Pith Autolysis in Herbaceous Dicotyledonous Plants. A Physiological Ecological Study of Pith Autolysis under Native Conditions with Special Attention to the Wild Plant Impatiens capensis Meerb.

Pith autolysis, in the past studied only in herbaceous cultivateddicotyledonous plants, commonly occurs in wild plants. A surveyof pith autolysis in wild plants was conducted in several differentbiomes in both North Carolina, USA and in Belgium. An objectivemethod and a subjective method were developed and used to estimatethe amount of pith autolysis in the internodes of bean (Phaseolusvulgaris L.) and the wild plant jewelweed (Impatiens capensisMeerb.) under experimental conditions. A ranking system alsowas used to quantify pith autolysis in other wild plants. Inthe field, a positive correlation was found between the rateof internode elongation and pith autolysis. Injections of sucrose solution were found to reduce the amountof pith autolysis developed in the upper internodes of jewelweedwhen compared to injections of control solution. Extracts fromthe nodal tissues of jewelweed increased the amount of pithautolysis developed by both jewelweed in the field and beanin the greenhouse. This increase in pith autolysis may be dueto a principle present in the jewelweed nodes that signals orcauses the supertending internodes to become hollow.Copyright1995, 1999 Academic Press Pith, autolysis, Phaseolus vulgaris L., Impatiens capensis Meerb., sucrose, internode, hollow stem     

Pith Autolysis in Plants: IV. The Activity of Polygalacturonase and Cellulase during Drought Stress Induced Pith Autolysis

The water potential, amount of pith autolysis and activitiesof apoplastic cellulase and polygalacturonase of tomato stemswere measured during 24 h of drought stress (DS) and for 24h following reirrigation. During DS the water potential droppedfrom —5.5 to —10.4 bars and rose to —8.3 barssoon after reirrigation. Drought stress induced considerablepith autolysis, more of which occurred after reirrigation. Pretreatmentwith mechanical perturbation (MP) of the stems or applicationof exogenous ethephon on the buds hardened the tomato plantsagainst DS-induced pith autolysis. Drought stress caused anincrease in apoplastic polygalacturonase and an even greaterincrease in apoplastic cellulase. Reirrigation caused a largetransient increase in the former and a decrease in the latter.The apoplastic reducing sugar content (as galacturonic acid)of the stem rose in parallel with the activity of the enzymes.Both DS and MP caused an increase in ethylene evolution, althoughthe former was significantly greater than the latter. However,when MP preceded DS, the amount of ethylene produced was significantlyless than DS alone induced. Pretreatment with either MP or exogenousethephon inhibited the increase in apoplastic cellulytic enzymes. It is concluded that DS induces ethylene evolution from thetomato stem, causing an increase in the stem apoplastic cellulyticenzymes, which in turn start the autolysis of the pith cellwalls. Pretreatment with MP or ethephon, each of which inducesethylene evolution, hardens the stem so that it does not producemore ethylene during DS, and thus becomes resistant to DS-inducedpith autolysis. ¹Supported by Bi-national Agricultural Research and Developmentgrant I-127, NASA grant NAGW 96 and NSF grant 8003689to MJJ. ²Permanent address: Horticulture Department, Faculty of Agriculture,The Hebrew University Rehovot, Israel ³Permanent address: Vegetable Crops Department, AgriculturalResearch Organization, the Volcani Center Bet Dagan, Israel     

Growth at elevated CO2 leads to down-regulation of photosynthesis and altered response to high temperature in Quercus suber L. seedlings

The effects of growth at elevated CO₂ on the response to hightemperatures in terms of carbon assimilation (net photosynthesis,stomatal conductance, amount and activity of Rubisco, and concentrationsof total soluble sugars and starch) and of photochemistry (forexample, the efficiency of excitation energy captured by openphotosystem II reaction centres) were studied in cork oak (Quercussuber L.). Plants grown in elevated CO₂ (700 ppm) showed a down-regulationof photosynthesis and had lower amounts and activity of Rubiscothan plants grown at ambient CO₂ (350 ppm), after 14 monthsin the greenhouse. At that time plants were subjected to a heat-shocktreatment (4 h at 45C in a chamber with 80% relative humidityand 800–1000 mol m^–2 s^–1 photon flux density).Growth in a CO₂-enriched atmosphere seems to protect cork oakleaves from the short-term effects of high temperature. ElevatedCO₂ plants had positive net carbon uptake rates during the heatshock treatment whereas plants grown at ambient CO₂ showed negativerates. Moreover, recovery was faster in high CO₂-grown plantswhich, after 30 min at 25C, exhibited higher net carbon uptakerates and lower decreases in photosynthetic capacity (A_max aswell as in the efficiency of excitation energy captured by openphotosystem II reaction centres (F_vJF_m than plants grown atambient CO₂. The stomata of elevated CO₂ plants were also lessresponsive when exposed to high temperature. Key words: Elevated CO₂, temperature, acclimation, photosynthesis, Quercus suber L.     

Elevated carbon dioxide affects the patterning of subsidiary cells in Tradescantia stomatal complexes

The influence of elevated CO₂ concentration (670 ppm) on thestructure, distribution, and patterning of stomata in Tradescantialeaves was studied by making comparisons with plants grown atambient CO₂. Extra subsidiary cells, beyond the normal complementof four per stoma, were associated with nearly half the stomatalcomplexes on leaves grown in elevated CO₂. The extra cells sharedcharacteristics, such as pigmentation and expansion, with thetypical subsidiary cells. The position and shape of the extrasubsidiary cells in face view differed in the green and purplevarieties of Tradescantia. Substomatal cavities of complexeswith extra subsidiary cells appeared larger than those foundin control leaves. Stomatal frequency expressed on the basisof leaf area did not differ from the control. Stomatal frequencybased on cell counts (stomatal index) was greater in leavesgrown in CO₂-enriched air when all subsidiary cells were countedas part of the stomatal complex. This difference was eliminatedwhen subsidiary cells were included in the count of epidermalcells, thereby evaluating the frequency of guard cell pairs.The extra subsidiary cells were, therefore, recruited from theepidermal cell population during development. Stomatal frequencyin plants grown at elevated temperature (29 C) was not significantlydifferent from that of the control (24 C). The linear aggregationsof stomata were similar in plants grown in ambient and elevatedCO₂. Since enriched CO₂ had no effect on the structure or patterningof guard cells, but resulted in the formation of additionalsubsidiary cells, it is likely that separate and independentevents pattern the two cell types. Plants grown at enrichedCO₂ levels had significantly greater internode lengths, butleaf area and the time interval between the appearance of successiveleaves were similar to that of control plants. Porometric measurementsrevealed that stomatal conductance of plants grown under elevatedCO₂ was lower than that of control leaves and those grown atelevated temperature. Tradescantia was capable of regulatingstomatal conductance in response to elevated CO₂ without changingthe relative number of stomata present on the leaf. Key words: Elevated CO₂, stomata, subsidiary cells, patterning     

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.     

Photosynthetic Responses of Different Varieties of Wheat to High Temperature: I. EFFECT OF GROWTH TEMPERATURE ON DEVELOPMENT AND PHOTOSYNTHETIC PERFORMANCE

Both fast and slow chlorophyll fluorescence kinetics were usedto assess the differential heat sensitivity of ten wheat (Triticumaestivum L.) varieties commonly grown in northern, temperateor (sub-) tropical climate zones. No consistent differencesbetween the groups were found. Studies on the role of growthtemperature in modulating the response of photosynthesis toheat stress were carried out on two of the varieties, APU (Finnish)and K65 (Indian), the former being more sensitive to heat stress.Growth and development of these varieties were similar underboth cool (13 °C day, 10 °C night) and warm (30 °Cday, 25 °C night), regimes. The cool-grown plants exhibitedhigher rates of net CO₂ exchange than the warm-grown plantswhen expressed on a chlorophyll basis and, in both regimes,photosynthesis declined with age prior to reduction in chlorophyllcontent. Net CO₂ exchange in leaves of K65 showed greater sensitivityto short-term heat stress than APU when plants were grown underthe cool regime. Plants grown under the warm regime exhibitedan upward shift in the optimum temperature for net CO₂ exchangein both varieties, with K65 showing somewhat superior performanceat high temperature compared with APU. Stomatal aperture wasessentially unaffected by assay temperature during these measurements. Key words: CO₂ exchange, growth temperature, Triticum aestivum     

Physiology and Growth of Wheat Across a Subambient Carbon Dioxide Gradient

Two cultivars of wheat (Triticum aestivum L.), 'Yaqui 54' and'Seri M82', were grown along a gradient of daytime carbon dioxideconcentrations ([CO₂]) from near 350-200 µmol CO₂ mol^-1air in a 38 m long controlled environment chamber. Carbon dioxidefluxes and evapotranspiration were measured for stands (plantsand soil) in five consecutive 7·6-m lengths of the chamberto determined potential effects of the glacial/interglacialincrease in atmospheric [CO₂] on C₃ plants. Growth rates andleaf areas of individual plants and net assimilation per unitleaf area and daily (24-h) net CO₂ accumulation of wheat standsrose with increasing [CO₂]. Daytime net assimilation (P_D, mmolCO₂ m^-2 soil surface area) and water use efficiency of wheatstands increased and the daily total of photosynthetic photonflux density required by stands for positive CO₂ accumulation(light compensation point) declined at higher [CO₂]. Nighttimerespiration (R_N, mmol CO₂ m^-2 soil surface) of wheat, measuredat 369-397 µmol mol^-1 CO₂, apparently was not alteredby growth at different daytime [CO₂], but R_N /P_D of stands declinedlinearly as daytime [CO₂] and P_D increased. The responses ofwheat to [CO₂], if representative of other C₃ species, suggestthat the 75-100% increase in [CO₂] since glaciation and the30% increase since 1800 reduced the minimum light and waterrequirements for growth and increased the productivity of C₃plants.Copyright 1993, 1999 Academic Press Atmospheric carbon dioxide, carbon accumulation, evapotranspiration, light compensation point, net assimilation, respiration, Triticum aestivum, water use efficiency, wheat     

The Influence of Temperature on Photosynthesis and Transpiration in ten Temperate Grass Varieties Grown in four Different Environments

The influence of temperature on photosynthesis and transpirationwas studied in ten varieties of Lolium perenne, L. multiflorum,Dactylis glomerata, and Festuca arundinacea from three climaticorigins grown in three different controlled environments (15?C, 72 W m^-2 visible irradiation, 16-h photoperiod; 25 ?C, 72W m^-2 visible irradiation, 16-h photoperiod; and 25 ?C, 180W m^-2 visible irradiation, 16-h photoperiod) and in the glasshousein July/August. The optimum temperature for photosynthesis was influenced primarilyby growth environment; growth at low temperature (15 ?C) resultedin a low optimum temperature, which differed little from varietyto variety. The maximum CO₂-exchange rate was influenced bygrowth environment and by variety. Within a variety, plantsgrown at higher light intensity or lower temperature had a greaterCO₂-exchange rate. Seven varieties showed a negative correlationbetween the optimum leaf temperature and the maximum CO₂-exchangerate. Activation energies for photosynthesis were influencedby growth environment only. There were marked varietal differences in the values of leafresistances (r_a + r_t) obtained from transpiration data at theoptimum leaf temperature for CO₂ exchange. In Lolium, and Dactylisthe Mediterranean varieties had higher leaf resistances thanthe Northern varieties with the maritime varieties intermediate.In general the Dactylis varieties had higher resistances thanthe corresponding Lolium and Festuca varieties. Only at highgrowth temperatures was (r_a+r_l) insensitive to temperature;otherwise an activation energy of about 10 kcal/mole was observed.A negative correlation was found between mean varietal diffusionresistances (r_a+r_l), and corresponding maximum CO₂-exchangerates.     

Acclimation of Lolium temulentum to enhanced carbon dioxide concentration

Acclimation of single plants of Lolium temulentum to changing[CO₂] was studied on plants grown in controlled environmentsat 20°C with an 8 h photoperiod. In the first experimentplants were grown at 135 µ;mol m^–2 s^–1 photosyntheticphoton flux density (PPFD) at 415µl l^–1 or 550µll^–1 [CO₂] with some plants transferred from the lowerto the higher [CO₂] at emergence of leaf 4. In the second experimentplants were grown at 135 and 500 µmol m^–2 s^–1PPFD at 345 and 575 µl l^–1 [CO₂]. High [CO₂] during growth had little effect on stomatal density,total soluble proteins, chlorophyll a content, amount of Rubiscoor cytochrome f. However, increasing [CO₂] during measurementincreased photosynthetic rates, particularly in high light.Plants grown in the higher [CO₂] had greater leaf extension,leaf and plant growth rates in low but not in high light. Theresults are discussed in relation to the limitation of growthby sink capacity and the modifications in the plant which allowthe storage of extra assimilates at high [CO₂]. Key words: Lolium, carbon dioxide, photosynthesis, growth, stomatal density     

Responses of shoot and root gas exchange, leaf blade expansion and biomass production to pulses of elevated C02 in hydroponic wheat

Short-term effects of elevated CO₂ during the early life phaseof plants may have long lasting consequences for growth andbiomass in later periods. We exposed hydroponically grown wheatseedlings to 5 d pulses of elevated CO₂ while leaf expansiongrowth as well as shoot and root gas exchange were measuredsimultaneously and continuously. Shoot photosynthesis, night-timeshoot respiration and below-ground respiration (largely by roots)roughly doubled when atmospheric CO₂ concentration was doubled.An interruption of CO₂ enrichment caused CO₂ assimilation andrespiration to return to control levels. However, while theresponse of photosynthesis was immediate, that of respirationshowed a hysteresis of about 3 d. Since shoot biomass increasedat elevated CO₂ (with no change in allocation pattern) equalfluxes per shoot or root system after a return to control CO₂concentrations indicate substantial downward adjustment of thecapacity for CO₂ fixation and release in high-CO₂ grown plants.Leaf expansion growth was completely unaffected by CO₂ enrichment,whereas tiller initiation was significantly increased (doubledin 18 d). We conclude that leaf growth in these wheat plantswas already carbon-saturated at ambient CO₂ concentration atoptimum mineral nutrient supply. The stimulation of growth ofwhole plants was exclusively due to enhanced tillering duringthis very early part of the life of these wheat plants. Key words: Allocation, atmospheric carbon dioxide enrichment, growth, photosynthesis, respiration, tillering, Triticum aestivum     

Changes in the Morphology of Roots and Leaves of Carob Seedlings Induced by Nitrogen Source and Atmospheric Carbon Dioxide

Carob seedlings were grown hydroponically for 9 weeks under360 and 800 µl l^-1CO₂. One of two nitrogen sources, nitrateor ammonium, was added to the nutrient medium at concentrationsof 3 mol m^-3. Root systems of the developing plants suppliedwith nitrate compared to those supplied with ammonium were characterizedby:(a)more biomass on the lower part of the root;(b)fewer lateralroots of first and second order;(c)longer roots;(d)higher specificroot length;(e)a smaller root diameter. The morphology of theroot systems of nitrate-fed plants changed in the presence ofelevated carbon dioxide concentrations, resembling, more closely,that of ammonium-fed plants. Total leaf area was higher in ammonium-than in nitrate-fed plants. Nitrate-fed plants had greater totalleaf area in the presence of high carbon dioxide than in normalCO₂, due to an increase in epidermal cell size that led to developmentof larger leaflets with lower stomatal frequency. The observedchanges in the morphology of roots and shoots agreed with theresults observed for total biomass production. Nitrate-fed plantsincreased their biomass production by 100% in the presence ofelevated CO₂compared to 15% in ammonium-fed plants, indicatingthat the response of carob to high CO₂concentrations is verydependent on the nitrogen source. Under elevated CO₂, nitrate-grownplants had a larger content of sucrose in both roots and shoots,while no significant difference was observed in the contentof sucrose in ammonium-grown plants, whether in ambient or enrichedcarbon dioxide. Hence, the differences in soluble carbohydratecontents can, at least partly, account for differences in rootand shoot morphology.Copyright 1997 Annals of Botany Company Ceratonia siliquaL.; carob; ammonium; carbohydrate; carbon dioxide; nitrate; morphology; sucrose     

Effect of CO2 Concentration on Growth, Carbon Distribution and Loss of Carbon from the Roots of Maize

The effects of three ranges of CO₂ concentration on growth,carbon distribution and loss of carbon from the roots of maizegrown for 14 d and 28 d with shoots in constant specific activity¹⁴CO₂ are described. Increasing concentrations of CO₂ led toenhancement of plant growth with the relative growth rate (RGR)of the roots affected more than the RGR of the shoots. Between16% and 21% of total net fixed carbon (defined as ¹⁴C retainedin the plant plus ¹⁴C lost from the root) was lost from theroots at all CO₂ concentrations at all times but the amountsof carbon lost per unit weight of plant decreased with time.Possible mechanisms to account for these observations are discussed. Key words: Growth, Roots, Carbon loss, [CO₂]     

The Effect of Salinity on Light and Dark CO2-Fixation of Salt-Adapted and Unadapted Cell Cultures of Atriplex and Tomato

The effect of salinity on light and dark CO₂,-fixation was determinedin cells of A triplex portulacoides and tomato (Lycopersiconesculenturn Mill.) grown in culture. CO₂,-fixation of tomatocells was also determined in cultures adapted to mannitol andpolyethylene glycol (PEG). Salinity up to 400 mM NaCI in thecase of A triplex and up to 50 mM in the case of tomato enhancedthe rate of light-induced CO₂,-flxation in unadapted cells.Higher salt concentrations led to a marked decline in CO₂-flxationin both species. In salt-adapted A triplex cells no declinein the rate of light CO²,-flxation was seen even at 500 mM NaCl.Dark CO₂,-fixation was approximately 40% and 80% of the lightfixation in control cell cultures of A triplex and tomato, respectively.No enhancement in dark CO₂,-flxation was seen as salinity wasincreased, but a decline was found at similar salt concentrationsthat decreased fixation in the light. Mannitol-and PEG-adaptedtomato cells fixed CO₂, at somewhat lower rates than the controlcells in the light but not in the dark. Key words: Salinity, CO₂-fixation, cell cultures, Atriplex, tomato     

Effect of CO2 Concentration During Growth on a CO2 Concentrating Mechanism in White Clover as Predicted from Differential 14CO2/12CO2Uptake

Lehnherr, B. M?chler, F. and N?sberger, J. 1985. Effect of CO₂concentration during growth on a CO₂ concentrating mechanismin white clover as predicted from differential ¹⁴CO₂/¹²CO² uptake.-J. exp. Bot. 36: 1835-1841. White clover was grown at 20 and100 Pa p(CO₂). The CO₂ response of net photosynthesis and differentialuptake of ¹⁴CO₂ and ¹²CO₂ by leaves were measured at varioustemperatures and at various O₂ and CO₂ partial pressures andcompared with predictions from ribulose bisphosphate carboxylase/oxygenasekinetics. Discrepancies between the observed gas exchange characteristicsfor the leaves and those predicted from the enzyme kineticswere interpreted as being due to a CO₂ concentrating mechanism.Plants grown at 20 Pa p(CO₂) showed a higher affinity for CO₂than plants grown at 100 Pa p(CO₂) when measured at 10 ?C. Nodifference in affinity was found at 30 ?C. The postulated CO₂concentrating effect was greater in plants grown at low CO₂than in plants grown at high CO₂ concentration and occurredonly at low temperature and low CO₂ partial pressure. It issuggested that plants grown at the lower CO₂ partial pressurehave a higher affinity for CO₂ due to a more efficient CO₂ concentratingsystem than plants grown at the higher CO₂ partial pressure. Key words: Photosynthesis, CO₂, concentration, RuBP carboxylase/oxygenase     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 1. Development and Application of 13CO2 Assimilation System at a Constant 13C Abundance

A long-term, steady-state ¹³CO₂ assimilation system at a constantCO₂ concentration with a constant ¹³C abundance was designedand applied to quantitative investigations on the allocationof photoassimilated carbon in nodulated soya bean (Glycine maxL.) plants. The CO₂ concentration in the assimilation chamberand its ¹³C abundance were maintained constant with relativevariances of less than ±0.5 per cent during an 8-h assimilationperiod. At the termination of 8-h ¹³CO₂ assimilation by plantsat early flowering stage, the currently assimilated carbon relativeto total tissue carbon (measured by the degree of isotopic saturation)were for young leaves (including flower buds), 13.9 per cent;mature leaves, 15.7 per cent; stems+petioles, 5.9 per cent;roots, 5.4 per cent and nodules, 6.9 per cent, 48 h after theend of the ¹³CO₂ assimilation period, they were 12.3, 7.5, 7.4,6.8 and 6.1 per cent, respectively. The treatment with a highconcentration of nitrate in the nutrient media significantlydecreased the allocation of ¹³C into nodules. Experiments on¹³CO₂ assimilation by plants at the pod-filling stage were alsoconducted. Labelling by ¹³C was weaker than at the early floweringstage, but an intense accumulation of ¹³C into reproductiveorgans was observed. Glycine max L., nodulated soya bean plants, ¹³CO₂ assimilation, carbon dynamics     

Effects of CO2-Enrichment on the Growth of Young Tomato Plants in Low Light

Carbon dioxide-enrichment of young tomato plants grown in controlled-environmentcabinets at low light intensity (14 cal cm^–2 day^–1,visible radiation) increased their net assimilation rates and,initially, relative growth-rates. Subsequently, the relativegrowth-rate fell to near the rate of non-enriched plants, owingto a fall in leaf-area ratio associated with an increase inleaf dry weight/area. Sowing non-enriched plants a few daysearlier to reach the same total dry weight would not have producedidentical plants. The effects of CO₂-enrichment to 1000 vpm could be simulatedby increasing light intensity by approximately one third exceptthat the plants had shorter internodes than those in extra CO₂.This was a morphogenetic effect of light since CO₂-enrichmentitself produced slightly shorter plants than controls for anequivalent total dry weight. CO₂-enrichment did not change the dry-weight distribution inthe plants and had little effect on rate of leaf produoctionor the number of flower primordia. There were no indicationsthat beneficial effects of CO₂-enrichment operated other thanthrough increased photosynthesis.     

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     

Variation in the Photoperiodic Control of Flowering of Two Cultivars of Antirrhinum majus L.

The susceptibility to photoperiodic induction of an early (PinkIce) and a late (Orchid Rocket) flowering variety of Antirrhinummajus was investigated. At various times during developmentplants of both varieties were subjected to 1, 2, 3 or 4 long-daycycles. The early variety became increasingly more responsiveto long-days while the late variety showed no positive responsethroughout the course of the experiment. One hour light-breaksgiven midway through the 16-h dark period evoked a decreasein the leaf number of Pink Ice plants. The leaf areas of bothvarieties increased to levels equivalent to long-day grown plants.The net CO₂ uptake of plants grown in light-breaks increasedin proportion to the increase in leaf area. However the specificuptake of CO₂ per unit area was similar for light-break andshort-day grown plants but was substantially lower than thelong-day plants. Suggestions are made to explain the differencesin photoperiodic response of the two varieties in terms of variationin assimilation rate.     

Humidity Pretreatment Affects the Responses of Stomata and CO2 Assimilation to Vapor Pressure Difference in C3 and C4 Plants

Experiments were carried out to investigate the long-term influenceof humidity on the short-term responses of stomata and CO₂ assimilationto vapor pressure difference in Oryza sativa (rice, C₃ species)and Panicum maximum (green panic, C₄ species). Plants were grownfor four weeks in growth chambers set at 35% and 85% relativehumidity at 25C air temperature, 38+2 Pa CO₂ partial pressureand 1,700µmol m^-2s^-1 photon flux density. Soil was saturatedwith water in both humidity treatments. Low humidity pretreatmentscaused low leaf conductance and low rates of transpiration andCO₂ assimilation in O. sativa, but small changes in stomatalresponses to humidity and in CO₂ assimilation were found inP. maximum. From the short-term gas exchange experiments, itwas noted that the responsiveness of leaf conductance to vaporpressure difference were affected by humidity pretreatmentsin O. sativa, whereas unaffected in P. maximum. In O. sativameasurements of CO₂ assimilation as a function of internal CO₂partial pressure (A-Ci curve) indicated that low humidity pretreatmentsreduced the CO₂ assimilation at high internal CO₂ partial pressure,but the initial slope of the A-Ci curve was unaffected. Furthermore,plant characteristics such as total dry weight and leaf areaof plants subjected to low umidity were lower than plants subjectedto high humidity. The reductions in O. sativa, however, werelarger than in P. maximum. Stomatal frequency from low humiditygrown plant was higher than that from high humidity grown plantsin both species although there is no significant difference.The data indicated that if the short term inhibition of netCO₂ assimilation at a high vapor pressure difference was imposedduring vegetative growth, the photosynthetic biochemistry andthe resultant plant growth were largely depressed in O. sativa,a C₃ species. (Received May 26, 1992; Accepted November 2, 1992)     

The influence of elevated C02 on growth and age-related changes in leaf gas exchange

Rumex obtusifolius plants were grown for several months in daylitenvironment chambers (Solardomes) force-ventilated with aircontaining 350 or 600 µ;mol mol^–1 C0₂. ElevatedCO₂ was found to accelerate the natural ontogenic decline inphotosynthesis, but did not reduce leaf duration. In both CO₂treatments photosynthetic rates declined progressively withincreasing leaf age, the decline being greater for plants grownin elevated C0₂ such that rates became lower than in ambientCO₂. The degree of CO₂-induced photosynthetic down-regulationas determined by A/C₁ analysis was found to be dependent onleaf age. The major contribution to the decline in photosynthesiswas likely to be a reduction in Rubisco activity as changesin stomataland mesophyll limitations were small. Instantaneouswater use efficiency (WUE₁) was greater for plants in elevatedCO₂, but these values declined rapidly with leaf age, whereasin ambient CO₂ values were always lower, but were maintainedfor longer. Growth analysis indicated an increased root:-shootratio for plants grown in elevated CO₂, this occurring almostentirely as a result of increased root growth. Greater rootproliferation and increased WUE₁, are characteristics whichshould give this persistent and troublesome weed an increasedcompetitive advantage under projected conditions of climatechange Key words: tusifoliu, elevated CO₂, gas exchange, leaf age, senescence