A coupled model of stomatal conductance, photosynthesis and transpiration

A model that couples stomatal conductance, photosynthesis, leaf energy balance and transport of water through the soil–plant–atmosphere continuum is presented. Stomatal conductance in the model depends on light, temperature and intercellular CO₂ concentration via photosynthesis and on leaf water potential, which in turn is a function of soil water potential, the rate of water flow through the soil and plant, and on xylem hydraulic resistance. Water transport from soil to roots is simulated through solution of Richards’ equation. The model captures the observed hysteresis in diurnal variations in stomatal conductance, assimilation rate and transpiration for plant canopies. Hysteresis arises because atmospheric demand for water from the leaves typically peaks in mid‐afternoon and because of uneven distribution of soil matric potentials with distance from the roots. Potentials at the root surfaces are lower than in the bulk soil, and once soil water supply starts to limit transpiration, root potentials are substantially less negative in the morning than in the afternoon. This leads to higher stomatal conductances, CO₂ assimilation and transpiration in the morning compared to later in the day. Stomatal conductance is sensitive to soil and plant hydraulic properties and to root length density only after approximately 10 d of soil drying, when supply of water by the soil to the roots becomes limiting. High atmospheric demand causes transpiration rates, LE, to decline at a slightly higher soil water content, θ_s, than at low atmospheric demand, but all curves of LE versus θ_s fall on the same line when soil water supply limits transpiration. Stomatal conductance cannot be modelled in isolation, but must be fully coupled with models of photosynthesis/respiration and the transport of water from soil, through roots, stems and leaves to the atmosphere.     

Seasonal respiration of foliage, fine roots, and woody tissues in relation to growth, tissue N, and photosynthesis

Autotrophic respiration may regulate how ecosystem productivity responds to changes in temperature, atmospheric [CO₂] and N deposition. Estimates of autotrophic respiration are difficult for forest ecosystems, because of the large amount of biomass, different metabolic rates among tissues, and seasonal variation in respiration rates. We examined spatial and seasonal patterns in autotrophic respiration in a Pinus strobus ecosystem, and hypothesized that seasonal patterns in respiration rates at a common temperature would vary with [N] for fully expanded foliage and fine roots, with photosynthesis for foliage, and with growth for woody tissues (stems, branches, and coarse roots). We also hypothesized that differences in [N] would largely explain differences in maintenance or dormant‐season respiration among tissues. For April–November, mean respiration at 15 °C varied from 1.5 to 2.8 μmol kg^?1 s^?1 for fully expanded foliage, 1.7–3.0 for growing foliage, 0.8–1.6 for fine roots, 0.6–1.1 (sapwood) for stems, 0.5–1.8 (sapwood) for branches, and 0.2–1.5 (sapwood) for coarse roots. Growing season variation in respiration for foliage produced the prior year was strongly related to [N] (r² = 0.94), but fine root respiration was not related to [N]. For current‐year needles, respiration did not covary with [N]. Night‐time foliar respiration did not vary in concert with previous‐day photosynthesis for either growing or fully expanded needles. Stem growth explained about one‐third of the seasonal variation in stem respiration (r² = 0.38), and also variation among trees (r² = 0.43). We did not determine the cause of seasonal variation in branch and coarse root respiration, but it is unlikely to be directly related to growth, as the pattern of respiration in coarse roots and branches was not synchronized with stem growth. Seasonal variations in temperature‐corrected respiration rates were not synchronized among tissues, except foliage and branches. Spatial variability in dormant‐season respiration rates was significantly related to tissue N content in foliage (r² = 0.67), stems (r² = 0.45), coarse roots (r² = 0.36), and all tissues combined (r² = 0.83), but not for fine roots and branches. Per unit N, rates for P. strobus varied from 0.22 to 3.4 μmol molN^?1 s^?1 at 15 °C, comparable to those found for other conifers. Accurate estimates of annual autotrophic respiration should reflect seasonal and spatial variation in respiration rates of individual tissues.     

Optimal co-allocation of carbon and nitrogen in a forest stand at steady state

Nitrogen (N) is essential for plant production, but N uptake imposes carbon (C) costs through maintenance respiration and fine-root construction, suggesting that an optimal C:N balance can be found. Previous studies have elaborated this optimum under exponential growth; work on closed canopies has focused on foliage only. Here, the optimal co-allocation of C and N to foliage, fine roots and live wood is examined in a closed forest stand. Optimal co-allocation maximizes net primary productivity (NPP) as constrained by stand-level C and N balances and the pipe model. Photosynthesis and maintenance respiration increase with foliar nitrogen concentration ([N]), and stand-level photosynthesis and N uptake saturate at high foliage and fine-root density. Optimal NPP increases almost linearly from low to moderate N availability, saturating at high N. Where N availability is very low or very high, the system resembles a functional balance with a steady foliage [N]; in between, [N] increases with N availability. Carbon allocation to fine roots decreases, allocation to wood increases, and allocation to foliage remains stable with increasing N availability. The predicted relationships between biomass density and foliage [N] are in reasonable agreement with data from coniferous stands across Finland. All predictions agree with our qualitative understanding of N effects on growth.     

Root responses along a subambient to elevated CO2 gradient in a C3–C4 grassland

Atmospheric CO₂ (C_a) concentration has increased significantly during the last 20 000 years, and is projected to double this century. Despite the importance of belowground processes in the global carbon cycle, community‐level and single species root responses to rising C_a are not well understood. We measured net community root biomass over 3 years using ingrowth cores in a natural C₃–C₄ grassland exposed to a gradient of C_a from preglacial to future levels (230–550 μmol mol^?1). Root windows and minirhizotron tubes were installed below naturally occurring stands of the C₄ perennial grass Bothriochloa ischaemum and its roots were measured for respiration, carbohydrate concentration, specific root length (SRL), production, and lifespan over 2 years. Community root biomass increased significantly (P<0.05) with C_a over initial conditions, with linear or curvilinear responses depending on sample date. In contrast, B. ischaemum produced significantly more roots at subambient than elevated C_a in minirhizotrons. The lifespan of roots with five or more neighboring roots in minirhizotron windows decreased significantly at high C_a, suggesting that after dense root growth depletes soil resource patches, plants with carbon surpluses readily shed these roots. Root respiration in B. ischaemum showed a curvilinear response to C_a under moist conditions in June 2000, with the lowest rates at C_a<300 μmol mol^?1 and peak activity at 450 μmol mol^?1 in a quadratic model. B. ischaemum roots at subambient C_a had higher SRLs and slightly higher carbohydrate concentrations than those at higher C_a, which may be related to drier soils at low C_a. Our data emphasize that belowground responses of plant communities to C_a can be quite different from those of the individual species, and suggest that complex interactions between and among roots and their immediate soil environment influence the responses of root physiology and lifespan to changing C_a.     

Influence of temperature and soil drying on respiration of individual roots in citrus: integrating greenhouse observations into a predictive model for the field

In citrus, the majority of fine roots are distributed near the soil surface – a region where conditions are frequently dry and temperatures fluctuate considerably. To develop a better understanding of the relationship between changes in soil conditions and a plant’s below‐ground respiratory costs, the effects of temperature and soil drying on citrus root respiration were quantified in controlled greenhouse experiments. Chambers designed for measuring the respiration of individual roots were used. Under moist soil conditions, root respiration in citrus increased exponentially with changes in soil temperature (Q₁₀ = 1·8–2·0), provided that the changes in temperature were short‐term. However, when temperatures were held constant, root respiration did not increase exponentially with increasing temperatures. Instead, the roots acclimated to controlled temperatures above 23 °C, thereby reducing their metabolism in warmer soils. Under drying soil conditions, root respiration decreased gradually beginning at 6% soil water content and reached a minimum at <2% soil water content in sandy soil. A model was constructed from greenhouse data to predict diurnal patterns of fine root respiration based on temperature and soil water content. The model was then validated in the field using data obtained by CO₂ trapping on root systems of mature citrus trees. The trees were grown at a site where the soil temperature and water content were manipulated. Respiration predicted by the model was in general agreement with observed rates, which indicates the model may be used to estimate entire root system respiration for citrus.     

Arbuscular Mycorrhizal Symbiosis with Arundo donax Decreases Root Respiration and Increases Both Photosynthesis and Plant Biomass Accumulation

The effect of arbuscular mycorrhiza (AM) symbiosis on plant growth is associated with the balance between costs and benefits. A feedback regulation loop has been described in which the higher carbohydrate cost to plants for AM symbiosis is compensated by increases in their photosynthetic rates. Nevertheless, plant carbon balance depends both on photosynthetic carbon uptake and respiratory carbon consumption. The hypothesis behind this research was that the role of respiration in plant growth under AM symbiosis may be as important as that of photosynthesis. This hypothesis was tested in Arundo donax L. plantlets inoculated with Rhizophagus irregularis and Funneliformis mosseae. We tested the effects of AM inoculation on both photosynthetic capacity and in vivo leaf and root respiration. Additionally, analyses of the primary metabolism and ion content were performed in both leaves and roots. AM inoculation increased photosynthesis through increased CO₂ diffusion and electron transport in the chloroplast. Moreover, respiration decreased only in AM roots via the cytochrome oxidase pathway (COP) as measured by the oxygen isotope technique. This decline in the COP can be related to the reduced respiratory metabolism and substrates (sugars and tricarboxylic acid cycle intermediates) observed in roots.     

What controls variation in carbon use efficiency among Amazonian tropical forests?

Why do some forests produce biomass more efficiently than others? Variations in Carbon Use Efficiency (CUE: total Net Primary Production (NPP)/ Gross Primary Production (GPP)) may be due to changes in wood residence time (Biomass/NPP_wood), temperature, or soil nutrient status. We tested these hypotheses in 14, one ha plots across Amazonian and Andean forests where we measured most key components of net primary production (NPP: wood, fine roots, and leaves) and autotrophic respiration (R_a; wood, rhizosphere, and leaf respiration). We found that lower fertility sites were less efficient at producing biomass and had higher rhizosphere respiration, indicating increased carbon allocation to belowground components. We then compared wood respiration to wood growth and rhizosphere respiration to fine root growth and found that forests with residence times <40 yrs had significantly lower maintenance respiration for both wood and fine roots than forests with residence times >40 yrs. A comparison of rhizosphere respiration to fine root growth showed that rhizosphere growth respiration was significantly greater at low fertility sites. Overall, we found that Amazonian forests produce biomass less efficiently in stands with residence times >40 yrs and in stands with lower fertility, but changes to long‐term mean annual temperatures do not impact CUE.     

Fine root traits in <Emphasis Type="Italic">Chamaecyparis obtusa</Emphasis> forest soils with different acid buffering capacities

Key message

The biomass, morphology, and respiration of the fine roots of Chamaecyparis obtusa did not change between different soil acid buffering capacities. Soil nitrate has noticeable effects on morphology and respiration.

Abstract

Low soil acid buffering capacity (ABC) accelerates soil acidification because of the lower concentrations of base cations (BC) and higher concentrations of aluminum (Al) present under such conditions. More information on fine root traits across soil ABC gradients is required to evaluate the effects of accelerated soil acidification in mature forests, especially in East Asia. We investigated the biomass, morphology (specific root length; SRL), and respiration rates of fine roots and analyzed the soil nitrogen status in seven Chamaecyparis obtusa stands with two highly contrasting ABC soils. There were no significant differences in the biomass, SRL, and respiration rates of fine roots between high- and low-ABC stands. However, fine roots in the low-ABC stands were concentrated in the uppermost soil layers and the biomass proportion of roots <0.5 mm in diameter was higher in low-ABC stands than in high-ABC stands. The fine root biomass increased with increasing soil Al, NH₄ ⁺-N, and C and with decreasing soil BC and bulk density. The SRL and respiration rates of fine roots were positively correlated with soil NO₃ ^?-N. We conclude that the fine root traits were affected not only by soil ABC but also by other soil properties in the forest.

     

Oxygen loss from Spartina alterniflora and its relationship to salt marsh oxygen balance

Spartina alterniflora has been reported to lose significant amounts of oxygen to its rhizosphere with potentially important effects on salt-marsh biogeochemical cycling and plant productivity. The potential significance of this oxidative pathway was evaluated using laboratory split-chamber experiments to quantify oxygen loss from intact root systems under a wide variety of pre-treatment and incubation conditions including antibiotics to inhibit microbial respiration. The aerenchyma system of S. alterniflora was found to transport O₂, N₂, Ar, and CH₄ from above-ground sources to its below-ground roots and rhizomes. While non-respiratory gases were observed to move from the lacunae to water bathing the root systems, net O₂ loss did not occur; instead oxygen present outside of the roots/rhizomes was consumed. Net oxygen loss was found when resistance to gas transport was reduced in the lacunae-rhizosphere pathway by placing the root systems in a gas phase and when plant respiration was significantly reduced. Root system respiration appeared to be the major variable in the plant oxygen balance. When root and rhizome respiration was inhibited using poisons or lowered by cooling, the oxygen deficit was greatly reduced and oxygen loss was indicated. The effect of seasonal temperature changes on root system oxygen deficit presents a possible explanation as to why Spartina produces root systems with respiration rates that cannot be supported by gas transport. Overall, while oxygen loss from individual plant roots is likely, integrating measured root system oxygen loss with geochemical data indicates that the mass amount of oxygen lost from S. alterniflora root systems is small compared to the total oxygen balance of vegetated salt marsh sediments.     

The effects of elevated CO2 on root respiration rates of two Mojave Desert shrubs

Although desert ecosystems are predicted to be the most responsive to elevated CO₂, low nutrient availability may limit increases in productivity and cause plants in deserts to allocate more resources to root biomass or activity for increased nutrient acquisition. We measured root respiration of two Mojave Desert shrubs, Ambrosia dumosa and Larrea tridentata, grown under ambient (～375 ppm) and elevated (～517 ppm) CO₂ concentrations at the Nevada Desert FACE Facility (NDFF) over five growing seasons. In addition, we grew L. tridentata seedlings in a greenhouse with similar CO₂ treatments to determine responses of primary and lateral roots to an increase in CO₂. In both field and greenhouse studies, root respiration was not significantly affected by elevated CO₂. However, respiration of A. dumosa roots <1 month old was significantly greater than respiration of A. dumosa roots between 1 and 4 months old. For both shrub species, respiration rates of very fine (<1.0 mm diameter) roots were significantly greater than those of fine (1–2 mm diameter) roots, and root respiration decreased as soil water decreased. Because specific root length was not significantly affected by CO₂ and because field minirhizotron measurements of root production were not significantly different, we infer that root growth at the NDFF has not increased with elevated CO₂. Furthermore, other studies at the NDFF have shown increased nutrient availability under elevated CO₂, which reduces the need for roots to increase scavenging for nutrients. Thus, we conclude that A. dumosa and L. tridentata root systems have not increased in size or activity, and increased shoot production observed under elevated CO₂ for these species does not appear to be constrained by the plant's root growth or activity.     

Stand age-related effects on soil respiration in a first rotation Sitka spruce chronosequence in central Ireland

The effect of stand age on soil respiration and its components was studied in a first rotation Sitka spruce chronosequence composed of 10‐, 15‐, 31‐, and 47‐year‐old stands established on wet mineral gley in central Ireland. For each stand age, three forest stands with similar characteristics of soil type and site preparation were used. There were no significant differences in total soil respiration among sites of the same age, except for the case of a 15‐year‐old stand that had lower soil respiration rates due to its higher productivity. Soil respiration initially decreased with stand age, but levelled out in the older stands. The youngest stands had significantly higher respiration rates than more mature sites. Annual soil respiration rates were modelled by means of temperature‐derived functions. The average Q ₁₀ value obtained treating all the stands together was 3.8. Annual soil respiration rates were 991, 686, 556, and 564 g C m^?2 for the 10‐, 15‐, 31‐, and 47‐year‐old stands, respectively. We used the trenching approach to separate soil respiration components. Heterotrophic respiration paralleled soil organic carbon dynamics over the chronosequence, decreasing with stand age to slightly increase in the oldest stand as a result of accumulated aboveground litter and root inputs. Root respiration showed a decreasing trend with stand age, which was explained by a decrease in fine root biomass over the chronosequence, but not by nitrogen concentration of fine roots. The decrease in the relative contribution of autotrophic respiration to total soil CO₂ efflux from 59.3% in the youngest stand to 49.7% in the oldest stand was explained by the higher activity of the root system in younger stands. Our results show that stand age should be considered if simple temperature‐based models to predict annual soil respiration in afforestation sites are to be used.     

川中丘陵区3个树种的细根形态和功能异质性分析

以川中丘陵区柏木低效林林窗改造初期种植的银木和香椿细根为研究对象,以未改造的柏木纯林为对照,采用LI-8100土壤碳通量测量系统测定银木、香椿和柏木1~5级细根的原位呼吸速率,并探讨细根形态结构和养分元素浓度与细根呼吸的相关关系,以揭示细根结构与功能异质性。结果表明:银木、香椿和柏木细根的直径、根长、组织碳浓度均随着根序级别的增加而增加,而它们细根的比根长、组织氮浓度和比根呼吸速率均随着根序的增加而降低,树种、根序级及其交互作用对3个树种细根形态、养分浓度和比根呼吸均有显著或极显著影响。回归分析显示,3个树种比根呼吸速率均随细根直径、比根长、N浓度变化呈现出系统性的变化,三者分别能解释64.7%、87.6%和、67.6%的比根呼吸变异。可见,细根在形态和功能上存在明显的异质性,且细根的形态特征、组织化学含量和生理功能之间存在着紧密的联系,为理解植物根系结构与功能变异提供了依据。     

Effect of respiratory homeostasis on plant growth in cultivars of wheat and rice

Some plants have the ability to maintain similar respiratory rates (measured at the growth temperature), even when grown at different temperatures, a phenomenon referred to as respiratory homeostasis. The underlying mechanisms and ecological importance of this respiratory homeostasis are not understood. In order to understand this, root respiration and plant growth were investigated in two wheat cultivars (Triticum aestivum L. cv. Stiletto and cv. Patterson) with a high degree of homeostasis, and in one wheat cultivar (T. aestivum L. cv. Brookton) and one rice cultivar (Oryza sativa L. cv. Amaroo) with a low degree of homeostasis. The degree of homeostasis (H) is defined as a quantitative value, which occurs between 0 (no acclimation) and 1 (full acclimation). These plants were grown hydroponically at constant 15 or 25 °C. A good correlation was observed between the rate of root respiration and the relative growth rates (RGR) of whole plant, shoot or root. The plants with high H showed a tendency to maintain their RGR, irrespective of growth temperature, whereas the plants with low H grown at 15 °C showed lower RGR than those grown at 25 °C. Among several parameters of growth analysis, variation in net assimilation rate per shoot mass (NAR_m) appeared to be responsible for the variation in RGR and rates of root respiration in the four cultivars. The plants with high H maintained their NAR_m at low growth temperature, but the plants with low H grown at 15 °C showed lower NAR_m than those grown at 25 °C. It is concluded that respiratory homeostasis in roots would help to maintain growth rate at low temperature due to a smaller decrease in net carbon gain at low temperature. Alternatively, growth rate per se may control the demand of respiratory ATP, root respiration rates and sink demands of photosynthesis. The contribution of nitrogen uptake to total respiratory costs was also estimated, and the effects of a nitrogen leak out of the roots and the efficiency of respiration on those costs are discussed.     

Supply-side controls on soil respiration among Oregon forests

To test the hypothesis that variation in soil respiration is related to plant production across a diverse forested landscape, we compared annual soil respiration rates with net primary production and the subsequent allocation of carbon to various ecosystem pools, including leaves, fine roots, forests floor, and mineral soil for 36 independent plots arranged as three replicates of four age classes in three climatically distinct forest types. Across all plots, annual soil respiration was not correlated with aboveground net primary production (R²=0.06, P>0.1) but it was moderately correlated with belowground net primary production (R²=0.46, P<0.001). Despite the wide range in temperature and precipitation regimes experienced by these forests, all exhibited similar soil respiration per unit live fine root biomass, with about 5 g of carbon respired each year per 1 g of fine root carbon (R²=0.45, P<0.001). Annual soil respiration was only weakly correlated with dead carbon pools such as forest floor and mineral soil carbon (R²=0.14 and 0.12, respectively). Trends between soil respiration, production, and root mass among age classes within forest type were inconsistent and do not always reflect cross‐site trends. These results are consistent with a growing appreciation that soil respiration is strongly influenced by the supply of carbohydrates to roots and the rhizosphere, and that some regional patterns of soil respiration may depend more on belowground carbon allocation than the abiotic constraints imposed on subsequent metabolism.     

An empirical model for estimating CO2 exchange of Bouteloua gracilis (H.B.K.) Lag. in the shortgrass prairie

Summary An empirical model for predicting net photosynthesis (P _N ) and dark respiration (R _D ) in the field was developed and tested for Bouteloua gracilis (H.B.K.) Lag., the dominant C₄ grass of the North American shortgrass prairie. P _N is predicted as a function of soil water potential, canopy air temperature, irradiance, and plant age, while R _D is expressed as a function of soil water potential and temperature. The model accounted for 85% of the variability in the data base used to estimate parameter values. Results of a validation test showed good agreement between observed and predicted P _N rates, suggesting this approach would be useful as a submodel of a grassland ecosystem model.     

An in situ approach for measuring root-associated respiration and nitrate uptake of forest trees

The ecophysiological characteristics of fine roots of mature forest plants are poorly understood because of difficulties of measurement. We explored a root in-growth approach to measure respiration and nitrate uptake of woody plant roots in situ. Roots of seven species were grown into sand-filled chambers. Root-associated respiration was measured as CO ₂ emission on four dates and nitrate uptake was quantified using ¹⁵N. All the roots were younger than 3 months at the time of measurement. Fine root respiration measured over the temperature range of 14.5–15.5 °C averaged 18.9–36.5 nmol gDM ^–1 s ^–1 across species. Nitrate uptake rates by these fine roots (1.3–6.8 nmol gDM ^–1 s ^–1) were comparable to other studies of forest trees. The root respiration rates were several times higher than measurements on detached roots of mature trees, concurring with literature observations that young roots respire much more rapidly than older roots. The root in-growth approach appears promising for providing information on the metabolic activity of fine roots of mature forest trees growing in soil.     

Mechanisms of age‐related changes in forest production: the influence of physiological and successional changes

Net primary production (NPP) declines as forests age, but the causal role of decreased gross primary production (GPP), or increased autotrophic respiration (R_a) is still a matter of debate. This uncertainty complicates predicted responses to future climate, as higher atmospheric carbon dioxide (CO₂) concentrations may amplify the carbon (C)‐sink in temperate forests if GPP controls the decline in NPP, but increased temperatures may decrease this C‐sink if R_a controls the NPP decline. We quantified NPP in forests dominated by loblolly pine (Pinus taeda) in North Carolina, USA that varied from 14 to 115 years old. We used a sap‐flow approach to quantify summer canopy photosynthesis by pines and later‐successional hardwood trees, and measured wood CO₂ efflux to investigate age‐related changes in pine R_a. Despite increasing production by later‐successional hardwoods, an 80% decline in pine NPP caused ecosystem NPP to decline with age by ～40%. The decline in pine NPP was explained by reduced stomatal conductance and photosynthesis, supporting the hypothesis that increasing hydraulic limitation and declining GPP drove the age‐related decline of NPP in this species. The difference between GPP and NPP indicated that pine R_a also declined with age; this was corroborated by measurements of reduced stem CO₂ efflux with increasing age. These results indicate that C cycling in these successional temperate forests is controlled by C input from GPP, and elements of global change that increase GPP may increase the C‐sink in aging warm‐temperate pine forests.     

The Osmometer Model of the Root: Water and Solute Relations of Roots of Phaseolus coccineus

Water and solute relations of young roots of Phaseolus coccineus have been measured using the root pressure probe. Biphasic root pressure relaxations were obtained when roots were treated with solutions containing different osmotic test solutes. From the relaxations, the hydraulic conductivity (Lp_r), the permeability coefficient (P_sr), and the reflection coefficient (σ_sr) of the roots could be evaluated. Lp_r was 1.8 to 8.4 . 10^?8 m . s^?1 . MPa^?1 and P_sr (in 10^?10 m . s^?1): methanol, 27–62; ethanol, 44–73; urea, 5–11; mannitol, 1.5; KCl, 7.1–9.2; NaCl, 2.1; NaNO₃, 3.7. The hydraulic conductivity was similar when using osmotic and hydrostatic pressure gradients as driving forces. The hydraulic conductivity of individual root cortex cells (Lp) was by two orders of magnitude larger than Lp_r (Lp = 0.3 to 4.7 . 10^?6 m . s^?1 . MPa^?1) which indicated a predominant cell-to-cell rather than an apoplasmic transport of water in the Phaseolus root. Except for distances shorter than 20 mm from the root apex, the hydraulic resistance of the roots was limited by the radial movement of water across the root cylinder and not by the hydraulic resistance within the xylem. Reflection coefficients were low: methanol: 0.16–0.34; ethanol: 0.15–0.47; urea: 0.41–0.51; mannitol: 0.68; KCl: 0.43–0.54; NaCl: 0.59; NaNO₃: 0.54. The transport coefficients (Lp_r, P_sr, σ_sr) have been critically examined for influences of unstirred layers and active transport. The low σ_sr suggests that the common treatment of the root as a rather perfect osmometer (σ_sr = 1) analogous to plant cells should be treated cautiously. The reasons for the low σ_sr and the possible implications of the absolute values of the transport parameters for the absorption of water and nutrients are discussed.