Showing papers on "Transpiration published in 2002"
TL;DR: In this paper, free-air CO2 enrichment (FACE) experiments have been conducted on several agricultural crops: wheat(Triticum aestivum L), perennial ryegrass (Lolium perenne), and rice(Oryza sativa L.) which are C3 grasses; sorghum (Sorghum bicolor (L.) Moench), a C4 grass; white clover (Trifolium repens), a c3 legume; potato (Solanum tuberosum L.), a C3
Abstract: Over the past decade, free-air CO2 enrichment (FACE) experiments have been conducted on several agricultural crops: wheat(Triticum aestivum L.), perennial ryegrass (Lolium perenne), and rice(Oryza sativa L.) which are C3 grasses; sorghum (Sorghum bicolor (L.) Moench), a C4 grass; white clover (Trifolium repens), a C3 legume; potato (Solanum tuberosum L.), a C3 forb with tuber storage; and cotton (Gossypium hirsutum L.) and grape (Vitis vinifera L.) which are C3 woody perennials. Using reports from these experiments, the relative responses of these crops was discussed with regard to photosynthesis, stomatal conductance, canopy temperature, water use, water potential, leaf area index, shoot and root biomass accumulation, agricultural yield, radiation use efficiency, specific leaf area, tissue nitrogen concentration, nitrogen yield, carbohydrate concentration, phenology, soil microbiology, soil respiration, trace gas emissions, and soil carbon sequestration. Generally, the magnitude of these responses varied with the functional type of plant and with the soil nitrogen and water status. As expected, the elevated CO2 increased photosynthesis and biomass production and yield substantially in C3 species, but little in C4, and it decreased stomatal conductance and transpiration in both C3 and C4 species and greatly improved water-use efficiency in all the crops. Growth stimulations were as large or larger under water-stress compared to well-watered conditions. Growth stimulations of non-legumes were reduced at low soil nitrogen, whereas elevated CO2 strongly stimulated the growth of the clover legume both at ample and under low N conditions. Roots were generally stimulated more than shoots. Woody perennials had larger growth responses to elevated CO2, while at the same time, their reductions in stomatal conductance were smaller. Tissue nitrogen concentrations went down while carbohydrate and some other carbon-based compounds went up due to elevated CO2, with leaves and foliage affected more than other organs. Phenology was accelerated slightly in most but not all species. Elevated CO2 affected some soil microbes greatly but not others, yet overall activity appears to be stimulated. Detection of statistically significant changes in soil organic carbon in any one study was impossible, yet combining results from several sites and years, it appears that elevated CO2 did increase sequestration of soil carbon. Whenever possible, comparisons were made between the FACE results and those from prior chamber-based experiments reviewed in the literature. Over all the data and parameters considered in this review, there are only two parameters for which the FACE- and chamber-based data appear to be inconsistent. One is that elevated CO2 from FACE appears to reduce stomatal conductance about one and a half times more than observed in prior chamber experiments. Similarly, elevated CO2 appears to have stimulated root growth relatively more than shoot growth under FACE conditions compared to chamber conditions. Nevertheless, for the most part, the FACE- and chamber-based results have been consistent, which gives confidence that conclusions drawn from both types of data are accurate. However, the more realistic FACE environment and the larger plot size have enabled more extensive robust multidisciplinary data sets to be obtained under conditions representative of open fields in the future high-CO2 world.
1,020 citations
TL;DR: It is concluded that inactivation of Rubisco was the primary constraint on the rate of Pn of maize leaves as leaf temperature increased above 30°C, and acclimation was associated with the expression of a new activase polypeptide.
Abstract: Our objective was to determine the sensitivity of components of the photosynthetic apparatus of maize (Zea mays), a C4 plant, to high temperature stress. Net photosynthesis (Pn) was inhibited at leaf temperatures above 38°C, and the inhibition was much more severe when the temperature was increased rapidly rather than gradually. Transpiration rate increased progressively with leaf temperature, indicating that inhibition was not associated with stomatal closure. Nonphotochemical fluorescence quenching (qN) increased at leaf temperatures above 30°C, indicating increased thylakoid energization even at temperatures that did not inhibit Pn. Compared with CO2 assimilation, the maximum quantum yield of photosystem II (Fv/Fm) was relatively insensitive to leaf temperatures up to 45°C. The activation state of phosphoenolpyruvate carboxylase decreased marginally at leaf temperatures above 40°C, and the activity of pyruvate phosphate dikinase was insensitive to temperature up to 45°C. The activation state of Rubisco decreased at temperatures exceeding 32.5°C, with nearly complete inactivation at 45°C. Levels of 3-phosphoglyceric acid and ribulose-1,5-bisphosphate decreased and increased, respectively, as leaf temperature increased, consistent with the decrease in Rubisco activation. When leaf temperature was increased gradually, Rubisco activation acclimated in a similar manner as Pn, and acclimation was associated with the expression of a new activase polypeptide. Rates of Pn calculated solely from the kinetics of Rubisco were remarkably similar to measured rates if the calculation included adjustment for temperature effects on Rubisco activation. We conclude that inactivation of Rubisco was the primary constraint on the rate of Pn of maize leaves as leaf temperature increased above 30°C.
675 citations
TL;DR: In this paper, a top-down analysis of eddy covariance and sapflow data from three Mediterranean ecosystems was performed in conjunction with a mechanistic ecosystem gas exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO2/H2O exchange.
Abstract: Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top-down approaches in conjunction with a mechanistic ecosystem gas-exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO2/H2O exchange. The three sites include two nearly monospecific Quercus ilex L. forests - one on karstic limestone (Puechabon), the other on fluvial sand with access to ground water (Castelporziano) - and a typical mixed macchia on limestone (Arca di Noe). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO2 exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO2 fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman-Monteith equation. In a top-down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity (Q10) in response to drought. Soil temperature and soil water content explained 70-80% of the seasonal variability of ecosystem respiration. During the drought, light-saturated ecosystem gross carbon uptake and day-time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water-use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water-use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO2, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi) arid ecosystems' carbon balances. (Resume d'auteur)
541 citations
TL;DR: In this article, the authors used the Compact Airborne Spectral Imager (CASI) for an agricultural area in Denmark with the purpose of quantifying vegetation amount and variations in the physiological status of the vegetation.
416 citations
TL;DR: Le Leaf rachis was the most vulnerable organ, with a threshold P(rachis) for embolism induction of -1.4 MPa, and the minimum Psileaf values corresponded to leaf turgor loss point, which suggested that stomata are responding to leaf water status as determined by transpiration rate and plant hydraulics and that P( rACHis) might be the physiological parameter regulated by stomatal closure during water stress.
Abstract: The objectives of the study were to identify the relevant hydraulic parameters associated with stomatal regulation during water stress and to test the hypothesis of a stomatal control of xylem embolism in walnut (Juglans regia x nigra) trees. The hydraulic characteristics of the sap pathway were experimentally altered with different methods to alter plant transpiration (Eplant) and stomatal conductance (gs). Potted trees were exposed to a soil water depletion to alter soil water potential (Psisoil), soil resistance (Rsoil), and root hydraulic resistances (Rroot). Soil temperature was changed to alter Rroot alone. Embolism was created in the trunk to increase shoot resistance (Rshoot). Stomata closed in response to these stresses with the effect of maintaining the water pressure in the leaf rachis xylem (P(rachis)) above -1.4 MPa and the leaf water potential (Psileaf) above -1.6 MPa. The same dependence of Eplant and gs on P(rachis) or Psileaf was always observed. This suggested that stomata were not responding to changes in Psisoil, Rsoil, Rroot, or Rshoot per se but rather to their impact on P(rachis) and/or Psileaf. Leaf rachis was the most vulnerable organ, with a threshold P(rachis) for embolism induction of -1.4 MPa. The minimum Psileaf values corresponded to leaf turgor loss point. This suggested that stomata are responding to leaf water status as determined by transpiration rate and plant hydraulics and that P(rachis) might be the physiological parameter regulated by stomatal closure during water stress, which would have the effect of preventing extensive developments of cavitation during water stress.
392 citations
TL;DR: Experimental conditions under which individual Arabidopsis plants with altered stomatal responses to drought can be identified by infrared thermography are optimised and ost1 and ost2 represent, to the authors' knowledge, the firstArabidopsis mutations altering ABA responsiveness in stomata and not in seeds.
Abstract: In response to drought, plants synthesise the hormone abscisic acid (ABA), which triggers closure of the stomatal pores. This process is vital for plants to conserve water by reducing transpirational water loss. Moreover, recent studies have demonstrated the advantages of the Arabidopsis stomatal guard cell for combining genetic, molecular and biophysical approaches to characterise ABA action. However, genetic dissection of stomatal regulation has been limited by the difficulty of identifying a reliable phenotype for mutant screening. Leaf temperature can be used as an indicator to detect mutants with altered stomatal control, since transpiration causes leaf cooling. In this study, we optimised experimental conditions under which individual Arabidopsis plants with altered stomatal responses to drought can be identified by infrared thermography. These conditions were then used to perform a pilot screen for mutants that displayed a reduced ability to close their stomata and hence appeared colder than the wild type. Some of the mutants recovered were deficient in ABA accumulation, and corresponded to alleles of the ABA biosynthesis loci ABA1, ABA2 and ABA3. Interestingly, two of these novel aba2 alleles were able to intragenically complement the aba2-1 mutation. The remaining mutants showed reduced ABA responsiveness in guard cells. In addition to the previously known abi1-1 mutation, we isolated mutations at two novel loci designated as OST1 (OPEN STOMATA 1) and OST2. Remarkably, ost1 and ost2 represent, to our knowledge, the first Arabidopsis mutations altering ABA responsiveness in stomata and not in seeds.
386 citations
TL;DR: The results showed that the relationship between specific hydraulic conductivity (Ks) and resistance to cavitation followed a power function with exponent ≈–2, consistent with the existence of a trade-off between conductivity and security in the xylem, and also consistent with a linear relationship between vessel diameter and the size of inter-vessel pores.
Abstract: We studied the hydraulic architecture and water relations of nine co-occurring woody species in a Spanish evergreen oak forest over the course of a dry season. Our main objectives were to: (1) test the existence of a trade-off between hydraulic conductivity and security in the xylem, and (2) establish the safety margins at which the species operated in relation to hydraulic failure, and compare these safety margins between species and tissues (roots vs. stems). Our results showed that the relationship between specific hydraulic conductivity (Ks) and resistance to cavitation followed a power function with exponent ≈–2, consistent with the existence of a trade-off between conductivity and security in the xylem, and also consistent with a linear relationship between vessel diameter and the size of inter-vessel pores. The diameter of xylem conduits, Ks and vulnerability to xylem embolism were always higher in roots than in stems of the same species. Safety margins from hydraulic failure were narrower in roots than in stems. Among species, the water potential (Ψ) at which 50% of conductivity was lost due to embolism ranged between –0.9 and Cistus albidus=Ilex aquifolium>Phillyrea latifolia>Juniperus oxycedrus. Gas exchange and seasonal Ψ minima were in general correlated with resistance to xylem embolism. Hydraulic safety margins differed markedly among species, with some of them (J. oxycedrus, I. aquifolium, P. latifolia) showing a xylem overly resistant to cavitation. We hypothesize that this overly resistant xylem may be related to the shape of the relationship between Ks and security we have found.
379 citations
TL;DR: Drought stress in both olive cultivars resulted in a decrease of the size of the epidermal and mesophyll cells with a parallel increase of the cell density, which was more characteristic in cv.
372 citations
TL;DR: Major questions remain unanswered on how water stress signals perceived at root and leaf locations are integrated at the guard cell to control stomatal behaviour.
Abstract: Abscisic acid (ABA) transported in the xylem from root to shoot and perceived at the guard cell is now widely studied as an essential regulating factor in stomatal closure under drought stress This provides the plant with a stomatal response mechanism in which water potential is perceived in the root as an indication of soil water status and available water resources There is also ample evidence that stomata respond directly to some component of leaf water status This provides additional information about water potential gradients developing between root and shoot as the result of water transport, allowing for a more stable regulation of shoot water status and better protection of the transport system itself The precise location at which leaf water status is sensed, however, and the molecular events transducing this signal into a guard cell response are not yet known Major questions therefore remain unanswered on how water stress signals perceived at root and leaf locations are integrated at the guard cell to control stomatal behaviour
352 citations
TL;DR: In this article, a process-based forest growth model, growth of trees is limited by water in the Mediterranean (GOTILWAþ), was applied to the Mediterranean region on Quercus ilex, Pinus halepensis, P. sylvestris and Fagus sylvatica forests, and the results showed a higher production promoted by projected climate change in response to the increasing atmospheric CO2 concentration and rainfall in the region.
345 citations
TL;DR: Elevated CO(2) will benefit sunflower growing under water deficit by marginally increasing P(n), and by slowing transpiration, which will decrease the rate and severity of water deficits, with limited effects on metabolism.
Abstract: Photosynthetic responses of sunflower plants grown for 52 d in ambient and elevated CO(2) (A=350 or E=700 micromol mol(-1), respectively) and subjected to no (control), mild or severe water deficits after 45 d were analysed to determine if E modifies responses to water deficiency. Relative water content, leaf water potential (Psi(w)) and osmotic potential decreased with water deficiency, but there were no effects of E. Growth in E decreased stomatal conductance (g(s)) and thereby transpiration, but increased net CO(2) assimilation rate (P(n), short-term measurements); therefore, water-use efficiency increased by 230% (control plants) and 380% (severe stress). Growth in E did not affect the response of P(n) to intercellular CO(2) concentration, despite a reduction of 25% in Rubisco content, because this was compensated by a 32% increase in Rubisco activity. Analysis of chlorophyll a fluorescence showed that changes in energy metabolism associated with E were small, despite the decreased Rubisco content. Water deficits decreased g(s) and P(n): metabolic limitation was greater than stomatal at mild and severe deficit and was not overcome by elevated CO(2). The decrease in P(n) with water deficiency was related to lower Rubisco activity rather than to ATP and RuBP contents. Thus, there were no important interactions between CO(2) during growth and water deficit with respect to photosynthetic metabolism. Elevated CO(2 )will benefit sunflower growing under water deficit by marginally increasing P(n), and by slowing transpiration, which will decrease the rate and severity of water deficits, with limited effects on metabolism.
TL;DR: In this paper, a computational fluid dynamic software (cfd2000 ) was used to study the climate and crop transpiration distributions in a 22×8 m 2 plastic tunnel situated in Avignon, France, together with a global solar radiation model and a crop heat exchange model.
TL;DR: Field experiments indicate that CO(2) enrichment will increase crop water use efficiency mainly by increasing photosynthesis and growth, and yield should be most responsive to CO( 2) when temperatures approximate the optimum for crop growth.
Abstract: Yield of water-limited crops is determined by crop water use and by plant water use efficiency, each of which will be affected by the anticipated rise in atmospheric carbon dioxide (CO(2)) concentration and concomitant increase in temperature. At the leaf level, a given proportional increase in CO(2) concentration generally elicits a similar relative increase in transpiration efficiency (ratio of net photosynthesis to transpiration). The increase in transpiration efficiency may result both from an increase in photosynthetic rate and a decrease in stomatal conductance. Feedbacks involved in scaling from leaf to crop constrain the increase in net carbon gain and reduce the anti-transpiration effect of CO(2) enrichment. As a result, the increase in crop water use efficiency at high CO(2) typically is less than 75% of that measured at the leaf level. By accelerating crop development and reducing harvest index, higher temperatures often erode yield benefits of improved water use efficiency at high CO(2). The fraction of available water that is used by crops could increase with CO(2) concentration because of greater root growth and faster canopy closure, but these effects have received scant study. Field experiments indicate that CO(2) enrichment will increase crop water use efficiency mainly by increasing photosynthesis and growth. Yield should be most responsive to CO(2) when temperatures approximate the optimum for crop growth. Elevating CO(2) can ameliorate negative effects of above-optimal temperatures, but temperatures near the upper limit for crops will depress yields irrespective of CO(2) concentration.
TL;DR: It is hypothesized that a population's fraction of the total community Leaf Area Index, more than species identity, determines which species takes up most of the spring and summer precipitation and is discussed in the context of Walter and Stadelmann's (1974) water partitioning hypothesis.
Abstract: We contrasted the seasonal use of simulated large rain events (24 mm) by three native species of the arid Colorado Plateau: the perennial grass Hilaria jame- sii and two shrubs Artemesia filifolia and Coleogyne ramosissima. Deuterium-enriched water was used to dis- tinguish shallow "pulse" water from water in deeper soil layers that were unaffected by the water input. We also measured the leaf gas exchange rates of watered and un- watered control plants for 5 days after the rain event. H. jamesii had twice the pulse water proportion in its xy- lem than the two shrubs in spring (approx. 70% vs 35%). In summer, the pulse water proportions of all species were around 70%. The increase in the relative pulse wa- ter uptake of the two shrubs was caused primarily by a reduction in the rate of water uptake from deeper sourc- es, consistent with the decrease in the availability of stored winter water. Rain increased the rates of gas ex- change in C. ramosissima in both seasons, in H. jamesii only in summer and had no significant effect on A. filifo- lia. In H. jamesii, summer rain also increased water use efficiency. This suggests three principle mechanisms for rainwater use: (1) immediate increase in gas exchange via stomatal opening (C. ramisissima), (2) immediate in- crease in water use efficiency through restoration of the photosynthetic apparatus (H. jamesii) and (3) conserva- tion of deeper soil water, potentially extending photosyn- thetic activity into later drought periods ( A. filifolia). On a ground-area basis, A. filifolia was by far the largest consumer of spring and summer rain, due to its greater ground cover, while rain use by H. jamesii was negligi- ble. We hypothesize that a population's fraction of the total community Leaf Area Index, more than species identity, determines which species takes up most of the spring and summer precipitation and we discuss this idea in the context of Walter and Stadelmann's (1974, In: Brown JW Jr (ed) Desert biology. Academic Press, New York, pp 213-310) water partitioning hypothesis.
TL;DR: In this article, the free air CO2 enrichment (FACE) facility (n = 3) in a 14m tall Pinus taeda L. stand was designed to reduce uncertainties in predicting such responses, and continuous measurements of precipitation, throughfall precipitation, sap flux, and soil moisture were made over 3.5 years under ambient CO2a and elevated CO2e conditions.
Abstract: Increasing atmospheric CO2 concentration decreases stomatal conductance in many species, but the savings of water from reduced transpiration may permit the forest to retain greater leaf area index (L). Therefore, the net effect on water use in forest ecosystems under a higher CO2 atmosphere is difficult to predict. The free air CO2 enrichment (FACE) facility (n = 3) in a 14-m tall (in 1996) Pinus taeda L. stand was designed to reduce uncertainties in predicting such responses. Continuous measurements of precipitation, throughfall precipitation, sap flux, and soil moisture were made over 3.5 years under ambient (CO2a) and elevated (CO2e) ambient + 200 µmol mol−1). Annual stand transpiration under ambient CO2 conditions accounted for 84–96% of latent heat flux measured with the eddy-covariance technique above the canopy. Under CO2e, P. taeda transpired less per unit of leaf area only when soil drought was severe. Liquidambar styraciflua, the other major species in the forest, used progressively less water, settling at 25% reduction in sap flux density after 3.5 years under CO2e. Because P. taeda dominated the stand, and severe drought periods were of relatively short duration, the direct impact of CO2e on water savings in the stand was undetectable. Moreover, the forest used progressively more water under CO2e, probably because soil moisture availability progressively increased, probably owing to a reduction in soil evaporation caused by more litter buildup in the CO2e plots. The results suggest that, in this forest, the effect of CO2e on transpiration was greater indirectly through enhanced litter production than directly through reduced stomatal conductance. In forests composed of species more similar to L. styraciflua, water savings from stomatal closure may dominate the response to CO2e.
TL;DR: A strong reduction of predawn water potential in roots and shoots, as well as on transpiration rate, was found and a drought-sensitive cluster, originating from regions with high annual precipitation, had low water potential and transpiration rates, aswell as high concentrations of fructose, ABA and proline after drought.
Abstract: Summary
• The effects of drought on European beech (Fagus sylvatica) were assessed in a pot experiment under controlled conditions.
• Plants from 11 autochthonous provenances originating from regions in Germany, which differed in annual precipitation, were exposed to a 3-wk drought period in a glasshouse after the first stage of shoot growth had been completed.
• Drought reduced the water content to 97% of control in leaves and axes and to 92% in the roots. A strong reduction of predawn water potential in roots and shoots, as well as on transpiration rate, was found. In the roots, the effect on water potential was the same for all provenances, but differences were observed in the shoot water potential. Leaf concentrations of abscisic acid (ABA), proline and sucrose increased in the drought-treated plants compared with the controls.
• Two extreme clusters from opposite climatic sites were identified by cluster analysis. A drought-sensitive cluster, originating from regions with high annual precipitation, had low water potential and transpiration rates, as well as high concentrations of fructose, ABA and proline after drought. Water potential and transpiration rates were less affected by drought in the other cluster, which comprised two provenances of relatively dry habitats, and concentrations of hexose, ABA and proline were low.
TL;DR: These data illustrate that the hydrological response of a closed-canopy plantation to elevated CO2 depends on the temporal and spatial scale of observation and confirm that integration of measurements over space and time reduce what, at the leaf level, might otherwise appear to be a large and significant response.
Abstract: ○ The hydrological response of forests to rising CO 2 is a critical biotic feedback in the study of global climate change. Few studies, however, have investigated this highly dynamic response at relevant temporal and spatial scales. ○ A combination of leaf and whole-tree measurements and stand-level extrapolations were used to assess how stomatal conductance, canopy transpiration and conductance, and evapotranspiration might be affected by future, higher CO 2 concentrations. ○ Midday measurements of stomatal conductance for leaves sampled in a 12-yr-old sweetgum (Liquidambar styraciflua) stand exposed to free-air CO 2 enrichment were up to 44% lower at elevated than at ambient CO 2 concentrations, whereas canopy conductance, averaged over the growing season, was only 14% lower in stands exposed to CO 2 enrichment. The magnitude of this response was dependent on vapor pressure deficit and soil water potential. Annual estimates of evapotranspiration showed relatively small reductions due to atmospheric CO 2 enrichment. ○ These data illustrate that the hydrological response of a closed-canopy plantation to elevated CO 2 depends on the temporal and spatial scale of observation. They emphasize the importance of interacting variables and confirm that integration of measurements over space and time reduce what, at the leaf level, might otherwise appear to be a large and significant response.
TL;DR: The possibility of improving the water use efficiency (q) of maize crops by applying fertiliser N was tested under irrigated and rainfed conditions as discussed by the authors, and three field experiments were undertaken in 2 years using a split-plot design with watering regimes (rainfed and irrigation) as main plots and N (0 and 100 kg N−1) as sub-plots.
TL;DR: Investigation of the effects of increasing soil NaCl on growth, water flow, ion transport and intracellular compartmentation of Na + and Cl − in 1-year-old seedlings of Populus euphratica Oliv found that salt tolerance of P. euphrata likely depends on its ability to restrict salt transport to leaves.
TL;DR: Stomatal conductance of pine trees was more strongly affected by vapour pressure deficit than that of other species because of their small soil-to-leaf conductance, which is explainable in terms of xylem tracheids in conifer trees.
Abstract: A model of stomatal conductance was developed to relate plant transpiration rate to photosynthetic active radiation (PAR), vapour pressure deficit and soil water potential. Parameters of the model include sensitivity of osmotic potential of guard cells to photosynthetic active radiation, elastic modulus of guard cell structure, soil-to-leaf conductance and osmotic potential of guard cells at zero PAR. The model was applied to field observations on three functional types that include 11 species in subtropical southern China. Non-linear statistical regression was used to obtain parameters of the model. The result indicated that the model was capable of predicting stomatal conductance of all the 11 species and three functional types under wide ranges of environmental conditions. Major conclusions included that coniferous trees and shrubs were more tolerant for and resistant to soil water stress than broad-leaf trees due to their lower osmotic potential, lignified guard cell walls, and sunken and suspended guard cell structure under subsidiary epidermal cells. Mid-day depression in transpiration and photosynthesis of pines may be explained by decreased stomatal conductance under a large vapour pressure deficit. Stomatal conductance of pine trees was more strongly affected by vapour pressure deficit than that of other species because of their small soil-to-leaf conductance, which is explainable in terms of xylem tracheids in conifer trees. Tracheids transport water by means of small pit-pairs in their side walls, and are much less efficient than the end-perforated vessel members in broad-leaf xylem systems. These conclusions remain hypothetical until direct measurements of these parameters are available.
TL;DR: It is suggested that the presence of a dimorphic root system in deciduous species may play a role in facilitating leaf expansion near the end of the dry season when the soil surrounding shallow lateral roots is still dry.
Abstract: The magnitude and direction of water transport by the roots of eight dominant Brazilian savanna (Cerrado) woody species were determined with a heat pulse system that allowed bidirectional measurements of sap flow. The patterns of sap flow observed during the dry season in species with dimorphic root systems were consistent with the occurrence of hydraulic redistribution of soil water, the movement of water from moist to drier regions of the soil profile via plant roots. In these species, shallow roots exhibited positive sap flow (from the soil into the plant) during the day and negative sap flow (from the plant into the soil) during the night. Sap flow in the taproots was positive throughout the 24-h period. Diel fluctuations in soil water potential, with maximum values occurring at night, provided evidence for partial rewetting of upper soil layers by water released from shallow roots. In other species, shallow roots exhibited negative sap flow during both the day and night, indicating that hydraulic redistribution was occurring continuously. A third sap flow pattern was observed at the end of the dry season after a heavy rainfall event when sap flow became negative in the taproot, and positive in the small roots, indicating movement of water from upper soil layers into shallow roots, and then into taproots and deeper soil layers. Experimental manipulations employed to evaluate the response of hydraulic redistribution to changes in plant and environmental conditions included watering the soil surface above shallow roots, decreasing transpiration by covering the plant and cutting roots where probes were inserted. Natural and manipulated patterns of sap flow in roots and stems were consistent with passive movement of water toward competing sinks in the soil and plant. Because dry shallow soil layers were often a stronger sink than the shoot, we suggest that the presence of a dimorphic root system in deciduous species may play a role in facilitating leaf expansion near the end of the dry season when the soil surrounding shallow lateral roots is still dry.
TL;DR: It is suggested that were plant density to be included, it too would further share the response, further diminishing the changes required per unit leaf area, so that canopy assimilation over a day is a scaled sum of daily water use and of photosynthetic nitrogen display.
Abstract: We introduce the simultaneous optimisation of water-use effi ciency and nitrogen-use effi ciency of canopy photosynthesis. As a vehicle for this idea we consider the optimal leaf area for a plant in which there is no self-shading among leaves. An emergent result is that canopy assimilation over a day is a scaled sum of daily water use and of photosynthetic nitrogen display. The respective scaling factors are the marginal carbon benefi ts of extra transpiration and extra such nitrogen, respectively. The simple approach successfully predicts that as available water increases, or evaporative demand decreases, the leaf area should increase, with a concomitant reduction in nitrogen per unit leaf area. The changes in stomatal conductance are therefore less than would occur if leaf area were not to change. As irradiance increases, the modelled leaf area decreases, and nitrogen/leaf area increases. As total available nitrogen increases, leaf area also increases. In all the examples examined, the sharing by leaf area and properties per unit leaf area means that predicted changes in either are less than if predicted in isolation. We suggest that were plant density to be included, it too would further share the response, further diminishing the changes required per unit leaf area.
TL;DR: In this paper, a model is developed to predict drought-induced mortality of woody plants under different climatic scenarios, assuming that plant mortality is controlled by the carbon balance: when the plant is unable to transport water to the leaves it ceases to acquire carbon and, if this situation lasts long enough, it can no longer survive.
TL;DR: The fact that no interaction between ABA stress messages and hydraulic constraints to sap flow was necessary to impair leaf growth suggests that the targets of ABA and hydraulic-limitation effects on leaf expansion are not the same.
Abstract: In order to investigate whether plant hydraulic conductance (gplant) is reduced under drought conditions via an ABA-related mechanism, a water-stress experiment was carried out using split-rooted grapevines. In addition, inversion of shoot growth orientation was imposed to reduce gplant independently of soil water availability, and thus of the putative ABA root-generated stress message. As expected, water stress imposed on split-roots affected ABA accumulation. ABA drought-stress message negatively affected stomatal conductance (gs) and transpiration (E), but modified neither leaf or stem water potentials (Ψleaf and Ψstem, respectively), nor gplant. When gplant was reduced in split-rooted, shoot-inverted (s-r/s-i) grapevines, Ψleaf and Ψstem decreased, without changes in ABA accumulation, gs and E. The ABA drought-stress message did not modify gplant, nor did gplant (impaired by shoot-growth inversion) decrease ABA delivery to the leaves. However, leaf growth was depressed in s-r/s-i grapevines. The fact that no interaction between ABA stress messages (caused by split-root technique) and hydraulic constraints to sap flow (caused by shoot inversion) was necessary to impair leaf growth suggests that the targets of ABA and hydraulic-limitation effects on leaf expansion are not the same.
TL;DR: Overall, gibberellic acid treatment stimulated the vegetative growth of both cultivars ofwheat under salt stress, but it caused a slight reduction in grain yield.
Abstract: A sand culture experiment assessed whether gibberellic acid(GA3) could alleviate the adverse effects of salt stress on thegrowth, ion accumulation and photosynthetic capacity of two spring wheatcultivars, Barani-83 (salt sensitive) and SARC-I (salt tolerant).Three-week-oldplants of both cultivars were exposed to 0, 100 and 200 molm−3 NaCl in Hoagland's nutrient solution. Threeweeks after the initiation of salt treatments, half of the plants of eachcultivar were sprayed overall with 100 mg L−1GA3 solution. Plants were harvested 3 weeks after theapplication of GA3. Fresh and dry weights of shoots and roots, plantheight and leaf area were decreased with increasing supply of salt, butgibberellic acid treatment caused a significant ameliorative effect on both thecultivars with respect to these growth attributes. However, GA3caused no significant change in grain yields but increased grain size in boththe cultivars. Saline growth medium caused a marked increase in theconcentrations of Na+ and Cl− in shoots androots of both the lines. However, with the application of GA3accumulation of Na+ and Cl− was enhanced inboth shoots and roots of both wheat lines, but more ions accumulated in saltsensitive Barani-83 than in salt tolerant SARC-1. Net CO2assimilation rate (A) of both wheat lines decreased consistently withincreasingsupply of NaCl, but application of GA3 alleviated the effect of saltstress on this variable in both the cultivars. However, the ameliorative effectof the hormone was more pronounced in Barani-83 than in SARC-1. Althoughwater-use efficiency (A/E = CO2assimilation/transpiration) and intrinsic water use efficiency(A/gs = CO2 assimilation/stomatalconductance) decreased significantly with increasing salt concentration of thegrowth medium in both the cultivars, GA3 was more effective inenhancing both the water-use attributes in Barani-83 than in SARC-1. Overall,GA3 treatment stimulated the vegetative growth of both cultivars ofwheat under salt stress, but it caused a slight reduction in grain yield.GA3 treatment enhanced the accumulation of Na+ andCl− in both shoots and roots of wheat plants under saltstress.It also caused a significant increase in photosynthetic capacity in both linesat the vegetative stage under both saline and non-saline media.
TL;DR: In the experiment reported here, NAD-ME grasses increased their WUE under drought to a greater extent than their NADP-ME counterparts, primarily related to control of water loss relative to carbon gain at the leaf, rather than the plant, level.
Abstract: We investigated the response to drought of nine NAD-malic enzyme (NAD-ME) and nine NADP-malic enzyme (NADP-ME) C4 grasses. Species were grown from seeds in potted soil in a glasshouse. Seedlings were either watered regularly or exposed to two successive drying cycles of 8-10 d each, after which plants were harvested. Under well-watered conditions, average water use efficiency (WUE; dry mass gain per unit water transpired) was similar for NAD-ME and NADP-ME C4 grasses, and ranged between 6.0 and 8.7 g dry mass kg-1 H2O. Drought enhanced WUE of most species, but to a significantly greater extent in NAD-ME (1.20-fold) than NADP-ME (1.11-fold) grasses. Inhibition of dry matter accumulation (average of 12%) and shoot elongation under drought was similar among the C4 grasses. Leaf dry matter carbon (δ13C) and oxygen (δ18O) isotope compositions were significantly different between the two C4 subtypes. Leaf δ13C averaged -13.3 and -12.2, and leaf δ18O averaged 26.0 and 26.9 in well-watered NAD-ME and NADP-ME grasses, respectively. Drought significantly reduced leaf δ13C in most C4 grasses by an average 0.5. Leaf δ18O was not significantly affected by drought, indicating that leaf δ18O does not reflect drought-induced changes in leaf transpiration of C4 grasses. In the experiment reported here, NAD-ME grasses increased their WUE under drought to a greater extent than their NADP-ME counterparts. Increased WUE of the C4 grasses under drought was primarily related to control of water loss relative to carbon gain at the leaf, rather than the plant, level.
TL;DR: In this paper, the carbon dioxide and water vapor exchange were measured over a young and an old-growth ponderosa pine (Pinus ponderosaDougl. ex P. & C. Laws) ecosystem located in a semiarid environment in central Oregon.
TL;DR: An efficient control of photochemical and non-photochemical quenching and adjustments in the partition of electron flow between assimilative andnon-assimilative processes alleviated the danger of photoinhibition, but the results suggest that losses in potential carbon gain because of high photorespiratory costs could impose strong limitations on leaf carbon balance of cerrado woody species.
Abstract: High irradiances, high air temperatures and low relative humidities characterize the seasonal savannas of central Brazil, locally known as cerrados. In the present study, we investigated the implications to photoprotection of diurnal adjustments in photochemical and non-photochemical processes in five cerrado woody species that differed in photosynthetic capacity and in the duration and extension of the midday depression of photosynthesis. We also evaluated the contribution of photorespiration to minimize the danger of photoinhibition and the potential carbon costs of the operation of this mechanism in response to changes in irradiance levels. Notwithstanding the large differences in diurnal patterns of photosynthesis and in stomatal conductance, four out of the five species showed a tight common linear relationship between net CO2 assimilation rates and transpiration over the large range of environmental conditions that prevailed during typical sunny days at the end of the rainy season. Large reversible decreases in photochemical efficiency were compensated by proportional increases in non-photochemical processes related to photoprotection, irrespective of the prevailing irradiance levels and degree of stomata closure. Light response curves were used to evaluate the relative contribution of photorespiratory CO2 production to electron flow in response to changes in irradiance levels. A large percentage of the electron flow was used to drive photorespiration in light-saturated leaves. In conclusion, an efficient control of photochemical and non-photochemical quenching and adjustments in the partition of electron flow between assimilative and non-assimilative processes alleviated the danger of photoinhibition. However, the results also suggest that losses in potential carbon gain because of high photorespiratory costs could impose strong limitations on leaf carbon balance of cerrado woody species.
TL;DR: It is suggested that stomatal closure during drought contains xylem embolism to a minimum value, which can be speculated that an increase in cavitation resistance by cultural practices or genetic selection may increase drought survival in maize.
Abstract: Water relations during drought and xylem vulnerability to embolism were studied on four maize (Zea mays L.) genotypes having contrasting grain yields under drought conditions. Drought provoked a drop in xylem pressure, leaf water potential and whole-plant transpiration. Transpiration was reduced to a minimum value when xylem pressures reached ca. –1.6 MPa. This value corresponded to the threshold xylem pressure below which xylem embolism developed to a substantial degree in leaf midribs. Therefore, xylem embolism always remained low in leaf veins, even when plants exhibited clear water-stress symptoms. This suggests that stomatal closure during drought contains xylem embolism to a minimum value. Cavitation resistance was not related to grain yield under drought conditions for the four genotypes evaluated. However, it can be speculated that an increase in cavitation resistance by cultural practices or genetic selection may increase drought survival in maize.
TL;DR: In this article, the influence of salinity and boron levels on tomato yield and transpiration was investigated in lysimeters, and a dominant stress-factor model was used to determine the more severe stress determines yield.
Abstract: Boron is essential to growth at low concentrations and limits growth and yield when in excess. Little is known regarding plant response to excess boron (B) and salinity occurring simultaneously. The influences of B and salinity on tomatoes (Lycopersicon esculentum Mill. Cv `5656') were investigated in lysimeters. Salinity levels were 1, 3, 6 and 9 dSm−1 and B levels were 0.028, 0.185, 0.37, 0.74, 1.11, 1.48 mol m−3. Excess boron was found to decrease yield and transpiration of tomatoes. This effect was inhibited when plants were exposed to simultaneous B and salinity stresses. Both irrigation water salinity and boron concentration influenced water use of the plants in the same manner as they influenced yield. While yield was found to decrease with increased boron concentration in leaf tissue, increased salinity led to decreased boron accumulation. Yield response was found to correlate better to B concentration in irrigation water and soil solution than to plant tissue B content. A dominant-stress-factor model was assumed and validated. The model applies the principle that when a plant is submitted to conditions of stress caused by B in conjunction with salinity, the more severe stress determines yield. The results of this study have significance in modeling and management of high salinity high boron conditions. Under saline conditions, differences in crop yield and in water use may not be experienced over a significant range of boron concentrations.