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.     

Genetic variation for carbon isotope composition in honey mesquite (Prosopis glandulosa)

Carbon isotope composition (delta(13)C) is a useful surrogate for integrated, plant water-use efficiency (WUE) when measured on plants grown in a common environment. In a variety of species, genetic variation in delta(13)C has been linked to the distribution of genotypes across gradients in atmospheric and soil water. We examined genetic variation for delta(13)C in seedlings of honey mesquite (Prosopis glandulosa Torr.), an invasive grassland shrub that thrives in the southwestern USA. Fifteen maternal families, representing progeny of 15 adult trees, were studied in three common garden experiments in a greenhouse. The 990-km east-west transect along which the adult trees were located encompasses a wide precipitation gradient, and includes mesic grassland, semiarid grassland, and Chihuahuan desert ecosystems. Genetic variation for delta(13)C in mesquite was substantial, with the rank order of half-sib families based on delta(13)C relatively stable across experiments, which were conducted under different environmental conditions. Conversely, rankings of families by mean seedling height (an index of growth rate) varied markedly among experiments. Seedlings derived from Chihuahuan desert adults emerged more quickly and had more negative delta(13)C (indicative of lower WUE) than seedlings derived from the other regions. Although delta(13)C and seedling height were not correlated, these results suggest that mesquite genotypes at the drier, western extreme of the species' range are adapted for quicker emergence and possibly faster growth than genotypes from mesic areas. Together, these traits may facilitate exploitation of infrequent precipitation events.     

Silvicultural treatments for sapling mesquite (Prosopis glandulosa var. glandulosa) to optimize timber production and minimize seedling encroachment

To maximize lumber production and minimize weed problems with Prosopis glandulosa, silvicultural treatment methods were evaluated nine years after initiation. A randomized complete block design was used with four replicates and six treatments. Plots were shredder-harvested leaving sixteen 2-m squares on 10-m spacing that included a final rotation crop tree. In three of the treatments the crop trees were pruned to a single stem. To prevent re-establishment of mesquite in the interstitial areas, plots were spot sprayed with herbicides, disked, or disked and seeded with rye grass in 1986. Herbicide treatments and disking continued yearly through 1989. In 1991, disk treatments were repeated. After both 2.5 and 9 yrs, significant treatment differences were found for growth of basal diameter, growth of basal area, and growth of dry weight. The greatest crop tree growth occurred in treatments that were pruned with interstitial competition suppressed. Mortality was greatest in the dense treatments, while re-establishment of mesquite was greatest in the more open treatments. The greatest basal diameter growth of 1.21 cm year⁻¹ in the disked and pruned treatments is comparable to other fine hardwoods in temperate and dry tropical forests.     

Carbon dioxide enrichment accelerates the decline in nutrient status and relative growth rate of Populus tremuloides Michx. seedlings

Changes in growth dynamics and mineral nutrient concentrations were measured in Populus tremuloides Michx., trembling aspen, grown for 100 days following germination in atmospheres containing 350 or 750 microl l(-1) CO(2). Seedlings were fertilized with nitrogen (N) at concentrations of 15.5 mM (high-N), 1.55 mM (medium-N), or 0.155 mM (low-N). Initially, relative growth rates were enhanced by CO(2) enrichment in each N regime, but the effects did not persist. In plants grown in high-N or medium-N, foliar concentrations of Ca and Mg decreased in response to CO(2) enrichment. During the 100-day study, whole-plant concentrations of N and P decreased in all treatments. The decreases in mineral nutrient concentrations over time were accelerated in CO(2)-enriched plants and accompanied the disappearance of the CO(2)-induced growth enhancement. It is concluded that the depression of relative growth rates often associated with long-term CO(2) enrichment of plants may result from decreases in plant nutrient status.     

Forage production and quality of 4 perennial grasses grown under and outside canopies of matureProsopis glandulosa Torr. var.glandulosa (mesquite)

Comparisons were made of dry matter production and forage quality of two coolseason grasses, Canada wildrye (Elymus canadensis L.), Virginia wildrye (Elymus virginicus L.), and two warm-season grasses, green panic (Panicum maximum var.trichoglume Eyles) and plains bristlegrass (Setaria leucopila Scribn. & Merr.), planted under and outside the canopies of mature honey mesquite (Prosopis glandulosa Torr. var.glandulosa). Green panic gave the greatest cumulative dry matter yield in both canopy (5120 kg ha^–1) and open (3370 kg ha^–1) locations, followed by plains bristlegrass under mesquite (1130 kg ha^–1) and in the open (570 kg ha^–1). One-time yields from Canada wildrye and Virginia wildrye, under and outside mesquite canopy, were 247 and 329 kg ha^–1, and 272 and 268 kg ha^–1, respectively. Dry matter production of green panic (p=0.001) and plains bristlegrass (p=0.026) was greater under mesquite only for the first of four harvests, although both warm-season grasses exhibited a trend for greater production under mesquite than in the open. Average nitrogen content of green panic was greater (p=0.0004) under mesquite than in the open, while plains bristlegrass exhibited a trend for greater crude protein content in the open than under mesquite. Overall, moisture content of the warm-season grasses was greater (p=0.0001) under mesquite than in the open, while Virginia wildrye was more (p=0.002) succulent under mesquite than in the open. Averagein-vitro dry matter digestibility of warm-season grasses was almost significantly greater (p=0.0501) in the open than under mesquite. Canopy soils contained significantly more organic C (p=0.0004) and total N (p=0.0001) than open soils, with differences of 8.3 Mg organic C ha^–1 and 1.3 Mg total N ha^–1. Correlations indicated that soil fertility was more limiting to grass production than light intensity.     

Leaf growth, photosynthesis and tissue water relations of greenhouse-grown Eucalyptus marginata seedlings in response to water deficits

Leaf growth, rate of leaf photosynthesis and tissue water relations of shoots of Eucalyptus marginata Donn ex Sm. (jarrah) seedlings were studied during a soil drying and rewatering cycle in a greenhouse experiment. Rates of leaf growth and photosynthesis were sensitive to water deficits. The rate of leaf growth decreased linearly with predawn leaf water potential to reach zero at -1.5 MPa. Rate of leaf growth did not recover completely within the first three days after rewatering. Midday photosynthetic rates declined to 40% of those of well-watered seedlings at a predawn leaf water potential of -1.0 MPa and reached zero at -2.2 MPa. Photosynthetic rate recovered rapidly following rewatering and almost fully recovered by the second day after rewatering. All tissue water relations parameters, except the bulk modulus of elasticity, changed significantly as the soil dried and recovered completely by the third day after rewatering. Changes in osmotic pressure at full turgor of 0.4 MPa indicated considerable capacity for osmotic adjustment. However, because there was little osmotic adjustment until predawn leaf water potential fell below -1.5 MPa, this capacity would not have enhanced seedling growth, although it may have increased seedling survival. The sensitivity of photosynthesis and relative water content to water deficits suggests that greenhouse-grown E. marginata seedlings behave like mesophytic plants, even though E. marginata seedlings naturally grow in a drought-prone environment.     

Carbon allocation, root exudation and mycorrhizal colonization of Pinus echinata seedlings grown under CO(2) enrichment

Increased exudation of carbon compounds from roots may provide a mechanism for enhancement of nutrient availability to plants growing in a CO(2)-enriched atmosphere. Therefore, the effect of atmospheric CO(2) concentration on carbon allocation and root exudation was investigated in Pinus echinata Mill. (shortleaf pine) seedlings. After 34 and 41 weeks, seedlings growing in 695 microl l(-1) CO(2) allocated proportionately more (14)C-labeled photosynthate to fine roots than did seedlings growing in ambient air. This was associated with greater fine root mass and mycorrhizal density in CO(2)-enriched plants after 34 weeks. Exudation of soluble, (14)C-labeled compounds from roots also was greater in these plants at 34 weeks, but the effect of CO(2) concentration on exudation did not persist at 41 weeks.     

TMS天目丝增抗剂在落叶松育苗中的应用

应用2000,1000,500,250,125,62mLL-1TMS天目丝增抗剂对200株长白落叶松苗进行6小时浸根,在生长季节(5月30日至10月20日)对经过天目丝增抗剂处理的造林苗每隔15天测定一次苗木的高生长、地径、主根长度,侧根数。实验结果表明,天目丝增抗剂处理可以极大地促进长白山落叶松苗木生长并能提高苗木质量。500mLL-1TMS处理的苗木生长效果最好,与常规育苗相比,高生长平均增加42.5%,地径生长平均增加30.7%,主根长平均增加14.0%,侧根数平均增加31.6%;Ⅰ级苗和Ⅱ级苗各占一半,没有Ⅲ级苗,显著地提高了经济效益。表3参8。     

Variation in water relations characteristics of terminal shoots of Port-Orford-cedar (Chamaecyparis lawsoniana) seedlings

We measured water relations attributes of the terminal shoots of 3-year-old Port-Orford-cedar (Chamaecyparis lawsoniana (A. Murr.) Parl.) seedlings that represented its geographic range. Pressure-volume curves were developed and osmotic potentials at full (psi(sf)) and zero turgor (psi(sz)), relative water content at zero turgor, and an index of tissue elasticity (IE) were calculated for 38 families during early, mid- and late summer at an inland nursery, and for 12 of these families during mid- and late summer at a coastal nursery. Compared with other conifer species, psi(sz) was high (-1.4 to -1.5 MPa) and declined in seedlings at both nurseries as the season progressed. Both IE and osmotic amplitude (psi(sf)-psi(sz)) increased during the season. Osmotic potential at zero turgor was lower and osmotic amplitude greater in seedlings at the inland nursery than at the coastal nursery. Correlations of water relations attributes with geographic location of the seed sources were weak and usually not significant. High elevation southern sources exhibited smaller differences in psi(sz) between nurseries than low elevation northern sources. The small differences in water relations attributes among sources and between nurseries suggest that some may be of marginal physiological importance; however, sources that produced larger seedlings appeared to be less desiccation tolerant. We conclude that, when moving genotypes during reforestation, decisions based on patterns in tree size and timing of growth will account for these small differences in water relations.     

Partial shoot removal increases net CO(2) assimilation and alters water relations of Citrus seedlings

Effects of defoliation on partial shoot removal by decapitation on seedling growth, water use and net gas exchange of remaining basal leaves, were examined in Citrus spp. Shoot and root growth rates were manipulated to test for effects of growth demands on net gas exchange. Partially defoliated plants had higher leaf pressure potentials, root conductivities and rates of water use than intact control plants. Shoot regrowth occurred at the expense of root loss. Basal leaves on defoliated plants consistently had higher rates of CO(2) assimilation (A) than leaves on intact plants. Stomatal conductance (g(s)) changed little after defoliation so the higher A of leaves on defoliated plants lowered the ratio of intercellular to ambient CO(2) concentration (C(i)/C(a)) in the mesophyll. In some cases, g(s) increased with A in defoliated plants but C(i)/C(a) was not affected. Stomatal conductance only limited A when intact seedlings were stressed by root confinement in small pots or when leaves were exposed to high vapor pressure deficits during gas exchange measurements. Increased carbon demand for shoot regrowth increased photosynthetic capacity and was more important than stomatal responses in determining A after partial shoot loss.     

Effects of cold storage and water stress on water relations and gas exchange of white spruce (Picea glauca) seedlings

To determine the effects of lifting time and storage on water-stress resistance of nursery-grown white spruce (Picea glauca (Moench) Voss) seedlings, we compared gas exchange, water relations and mortality of 3-year-old seedlings lifted in October 1991 and stored at -2 degrees C for 3 months with seedlings lifted in January 1992. The seedlings were placed in nutrient solution and subjected to -1.1 or -2.7 MPa water stress induced by polyethylene glycol 3350 for 9 days. Water stress, but not lifting time, had a significant effect on seedling net assimilation, symplastic volume and turgor loss point. In a second experiment, seedlings lifted in October 1991 were stored at -2 degrees C for 7 months and compared with seedlings lifted in May 1992. The seedlings were planted in pots, and their gas exchange and water relation parameters measured in response to gradual water stress. The results suggest that prolonged cold storage retards photosynthetic recovery of seedlings after planting. Higher rates of net assimilation in seedlings lifted in May were not directly related to their water status. Nonstomatal limitations were the primary factor influencing photosynthetic rate. We conclude that the inferior ability of cold-stored seedlings to tolerate water stress was due to poor osmotic adjustment and a lag in recovery of photosynthesis.     

Effect of elevated carbon dioxide concentration and root restriction on net photosynthesis, water relations and foliar carbohydrate status of loblolly pine seedlings

To determine the effects of CO(2)-enriched air and root restriction on photosynthetic capacity, we measured net photosynthetic rates of 1-year-old loblolly pine seedlings grown in 0.6-, 3.8- or 18.9-liter pots in ambient (360 micro mol mol(-1)) or 2x ambient CO(2) (720 micro mol mol(-1)) concentration for 23 weeks. We also measured needle carbohydrate concentration and water relations to determine whether feedback inhibition or water stress was responsible for any decreases in net photosynthesis. Across all treatments, carbon dioxide enrichment increased net photosynthesis by approximately 60 to 70%. Net photosynthetic rates of seedlings in the smallest pots decreased over time with the reduction occurring first in the ambient CO(2) treatment and then in the 2x ambient CO(2) treatment. Needle starch concentrations of seedlings grown in the smallest pots were two to three times greater in the 2x ambient CO(2) treatment than in the ambient CO(2) treatment, but decreased net photosynthesis was not associated with increased starch or sugar concentrations. The reduction in net photosynthesis of seedlings in small pots was correlated with decreased needle water potentials, indicating that seedlings in the small pots had restricted root systems and were unable to supply sufficient water to the shoots. We conclude that the decrease in net photosynthesis of seedlings in small pots was not the result of CO(2) enrichment or an accumulation of carbohydrates causing feedback inhibition, but was caused by water stress.     

Growth, survival, and genetic variability of Austrian pine (Pinus nigra Arnold) seedlings in response to water deficit

Drought is one of the crucial factors limiting plant growth and is expected to become even more limiting in many regions as the effects of climate change increase. Knowledge of genetic adaptation to environmental factors can help develop strategies for successful reforestation in arid terrain. In this study, Austrian pine (Pinus nigra Arnold) seedlings grown from seed collected from 40 open-pollinated families from five Balkan Peninsula provenances were evaluated for survival and growth under imposed drought. Within each provenance, seed was collected from an extremely harsh habitat and from a favorable habitat. One-year-old seedlings, from each provenance, habitat type and open pollinated family were transplanted into three “stressbeds”, each designed to simulate different levels of water availability by filling with varying ratios of crushed stone and soil. Seedling height growth and survival were recorded every 20–30?days from April–September. In all provenances, seedlings grown from seed originating from the harsh habitats had lower survival regardless of stressbed treatment compared to those grown from seed collected from favorable habitats, with the exception of one population from the Vi?egrad provenance which had high survival rates throughout. Overall height growth was very low and did not follow consistent patterns among treatment groups. Cluster analyses of survival data showed that populations from a similar habitat generally grouped together, regardless of provenance. The lowest and highest survival values were found in seedlings from harsh habitats, while those from the favorable habitats clustered in the mid-range. These results suggest that higher seed quality from productive stands may be the most influential factor determining seedling vigor under drought stress and that genetic adaptation to drought was not detected in adjacent populations having different habitat conditions.     

Photosynthesis, water relations and mortality in Abies lasiocarpa seedlings during natural establishment

Few studies have investigated the ecophysiology of natural seedling establishment in forest trees not associated with anthropogenic disturbance. Photosynthesis and water relations measurements were made on one- through four-year-old seedlings of Abies lasiocarpa Nutt. (subalpine fir) establishing naturally in an understory environment. First (current)-year seedlings generally had only cotyledons, whereas most second-year seedlings had both cotyledons and primary leaves. Mortality was high (> 60%) in first-year seedlings with the greatest mortality (> 90%) measured at the more open, sun-exposed sites within the understory. Seedling mortality was negligible after the first year of growth at shaded microsites and after the second year of growth at sunny microsites. Photosynthetic CO(2) uptake at light saturation was considerably lower in first-year than in fourth-year seedlings (0.6 micromol m(-2) s(-1) versus 1.7 micromol m(-2) s(-1)) and occurred at lower solar irradiance (240 micromol m(-2) s(-1) versus 600 micromol m(-2) s(-1) of photosynthetically active radiation). Differences in photosynthetic capacity were due to differences in both stomatal and non-stomatal limitations to CO(2) uptake. Carbon dioxide assimilation in first- and second-year seedlings was 28 and 29%, respectively, of the mean value measured for fourth-year seedlings. Although first-year seedlings had low transpiration rates, their water use efficiency (photosynthesis/transpiration) was less than half that of fourth-year seedlings and their water potentials were lower than those of all other age classes (0.46 mol CO(2) mmol(-1) H(2)O and -3.0 MPa, respectively). The stomatal limitation to CO(2) uptake was approximately 21% in first- and second-year seedlings, and increased to 39% in fourth-year seedlings. Intercellular CO(2) concentrations were greater in first- and second-year seedlings (255 and 250 microl l(-1), respectively) than in third- and fourth-year seedlings (203 and 186 microl l(-1), respectively). Thus, abrupt increases in water status and photosynthetic capacity after the first or second year of growth appear crucial for survival to maturity. Moreover, differences in temperature and water relations according to microsite may be major factors determining seedling establishment and, thus, the distributional and successional patterns observed for adult trees of Abies lasiocarpa.     

Adjustments in leaf water relations of mangrove (Avicennia germinans) seedlings grown in a salinity gradient

We used pressure-volume analysis and dewpoint hygrometry to determine leaf water relation parameters of mangrove (Avicennia germinans L.) seedlings grown at salinities of 0, 8, 20 and 32 per thousand. Seedlings responded to an increase in salinity from 0 to 32 per thousand by an increase in leaf succulence as reflected in an increase in leaf water content per unit area from 300 to 360 g m(-2). Additionally, osmotic potential at full turgor decreased from -2.3 to -3.5 MPa and osmotic potential at zero turgor decreased from -2.7 to -4.3 MPa. Cell elasticity decreased as salinity increased from 0 to 32 per thousand, as indicated by a progressive increase in volumetric modulus of elasticity from 19 to 27 MPa. Increased leaf succulence enabled leaves to sequester large amounts of solutes without adversely increasing cell osmotic pressure. On the other hand, osmotic adjustment facilitated turgor maintenance as water potential diminished. Salinity-induced decreases in tissue elasticity generated greater water potential differences between leaves and soil under saline conditions than under non-saline conditions.     

Use of non-timber forest products from invasive alien Prosopis species(mesquite) and native trees in South Africa: implications for management

Background: Prosopis species have been introduced to many areas outside their native range to provide benefits to local communities. Several Prosopis species and their hybrids(hereafter mesquite) have, however, become naturalised and invasive and now generate substantial costs. Management options are limited because of the complex conflicts of interest regarding benefits and costs. Management policies and strategies must take account of such conflicts, but further insights are needed on the dimensions of uses and impacts before such information can be usefully applied. Current policy in South Africa allows for the growth and use of mesquite in one province,but not in others where its control is mandatory. We report on a study to quantify the direct use and perceptions of non-timber forest products(NTFPs) from mesquite and native trees in South Africa.Methods: Semi-structures household interviews were conducted with various stakeholder groups to identify what tree products are used, to ascertain amounts used as well as to gauge perceptions of natural resource use between different tree species and use over time.Results: The direct household use value of native trees was higher than that of mesquite, and local stakeholders attached greater value to products from native trees than from mesquite. Therefore, native trees are and will still be preferentially harvested, and mesquite is unlikely to offer protection to native species by providing an alternative source of products. Mesquite pods do, however, provide valuable additional resources(fodder and medicinal products). The use of both native trees and mesquite is decreasing as the incomes of poorer households rise and as alternative energy sources become available. The benefits and reliance on mesquite are not as high as previously assumed and the impacts from mesquite invasions create large problems for local communities.Conclusion: This study provides further evidence that the impacts of mesquite exceed the benefits, lending support for a policy to reduce negative impacts.     

Gas exchange,biomass, whole-plant water-use efficiency and water uptake of peach (Prunus persica) seedlings in response to elevated carbon dioxide concentration and water availability

We examined the interactive effects of elevated CO2 concentration ([CO2]) and water stress on growth and physiology of 1-year-old peach (Prunus persica L.) seedlings grown in 10-dm3 pots in open-top chambers with ambient (350 micromol mol-1) or elevated (700 micromol mol-1) [CO2]. Seedlings were supplied weekly with a non-limiting nutrient solution. Water was withheld from half of the plants in each treatment for a 4-week drying cycle, to simulate a sudden and severe water stress during the phase of rapid plant growth. Throughout the growing season, seedlings in elevated [CO2] had higher assimilation rates, measured at the growth [CO2], than seedlings in ambient [CO2], and this caused an increase in total dry mass of about 33%. Stomatal conductance, total water uptake, leaf area and leaf number were unaffected by elevated [CO2]. Because seedlings in the two CO2 treatments had similar transpiration despite large differences in total dry mass, water-use efficiency (WUE) of well-watered and water-stressed seedlings grown in elevated [CO2] was an average of 51 and 63% higher, respectively, than WUE of comparable seedlings grown in ambient [CO2]. Elevated [CO2] enhanced total biomass of water-stressed seedlings by 31%, and thus ameliorated the effects of water limitation. However, the percentage increases in total dry mass between well-watered and water-stressed seedlings were similar in ambient (53%) and elevated (58%) [CO2], demonstrating that there was no interaction between elevated [CO2] and water stress. This finding should be considered when predicting responses of trees to global climate change in hot and dry environments, where predicted temperature increases will raise evaporative demands and exacerbate the effects of drought on tree growth.     

草本地下竞争对3种幼苗存活和生长的影响

为了研究不同土壤营养条件下草本地下竞争对华北落叶松、麦氏云杉、辽东栎3种1年生幼苗存活和生长的影响,以华北落叶松、麦氏云杉和辽东栎1年生幼苗为材料,以土壤营养条件(以施用肥料中的氮含量为基准:10 g·m^2和40 g·m^2)和草本地下竞争方式(完全地下竞争,部分地下竞争,无地下竞争)为处理组合,统计幼苗存活率并测定苗高。结果表明:1)完全和部分去除草本根系的地下竞争均可同样有效地提高华北落叶松幼苗的存活率,而不同土壤营养条件对其存活率和高生长均无影响,不同地下竞争方式不影响其高生长。2)对麦氏云杉幼苗而言,在40 g·m^2的土壤营养处理下其存活率显著降低;但高生长未受影响,而完全和部分去除草本根系的地下竞争均显著提高了其存活率,且高生长在完全去除草本地下竞争的处理下最高。3)辽东栎幼苗在所有处理组合下全部存活,高生长在40 g·m^2的土壤营养处理下以及完全和部分去除草本根系地下竞争的处理下显著较高。华北落叶松和麦氏云杉幼苗定居的关键并非是草本植物对土壤养分的争夺,而是草本植物地下部分对华北落叶松幼苗根系生长空间的争抢。在今后森林营造和恢复实践中,通过清除草本的地下部分,特别是宿根性草本,避免草本占据过多地下生长空间,协助华北落叶松和麦氏云杉成功渡过幼苗定居期,同时森林培育中的除草措施也在一定程度上有利于辽东栎幼苗的生长。     

Naphthenic acids inhibit root water transport,gas exchange and leaf growth in aspen (Populus tremuloides) seedlings

Effects of sodium naphthenates (NAs) on root hydraulic conductivity (Lp) and gas exchange processes were examined in aspen (Populus tremuloides Michx.) seedlings grown in solution culture. Exposure of roots to NAs for 3-5 weeks significantly decreased Lp and stomatal conductance. Root-absorbed NAs also decreased leaf chlorophyll concentration, net photosynthesis and leaf growth. Short-term (< or = 2 h) exposure of excised roots to NAs significantly decreased root water flow (Qv) with a concomitant decline in root respiration. We conclude that NAs metabolically inhibited Lp, likely by affecting water channel activity, and that this inhibition could be responsible for the observed reductions in gas exchange and leaf growth.