Involvement of respiratory processes in the transient knockout of net CO2 uptake in Mimosa pudica upon heat stimulation

Leaf photosynthesis of the sensitive plant Mimosa pudica displays a transient knockout in response to electrical signals induced by heat stimulation. This study aims at clarifying the underlying mechanisms, in particular, the involvement of respiration. To this end, leaf gas exchange and light reactions of photosynthesis were assessed under atmospheric conditions largely eliminating photorespiration by either elevated atmospheric CO₂ or lowered O₂ concentration (i.e. 2000 μmol mol^?1 or 1%, respectively). In addition, leaf gas exchange was studied in the absence of light. Under darkness, heat stimulation caused a transient increase of respiratory CO₂ release simultaneously with stomatal opening, hence reflecting direct involvement of respiratory stimulation in the drop of the net CO₂ uptake rate. However, persistence of the transient decline in net CO₂ uptake rate under illumination and elevated CO₂ or 1% O₂ makes it unlikely that photorespiration is the metabolic origin of the respiratory CO₂ release. In conclusion, the transient knockout of net CO₂ uptake is at least partially attributed to an increased CO₂ release through mitochondrial respiration as stimulated by electrical signals. Putative CO₂ limitation of Rubisco due to decreased activity of carbonic anhydrase was ruled out as the photosynthesis effect was not prevented by elevated CO₂.     

Soybean leaf growth and gas exchange response to drought under carbon dioxide enrichment

This study was conducted to determine the response in leaf growth and gas exchange of soybean (Glycine max Merr.) to the combined effects of water deficits and carbon dioxide (CO₂) enrichment. Plants grown in pots were allowed to develop initially in a glasshouse under ambient CO₂ and well-watered conditions. Four-week old plants were transferred into two different glasshouses with either ambient (360 μmol mol^-1) or elevated (700 μmol mol^-1) CO₂. Following a 2-day acclimation period, the soil of the drought-stressed pots was allowed to dry slowly over a 2-week period. The stressed pots were watered daily so that the soil dried at an equivalent rate under the two CO₂ levels. Elevated [CO₂] decreased water loss rate and increased leaf area development and photosynthetic rate under both well-watered and drought-stressed conditions. There was, however, no significant effect of [CO₂] in the response relative to soil water content of normalized leaf gas exchange and leaf area. The drought response based on soil water content for transpiration, leaf area, and photosynthesis provide an effective method for describing the responses of soybean physiological processes to the available soil water, independent of [CO₂].     

Effect of leaf water deficit on stomatal and nonstomatal regulation of net carbon dioxide assimilation

The effect of leaf water deficit on net CO₂ assimilation was studied under two conditions: in one, the stomata were allowed to contribute to the regulation of CO₂ assimilation; in the other, air was forced through the leaf at a constant rate to overcome the effects of change in stomatal resistance accompanying changes in leaf water deficit. When the stomata were allowed to regulate the gaseous diffusive resistance of the leaf, CO₂ assimilation decreased with increasing leaf water deficit. However, when air was forced through the leaf, the rate of assimilation was not inhibited by increasing leaf water deficit. The results indicate that the inhibition of net CO₂ assimilation with increasing leaf water deficit is a consequence of an increase in the diffusive resistance to gas exchange and not of a change in apparent mesophyll resistance.     

Increased leaf area dominates carbon flux response to elevated CO2 in stands of Populus deltoides (Bartr.)

We examined the effects of atmospheric vapor pressure deficit (VPD) and soil moisture stress (SMS) on leaf‐ and stand‐level CO₂ exchange in model 3‐year‐old coppiced cottonwood (Populus deltoides Bartr.) plantations using the large‐scale, controlled environments of the Biosphere 2 Laboratory. A short‐term experiment was imposed on top of continuing, long‐term CO₂ treatments (43 and 120 Pa), at the end of the growing season. For the experiment, the plantations were exposed for 6–14 days to low and high VPD (0.6 and 2.5 kPa) at low and high volumetric soil moisture contents (25–39%). When system gross CO₂ assimilation was corrected for leaf area, system net CO₂ exchange (SNCE), integrated daily SNCE, and system respiration increased in response to elevated CO₂. The increases were mainly as a result of the larger leaf area developed during growth at high CO₂, before the short‐term experiment; the observed decline in responses to SMS and high VPD treatments was partly because of leaf area reduction. Elevated CO₂ ameliorated the gas exchange consequences of water stress at the stand level, in all treatments. The initial slope of light response curves of stand photosynthesis (efficiency of light use by the stand) increased in response to elevated CO₂ under all treatments. Leaf‐level net CO₂ assimilation rate and apparent quantum efficiency were consistently higher, and stomatal conductance and transpiration were significantly lower, under high CO₂ in all soil moisture and VPD combinations (except for conductance and transpiration in high soil moisture, low VPD). Comparisons of leaf‐ and stand‐level gross CO₂ exchange indicated that the limitation of assimilation because of canopy light environment (in well‐irrigated stands; ratio of leaf : stand=3.2–3.5) switched to a predominantly individual leaf limitation (because of stomatal closure) in response to water stress (leaf : stand=0.8–1.3). These observations enabled a good prediction of whole stand assimilation from leaf‐level data under water‐stressed conditions; the predictive ability was less under well‐watered conditions. The data also demonstrated the need for a better understanding of the relationship between leaf water potential, leaf abscission, and stand LAI.     

Heterogenous stomatal closure in response to leaf water deficits is not a universal phenomenon

The extent and occurrence of water stress-induced “patchy” CO₂ uptake across the surface of leaves was evaluated in a number of plant species. Leaves, while still attached to a plant, were illuminated and exposed to air containing [¹⁴C]CO₂ before autoradiographs were developed. Plant water deficits that caused leaf water potential depression to −1.1 megapascals during a 4-day period did result in heterogenous CO₂ assimilation patterns in bean (Phaseolus vulgaris). However, when the same level of stress was imposed more gradually (during 17 days), no patchy stomatal closure was evident. The patchy CO₂ assimilation pattern that occurs when bean plants are subjected to a rapidly imposed stress could induce artifacts in gas exchange studies such that an effect of stress on chloroplast metabolism is incorrectly deduced. This problem was characterized by examining the relationship between photosynthesis and internal [CO₂] in stressed bean leaves. When extent of heterogenous CO₂ uptake was estimated and accounted for, there appeared to be little difference in this relationship between control and stressed leaves. Subjecting spinach (Spinacea oleracea) plants to stress (leaf water potential depression to −1.5 megapascals) did not appear to cause patchy stomatal closure. Wheat (Triticum aestivum) plants also showed homogenous CO₂ assimilation patterns when stressed to a leaf water potential of −2.6 megapascals. It was concluded that water stress-induced patchy stomatal closure can occur to an extent that could influence the analysis of gas exchange studies. However, this phenomenon was not found to be a general response. Not all stress regimens will induce patchiness; nor will all plant species demonstrate this response to water deficits.     

Effect of Periodic Oscillations of Artificial Light Emission on Photosynthetic Activity

The effects of periodic oscillation of artificial light and of the frequency of these oscillations on photosynthetic assimilation has been measured. When the frequency is sufficiently high, the gross CO₂ assimilation in fluctuating light is identical to the assimilation of the leaf under a continuous illumination equal to the efficient intensity of the intermittent light. It is necessary to consider the quality of the sensor used for measuring light intensity, particularly its response velocity.     

Mesophyll Resistance and Carboxylase Activity: A Comparison under Water Stress Conditions

The response of several leaf gas exchange parameters were monitored with decreasing leaf water potential in Phaseolus vulgaris L. leaflets. These included photosynthesis, transpiration, CO₂ compensation point, ribulose 1,5-diphosphate carboxylase activity, boundary layer plus stomatal, and mesophyll resistance to diffusion of CO₂. Mesophyll resistance was calculated under two assumptions: (a) the CO₂ concentration at the chloroplast was zero, and (b) it was equal to the CO₂ compensation point.     

A system for measuring leaf gas exchange based on regulating vapour pressure difference

A system for measurement of leaf gas exchange while regulating leaf to air vapour pressure difference has been developed; it comprises an assimilation chamber, leaf temperature controller, mass flow controller, dew point controller and personal computer. A relative humidity sensor and air and leaf temperature sensors, which are all used for regulating the vapour pressure difference, are mounted into the chamber. During the experiments, the computer continuously monitored the photosynthetic parameters and measurement conditions, so that accurate and intenstive measurements could be made.When measuring the light-response curve of CO₂ assimilation for single leaves, in order to regulate the vapour pressure difference, the leaf temperature and relative humidity in the chamber were separately and simultaneously controlled by changing the air temperature around the leaf and varying the air flow rate through the chamber, respectively. When the vapour pressure difference was regulated, net CO₂ assimilation, transpiration and leaf conductance for leaves of rice plant increased at high quantum flux density as compared with those values obtained when it was not regulated.When measuring the temperature-response curve of CO₂ assimilation, the regulation of vapour pressure difference was manipulated by the feed-forward control of the dew point temperature in the inlet air stream. As the vapour pressure difference was regulated at 12 mbar, the maximum rate of and the optimum temperature for CO₂ assimilation in rice leaves increased 5 molCO₂ m^–2 s^–1 and 5°C, respectively, as compared with those values obtained when the vapour pressure difference took its own course. This was reasoned to be due to the increase in leaf conductance and the decrease in transpiration rate. In addition, these results confirmed that stomatal conductance essentially increases with increasing leaf temperature under constant vapour pressure difference conditions, in other words, when the influence of the vapour pressure difference is removed.This system may be used successfully to measure inter- and intra-specific differences and characteristics of leaf gas exchange in plants with a high degree of accuracy.Abbreviations A CO₂ assimilation rate - A_max Maximum rate of CO₂ assimilation - A_opt Optimum teperature for CO₂ assimilation - CTWB Controlled-temperature water bath - DPC Dew point controller - E Transpiration rate; gl, leaf conductance - HCC Humidity control circuit - IRGA Infrared gas analyzer - LT Leaf temperature - LTC Leaf temperature controller - MFC Mass flow controller - QFD Quantum flux density - RH Relative humidity - RHC Relative humidity controller - VPD Vapour pressure difference - CO₂ Difference of CO₂ concentration between inlet and outlet air     

Stem photosynthesis in a desert ephemeral,Eriogonum inflatum

Summary The gas exchange characteristics of photosynthetic tissues of leaves and stems of Eriogonum inflatum are described. Inflated stems were found to contain extraordinarily high internal CO₂ concentrations (to 14000 bar), but fixation of this internal CO₂ was 6–10 times slower than fixation of atmospheric CO₂ by these stems. Although the pool of CO₂ is a trivial source of CO₂ for stem photosynthesis, it may result in higher water-use efficiency of stem tissues. Leaf and stem photosynthetic activities were compared by means of CO₂ fixation in CO₂ response curves, light and temperature response curves in IRGA systems, and by means of O₂ exchange at CO₂ saturation in a leaf disc O₂ electrode system. On an area basis leaves contain about twice the chlorophyll and nitrogen as stems, and are capable of up to 4-times the absolute CO₂ and O₂ exchange rates. However, the stem shape is such that lighting of the shaded side leads to a substantial increase in overall stem photosynthesis on a projected area basis, to about half the leaf rate in air. Stem conductance is lower than leaf conductance under most conditions and is less sensitive to high temperature or high VPD. Under most conditions, the ratio C _i /C _a is lower in stems than in leaves and stems show greater water-use efficiency (higher ratio assimilation/transpiration) as a function of VPD. This potential advantage of stem photosynthesis in a water limited environment may be offset by the higher VPD conditions in the hotter, drier part of the year when stems are active after leaves have senesced. Stem and leaf photosynthesis were similarly affected by decreasing plant water potential.     

Do Stomata Respond to CO(2) Concentrations Other than Intercellular?

Most studies on stomatal responses to CO₂ assume that guard cells respond only to intercellular CO₂ concentration and are insensitive to the CO₂ concentrations in the pore and outside the leaf. If stomata are sensitive to the CO₂ concentration at the surface of the leaf or in the stomatal pore, the stomatal response to intercellular CO₂ concentration will be incorrect for a `normally' operating leaf (where ambient CO₂ concentration is a constant). In this study asymmetric CO₂ concentrations for the two surfaces of amphistomatous leaves were used to vary intercellular and leaf surface CO₂ concentrations independently in Xanthium strumarium L. and Helianthus annuus L. The response of stomata to intercellular CO₂ concentration when the concentration at the leaf surface was held constant was found to be the same as the response when the surface concentration was varied. In addition, stomata did not respond to changes in leaf surface CO₂ concentration when the intercellular concentration for that surface was held constant. It is concluded that stomata respond to intercellular CO₂ concentration and are insensitive to the CO₂ concentration at the surface of the leaf and in the stomatal pore.     

A computer-operated routine of gas exchange and optical measurements to diagnose photosynthetic apparatus in leaves

Photosynthesis is a complex process whose rate is affected by many biochemical and biophysical factors. Fortunately, it is possible to determine, or at least estimate, many of the most important parameters using a combination of optical methods and gas transient analyses. We describe here a computer‐operated routine that has been developed to make detailed assessments of photosynthesis at a comprehensive level. The routine comprised the following measurements: steady‐state light and CO₂ response curves of net CO₂ assimilation at 21 and 2 kPa O₂; transients from limiting to different saturating CO₂ concentrations at 2 kPa O₂; post‐illumination CO₂ fixation transient; dark–light induction of O₂ evolution; O₂ yield from one saturating single‐turnover flash; chlorophyll fluorescence F₀, F_s and F_m during the light and CO₂ response curves; leaf transmission at 820 nm (P700⁺) during the light and CO₂ response curves; post‐illumination re‐reduction time of P700⁺. The routine was executed on a two‐channel fast‐response gas exchange measurement system (A. Laisk and V. Oja: Dynamic Gas Exchange of Leaf Photosynthesis. CSIRO, Canberra, Australia). Thirty‐six intrinsic characteristics of the photosynthetic machinery were derived, including quantum yield of CO₂ fixation (Y_CO2), time constant of P700 re‐reduction (τ′), relative optical cross‐sections of PSII and PSI antennae (a_II, a_I), PSII and PSI density per leaf area unit, plastoquinone pool, total mesophyll resistance, mesophyll diffusion resistance, V_m, K_m(CO₂) and CO₂/O₂ specificity of Rubisco, RuBP pool at CO₂ limitation (assimilatory charge). An example of the routine and calculations are shown for one leaf and data are presented for leaves of 8‐year‐old‐trees of two birch clones growing in Suonenjoki Forest Research Station, Finland, during summer 2000. Parameters Y_CO2, basic τ′, a_II, a_I, K_m(CO₂) and K_s varied little in different leaves [relative standard deviation (RSD) < 7%], other parameters scattered widely (RSD typically 10–40%). It is concluded that the little scattered parameters are determined by basic physico‐chemical properties of the photosynthetic machinery whereas the widely scattered parameters are adjusting to growth conditions. The proposed non‐destructive routine is suitable for diagnosing the photosynthetic machinery of leaves and may be applied in plant ecophysiology and in genetic engineering of plants.     

Growth and onto-morphogenesis of soybean (Glycine max Merril) in an open, naturally CO2-enriched environment

Springs emitting carbon dioxide are frequent in Central Italy and provide a way of testing the response of plants to CO₂ enrichment under natural conditions. Results of a CO₂ enrichment experiment on soybean at a CO₂ spring (Solfatara) are presented. The experimental site is characterized by significant anomalies in atmospheric CO₂ concentration produced by a large number of vents emitting almost pure CO₂ (93%) plus small amounts of hydrogen sulphide, methane, nitrogen and oxygen. Within the gas vent area, plants were grown at three sub-areas whose mean CO₂ concentrations during daytime were 350,652 and 2370 μmol mol^-1, respectively. Weekly harvests were made to measure biomass growth, leaf area and ontogenetic development. Biomass growth rate and seed yield were enhanced by elevated CO₂. In particular, onto-morphogenetic development was affected by elevated CO₂ with high levels of CO₂ increasing the total number of main stem leaf nodes and the area of the main stem trifoliolate leaves. Biochemical analysis of plant tissue suggested that there was no effect of the small amounts of H₂S on the response to CO₂ enrichment. Non-protein sulphydryl compounds did not accumulate in leaf tissues and the overall capacity of leaf extracts to oxidize exogenously added NADH was not decreased. The limitations and advantages of experimenting with crop plants at elevated CO₂ in the open and in the proximity of carbon dioxide springs are discussed.     

Leaf photosynthesis and carbohydrates of CO₂-enriched maize and grain sorghum exposed to a short period of soil water deficit during vegetative development

Among C₄ species, sorghum is known to be more drought tolerant than maize. The objective was to evaluate differences in leaf gas exchanges, carbohydrates, and two enzyme activities of these nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME) C₄ subtype monocots in response to water deficit and CO₂ concentration ([CO₂]). Maize and sorghum were grown in pots in sunlit environmental-controlled chambers. Treatments included well watered (WW) and water stressed (WS) (water withheld at 26 days) and daytime [CO₂] of 360 (ambient) and 720 (elevated) μmol mol⁻¹. Midday gas exchange rates, concentrations of nonstructural carbohydrates, and activities of sucrose-phosphate synthase (SPS) and adenosine 5′-diphosphoglucose pyrophosphorylase (ADGP) were determined for fully expanded leaf sections. There was no difference in leaf CO₂ exchange rates (CER) between ambient and elevated [CO₂] control plants for both maize and sorghum. After withholding water, leaf CER declined to zero after 8 days in maize and 10 days for sorghum. Sorghum had lower stomatal conductance and transpiration rates than maize, which resulted in a longer period of CER under drought. Nonstructural carbohydrates of both control maize and sorghum were hardly affected by elevated [CO₂]. Under drought, however, increases in soluble sugars and decreases in starch were generally observed for maize and sorghum at both [CO₂] levels. For stressed maize and sorghum, decreases in starch occurred earlier and were greater at ambient [CO₂] than at elevated [CO₂]. For maize, drought did not meaningfully affect SPS activity. However, a decline in SPS activity was observed for drought-stressed sorghum under both [CO₂] treatments. There was an increase in ADGP activity in maize under drought for both [CO₂] treatments. Such a response in ADGP to drought, however, did not occur for sorghum. The generally more rapid response of maize than sorghum to drought might be related to the more rapid growth of leaf area of maize.     

A micro pCO2 electrode

By utilizing a previously developed micro pH glass electrode it has been possible to make a micro pCO₂ electrode with a tip diameter of about 10 μm. This was accomplished by placing the micro pH electrode in a conical tube containing a weak NaHCO₃ solution. The tip of the conical tube was closed with Teflon^® oil wax mixture. This closure prevented the flow of solution, but allowed CO₂ to pass into the NaHCO₃ solution thus altering the pH of this solution. Changes in pH were seen and measured by the micro pH electrode and could be related to the pCO₂ of gas or solution in which the total electrode system was placed. This electrode, principally because of its small size, has many possible applications in biological research.     

A field portable system for the measurement of gas exchange of leaves under natural and controlled conditions: examples with field-grown Eucalyptus pauciflora Sieb. ex Spreng. ssp. pauciflora, E. behriana F. Muell. and Pinus radiata R. Don.

Abstract A field portable system is described which measures the response of gas exchange of one leaf to changes in environmental parameters under controlled conditions, and which simultaneously measures the gas exchange of another leaf as the climatic parameters vary naturally. The system consists of two independently operating cuvettes. It enables detailed studies of photosynthesis and stomata/transpiration of leaves attached to the plant in their natural position. It provides control of temperature, humidity, CO₂ and oxygen concentration (or, alternatively, of other gases) as well as of light. Infrared gas analyzers for CO₂ and H₂O are used which allow similar time constants for the measurement of the two gases. Examples of a diurnal course of gas exchange of a leaf in its natural exposition and of experiments with steady-state responses of gas exchange are presented. In Eucalyptus pauciflora Sieb. ex Spreng. ssp. pauciflora, a set of response curves of CO, assimilation (A) to CO₂, as measured at various leaf temperatures and light levels, shows carboxylation efficiency to be light saturated at the lower photon irradiances the lower the leaf temperature is. Carboxylation efficiency is maximal at 25°C. At ambient CO, partial pressure stomata open in a way that CO₂ assimilation occurs at a rate found within the curvature region of the CO₂ response function of A. The light-independent CO² compensation point as a function of temperature is presented. Applying a combined heat/low humidity pulse (15 or 60 min) on leaves of Eucalyptus behriana F. Muell. or Pinus radiata R. Don, respectively, leads to a lower level of intercellular carbon dioxide partial pressure (C_i) during the decline in A and leaf conductance to water vapour (g). A lower C_i level is maintained during recovery of A and g, A almost reaching the pre-pulse level but not g. The existence of an after-effect indicates that the response to the combined high temperature/low humidity pulse is a multi-step process.     

Photosynthetic Induction in a C(4) Dicot, Flaveria trinervia: I. Initial Products of CO(2) Assimilation and Levels of Whole Leaf C(4) Metabolites

Labeling patterns from ¹⁴CO₂ pulses to leaves and whole leaf metabolite contents were examined during photosynthetic induction in Flaveria trinervia, a C₄ dicot of the NADP-malic enzyme subgroup. During the first one to two minutes of illumination, malate was the primary initial product of ¹⁴CO₂ assimiltion (about 77% of total ¹⁴C incorporated). After about 5 minutes of illumination, the proportion of initial label to aspartate increased from 16 to 66%, and then gradually declined during the following 7 to 10 minutes of illumination. Nutrition experiments showed that the increase in ¹⁴CO₂ partitioning to aspartate was delayed about 2.5 minutes in plants grown with limiting N, and was highly dampened in plants previously treated 10 to 12 days with ammonia as the sole N source. Measurements of C₄ leaf metabolites revealed several transients in metabolite pools during the first few minutes of illumination, and subsequently, more gradual adjustments in pool sizes. These include a large initial decrease in malate (about 1.6 micromoles per milligram chlorophyll) and a small initial decrease in pyruvate. There was a transient increase in alanine levels after 1 minute of illumination, which was followed by a gradual, prolonged decrease during the remainder of the induction period. Total leaf aspartate decreased initially, but temporarily doubled in amount between 5 and 10 minutes of illumination (after its surge as a primary product). These results are discussed in terms of a plausible sequence of metabolic events which lead to the formation of the intercellular metabolite gradients required in C₄ photosynthesis.     

Profiles of isoprene emission and photosynthetic parameters in hybrid poplars exposed to free‐air CO2 enrichment†

Poplar (Populus × euroamericana) saplings were grown in the field to study the changes of photosynthesis and isoprene emission with leaf ontogeny in response to free air carbon dioxide enrichment (FACE) and soil nutrient availability. Plants growing in elevated [CO₂] produced more leaves than those in ambient [CO₂]. The rate of leaf expansion was measured by comparing leaves along the plant profile. Leaf expansion and nitrogen concentration per unit of leaf area was similar between nutrient treatment, and this led to similar source–sink functional balance. Consequently, soil nutrient availability did not cause downward acclimation of photosynthetic capacity in elevated [CO₂] and did not affect isoprene synthesis. Photosynthesis assessed in growth [CO₂] was higher in plants growing in elevated than in ambient [CO₂]. After normalizing for the different number of leaves over the profile, maximal photosynthesis was reached and started to decline earlier in elevated than in ambient [CO₂]. This may indicate a [CO₂]‐driven acceleration of leaf maturity and senescence. Isoprene emission was adversely affected by elevated [CO₂]. When measured on the different leaves of the profile, isoprene peak emission was higher and was reached earlier in ambient than in elevated [CO₂]. However, a larger number of leaves was emitting isoprene in plant growing in elevated [CO₂]. When integrating over the plant profile, emissions in the two [CO₂] levels were not different. Normalization as for photosynthesis showed that profiles of isoprene emission were remarkably similar in the two [CO₂] levels, with peak emissions at the centre of the profile. Only the rate of increase of the emission of young leaves may have been faster in elevated than in ambient [CO₂]. Our results indicate that elevated [CO₂] may overall have a limited effect on isoprene emission from young seedlings and that plants generally regulate the emission to reach the maximum at the centre of the leaf profile, irrespective of the total leaf number. In comparison with leaf expansion and photosynthesis, isoprene showed marked and repeatable differences among leaves of the profile and may therefore be a useful trait to accurately monitor changes of leaf ontogeny as a consequence of elevated [CO₂].     

Two Steps of Gas Exchange in Leaf Photosynthesis

Photosynthesis and transpiration of excised leaves of Taraxacum officinale L. and a few other species of plants were measured, using an open gas analysis system. The rates of CO₂ uptake and transpiration increased in two steps upon illumination of stomata-bearing epidermis of these leaves at a light intensity of 50 mW × cm⁻². Abscisic acid inhibited only the second step of gas exchange. Illumination of the astomatous epidermis of hypostomatous leaves caused only the first step of gas exchange. These data indicate that the first and second steps arise from cuticular and stomatal gas exchange, respectively. The rate of the cuticular photosynthesis in a Taraxacum leaf reached saturation at a light intensity of 5 mW × cm⁻², and the rates of the stomatal photosynthesis and transpiration reached saturation at a higher intensity of 35 mW × cm⁻². The cuticular photosynthesis of a Taraxacum leaf was 18% of the stomatal photosynthesis at 50 mW × cm⁻² and 270% at 5 mW × cm⁻². The other species of leaves showed the same trend. The importance of cuticular CO₂ uptake in leaf photosynthesis, especially under low light intensity was stressed from these data.     

A Direct Confirmation of the Standard Method of Estimating Intercellular Partial Pressure of CO(2)

The partial pressure of CO₂ inside leaves of several species was measured directly. Small gas exchange chambers were clamped above and below the same section of an amphistomatous leaf. A flowing gas stream through one chamber allowed normal CO₂ and water vapor exchange. The other chamber was in a closed circuit consisting of the chamber, an infrared gas analyzer, and a peristaltic pump. The CO₂ in the closed system rapidly reached a steady pressure which it is believed was identical to the CO₂ pressure inside the leaf, because there was no flux of CO₂ across the epidermis. This measured partial pressure was in close agreement with that estimated from a consideration of the fluxes of CO₂ and vapor at the other surface.     

HIGH CARBON DIOXIDE CONCENTRATIONS IN AERENCHYMA OF TYPHA LATIFOLIA

Diurnal and seasonal patterns of CO₂ concentration ([CO₂]) in leaf gas spaces were measured to better understand the relationship of sediment-derived CO₂ to photosynthesis in the emergent wetland species, Typha latifolia L. (cattail). Leaf [CO₂] was above 2,000 μl/liter at dawn on all but the first sampling date. At all sampling dates, leaf [CO₂] declined to near atmospheric [CO₂] at midday and rose to well above atmospheric [CO₂] in the late afternoon. The maximum leaf [CO₂] varied with sampling date and was over 18 times atmospheric levels (over 6,300 μl/liter) in August. Based on measurement of photon flux density and temperature, the diurnal pattern in leaf [CO₂] may be generally controlled by expected photosynthetic rates. It is hypothesized that seasonal variation in leaf [CO₂] may be a function of variation in microbial (soil) respiration. Using dye and slight pressurization, it was confirmed that gas spaces in rhizomes were interconnected with the gas spaces in leaves through the rhizome-shoot transition. From anatomical measurements, it was also estimated that over 50% of total leaf volume was occupied by gas spaces and that most of the total gas-space volume in plants was in the shoot. Photosynthetic rate in C3 plants, like cattail, can increase with increasing [CO₂] under natural conditions. For this reason, cattail and other emergent wetland plants possessing continuous gas-space pathways appear to have a significant carbon supplement as compared to other C3 plants growing in well-aerated soils.