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.     

Effects of phosphorus deficiency on the photosynthesis and respiration of leaves of sugar beet

Phosphorus deficiency was induced in sugar beet plants (Beta vulgaris L. var. F5855441), cultured hydroponically under standardized environmental conditions, by removal of phosphorus from the nutrient supply at the ten leaf stage 28 days after germination. CO₂ and water vapor exchange rates of individual attached leaves were determined at intervals after P cutoff. Leaves grown with an adequate nutrient supply attained net rates of photosynthetic CO₂ fixation of 125 ng CO₂ cm⁻² sec⁻¹ at saturating irradiance, 25 C, and an ambient CO₂ concentration of about 250 μl l⁻¹. After P cutoff, leaf phosphorus concentrations decreased as did net rates of photosynthetic CO₂ uptake, photorespiratory evolution of CO₂ into CO₂-free air, and dark respiration, so that 30 days after cutoff these rates were about one-third of the control rates. The decrease in photosynthetic rates during the first 15 days after cutoff was associated with increased mesophyll resistance (r_m) which increased from 2.4 to 4.9 sec cm⁻¹, while from 15 to 30 days there was an increase in leaf (mainly stomatal) diffusion resistance (r_l′) from 0.3 to 0.9 sec cm⁻¹, as well as further increases in r_m to 8.5 sec cm⁻¹. Leaf diffusion resistance (r_l′) was increased greatly by low P at low but not at high irradiance, r_l′ for plants at low P reaching values as high as 9 sec cm⁻¹.     

Glyphosate effects on carbon assimilation and gas exchange in sugar beet leaves

The mechanism responsible for the inhibition of net carbon exchange (NCE) which was reported previously (DR Geiger et al. 1986 Plant Physiol 82: 468-472) was investigated by applying glyphosate [N-(phosphonomethyl)glycine] to exporting leaves of sugar beet (Beta vulgaris L.). Leaf internal CO₂ concentration (C_i) remained constant despite decreases in stomatal conductance and NCE following glyphosate treatment, indicating that the cause of the inhibition was a slowing of carbon assimilation rather than decreased conductance of CO₂. Throughout a range of CO₂ concentrations, NCE rate at a given C_i declined gradually, with the time-series of response curves remaining parallel. Gas exchange measurements revealed that disruption of chloroplast carbon metabolism was an early and important factor in mediating these glyphosate effects, perhaps by slowing the rate of ribulose bisphosphate regeneration. An increase in the CO₂ compensation point accompanied the decrease in NCE and this increase was hastened by stepwise lowering of the ambient CO₂ concentration. Eventually the CO₂ compensation point approached the CO₂ level of air and the difference between internal and external CO₂ concentrations decreased. In control and in glyphosate-treated plants, both carbon assimilation and photorespiration at atmospheric CO₂ level were inhibited to a similar extent of air level of O₂. Maintaining leaves in low O₂ concentration did not prevent the decline in NCE rate.     

Correlation between the Carbon Isotope Discrimination in Leaf Starch and Sugars of C(3) Plants and the Ratio of Intercellular and Atmospheric Partial Pressures of Carbon Dioxide

Carbon isotope discrimination (Δ) was analyzed in leaf starch and soluble sugars, which represent most of the recently fixed carbon. Plants of three C₃ species (Populus nigra L. × P. deltoides Marsh., Gossypium hirsutum L. and Phaseolus vulgaris L.) were kept in the dark for 24 hours to decrease contents of starch and sugar in leaves. Then gas exchange measurements were made with constant conditions for 8 hours, and subsequently starch and soluble sugars were extracted for analysis of carbon isotope composition. The ratio of intercellular, p_i, and atmospheric, p_a, partial pressures of CO₂, was calculated from gas exchange measurements, integrated over time and weighted by assimilation rate, for comparison with the carbon isotope ratios in soluble sugars and starch. Carbon isotope discrimination in soluble sugars correlated strongly (r = 0.93) with p_i/p_a in all species, as did Δ in leaf starch (r = 0.84). Starch was found to contain significantly more ¹³C than soluble sugar, and possible explanations are discussed. The strong correlation found between Δ and p_i/p_a suggests that carbon isotope analysis in leaf starch and soluble sugars may be used for monitoring, indirectly, the average of p_i/p_a weighted by CO₂ assimilation rate, over a day. Because p_i/p_a has a negative correlation with transpiration efficiency (mol CO₂/mol H₂O) of isolated plants, Δ in starch and sugars may be used to predict differences in this efficiency. This new method may be useful in ecophysiological studies and in selection for improved transpiration efficiency in breeding programs for C₃ species.     

Effects of magnesium deficiency on the photosynthesis and respiration of leaves of sugar beet

The effects of Mg deficiency on the photosynthesis and respiration of sugar beets (Beta vulgaris L. cv. F58-554H1) were studied by withholding Mg from the culture solution and by following changes in CO₂ and water vapor exchange of attached leaves. Leaf blade Mg concentration decreased from about 1200 to less than 200 meq kg⁻¹ dry matter without change in the rate of photosynthetic CO₂ uptake per unit leaf area, while from 200 to 50 meq kg⁻¹ the rate decreased to one-third. Rates of photorespiratory evolution of CO₂ into CO₂-free air responded to Mg like those of photosynthetic CO₂ uptake, the rates decreasing to one-half, below 200 meq kg⁻¹. Respiratory CO₂ evolution in the dark increased almost 2-fold in low Mg leaves. Magnesium deficiency had less effect on leaf (mainly stomatal) diffusion resistance (r₁) than on mesophyll resistance (r_m); in Mg-deficient plants r_m increased from 2.9 to 7.1 sec cm⁻¹, whereas r₁ became significantly greater than the control value only in the most severe instances of Mg deficiency.     

Effect of Errors in Measuring Leaf Temperature and Ambient Gas Concentration on Calculated Resistances to CO(2) and Water Vapor Exchanges in Plant Leaves

Errors as small as 1 C in the measurement of leaf temperature (T_leaf) are shown to cause significant changes in the estimated value of the stomatal resistance (expressed in terms of total resistance to water vapor transfer, ∑rH₂O). The effect increases as T_leaf increases and as ambient relative humidity increases, if other conditions are maintained constant. The effect on the key CO₂ exchange parameter, the intracellular (or mesophyll) resistance, r_int, tends to be small under open stomata conditions but increases rapidly as stomatal closure occurs, particularly if the true value of r_int is relatively small.     

The Application of Transcutaneous CO2 Pressure Monitoring in the Anesthesia of Obese Patients Undergoing Laparoscopic Bariatric Surgery

To investigate the correlation and accuracy of transcutaneous carbon dioxide partial pressure (P_TCCO₂) with regard to arterial carbon dioxide partial pressure (P_aCO₂) in severe obese patients undergoing laparoscopic bariatric surgery. Twenty-one patients with BMI>35 kg/m² were enrolled in our study. Their P_aCO₂, end-tidal carbon dioxide partial pressure (P_etCO₂), as well as P_TCCO₂ values were measured at before pneumoperitoneum and 30 min, 60 min, 120 min after pneumoperitoneum respectively. Then the differences between each pair of values (P_etCO₂–P_aCO₂) and_. (P_TCCO₂–P_aCO₂₎ were calculated. Bland–Altman method, correlation and regression analysis, as well as exact probability method and two way contingency table were employed for the data analysis. 21 adults (aged 19–54 yr, mean 29, SD 9 yr; weight 86–160 kg, mean119.3, SD 22.1 kg; BMI 35.3–51.1 kg/m², mean 42.1,SD 5.4 kg/m²) were finally included in this study. One patient was eliminated due to the use of vaso-excitor material phenylephrine during anesthesia induction. Eighty-four sample sets were obtained. The average P_aCO₂–P_TCCO₂ difference was 0.9±1.3 mmHg (mean±SD). And the average P_aCO₂–P_etCO₂ difference was 10.3±2.3 mmHg (mean±SD). The linear regression equation of P_aCO₂–P_etCO₂ is P_etCO₂ = 11.58+0.57×P_aCO₂ (r² = 0.64, P<0.01), whereas the one of P_aCO₂–P_TCCO₂ is P_TCCO₂ = 0.60+0.97×P_aCO₂ (r² = 0.89). The LOA (limits of agreement) of 95% average P_aCO₂–P_etCO₂ difference is 10.3±4.6 mmHg (mean±1.96 SD), while the LOA of 95% average P_aCO₂–P_TCCO2 difference is 0.9±2.6 mmHg (mean±1.96 SD). In conclusion, transcutaneous carbon dioxide monitoring provides a better estimate of PaCO₂ than P_etCO₂ in severe obese patients undergoing laparoscopic bariatric surgery.     

Diffusive resistances at, and transpiration rates from leaves in situ within the vegetative canopy of a corn crop

At several heights and times of day within a crop of Zea mays, internal leaf diffusion resistance (r_i) and external boundary layer diffusion resistance (r_a) were evaluated by measuring the temperature of a transpiring and a non-transpiring leaf (simulated by covering both sides of a normal leaf with strips of poly-ethylene tape), and by measuring the immediate air temperature, humidity and windspeed.

Both r_a and r_i increased with depth into the crop. However, r_a generally was less than 10% of r_i.

Profiles of latent-heat flux density and source intensity of transpiration showed that transpiration corresponded roughly to foliage distribution (with an upward shift) and were not similar to the profile of radiation absorption.

The data were compared with heat budget data. The 2 approaches yielded quite similar height distributions of transpiration per unit leaf area and total transpiration resistance.

The total crop resistance to transpiration was computed as 0.027 min cm⁻¹. This compares to Monteith's values of 0.017 to 0.040 min cm⁻¹ for beans (Phaseolus vulgaris L.), and Linacre's values of 0.015 to 0.020 min cm⁻¹ for turf.

     

Low temperature stress modifies the photochemical efficiency of a tropical tree species <Emphasis Type="Italic">Hevea brasiliensis</Emphasis>: effects of varying concentration of CO<Subscript>2</Subscript> and photon flux density

Two clones of Hevea brasiliensis (RRII 105 and PB 235) were grown for one year in two distinct agroclimatic locations (warmer and colder, W and C) in peninsular India. We simultaneously measured gas exchange and chlorophyll (Chl) fluorescence on fully mature intact leaves at different photosynthetic photon flux densities (PPFDs) and ambient CO₂ concentrations (C _a) and at constant ambient O₂ concentration (21 %). Net photosynthetic rate (P _N), apparent quantum yield for CO₂ assimilation (Φ_c), in vivo carboxylation efficiency (CE), and photosystem 2 quantum yield (Φ_PS2) were low in plants grown in C climate and these reductions were more predominant in RRII 105 than in PB 235 which was also reflected in their growth. We estimated in these clones the partitioning of photosynthetic electrons between CO₂ reduction (J_A) and processes other than CO₂ reduction (J^*) at low and high PPFDs and C _a. At high C _a (700 µmol mol⁻¹) most of the photosynthetic electrons were used for CO₂ assimilation and negligible amount went for other processes when PPFD was low (200–300 µmol m⁻² s⁻¹) both in the C and W climates. But at high PPFD (900-1 100 µmol m⁻² s⁻¹), J^* was appreciably high even at a high C _a. Hence at normal ambient C _a and high irradiance, electrons can be generated in the photosynthetic apparatus far in excess of what can be safely utilised for photosynthetic CO₂ reduction. However, at high C _a there was increased diversion of electrons to photosynthetic CO₂ reduction which resulted in improved photosynthetic parameters even in plants grown in C climate.     

Photosynthesis at low water potentials in sunflower: lack of photoinhibitory effects

The losses in chloroplast capacity to fix CO₂ when photosynthesis is reduced at low leaf water potential (ψ₁) have been proposed to result from photoinhibition. We investigated this possibility in soil-grown sunflower (Helianthus annuus L. cv IS894) using gas exchange techniques to measure directly the influence of light during dehydration on the in situ chloroplast capacity to fix CO₂. The quantum yield for CO₂ fixation as well as the rate of light- and CO₂-saturated photosynthesis were strongly inhibited at low ψ₁. The extent of inhibition was the same whether the leaves were exposed to high or to low light during dehydration. When intercellular partial pressures of CO₂ were decreased to the compensation point, which was lower than the partial pressures resulting from stomatal closure, the inhibition of the quantum yield was also unaffected. Photoinhibition could be observed only after high light exposures were imposed under nonphysiological low CO₂ and O₂ where both photosynthesis and photorespiration were suppressed. The experiments are the first to test whether gas exchange at low ψ₁ is affected by potentially photoinhibitory conditions and show that the loss in chloroplast capacity to fix CO₂ was entirely the result of a direct effect of water availability on chloroplast function and not photoinhibition.     

Carbon Isotope Discrimination in Coffee Genotypes Grown under Limited Water Supply

Photosynthetic gas exchange, plant-water relations characteristics, and stable carbon isotope discrimination (Δ) were evaluated for five Coffea arabica L. genotypes growing under two soil moisture regimes in the field. The Δ of leaf tissue was strongly correlated (r = −0.95) with inherent water use efficiency (ratio of assimilation to stomatal conductance; A/g). The variation in inherent water use efficiency (WUE) among genotypes was 30% for plants irrigated weekly. The higher WUE exhibited by some of these plants resulted from reduced g rather than increased photosynthetic capacity at a given g. Withholding irrigation for 1 month caused Δ to decline substantially in expanding leaf tissue of all genotypes. A strong correlation (r = 0.92) was found between Δ and plant hydraulic efficiency estimated as the ratio of g to the diurnal range in leaf water potential (Ψ_l). The Δ values for plants irrigated weekly adequately predicted drought-induced changes in Δ (r = 0.99) and midday Ψ_l (r = 0.95). The results indicated that Δ might be used to evaluate several aspects of plant performance and response to specific environmental conditions, once suitable background physiological data have been gathered.     

Effects of potassium deficiency on the photosynthesis and respiration of leaves of sugar beet

Sugar beet plants (Beta vulgaris L. var. F5855441) were germinated and cultured under standardized environmental conditions for 28 days. Potassium deficiency was then induced by withholding K from the culture solution. Changes in CO₂ and water vapor exchange rates and surface temperatures of individual attached leaves were measured with time after K cut-off, along with changes in the concentrations of the leaf minerals K, Na, Ca, Mg, Fe, Mn, Cu, and Zn. During the 1st week after K cut-off the concentration of Na in the leaf blade increased from 200 to 1000 milliequivalents per kilogram dry matter while K decreased from 1500 to 300 milliequivalents per kilogram. During the subsequent 2 weeks, both Na and K concentrations decreased. The concentrations of other leaf minerals, except Mn, were little affected by K cut-off. Photosynthetic CO₂ uptake per unit area decreased linearly with time after cut-off and attained one-third of the control rate after 21 days. Low K apparently decreased photosynthesis through an increase in mesophyll resistance to CO₂ (r_m) from 2.8 to 5.3 seconds per centimeter in 21 days. Leaf (mainly stomatal) diffusion resistance (r′₁) increased only slowly during the first 15 days from 0.3 to 0.5 second per centimeter, eventually reaching 1.6 seconds per centimeter at 21 days. Low K progressively decreased the photorespiratory evolution of CO₂ into CO₂-free air, but steadily increased the rate of CO₂ evolution in dark.     

A stomatal optimization theory to describe the effects of atmospheric CO2 on leaf photosynthesis and transpiration

Background and Aims

Global climate models predict decreases in leaf stomatal conductance and transpiration due to increases in atmospheric CO₂. The consequences of these reductions are increases in soil moisture availability and continental scale run-off at decadal time-scales. Thus, a theory explaining the differential sensitivity of stomata to changing atmospheric CO₂ and other environmental conditions must be identified. Here, these responses are investigated using optimality theory applied to stomatal conductance.

Methods

An analytical model for stomatal conductance is proposed based on: (a) Fickian mass transfer of CO₂ and H₂O through stomata; (b) a biochemical photosynthesis model that relates intercellular CO₂ to net photosynthesis; and (c) a stomatal model based on optimization for maximizing carbon gains when water losses represent a cost. Comparisons between the optimization-based model and empirical relationships widely used in climate models were made using an extensive gas exchange dataset collected in a maturing pine (Pinus taeda) forest under ambient and enriched atmospheric CO₂.

Key Results and Conclusion

In this interpretation, it is proposed that an individual leaf optimally and autonomously regulates stomatal opening on short-term (approx. 10-min time-scale) rather than on daily or longer time-scales. The derived equations are analytical with explicit expressions for conductance, photosynthesis and intercellular CO₂, thereby making the approach useful for climate models. Using a gas exchange dataset collected in a pine forest, it is shown that (a) the cost of unit water loss λ (a measure of marginal water-use efficiency) increases with atmospheric CO₂; (b) the new formulation correctly predicts the condition under which CO₂-enriched atmosphere will cause increasing assimilation and decreasing stomatal conductance.     

A study on the relationship between leaf conductance,CO2 concentration and carboxylation rate in various species

Leaf conductance g_L is strongly influenced by environmental factors like CO₂, irradiance and air humidity. According to Ball et al. (1987), g_L is correlated with an index calculated as the product of net CO₂ exchange rate A and ambient water vapour concentration W_a, divided by ambient CO₂ concentration c_a. However, this empirical model does not apply to high values of g_L observed at c_a below CO₂ compensation concentration . Therefore, we applied modified indices in which A is replaced by estimates for the rate of carboxylation. Such estimates, P₁ and P₂, were determined by adding to A the quotient of and the sum of gas phase resistance r_g and intracellular resistance for CO₂ exchange r_i, P₁ = A+/(r_g + r_i), or the quotient of and r_i, P₂ = A + /r_i. If P₂ is chosen, c_a in the Ball index has to be replaced by the intercellular CO₂ concentration c_i. By using the modified indices P₁·W_a/c_a and P₂·W_a/c_i, we analysed data from the C₃ species Nicotiana tabacum and Nicotiana plumbaginifolia, the C₃–C₄ intermediate species Diplotaxis tenuifolia, and the C₄ species Zea mays. The data were collected at widely varying levels of irradiance and CO₂ concentration. For all species uniform relationships between g_L and the new indices were found for the whole range of CO₂ concentrations below and above . Correlations between g_L and P₁·W_a/c_a were closer than those between g_L and P₂·W_a/c_i because P₁/c_a implicitly contains g_L. Highly significant correlations were also obtained for the relationships between g_L and the ratios P₁/c_a and P₂/c_i.     

The calibration of thermocouples for leaf temperature measurements in gas exchange cuvettes

Summary A thermocouple shows a high accuracy in reading temperatures, but it does not always give a true value for the temperature of the leaf against which it is pressed on. Temperature differences between the leaf and the air moving around it cause deviations of the shown temperature from the actual leaf temperature. A method is described to calibrate thermocouples inside gas exchange cuvettes without obstructing the movement of the air around the leaf, so that the heat exchange between thermocouple and air is taken into account. The reading of the leaf temperature in a steady-state porometer was checked by this method at various temperatures of ambient air (T _a ) and of the leaf (T ₁ ) and was found to give an average value of T _a and T ₁ . The effect of incorrect estimation of the leaf temperature on computed diffusive resistances on H₂O (r ^w ) and CO₂ (r ^c ) and intercellular CO₂ partial pressures (p _i ^c ) is discussed.Abbreviations A net CO₂ uptake - E evaporative transpiration - R correlation coefficient - T Temperature - a heat-transfer coefficient - b ratio of heat-transfer coefficients - q heat transfer - p ^c CO₂ partial pressure - p ^w H₂O partial pressure - r ^c diffusive resistance on CO₂ - r ^w diff. resistance on H₂O - a ambient - i intercellular - l leaf - p porometer - s leaf surface (=boundary layer+stomata) - t thermocouple; 1, 2: number of cuvette     

Correlation between the Maintenance of Photosynthesis and in Situ Protoplast Volume at Low Water Potentials in Droughted Wheat

Studies were undertaken to examine the relationship between water deficit effects on photosynthesis and the extent of protoplast volume reduction which occurs in leaves at low water potential (Ψ_w). This relationship was monitored in two cultivars (`Condor' and `Capelle Desprez') of cultivated wheat (Triticum aestivum) that differed in sensitivity to drought, and in a wild relative of cultivated wheat (Triticum kotschyi) that has been previously found to be `drought resistant.' When subjected to periods of water stress, Condor and T. kotschyi plants underwent osmotic adjustment; Capelle plants did not. Photosynthetic capacity was maintained to different extents in the three genotypes as leaf Ψ_w declined during stress; Capelle plants were most severely affected. Calculations of internal leaf [CO₂] and stomatal conductance from gas exchange measurements indicated that differences in photosynthetic inhibition at low Ψ_w among the genotypes were primarily due to nonstomatal effects. The extent of protoplast volume reduction that occurred in leaves at low Ψ_w was also found to be different in the three genotypes; maintenance of protoplast volume and photosynthetic capacity in stressed plants of the genotypes appeared to be correlated. When the extent of water stress-induced inhibition of photosynthesis was plotted as a function of declining protoplast volume, this relationship appeared identical for the three genotypes. It was concluded that there is a correlative association between protoplast volume and photosynthetic capacity in leaves of wheat plants subjected to periods of water stress.     

Plant Water Use Efficiency over Geological Time – Evolution of Leaf Stomata Configurations Affecting Plant Gas Exchange

Plant gas exchange is a key process shaping global hydrological and carbon cycles and is often characterized by plant water use efficiency (WUE - the ratio of CO₂ gain to water vapor loss). Plant fossil record suggests that plant adaptation to changing atmospheric CO₂ involved correlated evolution of stomata density (d) and size (s), and related maximal aperture, a_max. We interpreted the fossil record of s and d correlated evolution during the Phanerozoic to quantify impacts on gas conductance affecting plant transpiration, E, and CO₂ uptake, A, independently, and consequently, on plant WUE. A shift in stomata configuration from large s-low d to small s-high d in response to decreasing atmospheric CO₂ resulted in large changes in plant gas exchange characteristics. The relationships between gas conductance, g_ws, A and E and maximal relative transpiring leaf area, (a_max⋅d), exhibited hysteretic-like behavior. The new WUE trend derived from independent estimates of A and E differs from established WUE-CO₂ trends for atmospheric CO₂ concentrations exceeding 1,200 ppm. In contrast with a nearly-linear decrease in WUE with decreasing CO₂ obtained by standard methods, the newly estimated WUE trend exhibits remarkably stable values for an extended geologic period during which atmospheric CO₂ dropped from 3,500 to 1,200 ppm. Pending additional tests, the findings may affect projected impacts of increased atmospheric CO₂ on components of the global hydrological cycle.     

Resistance Analysis of Nocturnal Carbon Dioxide Uptake by a Crassulacean Acid Metabolism Succulent, Agave deserti

Nocturnal CO₂ uptake by a Crassulacean acid metabolism succulent, Agave deserti Engelm. (Agavaceae), was measured so that the resistance properties of the mesophyll chlorenchyma cells and their CO₂ concentrations could be determined. Two equivalents of acidity were produced at night per mole of CO₂ taken up. The nocturnal CO₂ uptake became light-saturated at 3.5 mEinsteins cm⁻² of photosynthetically active radiation (400-700 nm) incident during the preceding day; at least 46 Einsteins were required per mole of CO₂ fixed. Variations in the daytime leaf temperature between 20 and 37 C had little effect on nocturnal CO₂ uptake. After the first few hours in the dark, the leaf liquid phase CO₂ resistance (r^liq_CO2) and the CO₂ concentration in the chlorenchyma cells (cⁱ_CO2) both increased, the latter usually reaching the ambient external CO₂ level at the end of the dark period. Increasing the leaf surface temperature above 15 C at night markedly increased the stomatal resistance, r^liq_CO2, and cⁱ_CO2.

The minimum r^liq_CO2 at night was about 1.6 seconds cm⁻¹. Based on the ratio of chlorenchyma surface area to total leaf surface area of 82, this r^liq_CO2 corresponded to a minimum cellular resistance of approximately 130 seconds cm⁻¹, comparable to values for mesophyll cells of C₃ plants. The contribution of the carboxylation reaction and/or other biochemical steps to r^liq_CO2 may increase appreciably as the nighttime temperature shifts a few degrees from the optimum or after a few hours in the dark, both of which caused large increases in r^liq_CO2. This necessitates a large internal leaf area for CO₂ diffusion into the chlorenchyma to support moderate nocturnal CO₂ uptake rates by these succulent leaves.

     

Estimation of whole-plant resistance to gaseous exchange independent of leaf temperature measurement

For studies into the uptake of mercury vapor by wheat (Triticum aestivum), a simple theory and plant chamber were employed to estimate total leaf resistance of whole plants to water vapor exchange. The estimates were independent of leaf temperature, for which mean values were indirectly determined. The approach involved the measurement, at steady-state conditions, of the net change in water vapor flux per unit of leaf surface (Δq_v) in response to a small induced change in absolute humidity (ΔC_a). Assuming that total leaf resistance (r_l) was constant and that change in leaf temperature (T_l) was negligible, total leaf resistance was calculated from the equation, [Formula: see text]     

Does the threshold of transcutaneous partial pressure of carbon dioxide represent the respiratory compensation point or anaerobic threshold?

On reaching the respiratory compensation point (RCP) during rapidly increasing incremental exercise, the ratio of minute ventilation (V_E) to CO₂ output (VCO₂) rises, which coincides with changes of arterial partial pressure of carbon dioxide (P _aCO₂). Since P _aCO₂ changes can be monitored by transcutaneous partial pressure of carbon dioxide (PCO_2,tc) RCP may be estimated by PCO_2,tc measurement. Few available studies, however, have dealt with comparisons between PCO_2,tc threshold (T _AT) and lactic, ventilatory or gas exchange threshold (V _AT), and the results have been conflicting. This study was designed to examine whether this threshold represents RCP rather than V _AT. A group of 11 male athletes performed incremental excercise (25 W · min^–1) on a cycle ergometer. The PCO_2,tc at (44°C) was continuously measured. Gas exchange was computed breath-by-breath, and hyperaemized capillary blood for lactate concentration ([la^–]_b) and P _aCO₂ measurements was sampled each 2 min. The T _AT was determined at the deflection point of PCO_2,tc curve where PCO_2,tc began to decrease continuously. The V _AT and RCP were evaluated with VCO₂ compared with oxygen uptake (VO₂) and V_E compared with the VCO₂ method, respectively. The PCO_2,tc correlated with P _aCO₂ and end-tidal PCO₂. At T _AT, power output [P, 294 (SD 40) W], VO₂ [4.18 (SD 0.57)l · min^–1] and [la^–] [4.40 (SD 0.64) mmol · l^–1] were significantly higher than those at V _AT[P 242 (SD 26) W, VO₂ 3.56 (SD 0.53) l · min^–1 and [la^–]_b 3.52 (SD 0.75), mmol · l^–1 respectively], but close to those at RCP [P 289 (SD 37) W; VO₂ 3.97 (SD 0.43) l · min^– and [la^–]_b 4.19 (SD 0.62) mmol · l^–1, respectively]. Accordingly, linear correlation and regression analyses showed that P, VO₂ and [la^–]_b at T _AT were closer to those at RCP than at V _AT. In conclusion, the T _AT reflected the RCP rather than V _AT during rapidly increasing incremental exercise.