Photosynthesis and Transpiration in Leaves and Ears of Wheat and Barley Varieties

Carbon exchange rate (CER) and transpiration were measured inflag leaves, whole ears, glumes (referring to the total areaof glumes and lemmas) and awns, in six hexaploid spring wheats(Triticum aestivum L.), three cultivated tetraploid spring wheats(T. turgidum L.), four wild tetraploid wheats (T. dicoccoides),eight six-rowed barleys (Hordeum vulgare L.) and five two-rowedbarleys (H. vulgare L.). Differences between varieties and between species in total earCER and transpiration were associated largely with differencesin ear surface area rather than with rates per unit area. Ratesof CER and transpiration per unit area of ears were 40–80%of those of flag leaves, depending on the species. However, since ear surface area was greater than flag leaf areaby a factor of 1.1, 3.9, 5.5 and 4.4, in hexaploid wheat, tetraploidwheat, six-rowed barley, and two-rowed barley, respectively,total ear CER reached up to 90% of that of the flag leaf. The contribution of awns to total ear CER depended largely ontotal awn surface area per ear, rather than on CER per unitawn area. Awns contributed about 40–80% of total spikeCER, depending on the species, but only 10–20% of spiketranspiration. The disproportionately small contribution ofawns to ear transpiration was caused by the very low rate oftranspiration per unit area of awns. Thus, while transpirationratio (CER/transpiration) was about the same in flag leavesand glumes, it was higher by several orders of magnitude inthe awns. A large amount of awns in the ear is therefore a drought adaptiveattribute in these cereals, for which tetraploid wheat exceededhexaploid wheat and six-rowed barley exceeded two-rowed barley. Key words: Carbon exchange rate, Transpiration, Barley, Wheat     

A Portable System for Measuring the Photosynthesis and Transpiration of Graminaceous Leaves

A portable system has been developed for measuring rates ofphotosynthesis and transpiration of Graminaceous leaves; itcomprises a transparent chamber, into which the leaves are sealed,and a gas supply. The chamber is made from poly 4-methylpent-1-ene(PMP), chosen for its transparency and small water absorption.The leaf is sealed into the chamber by an inflatable rubberseal. The chamber contains a humidity sensor, photocell, thermistor,and a fan which is positioned to give efficient gas-mixing andrapid thermal equilibration. The chamber is surrounded by aPerspex tube to absorb part of the incident infrared radiation.Another fan blows ambient air between the chamber and the Perspextube to counteract solar heating of the chamber wall. The gas supply to the chamber comes from a gas cylinder viaa dilutor. The dilutor contains four small orifices arrangedin parallel. Each orifice is mounted in the inlet of a two-waygas valve: one way goes to the leaf chamber directly, the othervia a CO₂ absorber. The total gas flow-rate is independent ofvalve position, and the CO₂ concentration entering the chambercan be varied from zero to the cylinder concentration by switchingthe valves. The system has been tested both in the laboratory and in thefield, with satisfactory results.     

Effect of Cd on Photosynthesis and Transpiration of Excised Leaves of Corn and Sunflower

Detached corn and sunflower leaves exposed to various concentrations of Cd, supplied as CdCl₂, exhibit reduced photosynthesis and transpiration. The reduction is dependent on the concentration of CdCl₂ solution and generally becomes more pronounced with time. In sunflower, net photosynthesis and transpiration are completely inhibited within 45 min after the introduction of 18 mM Cd. Within two hours net photosynthesis is reduced to 40% and 70% of maximum after the introduction of 9 and 4.5 mM Cd respectively. In corn the trend of photo-synthetic response to Cd is similar to that in sunflower except that the inhibition in corn is more pronounced at all treatment levels. A strong linear relationship between photosynthesis and transpiration inhibition is obtained in both species suggesting that Cd contamination induces stomatal closure.     

Comparisons between Photosynthesis and Transpiration in Birch

Changes in photosynthesis and transpiration of a potted birch seedling were simultaneously followed in the field using two infrared gas analyzers. Wet and dry temperature measurements alone explained 81% of the variation in the transpiration rate when the plant was not suffering from water deficit. During drought and the period after the water balance had been restored, net photosynthesis decreased more distinctly than transpiration. This result was in accordance with our previous results on the after-effects of water deficit on photosynthesis, and it was also interpreted as evidence for at least partly separate control mechanisms for photosynthesis and transpiration.     

Photosynthesis and Transpiration in Rice as Influenced by Soil Moisture and Air Humidity

The paper describes the effect of soil moisture content andair humidity on CO₂ exchange (P_N), CO₂ diffusion resistance(Cr) and transpiration (E) in four varieties of japonica rice(Oryza sativa L.). A decrease in soil moisture content reducedthe rate of photosynthesis to a varying degree in the varieties.Reduction in photosynthesis was attributed to increase in Cr.The effect of low soil moisture on photosynthesis and CO₂ diffusionwas further intensified by decrease in air humidity. By maintaininga high humidity in the air around the leaves however, the effectof soil moisture deficiency was reduced considerably, exceptin Rikuto Norin 21 which was very sensitive to soil-moisturedeficiency alone. Dryness of the air enhanced the transpirationrate, although the increase was relatively less in the plantsfacing a simultaneous water crisis at the root surface. In plantsgrowing under flooded conditions, a decrease in air humiditycaused a slight depression in P_N despite the simultaneous decreasein Cr. Oryza sativa L., rice, photosynthesis, transpiration, diffusion resistance, soil moisture, air humidity     

The Effect of Cadmium on Net Photosynthesis, Transpiration, and Dark Respiration of Excised Silver Maple Leaves

Excised leaves of silver maple (Acer saccharinum L.) exposed to 0, 0.045, 0.090, or 0.180 mM Cd²⁴ exhibited reduced net photosynthesis and transpiration, and increased dark respiration. Rates of net photosynthesis and transpiration diminished with time and were strongly correlated with solution concentration and tissue content of Cd²⁴, Net photosynthesis and transpiration were reduced to 18 and 21%, respectively, of the untreated controls after 64 h. Dark respiration increased as much as 193% of the untreated controls but was poorly correlated with solution concentration or tissue content of Cd²⁴, Diffusive resistances of leaves to carbon dioxide and water vapor transfer increased with both increasing Cd²⁴ concentration and time. These findings are discussed in relation to stomatal function.     

Increases in the Diffusion Resistances of Leaves in a Carbon Dioxide-Enriched Atmosphere

The stomatal closing reaction to CO₂, which has been observedin laboratory studies by many workers, was investigated in plant-growthcabinets under conditions similar to those in glass-houses withCO₂-enriched atmospheres. Lettuce and Xanthium plants grownin a normal atmosphere showed the expected stomatal closurein response to increasing CO₂ concentrations, and lettuce plantsgrown in 1000 ppm CO₂ showed the same response, even after 4weeks. Thus there was no evidence of acclimatization of thestomata and it is concluded that they must remain partiallyclosed during CO₂-enrichment. Estimates of diffusion resistancesto CO₂ intake in lettuce leaves showed that in light of 14.4J m^–2 S^–1, 880 ppm CO₂ in the atmosphere resultedin a concentration of 367 ppm near the mesophyll cell walls.If the stomata remained open this same internal concentrationcould be achieved with an external concentration of only 640ppm. There could, therefore, be some economic advantage if stomatalclosure were prevented during CO₂-enrichment.     

Modelling Diurnal Courses of Photosynthesis and Transpiration of Leaves on the Basis of Stomatal and Non-Stomatal Responses,Including Photoinhibition

A mathematical model for photoinhibition of leaf photosynthesis was developed by formalising the assumptions that (1) the rate of photoinhibition is proportional to irradiance; and (2) the rate of recovery, derived from the formulae for a pseudo first-order process, is proportional to the extent of inhibition. The photoinhibition model to calculate initial photo yield is integrated into a photosynthesis-stomatal conductance (g _s) model that combines net photosynthetic rate (P _N), transpiration rate (E), and g _s, and also the leaf energy balance. The model was run to simulate the diurnal courses of P _N, E, g _s, photochemical efficiency, i.e., ratio of intercellular CO₂ concentration and CO₂ concentration over leaf surface (C _i/C _s), and leaf temperature (T ₁) under different irradiances, air temperature, and humidity separately with fixed time courses of others. When midday depression occurred under high temperature, g _s decreased the most and E the least. The duration of midday depression of g _s was the longest and that in E the shortest. E increased with increasing vapour pressure deficit (VPD) initially, but when VPD exceeded a certain value, it decreased with increasing VPD; this was caused by a rapid decrease in g _s. When air temperature exceeded a certain value, an increase in solar irradiance raised T ₁ and the degree of midday depression. High solar radiation caused large decrease in initial photon efficiency (). P _N, E, and g _s showed reasonable decreases under conditions causing photoinhibition compared with non-photoinhibition condition under high irradiance. The T ₁ under photoinhibition was higher than that under non-photoinhibition conditions, which was evident under high solar irradiance around noon. The decrease in C _i/C _s at midday implies that stomatal closure is a factor causing midday depression of photosynthesis.     

Resistances to the Diffusion of Gas and Vapor in Leaves

Considerable error may be incurred when the component leaf resistances to gases are calculated from the rates of photosynthesis and transpiration measured for the entire leaf. This is due to the fact that there are two parallel pathways for diffusion — through the upper and the lower leaf surfaces — which in most leaves are asymmetrical. True values for the resistances, and predictions of the results of their modification, have been obtained from separate measurements of the photosynthesis and transpiration via the two leaf surfaces.     

Cyclic Changes in Transpiration of Sunflower Leaves in a Steady Environment

Concurrent cyclic changes of leaf temperature, transpirationrate, and stomatal aperture were found to occur in plants ofsunflower (Helianthus annuus L.) exposed to a steady aerialenvironment and with their roots in water. These cyclic changeswere less regular with plants in drying soils and did not occurat all with plants in soils with a water potential less than–120 J kg^–1. A sequence of possible events whichlead to the repeated cycling is suggested; these involve relativechanges in the turgor of epidermal and guard cells. The possibilitythat repeated cycling may be caused by a changing concentrationof carbon dioxide in the sub-stomatal cavities is also discussed.The irregularity and eventual cessation of these cyclic changesare considered to be due to the reduction in turgor of the epidermaland guard cells as the availability of water to these cellsis reduced.     

Inorganic Carbon Accumulation and Photosynthesis in a Blue-green Alga as a Function of External pH

The blue-green alga Coccochloris peniocystis photosynthesizes optimally over the pH range of 7.0 to 10.0, but the O₂-evolution rate is inhibited below pH 7.0 and ceases below pH 5.25. Measurement of the inorganic carbon pool in this alga in the light, using the silicone-fluid filtration technique demonstrated that the rate of accumulation of dissolved inorganic carbon remained relatively constant over a wide pH range. At external dissolved inorganic carbon concentrations of 0.56 to 0.89 millimolar the internal concentration after 30 seconds illumination was greater than 3.5 millimolar over the entire pH range. Intracellular pH measured in the light using [¹⁴C]5,5-dimethyloxazolidine-2,4-dione and [¹⁴C]methylamine dropped from pH 7.6 at an external pH of 7.0 to pH 6.6 at an external pH of 5.25. Above an external pH of 7.0 the intracellular pH rose gradually to pH 7.9 at an external pH 10.0. Ribulose-1,5-bisphosphate carboxylase activity of cell-free algal extracts exhibited optimal activity at pH 7.5 to 7.8 but was inactive below pH 6.5. It is suggested that the inability of Coccochloris to maintain its intracellular pH when in an acidic environment restricts its photosynthetic capacity by a direct pH effect on the principal CO₂ fixing enzyme.     

A Battery-Operated Instrument for Non-destructive Measurements of Photosynthesis and Transpiration of Ears and Leaves of Cereals using 14CO2 and a Lithium Chloride Hygrometer

The design and evaluation of a battery-operated cuvette suitablefor non-destructive measurements of photosynthesis and transpirationin the field are described. The ear or leaf of wheat was sealedin a clear acrylic tube of 2 cm diameter and ¹⁴CO₂-enrichedair was circulated in the closed system which had a total volumeof 1.17 x 10^–3 m³; a boundary layer conductance of 5.2cm s^–1 was measured with an ear in the tube. Photosynthesiswas calculated from the decrease in activity of 2.5 cm3 samplesof air taken from the chamber at 15, 60, 120, and 180 s afterassimilation began. The gas samples were injected into scintillationvials containing 0.5 cm³ of strong base and subsequently countedin a scintillation counter. Transpiration was calculated fromthe time required to increase the humidity of the circulatinggas by a known amount as measured by a lithium chloride hygrometer.The cuvette was flushed between measurements and recharged froma cylinder of air enriched with ¹⁴CO₂; ten measurements an hourwere possible with two operators. Comparisons between the lossin activity of the gas samples and the activity gained by astainless steel mesh saturated with strong base, or activityextracted from exposed leaves, indicated the method of gas-samplingwas sensitive, reproducible, and accurate. Transpiration ofears of wheat measured with the lithium chloride hygrometerin the closed cuvette gave comparable values to those obtainedwhen the same ears were measured in an open system with a dewpointhygrometer. Formulae for calculating photosynthesis and transpirationare given.     

Simulation of the Physiological Responses of C3 Plant Leaves to Environmental Factors by a Model Which Combines Stomatal Conductance,Photosynthesis and Transpiration

Transpiration element is included in the integrated stomatal conductance-photosynthesis model by considering gaseous transfer processes, so the present model is capable to simulate the influence of boundary layer conductance. Leuning in his revised Ball' s model replaced relative humidity with VPDs(the vapor pressure deficit from stomatal pore to leaf surface) and thereby made the relation with transpiration more straightforward, and made it possible for the regulation of transpiration and the influence of boundary layer conductance to be integrated into the combined model. If the differences in water vapor and CO2 concentration between leaf and ambient air are considered, VPDs, the evaporative demand, is influenced by stomatal and boundary layer conductance. The physiological responses of photosynthesis, transpiration, and stomatal function, and the changes of intercellular CO2 and water use efficiency to environmental factors, such as wind speed, photon flux density, leaf temperature and ambient CO2, are analyzed. It is shown that ff the boundary layer conductance drops to a level comparable with stomatal conductance, the results of simulation by the model presented here differ significantly from those by the previous model, and, in some cases, are more realistic than the latter.