Photosynthesis and Transpiration as a Function of Gaseous Diffusive Resistances in Orange Leaves

The CO₂ and H₂O vapour exchange of single attached orange, Citrus sinensis (L.), leaves was measured under laboratory conditions using infrared gas analysis. Gaseous diffusive resistances were derived from measurements at a saturating irradiance and at a leaf temperature optimum for photosynthesis. Variation in leaf resistance (within the range 1.6 to 60 s cm^-1) induced by moisture status, or by cyclic oscillations in stomatal aperture, was associated with changes in both photosynthesis and transpiration. At low leaf resistance (ri less than 10 s cm^-1) the ratio of transpiration to photosynthesis declined with reduced stomatal aperture, indicating a tighter stomatal control over H₂O vapour loss than over CO₂ assimilation. At higher leaf resistance (ri greater than 10 s cm^-1) changes in transpiration and photosynthesis were linearly related, but leaf resistance and mesophyll resistance were also positively correlated, so that strictly stomatal control of photosynthesis became more apparent than real. This evidence, combined with direct measurements of CO₂ diffusive resistances (in a -O₂ gas stream) emphasised the presence of a significant mesophyll resistance; i.e., an additional and rate limiting resistance to CO₂ assimilation over and above that encountered by H₂O vapour escaping from the leaf.     

Effect of leaf water deficit on stomatal and nonstomatal regulation of net carbon dioxide assimilation

The effect of leaf water deficit on net CO₂ assimilation was studied under two conditions: in one, the stomata were allowed to contribute to the regulation of CO₂ assimilation; in the other, air was forced through the leaf at a constant rate to overcome the effects of change in stomatal resistance accompanying changes in leaf water deficit. When the stomata were allowed to regulate the gaseous diffusive resistance of the leaf, CO₂ assimilation decreased with increasing leaf water deficit. However, when air was forced through the leaf, the rate of assimilation was not inhibited by increasing leaf water deficit. The results indicate that the inhibition of net CO₂ assimilation with increasing leaf water deficit is a consequence of an increase in the diffusive resistance to gas exchange and not of a change in apparent mesophyll resistance.     

Leaf Factors Affecting Light-Saturated Photosynthesis in Ecotypes of Solidago virgaurea from Exposed and Shaded Habitats

Plants of Solidago virgaurea L. from exposed and shaded habitats differ with respect to the response of the photosynthetic apparatus to the level of irradiance during growth. An analysis was carried out on leaf characteristies which might be responsible for the differences established in the rates of Hght-saturated CO₂ uptake. The clones were grown in controlled environment chambers at high and low levels of irradiance. Light-saturated rates of photosynthesis and transpiration were measured at natural and lower ambient CO₂ concentrations. A low temperature dependence of light-saturated CO₂ uptake at natural CO₂ concentrations, and a strong response to changes in stomatal width, suggested that the rate of CO₂ transfer from ambient air towards reaetion sites in chloroplasts was mainly limiting the pholosynthetic rate. Resistances to transfer of CO₂ for different parts of the pathway were calculated. There was a weak but significant correlation between stomatal conductance and the product stomatal frequency ± pore length. Mesopbyll conductance and dry weight per unit area were highly correlated in leaves not damaged by high irradiance. This suggests that mesophyll conductance increases with increasing cross sectional area (per unit leaf area) of the pathways of CO₂ transfer in the mesophyll from cell surfaces to reaction sites. The higher light-saturated photosynthesis in clones from exposed habitats when grown at high irradiance than when grown at low irradiance was attributable mainly to a lower mesophyll resistance. In shade clones the effect upon CO₂ uptake of the increase in leaf thickness when grown at high irradiance was counteracted by the associated inactivation of the photosynthetic apparatus. The difference in CO₂ uptake present between clones from exposed and shaded habitats when preconditioned to high irradiance resulted from differences in both mesophyll and stomatal resistances. A few hybrid clones of an F₁-population from a cross between a clone from an exposed habitat and a clone from a shaded habitat reacted, on the whole, in the same way as the exposed habitat parent. When grown at high irradiance, the hybrid clones showed higher photosynthetic rates than either parent; this was largely attributable to the unusually low stomatal resistance of the hybrid leaves.     

A three-dimensional model of CO2 transport in airspaces and mesophyll cells of a silver birch leaf

A realistic numerical three‐dimensional (3D) model was constructed to study CO₂ transport inside a birch leaf. The model included chloroplasts, palisade and spongy mesophyll cells, airspaces, stomatal opening and the leaf boundary layer. Diffusion equations for CO₂ were solved for liquid(mesophyll) and gaseous(air) phases. Simulations were made in typical ambient field conditions varying stomatal opening, photosynthetic capacity and temperature. Doubled ambient CO₂ concentration was also considered. Changes in variables caused non‐linear effects in the total flux, especially when compared with the results of double CO₂ concentration. The reduction in stomatal opening size had a smaller effect on the total flux in doubled concentrations than ambient CO₂. The reduced photosynthetic capacity had a similar effect on the flux in both cases. The palisade and spongy mesophyll cells had unequal roles mainly due to the light absorption profile. Results from the 3D simulation were also compared to the classical one‐dimensional resistance approach. Liquid and gas phase resistances were estimated and found strongly variable according to changes in temperature and degree of stomatal opening. For the birch leaves modelled, intercellular airspace resistance was small (2% of the total resistance in saturating irradiance conditions at 25 °C at stomatal opening diameter of 4 µm) whereas the liquid phase resistance was significant (23% for mesophyll and chloroplasts in the same ‘base case’). The absorption of CO₂ into water at cell surfaces caused additional (strongly temperature dependent) resistance which accounted for 36% of the total resistance in the base case.     

Stomatal function,density and pattern,and CO2 assimilation in Arabidopsis thaliana tmm1 and sdd1‐1 mutants

• Stomata modulate the exchange of water and CO₂ between plant and atmosphere. Although stomatal density is known to affect CO₂ diffusion into the leaf and thus photosynthetic rate, the effect of stomatal density and patterning on CO₂ assimilation is not fully understood.
• We used wild types Col‐0 and C24 and stomatal mutants sdd1‐1 and tmm1 of Arabidopsis thaliana, differing in stomatal density and pattern, to study the effects of these variations on both stomatal and mesophyll conductance and CO₂ assimilation rate. Anatomical parameters of stomata, leaf temperature and carbon isotope discrimination were also assessed.
• Our results indicate that increased stomatal density enhanced stomatal conductance in sdd1‐1 plants, with no effect on photosynthesis, due to both unchanged photosynthetic capacity and decreased mesophyll conductance. Clustering (abnormal patterning formed by clusters of two or more stomata) and a highly unequal distribution of stomata between the adaxial and abaxial leaf sides in tmm1 mutants also had no effect on photosynthesis.
• Except at very high stomatal densities, stomatal conductance and water loss were proportional to stomatal density. Stomatal formation in clusters reduced stomatal dynamics and their operational range as well as the efficiency of CO₂ transport.

     

Interpretation of an empirical model for stomatal conductance in terms of guard cell function

An empirical model for stomatal conductance (g), proposed by Leuning (1995, this issue) as a modification of Ball, Woodrow & Berry's (1987) model, is interpreted in terms of a simple, steady-state model of guard cell function. In this model, stomatal aperture is a function of the relative turgor between guard cells and epidermal cells. The correlation between g and leaf surface vapour pressure deficit in Leuning's model is interpreted in terms of stomatal sensing of the transpiration rate, via changes in the gradient of total water potential between guard cells and epidermal cells. The correlation between g, CO₂ assimilation rate and leaf surface CO₂ concentration in Leuning's model is interpreted as a relationship between the corresponding osmotic gradient, irradiance, temperature, intercellular CO₂ concentration and stomatal aperture itself. The explicit relationship between osmotic gradient and stomatal aperture (possibly describing the effect of changes in guard cell volume on the membrane permeability for ion transport) results in a decrease in the transpiration rate in sufficiently dry air. Possible extension of the guard cell model to include stomatal responses to soil water status is discussed.     

Effect of drought stress on net CO2 uptake by Zea leaves

The net CO₂ assimilation by leaves of maize (Zea mays L. cv. Adonis) plants subjected to slow or rapid dehydration decreased without changes in the total extractable activities of phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH) and malic enzyme (ME). The phosphorylation state of PEPC extracted from leaves after 2–3 h of exposure to light was not affected by water deficit, either. Moreover, when plants which had been slowly dehydrated to a leaf relative water content of about 60% were rehydrated, the net CO₂ assimilation by leaves increased very rapidly without any changes in the activities of MDH, ME and PEPC or phosphorylation state of PEPC. The net CO₂-dependent O₂ evolution of a non-wilted leaf measured with an oxygen electrode decreased as CO₂ concentration increased and was totally inhibited when the CO₂ concentration was about 10%. Nevertheless, high CO₂ concentrations (5–10%) counteracted most of the inhibitory effect of water deficit that developed during a slow dehydration but only counteracted a little of the inhibitory effect that developed during a rapid dehydration. In contrast to what could be observed during a rapidly developing water deficit, inhibition of leaf photosynthesis by cis-abscisic acid could be alleviated by high CO₂ concentrations. These results indicate that the inhibition of leaf net CO₂ uptake brought about by water deficit is mainly due to stomatal closure when a maize plant is dehydrated slowly while it is mainly due to inhibition of non-stomatal processes when a plant is rapidly dehydrated. The photosynthetic apparatus of maize leaves appears to be as resistant to drought as that of C₃ plants. The non-stomatal inhibition observed in rapidly dehydrated leaves might be the result of either a down-regulation of the photosynthetic enzymes by changes in metabolite pool sizes or restricted plasmodesmatal transport between mesophyll and bundle-sheath cells.     

Occurrence of Crassulacean acid metabolism in Cissus trifoliata L. (Vitaceae)

Summary Evidence for the operation of CAM in the deciduous climber, Cissus trifoliata L., was obtained in field and laboratory studies. Under natural conditions, diurnal oscillations of titratable acidity and colorimetric measurements of night CO₂ fixation, determined for a period of two and a half years, suggested that acid accumulation was related to plant water status, assessed through the daily courses of stomatal resistance and xylem water potential during dry and rainy seasons. These findings were confirmed by gas exchange studies under controlled conditions which showed that the plant fixed all its CO₂ during the day when it was well irrigated; as water stress increased, dark CO₂ uptake gradually replaced fixation during the day until the plant only performed dark fixation. In severe water stress, even the rate of the latter process decreased until leaves fell.Abbreviations CAM Crassulacean acid metabolism - FW leaf fresh weight - SWC relative soil water content - PAR photosynthetically active radiation - TR total radiation; r, leaf diffusive resistance - WSD water saturation deficit (leaf-air vapour concentration difference) - RWC relative water content of leaves     

PHYSIOLOGICAL RESPONSES TO RAINFALL IN OPUNTIA BASILARIS (CACTACEAE)

An immediate, marked response to small amounts of rainfall occurs in Opuntia basilaris, despite previous drought conditions. The effect of rainfall is upon plant water potential, which is the single most important parameter influencing stomatal opening, CO₂ assimilation, and organic acid synthesis. Nocturnal stomatal opening is initiated following rainfall, and stomata remain open during the daytime. Decreasing stomatal and mesophyll resistances correlate with increasing rates of nocturnal assimilation of ¹⁴CO₂. Photosynthetic rates of ¹⁴CO₂ assimilation are low, despite high plant water potentials and low stomatal diffusion resistances. The decreased mesophyll resistances and increased rates of nocturnal ¹⁴CO₂ assimilation correlate with the increases of nocturnal efficiency of water use and CO₂ assimilation. The diurnal efficiency of water use and CO₂ assimilation is lower than the nocturnal gas exchange efficiency values.     

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.     

Changes in gas exchange characteristics and water use efficiency of mangroves in response to salinity and vapour pressure deficit

Summary Measurements were made of the photosynthetic gas exchange properties and water use efficiency of 19 species of mangrove in 9 estuaries with different salinity and climatic regimes in north eastern Australia and Papua New Guinea. Stomatal conductance and CO₂ assimilation rates differed significantly between species at the same locality, with the salt-secreting species, Avicennia marina, consistently having the highest CO₂ assimilation rates and stomatal conductances. Proportional changes in stomatal conductance and CO₂ assimilation rate resulted in constant and similar intercellular CO₂ concentrations for leaves exposed to photon flux densities above 800 mol·m^-2·s^-1 in all species at a particular locality. In consequence, all species at the same locality had similar water use efficiencies. There were, however, significant differences in gas exchange properties between different localities. Stomatal conductance and CO₂ assimilation rate both decreased with increasing salinity and with increasing leaf to air vapour pressure deficit (VPD). Furthermore, the slope of the relationship between assimilation rate and stomatal conductance increased, while intercellular CO₂ concentration decreased, with increasing salinity and with decreasing ambient relative humidity. It is concluded from these results that the water use efficiency of mangroves increases with increasing environmental stress, in this case aridity, thereby maximising photosynthetic carbon fixation while minimising water loss.Contribution No. 459 from the Australian Institute of Marine Science     

Seasonal changes in photosynthetic characteristics of Pachysandra terminalis (Buxaceae), an evergreen woodland chamaephyte,in the cool temperate regions of Japan

Summary Seasonal changes in photosynthetic capacity, and photosynthetic responses to intercellular CO₂ concentration and irradiance were investigated under laboratory conditions on intact leaves of Pachysandra terminalis. Photosynthetic capacity and stomatal conductance under saturating light intensity and constant water vapor pressure deficit showed almost the same seasonal trend. They increased from early June just after the expansion of leaves, reached the maximum in late-Septemer, and then decreased to winter. In over-wintering leaves they recovered and increased immediately after snow-melting, reached a first maximum in late April, and then decreased to early July in response to the reduction of light intensity on the forest floor. There-after, they increased from mid August, reached a second maximum in late September, and then decreased to winter. The parallel changes of photosynthesis and stomatal conductane indicate a more or less constant intercellular CO₂ concentration throughout the year. The calculated values of relative stomatal limitation of photosynthesis were nearly constant throughout the year, irrespective of leaf age. The results indicate that the seasonal changes in light-saturated photosynthetic capacity are not due to a change of stomatal conductance, but to a change in the photosynthetic capacity of mesophyll. Indeed, carboxylation efficiency assessed by the inital slope of the Ci-photosynthesis curve changed in proportion to seasonal changes of the photosynthetic capacity in both current-year and over-wintered leaves. High photosynthetic capacity in current-year leaves as compared with one-year-old leaves was also due to the high photosynthetic capacity of mesophyll. Nevertheless, stomatal conductance changed in proportion to photosynthetic capacity, indicating that stomatal conductance is regulated by the mesophyll photosynthetic capacity such that the intercellular CO₂ concentrations are maintained constant. The quantum yield also changed seasonally parallel with that in the photosynthetic capacity.Contribution No. 2893 from the Institute of Low Temperature Science     

Gas-exchange analysis of chloroplastic fructose-1,6-bisphosphatase antisense potatoes at different air humidities and at elevated CO2

Gas-exchange measurements were performed to analyze the leaf conductances and assimilation rates of potato (Solanum tuberosum L. cv. Desireé) plants expressing an antisense construct against chloroplastic fructose-1,6-bisphosphatase (FBPase, EC 3.1.3.11) in response to increasing photon flux densities, different relative air humidities and elevated CO₂ concentrations. Assimilation rates (A) and transpiration rates (E) were observed during a stepwise increase of photon flux density. These experiments were carried out under atmospheric conditions and in air containing 500 μmol mol⁻¹ CO₂. In both gas atmospheres, two levels of relative air humidity (60–70% and 70–80%) were applied in different sets of measurements. Intercellular CO₂ concentration, leaf conductance, air-to-leaf vapour pressure deficit, and instantaneous water-use efficiency (A/E) were determined. As expected, assimilation rates of the FBPase antisense plants were significantly reduced as compared to the wild type. Saturation of assimilation rates in transgenic plants occurred at a photon flux density of 200 μmol m⁻² s⁻¹, whereas saturation in wild type plants was observed at 600 μmol m⁻² s⁻¹. Elevated ambient CO₂ levels did not effect assimilation rates of transgenic plants. At 70–80% relative humidity and atmospheric CO₂ concentration the FBPase antisense plants had significantly higher leaf conductances than wild-type plants while no difference emerged at 60–70%. These differences in leaf conductance vanished at elevated levels of ambient CO₂. Stomatal response to different relative air humidities was not affected by mesophyll photosynthetic activity. It is suggested that the regulation of stomatal opening upon changes in photon flux density is merely mediated by a signal transmitted from mesophyll cells, whereas the intercellular CO₂ concentration plays a minor role in this kind of stomatal response. The results are discussed with respect to stomatal control by environmental parameters and mesophyll photosynthesis. Received: 24 September 1998 / Accepted: 9 February 1999     

Elevated CO2 alleviates the impact of drought on barley improving water status by lowering stomatal conductance and delaying its effects on photosynthesis

We analysed the impact of elevated CO₂ on water relations, water use efficiency and photosynthetic gas exchange in barley (Hordeum vulgare L.) under wet and drying soil conditions. Soil moisture was less depleted under elevated compared to ambient [CO₂]. Elevated CO₂ had no significant effect on the water relations of irrigated plants, except on whole plant hydraulic conductance, which was markedly decreased at elevated compared to ambient CO₂ concentrations. The values of relative water content, water potential and osmotic potential were higher under elevated CO₂ during the entire drought period. The better water status of water-limited plants grown at elevated CO₂ was the result of stomatal control rather than of osmotic adjustment. Despite the low stomatal conductance produced by elevated CO₂, net photosynthesis was higher under elevated than ambient CO₂ concentrations. With water shortage, photosynthesis was maintained for longer at higher rates under elevated CO₂. The reduction of stomatal conductance and therefore transpiration, and the enhancement of carbon assimilation by elevated CO₂, increased instantaneous and whole plant water use efficiency in both irrigated and droughted plants. Thus, the metabolism of barley plants grown under elevated CO₂ and moderate or mild water deficit conditions is benefited by increased photosynthesis and lower transpiration. The reduction in plant water use results in a marked increase in soil water content which delays the onset and severity of water deficit.     

Photosynthetic Response to Water Stress in Phaseolus vulgaris

Water stressed Phaseolus vulgaris L. plants were monitored to detect the relationships between net photosynthesis, transpiration, boundary layer plus stomatal resistance, mesophyll resistance, CO₂ compensation point, ribulose, 1,5-diphosphate carboxylase activity and leaf water potential. At full expansion, the first trifoliate leaves of greenhouse grown bean plants were subjected to water stress by withholding irrigation. Gas exchange and enzyme activity of the central trifoliolate leaflets were monitored as leaf water potential decreased. Although increased stomatal resistance appeared to be the primary causal factor of reduced net photosynthesis, increased mesophyll resistance and decreased ribulose 1,5-diphosphate carboxylase activity further documented the role of non-stomatal factors.     

Stomatal diffusion resistance of snap beans. I. Influence of leaf-water potential

Concurrent measurements of abaxial and adaxial stomatal resistance and leaf-water potentials of snap beans (Phaseolus vulgaris L.) in the field and growth chamber show that the stomata on the 2 surfaces of the leaflet react differently to water deficit. The stomata on the abaxial surface, which are about 7 times more numerous than on the adaxial surface, are not significantly affected at leaf-water potentials greater than —11 bars, but with further decrease in leaf-water potential, the resistance rapidly increases. On the other hand, the resistance of the adaxial stomata increases sharply at a leaf-water potential of about —8 bars and is constant at higher water potentials. The average stomatal resistance for both surfaces of the leaf, which is the major diffusive resistance to water vapor, to a first approximation acts as an on-off switch and helps prevent further decline in leaf-water potential. The relation between the leaf-water potential and the stomatal resistance links the soil-water potential to the transpiration stream as needed for soil-plant-atmosphere models.     

Stomatal and Mesophyll Limitations of Photosynthesis in Phosphate Deficient Sunflower, Maize and Wheat Plants

The effects of phosphate deficiency on the composition and photosyntheticCO₂ assimilation rates of fully expanded leaves of sunflower,maize and wheat plants are described. The regulation of photosynthesisby stomatal and mesophyll characteristics of leaves of differentphosphate status is analysed and related to structure. Phosphatedeficient leaves had small concentrations of inorganic phosphate,Pi, in the tissue water. Rate of photosynthesis in leaves andstomatal conductance were smaller in plants grown with inadequatephosphate when measured under any given light intensity or CO₂partial pressure. Despite the decrease in stomatal conductance(and without evidence of patchy stomatal closure), the relativestomatal limitation of photosynthesis was similar in the plantsgrown with deficient or abundant phosphate. However, the mesophyllcapacity for photosynthesis was greatly limited by phosphatedeficiency. Leaves deficient in phosphate had larger numbersof small size cells per unit leaf area than leaves with adequatephosphate. The total soluble protein content of leaves decreasedwith phosphate deficiency in all three species; however, theleaf chlorophyll content was decreased only in sunflower andmaize and not in wheat. These results suggest that stomatalconductance did not restrict the CO₂ diffusion rate, ratherthe metabolism of the mesophyll was the limiting factor. Thisis shown by poor carboxylation efficiency and decreased apparentquantum yield for CO₂ assimilation, both of which contributedto the increase in relative mesophyll limitation of photosynthesisin phosphate deficient plants. Key words: Apparent quantum yield, carboxylation efficiency, phosphate nutrition, photosynthesis, stomatal and mesophyll limitation     

Influence of Seismic Stress on Photosynthetic Productivity, Gas Exchange, and Leaf Diffusive Resistance of Glycine max (L.) Merrill cv Wells II

Relative growth rate (RGR), leaf water potential (Ψw), transpiration rate (Tr), photosynthetic rate (Pn), and stomatal and mesophyll resistances to CO₂ exchange were measured or calculated to determine how periodic seismic (shaking) stress decreased dry weight accumulation by soybean (Glycine max [L.] Merrill cv Wells II). Seismic stress was applied with a gyratory shaker at 240 to 280 revolutions per minute for 5 minutes three times daily at 0930, 1430, and 1930 hours. Fifteen days of treatment decreased stem length 21%, leaf area 17%, and plant dry weight 18% relative to undisturbed plants. Seismic stress also decreased RGR 4%, which was due entirely to decreased net assimilation rate. Transpiration decreased 17% and leaf Ψw increased 39% 30 minutes after treatment. A reduction in Pn began within seconds after the onset of treatment and had declined 16% after 20 minutes, at which time gradual recovery began. Assimilation rate recovered fully before the next seismic treatment 5 hours later. Resistance analysis and calculation of leaf internal CO₂ levels indicated that the transitory decrease in Pn caused by periodic seismic stress was due to increased stomatal resistance on the abaxial leaf surface.     

Photosynthetic responses to slowly decreasing leaf water potentials in Encelia frutescens

The importance of reduced leaf conductance (stomatal and boundary layer) in limiting photosynthetic rates during water stress was studied in Encelia frutescens, a drought-deciduous leaved subshrub of the Mohave and Sonoran Deserts. Light-saturated CO₂ assimilation rates of greenhouse grown plants decreased from 42.6±1.6mol CO₂ m^-2 s^-1 (x±s.e.) to 1.7±1.7 mol CO₂ m^-2s^-1 as leaf water potential decreased from-1.5 MPa to-4.0 MPa. The dependence of light saturated, CO₂ assimilation rate on leaf intercellular CO₂ concentrations between 60 and 335 l l^-1 was also determined as leaf water potential decline. This enabled us to compare the effects of leaf water potentials on limitations to carbon assimilation imposed by leaf conductance and by intrinsic photosynthetic capacity. Both leaf conductance and intrinsic photosynthetic capacity decreased with decreasing leaf water potential, but the decrease in leaf conductance was proportionately greater. The relative stomatal limitation, defined as the percent limitation in photosynthetic rate due to the presence of gas-phase diffusional barriers, increased from (x±s.e.) to 41±3% as water potentials became more negative. Since both leaf conductance and intrinsic photosynthetic capacity were severely reduced in an absolute sense, however, high photosynthetic rates could not have been restored at low leaf water potentials without simultaneous increases in both components.     

The Effects of N Nutrition on the Water Relations and Gas Exchange Characteristics of Wheat (Triticum aestivum L.)

The purpose of this study was to characterize leaf photosynthetic and stomatal responses of wheat (Triticum aestivum L.) plants grown under two N-nutritional regimes. High- and low-N regimes were imposed on growth-chamber-grown plants by fertilizing with nutrient solutions containing 12 or 1 millimolar nitrogen, respectively. Gas-exchange measurements indicated not only greater photosynthetic capacity of high-N plants under well-watered conditions, but also a greater sensitivity of CO₂ exchange rate and leaf conductance to CO₂ and leaf water potential compared to low-N plants. Increased sensitivity of high-N plants was associated with greater tissue elasticity, lower values of leaf osmotic pressure and greater aboveground biomass. These N-nutritional-related changes resulted in greater desiccation (lowered relative water content) of high-N plants as leaf water potential fell, and were implicated as being important in causing greater sensitivity of high-N leaf gas exchange to reductions in water potential. Water use efficiency of leaves, calculated as CO₂ exchange rate/transpiration, increased from 9.1 to 13 millimoles per mole and 7.9 to 9.1 millimoles per mole for high- and low-N plants as water became limiting. Stomatal oscillations were commonly observed in the low-N treatment at low leaf water potentials and ambient CO₂ concentrations, but disappeared as CO₂ was lowered and stomata opened.