Changes In Root Hydraulic Conductivity Research Articles

Changes in root hydraulic conductivity in wheat (Triticum aestivum L.) in response to salt stress and day/night can best be explained through altered activity of aquaporins.

Salt stress reduces plant water flow during day and night. It is not known to which extent root hydraulic properties change in parallel. To test this idea, hydroponically grown wheat plants were grown at four levels of salt stress (50, 100, 150 and 200 mM NaCl) for 5-8d before harvest (d14-18) and subjected to a range of analyses to determine diurnal changes in hydraulic conductivity (Lp) at cell, root and plant level. Cell pressure probe analyses showed that the Lp of cortex cells was differentially affected by salt stress during day and night, and that the response to salt stress differed between the main axis of roots and lateral roots. The Aquaporin (AQP) inhibitor H2 O2 reduced Lp to a common, across treatments, level as observed in salt-stressed plants during the night. Analyses of transpiring plants and exuding root systems provided values of root Lp which were in the same range as values modeled based on cell-Lp. The results can best be explained through a change in root Lp in response to salt stress and day/night, which results from an altered activity of AQPs. qPCR gene expression analyses point to possible candidate AQP isoforms.

Differential Aquaporin Response to Distinct Effects of Two Zn Concentrations after Foliar Application in Pak Choi (Brassica rapa L.) Plants

Zinc (Zn) is considered an essential element with beneficial effects on plant cells; however, as a heavy metal, it may induce adverse effects on plants if its concentration exceeds a threshold. In this work, the effects of short-term and prolonged application of low (25 µM) and high (500 µM) Zn concentrations on pak choi (Brassica rapa L.) plants were evaluated. For this, two experiments were conducted. In the first, the effects of short-term (15 h) and partial foliar application were evaluated, and in the second a long-term (15 day) foliar application was applied. The results indicate that at short-term, Zn may induce a rapid hydraulic signal from the sprayed leaves to the roots, leading to changes in root hydraulic conductance but without effects on the whole-leaf gas exchange parameters. Root accumulation of Zn may prevent leaf damage. The role of different root and leaf aquaporin isoforms in the mediation of this signal is discussed, since significant variations in PIP1 and PIP2 gene expression were observed. In the second experiment, low Zn concentration had a beneficial effect on plant growth and specific aquaporin isoforms were differentially regulated at the transcriptional level in the roots. By contrast, the high Zn concentration had a detrimental effect on growth, with reductions in the root hydraulic conductance, leaf photosynthesis rate and Ca2+ uptake in the roots. The abundance of the PIP1 isoforms was significantly increased during this response. Therefore, a 25 µM Zn dose resulted in a positive effect in pak choi growth through an increased root hydraulic conductance.

Changes in root hydraulic conductance in relation to the overall growth response of maize seedlings to partial root-zone nitrogen application

The influence mechanism of heterogeneous nutrient distribution on compensatory growth of maize seedlings is a key topic in the interaction between crops and soil environment. We evaluated the significance of root hydraulic conductance (Lp) and related parameter changes in water uptake and growth regulation of plants under different nitrogen (N) conditions. Maize seedlings were grown in split-root containers containing N solutions. Three N treatments were applied: (1) full-strength N (control: C), in which both sub-root systems received 4.0 mM N (each sub-root: C44); (2) partial N deficit (D), in which each sub-root system received 2.0 mM N (D42) or full-strength N (D44); and (3) partial N resupply (S), in which both sub-root systems received 2.0 mM N for six days, followed by 2.0 mM N (S22) or full-strength N (S24). The shoot dry mass in D increased gradually with early development, and was greater than that in C and S within 15–21 day, suggesting that the superiority of partial N deficit (D) in term of maize seedling growth was apparent and caused compensatory growth. Moreover, the slope and intercept in D44 and D42 between ΨL vs. Lp and shoot N content vs. Lp were obviously greater than that in C44, S24 and S22, indicating that higher plant dry mass in D might be attributable to the maintenance of a similar ΨL and improved shoot N content. In addition, the slopes and intercepts in D44 and D42 between Lp vs. root surface area and shoot N content vs. root surface area were higher than that in S24 and S22, suggesting that compared with partial N resupply, partial N deficit was more advantageous to root water uptake and N accumulation at the same level of root surface area, thus resulted in higher dry mass of maize seedlings. However, partial N resupply significantly increased root cortex thickness/diameter ratio and reduced root vessel diameter, which resulted in lower Lp, ΨL and shoot N content during 12–21 day. Unexpectedly, at 21 day, the shoot dry mass in S could recover to the level of control. The regulation mechanisms in partial N resupply will be the focus of future studies. Thus, when adopting the method of partial N application, it is necessary to consider the soil N condition before partial application.

Apoplastic barriers, aquaporin gene expression and root and cell hydraulic conductivity in phosphate-limited sheepgrass plants.

Mineral nutrient supply can affect the hydraulic property of roots. The aim of the present work on sheepgrass (Leymus chinensis L.) plants was to test whether any changes in root hydraulic conductivity (Lp; exudation analyses) in response to a growth-limiting supply of phosphate (P) are accompanied by changes in (1) cell Lp via measuring the cell pressure, (2) the aquaporin (AQP) gene expression by performing qPCR and (3) the formation of apoplastic barriers, by analyzing suberin lamella and Casparian bands via cross-sectional analyses in roots. Plants were grown hydroponically on complete nutrient solution, containing 250 µM P, until they were 31-36 days old, and then kept for 2-3weeks on either complete solution, or transferred on solution containing 2.5µM (low-P) or no added P (no-P). Phosphate treatments caused significant decreases in root and cell-Lp and AQP gene expression, while the formation of apoplastic barriers increased, particularly in lateral roots. Experiments using the AQP inhibitor mercury (Hg) suggested that a significant portion of radial root water uptake in sheepgrass occurs along a path involving AQPs, and that the Lp of this path is reduced under low- and no-P. It is concluded that a growth-limiting supply of phosphate causes parallel changes in (1) cell Lp and aquaporin gene expression (decrease) and (2) apoplastic barrier formation (increase), and that the two may combine to reduce root Lp. The reduction in root Lp, in turn, facilitates an increased root-to-shoot surface area ratio, which allocates resources to the root, sourcing the limiting nutrient.

Changes in root hydraulic conductivity facilitate the overall hydraulic response of rice (Oryza sativa L.) cultivars to salt and osmotic stress

The aim of the present work was to assess the significance of changes in root AQP gene expression and hydraulic conductivity (Lp) in the regulation of water balance in two hydroponically-grown rice cultivars (Azucena, Bala) which differ in root morphology, stomatal regulation and aquaporin (AQP) isoform expression. Plants were exposed to NaCl (25 mM, 50 mM) and osmotic stress (5%, 10% PEG6000). Root Lp was determined for exuding root systems (osmotic forces driving water uptake; ‘exudation Lp’) and transpiring plants (hydrostatic forces dominating; ‘transpiration-Lp’). Gene expression was analysed by qPCR. Stress treatments caused a consistent and significant decrease in plant growth, transpirational water loss, stomatal conductance, shoot-to-root surface area ratio and root Lp. Comparison of exudation-with transpiration-Lp supported a significant contribution of AQP-facilitated water flow to root water uptake. Changes in root Lp in response to treatments were correlated much stronger with root morphological characteristics, such as the number of main and lateral roots, surface area ratio of root to shoot and plant transpiration rate than with AQP gene expression. Changes in root Lp, involving AQP function, form an integral part of the plant hydraulic response to stress and facilitate changes in the root-to-shoot surface area ratio, transpiration and stomatal conductance.

Rapid changes in root hydraulic conductivity and aquaporin expression in rice (Oryza sativa L.) in response to shoot removal - xylem tension as a possible signal.

Background and Aims It is not clear how plants adjust the rate of root water uptake to that of shoot water loss. The aim of this study on rice was to test the idea that root aquaporins (AQPs) and xylem tension play a role in this adjustment. Methods Three-week-old rice (Oryza sativa L.) plants, which were grown hydroponically, had their entire shoot system removed, and root hydraulic conductivity (exudation analyses) and gene expression (quantitative real-time PCR) of root plasma membrane intrinsic aquaporin proteins (PIPs) was followed within 60 min after shoot excision. Key Results All three PIP1 genes (OsPIP1;1, OsPIP1;2 and OsPIP1;3) and three of the six PIP2 genes tested (OsPIP2;1, OsPIP2;4 and OsPIP2;5) showed a rapid (5 min) and lasting (60 min) decrease in gene expression. Expression decreased by up to 85 % within 60 min. The other three PIP2 genes tested (OsPIP2;2, OsPIP2;3 and OsPIP2;6) showed a varied response, with expression decreasing either only initially (5 min) or after 60 min, or not changing at all. In a follow-up experiment, plants had their shoot system removed and the detached root system immediately connected to a vacuum pump through which some tension (80 kPa) was applied. This application of tension prevented any significant decrease in PIP expression. Conclusions Shoot removal leads to a rapid decrease in expression of all PIP1s and some PIP2s in roots of rice. Xylem tension plays some role in this process.

New Insights into the Regulation of Aquaporins by the Arbuscular Mycorrhizal Symbiosis in Maize Plants Under Drought Stress and Possible Implications for Plant Performance

The relationship between modulation by arbuscular mycorrhizae (AM) of aquaporin expression in the host plant and changes in root hydraulic conductance, plant water status, and performance under stressful conditions is not well known. This investigation aimed to elucidate how the AM symbiosis modulates the expression of the whole set of aquaporin genes in maize plants under different growing and drought stress conditions, as well as to characterize some of these aquaporins in order to shed further light on the molecules that may be involved in the mycorrhizal responses to drought. The AM symbiosis regulated a wide number of aquaporins in the host plant, comprising members of the different aquaporin subfamilies. The regulation of these genes depends on the watering conditions and the severity of the drought stress imposed. Some of these aquaporins can transport water and also other molecules which are of physiological importance for plant performance. AM plants grew and developed better than non-AM plants under the different conditions assayed. Thus, for the first time, this study relates the well-known better performance of AM plants under drought stress to not only the water movement in their tissues but also the mobilization of N compounds, glycerol, signaling molecules, or metalloids with a role in abiotic stress tolerance. Future studies should elucidate the specific function of each aquaporin isoform regulated by the AM symbiosis in order to shed further light on how the symbiosis alters the plant fitness under stressful conditions.

Coordinated Post-translational Responses of Aquaporins to Abiotic and Nutritional Stimuli in Arabidopsis Roots

In plants, aquaporins play a crucial role in regulating root water transport in response to environmental and physiological cues. Controls achieved at the post-translational level are thought to be of critical importance for regulating aquaporin function. To investigate the general molecular mechanisms involved, we performed, using the model species Arabidopsis, a comprehensive proteomic analysis of root aquaporins in a large set of physiological contexts. We identified nine physiological treatments that modulate root hydraulics in time frames of minutes (NO and H2O2 treatments), hours (mannitol and NaCl treatments, exposure to darkness and reversal with sucrose, phosphate supply to phosphate-starved roots), or days (phosphate or nitrogen starvation). All treatments induced inhibition of root water transport except for sucrose supply to dark-grown plants and phosphate resupply to phosphate-starved plants, which had opposing effects. Using a robust label-free quantitative proteomic methodology, we identified 12 of 13 plasma membrane intrinsic protein (PIP) aquaporin isoforms, 4 of the 10 tonoplast intrinsic protein isoforms, and a diversity of post-translational modifications including phosphorylation, methylation, deamidation, and acetylation. A total of 55 aquaporin peptides displayed significant changes after treatments and enabled the identification of specific and as yet unknown patterns of response to stimuli. The data show that the regulation of PIP and tonoplast intrinsic protein abundance was involved in response to a few treatments (i.e. NaCl, NO, and nitrate starvation), whereas changes in the phosphorylation status of PIP aquaporins were positively correlated to changes in root hydraulic conductivity in the whole set of treatments. The identification of in vivo deamidated forms of aquaporins and their stimulus-induced changes in abundance may reflect a new mechanism of aquaporin regulation. The overall work provides deep insights into the in vivo post-translational events triggered by environmental constraints and their possible role in regulating plant water status.

The capacity for nitrate regulation of root hydraulic properties correlates with species’ nitrate uptake rates

A mechanism whereby water flow towards root surfaces is stimulated when exposed to nutrient patches may be evolutionarily desirable in environments with heterogeneous soils. Indeed, the presence of nitrate has been shown to increase root hydraulic conductance in a few agricultural species characterized by high nitrate demand. Does a similar stimulation of root conductivity in response to external nitrate addition exist among wild type organisms? To answer this question, we studied and compared the effect of a sudden increase of nitrate concentration on root hydraulic properties in seven species. These included three agricultural species (Cucumis sativus L., Solanum esculentum L., Zea mays L.) and four wild type species (Arabidopsis thaliana L., Festuca arundinacea Schreb., Populus trichocarpa Torr.&Gray, Nephrolepis exaltata L.). The selected species differed in overall nitrate demand and specific nitrate uptake rates. Changes in root hydraulic conductance induced by nitrate varied from non-existent (N. exaltata L.) to more than 50% increases (Z. mays L.). The magnitude of the hydraulic response to nitrate presence was significantly correlated with species nitrate uptake rate. This finding suggests an emergent physiological trait that links together plant nitrate needs, nitrate availability and root hydraulic properties.

Kinetic analyses of plant water relocation using deuterium as tracer – reduced water flux of Arabidopsis pip2 aquaporin knockout mutants

Due to reduced evaporation and diffusion of water molecules containing heavier isotopes, leaf water possesses an elevated (18)O or (2)H steady-state content. This enrichment has been exploited in plant physiology and ecology to assess transpiration and leaf water relations. In contrast to these studies, in this work the (2)H content of the medium of hydroponically grown Arabidopsis thaliana was artificially raised, and the kinetics of (2)H increase in the aerial parts recorded during a short phase of 6-8 h, until a new equilibrium at a higher level was reached. A basic version of the enrichment models was modified to establish an equation that could be fitted to measured leaf (2)H content during uptake kinetics. The fitting parameters allowed estimation of the relative water flux q(leaf) into the Arabidopsis rosette. This approach is quasi-non-invasive, since plants are not manipulated during the uptake process, and therefore, offers a new tool for integrated analysis of plant water relations. The deuterium tracer method was employed to assess water relocation in Arabidopsis pip2;1 and pip2;2 aquaporin knockout plants. In both cases, q(leaf) was significantly reduced by about 20%. The organ and cellular expression patterns of both genes imply that changes in root hydraulic conductivity, as previously demonstrated for pip2;2 mutants, and leaf water uptake and distribution contributed in an integrated fashion to this reduced flux in intact plants.

Aquaporin‐mediated changes in hydraulic conductivity of deep tree roots accessed via caves

Although deep roots can contribute substantially to whole-tree water use, little is known about deep root functioning because of limited access for in situ measurements. We used a cave system on the Edwards Plateau of central Texas to investigate the physiology of water transport in roots at 18-20 m depth for two common tree species, Quercus fusiformis and Bumelia lanuginosa. Using sap flow and water potential measurements on deep roots, we found that calculated root hydraulic conductivity (RHC) fluctuated diurnally for both species and decreased under shading for B. lanuginosa. To assess whether these dynamic changes in RHC were regulated during initial water absorption by fine roots, we used an ultra-low flowmeter and hydroxyl radical inhibition to measure in situ fine root hydraulic conductivity (FRHC) and aquaporin contribution to FRHC (AQPC), respectively. During the summer, FRHC and AQPC were found to cycle diurnally in both species, with peaks corresponding to the period of highest transpirational demand at midday. During whole-tree shade treatments, B. lanuginosa FRHC ceased diurnal cycling and decreased by 75 and 35% at midday and midnight, respectively, while AQPC decreased by 41 and 30% during both time periods. A controlled growth-chamber study using hydroponically grown saplings confirmed daily cycling and shade-induced reductions in FRHC and AQPC. Winter measurements showed that the evergreen Q. fusiformis maintained high FRHC and AQPC throughout the year, while the deciduous B. lanuginosa ceased diurnal cycling and exhibited its lowest annual values for both parameters in winter. Adjustments in FRHC and AQPC to changing canopy water demands may help the trees maintain the use of reliable water resources from depth and contribute to the success of these species in this semi-arid environment.

Changes in Root Hydraulic Conductivity During Wheat Evolution

Abstract: A better understanding of the mechanisms of water uptake by plant roots should be vital for improving drought resistance and water use efficiency (WUE). In the present study, we have demonstrated correlations between root system hydraulic conductivity and root characteristics during evolution using six wheat evolution genotypes (solution culture) with different ploidy chromosome sets (Triticum boeoticum Bioss., T. monococcum L.: 2n=2x=14; T. dicoccides Koern., T. dicoccon (Schrank) Schuebl.: 2n = 4x = 28; T. vulgare Vill., T. aestivum L. cv. Xiaoyan No. 6: 2n = 6x = 42). The experimental results showed that significant correlations were found between root system hydraulic conductivity and root characteristics of the materials with the increase in ploidy chromosomes (2x→6x) during wheat evolution. Hydraulic conductivity of the wheat root system at the whole‐plant level was increased with chromosome ploidy during evolution, which was positively correlated with hydraulic conductivity of single roots, whole plant biomass, root average diameter, and root growth (length, area), whereas the root/shoot ratio had an inverse correlation with the hydraulic conductivity of root system with increasing chromosome ploidy during wheat evolution. Therefore, it is concluded that that the water uptake ability of wheat roots was strengthened from wild to modern cultivated species during evolution, which will provide scientific evidence for genetic breeding to improve the WUE of wheat by genetic engineering.(Managing editor: Ya‐Qin HAN)

Hydraulic conductance and mercury-sensitive water transport for roots of Opuntia acanthocarpa in relation to soil drying and rewetting.

Drought-induced changes in root hydraulic conductance (LP) and mercury-sensitive water transport were examined for distal (immature) and mid-root (mature) regions of Opuntia acanthocarpa. During 45 d of soil drying, LP decreased by about 67% for distal and mid-root regions. After 8 d in rewetted soil, LP recovered to 60% of its initial value for both regions. Axial xylem hydraulic conductivity was only a minor limiter of LP. Under wet conditions, HgCl2 (50 microM), which is known to block membrane water-transport channels (aquaporins), decreased LP and the radial hydraulic conductance for the stele (L(R, S)) of the distal root region by 32% and 41%, respectively; both LP and L(R, S) recovered fully after transfer to 2-mercaptoethanol (10 mM). In contrast, HgCl2 did not inhibit LP of the mid-root region under wet conditions, although it reduced L(R, S) by 41%. Under dry conditions, neither LP nor L(R, S) of the two root regions was inhibited by HgCl2. After 8 d of rewetting, HgCl2 decreased LP and L(R, S) of the distal region by 23% and 32%, respectively, but LP and L(R, S) of the mid-root region were unaltered. Changes in putative aquaporin activity accounted for about 38% of the reduction in LP in drying soil and for 61% of its recovery for the distal region 8 d after rewetting. In the stele, changes in aquaporin activity accounted for about 74% of the variable L(R, S) during drought and after rewetting. Thus, aquaporins are important for regulating water movement for roots of O. acanthocarpa.

Effects of Ca-signalling inhibitors on short-term cold-acclimation of hydraulic conductivity in roots of Brassica rapa plants

Summary The signal transmission during short-term acclimation in Brassica rapa plants confronted with low root zone temperatures was explored by using signal blockers in root medium with (or without) low temperatures, and examining changes in root hydraulic conductivity. EGTA, lanthanum, and verapamil did not block this root response suggesting that Ca2+ exchanges with the apoplast on the plasmalemma were not strictly required for signal transfer. In contrast, treatments with lithium ions or neomycin blocked the reestablishment of root hydraulic conductivity during cold acclimation. This result is consistent with the involvement of IP3 signaling in the response, and suggests the role of Ca2+ ions from internal stores as secondary messenger. The effect of t-BHQ (modulator of Ca2+ exchange between internal stores and protoplast) reinforces this hypothesis of IP3 intervention in delivering Ca2+ ions on the endomembrane system. The target of the Ca2+ ions would be enzymes (such as protein kinases) rather than calmodulin itself.

Competitive strategies for water availability in two Mediterranean Quercus species

Competition for water availability was studied in a mixed natural stand of Quercus suber L. and Quercus cerris L. growing in Sicily by measuring diurnal changes of leaf conductance to water vapour (gL), water potential (ΨL) and relative water content (RWC) in April, July and October 1997 as well as the seasonal changes in root hydraulic conductance per unit leaf surface area (KRL). Quercus cerris behaved as a drought‐tolerant species, with strong reductions of KRL, ΨL, and RWC in the summer. By contrast, Q. suber appeared to withstand summer drought by an avoidance strategy based on reducing gL, maintaining ΨL and RWC high and KRL at the same level as that measured in the spring. A ‘conductance ratio’ (CR) was calculated in terms of the ratio of gL to KRL. Seasonal changes of this ratio contrasted in the two species, thus suggesting that Q. suber and Q. cerris did not really compete for available water. In the summer, when Q. suber was extracting water from the soil to maintain high leaf hydration, Q. cerris had restricted water absorption, thus suffering drought but tolerating its effects. The possibility that cohabitation of drought‐tolerant with drought‐avoiding species can be generalized is also discussed.

Changes in root hydraulic conductance (KR) of Olea oleaster seedlings following drought stress and irrigation

Quasi‐steady‐state measurements of root hydraulic conductance (KR) of Olea oleaster Hoffmgg. et Link potted seedlings were performed using a pressure chamber with the aim of: (a) measuring the impact of different water‐stress levels on a KR; (b) measuring the kinetics of KR recovery several days after soil rewetting; (c) relating changes in KR to changes in root anatomy and morphology. Increasing water‐stress was applied in terms of ratio of leaf water potential (ΨL) measured at midday to that at zero turgor (ΨTLP), i.e. ΨL/ΨTLP=0·5, 1·0, 1·2, 1·6; KR was measured initially and at 24, 48, 72, 96 h after irrigation.Values of KR in seedlings stressed to ΨL/ΨTLP=1·2 increased for 48 h after irrigation from 0·23 to 0·97×10−5 kg s−1 m−2 MPa−1 i.e. from 16% to 66% of that measured in unstressed seedlings. A marked shift of the x‐axis intercept of the straight line relating flow to pressure (zero flow at non‐zero pressure) was recorded initially after irrigation and persisted up to 48 h. Recovery of KR occurred within 24 h after irrigation in seedlings at ΨL/ΨTLP=0·5 and 48 h later in those at ΨL/ΨTLP=1·0.Severe drought stress (ΨL/ΨTLP=1·6) caused anatomical changes to roots which formed a two‐layered exodermis with thicker suberized walls and a three‐ to four‐layered endodermis with completely suberized tangential walls. Recovery of KR in these roots required resumed growth of root tips and emergence of new lateral roots.

Changes in Root Hydraulic Conductivity for Two Tropical Epiphytic Cacti as Soil Moisture Varies

The tropical epiphytic cacti Epiphyllum phyllanthus and Rhipsalis baccifera experience extreme variations in soil moisture due to limited soil volumes and episodic rainfalls. To examine possible root rectification, whereby water uptake from a wet soil occurs readily but water loss to a dry soil is minimal, responses of root hydraulic conductivity (Lp) to soil drying and rewetting were investigated along with the underlying anatomical changes. After 30 d of soil drying, Lp decreased 50%–70% for roots of both species, primarily because increased suberization of the periderm reduced radial conductivity. Sheaths composed of soil particles, root hairs, and mucilage covered young roots and helped reduce root desiccation. Axial (xylem) conductance increased during drying due to vessel differentiation and maturation, and drought-induced embolism was relatively low. Within 4 d of rewetting, Lp for roots of both species attained predrought values; radial conductivity increased for young roots due to the growth of new branch roots initiated during drying and for older roots due to the development of radial breaks in the periderm. The decreases in Lp during drought reduced plant water loss to a dry soil, and yet maximal water uptake and transpiration occurred within a few days of rewetting, helping these epiphytes to take advantage of episodic rainfalls in a moist tropical forest.

Changes in root hydraulic conductivity for two tropical epiphytic cacti as soil moisture varies

The tropical epiphytic cacti Epiphyllum phyllanthus and Rhipsalis baccifera experience extreme variations in soil moisture due to limited soil volumes and episodic rainfalls. To examine possible root rectification, whereby water uptake from a wet soil occurs readily but water loss to a dry soil is minimal, responses of root hydraulic conductivity (Lp) to soil drying and rewetting were investigated along with the underlying anatomical changes. After 30 d of soil drying, Lp decreased 50%–70% for roots of both species, primarily because increased suberization of the periderm reduced radial conductivity. Sheaths composed of soil particles, root hairs, and mucilage covered young roots and helped reduce root desiccation. Axial (xylem) conductance increased during drying due to vessel differentiation and maturation, and drought‐induced embolism was relatively low. Within 4 d of rewetting, Lp for roots of both species attained predrought values; radial conductivity increased for young roots due to the growth of new branch roots initiated during drying and for older roots due to the development of radial breaks in the periderm. The decreases in Lp during drought reduced plant water loss to a dry soil, and yet maximal water uptake and transpiration occurred within a few days of rewetting, helping these epiphytes to take advantage of episodic rainfalls in a moist tropical forest.

Responses of red alder and black cottonwood seedlings to flooding

Red alder (Alnus rubra Bong.) and black cottonwood (Populus trichocarpa Torr. & Gray) seedlings were monitored to evaluate response during a 20‐day period of artificial flooding and a 20‐day recovery period following flooding. During the flooding period, both species showed changes in nutrient uptake and transport, initiated stemderived adventitious roots that became aerenchymatous, and exhibited hypertrophied lenticels. Flooded red alder seedlings also showed reduced height and leaf area growth and developed lower‐stem hypertrophy. Flooded black cottonwood seedlings exhibited root dieback, aerenchyma in below ground root tips, and changes in root hydraulic conductance and xylem pressure potential. Contrary to expectations, however, stomatal closure following flooding was not observed in either species. Flooded red alder seedlings increased growth rapidly when drained, and by the end of the recovery period, formerly flooded and non‐flooded red alder seedlings differed only minimally in this respect. In contrast, several characteristics of black cottonwood – including growth rate and nutrient content – still differed between formerly flooded and non‐flooded seedlings at the end of the recovery period. Based on observed treatment differences at the end of the experiment, red alder seedlings were judged to be more tolerant of flooding than black cottonwood.

Amelioration of Chilling-Induced Water Stress by Abscisic Acid-Induced Changes in Root Hydraulic Conductance

Pretreatment of soybean (Glycine max L. var Ransom) root systems with abscisic acid (ABA) ameliorates the deleterious effect of low temperatures on root hydraulic conductance. ABA treatment of root systems subsequently chilled to 10 degrees C with shoots at 25 degrees C resulted in higher leaf water potentials and lower stomatal resistances. If the root systems are left at 25 degrees C, ABA causes stomatal closure. Membrane alterations are suggested as a mechanism for the ABA action in plant response to chilling stress.