Aquaporins: Another piece in the osmotic puzzle

Osmolarity not only plays a key role in cellular homeostasis but also challenges cell survival. The molecular understanding of osmosis has not yet been completely achieved, and the discovery of aquaporins as molecular entities involved in water transport has caused osmosis to again become a focus of research. The main questions that need to be answered are the mechanism underlying the osmotic permeability coefficients and the extent to which aquaporins change our understanding of osmosis. Here, attempts to answer these questions are discussed. Critical aspects of the state of the state of knowledge on osmosis, a topic that has been studied since 19th century, are reviewed and integrated with the available information provided by in vivo, in vitro and in silico approaches.     

Plant aquaporins: Roles in plant physiology

Background

Aquaporins are membrane channels that facilitate the transport of water and small neutral molecules across biological membranes of most living organisms.

Scope of review

Here, we present comprehensive insights made on plant aquaporins in recent years, pointing to their molecular and physiological specificities with respect to animal or microbial counterparts.

Major conclusions

In plants, aquaporins occur as multiple isoforms reflecting a high diversity of cellular localizations and various physiological substrates in addition to water. Of particular relevance for plants is the transport by aquaporins of dissolved gases such as carbon dioxide or metalloids such as boric or silicic acid. The mechanisms that determine the gating and subcellular localization of plant aquaporins are extensively studied. They allow aquaporin regulation in response to multiple environmental and hormonal stimuli. Thus, aquaporins play key roles in hydraulic regulation and nutrient transport in roots and leaves. They contribute to several plant growth and developmental processes such as seed germination or emergence of lateral roots.

General significance

Plants with genetically altered aquaporin functions are now tested for their ability to improve plant resistance to stresses. This article is part of a Special Issue entitled Aquaporins.     

Different responses to drought and freeze stress of Quercus ilex L. growing along a latitudinal gradient

The vulnerability to drought and freeze stress was measured in young plants of Quercus ilex L. growing in the field in two natural sites within the Italian distribution area of this species, i.e. Sicily (Southern Mediterranean Basin) and Venezia Giulia (Northeastern Italy), respectively. In particular, the resistance strategies adopted by Q. ilex to withstand the two stresses were estimated in terms of seasonal and/or diurnal changes in leaf conductance to water vapour (g_L), water potential (_L) and relative water content (RWC) as well as of xylem embolism in the stem and root hydraulic conductance (K_RL). Sicilian (SI) plants showed to reduce water loss by stomatal closure (g_L decreased) in summer, thus maintaining average RWCs at 88–90%. Moreover, SI plants showed considerable resistance to xylem cavitation in the stem (the loss of hydraulic conductance, PLC, was less than 12% throughout the year) and to maintain the hydraulic conductance of their roots (K_RL), constantly high even in summer. Plants growing in Venezia Giulia (VG plants), on the contrary, underwent leaf dehydration in the winter due to freeze stress so that RWC measured in April was still 78% on a diurnal basis. This was apparently due to consistent xylem embolism in the stem. In fact, PLC was as high as 40% between November and March. Only in the summer was PLC similar to that recorded in SI plants. Moreover, K_RL of VG plants decreased in November from about 1.5 to 0.8×10^–4 kg s^–1 m^–2 MPa^–1, i.e. about 50%, and in February K_RL dropped further to 0.4×10^–4 kg s^–1 m^–2 MPa^–1. On the basis of the above, we conclude that: (a) Q. ilex was more sensitive to freeze than to drought stress so that freeze stress can be considered as a factor limiting the distribution area of this species; (b) drought and freeze stress were faced by Q. ilex adopting two different resistance strategies, i.e. drought avoidance based on water saving in Sicily and freeze tolerance in Venezia Giulia.     

Water relations and xylem transport of nutrients in pepper plants grown under two different salts stress regimes

Two iso-osmotic concentrations of NaCl and Na₂SO₄ were used for discriminating between the effects of specific ion toxicities of salt stress on pepper plants (Capsicum annuum L.) grown in hydroponic conditions, in a controlled-environment greenhouse. The two salts were applied to plants at different electrical conductivities, and leaf water relations, osmotic adjustment and root hydraulic conductance were measured. Leaf water potential (_w), leaf osmotic potential (_o) and leaf turgor potential (_p) decreased significantly when EC increased, but the decrease was less for NaCl- than for Na₂SO₄-treated plants. The reduction in stomatal conductance was higher for NaCl-treated plants. There were no differences in the effect of both treatments on the osmotic adjustment, and a reduction in root hydraulic conductance and the flux of solutes into the xylem was observed, except for the saline ions (Na⁺, Cl^– and SO₄ ^2–). Therefore, pepper growth decreased with increasing salinity because the plants were unable to adjust osmotically or because of the toxic effects of Cl^–, SO₄ ^2– and/or Na⁺. However, turgor of NaCl-treated plants was maintained at low EC (3 and 4 dS m^–1) probably due to the maintenance of water transport into the plant (decrease of stomatal conductance), which, together with the lower concentration of Na⁺ in the plant tissues compared with the Na₂SO₄ treatment, could be the cause of the smaller decrease in growth.     

What a difference a bay makes: natural variation in dietary resources mediates growth in a recently settled herbivorous fish

Processes acting during the early stages of coral reef fish life cycles have a disproportionate influence on their adult abundance and community structure. Higher growth rates, for example, confer a major fitness advantage in larval and juvenile fishes, with larger fish undergoing significantly less mortality. The role of dietary resources in the size-structuring process has not been well validated, especially at the early post-settlement phase, where competition and predation are seen as preeminent drivers of juvenile fish assemblage structure. Here, we report on a size differential of 10–20% between recently settled Siganus spinus rabbitfish recruits from different bays around the Pacific island of Guam. This difference was maintained across multiple recruitment events within and between years. After confirming the validity of our observations through otolith increment analysis, subsequent investigation into the drivers of this variation revealed significant differences in the structure of algal assemblages between bays, congruent with the observed differences in size of the recently settled fish. Gut analyses showed a greater presence of algal types with higher levels of nitrogen and phosphorus in the stomachs of fish from Tanguisson, the bay with the largest observed recruits. To ensure this mechanism was one of causation and not correlation, we conducted a fully factorial experiment in which S. spinus recruits sampled from different bays were reared on all combinations of algal diets representative of the different bays. Recruits on the ‘Tanguisson’ diet grew faster than recruits on other diets, regardless of their origin. We propose that the greater availability of high-quality dietary resources at this location is likely conferring benefits that impact on the population-level dynamics of this species. The spatial and temporal extent of this process clearly implicates food as a limiting resource, capable of mediating fish population dynamics at multiple spatial scales and ontogenetic phases.     

Measurement of apoplasmic and cell-to-cell components of root hydraulic conductance by a pressure-clamp technique

A pressure-clamp technique was devised for the direct measurement of cell-to-cell and apoplasmic components of root hydraulic conductance; the experimental results were analyzed in terms of a theoretical model of water and solute flow, based on a composite membrane model of the root. When water is forced under a constant pressure into a cut root system, an exponential decay of flow is observed, until a constant value is attained; when pressure is released, a reverse water flow out of the root system is observed which shows a similar exponential behavour. The model assumes that the transient flow occurs through a cell-to-cell pathway and the observed decrease is the result of accumulation of solutes in front of the root semi-permeable membrane, whilst the steady-state component results from the movement of water through the parallel apoplasmic pathway. Root conductance components are estimated by fitting the model to experimental data. The technique was applied to the root systems of potted cherry (Prunus avium L.) seedlings; average apoplasmic conductance was 15.5 × 10^–9m³· s^–1· MPa^–1, with values ranging from 12.0 × 10^–9 to 18.5 × 10^–9m³· s^–1· MPa^–1; average cell-to-cell conductance was 11.7 × 10⁹ m³· s^–1· MPa^–1, with values ranging from 8.5 × 10^–9 to 15.3 × 10^–9 m³ · s^–1·MPa^–1. Cell-to-cell conductance amounted on average to 43% of total root conductance, with values between 41 and 45%. Leaf specific conductance (conductance per unit of leaf area supported) of the root systems ranged from 2.7 × 10^–8 to 5.6 × 10^–8 m· s^–1·MPa^–1, with an average of 3.7 × 10^–8 m · s^–1·MPa^–1. The newly developed technique allows the interaction of mass flow of water and of solutes to be explored in the roots of soil-grown plants.Abbreviations and Symbols A L_p root hydraulic conductance - A^aL _p ^a root apoplasmic conductance - A^ccL _p ^cc root cell-to-cell conductance - C_s(t) concentration of solutes in apical root compartment at time t - J_v flow of water through the root - J _v ^a apoplasmic flow of water - J_v/cc cell-to-cell flow of water - LSC leaf specific conductance of the root system - P root hydrostatic pressure - P_appl applied pressure - _s(t) root osmotic pressure at time t - _m osmotic pressure of rooting medium - reflection coefficient of root membrane - time constant of cell-to-cell flow decay This research was funded within the EC Project Long-term effects of CO₂-increase and climate change on European forests (LTEEF) (EV5V-CT94-0468); F.M. was supported by a Ministero dell' Universitá e della Ricerca Scientifica e Tecnologica — British Council agreement (Project The ecological significance of cavitation in woody plants); M.C. was supported by a Consiglio Nazionale delle Ricerche — British Council agreement. We gratefully thank Prof. P.G. Jarvis (University of Edinburgh, UK) for revising an earlier version of this paper and Prof. E. Steudle (University of Bayreuth, Germany) for helpful comments.     

Impact of light quality on leaf and shoot hydraulic properties: a case study in silver birch (Betula pendula)

Responses of leaf and shoot hydraulic conductance to light quality were examined on shoots of silver birch (Betula pendula), cut from lower (‘shade position’) and upper thirds of the crowns (‘sun position’) of trees growing in a natural temperate forest stand. Hydraulic conductances of leaf blades (K_lb), petioles (K_P) and branches (i.e. leafless stem; K_B) were determined using a high pressure flow meter in steady state mode. The shoots were exposed to photosynthetic photon flux density of 200–250 µmol m^?2 s^?1 using white, blue or red light. K_lb depended significantly on both light quality and canopy position (P < 0.001), K_B on canopy position (P < 0.001) and exposure time (P = 0.014), and none of the three factors had effect on K_P. The highest values of K_lb were recorded under the blue light (3.63 and 3.13 × 10^?4 kg m^?2 MPa^?1 s^?1 for the sun and shade leaves, respectively), intermediate values under white light (3.37 and 2.46 × 10^?4 kg m^?2 MPa^?1 s^?1, respectively) and lowest values under red light (2.83 and 2.02 × 10^?4 kg m^?2 MPa^?1 s^?1, respectively). Light quality has an important impact on leaf hydraulic properties, independently of light intensity or of total light energy, and the specific light receptors involved in this response require identification. Given that natural canopy shade depletes blue and red light, K_lb may be decreased both by reduced fluence and shifts in light spectra, indicating the need for studies of the natural heterogeneity of K_lb within and under canopies, and its impacts on gas exchange.     

Induction of toxin production in dinoflagellates: the grazer makes a difference

The dinoflagellate Alexandrium minutum has previously been shown to produce paralytic shellfish toxins (PST) in response to waterborne cues from the copepod Acartia tonsa. In order to investigate if grazer-induced toxin production is a general or grazer-specific response of A. minutum to calanoid copepods, we exposed two strains of A. minutum to waterborne cues from three other species of calanoid copepods, Acartia clausi, Centropages typicus and Pseudocalanus sp. Both A. minutum strains responded to waterborne cues from Centropages and Acartia with significantly increased cell-specific toxicity. Waterborne cues from Centropages caused the strongest response in the A. minutum cells, with 5 to >20 times higher toxin concentrations compared to controls. In contrast, neither of the A. minutum strains responded with significantly increased toxicity to waterborne cues from Pseudocalanus. The absolute increase in PST content was proportional to the intrinsic toxicity of the different A. minutum strains that were used. The results show that grazer-induced PST production is a grazer-specific response in A. minutum, and its potential ecological importance will thus depend on the composition of the zooplankton community, as well as the intrinsic toxin-producing properties of the A. minutum population.     

Abscisic acid and hydraulic conductivity of maize roots: a study using cell- and root-pressure probes

Using root- and cell-pressure probes, the effects of the stress hormone abscisic acid (ABA) on the water-transport properties of maize roots (Zea mays L.) were examined in order to work out dose and time responses for root hydraulic conductivity. Abscisic acid applied at concentrations of 100–1,000 nM increased the hydraulic conductivity of excised maize roots both at the organ (root Lp_r: factor of 3–4) and the root cell level (cell Lp: factor of 7–27). Effects on the root cortical cells were more pronounced than at the organ level. From the results it was concluded that ABA acts at the plasmalemma, presumably by an interaction with water channels. Abscisic acid therefore facilitated the cell-to-cell component of transport of water across the root cylinder. Effects on cell Lp were transient and highly specific for the undissociated (+)-cis-trans-ABA. The stress hormone ABA facilitates water uptake into roots as soils start drying, especially under non-transpiring conditions, when the apoplastic path of water transport is largely excluded. Received: 26 February 2000 / Accepted: 17 August 2000     

What makes a fig: insights from a comparative analysis of inflorescence morphogenesis in Moraceae

Background and AimsMoraceae, the family of mulberry and fig trees, displays small homogeneous flowers but extremely diverse inflorescences ranging from simple and branched to complex and condensed. Inflorescences also vary in flower organization in the receptacle, in the degree of flower condensation and in receptacle shape. Thus, the objective of the present study was to compare the inflorescence morphogenesis of Moraceae species, to investigate whether clades with a similar pollination mode share the same patterns of inflorescence development and the developmental stages at which we observe the key changes resulting in the diversified inflorescence architecture that culminates in the Ficus syconium.MethodsInflorescences at different developmental stages were sampled from Brosimum gaudichaudii, Castilla elastica, Clarisia ilicifolia, Ficus pertusa, Maclura tinctoria and Morus nigra and processed for surface and anatomical analyses.Key ResultsThe inflorescence morphogenesis of the studied species is highly variable. The shape of the inflorescence meristem (bulging, hemispheric or elongated), the initiation order and arrangement of flowers along the receptacle and the occurrence of bracts vary between related species. This diversity originates early during inflorescence development. Brosimum gaudichaudii, C. elastica and F. pertusa have flowers enclosed or immersed within the receptacle, although inflorescences begin their development as flat and open structures, as occurs in the other three study species.ConclusionComparison of the inflorescence morphogenesis in Moraceae species allows us to infer that evolutionary ontogenetic changes driven by pollinators culminate in the enclosure of flowers inside the receptacle, as occurs in the Ficus syconium.     

Limitation of plant water use by rhizosphere and xylem conductance: results from a model

Hydraulic conductivity ( K ) in the soil and xylem declines as water potential ( Ψ ) declines. This results in a maximum rate of steady-state transpiration ( E _crit) and corresponding minimum leaf Ψ ( Ψ _crit) at which K has approached zero somewhere in the soil–leaf continuum. Exceeding these limits causes water transport to cease. A model determined whether the point of hydraulic failure (where K = 0) occurred in the rhizosphere or xylem components of the continuum. Below a threshold of root:leaf area ( A _R: A _L), the loss of rhizosphere K limited E _crit and Ψ _crit. Above the threshold, loss of xylem K from cavitation was limiting. The A _R: A _L threshold ranged from > 40 for coarse soils and/or cavitation-resistant xylem to < 0·20 in fine soils and/or cavitation-susceptible xylem. Comparison of model results with drought experiments in sunflower and water birch indicated that stomatal regulation of E reflected the species' hydraulic potential for extracting soil water, and that the more sensitive stomatal response of water birch to drought was necessary to avoid hydraulic failure. The results suggest that plants should be xylem-limited and near their A _R: A _L threshold. Corollary predictions are (1) within a soil type the A _R: A _L should increase with increasing cavitation resistance and drought tolerance, and (2) across soil types from fine to coarse the A _R: A _L should increase and maximum cavitation resistance should decrease.     

Capacitive effect of cavitation in xylem conduits: results from a dynamic model

Embolisms decrease plant hydraulic conductance and therefore reduce the ability of the xylem to transport water to leaves provided that embolized conduits are not refilled. However, as a xylem conduit is filled with gas during cavitation, water is freed to the transpiration stream and this transiently increases xylem water potential. This capacitive effect of embolism formation on plant function has not been explicitly quantified in the past. A dynamic model is presented that models xylem water potential, xylem sap flow and cavitation, taking into account both the decreasing hydraulic conductance and the water release effect of xylem embolism. The significance of the capacitive effect increases in relation to the decreasing hydraulic conductance effect when transpiration rate is low in relation to the total amount of water in xylem conduits. This ratio is typically large in large trees and during drought.     

What makes a weed a weed: life history traits of native and exotic plants in the USA

I compared ten life history traits (vegetative reproduction, breeding system, compatibility, pollination system, shade tolerance, habitat, life span, life form, morphology, and toxicity) from two existing databases for the 19,960 plant species that occur in the USA. I used two-way tests of independence to determine if there were significant life history traits that distinguish weeds from non-weeds, exotic weeds from native weeds, and invasive exotic weeds from non-invasive exotic weeds. Life span was the most significant life history trait for weeds in general; weeds were more likely to be annuals and biennials and less likely to perennials than non-weeds. In addition, vegetative reproduction, breeding system, compatibility, shade tolerance, and life form were related to life span. Annual and biennial weeds (whether native, exotic, or exotic invasives) were more likely to be wetland adapted, armed, and toxic than annual or biennial non-weeds. Perennial weeds (whether native, exotic, or exotic invasives) were less likely to be forbs or subshrubs, and more likely to be wetland adapted, toxic, shade intolerant, grasses, vines and trees than perennial non-weeds. Exotic annual and perennial weeds were less likely to be wetland species than native weeds, but more likely to be wetland species than non-weeds. Invasive exotic weeds, in contrast, were less likely to be forbs and more likely to be perennial, monoecious, self-incompatible, and trees and than non-invasive exotics.     

Genotypic variation in transpiration of coppiced poplar during the third rotation of a short‐rotation bio‐energy culture

The productivity of short‐rotation coppice (SRC) plantations with poplar (Populus spp.) strongly depends on soil water availability, which limits the future development of its cultivation, and makes the study of the transpirational water loss particularly timely under the ongoing climate change (more frequent drought and floods). This study assesses the transpiration at different scales (leaf, tree and stand) of four poplar genotypes belonging to different species and from a different genetic background grown under an SRC regime. Measurements were performed for an entire growing season during the third year of the third rotation in a commercial scale multigenotype SRC plantation in Flanders (Belgium). Measurements at leaf level were performed on specific days with a contrasted evaporative demand, temperature and incoming shortwave radiation and included stomatal conductance, stem and leaf water potential. Leaf transpiration and leaf hydraulic conductance were obtained from these measurements. To determine the transpiration at the tree level, single‐stem sap flow using the stem heat balance (SHB) method and daily stem diameter variations were measured during the entire growing season. Sap flow‐based canopy transpiration (E_c), seasonal dry biomass yield, and water use efficiency (WUE; g aboveground dry matter/kg water transpired) of the four poplar genotypes were also calculated. The genotypes had contrasting physiological responses to environmental drivers and to soil conditions. Sap flow was tightly linked to the phenological stage of the trees and to the environmental variables (photosynthetically active radiation and vapor pressure deficit). The total E_c for the 2016 growing season was of 334, 350, 483 and 618 mm for the four poplar genotypes, Bakan, Koster, Oudenberg and Grimminge, respectively. The differences in physiological traits and in transpiration of the four genotypes resulted in different responses of WUE.     

Modelling the soil-plant-atmosphere continuum in a Quercus–Acer stand at Harvard Forest: the regulation of stomatal conductance by light, nitrogen and soil/plant hydraulic properties

Our objective is to describe a multi-layer model of C₃-canopy processes that effectively simulates hourly CO₂ and latent energy (LE) fluxes in a mixed deciduous Quercus-Acer (oak–maple) stand in central Massachusetts, USA. The key hypothesis governing the biological component of the model is that stomatal conductance (g_s) is varied so that daily carbon uptake per unit of foliar nitrogen is maximized within the limitations of canopy water availability. The hydraulic system is modelled as an analogue to simple electrical circuits in parallel, including a separate soil hydraulic resistance, plant resistance and plant capacitance for each canopy layer. Stomatal opening is initially controlled to conserve plant water stores and delay the onset of water stress. Stomatal closure at a threshold minimum leaf water potential prevents xylem cavitation and controls the maximum rate of water flux through the hydraulic system. We show a strong correlation between predicted hourly CO₂ exchange rate (r²= 0.86) and LE (r²= 0.87) with independent whole-forest measurements made by the eddy correlation method during the summer of 1992. Our theoretical derivation shows that observed relationships between CO₂ assimilation and LE flux can be explained on the basis of stomatal behaviour optimizing carbon gain, and provides an explicit link between canopy structure, soil properties, atmospheric conditions and stomatal conductance.     

Using combined measurements of gas exchange and chlorophyll fluorescence to estimate parameters of a biochemical C3 photosynthesis model: a critical appraisal and a new integrated approach applied to leaves in a wheat (Triticum aestivum) canopy

We appraised the literature and described an approach to estimate the parameters of the Farquhar, von Caemmerer and Berry model using measured CO₂ assimilation rate (A) and photosystem II (PSII) electron transport efficiency (Φ₂). The approach uses curve fitting to data of A and Φ₂ at various levels of incident irradiance (I_inc), intercellular CO₂ (C_i) and O₂. Estimated parameters include day respiration (R_d), conversion efficiency of I_inc into linear electron transport of PSII under limiting light [κ_2(LL)], electron transport capacity (J_max), curvature factor (θ) for the non‐rectangular hyperbolic response of electron flux to I_inc, ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) CO₂/O₂ specificity (S_c/o), Rubisco carboxylation capacity (V_cmax), rate of triose phosphate utilization (T_p) and mesophyll conductance (g_m). The method is used to analyse combined gas exchange and chlorophyll fluorescence measurements on leaves of various ages and positions in wheat plants grown at two nitrogen levels. Estimated S_c/o (25 °C) was 3.13 mbar µbar^?1; R_d was lower than respiration in the dark; J_max was lower and θ was higher at 2% than at 21% O₂; κ_2(LL), V_cmax, J_max and T_p correlated to leaf nitrogen content; and g_m decreased with increasing C_i and with decreasing I_inc. Based on the parameter estimates, we surmised that there was some alternative electron transport.     

Size makes a difference: Chiral recognition in complexes of fenchone with cyclodextrins studied by means of NMR titration

Gibbs energies of complex formation between enantiomers of bicyclic terpenoid, fenchone, and naturally occurring cyclodextrins, βCD and γCD, were determined by means of ¹³C and ¹H nuclear magnetic resonance (NMR) titration data. These results were compared with the corresponding data obtained previously for the diastereomeric fenchone/αCD complexes. The size of the inner cavity of host molecules significantly influences stoichiometry, association constants, and enantiomeric differentiation of the studied complexes. These complementary data allow us to discuss qualitatively the influence of the host size on the guest–host interactions. A method of the simultaneous use of titration data collected for several resonances of different isotopes in the determination of association constants was worked out and thoroughly analyzed. Comparison of the results of global data analyses with weighted means of individual ones revealed that both these approaches are equally trustworthy.     

Major intrinsic proteins (MIPs) in plants: a complex gene family with major impacts on plant phenotype

The ubiquitous cell membrane proteins called aquaporins are now firmly established as channel proteins that control the specific transport of water molecules across cell membranes in all living organisms. The aquaporins are thus likely to be of fundamental significance to all facets of plant growth and development affected by plant–water relations. A majority of plant aquaporins have been found to share essential structural features with the human aquaporin and exhibit water-transporting ability in various functional assays, and some have been shown experimentally to be of critical importance to plant survival. Furthermore, substantial evidence is now available from a number of plant species that shows differential gene expression of aquaporins in response to abiotic stresses such as salinity, drought, or cold and clearly establishes the aquaporins as major players in the response of plants to conditions that affect water availability. This review summarizes the function and regulation of these genes to develop a greater understanding of the response of plants to water insufficiency, and particularly, to identify tolerant genotypes of major crop species including wheat and rice and plants that are important in agroforestry. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mathematical treatment of water movement in plant cells and tissue: a review

Abstract. Over the past three decades, many contributions have been made to the development of a mathematical basis for describing water transport in plant cells and tissue. This review paper attempts to summarize the more significant contributions and to outline the concepts upon which the various mathematical analyses are founded.
The paper itself is divided into three major sections. Section I deals with the quantitative water relations of single plant cells. Basic equations are developed which describe the water statics and water dynamics of such cells. Included is a discussion of the theory and methods for measuring the various parameters (permeabilities, cell wall elastic moduli, etc.) which enter into the development. The section closes with a presentation of circuit analog models for single plant cells.
Section II is devoted to a review and development of the water relations of plant tissues which contain numerous cells in series. Following a historical overview, various existing models are derived and physical tissue properties which enter the derivation are identified. The concept of 'local equilibrium' is discussed and circuit analog models for single cells are generalized and applied to several cells in series.
The final section contains two example applications of water transport theory as it applies to plant tissue. One application involves radial water movement in a soybean hypocotyl while the other deals with water transport in a growing root tip. A summary at the end of the section is largely devoted to a discussion of the limitations of mathematical models dial are presently available.