Hydraulic properties and photosynthetic rates in co-occurring lianas and trees in a seasonal tropical rainforest in southwestern China

In this study, we examined wood anatomy, hydraulic properties, photosynthetic rate, and water status and osmotic regulation in three liana species and three tree species co-occurring in a seasonal tropical rain forest. Our results showed that the three liana species had larger vessel diameter, lower sapwood density, and consequently higher branch sapwood specific hydraulic conductivity (K _S) than the three tree species. Across species, K _S was positively correlated with leaf nitrogen concentration and maximum net CO₂ assimilation rate. However, it was also positively correlated with xylem water potential at 50% loss of hydraulic conductivity, indicating a trade-off between hydraulic efficiency and safety. Compared to the tree species, the liana species had higher predawn leaf water potential and lower osmotic adjustment in the dry season. The combination of more efficient water transport, higher photosynthetic rates, and their ability to access to more reliable water source at deeper soil layers in the dry season in the lianas should contribute to their fast growth.     

Root and stem xylem embolism,stomatal conductance,and leaf turgor in Acer grandidentatum populations along a soil moisture gradient

The objective of this study was to determine how adjustment in stomatal conductance (g ^s) and turgor loss point (_tlp) between riparian (wet) and neighboring slope (dry) populations of Acer grandidentum Nutt. was associated with the susceptibility of root versus stem xylem to embolism. Over two summers of study (1993–1994), the slope site had substantially lower xylem pressures (_px) and g ^s than the riparian site, particularly during the drought year of 1994. The _tlp was also lower at the slope (-2.9±0.1 MPa; all errors 95% confidence limits) than at riparian sites (-1.9±0.2 MPa); but it did not drop in response to the 1994 drought. Stem xylem did not differ in vulnerability to embolism between sites. Although slope-site stems lost a greater percentage of hydraulic conductance to embolism than riparian stems during the 1994 drought (46±11% versus 27±3%), they still maintained a safety margin of at least 1.7 MPa between midday _px and the critical pressure triggering catastrophic xylem embolism (_pxCT). Root xylem was more susceptible to embolism than stem xylem, and there were significant differences between sites: riparian roots were completely cavitated at -1.75 MPa, compared with -2.75 MPa for slope roots. Vulnerability to embolism was related to pore sizes in intervessel pit membranes and bore no simple relationship to vessel diameter. Safety margins from _pxCT averaged less than 0.6 MPa in roots at both the riparian and slope sites. Minimal safety margins at the slope site during the drought of 1994 may have led to the almost complete closure of stomata (g _s=9±2 versus 79±15 mmol m^-2 s^-1 at riparian site) and made any further osmotic adjustment of _tlp non-adaptive. Embolism in roots was at least partially reversed after fall rains. Although catastrophic embolism in roots may limit the minimum for gas exchange, partial (and reversible) root embolism may be adaptive in limiting water use as soil water is exhausted.     

A plumber's-eye view of xylem water transport in woody plants

We present a practical for university-level students aimed at measuring and comparing xylem hydraulic properties of co-existing plant species. After sampling branches of several woody species in the field, their main hydraulic properties were measured using a simple set-up. Hydraulic conductivity (K_h ) was calculated as the ratio between water flow through a plant segment and the pressure gradient driving the flow. The percent reduction in conductivity due to xylem embolism (i.e. air-filled conduits) was estimated by comparing K_h before and after flushing the measure segments to remove all native embolism. Raw hydraulic conductivity was standardised by cross-sectional wood area or supported leaf area to obtain more meaningful measures of conducting capacity. The results showed differences among study species, particularly between conifers and angiosperms. These differences are briefly discussed in terms of wood anatomy and the general biology of the species. Overall the practical provides a good opportunity for students to appreciate the main aspects of xylem water transport and the constraints it imposes on plant water relations.     

Effects of wildfire on growth,transpiration and hydraulic properties of Pinus pinaster Aiton forest

In the last decades, forests have been seriously threatened by the effects of climate change. In Mediterranean regions, increasing fire occurrence and drought have a substantial impact on forests, possibly pushing them towards tipping points. Indeed, although trees could survive to fire events of medium severity in the short term, severe defoliation and environmental stressors could affect the capacity to transport water and to fix and use carbon, amplifying the possibility of a delayed post fire mortality. Therefore, it is urgent to better understand trees eco-physiological processes in response to wildfire in order to plan suitable forest management strategies. In this research an innovative continuous monitoring system of sap flow and micro-stationary climatic conditions of trees was combined with dendro-anatomical analyses and xylem cavitation vulnerability studies to assess fire impact on a Pinus pinaster forest affected by a severe fire and compared to an unburned control site. Xylem analyses showed that the forest-fire did not influence the xylem hydraulic parameters and did not increased vulnerability to embolism. However, burned trees with high defoliation rate, presented reduced assimilation capacity and carbon availability for growth, as well as a limited development of thicker xylem conduit walls compared to unburned trees. Furthermore, continuous monitoring highlighted an increase in sap flow of defoliated trees compared to the control ones during the hottest and driest months of the year indicating a higher stomatal conductance. Overall, our results suggested that although fire did not affect the xylem hydraulics properties of the vascular system, the severe crown damage caused growth reduction and transpiration dysfunction in the monitored period. Those effect could negatively affect the survival of burned trees, especially in areas subjected to hot and dry periods.     

Comparative dendroclimatic study of Scots pine, Norway spruce, and silver fir in the Vrancea Range, Eastern Carpathian Mountains

Using dendroclimatical methods, we compared the growth response to climate fluctuations of three of the main Romanian Carpathian Mountains coniferous species, Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies Karst.) and silver fir (Abies alba Mill.), growing intermixed in a unique stand. Climate and soil conditions were therefore the same for all the trees studied. The experimental site was chosen to be representative of the south-exposed sites in the Romanian Carpathian Mountains, where pine occurs naturally. In order to investigate the consequences of fluctuations in climate at different temporal scales, we examined both inter-annual and decadal time-steps. An index of soil water deficit was computed to investigate the consequences of drought. Our study reveals that species exhibited different responses to precipitation, temperature or drought. Overall, pine was the most sensitive to precipitation, while spruce showed a higher response to temperature at high frequency for both the current and the previous growing seasons, and to soil water deficit. Fir was the least sensitive species of the comparison. However, for all species, decadal modulations of growth show precipitation as a common and strong driver on the dry south-facing slopes. The results show that Scots pine would be affected more than fir by increased drought frequency and could in the future be replaced on the xeric sites.     

Stem hydraulic properties of vines vs. shrubs of western poison oak,Toxicodendron diversilobum

Summary This study investigated the effect of mechanical support on water transport properties and wood anatomy of stems of western poison oak, Toxicodendron diversilobum (T. & G.) Greene. This plant grows as a vine when support is present but as a shrub when support is absent. I compared vines and shrubs growing naturally in the field and those produced from cuttings of 11 source plants in a common garden. Huber value (xylem transverse area/distal leaf area) was lower but specific conductivity (water volume · time^-1 · xylem transverse area^-1 · pressure gradient^-1) was higher in supported than unsupported plants both in the field and the common garden. The opposing effects of Huber value and mon garden. The opposing effects of Huber value and specific conductivity resulted in the same values of leafspecific conductivity (LSC, water volume · time^-1 · distal leaf area^-1 · pressure gradient^-1) for supported and unsupported shoots at a given site. Therefore, for the same rates of evapotranspiration, supported and unsupported shoots will have the same pressure gradients in their stems. Vessel lumen composed a higher proportion of stem cross-section in supported than unsupported plants (due to slightly wider vessels and not to greater vessel density). These results suggest that the narrow stems of supported plants are compensated hydraulically by the production of wider vessels: at a given site, poison oak plants co-ordinate their leaf and xylem development such that their stems achieve the same overall conductive efficiencies (LSCs), regardless of support conditions.     

Cavitation resistance of peduncle,petiole and stem is correlated with bordered pit dimensions in Magnolia grandiflora

Variation in resistance of xylem to embolism among flowers, leaves, and stems strongly influences the survival and reproduction of plants. However, little is known about the vulnerability to xylem embolism under drought stress and their relationships to the anatomical traits of pits among reproductive and vegetative organs. In this study, we investigated the variation in xylem vulnerability to embolism in peduncles, petioles, and stems in a woody plant, Magnolia grandiflora. We analyzed the relationships between water potentials that induced 50% embolism (P₅₀) in peduncles, petioles, and stems and the conduit pit traits hypothesized to influence cavitation resistance. We found that peduncles were more vulnerable to cavitation than petioles and stems, supporting the hypothesis of hydraulic vulnerability segmentation that leaves and stems are prioritized over flowers during drought stress. Moreover, P₅₀ was significantly correlated with variation in the dimensions of inter-vessel pit apertures among peduncles, petioles and stems. These findings highlight that measuring xylem vulnerability to embolism in reproductive organs is essential for understanding the effect of drought on plant reproductive success and mortality under drought stress.     

Inter-species variation of photosynthetic and xylem hydraulic traits in the deciduous and evergreen Euphorbiaceae tree species from a seasonally tropical forest in south-western China

The objective of the present study was to examine the functional coordination among hydraulic traits, xylem characteristics and gas exchange rates across three deciduous Euphorbiaceae tree species (Hevea brasiliensis, Macaranga denticulata and Bischofia javanica) and three evergreen Euphorbiaceae tree species (Drypetes indica, Aleurites moluccana and Codiaeum variegatum) from a seasonally tropical forest in south-western China. The deciduous tree species were more vulnerable to water stress-induced embolism than the evergreen tree species. However, the deciduous tree species generally had higher maximal rates of sapwood and leaf-specific hydraulic conductivity (K _S and K _L), respectively. Compared with the evergreen tree species, the deciduous tree species, however, possessed a lower density of sapwood and a wider diameter of xylem vessels. Regardless of leaf phenology, the hydraulic vulnerability and conductivity were significantly correlated with sapwood density and mean vessel diameter. Furthermore, the hydraulic vulnerability was positively correlated with water transport efficiency. In addition, the deciduous tree species exhibited higher maximal photosynthetic rates (A _max) and stomatal conductance (g _max), but lower water use efficiency (WUE). Interestingly, the A _max, g _max and WUE were strongly correlated with K _S and K _L across the deciduous and evergreen tree species. These results suggest that xylem structure, rather than leaf phenology, accounts for the difference in hydraulic traits between the deciduous tree species and the evergreen tree species. Meanwhile, our results show that there is a significant trade-off between hydraulic efficiency and safety, and a strong functional correlation between the hydraulic capacity and gas exchange rates across the deciduous and evergreen tree species.     

Xylem structure and water transport in a twiner,a scrambler,and a shrub of Lonicera (Caprifoliaceae)

Summary Wood structure and function was investigated in different growth forms of temperate honeysuckles (Lonicera spp.). All three species had many narrow vessels and relatively few wide ones, with the measured K _h (flow rate/pressure gradient) approximately 24–55% of the theoretical K _h predicted by Poiseuille's law. Only the twiner, Lonicera japonica, had some vessels greater than 50 m in diameter. The twiner also had the narrowest stem xylem diameters, suggesting the greater maximum vessel diameter hydraulically compensated for narrow stems. Conversely, the free-standing shrub, L. maackii, had the greatest annual increments of xylem but the least percent conductive xylem implying that a great portion of the wood was involved with mechanical support. The scrambler, L, sempervirens had low maximum vessel diameter, high Huber values (= xylem area/leaf area), and low specific conductivities (= measured K _h/xylem area), much like the shrub. The greatest vessel frequency occurred in the scrambler (901 vessels · mm^-2), the highest thus far recorded in vines. The lowest Huber value and highest specific conductivity occurred in the twiner, suggesting little self-support but relatively efficient water conduction. LSC (= measured K _h/leaf area) and maximum vessel diameter of Lonicera vines were near the low end of the range for vines in general.     

Phenology and water relations of tree sprouts and seedlings in a tropical deciduous forest of South India

The phenology of sprouts (>1 year old, up to 1.5 m in height) and seedlings (<1 year old) of six woody species (four deciduous, one brevi-deciduous, and one evergreen) was examined during the dry season in a tropical deciduous forest of South India. Xylem water potential (_x), leaf relative water content (RWC; % turgid weight), and xylem specific conductivity (K _S; kg s^–1 m^–1 MPa^–1) of sprouts were measured on two occasions during the dry season. In addition, K _S of seedlings (<1 year old) of one deciduous and one evergreen species was determined to allow comparison with sprouts. _x of deciduous species was significantly higher at the second sampling date and was accompanied by a significant increase in K _S and RWC, while the brevi-deciduous and evergreen species did not show any difference in _x. Seedlings of Terminalia crenulata (deciduous) and Ixora parviflora (evergreen) had significantly lower K _S compared to sprouts, while seedlings of all four deciduous species shed their leaves much earlier in the dry season than did conspecific sprouts. More favorable water relations of sprouts compared to seedlings during the peak of the dry season may explain the lower rates of die-back and mortality of sprouts observed in dry deciduous forests of India.

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This revised version was published online in May 2005 with corrections to Received-/Accepted-dates.     

Hydraulic properties and freezing-induced cavitation in sympatric evergreen and deciduous oaks with contrasting habitats

We investigated the hydraulic properties in relation to soil moisture, leaf habit, and phylogenetic lineage of 17 species of oaks (Quercus) that occur sympatrically in northern central Florida (USA). Leaf area per shoot increased and Huber values (ratio of sapwood area to leaf area) decreased with increasing soil moisture of species’ habitats. As a result, maximum hydraulic conductance and maximum transpiration were positively correlated with mean soil moisture when calculated on a sapwood area basis, but not when calculated on a leaf area basis. This reveals the important role that changes in allometry among closely related species can play in co‐ordinating water transport capacity with soil water availability. There were significant differences in specific conductivity between species, but these differences were not explained by leaf habit or by evolutionary lineage. However, white oaks had significantly smaller average vessel diameters than red oaks or live oaks. Due to their lower Huber values, maximum leaf specific conductivity (K_L) was higher in evergreen species than in deciduous species and higher in live oaks than in red oaks or white oaks. There were large differences between species and between evolutionary lineages in freeze–thaw‐induced embolism. Deciduous species, on average, showed greater vulnerability to freezing than evergreen species. This result is strongly influenced by evolutionary lineage. Specifically, white oaks, which are all deciduous, had significantly higher vulnerability to freezing than live oaks (all evergreen) and red oaks, which include both evergreen and deciduous species. These results highlight the importance of taking evolutionary lineage into account in comparative physiological studies.     

Stand structure modulates the long-term vulnerability of Pinus halepensis to climatic drought in a semiarid Mediterranean ecosystem

We investigated whether stand structure modulates the long-term physiological performance and growth of Pinus halepensis Mill. in a semiarid Mediterranean ecosystem. Tree radial growth and carbon and oxygen stable isotope composition of latewood (δ(13)C(LW) and δ(18)O(LW), respectively) from 1967 to 2007 were measured in P. halepensis trees from two sharply contrasting stand types: open woodlands with widely scattered trees versus dense afforested stands. In both stand types, tree radial growth, δ(13)C(LW) and δ(18)O(LW) were strongly correlated with annual rainfall, thus indicating that tree performance in this semiarid environment is largely determined by inter-annual changes in water availability. However, trees in dense afforested stands showed consistently higher δ(18)O(LW) and similar δ(13)C(LW) values compared with those in neighbouring open woodlands, indicating lower stomatal conductance and photosynthesis rates in the former, but little difference in water use efficiency between stand types. Trees in dense afforested stands were more water stressed and showed lower radial growth, overall suggesting greater vulnerability to drought and climate aridification compared with trees in open woodlands. In this semiarid ecosystem, the negative impacts of intense inter-tree competition for water on P. halepensis performance clearly outweigh potential benefits derived from enhanced infiltration and reduced run-off losses in dense afforested stands.     

Patterns of tree dieback in Queensland,Australia: the importance of drought stress and the role of resistance to cavitation

During the extreme 1992–1997 El Niño drought event, widespread stem mortality, or tree dieback, of both mature and juvenile eucalypts occurred within the tropical savannas of northeast Australia. Most of the dieback occurred in individuals of the ironbark species complex (Eucalyptus crebra – E. xanthoclada) while individuals of the bloodwood species Corymbia erythrophloia, exhibited significantly less stem mortality. Indicative of greater water stress, predawn and midday xylem water potentials of ironbark adults and saplings were significantly more negative than predawn values of bloodwoods. The very negative xylem water potentials in ironbarks suggest that stem mortality in both adult and juvenile ironbarks results from drought-induced embolism and that ironbarks perhaps have a shallower and less extensive root system than bloodwoods. Although predawn and midday water potentials for ironbark adults and saplings were similar, a census of mature and juvenile ironbark trees indicated that mortality was higher in adult trees. Cavitation vulnerability curves indicated that ironbark saplings may be better buffered against cavitation than adult trees. If they possess smaller root systems, saplings are more likely than adults to experience low xylem water potentials, even in non-drought years. Xylem conduits produced in adult trees during periods of normal rainfall, although perhaps more efficient in water conduction, may be more vulnerable to cavitation during infrequent severe droughts.     

The relevance of xylem network structure for plant hydraulic efficiency and safety

The xylem is one of the two long distance transport tissues in plants, providing a low resistance pathway for water movement from roots to leaves. Its properties determine how much water can be transported and transpired and, at the same time, the plant's vulnerability to transport dysfunctions (the formation and propagation of emboli) associated to important stress factors, such as droughts and frost. Both maximum transport efficiency and safety against embolism have classically been attributed to the properties of individual conduits or of the pit membrane connecting them. But this approach overlooks the fact that the conduits of the xylem constitute a network. The topology of this network is likely to affect its overall transport properties, as well as the propagation of embolism through the xylem, since, according to the air-seeding hypothesis, drought-induced embolism propagates as a contact process (i.e., between neighbouring conduits). Here we present a model of the xylem that takes into account its system-level properties, including the connectivity of the xylem network. With the tools of graph theory and assuming steady state and Darcy's flow we calculated the hydraulic conductivity of idealized wood segments at different water potentials. A Monte Carlo approach was adopted, varying the anatomical and topological properties of the segments within biologically reasonable ranges, based on data available from the literature. Our results showed that maximum hydraulic conductivity and vulnerability to embolism increase with the connectivity of the xylem network. This can be explained by the fact that connectivity determines the fraction of all the potential paths or conduits actually available for water transport and spread of embolism. It is concluded that the xylem can no longer be interpreted as the mere sum of its conduits, because the spatial arrangement of those conduits in the xylem network influences the main functional properties of this tissue. This brings new arguments into the long-standing discussion on the efficiency vs. safety trade-off in the plants' xylem.     

Plastic bimodal growth in a Mediterranean mixed-forest of Quercus ilex and Pinus halepensis

Mediterranean tree species have evolved to face seasonal water shortages, but may fail to cope with future increases in drought frequency and intensity. We investigated stem radial increment dynamics in two typical Mediterranean tree species, Aleppo pine (Pinus halepensis), a drought-avoiding species, and holm oak (Quercus ilex), a drought-tolerant species, in a mixed forest and on contrasting slope aspects (south- and north-facing). Intra- and inter-annual growth patterns were modelled using the VS-Lite2 model for each tree species and slope-aspect. Both species showed a bimodal growth pattern, with peaks coinciding with favourable conditions in spring and autumn. A bimodal growth pattern is always observed in P. halepensis, while in Q. ilex is facultative, which suggests different strategies adopted by these species to cope with summer drought. More specifically, trees on south-facing slope showed a more evident bimodal pattern and more intra-annual density fluctuations. In recent decades, the intensity of both growth peaks has diminished and drifted away due to the increased summer drought. The VS-Lite2 model reveals a niche partitioning between both species. Differences in growing season’s length and timings of growth peaks in both species are relevant for their coexistence and should be considered for estimating mixed-forest responses under climate change scenarios.     

Water resource partitioning,stem xylem hydraulic properties,and plant water use strategies in a seasonally dry riparian tropical rainforest

This study investigated seasonal variation in the origin of water used by plants in a riparian tropical rainforest community and explored linkages between plant water source, plant xylem hydraulic conductivity and response to the onset of dry conditions. The study focused on five co-dominant canopy species, comprising three tree species (Doryphora aromatica, Argyrodendron trifoliolatum, Castanospora alphandii) and two climbing palms (Calamus australis and Calamus caryotoides). Stable isotope ratios of oxygen in water (¹⁸O) from soil, groundwater, stream water and plant xylem measured in the wet season and the subsequent dry season revealed water resource partitioning between species in the dry season. Measurement of stem-area-specific hydraulic conductivity (K_S) in the wet season and subsequent dry season showed a significant dry-season loss of K_S in three of the five species (Castanospora alphandii, Calamus australis and C. caryotoides) and a decrease in mean K_S for all species. This loss of hydraulic conductivity was positively correlated with the difference between wet-season and dry-season midday leaf water potentials and with leaf carbon isotope discrimination, indicating that plants that were less susceptible to loss of conductivity had greater control over transpiration rate and were more water-use efficient.     

Chlorophyll accumulation in cotyledons, hypocotyls and primary needles of Pinus pinea seedlings in light and dark

Seeds of Pinus pinea L. were germinated and grown in darkness or under natural photoperiodic (day/night) conditions for 8 weeks. Samples were taken for analysis at frequent intervals and chlorophyll (Chl) and protochlorophyllide (Pchlide) contents of cotyledons, hypocotyls and primary needles determined spectrophotometrically alter separating the esterified from unesterified pigments by partitioning between ammoniacal acetone and petroleum ether. Chlorophyll accumulated in cotyledons, hypocotyls and primary needles of dark as Well as light grown seedlings. The maximum Chl concentrations achieved in darkness in these organs were approximately 60, 20 and 10% respectively, of concentrations achieved in light. The tinsel of rapid Chl accumulation by cotyledons in darkness coincided with the emergence of the radicle from the seed. Light had no effect on the timing of this process or on the duration of the lag period. However, it extended the exponential growth phase of cotyledons by at least a week and. once the seedlings were about 2 weeks old, enabled Chl to accumulate at a faster rate. Time courses of Chi and Pchlide accumulation by cotyledons in darkness were similar. Pchlide did not accumulate prior to the onset of Chi accumulation and Chl slopped accumulating once the Pchlide pool reached maximum size. The implications of these findings are discussed and a hypothesis to account for the inability of primary needles of smaller seeded pines to accumulate Chl in darkness is proposed.     

O-coumaric acid ester,a potential early signaling molecule in Pinus pinea and Pisolithus arhizus symbiosis established in vitro

During ectomycorrhizal (ECM) establishment, biochemical signals lead to the development of complex structures in both the plant and the fungus that ultimately result in the formation of an ectomycorrhiza. The cross-talk between partners begins before physical contact. Our objective was to investigate the chemical nature of the signals during the first stages of in vitro mycorrhization of Pinus pinea with Pisolithus arhizus. For this purpose a double-phase solid–liquid medium was expressly developed for the co-culture in order to simplify the extraction and further molecules analysis. O-coumaric acid ester was identified using HPLC–UV and LC–DAD–MS on the second day of co-culture and its presence was detected for up to 10 days. These results contribute to the characterization of biochemical signals during pre-colonization involving conifer species and an ECM fungus, and demonstrate the suitability of the double-phase medium developed for the growth of both organisms and for the analysis of released chemical mediators.     

Hydraulic propagation of pressure along immature and mature xylem vessels of roots of Zea mays measured by pressure-probe techniques

Turgor pressure was measured in cortical cells and in xylem elements of excised roots and roots of intact plants of Zea mays L. by means of a cell pressure probe. Turgor of living and hence not fully differentiated late metaxylem (range 0.6–0.8 MPa) was consistently higher than turgor of cortical cells (range 0.4–0.6 MPa) at positions between 40 and 180 mm behind the root tip. Closer to the tip, no turgor difference between the cortex and the stele was measured. The turgor difference indicated that late-metaxylem elements may function as nutrient-storage compartments within the stele. Excised roots were attached to the root pressure probe to precisely manipulate the xylem water potential. Root excision did not affect turgor of cortical cells for at least 8 h. Using the cell pressure probe, the propagation of a hydrostatic pressure change effected by the root pressure probe was recorded in mature and immature xylem elements at various positions along the root. Within seconds, the pressure change propagated along both early and late metaxylems. The half-times of the kinetics, however, were about five times smaller for the early metaxylem, indicating they are likely the major pathway of longitudinal water flow. The hydraulic signal dissipated from the source of the pressure application (cut end of the root) to the tip of the root, presumably because of radial water movement along the root axis. The results demonstrate that the water status of the growth zone and other positions apical to 20 mm is mainly uncoupled from changes of the xylem water potential in the rest of the plant.Abbreviations and Symbols CPP cell pressure probe - EMX early metaxylem - LMX Late metaxylem - P_c cell turgor - P_r root pressure - RPP root pressure probe - t_1/2,c half-time of water exchange across a single cell - t_1/2 half-time of water exchange across multiple cells We thank Antony Matista for his expert assistance in the construction and modification of instruments. The work was supported by grant DCB8802033 from the National Science Foundation and grant 91-37100-6671 from USDA, and by the award of a Feodor Lynen-Fellowship from the Alexander von Humboldt-Foundation (Germany) to J.F.