Maximum Rates Of CO2 Uptake Research Articles

Carbon dioxide uptake and fixation by ferrous iron-oxidizing acidophilic bacteria

CO2 is a major nutrient for autotrophic, mineral sulfide-oxidizing bacteria. The uptake and fixation of this carbon source is therefore critical for industrial mineral sulfide bioprocessing. The CO2 concentration influenced growth of five iron- or sulfur-oxidizing acidophiles to different extents. Those less affected by a growth-limiting CO2 concentration included Acidithiobacillus ferrooxidans and Acidithiomicrobium ferrooxidans. Acidithiobacillus ferrooxidans showed efficient uptake of 14CO2 from its concentration in air when previously grown under air while the maximum rate of CO2 uptake by cells previously grown under an enhanced CO2 concentration was only approached when the CO2 was increased to 0.1% v/v in air. In contrast, growth of Sulfobacillus thermosulfidooxidans was more adversely affected by limiting CO2 under air and the CO2 uptake rate was the same in cells grown with limiting or excess CO2. At. ferrooxidans also appeared to fix accumulated CO2 more efficiently than S. thermosulfidooxidans and RT-qPCR confirmed that adaptation of At. ferrooxidans to growth under limiting CO2 included production of form 1A ribulose bis-phosphate carboxylase in place of the form 1Aq enzyme which was predominant during growth under an enhanced CO2 concentration.

CO 2 Uptake and Fixation by a Thermoacidophilic Microbial Community Attached to Precipitated Sulfur in a Geothermal Spring

Carbon fixation at temperatures above 73 degrees C, the upper limit for photosynthesis, is carried out by chemosynthetic thermophiles. Yellowstone National Park (YNP), Wyoming possesses many thermal features that, while too hot for photosynthesis, presumably support chemosynthetic-based carbon fixation. To our knowledge, in situ rates of chemosynthetic reactions at these high temperatures in YNP or other high-temperature terrestrial geothermal springs have not yet been reported. A microbial community attached to precipitated elemental sulfur (S(o) floc) at the source of Dragon Spring (73 degrees C, pH 3.1) in Norris Geyser Basin, YNP, exhibited a maximum rate of CO(2) uptake of 21.3 +/- 11.9 microg of C 10(7) cells(-1) h(-1). When extrapolated over the estimated total quantity of S(o) floc at the spring's source, the S(o) floc-associated microbial community accounted for the uptake of 121 mg of C h(-1) at this site. On a per-cell basis, the rate was higher than that calculated for a photosynthetic mat microbial community dominated by Synechococcus spp. in alkaline springs at comparable temperatures. A portion of the carbon taken up as CO(2) by the S(o) floc-associated biomass was recovered in the cellular nucleic acid pool, demonstrating that uptake was coupled to fixation. The most abundant sequences in a 16S rRNA clone library of the S(o) floc-associated community were related to chemolithoautotrophic Hydrogenobaculum strains previously isolated from springs in the Norris Geyser Basin. These microorganisms likely contributed to the uptake and fixation of CO(2) in this geothermal habitat.

Carbon Dioxide Exchange Between the Atmosphere and an Alpine Shrubland Meadow During the Growing Season on the Qinghai‐Tibetan Plateau

Abstract: In the present study, we used the eddy covariance method to measure CO2 exchange between the atmosphere and an alpine shrubland meadow ecosystem (37°36’N, 101°18’E; 3 250 m a.s.l.) on the Qinghai‐Tibetan Plateau, China, during the growing season in 2003, from 20 April to 30 September. This meadow is dominated by formations of Potentilla fruticosa L. The soil is Mol‐Cryic Cambisols. During the study period, the meadow was not grazed. The maximum rates of CO2 uptake and release derived from the diurnal course of CO2 flux were ‐9.38 and 5.02 μmol·m‐2·s‐1, respectively. The largest daily CO2 uptake was 1.7 g C·m‐2·d‐1 on 14 July, which is less than half that of an alpine Kobresia meadow ecosystem at similar latitudes. Daily CO2 uptake during the measurement period indicated that the alpine shrubland meadow ecosystem may behave as a sink of atmospheric CO2 during the growing season. The daytime CO2 uptake was correlated exponentially or linearly with the daily photo synthetic photon flux density each month. The daytime average water use efficiency of the ecosystem was 6.47 mg CO2/g H2O. The efficiency of the ecosystem increased with a decrease in vapor pressure deficit.(Managing editor: Ya‐Qin HAN)

Photosynthesis, Chlorophyll Fluorescence and Within‐Canopy Distribution of Epiphytic Ferns in a Mexican Cloud Forest

Abstract: Parameters associated with the photosynthetic performance of eight common epiphytic ferns in a Mexican cloud forest were investigated in relation to the distribution of these species within the canopy. If the substantial microclimatic gradients within tropical forest canopies provide microhabitats exploited by different epiphytic species, we would expect to find correlations between distribution and physiological traits. Maximum rates of CO2 uptake (Amax) and photon flux densities at light compensation points (LCP) were in the range of shade plants (Amax = 0.6 ‐ 5.2 μmol m‐2 s‐1; LCP = 4 ‐ 6.5 μmol m‐2 s‐1), but saturation light intensities were more typical for sun plants (270 ‐ 550 μmol m‐2 s‐1). Amax and nitrogen content per unit dry weight were correlated with the distribution of the species within the canopy, but LCP, apparent quantum yield and dark respiration were not. When leaves were left to desiccate, the fluorescence yield of dark‐adapted leaves (Y0) remained high until the relative water content (RWC) had dropped below 30 to 20 %. Fluorescence after short illumination with 200 μmol m‐2 s‐1 declined when RWC dropped below 70 to 40 %. After exposure to full sunlight for 1 h, Y0 of species growing in the outer canopy (Pleopeltis mexicana and Polypodium plebeium) and a plant characteristic of the mid‐canopy (Elaphoglossum petiolatum) recovered better than in species from shadier locations (Trichomanes bucinatum, Asplenium cuspidatum, Phlebodium areolatum). With the exception of Ph. areolatum and a species growing at both exposed and shaded sites (Polypodium puberulum), Y0 recovered at least partially after a loss of 80 ‐ 96 % of saturation water, with the humidity‐loving filmy fern (T. bucinatum) showing no signs of permanent damage at all. The results suggest that tolerance or avoidance of desiccation and high light may be at least as important in controlling the distribution of the species studied as photosynthetic performance without stress.

Response of Photosynthesis to Radiation and Intercellular CO<sub>2</sub> Concentration in Sun and Shade Shoots of Norway Spruce

Functional differentiation of assimilation activity of sun versus shade foliage was analysed in a Norway spruce monoculture stand (age 15 years). The investigated stand density (leaf area index 8.6) and crown structure led to variation in the photosynthetically active photon flux density (PPFD) within the crowns of the sampled trees. At the saturating PPFD, the maximum rate of CO2 uptake (P Nmax) of exposed shoots (E-shoots) was 1.7 times that of the shaded shoots (S-shoots). The apparent quantum yield (α) of E-shoots was 0.9 times that of the S-shoots. A lower ability to use excess energy at high PPFD in photosynthesis was observed in the S-layer. The CO2- and PPFD-saturated rate of CO2 uptake (P Nsat) of the E-shoots was 1.12 times and the carboxylation efficiency (τ) 1.6 times that of the S-shoots. The CO2-saturated rate of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) carboxylation (VCmax) and of actual electron transport (Jamax) in the S-needles amounted to 89 and 95 % of VCmax and Jamax in the E-needles. Thus, in addition to the irradiation conditions and thus limitation by low Ja, the important limitation of photosynthesis in shade needles is due to carboxylation. This limitation of photosynthesis is accompanied by lower stomatal conductance.

Effects of temperature on photosynthesis of two morphologically contrasting plant species along an altitudinal gradient in the tropical high Andes

The effects of temperature on photosynthesis of a rosette plant growing at ground level, Acaena cylindrostachya R. et P., and an herb that grows 20-50 cm above ground level, Senecio formosus H.B.K., were studied along an altitudinal gradient in the Venezuelan Andes. These species were chosen in order to determine - in the field and in the laboratory - how differences in leaf temperature, determined by plant form and microenvironmental conditions, affect their photosynthetic capacity. CO2 assimilation rates (A) for both species decreased with increasing altitude. For Acaena leaves at 2900 m, A reached maximum values above 9 μmol m-2 s-1, nearly twice as high as maximum A found at 3550 m (5.2) or at 4200 m (3.9). For Senecio leaves, maximum rates of CO2 uptake were 7.5, 5.8 and 3.6 μmol m-2 s-1 for plants at 2900, 3550 and 4200 m, respectively. Net photosynthesis-leaf temperature relations showed differences in optimum temperature for photosynthesis (A o.t.) for both species along the altitudinal gradient. Acaena showed similar A o.t. for the two lower altitudes, with 19.1°C at 2900 m and 19.6°C at 3550 m, while it increased to 21.7°C at 4200 m. Maximum A for this species at each altitude was similar, between 5.5 and 6.0 μmol m-2 s-1. For the taller Senecio, A o.t. was more closely related to air temperatures and decreased from 21.7°C at 2900 m, to 19.7°C at 3550 m and 15.5°C at 4200 m. In this species, maximum A was lower with increasing altitude (from 6.0 at 2900 m to 3.5 μmol m-2 s-1 at 4200 m). High temperature compensation points for Acaena were similar at the three altitudes, c. 35°C, but varied in Senecio from 37°C at 2900 m, to 39°C at 3550 m and 28°C at 4200 m. Our results show how photosynthetic characteristics change along the altitudinal gradient for two morphologically contrasting species influenced by soil or air temperatures.

Irradiance and temperature effects on photosynthesis of tussock tundra Sphagnum mosses from the foothills of the Philip Smith Mountains, Alaska.

Photosynthetic characteristics of three species of Sphagnum common in the foothills of the Brooks Range on the North Slope of Alaska were investigated. Generally, light-saturated rates of net photosynthesis decreased in the order S. squarrosum, S. angustifolium, and S. warnstorfii when plants were grown under common growth chamber conditions. For field-grown S. angustifolium, average light compensation point at 10°C was 37 μmol m-2s-1 photosynthetic photon flux density (PPFD), and light saturation occurred between 250 and 500 μmol m-2 s-1. At 20°C, compensation point increased to 127 μmol m-2s-1 and the PPFD required for light saturation increased to approximately 500 μmol m-2s-1, while maximum rates of CO2 uptake increased only slightly. Light response curves of chamber-grown plants exhibited substantially lower compensation points and higher light-saturated rates of CO2 assimilation than field-grown material, due perhaps to a higher percentage of green, photosynthetically competent tissue. All three species exhibited broad responses to temperature, with optima near 20°C, and maintained at least 75% of maximum assimilation between approx. 13° and 30°C. Rates at 5°C were approx. 50% of maximum. Studies of the microclimate of Sphagnum at the field research site suggest that CO2 uptake should occur at near light-saturated rates during the day in open tussock tundra but that PPFD may often be limiting under Salix and Betula canopies in a water track drainage. Simulations using a simple model provided a seasonal estimate of 0.78 g dry weight (DW) of S. angustifolium produced from each initial g of photosynthetic tissue under willow canopies, assuming no water limitations. Although the simulation model suggests that production would be 66% higher in open tussock tundra, S. angustifolium is rarely found in this potentially more stressful habitat. To explain the relative abundance of Sphagnum in shaded water track areas as compared to open tussock tundra, we postulate that the vascular plant canopies provide protection from adverse effects of high temperatures, excess irradiance and reduced water availability. Under conditions of normal water availability, removal of the vascular plant cover did not affect the tissue water content of S. squarrosum, but resulted in a strong decrease in photosynthetic capacity, accompanied by chlorophyll bleaching. These results suggest that photoinhibition may limit production under certain conditions.

Productivity and temperature biology of two snowbed bryophytes

Chlorophyll distribution within the carpets, CO2 gas exchange under controlled conditions, and heat resistance of the snowbed bryophyte Anthelia juratzkana (Limpr.) Trev. were investigated. Also the gas-exchange parameters of the co-occurring Polytrichum sexangulare Floercke were assessed. Only the uppermost 4 mm layer of Anthelia carpets contains sufficient pigments for photosynthesis. At light saturation and optimal temperatures (6–11°C) the maximum rates of CO2 uptake are 0.7 mg CO2 g-1dw h-1 in Anthelia and 1.5 mg CO2 g-1dw h-1 in Polytrichum. Gas exchange reaches light saturation at about 300 μE m-2s-1 in both species. At +2°C the light compensation point is reached at ca. 10μE m-2s-1 and increases significantly with increasing temperature. The lower temperature compensation point is reached at-4°C in Anthelia and does not drop much below-5°C in Polytrichum. Anthelia cannot sustain net photosynthesis beyond 30°C and Polytrichum not beyond 32°C. Nine month storage under dark, cold and wet conditions does not affect the photosynthetic capability of Anthelia. As a response, however, the net photosynthesis rate is depressed due to an increase of the respiration rates. Polytrichum sexangulare did not tolerate the storage so well. The heat resistance limit of Anthelia is low (39°C). There is evidence that the distribution of the two bryophytes within snowbed communities is determined by their capability to make use of low light intensities and their low temperature demand for optimal photosynthetic rates. Being resistant to long lasting cold, wet, and dark conditions, Anthelia is particularly adapted to grow in the border zone along permanent snowpatches. Polytrichum is more productive and is therefore capable of competing successfully at sites which are less extreme and therefore accessible for higher plants.

Shifts in the optimal temperature for nocturnal CO2 uptake caused by changes in growth temperature for cacti and agaves

When the day/night air temperatures were raised from 10°C/10°C to 30°C/30°C, the optimal tempearture for nocturnal CO2 uptake by six species of cacti and three species of agave shifted from an average of 12°C to an average of 20°C. The maximum rate of CO2 uptake was higher for Agave americana at the higher ambient temperature, lower for A. deserti, and much lower for A. utahensis, consistent with the relative mean temperatures of their native habitats. For the cactus Coryphantha vivipara, which had the greatest temperature shift observed (13°C), the halftime was 8 days for the upward shift and 4 days for the downward shift. The halftimes for the comparable shifts averaged 1.6 days for three other species of cacti and less than 1 day for two agave species. The shifts in the optimal temperature for nocturnal CO2 uptake were in response to changes in nighttime temperature, at least for C. vivipara, and reflected temperature responses of both the stomates and the chlorenchyma.

Effect of high-light treatments in inducing photoinhibition of photosynthesis in intact leaves of low-light grown Phaseolus vulgaris and Lastreopsis microsora

Photoinhibition studies, using gas-exchange techniques, were conducted with leaflets of Phaseolus vulgaris L. plants that were grown under low photonfluence rates. Comparative measurements were made on attached, intact leaflets and in subsequently isolated chloroplasts. Photoinhibition studies were also conducted with attached fronds of the deep-shade fern Lastreopsis microsora (Endl.) Tindale. Leaflets of lowlight-grown Phaseolus vulgaris and fronds of the shade fern were found to be subject to similar photoinhibition when exposed to photon-fluence rates in excess of those at which they were grown. Photoinhibition following exposure to a photon fluence-rate approximating full sunlight is manifested as a reduction in the capacity for both light-saturated and light-limited carbon uptake and is reflected at the chloroplast level as substantial inhibition of electron flow through photosystem (PS) II, with little effect on PS I. The extent of photoinhibition is markedly dependent on the length of exposure to a high-light regime and on the actual photon-fluence rate maintained during treatment. A greater degree of photoinhibition is evident if carbon metabolism is prevented by the removal of CO2 than when maximum rates of CO2 uptake prevail throughout the exposure to a high photonfluence rate. Apparently a certain level of CO2 turnover is beneficial in providing a sink for photochemically generated energy. When leaf material is exposed to photon-fluence rates well in excess of the rate present during growth apparently the potentials of the various biophysical and photochemical means of dissipating excitation energy are exceeded and photoinhibition of photosynthesis results.

Mesophyll cell properties for some C3 and C4 species with high photosynthetic rates

AbstractLeaves of twelve C3 species and six C4 species were examined to understand better the relationship between mesophyll cell properties and the generally high photosynthetic rates of these plants. The CO2 diffusion conductance expressed per unit mesophyll cell surface area (gCO2cell) cell was determined using measurements of the net rate of CO2 uptake, water vapor conductance, and the ratio of mesophyll cell surface area to leaf surface area (Ames/A). Ames/A averaged 31 for the C3 species and 16 for the C4 species. For the C3 species gCO2cell ranged from 0.12 to 0.32 mm s‐1, and for the C4 species it ranged from 0.55 to 1.5 mm s‐1, exceeding a previously predicted maximum of 0.5 mm s‐1. Although the C3 species Cammissonia claviformis did not have the highest gCO2cell, the combination of the highest Ames and highest stomatal conductance resulted in this species having the greatest maximum rate of CO2 uptake in low oxygen, 93 μmol m‐2 s‐1 (147 mg dm‐2 h‐1). The high gCO2cell of the C4 species Amaranthus retroflexus (1.5 mm s‐1) was in part attributable to its thin cell wall (72 nm thick).

Moisture content and CO2 exchange of lichens : I. Influence of temperature on moisture-dependent net photosynthesis and dark respiration in Ramalina maciformis.

Net photosynthesis (10 klx light intensity, 150 μE m-2 s-1 PAR) and dark respiration of the lichen Ramalina maciformis at different temperatures are measured in relation to thallus water content. Both first increase with increasing hydration. Dark respiration then remains constant with increased water content until thallus saturation. In contrast, a further increase in water content leads to a depression of net photosynthesis, as shown in previous studies, after a maximum of CO2 uptake has been attained. However, the extent of this depression depends strongly on temperature. In saturated thalli (160% water content in relation to lichen dry weight) the depression amounts to about 15% and 63% of the maximum unsaturated rate at 5°C and 25°C thallus temperature, respectively. The moisture compensation-point of net photosynthesis is also decisively determined by temperature (for 0°C at 20% water content; for 25°C at 15%), and the water content that allows maximum rates of CO2 uptake (for 0°C at 80%; for 25°C at less than 40% water content). An electrical analogue of CO2 exchange in a lichen thallus is presented, and it is suggested that the experimental results may be interpreted in terms of temperature-dependent CO2 diffusion resistances in imbibed lichen thalli.

Internal Leaf Area and Cellular CO2 Resistance: Photosynthetic Implications of Variations with Growth Conditions and Plant Species

AbstractThe influences of illumination, temperature, and soil water potential during development on leaf thickness, mesophyll cell wall area per unit leaf area (Ames/A), and the cellular CO2, resistance expressed on a mesophyll cell wall area basis (rCO2cell,) were examined for Plectranthus parviflorus Henckel. Although the ranges of all three growth conditions caused at least 9‐fold variations in the leaf biomass produced in 4 weeks, only the illumination had a major effect on internal leaf morphology, e.g. the thickness went from 279 to 831 μm and Ames/A from 10.5 to 34.8 as the photosynthetically active radiation was raised from 3 to 53 nEinsteins cm−2 s−1, while rCO2cell remained close to 154 s cm−1. Variations in the growth temperature, soil water potential, and the nutritional status of the plant, affected photosynthesis mainly by changes in rCO2cell.To compare the influence of internal leaf area on photosynthesis for other plants, especially those with low Ames/A values, the maximum rates of CO2 uptake at light saturation and photosynthetically optimal temperatures were also determined for a moss, Mnium ciliare (C. Muell.) Schimp., and two ferns, Adiantum decorum Moore and Alsophila australe R. Br. As Ames/A went from 2.00 for the moss to 3.8, 7.5, 11.7, and 20.8 for the fens, the illumination at light saturation and the maximum rate of photosynthesis both progressively increased. The cellular CO2 resistance, which theoretically might have a lower limit of 20 s cm−1, ranged from 85 to 190 s cm−1.

Net Carbon Exchange Rates of Field-Grown Crops in Relation to Irradiance and Dry Weight Accumulation

A range of summer and winter vegetable crops has been grown under favourable conditions, and the relation between net carbon exchange and irradiance determined at several stages of growth, using large field assimilation chambers and semi-closed gas analysis systems. For all crops, leaf area index was an important determinant of net carbon exchange rate during early stages of growth, and rates increased markedly from day to day during the period of rapid vegetative development. Except for cauliflower and cucumber, for which leaf area ceased to be so important as the crops developed, maximum rates of CO2 uptake were achieved at maximum leaf areas, and were in excess of 90 mg CO2 dm-2 (ground area) h-1 for sweet corn, about 64 for rockmelon, 50 for broad bean, cauliflower and cabbage, and 45 mg dm-2 h-1 for cucumber. For both rockmelon and cucumber, net carbon exchange rates in the morning were often greater than those at the same irradiances in the afternoon, which suggests partial stomatal closure at about midday. In all crops, variations in temperature of up to 10°C either side of the ambient temperature, imposed at any time during the day, generally had little effect on negative CO2 flux (uptake), but positive CO2 flux at night was strongly influenced by temperature. In cucurbits uptake was reduced at temperatures above 35°C, but rapidly recovered when the temperature was again lowered. High growth rates were achieved by all crops for most months of the year, and the efficiency of utilization of incident energy was also high compared with other reported values. However, growth rates were not related to the maximum rates of CO2 uptake in the day, for net carbon exchange over each 24 h depended also on factors such as the ratio between day- and nightlength and night temperatures.

Photosynthesis in the Rose; Effect of Light Intensity, Water Potential and Leaf Age1

Abstract The net rate of 14CO2 uptake was determined on individual leaves of Rosa hybrida, cv. ‘Forever Yours’, budded on Rosa manetti and grown in gravel. Rose leaves were found to reach an average peak of 11 mg CO2 dm−2 hr−1 about 6 days after the red color disappeared on the leaf underside, or 32 days after harvesting the previous flower on the parent cane. Thereafter, CO2 uptake declined during 14 days to 5 to 6 mg CO2 dm−2 hr−1. At ambient CO2 concentrations of 500 ppm, the maximum net uptake was near 3400 ft-c. However, internal plant water potential influenced the CO2 uptake by reducing it at each increase of radiant energy. This resulted in light saturation at lower energies the lower the plant water potential. Radiant energy affected both net CO2 uptake and water potential. Wilting was generally observed to occur at about –13 bars, and maximum rates of CO2 uptake were found at potentials of –8 bars or higher, over a range of 350 to 450 microeinsteins, or 3000 to 3500 ft-c.

Eco-physiological investigations on wild and cultivated plants in the Negev Desert : II. The influence of climatic factors on carbon dioxide exchange and transpiration at the end of the dry period

The influence of climatic factors on net photosynthesis, dark respiration and transpiration was investigated in the Negev Desert at the end of the dry summer period when plant water stress was at a maximum. Species studied included: dominant species of the natural vegetation (Artemisia herba-alba, Hammada scoparia, Noaea mucronata, Reaumuria negevensis, Salsola inermis, Zygophyllum dumosum), cultivated plants receiving rainfall and run-off water during the winter season in the run-off farm Avdat (Prunus armeniaca, Vitis vinifera), and irrigated cultivated plants receiving additional water during the summer season (Citrullus colocynthis, Datura metel). 1. Light saturation of net photosynthesis was reached at 60-90 klx conforming to the high solar radiation intensities of the desert. 2. Maximum rates of CO2 uptake per unit of dry weight for the irrigated mesomorphic plants was ten times that of the wild plants. However, in comparison to the other species, maximal rates of CO2 uptake for wild plants were higher when calculated on a leaf area basis than when represented on a dry weight basis. Maximum rates of net photosynthesis per unit chlorophyll content for some of the wild plants (Salsola and Noaea) were comparable to those of the cultivated Vitis and irrigated Citrullus and Datura, Hammada exhibited even higher rates than Prunus. This demonstrates the great photosynthetic capacity of the wild plants even at the end of the dry season. 3. The upper temperature compensation point for net photosynthesis of the wild plants was unusually high as an adaptation to the temperatures of the habitat. Compensation points higher than 49°C exceed the maxima known so far for other flowering species. Maximum rates of net photosynthesis of Hammada were measured when the temperature of the photosynthetic organs was 37°C; at 49°C photosynthesis was only reduced by 50%. 4. Leaf temperature affects plant gas exchange by influencing stomatal aperture. Diffusion resistance of leaves to water vapour was reduced at low temperatures and increased at high temperatures. Reduction of net photosynthesis and transpiration of desert plants at midday may, therefore, be the result of temperature-induced stomatal closure. The possible influence of peristomatal transpiration on stomatal aperture is also discussed. Peristomatal transpiration is directly related to the vapour pressure gradient between the leaf mesophyll and the ambient air which increases with increasing temperatures. 5. Diffusion resistance to water vapour was reduced at high temperatures approaching the limits of heat resistance, due to increased stomatal aperture. This resulted in greater transpirational cooling. 6. Under conditions of increased leaf water stress, diffusion resistance increased, either by sudden stomatal closure at specific threshold values of water stress or through a continuous increase in resistance. This increased resistance is coupled with decreases in transpiration and photosynthesis. 7. In several plant species increased diffusion resistance during the course of the day caused decreased transpiration without a corresponding decrease in photosynthesis. Under these conditions, the ratio of CO2 uptake to transpiration became more favourable as the day progressed. The possibility that this favourable gas exchange response is the result of an increased mesophyll resistance to water vapour loss is discussed.