Recovery Of Gas Exchange Research Articles

Low temperature impact on photosynthetic parameters of coffee genotypes

The objective of this work was to evaluate photoprotective mechanisms related to low positive temperatures in Coffea canephora (Conilon clones 02 and 153) and C. arabica ('Catucaí' IPR 102) genotypes, involved in cold temperature tolerance. To accomplish this, one-year-old plants were successively submitted to: temperature decrease of 0.5ºC day-1, from 25/20ºC to 13/8ºC; a three-day chilling cycle at 13/4ºC; and a recovery period of 14 days (25/20ºC). During the experiment, leaf gas exchange, chlorophyll a fluorescence and leaf photosynthetic pigment content were evaluated. Total activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and ribulose-5-phosphate kinase (Ru5PK) were quantified to measure the activity of photosynthesis key enzymes. All genotypes showed low temperature sensitivity, but displayed diverse cold impact and recovery capabilities regarding the photosynthetic-related parameters studied. Catucaí IPR 102 cultivar showed better ability to cope with cold stress than the Conilon clones, especially Conilon 02, and had full recovery of leaf gas exchange, fluorescence parameters, enzymatic activity, and higher contents of the photoprotective pigments zeaxanthin and lutein.

Leaf hydraulics and drought stress: response, recovery and survivorship in four woody temperate plant species

Efficient conduction of water inside leaves is essential for leaf function, yet the hydraulic-mediated impact of drought on gas exchange remains poorly understood. Here we examine the decline and subsequent recovery of leaf water potential (Psi(leaf)), leaf hydraulic conductance (K(leaf)), and midday transpiration (E) in four temperate woody species exposed to controlled drought conditions ranging from mild to lethal. During drought the vulnerability of K(leaf) to declining Psi(leaf) varied greatly among the species sampled. Following drought, plants were rewatered and the rate of E and K(leaf) recovery was found to be strongly dependent on the severity of the drought imposed. Gas exchange recovery was strongly correlated with the relatively slow recovery of K(leaf) for three of the four species, indicating conformity to a hydraulic-stomatal limitation model of plant recovery. However, there was also a shift in the sensitivity of stomata to Psi(leaf) suggesting that the plant hormone abscisic acid may be involved in limiting the rate of stomatal reopening. The level of drought tolerance varied among the four species and was correlated with leaf hydraulic vulnerability. These results suggest that species-specific variation in hydraulic properties plays a fundamental role in steering the dynamic response of plants during recovery.

Drought‐induced hydraulic limitations constrain leaf gas exchange recovery after precipitation pulses in the C3 woody legume, Prosopis velutina

The hypothesis that drought intensity constrains the recovery of photosynthesis from drought was tested in the C(3) woody legume Prosopis velutina, and the mechanisms underlying this constraint examined. Hydraulic status and gas exchange were measured the day before a 39 mm precipitation pulse, and up to 7 d afterwards. The experiment was conducted under rainout shelters, established on contrasting soil textures and with different vegetation cover at the Santa Rita Experimental Range in southeastern Arizona, USA. Rates of photosynthesis and stomatal conductance after re-watering, as well as the number of days necessary for photosynthesis to recover after re-watering, were negatively correlated with predawn water potential, a measure of drought intensity (R(2) = 0.83, 0.64 and 0.92, respectively). Photosynthetic recovery was incomplete when the vascular capacity for water transport had been severely impaired (percentage loss of hydraulic conductance > 80%) during the drought, which largely increased stomatal limitations. However, changes in biochemical capacity or in mesophyll conductance did not explain the observed pattern of photosynthesis recovery. Although the control that hydraulic limitations impose on photosynthesis recovery had been previously inferred, the first empirical test of this concept is reported here.

Hydraulic failure defines the recovery and point of death in water-stressed conifers.

This study combines existing hydraulic principles with recently developed methods for probing leaf hydraulic function to determine whether xylem physiology can explain the dynamic response of gas exchange both during drought and in the recovery phase after rewatering. Four conifer species from wet and dry forests were exposed to a range of water stresses by withholding water and then rewatering to observe the recovery process. During both phases midday transpiration and leaf water potential (Psileaf) were monitored. Stomatal responses to Psileaf were established for each species and these relationships used to evaluate whether the recovery of gas exchange after drought was limited by postembolism hydraulic repair in leaves. Furthermore, the timing of gas-exchange recovery was used to determine the maximum survivable water stress for each species and this index compared with data for both leaf and stem vulnerability to water-stress-induced dysfunction measured for each species. Recovery of gas exchange after water stress took between 1 and >100 d and during this period all species showed strong 1:1 conformity to a combined hydraulic-stomatal limitation model (r2 = 0.70 across all plants). Gas-exchange recovery time showed two distinct phases, a rapid overnight recovery in plants stressed to <50% loss of leaf hydraulic conductance (Kleaf) and a highly Psileaf-dependent phase in plants stressed to >50% loss of Kleaf. Maximum recoverable water stress (Psimin) corresponded to a 95% loss of Kleaf. Thus, we conclude that xylem hydraulics represents a direct limit to the drought tolerance of these conifer species.

Responses to changes in Ca2+ supply in two Mediterranean evergreens, Phillyrea latifolia and Pistacia lentiscus, during salinity stress and subsequent relief.

Changes in root-zone Ca(2+) concentration affect a plant's performance under high salinity, an issue poorly investigated for Mediterranean xerophytes, which may suffer from transient root-zone salinity stress in calcareous soils. It was hypothesized that high-Ca(2+) supply may affect differentially the response to salinity stress of species differing in their strategy of Na(+) allocation at organ level. Phillyrea latifolia and Pistacia lentiscus, which have been reported to greatly differ for Na(+) uptake and transport rates to the leaves, were studied. Methods In plants exposed to 0 mM or 200 mM NaCl and supplied with 2.0 mM or 8.0 mM Ca(2+), under 100 % solar irradiance, measurements were conducted of (a) gas exchange, PSII photochemistry and plant growth; (b) water and ionic relations; (c) the activity of superoxide dismutase and the lipid peroxidation; and (d) the concentration of individual polyphenols. Gas exchange and plant growth were also estimated during a period of relief from salinity stress. Key Results The performance of Pistacia lentiscus decreased to a significantly smaller degree than that of Phillyrea latifolia because of high salinity. Ameliorative effects of high-Ca(2+) supply were more evident in Phillyrea latifolia than in Pistacia lentiscus. High-Ca(2+) reduced steeply the Na(+) transport to the leaves in salt-treated Phillyrea latifolia, and allowed a faster recovery of gas exchange and growth rates as compared with low-Ca(2+) plants, during the period of relief from salinity. Salt-induced biochemical adjustments, mostly devoted to counter salt-induced oxidative damage, were greater in Phillyrea latifolia than in Pistacia lentiscus. An increased Ca(2+) : Na(+) ratio may be of greater benefit for Phillyrea latifolia than for Pistacia lentiscus, as in the former, adaptive mechanisms to high root-zone salinity are primarily devoted to restrict the accumulation of potentially toxic ions in sensitive shoot organs.

Photosynthetic performance in jack bean [Canavalia ensiformis (L.) D.C.] under drought and after rehydration

The effects of drought and rehydration on Canavalia ensiformis (L.) D.C. (jack bean) plants were evaluated using the following gas exchange parameters: net carbon assimilation rate (A), stomatal conductance (g s), Ci/Ca ratio and transpiration rate (E); chlorophyll a fluorescence: Fv/Fm and Fv/F0 ratio. The plants were cultivated under greenhouse conditions and after 30 days from the emergence, irrigation was suspended in the plants submitted to drought, to obtain the following predawn leaf water potential (psipd): -0.40 MPa (control), -1.00 MPa (moderate drought) and -2.30 Mpa (severe drought). Afterwards, the gas exchange and fluorescence analysis were initiated , and 24 h after rehydration the same analyses were repeated. The A, E, g s and Ci/Ca values decreased significantly under both drought treatments, without however changing the Fv/Fm and Fv/F0 values. The gas exchange parameters recovered after rehydration. It seems that drought affected photosynthesis by stomatal inhibition, as shown by the decreased gs and Ci/Ca values, besides the maintenance of PSII phtotochemical efficiency. The recovery of gas exchange after rehydration could be due to plant protection mechanisms.

Comparison of Isoprene Emission, Intercellular Isoprene Concentration and Photosynthetic Performance in Water‐Limited Oak (Quercus pubescens Willd. and Quercus robur L.) Saplings

Abstract: The influence of prolonged water limitation on leaf gas exchange, isoprene emission, isoprene synthase activities and intercellular isoprene concentrations was investigated under standard conditions (30 °C leaf temperature and 1000 μmol photons m‐2 s‐1 PPFD) in greenhouse experiments with five‐year‐old pubescent oak (Quercus pubescens Willd.) and four‐year‐old pedunculate oak (Quercus robur L.) saplings. Net assimilation rates proved to be highly sensitive to moderate drought in both oak species, and were virtually zero at water potentials (Ψpd) below ‐ 1.3 MPa in Q. robur and below ‐ 2.5 MPa in Q. pubescens. The response of stomatal conductance to water stress was slightly less distinct. Isoprene emission was much more resistant to drought and declined significantly only at Ψpd below ‐ 2 MPa in Q. robur and below ‐ 3.5 MPa in Q. pubescens. Even during the most severe water stress, isoprene emission of drought‐stressed saplings was still approximately one‐third of the control in Q. robur and one‐fifth in Q. pubescens. Isoprene synthase activities were virtually unaffected by drought stress. Re‐watering led to partial recovery of leaf gas exchange and isoprene emission. Intercellular isoprene concentrations were remarkably enhanced in water‐limited saplings of both oak species during the first half of the respective drought periods with maximum mean values up to ca. 16 μl l‐1 isoprene for Q. pubescens and ca. 11 μl l‐1 isoprene for pedunculate oak, supporting the hypothesis that isoprene serves as a short‐term thermoprotective agent in isoprene‐emitting plant species.

Gas exchange, water relations and osmotic adjustment in Phillyrea latifolia grown at various salinity concentrations.

Leaf gas exchange, water relations and osmotic adjustment were studied in hydroponically grown Phillyrea latifolia L. plants exposed to 5 weeks of salinity stress (0, 80, 160, 240 and 320 mM NaCl) followed by 5 weeks of treatment with half-strength Hoagland solution. Whole-plant relative growth rate and root/shoot and lateral/structural root ratios were also evaluated. Net CO2 assimilation rate, stomatal conductance and transpiration rate were markedly decreased by all of the salt treatments. Growth was also strongly depressed by all salt treatments, especially lateral root growth. Leaf water potential decreased soon after salinity stress was imposed, whereas there was a lag of several weeks before leaf osmotic potential decreased in response to the salt treatments. After 5 weeks of salinization, leaf turgor of salt-treated plants was similar to that of controls. Although Na+ + Cl- contributed little to the salt-induced changes in osmotic potential at full turgor (Psi(piFT)), the contributions of K+, mannitol (Man) and glucose (Glc) to Psi(piFT) markedly increased as external salinity increased. Salt accumulation was negligible in the youngest leaves, which mostly accumulated soluble carbohydrates and K+; in contrast, old leaves served as storage sinks for Na+ and Cl-. Photosynthetic performance of salt-treated plants fully recovered once salt was leached from the root zone, with the recovery rate depending on the severity of the salt stress previously experienced by the plants. Recovery of gas exchange occurred even though the leaves still had a salt load similar to that detected in leaves at the end of the 5-week salinity period, and had markedly lower concentrations of K+ and soluble carbohydrates than control leaves. We conclude that salt-induced water stress primarily controlled gas exchange of salt-treated P. latifolia leaves, whereas the salt load in the leaves did not cause irreversible damage to the photosynthetic apparatus.

Gas Exchange and Growth of Two Transplanted, Field-grown Tree Species in an Arid Climate

Gas exchange and growth of transplanted and nontransplanted, field-grown Norway maple (Acer platanoides L. `Schwedleri') and littleleaf linden (Tilia cordata Mill. `Greenspire') trees were investigated in an arid climate. In the spring of 1995, three trees of each species were moved with a tree spade to a new location within a field nursery and three nontransplanted trees were selected as controls. Predawn leaf water potential, morning-to-evening stomatal conductance and leaf temperature, leaf-to-air vapor pressure difference, midday stomatal conductance and photosynthetic rate, and growth data were collected over a 2-year period. After transplanting, weekly predawn leaf water potential indicated that transplanted trees were under greater water stress than were nontransplanted (control) trees. However, predawn leaf water potential of maple trees recovered to control levels 18 weeks after transplanting, while that of transplanted linden trees remained more negative than that of controls. In 1995, stomatal conductance and photosynthetic rates were lower throughout the day for transplanted trees. In 1996, gas exchange rates of transplanted maple trees recovered to near control levels while rates for transplanted linden trees did not. Sensitivity of stomata to leaf-to-air vapor pressure difference varied with species and with transplant treatment. Each year transplanted trees of both species had less apical growth than did control trees. Although gas exchange and apical growth of transplanted trees was reduced following transplanting, recovery of gas exchange to control rates differed with species.

The Effects of Intermittent Flooding on Seedlings of Three Forest Species

Under greenhouse conditions, seedlings of three forest species, baldcypress (Taxodium distichum), nuttall oak (Quercus nuttallii), and swamp chestnut oak (Quercus michauxii) were subjected to an intermittent flooding and subsequent physiological and growth responses to such conditions were evaluated. Baldcypress showed no significant reductions in stomatal conductance (g s) or net photosynthetic rate (P N) in response to flood pulses. In nuttall oak seedlings g s and P N were significantly decreased during periods of inundation, but recovered rapidly following drainage. In contrast, in swamp chestnut oak g s was reduced by 71.8 % while P N was reduced by 57.2 % compared to controls. Baldcypress displayed no significant changes in total mass while oak species had significantly lower leaf and total mass compared to their respective controls. Thus baldcypress and nuttall oak showed superior performance under frequent intermittent flooding regimes due to several factors including the ability for rapid recovery of gas exchange soon after soil was drained. In contrast, swamp chestnut oak seedlings failed to resume gas exchange functions after the removal of flooding.

Oxidative Stress and Diurnal Variation in Chilling Sensitivity of Tomato Seedlings

Diurnal variation in the chilling sensitivity of `Rutgers' tomato (Lycopersicon esculentum Mill.) seedlings was examined. Chilling sensitivity was highest in seedlings chilled at the end of the dark period, and these seedlings became more resistant to chilling injury on exposure to the light. The development of chilling tolerance in tomato seedlings was a response to light and not under the control of a circadian rhythm. The recovery of leaf gas exchange following chilling was faster in seedlings chilled at the end of the light period. Diurnal variation in chilling sensitivity was associated with changes in catalase and superoxide dismutase activities. An increase in catalase and superoxide dismutase activities was observed at the end of the light period. Catalase activity was significantly higher in all stages of chilling following the light period compared to those chilled after the end of the dark period. Forty-eight hours of 14 °C acclimation or pretreatment with hydrogen peroxide conferred increased chilling tolerance to tomato seedlings. Hydrogen peroxide-treated seedlings showed little evidence of a diurnal variation in chilling sensitivity. These results support a role for light and oxidative stress in conferring increased chilling tolerance to tomato seedlings.

Responses of three bottomland species with different flood tolerance capabilities to various flooding regimes

Seedlings of baldcypress (Taxodium distichum), nuttall oak (Quercus nuttalli), and cherrybark oak (Quercus falcata var.pagodaefolia) were subjected to four flooding treatments: control, continuously flooded, intermittently flooded, and partially flooded for 70 days in a greenhouse. The treatments imposed various durations and intensities of soil redox potential (Eh) conditions representing a range encountered by plants in their habitats. Morphological changes and gas exchange responses to the treatments differed among the study species. Rapid development of adventitious root and hypertrophied lenticels were observed in baldcypress and nuttall oak under all flooded treatments. Cherrybark oak had the highest percentage reduction in net photosynthesis ranging from 65–87%, whereas reductions in nuttall oak ranged between 35–68% and in baldcypress between 6–21% in response to various treatments. Recovery of gas exchange was noted in baldcypress but no significant recovery was found in oaks. The recovery in baldcypress contributed to the continued growth and biomass accumulation under various treatments. Little evidence of consistent changes in biomass allocation patterns in response to the treatments was found in baldcypress but total biomass decreased significantly under the continuously flooded treatment. In oaks, total biomass decreased significantly in all flooded treatments. The present findings demonstrated that physiological functions are adversely affected by low soil Eh conditions and the extent of such effects are dependent on the intensity and duration of soil reduction as well as the species' capability to respond to such conditions rapidly. Management plans concerned with regeneration of bottomland forested ecosystems should consider the species flood response capabilities at seedling stages as well as the timing, durations, and intensities of soil reduction at the specific site.

Carbon dioxide elimination measures resolution of experimental pulmonary embolus in dogs.

Patients with severe pulmonary embolism can suffer progressive hypercapnia refractory to supramaximal mechanical ventilation, and may require open-thoracic or transvenous emergency embolectomy in addition to anticoagulation and/or thrombolysis. The functional recovery of gas exchange would be signaled by an increase in pulmonary CO2 elimination and decrease in CO2 retention; such data could guide the course of operative embolectomy. Accordingly, we studied five chloralose-urethane anesthetized, mechanically ventilated dogs with open thoraces in which the right pulmonary arteries (RPAs) were reversibly occluded with cloth snares. After waiting for steady state, we abruptly released the snare to restore RPA perfusion and experimentally simulate resolution of pulmonary embolism. For 70 min we serially measure the CO2 volume exhaled per breath (VCO2,br), arterial, mixed venous, and end-tidal PCO2 (PACO2, PVCO2, PETCO2), cardiac output (QT), and the alveolar dead space fraction (VDalv/VTalv = [PaCO2 - PETCO2/PaCO2). RPA reperfusion caused VCO2,br to significantly and abruptly increase from 8.9 +/- 2.7 to 11.6 +/- 3.6 mL; 70 min later VCO2,br had returned to baseline. PaCO2 and PVCO2 steadily decreased during 70 min of RPA reperfusion. PETCO2 increased from 25 +/- 5 to 33 +/- 5 mm Hg immediately after RPA reperfusion, as VDalv/VTalv decreased from 54% +/- 10% to 32% +/- 12%, but PETCO2 was still significantly greater than baseline at 70 min of RPA reperfusion. QT did not significantly change. We conclude that intraoperative measurement of VCO2,br should immediately detect and follow the resolution of CO2 retention in the lung and peripheral tissues after RPA reperfusion. PETCO2 could not detect the decrease of VCO2,br back to baseline because PETCO2 does not measure exhaled volume or the PCO2 waveform.

Carbon Dioxide Elimination Measures Resolution of Experimental Pulmonary Embolus in Dogs

Patients with severe pulmonary embolism can suffer progressive hypercapnia refractory to supramaximal mechanical ventilation, and may require open-thoracic or transvenous emergency embolectomy in addition to anticoagulation and/or thrombolysis. The functional recovery of gas exchange would be signaled by an increase in pulmonary CO2 elimination and decrease in CO2 retention; such data could guide the course of operative embolectomy. Accordingly, we studied five chloralose-urethane anesthetized, mechanically ventilated dogs with open thoraces in which the right pulmonary arteries (RPAs) were reversibly occluded with cloth snares. After waiting for steady state, we abruptly released the snare to restore RPA perfusion and experimentally simulate resolution of pulmonary embolism. For 70 min we serially measure the CO (2) volume exhaled per breath (VCO2,br), arterial, mixed venous, and end-tidal PCO2 (PaCO2, PVCO2, PETCO2), cardiac output (QT), and the alveolar dead space fraction (VDalv/VTalv = [PaCO2 - PETCO2]/PaCO2). RPA reperfusion caused VCO2,br to significantly and abruptly increase from 8.9 +/- 2.7 to 11.6 +/- 3.6 mL; 70 min later VCO2,br had returned to baseline. PaCO2 and PVCO2 steadily decreased during 70 min of RPA reperfusion. PETCO (2) increased from 25 +/- 5 to 33 +/- 5 mm Hg immediately after RPA reperfusion, as VDalv/VTalv decreased from 54% +/- 10% to 32% +/- 12%, but PETCO2 was still significantly greater than baseline at 70 min of RPA reperfusion. QT did not significantly change. We conclude that intraoperative measurement of VCO (2),br should immediately detect and follow the resolution of CO2 retention in the lung and peripheral tissues after RPA reperfusion. PETCO2 could not detect the decrease of VCO2,br back to baseline because PETCO2 does not measure exhaled volume or the PCO2 waveform. (Anesth Analg 1996;83:247-53)

Drought effects on plant water relations and carbon gain in two lines of Lupinus albus L.

Two lines of white lupin (Lupinus albus L.), native from regions with contrasting climates in Portugal (line 6, with a longer life cycle, from an oceanic mild climate and line 43, with a shorter life cycle, from a continental climate with a long hot and dry season) were exposed to slowly developing water deficit. Leaf water status, stomatal conductance and net photosynthesis of line 43 plants were affected by drought earlier than those of line 6. By the end of the stress period, stomatal conductance was higher and net photosynthesis was reduced less in line 6 than in line 43. The photosynthetic capacity of water stressed plants was decreased in both lines by maximal drought. This inhibition of photosynthetic capacity was not due to photoinhibitory damage as demonstrated by the maintenance of chlorophyll a fluorescence parameters in water stressed plants. The recovery of gas exchange and stomatal conductance took place 36 h after rewatering, especially in line 6 plants.Leaf area per plant was strongly affected by drought, especially in line 43 where the number of leaves was reduced to 28 per cent of the mean value of control plants at the end of the stress period, while in line 6 the total number of leaves was reduced to 46 per cent of the mean value of controls. As a consequence, seed production was significantly affected by water stress in line 43 (reduction to 60 per cent of the control plants) and less in line 6 (reduction to about 90 per cent of the controls).It is concluded that line 6 showed more resistance to water shortage, since it was affected later and recovered faster than line 43 during the drying and rewatering process, and also because its production was not reduced so much. While the adaptation of line 43 to an environment with long dry summers results from an escape strategy, which enabled the plants to complete their life cycle before the onset of drought, plants of line 6 avoided water deficits, presumably by maximizing water uptake.

Gas exchange during a soil drying cycle in seedlings of four black walnut (Juglans nigra L.) families.

Photosynthetic and stomatal responses to a soil drying cycle were examined in half-sib seedlings of four walnut (Juglans nigra L.) families. Well-watered seedlings of an Iowa seed source had significantly higher rates of net photosynthesis than seedlings from New York or Michigan sources. This superior photosynthetic potential was associated with both greater stomatal conductance and mesophyll capacity for CO(2) fixation. In a drying soil, net photosynthesis and leaf conductance to water vapor of all families declined substantially, even under mild water stress. These responses were more strongly related to soil water status, as estimated by predawn leaf water potential, than to leaf water potential at the time of gas exchange measurement. There were no differences among families in the pattern of gas exchange response to developing water stress; however, families differed in capacity for recovery of gas exchange from water stress following rehydration. Sensitivity of photosynthesis of black walnut seedlings to water stress may be associated with poor growth and survival of this species in xeric habitats.

Fibrinolytic Effects of Urokinase and Heparin in Acute Pulmonary Embolism: A Randomized Clinical Trial

Dissolution of pulmonary emboli with heparin and urokinase is ascribed, respectively, to anticoagulation and fibrinolysis. Since truly independent assessment of these effects in man is lacking, we administered each drug alone. Fibrinogen and plasminogen plasma levels and the resolution of pulmonary emboli were measured in three randomized groups of 10 patients each: groups A and C infused with small repeated doses of urokinase and a large single dose of urokinase, respectively, and group B who received heparin. After 6 h of treatment, fibrinogen fell in all the groups, while, after 12 h, remained equally reduced in groups A and B and declined further in group C. Plasminogen behaved similarly. Up to 60 h, statistical analysis showed that these effects were related to timing and amounts of urokinase and heparin infusion. These observations suggest that heparin may induce a lytic state. As to signs of pulmonary emboli resolution, no differences between groups were found in lung perfusion and gas exchange recovery at any time (from 1 day to 1 year) and in pulmonary artery pressure reduction at 1 week. The greater angiographic and scintigraphic recovery observed with urokinase, versus heparin alone, after 1 day of treatment in the Urokinase Pulmonary Embolism Trial may be ascribed to a synergistic effect with urokinase of heparin administered during the diagnostic work-out. The indications of heparin and urokinase should be evaluated in the light of these results.

Circulation and respiration response to arm exercise and lower body negative pressure

The effects of supine arm exercise and lower body negative pressure (LBNP) were studied in six subjects with 10 min of LBNP at -40 mmHg (L), arm cranking for 8 min at a work load of 225 kpm/min (E) and both combined (L + E) preceded by 2 min of LBNP. Initial responses of ventilation (VI) and VO2 were curtailed and heart rate was significantly higher after the first min of L + E than E, reflecting the less accessible venous reservoir and reduced stroke volume due to L. Leg volume was significantly reduced after 30 s of E and continued to decline and remained below baseline during 6 min of recovery. With L + E, leg volume remained constant after E began, indicating both a shift of blood from legs to arms and reduced extra-vasation with LBNP. End-tidal PO2, VI and VI/VO2 were higher and PCO2 lower during the latter stages of L + E than during E, indicating less effective lung perfusion and greater alveolar deadspace caused by LBNP. The release of pooled blood from the lower body after L + E caused a greater VI, VO2 and lower RE than after E and produced marked transients in PCO2 and PO2, thereby slowing the recovery of gas exchange.

Photosynthesis and transpiration ofPinus radiata D. Don under natural conditions in a forest stand.

Gas-exchange ofPinus radiata foliage was measured with climatised cuvettes under natural light in the sun-crown of 8 m tall trees in a forest stand. Measurement began during a period of drought (ψWS -8.2 bar, ψWe -10.5 bar) and continued after elimination of soil moisture-deficit by watering (ψWS -0.5 bar, ψWe -5.5 bar). Soil and air moisture-deficits severely restricted gas-exchange. Watering resulted in an immediate decline in stomatal resistance (r s ) and an increase in net photosynthesis (P N ) of 13%. A slower progressive gas-exchange recovery occurred additionally during the 10 days after watering leading to a further decline inr s to 3 s cm-1 and an ultimate increase inP N of 38% when measured under comparable conditions at 8 mb v.p.d. Radiata pine had a high photosynthetic capacity with a measured maximumP N of 10.2 mg CO2 dm-2 h-1 total needle surface (11.4 mg CO2 g-1 DM h-1).Optimum temperature forP N in March (late summer) occurred at ca. 18°C. Rate ofP N was 95% saturated at irradiance of 900 μE m-2 s-1 and 50% saturated at only 270 μE m-2 s-1. Radiata pine needles responded directly to changes in atmospheric humidity by adjusting their stomatal diffusive resistance. As a result, between 8 and 18 mb v.p.d.P N declined by 2.3% mb-1 increase.