, concentrationsof N expressed on a shoot water content basis showed a modestdecrease with increasing temperature, from 345 mol m^–3at 3 C to 290 mol m^–3 at 25 C. Trifolium repens L, white clover, root temperature, N₂ fixation, potassium uptake, nitrate uptake, flowing solution culture     

Measurement of Yield Threshold and Cell Wal Extensibility of Intact Wheat Roots under Different Ionic, Osmotic and Temperature Treatments

Yield stress threshold (Y) and volumetric extensibility () arethe rheological properties that appear to control root growth.In this study they were measured in wheat roots by means ofparallel measurement of the growth rate (r) of intact wheatroots and of the turgor pressures (P) of individual cells withinthe expansion zone. Growth and turgor pressure were manipulatedby immersion in graded osmoticum (mannitol) solutions. Turgorwas measured with a pressure probe and growth rate by visualobservation. The influence of various growth conditions on Yand was investigated; (a) At 27 °C.In 0.5 mol m^–3 CaCl₂ r, P, Y and were20.7±4.6 µm min^–1, 0.77±0.05 MPa,0.07±0.03 MPa and 26±1.9 µm min^–1MPa^–1 (expressed as increase in length), respectively.Following 24 h growth in 10 mol m^–3 KC1 these parametersbecame 12.3±3.5 µm min^–1, 0.72±0.04MPa, 0.13±0.01 MPa and 21±0.7 µm min^–1MPa^–1. After 24 h osmotic adjustment in 150 mol m^–3mannitol/0.5 mol m^–3 CaCl₂ r= 19.6±4.2 µmmin^–1, P = 0.68±0.05 MPa and Y and were 0.07±0.04MPa and 30±0.2 µm min^–1 MPa^–01, respectively.After 24 h growth in 350 mol m^–3 mannitol/0.5 mol m^–3CaCl₂ r= 13.3±4.1 µm min^–1, P= 0.58±0.07MPa, Y=0.12±0.01 MPa and ø 32±0.2 tim min^–1MPa^–1. During osmotic adjustment in 200 mol m^–3mannitol/0.5 mol m^–3 CaCl₂, with or without KCl, the recoveryof growth rate corresponded to turgor pressure recovery (t1/2approximately 3 h). (b) At 15 °C. Lowered temperature dramatically influencedthe growth parameters which became r= 8.3±2.8 um min^–1,P=0.78 MPa, r=<0.2 MPa and =15±0.1 µm min^–1MPa^–1. Therefore, Y and are influenced by 10 mol m^–3 K⁺ ionsand low temperature. In each case the effective pressure forgrowth (P-Y) was large indicating that small fluctuations ofsoil water potential will not stop root elongation. Key words: Yield threshold, cell wall extensibility, wheat root growth, temperature, turgor pressur     

Effects of a Range of O2 Concentrations on Porosity of Barley Roots and on their Sugar and Protein Concentrations

Barley was grown at a range of oxygen concentrations (0.5–9mg l^–1), in nutrient solutions. Growth of both shootsand seminal roots was restricted by O₂ concentrations lowerthan 2–3 mg l^–1) but nodal root growth was not. Root porosities were increased even at those O₂ concentrationswhich did not restrict growth, and were inversely proportionalto the protein levels of the roots. Sugar concentrations increasedappreciably only at those O₂ concentrations which also restrictedgrowth. Hordeum vulgare L., barley, root porosity, sugar, protein, oxygen concentration     

Further Characteristics of Salt-Dependent Bicarbonate Use by the Seagrass Zostera muelleri

Millhouse, J. and Strother, S. 1987. Further characteristicsof salt-dependent bicarbonate use by the seagrass Zostera muelleri.—J.exp. Bot. 38: 1055–1068. The contribution of HCO^–₃to photosynthetic O₂ evolutionin the seagrass Zostera muelleri Irmisch ex Aschers. increasedwith increasing salinity of the bathing seawater when the inorganiccarbon concentration was kept constant. K_1/2 (seawater salts)for HCO^–₃ -dependent photosynthesis was 66% of seawatersalinity. Both short- and long-term pretreatment at low salinitiesstimulated photosynthesis in full strength seawater. Twentyfour hours pre-incubation of seagrass plants in 3·0 molm^–3 NaHCO₃ resulted in increased photosynthesis at allsalinities, apparently due to stimulation of HCO^–₃ use(K_1/2 (seawater salts) = 26%). V_max (HCO^–₃) was not affectedby low salinity pretreatment. The kinetics of HCO^–₃ stimulationby the major seawater cations was investigated. Ca²⁺ was themost effective cation with the highest V_max (HCO^–₃) andwith K_1/2(Ca²⁺) = 14 mol m^–3. Mg²⁺ was also very effectiveat less than 50 mol m^–3 but higher concentrations wereinhibitory. This inhibition cannot be accounted for solely byprecipitation of MgCO₃. Na⁺ and K⁺ were both capable of stimulatingHCO^–₃ use. Stimulation was in two distinct parts. Up to500 mol m^–3, both citrate and chloride salts gave similarresults (K_1/2(Na⁺) 81 mol m^–3, V_max(HCO^–₃) 0·26µmol O₂ mg^–1 chl min^–1), but use of citratesalts above 500 mol m^–2 caused a second stimulation ofHCO^–₃ use (K_1/2(Na⁺) 830 mol m^–3, V_max(HCO^–₃)0·68 µmol O₂ mg^–1 chl min^–1). V_max(HCO^–₃)for the second-phase Na⁺ or K⁺ stimulation was of the same orderas for Ca²⁺-stimulated HCO^–₃ use. To further characterizesalt-dependent HCO^–₃ use, the sensitivity of photosynthesisto Tris and TES buffers was investigated. The effects of Trisappear to be due to the action of Tris⁺ causing stimulationof HCO^–₃ -dependent photosynthesis in the absence of salt,but inhibition of HCO^–₃ use in saline media. TES has noeffect on photosynthesis. External carbonic anhydrase, althoughimplicated in salt-dependent HCO^–₃ use in Z. muelleri,could not be detected in whole leaves. Key words: Zostera muelleri, HCO^–₃ use, salinity     

Control of Wheat Root Elongation Growth: I. EFFECTS OF IONS ON GROWTH RATE, WALL RHEOLOGY AND CELL WATER RELATIONS

Pritchard, J., Tomos, A. D. and Wyn Jones, R. G. 1987. Controlof wheat root elongation growth. I. Effects of ions on growthrate, wall rheology and cell water relations.—J. exp.Bot. 38: 948–959. The nature of the ions in the bathing medium of hydroponicallygrown wheat seedlings strongly influenced root growth rate.In 0·5 mol m^–3 CaSO₄ the growth rate was 32 mm24 h^–1 (used as 100% control rate). K⁺ and SO^– ions(10 mol m^–3) each inhibited extension growth (to about40% and 70% of the control value respectively). In the absenceof K⁺, Cl^– greatly reduced the inhibition due to SO₄^2–.Measurement of tissue plasticity and elasticity in the expandingzone with an Instron-type tensiometer indicated that both werea function of growth rate although relationship of plasticityto growth rate was the steeper and the more pronounced. Turgor pressure at the proximal end of the expanding zone wasnot correlated to growth, being approximately 0·65 MPain all treatments. In mature tissue turgor pressure varied withtreatment, but was also not related to growth rate. Cell membranehydraulic conductivity (5 x 10^–7 ± 1·3 (10)m s^–1 MPa^–2) was not influenced by the presenceof K⁺. We propose that K⁺ and SO₄^{2 –} influence root growthrates by modulating the rheological properties of the wallsof the expanding cell. The physiological significance of these properties is discussed. Key words: Growth, wall extensibility, turgor pressure, wheat roots     

Na+, K+ and Cl- in Xylem Sap Flowing to Shoots of NaCl-Treated Barley

Munns, R. 1985. Na⁺, K⁺ and Cl^– in xylem sap flowing toshoots of NaCl-treated barley.—J. exp. Bot. 36: 1032–1042. Na⁺, Cl^– and K⁺ concentrations were measured in xylemsap obtained by applying pressure to the roots of decapitatedbarley plants grown at external [NaCl] of 0, 25, 50, 100, 150and 200 mol m^–3. For any given NaCl treatment, ion concentrationsin the xylem sap were hyperbolically related to the flux ofwater. Ion concentrations in sap collected at very low volumefluxes (without applied pressure) were 5–10 times higherthan in sap collected at moderate fluxes (under pressure). Fora given moderate volume flux, Na+ concentration in the xylemsap, [Na⁺]_x, was only 4.0 mol m^–3 at external [NaCl] of25–150 mol m^–3, and increased to 7.0 mol m^–3at 200 mol m^–3. [Cl-]x showed a similar pattern. Thisshows there would be little difference in the rate of uptaketo the shoot of plants at 25–150 mol m^–3 externalNaCl and indicates little change even at 200 mol m^-3 NaCl becausetranspiration rates would be much lower. Thus the reduced growthof the shoot of plants at high NaCl concentrations is not dueto higher uptake rates of Na⁺ or Cl^–. The fluxes of Na⁺, Cl^– and K^– increased non-linearlywith increasing volume flux indicating little movement of saltin the apoplast. The flux of K⁺ increased even when [K⁺]_x wasgreater than external [K⁺], indicating that membrane transportprocesses modify the K⁺ concentration in the transpiration streamas it flows through the root system. Key words: -Xylem sap, Na⁺, K⁺, Cl^– fluxes, salinity, barley     

Spatial and Temporal Aspects of Growth in the Primary Root of Cotton Seedlings: Effects of NaCl and CaCl2

Seedlings of cotton (Gossypium hirsutum L. cv. Acala SJ-2) weregrown in modified Hoagland nutrient solution with various combinationsof NaCl and CaCl₂. Marking experiments and numerical analysiswere conducted to characterize the spatial and temporal patternsof cotton root growth at varied Na/Ca ratios. At 1 mol m^–3Ca, 150 mol m^–3 NaCl reduced overall root elongation rateto 60% of the control, while increasing Ca to 10 mol m^–3at the same NaCl concentration restored the elongation rateto 80% of the control. Analysis of the spatial distributionof elongation revealed that the presence of 150 mol m^–3NaCl in the medium shortened the growth zone by about 2 mm fromthe approximate 10 mm in the control and also reduced the relativeelemental elongation rate (i.e. the longitudinal strain rate,defined as the derivatives of displacement velocity of a cellularparticle with respect to position on root axis). Supply of 10mol m^–3 Ca at the high salt condition restored partiallythe relative elemental elongation rate, but not the length ofthe growth zone. Compared to the control, the growth trajectoriesshowed that at 1 mol m^–3 CaCl2 it took more time for acellular particle to move through the growth zone at 150 molm^–3 NaCl, while at 10 mol m^–3 CaCl it took lesstime and there was no difference between the NaCl treatments Key words: Gossypium hirsutum, salinity stress, root growth kinematics