In situ estimation of net CO2 assimilation, photosynthetic electron flow and photorespiration in Turkey oak (Q. cerris L.) leaves: diurnal cycles under different levels of water supply

Diurnal time courses of net CO₂ assimilation rates, stomatal conductance and light-driven electron fluxes were measured in situ on attached leaves of 30-year-old Turkey oak trees (Quercus cerris L.) under natural summer conditions in central Italy. Combined measurements of gas exchange and chlorophyll a fluorescence under low O₂ concentrations allowed the demonstration of a linear relationship between the photochemical efficiency of PSII (fluorescence measurements) and the apparent quantum yield of gross photosynthesis (gas exchange). This relationship was used under normal O₂ to compute total light-driven electron fluxes, and to partition them into fractions used for RuBP carboxylation or RuBP oxygenation. This procedure also yielded an indirect estimate of the rate of photorespiration in vivo. The time courses of light-driven electron flow, net CO₂ assimilation and photorespiration paralleled that of photosynthetic photon flux density, with important afternoon deviations as soon as a severe drought stress occurred, whereas photochemical efficiency and maximal fluorescence underwent large but reversible diurnal decreases. The latter observation indicated the occurrence of a large non-photochemical energy dissipation at PSII. We estimated that less than 60% of the total photosynthetic electron flow was used for carbon assimilation at midday, while about 40% was devoted to photorespiration. The rate of carbon loss by photorespiration (R₁) reached mean levels of 56% of net assimilation rates. The potential application of this technique to analysis of the relative contributions of thermal de-excitation at PSII and photorespiratory carbon recycling in the protection of photosynthesis against stress effects is discussed.     

Photosynthetic rate control in cotton : photorespiration

The purpose of this research was to determine the magnitude of photorespiration in field-grown cotton (Gossypium hirsutum L.) as a function of environmental and plant-related factors. Photorespiration rates were estimated as the difference between measured gross and net photosynthetic rates.

A linear increase in photorespiration was observed as air temperature increased from 22 to 40°C at saturating photon flux density. At 22°C, photorespiration was less than 15 per cent of net photosynthesis and very comparable to the dark respiration rate. At 40°C, photorespiration represented about 50 per cent of net photosynthesis. Gross photosynthesis had a temperature optimum of 32 to 34°C. Water stress, as indicated by Ψ_L, did not alter the ratio of gross photosynthesis to net photosynthesis when the confounding effects of leaf temperature differences were accounted for in the data analyses. A reduction in both gross and net photosynthesis was apparent as Ψ_L declined from −2.0 megapascals indicating direct effects of water stress on the photosynthetic process. Photorespiration expressed as a proportion of net photosynthesis increased as water stress intensified.

Cotton cultivars possessing a fruit load had significantly higher gross and net photosynthetic rates and lower photorespiration rates than did photoperiod-sensitive cotton strains without a fruit load. Within the fruiting types, which were genetically very similar, only minor differences were observed in the photorespiration:net photosynthesis ratios. However, in the photoperiod-sensitive strains, considerable genetic variability existed when photorespiration was expressed as a proportion of net photosynthesis. These results suggest that the kinetics of ribulose-1,5-bisphosphate carboxylase:oxygenase may be different and, thus, the possibility of genetically reducing photorespiration exists.

     

Influence of certain environmental factors on photosynthesis and photorespiration in Simmondsia chinensis

The response of net photosynthesis and apparent light respiration to changes in [O₂], light intensity, and drought stress was determined by analysis of net photosynthetic CO₂ response curves. Low [O₂] treatment resulted in a large reduction in the rate of photorespiratory CO₂ evolution. Lightintensity levels influenced the maximum net photosynthetic rate at saturating [CO₂]. These results indicate that [CO₂], [O₂] and light intensity affect the levels of substrates involved in the enzymatic reactions of photosynthesis and photorespiration. Intracellular resistance to CO₂ uptake decreased in low [O₂] and increased at low leaf water potentials. This response reflects changes in the efficiency with which photosynthetic and photorespiratory substrates are formed and utilized. Water stress had no effect on the CO₂ compensation point or the [CO₂] at which net photosynthesis began to saturate at high light intensity. The relationship between these data and recently published in-vitro kinetic measurements with ribulose-diphosphate carboxylase is discussed.Abbreviations C _w intracellular CO₂ concentration - F _gross gross photosynthesis - F _net net photosynthesis - I light intensity - R _L light respiration rate - r _c carboxylation resistance - r ₈ leaf gas-phase resistance - r _i intracellular resistance; to CO₂ uptake - r _t resistance to CO₂ flux between the intercellular spaces and the carboxylation sites - T _L leaf temperature - _t leaf water potential - CO₂ compensation point     

SEASONAL PATTERNS OF CO2 AND WATER VAPOR EXCHANGE OF JUNCUS ROEMERIANUS SCHEELE IN A GEORGIA SALT MARSH

CO₂ and water vapor exchange studies of intact plants of black needle rush (Juncus roemerianus Scheele) were conducted in an undisturbed marsh community on Sapelo Island, Georgia. The seasonal patterns of the light and temperature responses of net photosynthesis, transpiration, leaf diffusive conductance, water-use efficiency and respiration were determined five times over the year. Internal resistances to CO₂ uptake were also evaluated. Net photosynthesis was highest in early spring, but declined only slightly through the year. A distinct and moderate temperature optimum of net photosynthesis was observed with decreasing rates above 30 C. Leaf conductances to water vapor were similar at all seasons and were high at cooler temperatures and decreased with increasing temperature. Transpiration was relatively high and constant during all seasons. The water-use efficiency of photosynthesis was high below 25 C, but decreased sharply above that temperature. Dark respiration was relatively low. Seasonal changes reflected changes in leaf density. Decreasing stomatal conductances and increasing respiration rates reduced net photosynthesis at higher temperatures. The stomatal resistance increased and internal resistances to CO₂ uptake decreased over the year, but the total resistance remained constant. The internal resistance to CO₂ uptake was consistently higher than the stomatal resistance. These seasonal response patterns show that J. roemerianus is well adapted to the seasonal changes in ambient temperature and irradiance and other microenvironmental factors in the high marsh. These physiological characteristics permit this C₃ species to maintain a high productivity in a seasonally hot and stressful environment.     

Effects of irradiance and temperature on photosynthesis in C3, C4 and C3/C4 Panicum species

Species in the Laxa and Grandia groups of the genus Panicum are adapted to low, wet areas of tropical and subtropical America. Panicum milioides is a species with C₃ photosynthesis and low apparent photorespiration and has been classified as a C₃/C₄ intermediate. Other species in the Laxa group are C₃ with normal photorespiration. Panicum prionitis is a C₄ species in the Grandia group. Since P. milioides has some leaf characteristics intermediate to C₃ and C₄ species, its photosynthetic response to irradiance and temperature was compared to the closely related C₃ species, P. laxum and P. boliviense and to P. prionitis. The response of apparent photosynthesis to irradiance and temperature was similar to that of P. laxum and P. boliviense, with saturation at a photosynthetic photo flux density of about 1 mmol m^-2 s^-1 at 30°C and temperature optimum near 30°C. In contrast, P. prionitis showed no light saturation up to 2 mmol m^-2 s^-1 and an optimum temperature near 40°C. P. milioides exhibited low CO₂ loss into CO₂-free air in the light and this loss was nearly insensitive to temperature. Loss of CO₂ in the light in the C₃ species, P. laxum and P. boliviense, was several-fold higher than in P. milioides and increased 2- to 5-fold with increases in temperature from 10 to 40°C. The level of dark respiration and its response to temperature were similar in all four Panicum species examined. It is concluded that the low apparent photorespiration in P. milioides does not influence its response of apparent photosynthesis to irradiance and temperature in comparison to closely related C₃ Panicum species.Abbreviations AP apparent photosynthesis - I CO₂ compensation point - gl leaf conductance; gm, mesophyll conductance - PPFD photosynthetic photon flux density - PR apparent photorespiration rate - RuBPC sibulose bisphosphate carboxylase     

Physiological responses of Opuntia ficus-indica to growth temperature

The influences of various day/night air temperatures on net CO₂ uptake and nocturnal acid accumulation were determined for Opuntia ficus-indica, complementing previous studies on the water relations and responses to photosynthetically active radiation (PAR) for this widely cultivated cactus. As for other Crassulacean acid metabolism (CAM) plants, net nocturnal CO₂ uptake had a relatively low optimal temperature, ranging from 11°C for plants grown at day/night air temperatures of 10°C/0°C to 23°C at 45°C/35°C. Stomatal opening, which occurred essentially only at night and was measured by changes in water vapor conductance, progressively decreased as the measurement temperature was raised. The CO₂ residual conductance, which describes chlorenchyma properties, had a temperature optimum a few degrees higher than the optimum for net CO₂ uptake at all growth temperatures. Nocturnal CO₂ uptake and acid accumulation summed over the whole night were maximal for growth temperatures near 25°C/15°C, CO₂ uptake decreasing more rapidly than acid accumulation as the growth temperature was raised. At day/night air temperatures that led to substantial nocturnal acid accumulation (25°C/15°C.). 90% saturation of acid accumulation required a higher total daily PAR than at non-optimal growth temperatures (10°C/0°C and 35°C/25°C). Also, the optimal temperature of net CO₂ uptake shifted downward when the plants were under drought conditions at all three growth temperatures tested, possibly reflecting an increased fractional importance of respiration at the higher temperatures during drought. Thus, water status, ambient PAR, and growth temperatures must all be considered when predicting the temperature response of gas exchange for O. ficus-indica and presumably for other CAM plants.     

Effects of Experimental Water Table and Temperature Manipulations on Ecosystem CO2 Fluxes in an Alaskan Rich Fen

Peatlands store 30% of the world’s terrestrial soil carbon (C) and those located at northern latitudes are expected to experience rapid climate warming. We monitored growing season carbon dioxide (CO₂) fluxes across a factorial design of in situ water table (control, drought, and flooded plots) and soil warming (control vs. warming via open top chambers) treatments for 2 years in a rich fen located just outside the Bonanza Creek Experimental Forest in interior Alaska. The drought (lowered water table position) treatment was a weak sink or small source of atmospheric CO₂ compared to the moderate atmospheric CO₂ sink at our control. This change in net ecosystem exchange was due to lower gross primary production and light-saturated photosynthesis rather than increased ecosystem respiration. The flooded (raised water table position) treatment was a greater CO₂ sink in 2006 due largely to increased early season gross primary production and higher light-saturated photosynthesis. Although flooding did not have substantial effects on rates of ecosystem respiration, this water table treatment had lower maximum respiration rates and a higher temperature sensitivity of ecosystem respiration than the control plot. Surface soil warming increased both ecosystem respiration and gross primary production by approximately 16% compared to control (ambient temperature) plots, with no net effect on net ecosystem exchange. Results from this rich fen manipulation suggest that fast responses to drought will include reduced ecosystem C storage driven by plant stress, whereas inundation will increase ecosystem C storage by stimulating plant growth.     

Variation in leaf respiration in relation to growth and photosynthesis of Lolium

Six Lolium genotypes with contrasting apparent photorespiration and CO_a compensation concentration, [C0₂]_c, were compared for net photosynthesis, dark respiration, leaf starch accumulation, rate of leaf expansion and shoot regrowth. Plants were grown in day/night temperatures of 15/10 and 25/20 ^oC. There were significant (P < 0–05) differences between the genotypes in all these parameters. At 25/20 ^oC apparent photorespiration was correlated with [CO₂]_c. Correlation coefficients, pooled from both temperature regimes, revealed that genotypes with high rates of net photosynthesis accumulated more leaf starch during light periods than genotypes with slow photosynthesis, but rates of leaf expansion and dry matter increase were only correlated, negatively, with dark respiration. Apparent photorespiration was negatively correlated with dark respiration. These findings suggest that attributes related to photorespiration such as [CO₂]_c and O₂ uptake from CO₂-free air in the light are unlikely to be useful selection criteria for growth of C₃ grasses, that net photosynthesis was probably not limiting growth and that maintenance respiration may have been an important determinant of genotypic differences in growth rate. Selections for slow and fast rates of dark respiration of mature leaves were therefore made at 8 and at 25 ^oC from within two different populations of L. perenne, S.23. This characteristic showed repeatabilities (broad-sense heritability) of from 0–41 to o-66. Six independent comparisons of simulated swards of the slow- and fast-respiring selections were made under periodic cutting regimes, either in a growth room at 25 ^oC or in a glasshouse from August to May. Growth of all plots of slow-respiring genotypes was consistently more rapid than that of the fast-respiring, at 25 ^oC in the growth room, and during autumn and spring in the glasshouse. There was no difference in winter growth. The implications of these results for the use of gas exchange measurements as selection criteria in plant breeding programmes are discussed.     

PHOTOSYNTHESIS AND PHOTOSYNTHESIS-COUPLED RESPIRATION IN NATURAL BIOFILMS QUANTIFIED WITH OXYGEN MICROSENSORS1

Photosynthesis and respiration were analyzed in natural biofilms by use of O₂ microsensors. Depth profiles of gross photosynthesis were obtained from the rate of decrease in O₂ concentration during the first few seconds following extinction of light, and net photosynthesis of the photic zone was calculated from O₂ concentration gradients measured at steady state. Respiration within the photic zone was calculated as the difference between gross and net photosynthesis. Two types of biofilms were investigated: one dominated by diatoms, and one dominated by cyanobacteria. High O₂/CO₂ ratios caused increased respiration especially within the diatom biofilm, which could indicate that photorespiration was a dominant O₂-consuming process. The rate of respiration was constant within both biofilms during the first 4.6 s following extinction of light, even when respiration was stimulated by high O₂/CO₂ ratio. The assumption of a constant rate of respiration during the dark period is an essential one for the determination of gross photosynthetic activity by use of O₂ microsensors. We here present the first evidence to substantiate this assumption. The results strongly suggest that gross photosynthesis as measured by use of O₂ microsensors may include carbon equivalents that are subsequently lost through photorespiration. Computer modeling of photosynthesis profiles measured after 1.1, 1.6, and 2.6 s of dark incubation illustrated how the actual photosynthesis profile could have appeared if it had been possible to do the determination at time 0. Diffusion of O₂ during the up to 4.6-s long dark incubations did not affect gross photosynthetic rate when integrated over all depths, but the apparent vertical distribution of the photosynthetic activity was strongly affected.     

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.     

Sensitivity of C Sequestration in Reintroduced Sphagnum to Water-Level Variation in a Cutaway Peatland

The reintroduction of Sphagnum fragments has been found to be a promising method for restoring mire vegetation in a cutaway peatland. Although it is known that moisture controls Sphagnum photosynthesis, information concerning the sensitivity of carbon dynamics on water‐level variation is still scarce. In a 4‐year field experiment, we studied the carbon dynamics of reintroduced Sphagnum angustifolium material in a restored (rewetted) cutaway peatland. Cutaway peatland restored by Sphagnum reintroduction showed high sensitivity to variation in water level. Water level controlled both photosynthesis and respiration. Gross photosynthesis (P_G) had a unimodal response to water‐level variation with optimum level at ?12 cm. The range of water level for high P_G (above 60% of the maximum light‐saturated P_G) was between 22 and 1 cm below soil surface. Water level had a dual effect on total respiration. When the water level was below soil surface, peat respiration increased rapidly along the lowering water level until the respiration rate started to slow down at approximately ?30 cm. Contrary to peat respiration, the response of Sphagnum respiration to water‐level variation resembled that of photosynthesis with an optimum at ?12 cm. In optimal conditions, Sphagnum reintroduction turned the cutaway site from carbon source to a sink of 23 g C/m² per season (mid‐May to the end of September). In dry conditions, lowered photosynthesis together with the higher peat respiration led to a net loss of 56 g C/m². Although the water level above the optimum amplitude restricted CO₂ fixation, a decrease in peat respiration led to a positive CO₂ balance of 9 g C/m².     

STEM AND LEAF GAS EXCHANGE OF TWO ARID LAND SHRUBS

Gas exchange studies were conducted on two shrub species found in cool shrub-steppe communities of the American West, big sagebrush (Artemisia tridentata subsp. tridentata Nutt.) and broom snakeweed (Gutierrezia sarothrae [Pursh] Britt. and Rusby), with a goal of evaluating characteristics and relative contributions of green stem and leaf material to total shoot CO₂ exchange at different temperatures. Variations in tissue temperature exerted a pronounced effect on CO₂ exchange—net photosynthesis and dark respiration—of green stems and leaves of both species. Definite temperature optima of net photosynthesis were noted, and dark respiration rates consistently increased with increases in temperature. Green stems of both species exhibited sizable dark respiration rates, although stem rates at all temperatures were lower than corresponding leaf rates. Artemisia tridentata did not exhibit sizeable green stem net photosynthesis even under conditions of optimal temperature and water availability, and leaf net photosynthesis rates were much lower than those of G. sarothrae. However, A. tridentata in general possessed a greater leaf biomass than G. sarothrae. Green stems of G. sarothrae exhibited considerable rates of net photosynthesis under both optimal and sub-optimal temperature and water availability conditions. A higher optimum temperature of net photosynthesis was noted for stems than for leaves of G. sarothrae. The adaptive significance of these interspecific differences in CO₂ exchange characteristics is discussed.     

Carbon Dioxide Exchange in an Incompatible Barley/Powdery Mildew Combination

CO₂ exchanges in intact barley leaves were studied by infra red gas analysis after inoculation with an incompatible race of powdery mildew. Net photosynthesis was inhibited, and dark and photorespiration were stimulated, by inoculation. The increase in respiratory processes was approximately equal to the change in net photosynthesis indicating that CO₂ fixation, “gross photosynthesis”, was unaffected by inoculation. It is concluded that changes in net photosynthesis may be the primary cause of the reduced quantity and quality of grain in incompatible barley/mildew combinations.     

An evaluation of the recycling in measurements of photorespiration

All measurements of photorespiration and gross photosynthesis in leaves, whether using isotopes or not, are underestimated because of the recycling of O₂ or CO₂. On the basis of a simple diffusion model, we propose a method for the calculation of the recycling and the corresponding underestimation of the measurements. This procedure can be applied when the stomatal resistance is known, and allows for a correction of certain results in the literature. It is found that measurements of the photorespiratory CO₂ release are usually underestimated by 20 to 100%, which sets the estimated rate of CO₂ photorespired at 30 to 50% of the net photosynthesis in C3 plants under normal conditions. In water stress studies, the correction of the photorespiration is still more important (1.5-3.3) because the stomata are closed more. Analysis of the diffusion of O₂ shows that its recycling is low and that the underestimation of photorespiration with ¹⁸O₂ is negligible.     

Severe drought effects on ecosystem CO2 and H2O fluxes at three Mediterranean evergreen sites: revision of current hypotheses?

Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top‐down approaches in conjunction with a mechanistic ecosystem gas‐exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO₂/H₂O exchange. The three sites include two nearly monospecific Quercus ilex L. forests – one on karstic limestone (Puéchabon), the other on fluvial sand with access to ground water (Castelporziano) – and a typical mixed macchia on limestone (Arca di Noè). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO₂ exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO₂ fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman‐Monteith equation. In a top‐down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity ( Q ₁₀) in response to drought. Soil temperature and soil water content explained 70–80% of the seasonal variability of ecosystem respiration. During the drought, light‐saturated ecosystem gross carbon uptake and day‐time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water‐use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water‐use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO₂, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi)arid ecosystems' carbon balances.     

Moss functioning in different taiga ecosystems in interior Alaska

Summary Carbon dioxide exchange rates in excised 2-year-old shoot sections of five common moss species were measured by infrared gas analysis in mosses collected from different stands of mature vegetation near Fairbanks, Alaska. The maximum rates of net photosynthesis ranged from 2.65 mg CO₂ g^-1h^-1 in Polytrichum commune Hedw. to 0.25 in Spagnum nemoreum Scop. Intermediate values were found in Sphagnum subsecundum Nees., Hylocomium splendens (Hedw.) B.S.G., and Pleurozium schreberi (Brid.) Mitt. Dark respiration rates at 15°C ranged from 0.24 mg CO₂ g^-1h^-1 in S. subsecundum to 0.57 mg CO₂ g^-1h^-1 in H. splendens. The dark respiration rates were found to increase in periods of growth or restoration of tissue (i.e., after desiccation). There was a strong decrease in the rates of net photosynthesis during the winter and after long periods of desiccation.Due to increasing amounts of young, photosynthetically active tissue there was a gradual increase in the rates of net photosynthesis during the season to maximum values in late August. As an apparent result of constant respiration rates and increasing gross photosynthetic rates, the optimum temperature for photosynthesis at light saturation and field capacity increased during the season in all species except Polytrichum, with a corresponding drop in the compensation light intensities. Sphagnum subsecundum seemed to be the most light-dependent species.Leaf water content was found to be an important limiting factor for photosynthesis in the field. A comparison between sites showed that the maximum rates of net photosynthesis increased with increasing nutrient content in the soil but at the permafrostfree sites photosynthesis was inhibited by frequent moisture stress.     

Field measurement of net photosynthesis of mosses at Langhovde,East Antarctica

Net photosynthesis and dark respiration (CO₂ flux) of Antarctic mosses were measured at Langhovde, East Antarctica, from 9 to 17 January 1988. Moss blocks were taken from communities in the Yukidori Valley (69°14′30″S, 39°46′00″E) at Langhovde. Each block was composed ofCeratodon purpureus andBryum pseudotriquetrum, orB. pseudotriquetrum. The upper part of the block was used to measure net photosynthesis and dark respiration. The net photosynthesis of each sample was measured in the field for one or three days with two infrared CO₂ gas analyzers and an assimilation chamber. The relationships of net photosynthetic rate and dark respiration rate, to the water content of the sample, the intensity of solar radiation and the moss temperature were estimated from the field data. The maximum rate of net photosynthesis was about 4 μmol CO₂ m⁻²s⁻¹ at saturating radiation intensity and at optimum temperature, about 10°C. Environmental features of moss habitats in the Yukidori Valley are discussed in relation to these results.     

The efficiency of water use in water stressed plants is increased due to ABA induced stomatal closure

Gas exchange and abscisic acid content of Digitalis lanata EHRH. have been examined at different levels of plant water stress. Net photosynthesis, transpiration and conductance of attached leaves declined rapidly at first, then more slowly following the withholding of irrigation. The intercellular partial pressure of CO₂ decreased slightly. The concentration of 2-cis(S)ABA increased about eight-fold in the leaves of non-irrigated plants as compared with well-watered controls. A close linear correlation was found between the ABA content of the leaves and their conductance on a leaf area basis. In contrast, the plot of net assimilation versus ABA concentration was curvilinear, leading to an increased efficiency of water use during stress. After rewatering, photosynthesis reached control values earlier than transpiration, leaf conductance and ABA content. From these data it is concluded that transpiration through the stomata is directly controlled by the ABA content, whereas net photosynthesis is influenced additionally by other factors.Possible reasons for the responses of photosynthesis and water use efficiency to different stress and ABA levels are discussed.Abbreviations A net CO₂ assimilation - ABA abscisic acid - C_i intercellular CO₂ concentration - g stomatal conductance - T transpiration - WUE water use efficiency     

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.     

Carbon dioxide exchange in Cladina lichens from subarctic and temperate habitats

Summary The survival potential of lichens in a given habitat is determined by the response of CO₂ exchange to photosynthetically active radiation (PhAR), thallus temperature, and thallus relative water content (RWC). Therefore morphologically similar lichens from contrasting climatic environments 1) should differ in their CO₂ exchange responses, and 2) these differences should reflect adaptations to their climatic regimes. The CO₂ exchange responses of a subarctic (55°N, 67°W) Cladina stellaris (Opiz) Brodo population and a temperate (29°N, 82°W) Cladina evansii (Abb.) Hale and W. Culb, population were used to test these two related hypotheses.Infrared gas analysis with lichens collected in September–October 1975 established that the two populations differed in their responses to incident PhAR, thallus temperature, and thallus RWC. Net photosynthesis in C. stellaris had an optimum at a lower temperature and a greater relative photosynthetic capacity at low temperatures than did C. evansii. Cladina evansii maintained net photosynthesis above 35°C thallus temperature; C. stellaris did not. In both species the optimum temperature for net photosynthesis increased with increasing irradiance. The C. stellaris light saturation point was consistently lower than that of C. evansii. Both species had maximal rates of net photosynthesis at 70–80% relative water content. In C. evansii the CO₂ exchange rates, expressed as percentages of the maximum rate, declined more rapidly under suboptimal conditions. The absolute CO₂ exchange rates of C. evansii were greater than those of C. stellaris. At 20°C and 90–95% RWC, resaturation respiration occurred in both species and continued until 6–7 h after wetting.Contrasts in the temporal patterns of thallus condition at each collection site suggest that not all differences in the two response surfaces reflect climatic adaptation. The two populations appear well adapted to incident PhAR and thallus temperature regimes but the 70–80% RWC optimum for net photosynthesis common to both species is puzzling since their water regimes differ markedly. The overall adaptedness of the CO₂ exchange responses in the two species cannot be judged without a comprehensive quantitative analysis of carbon balance under differing climatic regimes.