Effects of Temporary Droughts on Photosynthesis of Alfalfa Plants

The effect of temporary droughts on photosynthesis, total conductanceto water vapour, intercellular CO² concentration, CO² compensationpoint, light-response curves, photorespiration, dark respiration,chlorophyll content, and ribulose 1,5-bisphosphate (RuBP) carboxylase(EC 4.1.1.39 [EC] ) activity has been examined in nodulated alfalfaplants (Medicago saliva cv. Aragón). Plants were subjectedto moderate (S1/RS1) or severe (S2/RS2) cycles of drought (drought/recovery).Photosynthetic light-response curves showed decreased light-saturatedphotosynthetic capacity and decreased apparent quantum yield.Upon rewatering, photosynthesis did not recover whereas conductancedid in moderately stressed plants. Calculated electron transportrate also declined in drought-stressed plants, but upon rewatering,moderately stressed plants exhibited a total recovery. Comparison of photosynthetic intercellular CO² response curvesin well-watered and stressed leaves led to the assertion thateffects in chloroplast metabolism contribute significantly tophotosynthetic inhibition. Although the validity of this entireline of research has been questioned by some recent studiesbecause the occurrence of heterogeneous stomatal closure wouldaffect these curves, in our case, the effect of water stresswas investigated in experimental systems where stomata had beenremoved. Measurements of in vitro RuBP carboxylase activityand protein content showed a strong decline during drought treatmentsand upon rewatering no recovery was observed. Therefore, ourresults suggest the major implication of non-stomatal factorsin the decline of photosynthesis in alfalfa plants under cyclicdrought conditions. Key words: Alfalfa, water deficits, photosynthesis, ribulose, 1,5-bisphosphate carboxylase activity, stomatal limitation     

Stomatal Responses to Applied ABA and CO2 in Epidermis Detached from Well-Watered and Water-Stressed Plants of Commelina communis L.

Epidermal strips from either well-watered or water-stressedplants of Commelina communis L. were subjected to a range ofABA concentrations (10^–6–10^–3 mol m^–3)in the presence (330 parts 10^–6 in air) or virtual absence(3 parts 10–6 in air) of CO₂. The stomatal response toCO₂ was greater in epidermis from water-stressed plants, althoughthere was a distinct CO₂ response in epidermis from well-wateredplants. Additions of ABA via the incubation medium had littleeffect on the relative CO₂ response. Stomata responded to ABAboth in the presence and virtual absence of CO₂, but the relativeresponse to ABA was greatest in the high CO₂ treatment. Whenwell-watered plants were sprayed with a 10^–1 mol m^–3ABA solution 1 d prior to use, the stomatal response of detachedepidermis to both CO₂ and ABA was very similar to that of epidermisdetached from water-stressed leaves. It is hypothesized thata prolonged exposure to ABA is necessary before there is anymodification of the CO₂ response of stomata.     

The Response of the C3-CAM Tree, Clusia rosea, to Light and Water Stress: I. GAS EXCHANGE CHARACTERISTICS

Gas exchange measurements were undertaken on 2-year-old plantsof Clusia rosea. The plants were shown to have the ability toswitch from C₃-photosynthesis to CAM and vice versa regardlessof leaf age and, under some conditions, CO₂ was taken up continuously,throughout the day and night. The light response was saturatedby 120 µmol m^–2 s^–1 typical of a shade plant. Gas exchange patterns in response to light, water and VPD wereexamined. All combinations of daytime and night-time CO₂ uptakewere observed, with rates of CO₂ uptake ranging from 2 to 11µmol m^–2 s^–1 depending upon water status andlight. Categorization of this plant asC₃, CAM or an intermediateis impossible. Differing VPD affected the magnitude of changesfrom CAM to C₃-photosynthesis (0 to 0.5 and 0 to 6.0 µmolm^–2 s^–1 CO₂, respectively) when plants were watered.Under well-watered conditions, but not under water stress, highPPFD elicited changes from CAM to C₃ gas exchange. This is unusualnot only for a shade plant but also for a plant with CAM. Itis of ecological importance for C. rosea, which may spend theearly years of its life as an epiphyte or in the forest understorey,to be able to maximize photosynthesis with minimal water loss. Key words: Clusia rosea, CAM, C₃, stress     

Correlation of Activity and Amount of Ribulose 1,5-bisphosphate Carboxylase with Chloroplast Stroma Crystals in Water-Stressed Willow Leaves

Changes in the activity and amount of ribulose 1,5-bisphosphate(RuBP)carboxylase (E.C. 4.1.1.39 [EC] ) were studied in well-watered plantsof Salix ‘aquatica gigantea’ and in similar plantsduring three different water stress treatments and after rewatering.The chloroplast ultrastructure of these plants was examinedby electron microscopy. The amounts of crystallized proteinin the chloroplast stroma were assessed according to the areaof crystal structure seen in the thin sections. RuBP carboxylase activity decreased with decreasing leaf waterpotentials but recovered upon rewatering, except when leaveshad been exposed to severe water stress. The percentage of totalchloroplast area made up of crystal inclusions decreased withdecreasing leaf water potentials. After rewatering, the crystalseither disappeared or the amount decreased markedly. Both RuBPcarboxylase activity and the area of crystal inclusions increasedinitially with increased extractable RuBP carboxylase proteinbut decreased with further increases above 6700–7000 µgRuBP carboxylase protein mg^–1 chlorophyll. In well-wateredand water-stressed plants the activity of RuBP carboxylase,based on amount of chlorophyll, increased with an increasingamount of crystal inclusions in the chloroplast stroma. In rewateredplants no such correlation was observed, and the low percentageof crystal inclusions in the chloroplast area was independentof RuBP carboxylase activity. Key words: Chloroplast stroma crystals, ribulose 1,5-bisphosphate carboxylase, Salix, water stress     

Environmental Influences on Carbon Recycling in a Terrestrial CAM Bromeliad, Bromelia humilis Jacq.

The effects of night-time temperature, leaf-to-air vapour pressuredeficit (VPD) and water stress on CO₂ recycling in Bromeliahumilis Jacq. grown under two light and nitrogen regimes wereinvestigated. At night-time temperatures above 30°C, integratednet dark CO₂ uptake was severely reduced and CO₂ for malatesynthesis was mainly derived from dark respiration. At 35°C,up to 84% of the CO₂ liberated by dark respiration was refixedinto malic acid. Below 30 °C only nitrogen deficient plantsshowed significant recycling. No significant differences wereobserved between high and low light grown plants in CO₂ recycling.A doubling of leaf-to-air VPD from 7-46 Pa kPa^–1 to 15.49Pa kPa^–1 resulted in a 2- to 20-fold decrease in leafconductance and about 50 to 65% reduction in integrated darkCO₂ uptake. However, about twice as much CO₂ was recycled atthe higher VPD as in the lower. Ten days of water stress resultedin 80 to 100% recycling of respiratory CO₂. Under high VPD andwater stress treatments, the amount of water potentially savedthrough recycling of CO₂ reached 2- to 6-fold of the actualtranspiration. In general, nitrogen deficient plants had higherper cent recycling of respiratory CO₂ in response to high night-timetemperature, increased VPD or water stress. The results emphasizethe ecological relevance of carbon recycling in CAM plants. Key words: Bromelia humilis, CAM, PPFD, dark respiration, temperature, VPD, water stress     

The Influence of Nitrogen, Light and Water Stress on CO2 Exchange and Organic Acid Accumulation in the Tropical C3-CAM Tree, Clusia minor

The carbon balance and changes in leaf structure in Clusia minorL., were investigated in controlled conditions with regardto nitrogen supply and responses to low and high photosyntheticallyactive radiation (PAR). Nitrogen deficiency and high PAR ledto the production of smaller leaves with higher specific leafdry weight (SLDW) and higher leaf water content, but with lowerchlorophyll content. Nitrogen and PAR levels at growth alsoaffected CO₂ exchange and leaf area. In – N conditions,total daily net CO₂ uptake and leaf area accumulation were slightlyless for high-PAR-grown plants. In contrast, high-PAR-grownplants supplied with nitrogen showed about a 4-fold higher totaldaily CO₂ uptake and about twice the total leaf area of low-PAR-grownplants. Although total daily net CO₂ uptake of +N plants wasonly slightly higher than –N plants under the low PARlevel, –N plants produced almost three times more leafarea but with lower SLDW. Under well-watered conditions, low-PAR-grownplants showed only CO₂ evolution during the night and malicacid levels decreased. However, there was considerable night-timeaccumulation of titratable protons due to day/night changesin citric acid levels. High-PAR-grown plants showed net CO₂uptake, malate and citrate accumulation during the dark period.However, most of the CO₂ fixed at night probably came from respiratoryCO₂. Positive night-time CO₂ exchange was readily observed forlow-PAR-grown plants when they were transferred to high PARconditions or when they were submitted to water stress. In plantsgrown in high and low PAR, CAM leads to a substantial increasein daily water use efficiency for water-stressed plants, althoughtotal net CO₂ uptake decreased.     

Induction and Repression of CAM in Sedum telephium L. in Response to Photoperiod and Water Stress

Lee, H. S. J. and Griffiths, H. 1987. Induction and repressionof CAM in Sedurn relephluni L. in response to photopcnod andwater stress.—J. exp. Bot. 38: 834–841. The introduction and repression of CAM in Sedurn telephiunmL, a temperate succulent, was investigated in watered, progressivelydrouglited and rewatered plants in growth chambers. Measurementswere made of water vapour and CO₂ exchange, titratable acidity(TA) and xylem sap tension. Effects of photoperiod were alsostudied. CAM was induced by drought under long or short days,although when watered no CAM activity was expressed. C₃-CAM intermediate plants were used for the investigation ofwater supply. Those which had received water and those drought-stressedboth displayed a similar nocturnal increase in TA, with a day-nightmaximum (H+) of 69 µmol g^–1 fr. wt. The wateredplants took up CO₂ at a maximum rate of 2?2 µmol m^–2s^–1 only in the light period, while the droughted plantsshowed a maximum nocturnal CO₂ uptake rate of 0?69 µmolm^–2 s^–1. Subsequently, as CAM was repressed, thewatered S. telephiwn displayed little variation in TA, withconstant levels at 42 µmol g^–1 fr. wt. (day 10).After 10 d of drought stress, the CAM characteristics of S.telephiurn were aLso affected, with reduced net CO₂ uptake andH+. The transition between C₃ and CAM in S. telephium can be describedas a progression in terms of the proportion of respiratory CO₂which is recycled and refixed at night as malic acid, in comparisonwith net CO₂ uptake. Recycling increased from 20% (day 1) to44% (day 10) as a result of the drought stress and was highin both the CAM-C₃ stage (no net CO2 uptake at night) and alsoin the drought-stressed CAM stage (reduced net CO₂ uptake atnight). The complete C₃-CAM transition occurred in less than8 d, and the stages could be characterized by xylem sap tensionmeasurements: CAM = 0?50 MPa C₃-CAM = 0?36 MPa C₃ = 0?29 MPa. Key words: CAM, Sedum telephium L., recycling     

Properties of Phosphoenolpyruvate Carboxylase and Carbohydrate Accumulation in the C3-CAM Intermediate Sedum telephium L. Grown Under Different Light and Watering Regimes

A comparison of the activity and properties of the enzyme phosphoenolpyruvatecarboxylase (PEPC) was made for plants of Sedum telephium L.grown under low (70 µmol m^–2 s^–1) or high(500µmol m^–2 s^–1) PPFD and subjected to varyingdegrees of water stress. Under well-watered conditions onlyplants grown under high PPFD accumulated titratable acidityovernight and the extractable activity of PEPC was almost 2-foldhigher in these plants than in plants grown under low PPFD.Increasing drought stress resulted in a substantial increasein the activity of PEPC extracted both during the light anddark periods and a decrease in the sensitivity to inhibitionby malic acid. The magnitude of these changes was determinedby the severity and duration of drought and by light intensity.A comparison of the kinetic properties of PEPC from severelydroughted plants revealed that plants droughted under high PPFDhad a lower K_m for PEP than plants under low PPFD. Additionof 2·0 mol m^–3 malate resulted in an increase inthe K_m for PEP, with plants draughted under low PPFD havinga significantly higher K_m in the presence of malic acid comparedto those under high _PPFD. Response to the activator glc-6-P,which lowered the K_m for PEP, also varied between plants grownunder the two light regimes. Under well-watered conditions PEPCextracted from plants under high PPFD was more sensitive toactivation by glc-6-P than those under low PPFD. After the severedrought treatment, however, the K_m for PEP in the presence ofglc-6-P was similar for enzyme extracted from plants grown underboth light regimes. Soluble sugars and starch were depletedovernight and were both possible sources of substrate for PEPC.With increasing drought, however, the depletion of starch relativeto soluble sugars increased under both light regimes. The propertiesof PEPC and the characteristics of carbohydrate accumulation/depletionare discussed in relation to the regulation of CAM in S. telephiumgrown under different light and watering regimes. Key words: PEP carboxylase, CAM, carbohydrates, Sedum telephium     

Nocturnal Accumulation of Acid in Leaves of Wall Pennywort (Umbilicus rupestris) Following Exposure to Water Stress

Physiological responses to water stress (drought) have beeninvestigated in Umbilicus rupestris (wall pennywort) by comparingcontrol (well-watered) and draughted plants with respect to(i) diurnal fluctuations in the acid content of the leaves,(ii) CO₂ exchange patterns and (iii) stomatal conductance. Controlplants show no diurnal fluctuations in acid content, whereasafter 6 d of drought a clear CAM-type pattern (nocturnal acidificationfollowed by deacidification in the light) is observed. In controlplants, the CO₂ exchange pattern over a 24 h period is of atypical C-3 ‘square-wave’ type, with extensive CO₂uptake in the light and CO₂ output in the dark. In droughtedplants the day-time CO₂ uptake is confined to a morning ‘burst’,whilst night-time CO₂ output is markedly reduced. There is howeverno net noctural uptake of CO₂. In control plants, stomatal conductanceis high during the day (especially in the first half of theday) falling to a low level at the onset of darkness, and thenrising slowly through the remainder of the night. In droughtedplants, stomatal conductance is very low, except that thereis morning ‘burst’ of high conductance and a periodduring the night when conductance is higher than in controlplants. These results are discussed in relation to the response of U.rupestris to drought both in laboratory and in field conditions. Umbilicus rupestris, wall pennywort, CO₂ exchange, Crassulacean acid metabolism, drought, stomatal conductance, water stress     

The Influence of Prior Water Stress and Abscisic Acid Foliar Spraying on Stomatal Responses to CO2, IAA, ABA, and Calcium in Leaves of Solanum melongena

The influence of a water stress or foliar ABA spraying pretreatmenton stomatal responses to water loss, exogenous ABA, IAA, Ca²⁺,and CO₂ were studied using excised leaves of Solanum melongena.Both pretreatments increased stomatal sensitivity of water loss,in the presence and absence of CO₂, but decreased stomatal sensitivityto exogenous ABA. CO₂ greatly reduced the effect of exogenouslyapplied ABA. IAA decreased leaf diffusion resistance for controland ABA sprayed leaves, but did not influence the LDR of previouslywater-stressed leaves. CA²⁺ did not influence LDR of any leavesof any treatments. Key words: Water stress, stomatal response, pretreatments     

Reactions of birch leaves to changes in light during early ontogeny: comparison between in vivo and in vitro techniques to measure carbon uptake

Trends in several photosynthetic parameters and their responseto changed growth light were followed for 15 d in leaves ofyoung birch saplings using a rapid-response gas exchange measuringequipment. These in vivo measurements were compared to biochemicalassays that were made from the same leaves after the gas exchangestudies. The measurements were made on leaves that were selectedprior to the study and were at that time of similar age. Forthe first 7 d the photosynthetic parameters were followed fromthe growth conditions of moderate light (200 µmol m^–2s^–1; referred to as controls later in the text). On day7 some of the saplings were transferred to grow either underhigh (450 µmol m^–2 s^–1; referred to as highlight plants) or low (75 µmol m^–2 s^–1; referredto as low light plants) light and the capability of the preselectedleaves for acclimation was followed for 6 d. For comparison,at the end of the experiment the measurements were made on bothcontrols and on young leaves that had developed under high andlow light. Generally the in vivo measured rate of CO₂ uptake (gross photosynthesis)both at 310 ppm CO₂ and 2000 ppm CO₂ corresponded very wellto the biochemically determined CO₂ fixation capacity in vitroafter rapid extraction (measured as the initial and total activityof Rubisco, respectively). However, if the flux of CO₂ intothe chloroplasts was limited by the closure of the stomata,as was the case of the high light plants, then the in vitromeasured Rubisco activity was greater than the in vivo measuredCO₂ uptake. V_max, calculated from the mesophyll conductanceat 1% O₂, exceeded the initial activity of Rubisco (assayedat saturating RuBP and CO₂) constantly by 60%. The catalyticactivity of Rubisco in birch leaves was overall very low, evenwhen calculated from the total activity of Rubisco (K_cat 0.63–1.18 s^–1), when compared to herbaceous C₃ species. Signs of light acclimation were not observed in most of thephotosynthetic parameters and in chloroplast structure whenmature birch leaves were subjected to changes in growth lightfor 6 d. However, the change of the growth light either to highor low light caused day-to-day fluctuations in most of the measuredphotosynthetic parameters and in the case of the high lightplants signs of photoinhibition and photodestruction were alsoobserved (decrease in the amount of chlorophyll and increasein chlorophyll a/b ratio). As a result of these fluctuationsthese plants achieved a new and lower steady-state conditionbetween the light and dark reactions, as judged from the molarratio of RuBP to Rubisco binding site. Key words: Acclimation, photosynthesis, light, Rubisco, birch     

Simultaneous CO2- and 16O2/18O2-gas exchange and fluorescence measurements indicate differences in light energy dissipation between the wild type and the phytochrome-deficient aurea mutant of tomato during water stress

The CO₂-, H₂O- and ¹⁶O₂/¹⁸O₂ isotopic-gas exchange and the fluorescencequenching by attached leaves of the wild-type and of the phytochrome-deficienttomato aurea mutant was compared in relation to water stressand the photon fluence rate. The chlorophyll content of aurealeaves was reduced and the ultra-structure of the chloroplastswas altered. Nevertheless, the maximum rate of net CO₂ uptakein air by the yellow-green leaves of the aurea mutant was similarto that by the dark-green wild-type leaves. However, less O₂was produced by the leaves of the aurea mutant than by leavesof the wild-type. This result indicates a reduced rate of photosyntheticelectron flux in aurea mutant leaves. No difference in bothphotochemical and non-photochemical fluorescence quenching wasfound between wild-type and aurea mutant leaves. Water stresswas correlated with a reversible decrease in the rates of bothnet CO₂ uptake and transpiration by wild-type and aurea mutantleaves. The rate of gross ¹⁶O₂ evolution by both wild-type andaurea mutant leaves was fairly unaffected by water stress. Thisresult shows that in both wild-type and aurea leaves, the photochemicalprocesses are highly resistant to water stress. The rate ofgross ¹⁸O₂ uptake by wild-type leaves increased during waterstress when the photon fluence rate was high. Under the sameconditions, the rate of gross ¹⁸O₂ uptake by ^aurea mutant leavesremained unchanged. The physiological significane of this differencewith respect to the (presumed) importance of oxygen reductionin photoprotection is discussed. Key words: Water stress, gas exchange, fluorescence quenching, Lycopersicon esculentum, mutant (tomato, aurea), energy dissipation     

The Response of the C3-CAM Tree, Clusia rosea, to Light and Water Stress: II. INTERNAL CO2 CONCENTRATION AND WATER USE EFFICIENCY

Gas exchange in Clusia rosea has been measured under variousconditions of water status, light and leaf-air vapour pressuredeficit (_w, mbar bar^–1) which produce daytime (C₃), night-time(CAM) or 24 h uptake of CO₂. At high light levels, at a _w of6.6, well-watered plants utilized C₃ photosynthesis while CAMand 24 h uptake occurred under lower light levels and with lowto normal water availability and differing _w (13.5 and 3.4,respectively). CO₂ uptake was highest, stomatal conductanceto water vapour (gH₂o) lowest, and water use efficiency (WUE)highest in plants using C₃ photosynthesis. This latter factis contrary to the accepted view that CAM is most water useefficient, i.e. it optimizes CO₂ uptake with minimal water loss.It is suggested that the low CO₂ uptake in CAM photosynthesismay be related not only to the higher _w but also to the factthat Clusia species accumulate citrate which may originate fromß-carboxylation of fatty acids (i.e. an internal sourceof CO₂) and does not contribute to night-time external CO₂ assimilation.Curves of assimilation (A) versus internal partial pressureof CO₂ (A/c₁) for the three photosynthetic types, under atmosphericconditions, did not produce a single trend. The trends whichwere produced represent the supply function for the interaction,under differing modes of photosynthesis, of the two major enzymesystems involved in CAM. Key words: Clusia rosea, Crassulacean acid metabolism, C₃ photosynthesis, internal CO₂ concentration, 24 h carbon dioxide uptake, water use efficiency.     

Water Deficit Enhanced Cotton Resistance to Spider Mite Herbivory

We investigated the responses of cotton (Gossypium hirsutumL.)to the combined effects of soil water deficit and two-spottedspider mite (Tetranychus urticaeKoch) infestation. Two mitetreatments (-M: uninfested, +M: artificially infested 83 d aftersowing), and two water regimes (+W: well watered, -W: waterstressed) were combined factorially in four treatments. Mitecolonies developed at similar rates in well-watered and water-stressedcrops. Despite the similar intensity of infestation, visualsymptoms of mite injury were more marked in well-watered hostplants (+M+W) than in their water-stressed counterparts (+M-W).Lint yield of unstressed controls (-M+W) was 175 g m^-2. In uninfestedcrops, water deficit reduced yield by 30%, mites reduced theyield of well-watered crops by 92%, and the combination of miteinfestation and water deficit reduced yield by 72% (water effect:P<0.01;mite and interaction effect:P<0.0001). Differences in yieldresponses to mites between well-watered and water-stressed cropswere mostly related to differences in reproductive partitioning.The interaction between mites and water deficit was also significantfor other crop variables including canopy temperature, leafwater potential, concentration of nitrogen in reproductive structuresand seed oil concentration. The magnitude and consistency ofthe interaction between both stresses indicates that, underour experimental conditions, mechanisms of adjustment to waterdeficit may have enhanced cotton resistance to mites. This isfurther supported by (a) an increase in specific leaf weightand a parallel increase in leaf penetration resistance due towater deficit; (b) a negative association between macroscopicsymptoms of mite injury and leaf penetration resistance; and(c) a choice test showing that adult female mites preferredto feed and oviposit on leaves from well-watered plants.Copyright1998 Annals of Botany Company Gossypium hirsutumL.;Tetranychus urticaeKoch; leaf water potential; leaf penetration resistance; canopy temperature; multiple stresses; specific leaf weight; radiation use efficiency; nitrogen concentration; reproductive allocation.     

Responses of CO2 assimilation to changes in irradiance: laboratory and field data and a model for beans (Phaseolus vulgaris L.)

The responses of net CO₂ assimilation to sudden changes in irradiancewere studied in Phaseolus vulgaris L. in the laboratory andthe field. For irradiance changes between 50 µmol m^–2s^–1 to 350 µmol m^–2 s^–1 in the laboratory,assimilation rate increased with half-times of 2.7 and 4.1 minin well-watered and water-stressed plants, respectively. Ina field experiment with a change in irradiance from 400 to 1200µmol m^–2 s^–1 the response was faster (half-time=c.1.2 min). In all cases when irradiance was returned to a lowvalue, assimilation declined rapidly with a half-time of approximately1 min, which approached the time resolution of the gas-exchangesystem. The corresponding changes in stomatal conductance in responseto both increasing and decreasing irradiance were much slowerthan the assimilation responses, indicating that biochemicalprocesses, rather than CO₂ supply, primarily determined theactual rate of assimilation in these experiments. The conceptof stomatal limitation to photosynthesis is discussed in relationto these results. A simple model for assimilation in a fluctuating light environmentis proposed that depends on a steadystate light response curve,an ‘induction lag’ on increasing irradiance, andan induction-state memory. The likely importance of taking accountof such induction lags in natural canopy microclimates is considered. Key words: Models, Phaseolus vulgaris, photosynthetic induction, CO₂ assimilation, stomatal limitation, sunflecks, water stress     

Stomatal Functioning of In Vitro and Greenhouse Apple Leaves in Darkness, Mannitol, ABA, and CO2

Leaves from in vitro and greenhouse cultured plants of Malusdomestica (Borkh.) cv. Mark were subjected to 4 h of darkness;4 h of 1 M mannitol induced water stress; 1 h of 10^–4M to 10^–7 M cis-trans abscisic acid (ABA) treatment; 1h of 0.12% atmospheric CO₂. Stomatal closure was determinedby microscopic examination of leaf imprints. In all treatments,less than 5% of the stomata from leaves of in vitro culturedplants were closed. The diameter of open stomata on leaves fromin vitro culture remained at 8 µm. In contrast, an averageof 96% of the stomata on leaves of greenhouse grown plants wereclosed after 4 h in darkness; 56% after 4 h of mannitol inducedwater stress; 90% after 1 h of 10^–4 M ABA treatment; 61%after 1 h in an atmosphere of 0.12% CO₂. Stomata of in vitroapple leaves did not seem to have a closure mechanism, but acquiredone during acclimatization to the greenhouse environment. Thelack of stomatal closure in in vitro plants was the main causeof rapid water loss during transfer to low relative humidity.     

Responses of nitrate assimilation and N translocation in tomato (Lycopersicon esculentum Mill) to reduced ambient air humidity

The impact of low humidity in ambient air on water relations,nitrate uptake, and translocation of recently absorbed nitrogen,was investigated in 5-week-old tomato (Lycopersicon esculentumMill cv. Ailsa Craig) plants grown hydroponically in a completenutrient solution. Plants were subjected to dry air (relativehumidity 2–4% for 6 h. The transpiration rate increasedseveral-fold and the shoot water content decreased by almost20%, whereas root water content was unaffected. No effect onin vitro nitrate reductase (NR) activity was detected when usingan EDTA-contraining assay buffer. Replacement of EDTA with Mg²⁺revealed a significant decline in shoot NR activity, which suggestsphosphorylation of the enzyme during the stress treatment. Plantswere grown in a split-root system, in which one root half wasfed ¹⁵N-nitrate during the treatment, in order to determinenitrate uptake and translocation of recently absorbed nitrogenin the plants. Uptake of nitrate was substantially inhibited,but the proportion of absorbed ¹⁵N that was translocated tothe shoots was only slightly affected. In untreated plants,71% of the ¹⁵N recovered in roots had been retranslocated fromthe shoots, whereas in plants subjected to stress the deliveryof ¹⁵N from shoots to roots appeared to be completely inhibited.The data show that lowered humidity in air has significant effectson both uptake of nitrate as well as translocation of nitrogenwithin the plants. Some of these effects appear to be commonwith those observed in plants subjected to reduced water potentialsin the root environment and point to the possibility of theshoot water relations being highly influential on nitrogen uptakeand translocation. Key words: Air humidity, nitrate assimilation, nitrate reductase activity, nitrogen translocation, tomato, water stress     

Differing substomatal and chloroplastic CO2 concentrations in water-stressed wheat

Gas exchanges of wheat (Triticum aestivum L. cv. Courtot) shoots were measured before and during a water stress. While photosynthesis, transpiration and dark respiration decreased because of the stress, photorespiration increased initially, up to a maximum of 50% above its initial value. The CO₂ concentration in the intercellular space was calculated from gas-diffusion resistances, and remained approximately constant before and during the stress. On the other hand, the CO₂ concentration in the chloroplast, in the vicinity of Ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco), was evaluated from the ratio of CO₂ to O₂ uptake, using the known kinetic constants of the oxygenation and carboxylation reactions which compete for Rubisco. In the well-watered plants, the calculated chloroplastic concentration was slightly smaller than the substomatal concentration. During water stress, this concentration decreased while the substomatal CO₂ concentration remained constant. Hypotheses to explain this difference between substomatal and chloroplastic CO₂ concentrations are discussed.     

Utilization of Sediment CO2 by Selected North American Isoetids

The photosynthetic uptake of root-zone CO₂ was determined forEriocaulon septangulare, Gratiola aurea, Isoetes macrospora,Littorella uniflora var. americana and Lobelia dortmanna aspart of a study of the photosynthetic carbon economy of submergedaquatic isoetids. The pH and dissolved inorganic carbon (DIC)of the sediment interstitial water in four Wisconsin lakes reflectedthe water column character, where the DIC increased with depthin the sediment to concentrations five to ten times those ofthe water column. Sediment free CO₂ concentrations were 5–50times those in the water column and were similar at all sites(about 05–1.0mM CO₂ in the root-zone). In ‘pH-drift’studies these plants were unable to take up HCO₂^–. Laboratory determinations of the carbon uptake from the rootand shoot-zones were made for all five species. These experimentsshowed that CO₂ in the root-zone accounted for 65–95 percent of external carbon uptake for the five species. For G.aurea and E. septangulare, root-zone CO₂ was > 85 per centof carbon uptake. Carbon, CO₂, photosynthesis, sediment, isoetid, Eriocaulon septangulare, Gratiola aurea, Isoetes macrospora, Littorella uniflora, Lobelia dortmanna     

Crassulacean acid metabolism,CO2-recycling,and tissue desiccation in the Mexican epiphyte Tillandsia schiedeana Steud (Bromeliaceae)

After 23 days without water in a greenhouse, rates of nocturnal CO₂ uptake in Tillandsia schiedeana decreased substantially and maximum rates occurred later in the dark period eventually coinciding with the onset of illumination. Nocturnal CO₂ uptake accounted for less than half the total nighttime increase in acidity measured in well-watered plants. With increased tissue desiccation, only 11–12% of measured acid accumulation was attributable to atmospheric CO₂ uptake. Plants desiccated for 30 days regained initial levels of nocturnal acid accumulation and CO₂ uptake after rehydration for 10h. These results stress the importance of CO₂ recycling via CAM in this epiphytic bromeliad, especially during droughts.Partially supported by Biomedical Sciences Support Grant RR07037.