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At least 433 records · Page 24Linked to original sources
PubMed2002

Drought regulation of GST8, encoding the Arabidopsis homologue of ParC/Nt107 glutathione transferase/peroxidase.

Glutathione transferases (GST; EC 2.5.1.18) have been involved in many biotic and abiotic interactions of plants with their environment, but very little information is available on their regulation and possible role in drought tolerance. The GST8 gene of Arabidopsis thaliana encodes, as assessed by phylogenetic analysis, the homologue of an extremely conserved subgroup of Tau GSTs. During rapid dehydration of seedlings and progressive drought stress of mature plants, GST8 transcript levels increased following slower kinetics than RNAs for dehydration responsive genes RD29A and B, and strictly paralleled, in a mostly ABA independent manner, expression of oxidative stress marker PRX. GST8 RNA levels were also consistently increased by oxidative stress, high doses of auxin or cytokinin, and to a lesser extent, by wounding. RNA levels of ERD13, a previously described rapid dehydration responsive GST of the Phi class, were not co-regulated with those of GST8. Our results suggest that a drought-associated oxidative stress induces accumulation of GST8, whose function could be to counteract the effect of higher ROS production in stressed plants

Journal Article↗
PubMed2000

Reduced growth of Alaskan white spruce in the twentieth century from temperature-induced drought stress.

The extension of growing season at high northern latitudes seems increasingly clear from satellite observations of vegetation extent and duration. This extension is also thought to explain the observed increase in amplitude of seasonal variations in atmospheric CO2 concentration. Increased plant respiration and photosynthesis both correlate well with increases in temperature this century and are therefore the most probable link between the vegetation and CO2 observations. From these observations, it has been suggested that increases in temperature have stimulated carbon uptake in high latitudes and for the boreal forest system as a whole. Here we present multi-proxy tree-ring data (ring width, maximum late-wood density and carbon-isotope composition) from 20 productive stands of white spruce in the interior of Alaska. The tree-ring records show a strong and consistent relationship over the past 90 years and indicate that, in contrast with earlier predictions, radial growth has decreased with increasing temperature. Our data show that temperature-induced drought stress has disproportionately affected the most rapidly growing white spruce, suggesting that, under recent climate warming, drought may have been an important factor limiting carbon uptake in a large portion of the North American boreal forest. If this limitation in growth due to drought stress is sustained, the future capacity of northern latitudes to sequester carbon may be less than currently expected.

Alaska↗
PubMed2003

Stress-responsive zinc finger gene ZPT2-3 plays a role in drought tolerance in petunia.

The petunia gene, ZPT2-3, encodes a Cys2/His2-type zinc finger protein. Here, we describe the expression of ZPT2-3 in response to various stresses and the effects of ZPT2-3 overexpression in transgenic petunia. Mechanical wounding induced accumulation of ZPT2-3 transcript, and the activity of ZPT2-3::luciferase was conferred by the 1668-bp ZPT2-3 upstream sequence, both locally and systemically. This induction was mediated by a jasmonic acid (JA)-dependent and ethylene-independent pathway. ZPT2-3 expression was also induced by cold, drought, and heavy metal treatments. The same ZPT2-3 promoter sequence showed similar responsiveness to wounding, cold, drought, and JA treatments in Arabidopsis when investigated in a beta-glucuronidase (GUS) reporter gene, indicating conservation of similar signaling pathways between the two plant species. ZPT2-3 functioned as an active repressor in a transient assay using Arabidopsis leaves. Constitutive overexpression of ZPT2-3 in transgenic petunia plants increased tolerance to dehydration. These results demonstrate the involvement of ZPT2-3 in plant response to various stresses, and suggest its potential utility to improve drought tolerance.

Acclimatization↗
PubMed2006

Physiological responses of forest trees to heat and drought.

The heat wave of summer 2003 was the largest and the most persistent ever experienced in Central Europe and has fuelled concern about the effects of climate change on European ecosystems. Since forests constitute the most important European ecosystems, in this review article we assess current knowledge on the effects of heat and drought on key metabolic processes for growth and productivity of forest trees. In particular, the general consequences of heat and drought on (1) photosynthesis and respiration at the cellular and community level, and (2) on nutrient uptake, partitioning and competition for nutrients are summarized. The latter are a major sink for photosynthetic energy and, therefore, are indirectly but strongly connected to the performance of photosynthesis. In addition, the interaction of heat and drought with stress compensation mechanisms and emission of biogenic volatile organic compounds (BVOC) are discussed, since these processes are directly connected to carbon metabolism. Effects on the emission of BVOC are also included because they constitute an important feedback mechanism on ozone formation and, thus, on atmospheric pollution. As far as available, data collected during the 2003 heat wave are included and discussed.

Hot Temperature↗
PubMed2004

Different classes of proteases are involved in the response to drought of Phaseolus vulgaris L. cultivars differing in sensitivity.

Protein breakdown and recycling, which depend on the levels of proteolytic enzymes, are an essential part of the plant response to environmental stress. In order to study changes in the activity of proteases in Phaseolus vulgaris L. subjected to water deficit, three cultivars of European origin that exhibit different degrees of sensitivity to drought were chosen on the basis of changes in water potential, psiw, water and protein contents of leaves during progressive water deficit, and loss of membrane integrity after osmotic stress. Twenty-day-old plants were subjected to water deficit by withholding irrigation. Specific enzyme activities in leaf extracts were determined for plants under different degrees of drought stress using different substrates and protease inhibitors. Proteolytic activities were partially characterized by gel exclusion chromatography. Activities of two of the three identified serine proteinases changed under water deficit. The activity of the one with apparent molecular mass of approximately 65 kDa was observed to increase progressively with increasing withdrawal of water in the more sensitive cultivars, but to decrease in the more resistant cultivar. The same activity was elevated in senescent leaves. Under conditions of severe water deficit, the most sensitive cultivar exhibited a marked increase in the activity of two different aminopeptidases, while the more resistant cultivar showed a significant decrease in the activity of these aminopeptidases. These results point to complex and probably specific roles for different proteases in the plant response to drought.

Aminopeptidases↗
PubMed2004

Impact of the heroin 'drought' on patterns of drug use and drug-related harms.

Since late 2000, anecdotal reports from drug users and health professionals have suggested that there was a reduction in the supply of heroin in Adelaide in the first half of 2001, referred to as a heroin 'drought'. The aim of this paper was to critically review evidence for this, using data obtained from 100 injecting drug users surveyed for the 2001 Illicit Drug Reporting System (IDRS). This project is carried out annually in all Australian jurisdictions, and collects up-to-date information on the markets for heroin, methamphetamine, cocaine and cannabis. This paper also investigates the possible implications of this 'drought' on patterns of drug use and drug-related harms. The 2001 IDRS found consistent reports by users of an increase in the price of heroin, together with decreases in purity and availability. These factors resulted in a decrease in the frequency of self-reported heroin use among those surveyed in 2001, and a concomitant increase in the use of other drugs, in particular methamphetamine and morphine. The heroin 'drought' appears to have had a substantial impact on several indices of drug-related harm. There was a marked decrease in the number of opioid-related fatalities, and hospital data also showed reductions in heroin-related presentations. Treatment service data showed an increase in the number of admissions related to amphetamines. There is a need for health promotion and education on the adverse effects of methamphetamine use, and the development of improved treatment protocols for methamphetamine abuse and dependence.

Australia↗
PubMed2002

Photosynthetic carbon reduction and carbon oxidation cycles are the main electron sinks for photosystem II activity during a mild drought.

Stomatal closure can explain the inhibition of net CO2 uptake by a leaf subjected to a mild drought: the photosynthetic apparatus appears resistant to lack of water. Changes in both the water content of leaves maintained in a constant environment and the ambient CO2 molar fraction during measurements on well-hydrated leaves lead to similar effects on net CO2 uptake and whole chain electron transport as estimated by leaf chlorophyll fluorescence measurements. In particular, it is shown that photosystem II (PSII) functioning and its regulation are not qualitatively changed during desiccation and that the variations in PSII photochemistry can simply be understood by changes in substrate availability in this condition. Moreover, an analysis of the literature shows that when inhibition of net CO2 uptake by C3 leaves under drought (Phaseolus vulgaris L., Helianthus annus L. and Solanum tuberosum L.) was lower than 80 %, elevated CO2 completely restored the photosynthetic capacity. The CO2 molar fraction in the chloroplasts declines as stomata close in drying leaves. As a consequence, in C3 plants, ribulose-1,5-bisphosphate oxygenation increases and becomes the main sink for photosynthetic electrons. Depending on the prevailing photon flux density, the O2 uptake through photorespiratory activity can entirely replace carbon dioxide as an electron acceptor, or not. The rate of the Mehler reaction remains low and unchanged during desiccation. However, drought could also involve CO2-sensitive modification of the photosynthetic metabolism depending on plant growth conditions and possibly also on plant species.

Carbon↗
PubMed2002

Regulation of photosynthesis of C3 plants in response to progressive drought: stomatal conductance as a reference parameter.

We review the photosynthetic responses to drought in field-grown grapevines and other species. As in other plant species, the relationship between photosynthesis and leaf water potential and/or relative water content in field-grown grapevines depends on conditions during plant growth and measurements. However, when light-saturated stomatal conductance was used as the reference parameter to reflect drought intensity, a common response pattern was observed that was much less dependent on the species and conditions. Many photosynthetic parameters (e.g. electron transport rate, carboxylation efficiency, intrinsic water-use efficiency, respiration rate in the light, etc.) were also more strongly correlated with stomatal conductance than with water status itself. Moreover, steady-state chlorophyll fluorescence also showed a high dependency on stomatal conductance. This is discussed in terms of an integrated down-regulation of the whole photosynthetic process by CO2 availability in the mesophyll. A study with six Mediterranean shrubs revealed that, in spite of some marked interspecific differences, all followed the same pattern of dependence of photosynthetic processes on stomatal conductance, and this pattern was quite similar to that of grapevines. Further analysis of the available literature suggests that the above-mentioned pattern is general for C3 plants. Even though the patterns described do not necessarily imply a cause and effect relationship, they can help our understanding of the apparent contradictions concerning stomatal vs. non-stomatal limitations to photosynthesis under drought. The significance of these findings for the improvement of water-use efficiency of crops is discussed.

Biological Transport↗
PubMed2002

Rubisco activity: effects of drought stress.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity is modulated in vivo either by reaction with CO2 and Mg2+ to carbamylate a lysine residue in the catalytic site, or by the binding of inhibitors within the catalytic site. Binding of inhibitors blocks either activity or the carbamylation of the lysine residue that is essential for activity. At night, in many species, 2-carboxyarabinitol-1-phosphate (CA1P) is formed which binds tightly to Rubisco, inhibiting catalytic activity. Recent work has shown that tight-binding inhibitors can also decrease Rubisco activity in the light and contribute to the regulation of Rubisco activity. Here we determine the influence that such inhibitors of Rubisco exert on catalytic activity during drought stress. In tobacco plants, 'total Rubisco activity', i.e. the activity following pre-incubation with CO2 and Mg2+, was positively correlated with leaf relative water content. However, 'total Rubisco activity' in extracts from leaves with low water potential increased markedly when tightly bound inhibitors were removed, thus increasing the number of catalytic sites available. This suggests that in tobacco the decrease of Rubisco activity under drought stress is not primarily the result of changes in activation by CO2 and Mg2+ but due rather to the presence of tight-binding inhibitors. The amounts of inhibitor present in leaves of droughted tobacco based on the decrease in Rubisco activity per mg soluble protein were usually much greater than the amounts of the known inhibitors (CA1P and 'daytime inhibitor') that can be recovered in acid extracts. Alternative explanations for the difference between maximal and total activities are discussed.

Carbon Dioxide↗
PubMed2004

Effect of drought stress on lipid metabolism in the leaves of Arabidopsis thaliana (ecotype Columbia).

BACKGROUND AND AIMS: Cell membranes are major targets of environmental stresses. Lipids are important membrane components, and changes in their composition may help to maintain membrane integrity and preserve cell compartmentation under water stress conditions. The aim of this work was to investigate the effects of water stress on membrane lipid composition and other aspects of lipid metabolism in the leaves of the model plant, Arabidopsis thaliana. METHODS: Arabidopsis thaliana (ecotype Columbia) plants were submitted to progressive drought stress by withholding irrigation. Studies were carried out in plants with hydration levels ranging from slight to very severe water deficit. Enzymatic activities hydrolysing MGDG, DGDG and PC were measured. Expression of several genes essential to lipid metabolism, such as genes coding for enzymes involved in lipid biosynthesis (MGDG synthase, DGDG synthase) and degradation (phospholipases D, lipoxygenase, patatin-like lipolytic-acylhydrolase), was studied. KEY RESULTS: In response to drought, total leaf lipid contents decreased progressively. However, for leaf relative water content as low as 47.5 %, total fatty acids still represented 61 % of control contents. Lipid content of extremely dehydrated leaves rapidly increased after rehydration. The time-course of the decrease in leaf lipid contents correlated well with the increase in lipolytic activities of leaf extracts and with the expression of genes involved in lipid degradation. Despite a decrease in total lipid content, lipid class distribution remained relatively stable until the stress became very severe. CONCLUSIONS: Arabidopsis leaf membranes appeared to be very resistant to water deficit, as shown by their capacity to maintain their polar lipid contents and the stability of their lipid composition under severe water loss conditions. Moreover, arabidopsis displayed several characteristics indicative of a so far unknown adaptation capacity to drought-stress at the cellular level, such as an increase in the DGDG : MGDG ratio and fatty acid unsaturation.

Arabidopsis↗
PubMed2006

Differential adaptation of two varieties of common bean to abiotic stress: I. Effects of drought on yield and photosynthesis.

The yield of 24 commercial varieties and accessions of common bean (Phaseolus vulgaris) has been determined at different sites in Chile and Bolivia. Statistical analysis was performed in order to characterize whether a particular variety was more or less stable in yield under different environmental conditions. Amongst these, two varieties have been identified for more detailed study: one variety has a higher than average yield under unstressed conditions but is strongly affected by stress, and another has a reduced yield under unstressed conditions but is less affected by stress. The contrasting rate of abscission of the reproductive organs under drought stress was clearly consistent with these differences. The more tolerant genotype shows a great deal of plasticity at the biochemical and cellular level when exposed to drought stress, in terms of stomatal conductance, photosynthetic rate, abscisic acid synthesis, and resistance to photoinhibition. By contrast, the former lacks such plasticity, but shows an enhanced tendency for a morphological response, the movement of leaves, which appears to be its principal response to drought stress.

Abscisic Acid↗
PubMed2006

Overexpression of the regulator of G-protein signalling protein enhances ABA-mediated inhibition of root elongation and drought tolerance in Arabidopsis.

Regulator of G-protein signalling (RGS) proteins identified recently in Arabidopsis have been involved in the regulation of several physiological processes, but largely nothing is known about their roles at both the physiological and the molecular level. In the experiments reported here, the overexpression approach was used to present evidence that RGS1 protein plays critical roles in plant development and in modulating abscisic acid (ABA) and drought stress signal transduction. RGS1 affected the shapes of leaves, the development of floral buds, the elongation of stems, siliques, and hypocotyls, and the time of flowering. Post-germination growth was inhibited by 1 microM ABA, and root growth was hypersensitive to ABA for 35S-RGS1 transgenic plants. RGS1 overexpression conferred more drought tolerance to transgenic plants, as compared with the wild type (Columbia). Reverse transcription-PCR (RT-PCR) results indicated that RGS1 overexpresssion significantly stimulated the expression of NCED and ABA2, that encode two key enzymes catalysing ABA biosynthesis. Furthermore, the expression of several stress-regulated genes was either up- or down-regulated in RGS1-overexpressing transgenic plants. Combining the results above with previous results, it is suggested that RGS1 exerted its effects on plant responsiveness to ABA and drought tolerance largely through changing the expression either of genes responsible for ABA biosynthesis, which leads to changes in endogenous ABA levels, or of stress-responsive genes.

Abscisic Acid↗
PubMed2006

Sorghum stay-green QTL individually reduce post-flowering drought-induced leaf senescence.

Sorghum is an important source of food, feed, and biofuel, especially in the semi-arid tropics because this cereal is well adapted to harsh, drought-prone environments. Post-flowering drought adaptation in sorghum is associated with the stay-green phenotype. Alleles that contribute to this complex trait have been mapped to four major QTL, Stg1-Stg4, using a population derived from BTx642 and RTx7000. Near-isogenic RTx7000 lines containing BTx642 DNA spanning one or more of the four stay-green QTL were constructed. The size and location of BTx642 DNA regions in each RTx7000 NIL were analysed using 62 DNA markers spanning the four stay-green QTL. RTx7000 NILs were identified that contained BTx642 DNA completely or partially spanning Stg1, Stg2, Stg3, or Stg4. NILs were also identified that contained sub-portions of each QTL and various combinations of the four major stay-green QTL. Physiological analysis of four RTx7000 NILs containing only Stg1, Stg2, Stg3, or Stg4 showed that BTx642 alleles in each of these loci could contribute to the stay-green phenotype. RTx7000 NILs containing BTx642 DNA corresponding to Stg2 retained more green leaf area at maturity under terminal drought conditions than RTx7000 or the other RTx7000 NILs. Under post-anthesis water deficit, a trend for delayed onset of leaf senescence compared with RTx7000 was also exhibited by the Stg2, Stg3, and Stg4 NILs, while significantly lower rates of leaf senescence in relation to RTx7000 were displayed by all of the Stg NILs to varying degrees, but particularly by the Stg2 NIL. Greener leaves at anthesis relative to RTx7000, indicated by higher SPAD values, were exhibited by the Stg1 and Stg4 NILs. The RTx7000 NILs created in this study provide the starting point for in-depth analysis of stay-green physiology, interaction among stay-green QTL and map-based cloning of the genes that underlie this trait.

Chromosome Mapping↗
PubMedDate not supplied

Interactive effects of elevated CO(2) and soil drought on growth and transpiration efficiency and its determinants in two European forest tree species.

The responses of growth and transpiration efficiency (W = biomass accumulation/water consumption) to ambient and elevated atmospheric CO(2) concentrations (350 and 700 micro mol mol(-1), respectively) were investigated under optimal nutrient supply in well-watered and in drought conditions in two temperate-forest tree species: Quercus petraea Liebl. and Pinus pinaster Ait. Under well-watered conditions, doubling the CO(2) concentration for one growing season increased biomass growth by 138% in Q. petraea and by 63% in P. pinaster. In contrast, under drought conditions, elevated CO(2) increased biomass growth by only 47% in Q. petraea and had no significant effect on biomass growth in P. pinaster. Transpiration efficiency was higher in Q. petraea than in P. pinaster in all treatments. This difference was linked (i) to lower carbon isotope discrimination (Delta), and thus lower values of the intercellular/ambient CO(2) concentration (c(i)/c(a)) ratio, in Q. petraea, (ii) to lower values of leaf mass ratio (LMR, leaf mass/whole plant mass), which we suggest was positively related to the proportion of daytime carbon fixation lost by respiration (Phi), in Q. petraea, and (iii) to slightly lower C concentrations in Q. petraea than in P. pinaster. The CO(2)-promoted increase in W was higher in Q. petraea (+80%) than in P. pinaster (+50%), and the difference was associated with a more pronounced decrease in Phi in response to elevated CO(2) in Q. petraea than in P. pinaster, which could be linked with the N dilution effect observed in Q. petraea. Because Phi also directly affects growth, the CO(2)-induced enhancement of Phi in Q. petraea is a crucial determinant of the growth stimulation observed in this species. Leaf gas exchange regulation was not the only factor involved in the responses of growth and W to elevated CO(2) and drought, other physiological processes that have crucial roles include carbon and N allocation and respiration.

Journal Article↗
PubMed1998

Dynamics of change in stomatal response and water status of Picea abies during a persistent drought period: a contribution to the traditional view of plant water relations.

Four experiments on the simulation of a persistent drought period were carried out with cloned Picea abies (L.) Karst. trees: two in the field under varying weather conditions and two in a climate chamber under variously manipulated humidity conditions. Patterns of diurnal dynamics in gas exchange rates and water potential were monitored and analyzed. The first phase of the drought was characterized by relatively high daily maxima for photosynthesis and transpiration. With decreasing humidity during the day, the values dropped steeply, and the declines were larger and occurred earlier on each passing day of the drought period. When soil water potential was lower than -2000 hPa, maximum stomatal aperture was greatly reduced despite a humid atmosphere. Under these conditions, rates of photosynthesis and transpiration decreased less steeply from the daily maxima and differences between maxima and minima were small. In the field, the daily sums of transpiration and photosynthesis were more dependent on atmospheric conditions than on soil water potential. In the growth chamber experiments, the daily sums of transpiration and photosynthesis decreased continuously as the soil dried, at first steeply until a soil water potential of -2000 hPa was reached, then slowly. Predawn water potential values fluctuated under field conditions, but tended to decrease with time, whereas needle osmotic potential increased slightly. Because relative humidities did not reach 100% in the growth chamber, predawn water potentials of plants in the growth chamber were never higher than -1.0 MPa although the soil was saturated. In the experiment with a high average air humidity during the daily stress period, relatively high predawn water potentials were maintained until lower soil water potentials of -8000 hPa were reached. Results were used to assess the importance of evaporative demand versus soil drying on stomatal responses within the context of current concepts of plant water relations. The observed trends in diurnal dynamics can be explained solely by the interdependency of leaf conductance and water potential. Stomata react directly to the ratio of water supply to demand. The central role of peristomatal transpiration in this system is emphasized.

Journal Article↗
PubMed1999

Growth, water relations, and survival of drought-exposed seedlings from six maternal families of honey mesquite (Prosopis glandulosa): responses to CO(2) enrichment.

Low water availability is a leading contributor to mortality of woody seedlings on grasslands, including those of the invasive shrub Prosopis. Increasing atmospheric CO(2) concentration could favor some genotypes of this species over others if there exists intraspecific variation in the responsiveness of survivorship to CO(2). To investigate such variation, we studied effects of CO(2) enrichment on seedling survival in response to uniform rates of soil water depletion in six maternal families of honey mesquite (P. glandulosa Torr. var. glandulosa). Three families each from the arid and mesic extremes of the species' distribution in the southwestern United States were studied in environmentally controlled glasshouses. Relative water content at turgor loss and osmotic potential were not affected by CO(2) treatment. Increased atmospheric CO(2) concentration, however, increased growth, leaf production and area, and midday xylem pressure potential, and apparently reduced transpiration per unit leaf area of seedlings as soil dried. Consequently, CO(2) enrichment about doubled the fraction of seedlings that survived soil water depletion. Maternal families of honey mesquite differed in percentage survival of drought and in several other characteristics, but differences were of similar or of smaller magnitude compared with differences between CO(2) treatments. There was no evidence for genetic variation in the responsiveness of survivorship to CO(2). By increasing seedling survival of drought, increasing atmospheric CO(2) concentration could increase the abundance of honey mesquite where establishment is limited by water availability. Genetic types with superior ability to survive drought today, however, apparently will maintain that advantage in the future.

Journal Article↗
PubMed2001

Daily time course of whole-shoot gas exchange rates in two drought-exposed Mediterranean shrubs.

Effects of drought on water relations, whole-shoot gas-exchange characteristics, and pigment and zeatin concentrations were investigated in the Mediterranean shrubs rosemary (Rosmarinus officinalis L.) and lavender (Lavandula stoechas L.). Two-year-old, greenhouse-grown plants were placed in a whole-shoot gas-exchange measurement system and subjected to 10 days of drought, resulting in severe water stress, and then re-watered for 5 days in order to study their recovery. Water stress resulted in a significant decline in maximum whole-shoot net CO2 assimilation rates (An) for both species that was associated with reductions in leaf area and stomatal conductance. Because shoot dark respiration rate (Rd) was less sensitive to water stress than An, shoot Rd/An ratio increased from about 15 to 95% during water stress. No major changes in chlorophyll and carotenoid concentrations of rosemary leaves were observed during the experiments, but chlorophyll and carotenoid concentrations fell significantly in water-stressed lavender leaves. Zeatin concentrations were higher in rosemary leaves than in lavender leaves during water stress. After re-watering, whole-shoot An and Rd rapidly recovered to their pre-drought rates.

Carbon Dioxide↗
PubMed2002

Canopy stomatal conductance and xylem sap abscisic acid (ABA) in mature Scots pine during a gradually imposed drought.

We investigated the effect of drought on canopy stomatal conductance (g(c)), and examined the hypothesis that g(c) is controlled by the chemical messenger abscisic acid (ABA) produced in roots. Beginning in November 1994, we subjected a mature stand of Scots pine (Pinus sylvestris L.) to an imposed 11-month drought. Control plots were maintained at average-season soil water content. Xylem sap was extracted from shoots at regular intervals from April to November 1995. Soil water, sap flow and leaf water potentials (predawn to dusk) were recorded at the same time. Canopy stomatal conductance was calculated from sap flow data and xylem sap ABA concentrations ([ABA(xyl)]) were measured by radioimmunoassay. Mean [ABA(xyl)] in control trees was 250 micromol m(-3). No diurnal variation in [ABA(xyl)] was detected. With soil drying, [ABA(xyl)] increased to a maximum in summer (600 micromol m(-3)), but decreased again toward autumn; however, no significant increase in ABA flux to the leaves occurred. A decline in g(c) was detected when volumetric soil water content declined below 0.12. The decline in g(c) could not have been mediated by increasing [ABA(xyl)] because stomatal closure appeared to precede any increase in [ABA(xyl)]. Peak sap flow velocity data were used to estimate delivery times for root-to-shoot signals in 15-m tall trees. Under normal field conditions, a signal would take 12 days to travel from the site of production (roots) to the presumed site of action (shoots). However, under drought conditions it may take a chemical signal in excess of 6 weeks. We conclude that a feedforward model of short-term stomatal response to soil drying, based solely on the action of a chemical messenger from the roots, is not applicable in mature conifer trees because signal transmission is too slow.

Abscisic Acid↗