Showing papers on "Transpiration published in 1974"
TL;DR: Using a simple box model for transpiring leaves a quantitative understanding of the isotope fractionation is possible which is well confirmed by the results of model experiments as well as by measurements on trees.
Abstract: The vapor pressure difference between H2
18O and H2
16O is the reason for the accumulation of the heavy molecule in transpiring leaves. Since photosynthesis on land is the main source of atmospheric oxygen, this mechanism is important for the remarkable enrichment of18O in atmospheric O2 (Dole effect). Using a simple box model for transpiring leaves a quantitative understanding of the isotope fractionation is possible which is well confirmed by the results of model experiments as well as by measurements on trees. Maximum enrichment of H2
18O in the water of leaves (relative to soil water) is 25 ‰ (theoretically, for dry air) and was found under natural conditions to be 21 ‰ (for 28 % relative humidity); minimum theoretical enrichment is zero (observed 2.5 ‰).
603 citations
375 citations
TL;DR: A strong linear relationship between photosynthesis and transpiration inhibition is obtained in both species suggesting that Cd contamination induces stomatal closure.
Abstract: 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.
179 citations
TL;DR: The primary mode of action is the interference with stomatal function of detached sunflower leaves (Helianthus annuus L) as discussed by the authors, and photosynthesis is reduced by 50% of maximum when left tissue concentrations in mM · g −1 are 0.31 (63 ppm) Tl, 0·85 (96 ppm) Cd, 0.93 (193 ppm) Pb, or 1·35 (79 ppm) Ni.
148 citations
TL;DR: The two peaked curves of net photosynthesis and transpiration characteristic of plants living under arid conditions can be explained in this species by the humidity-and temperature-controlled stomatal response, which leads to a decreasing total daily transpirational water loss on a dry day as compared to a moist one.
Abstract: Experiments with Prunus armeniaca were carried out under conditions of constant temperature but varying air humidity. Experiments were also contucted with a constant water vapor difference between the evaporating sites in a leaf and the air, but with varying leaf temperature. These served as a basis for predicting the daily course of total diffusion resistance under the natural climatic conditions of a desert. For the simulation, the rsults of the experiments at constant conditions with only one variable factor are fitted with empirical equations which serve as "calibration curves" to predict the change in diffusion resistance caused by a change in humidity and temperature calculated from the meteorological data of a desert day. The simulation shows that for P. armeniaca humidity and temperature are the dominating factors in controlling the daily course of diffusion resistance. For meteorologically very different days the simulation allows the increase in diffusion resistance in the morning to be predicted with an accuracy of 90%-105% as compared to directly observed measurements. In the afternoon, especially after extreme climatic conditions during the morning, the deviation between predicted and observed values of diffusion resistance may be greater, but not more than -20% to -30%. This possibly indicates the existence of an additional factor of significance which was not included in the simulation. The two peaked curves of net photosynthesis and transpiration characteristic of plants living under arid conditions can be explained in this species by the humidity-and temperature-controlled stomatal response. This stomatal regulation leads to a decreasing total daily transpirational water loss on a dry day as compared to a moist one. The significance of this controlling mechanism for the primary production and the water relations of P. armeniaca is discussed.
142 citations
TL;DR: Results are discussed from the viewpoint that the pathways of transport of lipophilic and lipophobic compounds across the roots may differ and there is no simple inverse correlation between the total concentration of the different sub stances in the roots and that in the transpiration stream.
Abstract: A comparative study has been made of the uptake by and translocation from roots of intact barley plants of six herbicides and a systemic fungicide (four triazines, diuron, 2,4-dichloro phenoxyacetic acid (2,4-D) and ethirimol). Relationships between uptake and transpiration rate are discussed in the light of the physico-chemical properties of these compounds, notably their partition coefficients in oil/water systems and their dissociation constants. Apart from 2,4-D, sorption of these compounds appears to be a passive process. At pH4 the uptake of 2,4-D seems to be influenced by metabolism ; not only may the concentration of this compound in the transpiration stream be considerably greater than that in the medium surrounding the roots but absorption by roots is markedly reduced at low temperatures and by sodium azide. The initial rate of uptake of these compounds correlates reasonably well with their partition coefficients in olive oil/water or w-dodecane/water systems ; likewise the concentration in the transpiration stream is greater for lipophilic than for lipophobic substances. Whereas the hydrogen ion and calcium concentrations of the ambient medium appear to have no effect on the uptake of compounds with low pK's, the uptake of those substances which protonate between pH4 and pH6 is affected by them. These findings are discussed from the viewpoint that the pathways of transport of lipophilic and lipophobic compounds across the roots may differ. Although there is some evidence that retention by roots can limit transport to shoots, there is no simple inverse correlation between the total concentration of the different sub stances in the roots and that in the transpiration stream. This question is discussed in a subsequent paper.
138 citations
TL;DR: It is concluded that the leaf protoplasts represent a high resistance to water transport and that a considerable portion of the water involved in transpiration must bypass them.
Abstract: Leaf water potentials were measured at various rates of water absorption in whole plants and detached leaves of well-watered Helianthus annuus L. The experiments were conducted in the steady state, where changes in leaf hydration did not affect the measurements but both the transpiration and growth components of absorption could be observed. Calculations of the total plant resistance to water transport showed that the resistance at low fluxes was about 30 times the resistance at high fluxes. Most of the change took place in the leaves, since similar changes could be demonstrated in detached leaves. The roots accounted for little of the change, since they varied in resistance by a factor of only 2.5 as flow varied.To ascertain whether the protoplasts of the leaves varied in resistance by an amount which could account for the change in resistance to water transport, measurements of rates of water movement in and out of the protoplasm were made when gradients in water potential between the protoplasts and the water source were varied. These showed that water movement did not occur at rates which could account for high rates of transpiration even when large differences in potential drove flow. The high temperature sensitivity of efflux confirmed that the leaf protoplasts limited flow in these experiments. When the edge of the leaf was excised and flow occurred primarily through the vascular system of the leaf, the resistance was much lower than in the protoplasts. It is therefore concluded that the leaf protoplasts represent a high resistance to water transport and that a considerable portion of the water involved in transpiration must bypass them.Calculations based on a model of water transport showed that the protoplast resistance was almost 30 times larger than the resistance of the path leading from the soil to the leaf protoplasts. The decrease in resistance of the leaves with increasing rates of absorption was therefore attributed to a decrease in water movement in and out of leaf cells, which involved a high resistance, and an increase in movement around the leaf protoplasts, which involved a low resistance. Since the experiments were conducted at the steady state, the high resistances were apparent at low rates of flow where only growth occurred, whereas the low resistances could be observed at high rates of flow because growth did not occur and flow consisted solely of transpiration. Because of the high resistance of the protoplast pathway, leaf water potentials were governed more by protoplast water movement than by transpiration over a considerable range of rates of water absorption. This may explain some of the differences in earlier work on leaf water potentials and water transport.
131 citations
TL;DR: Changes in net photosynthetic rates associated with senescence, defoliation treatments and irradiance levels were largely attributable to changes in intracellular resistance to CO2 transfer.
Abstract: A study was made of the short- and long-term effects of partial defoliation (cutting at 15 cm above the crown) of lucerne plants (Medicago sativa L. cv. Hunter River) on the net photosynthesis, transpiration, photorespiration and CO2 transfer resistances of remaining leaves. The response in gas-exchange properties of leaves of different ages to partial defoliation of the plant was also investigated. Partial defoliation always induced rejuvenation in photosynthetic rate of remaining leaves. Young and middle-aged leaves rejuvenated to rates comparable to those of recently expanded leaves but old leaves only partially rejuvenated. Time after defoliation to attain peak rates increased as leaves aged; values were 5, 9 and 12 days for plants partially defoliated on days 16, 30 and 65 of regrowth respectively. Peak rates were maintained for only 3 or 4 days before declining. Rates of photorespiration and photosynthesis were closely coupled. Transpiration rates varied over time in a similar but more erratic pattern to net photosynthetic rates. Changes in net photosynthetic rates associated with senescence, defoliation treatments and irradiance levels were largely attributable to changes in intracellular resistance to CO2 transfer. Intracellular resistances ranged from 2.6 to 30 s cm-1, constituting 67-95 % of the total resistance to photosynthesis. Stomatal resistance to CO2 diffusion remained low, 0.2 - 1.0 s cm-1, for all but very old leaves. Partial defoliation followed by continual removal of new crown and stubble shoots induced very high net photosynthetic rates, c. 15 days later. Highest net photosynthetic rate was 238 ng CO2 cm-2 s-1. Possible mechanisms responsible for photosynthetic rejuvenation following partial defoliation are discussed, together with ecological implications of this phenomenon.
126 citations
TL;DR: In this paper, steady upward and downward flows of water in the presence of uptake of water by plant roots are analyzed, based upon an exponential increase of the hydraulic conductivity with the pressure head and an exponential decrease of the rate of uptake with depth.
Abstract: Steady upward and downward flows of water in the presence of uptake of water by plant roots are analyzed. Qualitative features of possible pressure head, total head, and water content profiles for steady conditions are discussed. Specific calculations are based upon an exponential increase of the hydraulic conductivity with the pressure head and an exponential decrease of the rate of uptake with depth. For downward flows, the distribution of the matric flux potential can be expressed as a function of the rate of infiltration, the leaching fraction, and two parameters characterizing the soil and the uptake distribution, respectively. Although the water content in the root zone may be nearly uniform, the matric component of the flux may be significant. For upward flows, the distribution of the matric flux potential can be expressed as a function of the rate of infiltration, the rate of transpiration, the hydraulic conductivity of the saturated soil and the same two parameters characterizing the soil and the uptake distribution. The depth at which the flux vanishes, the minimum value of the matric flux potential for a given depth of the water table, and the maximum depth of the water table are evaluated. Finally, neglecting the effects of salt precipitation and dissolution and of diffusion and dispersion, the distribution of salts associated with the steady downward flows is evaluated.
120 citations
TL;DR: It is concluded that the earlier wilting of cut flowers of the short-lived cultivar is mainly due to lower ability to close stomates in response to water stress conditions, and not to earlier formation of vascular blockage.
Abstract: A sharp decline in water potential of petal tissue associated with wilting of cut rose flowers is described. Such a decline did not develop in senescing intact flowers. A circadian rhythm in water absorption by cut flowers was observed.
The decline in fresh weight observed in the last phase of the vase life of cut flowers occurred earlier in a short-lived cultivar than in a longer-lived one. A decline in potential conductivity to water was observed with time in stems of cut roses. Concomitantly cellulase activity increased after cutting. Flowers held in cellulase solution wilted earlier than the controls. However, no difference was found in these two parameters between two cultivars differing in their longevity. The difference in longevity between the two cultivars was large especially under conditions promoting high transpiration rates, and was narrowed when flowers were either held in mild conditions, or the leaves were stripped off.
Although stomates were equally open in intact flowers of the two cultivars, in cut flower shoots of the short-lived cultivar stomates were more widely open. In accordance transpiration rates were higher, and wilting occurred earlier in the short-lived cultivar than in the long-lived one. It is concluded that the earlier wilting of cut flowers of the short-lived cultivar is mainly due to lower ability to close stomates in response to water stress conditions, and not to earlier formation of vascular blockage.
112 citations
TL;DR: Different maximum transpiration rates under the same climatic conditions were observed with different species and may be accounted for by stomatal response to humidity gradients between leaf and air.
Abstract: The efficiency with which plants transport water is related to the water potential differences required to drive water fluxes from the soil to the leaf. A comparative study of two woody and three herbaceous species (Citrus sinensis L. cv. Koethen, Pyrus kawakami L., Helianthus annuus L. cv. Mammoth Russian, Capsicum frutescens L. cv. Yolo Wonder, and Sesamum indicum L. cv. Glauca) indicated contrasts in water transport efficiency. Depression of leaf water potential in response to transpiration increases was found in the woody species; the herbaceous species, however, had more efficient water transport systems and presented no measurable response of leaf water potential to transpiration changes. Different maximum transpiration rates under the same climatic conditions were observed with different species and may be accounted for by stomatal response to humidity gradients between leaf and air. Leaf diffusion resistance in sesame increased markedly as the humidity gradient was increased, while leaf resistance of sunflower responded less to humidity. Stomata appeared to respond directly to the humidity gradient because changes in leaf water potential were not detected when leaf resistance increased or decreased.
TL;DR: Data indicate that A. artemisiifolia can not only efficiently exploit the environmental resources of the field during seedling and mature plant stages of its life cycle but also effectively function under the sometimes extreme conditions encountered in the field.
Abstract: Ambrosia arteinisiifolia L. dominates abandoned fields after spring plowing in many parts of the eastern United States. Its seeds germinate at or near the soil surface in early spring, and the cotyledons become green shortly after they emerge. They are photosynthetically active, with a net photosynthesis dark respiration ratio (P/R) of 1.9. The primary leaves are photosynthetically more active than the cotyledons with a P/R ratio of 3. Photosynthesis of young A. artemisiifolia seedlings is 21 mg C02 dm-2 hr-1 at 1,076 X 102 lux, 250 C, and 300 ppm CO2. They approach light saturation at 1,076 X 102 lux (1.1 cal cm-2 min-) and exhibit appreciable photosynthesis at temperatures from 5? to 350 C. Their rate of photosynthesis increases with an increase of the ambient C02 concentrations comparable to that measured near the soil surface in the field. Although seedling photosynthesis is quite sensitive to decreasing water potential (*J.) between 0 and -10 bars, there is no further decrease between -10 and -20 bars. Furthermore, at this range of 'P photosynthesis is about 20% of that at 0 bars. In a plowed field these seedlings experience high light intensities, appreciable variation in daily temperature, high CO2 concentrations due to its flux from the soil, and low water potentials since the roots are in the upper part of the soil. Field-grown, mature A. artemisiifolia plants do not light-saturate even at 1,076 X 102 lux. Their photosynthetic rate at optimum light intensity, temperature, ala, and 300 ppm C02 is about 35 mg C02 dm-2 hr-1. This high rate of photosynthesis is accompanied by a high rate of transpiration (3 g H20 dm-2 hr-1). Photosynthesis in these plants is sensitive to temperature and reaches a peak at 200 C. The effect of CO2 enrichment on the rate of photosynthesis is not as pronounced in mature plants as in seedlings. Photosynthetic rates decline sharply between -10 and -15 bars BILL; however, appreciable rates are maintained even at '' of -20 bars for several hours. Photosynthetic recovery from water stress is rapid after watering and is attributed to the very low total plant resistance to water transport (1 X 106 sec cm-1). Mature plants have moderate photorespiration since their CO2 compensation point is reached at 50 ppm C02. These data indicate that A. artemisiifolia can not only efficiently exploit the environmental resources of the field during seedling and mature plant stages of its life cycle but also effectively function under the sometimes extreme conditions encountered in the field. These ecophysio- logical attributes coupled with complex germination behavior and genetic plasticity enable the species to be a successful pioneer in early successional ecosystems where the environment can be severe and unpredictable.
TL;DR: In this paper, it is shown that plants can use stored soil water more efficiently if they experience moderate water deficits, causing root systems to absorb water from deeper in the soil, and it is possible that limited irrigation in more dryland areas and less supplemental water use in irrigated areas would promote more efficient use of a limited water supply while maintaining moderately high levels of production.
TL;DR: In this paper, the authors found that at lower Pb treatment levels, corn appear to be more sensitive than soybeans, while at high treatment levels (62.5-250 mg/plant) soybeans are more sensitive.
Abstract: Corn (Zea mays L.) and soybean (Glycine max L.) plants grown in media containing a range of Pb concentrations, supplied as PbCl₂, showed decreased net photosynthesis and transpiration with increasing Pb treatment levels. At lower Pb treatment levels, corn appearsto be more sensitive than soybeans. However, at high treatment levels (62.5–250 mg/plant) soybeans are more sensitive than corn. At 250 mg Pb/plant in the medium, photosynthesis is only 10% of maximum in soybeans but 47% in corn, even though corn Pb tissue content is much higher than that of soybeans. Transpiration exhibited similar trends to photosynthesis suggesting that, especially in corn, an appreciable part of the inhibition of the two processes is related to increased stomatal resistances with increased Pb concentrations. Lead accumulation trends were similar at treatment levels of 0 to 62.5 mg/plant but were slightly different at higher levels. The total amount of Pb accumulated was higher in corn than in soybeans. Maximum accumulation in both species occurred at 62.5 mg Pb/plant.
TL;DR: Stomatal responses to changes in light intensity and humidity were studied in green and chlorotic Fraxinus americana, Acer saccharum, Quercus macrocarpa, Citrus mitis, and Cercis canadensis seedlings and found that stomata opened and closed faster in green than in chlorotic plants.
Abstract: Stomatal responses to changes in light intensity and humidity were studied in green and chlorotic Fraxinus americana, Acer saccharum, Quercus macrocarpa, Citrus mitis, and Cercis canadensis seedlings. Stomatal closure occurred at higher light intensities in Acer than in other species. Transpiration was greater in Fraxinus and Quercus than in Citrus, Acer, or Cercis. Stomata opened faster than they closed in Fraxinus and Quercus and they closed faster than they opened in Citrus. Opening and closing rates were not significantly different from each other in Acer and Cercis. Stomata opened and closed faster in green than in chlorotic plants. In green plants, after a decrease in light intensity, species time to equilibrium of stomatal aperture was related as follows: Citrus < Acer < Quercus = Cercis < Fraxinus; and in chlorotic plants: Citrus < Acer = Quercus = Cercis < Fraxinus. After an increase in light intensity, stomatal opening time in green plants was related as follows: Citrus = Acer < Quercus < Cercis...
22 Oct 1974
TL;DR: Growth chamber studies were conducted to determine the relationships between leaf water potential and diffusive resistance, transpiration rate, and photosynthesis in corn (Zea mays L.) and sorghum (Sorghum vulgare L.).
Abstract: Growth chamber studies were conducted to determine the relationships between leaf water potential and diffusive resistance, transpiration rate, and photosynthesis in corn (Zea mays L.) and sorghum (Sorghum vulgare L.). Few differences were apparent between species in response to leaf water potentials above −8 to −6 bars at all light flux densities used. At lower potentials their ability to withstand water stress differed. Rapid increases in both total resistance to water vapor and mesophyll resistance to carbon dioxide within a narrow range of water potential were observed in corn with simultaneous decreases in transpiration and photosynthesis. More gradual changes occurred in sorghum, with little increase in mesophyll resistance except at the highest light flux density. Photosynthetic rate of sorghum was still 25% of maximum at −11.5 bars whereas corn was severely wilted and photosynthesis had ceased at a similar water potential.
01 Jan 1974
TL;DR: In this paper, a comparative study has been made of the uptake by and translocation from roots of intact barley plants of six herbicides and a systemic fungicide (4 triazines, diuron, 2,4-dichloro phenoxyacetic acid (2, 4-D) and ethirimol).
Abstract: A comparative study has been made of the uptake by and translocation from roots of intact barley plants of six herbicides and a systemic fungicide (four triazines, diuron, 2,4-dichloro phenoxyacetic acid (2,4-D) and ethirimol). Relationships between uptake and transpiration rate are discussed in the light of the physico-chemical properties of these compounds, notably their partition coefficients in oil/water systems and their dissociation constants. Apart from 2,4-D, sorption of these compounds appears to be a passive process. At pH4 the uptake of 2,4-D seems to be influenced by metabolism ; not only may the concentration of this compound in the transpiration stream be considerably greater than that in the medium surrounding the roots but absorption by roots is markedly reduced at low temperatures and by sodium azide. The initial rate of uptake of these compounds correlates reasonably well with their partition coefficients in olive oil/water or w-dodecane/water systems ; likewise the concentration in the transpiration stream is greater for lipophilic than for lipophobic substances. Whereas the hydrogen ion and calcium concentrations of the ambient medium appear to have no effect on the uptake of compounds with low pK's, the uptake of those substances which protonate between pH4 and pH6 is affected by them. These findings are discussed from the viewpoint that the pathways of transport of lipophilic and lipophobic compounds across the roots may differ. Although there is some evidence that retention by roots can limit transport to shoots, there is no simple inverse correlation between the total concentration of the different sub stances in the roots and that in the transpiration stream. This question is discussed in a subsequent paper.
TL;DR: Net photosynthesis and stomatal conductance decreased and dark respiration increased with increasing duration of prechilling and the reduction in net photosynthesis was not a consequence of decreased stomats, because the intercellular CO(2) concentration in prechilled leaves was equal to or greater than that in greenhouse-grown controls.
Abstract: Greenhouse-grown plants of Xanthium strumarium L. were exposed in a growth cabinet to 10 C during days and 5 C during nights for periods of up to 120 hours. Subsequently, CO2 exchange, transpiration, and leaf temperature were measured on attached leaves and in leaf sections at 25 or 30 C, 19 C dew point of the air, 61 milliwatts per square centimeter irradiance, and CO2 concentrations between 0 and 1000 microliters per liter ambient air. Net photosynthesis and stomatal conductance decreased and dark respiration increased with increasing duration of prechilling. The reduction in net photosynthesis was not a consequence of decreased stomatal conductance because the intercellular CO2 concentration in prechilled leaves was equal to or greater than that in greenhouse-grown controls. The intercellular CO2 concentration at which one-half maximum net photosynthesis occurred remained the same in prechilled leaves and controls (175 to 190 microliters per liter). Stomata of the control plants responded to changes in the CO2 concentration of the air only slightly. Prechilling for 24 hours or more sensitized stomata to CO2; they responded to changes in CO2 concentration in the range from 100 to 1000 microliters per liter.
TL;DR: In this article, Peltier-cooled thermocouple dewpoint hygrometers were used to measure leaf water potentials at several transpiration rates on intact corn (Zea mays L.), soybean (Glycine max (L.) Merr.), and sunflower (Helianthus annus L.), grown in a controlled environment in silica sand rooting media frequently watered with nutrient solution.
Abstract: Peltier-cooled thermocouple dewpoint hygrometers were used to measure leaf water potentials at several transpiration rates on intact corn (Zea mays L.), soybean (Glycine max (L.) Merr.), and sunflower (Helianthus annus L.), grown in a controlled environment in silica sand rooting media frequently watered with nutrient solution. Hygrometers were left in position for the duration of measurements on each plant, but tests showed this to have little effect on measured potentials. Measured potentials were found to be linearly related to the transpiration rates (correlation coefficients greater than 0.98). Extrapolated values of leaf water potential at zero transpiration were within a few tenths bar of measured nutrient-solution potentials. These results indicated that plant resistances to water flow remained constant from near zero transpiration up to the maximum obtained average rates of 1.8–3.0 g dm−2 h−1. The magnitude of the resistance varied considerably from plant to plant even within a single cultivar of...
TL;DR: Endogenous changes in stomatal aperture seemed to be the major control of the rhythm in photosynthesis, and the activity of ribulose-1,5-diphosphate carboxylase increased during the normal photoperiod, leveling off after 12 hours; however, the activity was not correlated with the rhythmic change in Photosynthesis.
Abstract: At 14-hour day length, 25 C leaf temperature, 9 mm Hg vapor-pressure deficit, and 1.17 joules cm−2 min−1 irradiance, the diurnal change in daily photosynthesis of the cultivated peanut (Arachis hypogaea L.) is a result of an endogenously controlled circadian rhythm in net photosynthesis which peaks near noon and troughs near midnight. By resetting the day-night light regime, the rhythm rephased in continuous light. The free-running rhythm approximates 26 hours. Both transpiration and dark respiration show similar rhythmicity, with transpiration closely in phase with the rhythm in photosynthesis. The rhythm in carbon dioxide compensation point is approximately 12 hours out of phase, peaking at midnight and troughing at midday. Endogenous changes in stomatal aperture seemed to be the major control of the rhythm in photosynthesis. The activity of ribulose-1,5-diphosphate carboxylase increased during the normal photoperiod, leveling off after 12 hours; however, the activity was not correlated with the rhythmic change in photosynthesis.
01 Jan 1974
TL;DR: In this paper, an endogenously controlled circadian rhythm in net photosynthesis of the cultivated peanut (Arachis hypogaea L) was found to peak near noon and troughs near midnight.
Abstract: At 14-hour day length, 25 C leaf temperature, 9 mm Hg vapor-pressure deficit, and 1.17 joules cm-2 min-' irradiance, the diurnal change in daily photosynthesis of the cultivated peanut (Arachis hypogaea L.) is a result of an endogenously controlled circadian rhythm in net photosynthesis which peaks near noon and troughs near midnight. By resetting the daynight light regime, the rhythm rephased in continuous light. The free-running rhythm approximates 26 hours. Both transpiration and dark respiration show similar rhythmicity, with transpiration closely in phase with the rhythm in photosynthesis. The rhythm in carbon dioxide compensation point is approximately 12 hours out of phase, peaking at midnight and troughing at midday. Endogenous changes in stomatal aperture seemed to be the major control of the rhythm in photosynthesis. The activity of ribulose-1,5-diphosphate carboxylase increased during the normal photoperiod, leveling off after 12 hours; however, the activity was not correlated with the rhythmic change in photosynthesis.
TL;DR: The aims of the investigations were to survey the microclimatic and particularly the thermal changes caused by the creation of a track in sand dune vegetation, and to evaluate the relative effects of the two major components of path creation, vegetation removal and soil compaction, in producing these changes.
Abstract: The importance of changes in microclimate caused by formation of tracks and their effects on vegetation is at present unknown. The principal physical changes that occur when a track is created are the removal of the tall surface vegetation and the compaction of the soil. Previous workers have investigated the microclimatic effects of such factors in isolation but not their combined effects in the track situation. Microclimate and plant cover are interdependent (Whitman & Wolters 1967) and a reduction of standing herbage and mulch leads to a lighter, warmer and drier microenvironment as well as increasing water loss by evaporation (Ellison 1960). Vegetation cover usually has a damping effect on daily and annual temperature range and it also reduces the depth to which the thermal variations penetrate the soil (Richardson 1958; Willis et al. 1959). The lower maximum temperatures recorded in deep vegetation are due to the upper layers absorbing and reflecting large amounts of the incident radiation and to the cooling effect of evaporation during transpiration of the plants (cf. Geiger 1950). At night, vegetation absorbs some of the long-wave radiation of heat from the soil and also radiates heat downwards thus reducing the extremes of soil temperature. Compaction alters the relative amount of soil components present in a given volume. Since the principal components have different thermal properties, the thermal characteristics of the soil alter as it is compressed. Bulk density and water content are known to change as a result of track creation in sand dunes (Liddle 1973), and therefore an associated change in microclimate may be expected. The aims of the investigations were firstly to survey the microclimatic and particularly the thermal changes caused by the creation of a track in sand dune vegetation, and secondly to evaluate the relative effects of the two major components of path creation, vegetation removal and soil compaction, in producing these changes.
TL;DR: In this article, the relationship between water potential and transpiration followed a sequence of steady states without hysteresis, and the assumption that the ratio of vapor pressure deficit to leaf diffusion resistance adequately estimates transpiration when leaf-to-air temperature differences are small.
Abstract: This paper reports on further studies of a model for interpreting leaf water potential data for Citrus. Experimental data confirmed the assumption that the ratio of vapor pressure deficit to leaf diffusion resistance adequately estimates transpiration when leaf-to-air temperature differences are small. Data collected diurnally indicated that the relationship between leaf water potential and transpiration followed a sequence of steady states without hysteresis. No difference in water transport characteristics was found for Valencia orange on three rootstocks in well-watered soil, but the two rootstocks Cleopatra mandarin and Rangpur gave slightly greater leaf water stress in Valencia orange leaves than‘Troyer’ citrange rootstock at high transpiration rates under mild soil water deficits. In laboratory studies, previously unstressed seedlings had higher leaf water potentials than field trees at equivalent transpiration rates. After several drying cycles, however, leaf water potentials were similar to those observed in the field.
TL;DR: Six-month-old seedlings of two low rainfall populations of Eucalyptus viminalis showed greater resistance to desiccation and were less damaged by sustained drought than seedlings from two high rainfall populations.
Abstract: Six-month-old seedlings of two low rainfall populations of Eucalyptus viminalis showed greater resistance to desiccation and were less damaged by sustained drought than seedlings from two high rainfall populations. The more resistant seedlings did not simply avoid drought by quickly closing stomata, but were capable of maintaining relatively high rates of transpiration under moderate moisture stress. The two populations from moist habitats wilted at a higher relative turgidity of leaves than did the populations from the drier habitats. Furthermore, for populations from one dry habitat and one moist habitat, changes in relative turgidity in relation to water potential of leaves during drying were not significantly different. It is suggested that the greater tolerance of the low rainfall populations to drought is related to physiological resistance of the protoplasm to desiccation. Some differences in drought resistance were also noted between the two low rainfall populations. Young seedlings (3 months old) from a population on a deep basalt-derived soil were less resistant than young seedlings from a neighbouring population on a shallower granite soil. It is suggested that the selection for drought tolerance at the granite site has been more severe than at the basalt site.
TL;DR: A low-resolution simulation model of transpiration was developed and run, with data from field studies in southwestern Oregon, and served as a means of relating environmental variables to plant response, resulting in an ordinate which proved helpful for comparing ecosystems, predicting community composition and, in special cases, growth.
Abstract: A low-resolution simulation model of transpiration was developed and run, with data from field studies in southwestern Oregon. The output of the model served as a means of relating environmental variables to plant response. This relation was used to define an ordinate which, in conjunction with previously developed ecosystem ordinates, proved helpful for compar- ing ecosystems, predicting community composition and, in special cases, growth. The data requirements of the model are modest; we deliberately developed a model that can be used on data obtained from field studies where electric power is unavailable and use of sophisticated instrumentation is impossible. The model requires inputs of air and soil temperature, atmo- spheric humidity, seasonal plant water potential (expressed as plant moisture stress, the absolute value of plant water potential), and a model of stomatal behavior. Where it was impossible to obtain accurate data, stochastic models were used to provide the necessary input. The model simulated both potential and actual transpiration, the ratio of which is the most valuable single index of the seasonal moisture regime. Where no measurable stomatal control was exerted by Douglas-fir, the ratio was 1.0, indicating that adequate water was available to meet the transpiration demand. The ratio approached 0.3 on the drier locations. Significant changes in vegetation and growth were associated with this index.
TL;DR: Rates of uptake and loss of tritiated water (HTO) were measured in grass vegetation in the field after the plants had been exposed to an atmosphere containing HTO.
Abstract: Rates of uptake and loss of tritiated water (HTO) were measured in grass vegetation in the field after the plants had been exposed to an atmosphere containing HTO. Uptake of HTO from the atmosphere by leaves required 66 min to achieve 50% equilibrium activity while stems required 1970 min. Loss of HTO from leaves and stems during daylight however took place at rapid and comparable rates. The half time for loss in stems was 25 min while in leaves it was 35 min. Delabeling of contaminated leaves and stems in darkness took place at much reduced rates. The half time for delabeling of leaves in the dark was 400 min while for stems it was 2180 min. Rapid labeling and delabeling of plant tissue after exposure from the atmosphere takes place through leaf stomata and by direct transfer through the epidermis. Loss of HTO from plants is primarily dependent on transpiration. In the event of accidental HTO release to the atmosphere near the ground rapid contamination of vegetation could be expected, When the atmospheric source is dissipated, rapid decontamination could also be expected during periods of normal plant transpiration but not at night or other times when transpiration flow is not taking place.
TL;DR: No adaptive acclimation response was apparent in the temperature dependence of photosynthesis, and high water use efficiency results from a combination of the C4 photosynthetic pathway, relatively low leaf conductances, and maximum growth and photosynthesis activity during the cooler months of the year.
Abstract: Field measurements of CO2 and water vapor exchange were made on Atriplex hymenelytra (Torr.) Wats. shrubs growing in Death Valley, California during March 1971 and July 1972. Rates of CO2 uptake and leaf conductances were substantially higher in March as compared to July. In spite of the large differences in ambient temperatures in March and July, no adaptive acclimation response was apparent in the temperature dependence of photosynthesis. Photosynthesis transpiration ratios at typical midday leaf temperatures were much higher in March than in July. High water use efficiency results from a combination of the C4 photosynthetic pathway, relatively low leaf conductances, and maximum growth and photosynthetic activity during the cooler months of the year.