The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content

The responses of leaf conductance, leaf water potential and rates of transpiration and net photosynthesis at different vapour pressure deficits ranging from 10 to 30 Pa kPa^-1 were followed in the sclerophyllous woody shrub Nerium oleander L. as the extractable soil water content decreased. When the vapour pressure deficit around a plant was kept constant at 25 Pa kPa^-1 as the soil water content decreased, the leaf conductance and transpiration rate showed a marked closing response to leaf water potential at-1.1 to-1.2 MPa, whereas when the vapour pressure deficit around the plant was kept constant at 10 Pa kPa^-1, leaf conductance decreased almost linearly from-0.4 to-1.1 MPa. Increasing the vapour pressure deficit from 10 to 30 Pa kPa^-1 in 5 Pa kPa^-1 steps, decreased leaf conductance at all exchangeable soil water contents. Changing the leaf water potential in a single leaf by exposing the remainder of the plant to a high rate of transpiration decreased the water potential of that leaf, but did not influence leaf conductance when the soil water content was high. As the soil water content was decreased, leaf conductances and photosynthetic rates were higher at equal levels of water potential when the decrease in potential was caused by short-term increases in transpiration than when the potential was decreased by soil drying.As the soil dried and the stomata closed, the rate of photosynthesis decreased with a decrease in the internal carbon dioxide partial pressure, but neither the net photosynthetic rate nor the internal CO₂ partial pressure were affected by low water potentials resulting from short-term increases in the rate of transpiration. Leaf conductance, transpiration rate and net photosynthetic rate showed no unique relationship to leaf water potential, but in all experiments the leaf gas exchange decreased when about one half of the extractable soil water had been utilized. We conclude that soil water status rather than leaf water status controls leaf gas exchange in N. oleander.     

Some effects of air temperature and humidity on crop and leaf photosynthesis, transpiration and resistance to gas transfer

Measurements of carbon dioxide exchange and transpiration were made, at various air temperatures, on wheat and barley using a field enclosure system. From these were derived the stomatal and mesophyll resistances to carbon dioxide transfer. Optimum temperatures for net CO₂ uptake were about 24°C for wheat and barley. Above these optima, as temperature increased so net CO₂ uptake rates decreased, because of increasing stomatal and mesophyll resistances; transpiration rates decreased in wheat but were constant in barley. In laboratory growth cabinets, wheat plants were subjected to different regimes of temperature and humidity. Optimum temperature for net CO₂ uptake of individual leaves was 25°C. At constant humidity, a decline in net uptake rates above 25°C was associated with large increases in mesophyll resistance. At a constant 25°C, as the vapour pressure deficit (v.p.d. was increased above 1 k Pa (10 mb) v.p.d. the net uptake declined, with an increase in mesophyll resistance and a small increase in stomatal resistance. When the v.p.d. exceeded 1 k Pa at a temperature of 30°C, conditions that are experienced by field plants, then there were large increases in both mesophyll and stomatal resistances and the net uptake rates declined. Photo-respiration, as measured by CO₂ uptake in oxygen-free air, was independent of temperature, but both dark respiration and CO₂ compensation concentration increased with temperature.     

Does transpiration control stomatal responses to water vapour pressure deficit?

Three types of observations were used to test the hypothesis that the response of stomatal conductance to a change in vapour pressure deficit is controlled by whole-leaf transpiration rate or by feedback from leaf water potential. Varying the leaf water potential of a measured leaf by controlling the transpiration rate of other leaves on the plant did not affect the response of stomatal conductance to vapour pressure deficit in Glycine max. In three species, stomatal sensitivity to vapour pressure deficit was eliminated when measurements were made at near-zero carbon dioxide concentrations, despite the much higher transpiration rates of leaves at low carbon dioxide. In Abutilon theophrasti, increasing vapour pressure deficit sometimes resulted in both decreased stomatal conductance and a lower transpiration rate even though the response of assimilation rate to the calculated substomatal carbon dioxide concentration indicated that there was no ‘patchy’ stomatal closure at high vapour pressure deficit in this case. These results are not consistent with stomatal closure at high vapour pressure deficit caused by increased whole-leaf transpiration rate or by lower leaf water potential. The lack of response of conductance to vapour pressure deficit in carbon dioxide-free air suggests that abscisic acid may mediate the response.     

Short- and Long-Term Inhibition of Respiratory Carbon Dioxide Efflux by Elevated Carbon Dioxide

Dark carbon dioxide efflux rates of recently fully expandedleaves and whole plants of Amaranthus hypochondriacus L., Glycinemax (L.) Merr., and Lycopersicon esculentum Mill. grown in controlledenvironments at 35 and 70 Pa carbon dioxide pressure were measuredat 35 and 70 Pa carbon dioxide pressure. Harvest data and whole-plant24-h carbon dioxide exchange were used to determine relativegrowth rates, net assimilation rates, leaf area ratios, andthe ratio of respiration to photosynthesis under the growthconditions. Biomass at a given time after planting was greaterat the higher carbon dioxide pressure in G. max and L. esculentum,but not the C₄ species, A. hypochondriacus. Relative growthrates for the same range of masses were not different betweencarbon dioxide treatments in the two C₃ species, because highernet assimilation rates at the higher carbon dioxide pressurewere offset by lower leaf area ratios. Whole plant carbon dioxideefflux rates per unit of mass were lower in plants grown andmeasured at the higher carbon dioxide pressure in both G. maxand L. esculentum, and were also smaller in relation to daytimenet carbon dioxide influx. Short-term responses of respirationrate to carbon dioxide pressure were found in all species, withcarbon dioxide efflux rates of leaves and whole plants lowerwhen measured at higher carbon dioxide pressure in almost allcases. Amaranthus hypochondriacus L., Glycine max L. Merr., Lycopersicon esculentum Mill., soybean, tomato, carbon dioxide, respiration, growth     

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

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

A Controlled-environment Leaf Chamber to Allow Measurement of Gas Exchange by Leaves Undergoing Rapid Fluctuations in Temperature

A leaf chamber is described which allows continuous measurementof transpiration from an attached leaf while leaf temperatureis controlled independently of air temperature. Leaf temperaturecan be varied from approximately 3 °C below air temperatureto 12 °C above air temperature while air temperature remainsrelatively constant (±2 °C). Leaf temperature canbe varied rapidly (by up to 12 °C in 30 s) in order to simulatethe rapid, short-term temperature fluctuations to which leavesare frequently exposed in the field. The chamber operates overa wide range of conditions of visible and total radiation, ofair and leaf temperatures, and of ambient carbon dioxide concentrationand water vapour density     

The thermal response of leaf extension rate in genotypes of the C4-grassMiscanthus: an important factor in determining the potential productivity of different genotypes

Thermal responses of plant extension rate are reported for 32genotypes of the C₄ grass Miscanthus. Displacement transducerswere used to measure plant extension rate as temperatures werestepped between 20C and 5C. Leaf extension accounted for 83%of the plant extension. The Q₁₀ between 10C and 20C for thegenotypes varied from 3.0 to 4.7. The relationship between temperature(5-20 C) and plant extension rate was found to be describedadequately by fitting a third order polynomial. An estimateof the effect of differences in the thermal response of plantextension rate on the potential yield of the genotypes was calculatedfor Irish climatic conditions using a simple growth model. Potentialyield varied between 3 and 23 t ha^–1 year^–1. Thisdemonstrates the critical role which differences in leaf expansionrate can play in the selection of more productive genotypes.The significance of vapour pressure deficit on the estimatesof thermal response of plant extension rate are discussed. Key words: Leaf growth, Miscanthus, temperature, radiation-use efficiency, vapour pressure deficit, C₄ plants     

The Responses of Net Photosysthesis to Light and Temperature in Spartina townsendii(sensu lato), a C4 Species from a Cool Temperate Climate

Carbon dioxide and water vapour exchanges for single attachedleaves of the temperate C₄ grass Spartina townsendii were measuredunder controlled environment conditions in an open gas-exchangesystem. The responses of net photosynthesis, stomatal resistance,and residual resistance to leaf temperature and photon fluxdensity are described. The light and temperature responses ofnet photosynthesis in S. townsendii are compared to informationon these responses in both temperate C₃ grasses and sub-tropicalC₄ grasses. Adaptation of photosynthesis in this C₄ speciesto a cool temperate climate is indicated both by the light andtemperature responses of net photo-synthesis. Unlike the C₄grasses examined previously, significant rates of net photosynthesiscan be detected at leaf temperatures below 10?C. Rates of netphotosynthesis equal or exceed those reported for temperateC₃ grasses at all of the temperature (5–40?C) and photonflax density (13–2500µmol m^–2 s^–1) conditionsexamined. Maximum rates of net photosynthesis in S. townsendiiare almost double those reported for C₃ herbage grasses. Unliketemperate C₃ grasses, the major limitation to net photosynthesisat low leaf temperatures (10?C and below) is the stomatal resistance,showing that the low residual resistance characteristic of C₄species is maintained in S. townsendii even at low leaf temperatures.     

Root Water Uptake, Leaf Water Storage and Gas Exchange of a Desert Succulent: Implications for Root System Redundancy

A technique used for hydroponics was adapted to measure instantaneousroot water uptake from the soil for a leaf succulent CAM species,Agave deserti. Comparisons were made to previously modelledwater fluxes for A. deserti and to Encelia farinosa, a non-succulentC₃species. Net CO₂uptake and transpiration forA. deserti underwell-watered conditions occurred primarily at night whereasroot water uptake was relatively constant over 24 h. Leaf thicknessdecreased when transpiration commenced and then increased whenrecharge from the stem and soil occurred, consistent with previousmodels. A drought of 90 d eliminated net CO₂uptake and transpirationand reduced the water content of leaves by 62%. Rewetting theentire root system for 7 d led to a full recovery of leaf waterstorage but only 56% of maximal net CO₂uptake. Root water uptakewas maximal immediately after rewetting, which replenished rootwater content, and decreased to a steady rate by 14 d. Whenonly the distal 50% of the root system was rewetted, the timefor net CO₂uptake and leaf water storage to recover increased,but by 30 d gas exchange and leaf water storage were similarto 100% rewetting. Rewetting 10 or 20% of the root system resultedin much less water uptake; these plants did not recover leafwater storage or gas exchange by 30 d after rewetting. A redundancyin the root system of A. deserti apparently exists for dailywater uptake requirements under wet conditions but the entireroot system is required for rapid recovery from drought.Copyright1999 Annals of Botany Company Agave deserti Engelm., desert, drought, gas exchange, rewetting, roots, succulent, water uptake.     

Alteration of transpiration rate, by changing air vapour pressure deficit, influences leaf extension rate transiently in Miscanthus

A controlled environment chamber for whole plants is described in which vapour pressure deficit (VPD) and temperature can be controlled independently. Plant responses to changes in VPD at constant temperature were measured in terms of leaf extension and plant transpiration rates. Manipulation of VPD independently of temperature was shown to be capable of altering the leaf extension rates of the C4 grass Miscanthus x giganteus grown in hydroponics. The effects of VPD on leaf extension are attributed to changes in transpiration rate and hence leaf water status. It was found that, at a temperature of 20C, the influence of a fixed change in VPD was proportionally less than those observed at temperatures which are close to the threshold for growth (between 6 and 10C). These responses are discussed in relation to our current understanding of the mechanisms of cell growth. The fact that the VPD effects on leaf expansion rates were largely transient suggest that simple models driven by temperature alone are adequate to predict leaf expansion within the temperature range 6-20°C, for this genotype of Miscanthus, in the field.Key words: Leaf growth, Miscanthus x giganteus, temperature, vapour pressure deficit, C4 plants.      

Seasonal variations of CO2 and water vapour exchange rates over a temperate deciduous forest

Long-term and direct measurements of CO₂ and water vapour exchange are needed over forested ecosystems to determine their net annual fluxes of carbon dioxide and water. Such measurements are also needed to parameterize and test biogeochemical, ecological and hydrological assessment models. Responding to this need, eddy covariance measurements of CO₂ and water vapour were made ever a deciduous forest growing near Oak Ridge, TN, between April 1993 and April 1994. Periodic measurements were made of leaf area index, stomatal resistance, soil moisture and pre-dawn leaf water potential to characterize the gas exchange capacity of the canopy. Four factors had a disproportionate influence on the seasonal variation of CO₂ flux densities. These factors were photon flux densities (during the growing season), temperature (during the dormant season), leaf area index and the occurrence of drought The drought period occurred during the peak of the growing season and caused a significant decline in daily and hourly CO₂ flux densities, relative to observations over the stand when soil moisture was plentiful. The annual net uptake of carbon was calculated by integrating flux measurements and filling missing and spurious data with the relations obtained between measured CO₂ fluxes and environmental forcing variables. The net flux of carbon for the period between April 1993 and April 1994 was -525 g C m^?2 y^?1. This value represents a net flux of carbon from the atmosphere and into the forest. The net annual carbon exchange of this southern temperate broadleaved forest exceeded values measured over a northern temperate forest (which experiences a shorter growing season and has less leaf area) by 200 g C m^?2 y^?1 (cf. Wofsy et al 1993). The seasonal variation of canopy evaporation (latent heat flux) was controlled mostly by changes in leaf area and net radiation. A strong depression in evaporation rates was not observed during the drought Over a broadleaved forest large vapour pressure deficits promote evaporation and trees in a mixed stand are able to tap a variety of deep and shallow water sources.     

The Effect of Vapour Pressure Deficit on Carbon Isotope Discrimination in the Desert Shrub Larrea tridentata(Creosote Bush)

Larrea tridentata (creosote bush) seedlings were subjected tothree regimens of atmospheric humidity in a growth chamber experiment.Relative humidity was varied to achieve daytime vapour pressuredeficits (VPD) during growth of 29, 48 and 77 kPa. Photosyntheticgas exchange, carbon isotope composition and biomass productionwere measured after 8–10 weeks of treatment. Whereas stomatalconductance (g) declined linearly with increasing ambient VPD,CO₂ assimilation rate (A) was not measurably affected by changesin ambient VPD. This resulted in a decrease in intrinsic wateruse efficiency (ratio of CO₂ assimilation to stomatal conductance;A/_g) with increasing VPD. Leaf carbon isotope discrimination(A) was negatively correlated (r² = 088) with A/g ratios. Carbonisotope discrimination also correlated positively with ratiosof internal (C₁) to ambient (c_a) CO₂ levels determined by gasexchange measurements (c₁/c). The ratio of c₁ to c_a was lowerat higher VPD levels. Leaf biomass decreased with increasingambient VPD and correlated positively with. Root to leaf biomassratio increased at higher VPD levels and correlated negativelywith. Key words: Larrea tridentata, vapour pressure deficit, carbon isotope discrimination, intrinsic water-use efficiency     

Carbon Dioxide and Water Vapour Exchange of Leaves on Field-Grown Citrus Trees

Carbon dioxide and water vapour exchange rates were measuredon attached leaves of field-grown citrus trees. The exchangerates were measured continuously during several weeks in thespring of two successive years. These data confirmed the ratherlow rates of maximum CO₂ exchange (6–11 µmol m^–2s^–1) by citrus leaves. However, the maximum rate was maintainedthrough the midday period on only about half the days. On theother days, characterized by high temperatures and high atmosphericwater vapour pressure deficits, pronounced midday depressionsin CO₂ exchange rates were observed. Since midday transpirationremained stable at a constant rate even with increasing vapourpressure deficit, these results indicate that stomatal closurewas occurring. In fact, the data suggest tfiat specific, maximumtranspiration rates were associated with differing rootstocks.Thus, the rate of water supply to the leaves may be an importantfactor in determining the maximum transpiration rate, and therebymediating control of stomatal conductance and the resultantmidday depression in CO² exchange rates.     

Responses of Net Photosynthesis and Conductance to Independent Changes in the Humidity Environments of the Upper and Lower Surfaces of Leaves of Sunflower and Soybean

A dual-surface leaf chamber was used to investigate the responsesof net photosynthesis and leaf conductance to independent changesin the humidity environments of the upper and lower surfacesof leaves of sunflower and soybean. In sunflower decreasingthe humidity around the upper leaf surface while maintainingthat of the lower surface constant and high reduced both thephotosynthetic rate and the conductance of the lower surface.These reductions could not be attributed to changes in bulkleaf water potential since the transpiration rate of the wholeleaf remained constant. Similarly, the reductions were not relatedto localized water deficits in the lower epidermis or lowermesophyll since the transpiration rate of the lower surfacewas reduced. Possible mechanisms whereby the gas exchange characteristicsof the lower leaf surface of sunflower respond to the humidityenvironment of the upper surface are discussed. In contrastto sunflower, the photosynthetic rate of the lower surface ofsoybean was insensitive to the humidity environment of the uppersurface. In leaves of sunflower grown under a moderate temperature anda medium light level, simultaneous decreases of humidity atboth leaf surfaces reduced the photosynthetic rate of the wholeleaf without affecting the substomatal partial pressure of CO₂.In contrast, with leaves developed under a cool temperatureand a high light level, both the photosynthetic rate and thesubstomatal partial pressure of CO₂ were reduced. Evidently,the occurrence in sunflower of the response pattern suggestinga non-stomatal inhibition of photosynthesis by low humiditydepends upon the environment during growth. The possibilitythat this non-stomatal inhibition may be an artifact due toan error in the assumption of water vapour saturation withinthe leaf airspace is considered. Key words: Vapour pressure deficit, photosynthesis, conductance, non-stomatal inhibition, Helianthus annuus, Glycine max     

Nitrate Nutrition and Temperature Effects on Wheat: Photosynthesis and Photorespiration of Leaves

Lawlor, D. W., Boyle, F. A., Young, A. T., Keys, A. J. and Kendall,A. C. 1987. Nitrate nutrition and temperature effects on wheat:photosynthesis and photorespiration of leaves.—J. exp.Bot. 38: 393–408. Photosynthetic and photorespiratory carbon dioxide exchangeby the third leaf of spring wheat (Triticum aestivum cv. Kolibri),was analysed for plants grown at 13/10 °C (day/night temperature)and 23/18 °C with two rates of nitrate fertilization (abasal rate, — N, and a 4-fold larger rate, +N) and, insome experiments, with two photon fluxes. Net photosynthesiswas greatest at the time of maximum lamina expansion, and forleaves grown with additional nitrate. Maximum rate of photosynthesis,carboxylation efficiency and photochemical efficiency at maturitywere slightly decreased by nitrate deficiency but photosystemactivity was similar under all conditions. As leaves aged, photosynthesisand photochemical efficiency decreased; carboxylation efficiencydecreased more than photochemical efficiency particularly withbasal nitrate. Low oxygen increased the carboxylation and photochemicalefficiencies, and increased the maximum rate of assimilationby a constant proportion in all treatments. Photorespiration,measured by CO₂ efflux to CO₂-free air, by ¹⁴CO₂ uptake, andfrom compensation concentration, was proportional to assimilationin all treatments. It was greater, and formed a larger proportionof net photosynthesis, when measured in warm than in cold conditionsbut was independent of growth conditions. Assimilation was relatedto RuBPc-o activity in the tissue. Relationships between photosynthesis,photorespiration and enzyme complement are discussed. Key words: Wheat, leaves, nitrate nutrition, temperature effect, photosynthesis, photorespiration     

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content

Summary The responses of photosynthesis, transpiration and leaf conductance to changes in vapour pressure deficit were followed in well-watered plants of the herbaceous species, Helianthus annuus, Helianthus nuttallii, Pisum sativum and Vigna unguiculata, and in the woody species having either sclerophyllous leaves, Arbutus unedo, Nerium oleander and Pistacia vera, or mesomorphic leaves, Corylus avellana, Gossypium hirsutum and Prunus dulcis. When the vapour pressure deficit of the air around a single leaf in a cuvette was varied from 10 to 30 Pa kPa^-1 in 5 Pa kPa^-1 steps, while holding the remainder of the plant at a vapour presure deficit of 10 Pa kPa^-1, the leaf conductance and net photosynthetic rate of the leaf decreased in all species. The rate of transpiration increased initially with increase in vapour pressure deficit in all species, but in several species a maximum transpiration rate was observed at 20 to 25 Pa kPa^-1. Concurrent measurements of the leaf water potential by in situ psychrometry showed that an increase in the vapour pressure deficit decreased the leaf water potential in all species. The decrease was greatest in woody species, and least in herbaceous species. When the vapour pressure deficit around the remainder of the plant was increased while the leaf in the cuvette was exposed to a low and constant vapour pressure deficit, similar responses in both degree and magnitude in the rates of transpiration and leaf conductance were observed in the remainder of the plant as those occurring when the vapour pressure deficit around the single leaf was varied. Increasing the external vapour pressure deficit lowered the water potential of the leaf in the cuvette in the woody species and induced a decrease in leaf conductance in some, but not all, speies. The decrease in leaf conductance with decreasing water potential was greater in the woody species when the vapour pressure deficit was increased than when it remained low and constant, indicating that changing the leaf-to-air vapour pressure difference had a direct effect on the stomata in these species. The low hydraulic resistance and maintenance of a high leaf water potential precluded such an analysis in the herbaceous species. We conclude that at least in the woody species studied, an increase in the vapour pressure deficit around a leaf will decrease leaf gas exchange through a direct effect on the leaf epidermis and sometimes additionally through a lowering of the mesophyll water potential.     

Measurement of the rate of CO2 solubilization in leaf tissue 4

A novel method for the measurement of the rate of CO₂ exchangecaused by solubilization and bicarbonate formation in leavesis presented. The method is based on the measurement of changesin O² concentration, caused by the uptake or evolution of CO₂from the gas flow as a result of solubilization in or desolubilizationfrom leaf tissue, by means of a zirconium-oxide O₂ analyser.The advantages of the new method are a fast response and a lackof interference by the CO₂ contained in the leaf chamber andleaf intercellular spaces. The method can be used for the investigationof the kinetics of CO₂ transport from leaf intercellular spacesto chloroplasts and the role of carbonic anhydrase in this transportand the investigation of mechanistic and ecological aspectsof buffering, proton translocation and other processes controllingpH in chloroplasts of intact leaves. Key words: Leaf, carbon dioxide, pH.     

Growth Response of a Chrysanthemum Crop to the Environment. III, Effects of Radiation and Temperature on Dry Matter Partitioning and Photosynthesis

Vegetative crops of chrysanthemum were grown for 5 or 6 weekperiods in daylit assimilation chambers. Crop responses to differentradiation levels and temperatures were analysed into effectson dry matter partitioning, specific leaf area, leaf photosynthesisand canopy light interception. The percentage of newly formed dry matter partitioned to theleaves was almost constant, although with increasing radiationor decreasing temperature, a greater percentage of dry matterwas partitioned to stem tissue at the expense of root tissue.There was a positive correlation between the percentage of drymatter in shoot material and the overall carbon: dry matterratio. Canopy photosynthesis was analysed assuming identical behaviourfor all leaves in the crop. Leaf photochemical efficiency wasonly slightly affected by crop environment. The rate of grossphotosynthesis per unit leaf area at light saturation, P_A (max),increased with increasing radiation integral, but the same parameterexpressed per unit leaf dry matter, P_w (max) was almost unaffectedby growth radiation. In contrast, P_A (max) was hardly affectedby temperature but P_w (max) increased with increasing growthtemperature. This was because specific leaf area decreased withdecreasing temperature and increased with decreasing radiation.There was a positive correlation between canopy respirationintegral and photosynthesis integral, and despite a four-foldchange in crop mass during the experiments, the maintenancecomponent of canopy respiration remained small and constant. Canopy extinction coefficient showed no consistent variationwith radiation integral but was negatively correlated with temperature.This decrease in the efficiency of the canopy at interceptingradiation exactly cancelled the increase in specific carbonassimilation rate that occurred with increasing growth temperature,giving a growth rate depending solely on the incident lightlevel. Chrysanthemum, dry matter partitioning, photosynthesis, specific leaf area     

Stomatal conductance, photosynthesis and respiration of temperate deciduous tree seedlings grown outdoors at an elevated concentration of carbon dioxide

Seedlings of temperate deciduous tree species were grown outdoors at ambient and at an elevated concentration of carbon dioxide to examine how aspects of their gas exchange would be altered by growth at elevated carbon dioxide concentration. Leaf conductances to water vapour and net carbon dioxide exchange rates were determined periodically near midday. Whole-plant carbon dioxide efflux rates in darkness were also determined. The stomatal conductance of leaves of plants grown and measured at 700 cm³ m^?3 carbon dioxide did not differ from that of plants grown and measured at 350 cm³ m^?3 in Malus domestica, Quercus prinus and Quercus robur at any measurement time. In Acer saccharinum, lower conductances occurred for plants grown and measured at elevated carbon dioxide concentration only at measurement temperatures above 33°C. Photo-synthetic adjustment to elevated carbon dioxide concentration was evident only in Q. robur. All species examined had lower rates of dark respiration per unit of mass when grown and measured at elevated carbon dioxide concentration.     

Responses of stomata of senescing and nonsenescing leaves of Nicotians glauca to changes in intercellular concentrations of leaf carbon dioxide

Abstract. Gas exchange measurements were performed to test the hypothesis that failure of stomata to open in senescing leaves of Nicotiana glauca is caused by elevated concentrations of carbon dioxide in the intercellular spaces of leaf mesophyll tissue (c_i). Senescing leaves selected for experiments were completely chlorotic and lacked positive rates of photosynthesis. When stomata in detached epidermis from senescing leaves were illuminated in CO2-free air, they opened to similar apertures as those in detached epidermis from nonsenescing leaves. To compare the effects of changes in c_i on stomatal responses of the two leaf types, leaf 'flags' of either nonsenescing or senescing leaves were illuminated at a photosynthetic photon flux density of 500 μmol m⁻² s⁻¹ in a gas exchange cuvette. Leaf temperatures were maintained at 23.5 ± 0.5°C, and vapour pressure differences between leaves and the air were maintained between 0.70 and 0.75kPa. C_i was adjusted by changing external concentrations of carbon dioxide in air circulating through the cuvette. Conductances and photosynthetic rates of nonsenescing leaves changed in response to changes in c_i, but neither the conductances nor the photosynthetic rates of senescing leaves were affected significantly by changes in q. We conclude that guard cells of senescing leaves of Nicotiana glauca do not lose the capacity to respond to changes in carbon dioxide concentration and that increases in c_i resulting from declining rates of mesophyll photosynthesis are not the sole cause of maintenance of stomatal closure during leaf senescence. The data suggest that factors external to guard cells may prevent them from responding to changes in carbon dioxide concentrations in intact senescing leaves.