Drop In Water Potential Research Articles

The role of height-driven constraints and compensations on tree vulnerability to drought.

Frequent observations of higher mortality in larger trees than in smaller ones during droughts have sparked an increasing interest in size-dependent drought-induced mortality. However, the underlying physiological mechanisms are not well understood, with height-associated hydraulic constraints often being implied as the potential mechanism driving increased drought vulnerability. We performed a quantitative synthesis on how key traits that drive plant water and carbon economy change with tree height within species and assessed the implications that the different constraints and compensations may have on the interacting mechanisms (hydraulic failure, carbon starvation and/or biotic-agent attacks) affecting tree vulnerability to drought. While xylem tension increases with tree height, taller trees present a range of structural and functional adjustments, including more efficient water use and transport and greater water uptake and storage capacity, that mitigate the path-length-associated drop in water potential. These adaptations allow taller trees to withstand episodic water stress. Conclusive evidence for height-dependent increased vulnerability to hydraulic failure and carbon starvation, and their coupling to defence mechanisms and pest and pathogen dynamics, is still lacking. Further research is needed, particularly at the intraspecific level, to ascertain the specific conditions and thresholds above which height hinders tree survival under drought.

Field scale plant water relation of maize (Zea mays) under drought – impact of root hairs and soil texture

Background and aimsImpact of drought on crop growth depends on soil and root hydraulic properties that determine the access of plant roots to soil water. Root hairs may increase the accessible water pool but their effect depends on soil hydraulic properties and adaptions of root systems to drought. These adaptions are difficult to investigate in pot experiments that focus on juvenile plants.MethodsA wild-type and its root hairless mutant maize (Zea mays) were grown in the field in loam and sand substrates during two growing seasons with a large precipitation deficit. A comprehensive dataset of soil and plant properties and monitored variables were collected and interpreted using simulations with a mechanistic root water uptake model.ResultsTotal crop water use was similar in both soils and for both genotypes whereas shoot biomass was larger for the wild type than for the hairless mutant and did not differ between soils. Total final root length was larger in sand than in loam but did not differ between genotypes. Simulations showed that root systems of both genotypes and in both soils extracted all plant available soil water, which was similar for sand and loam, at a potential rate. Leaf water potentials were overestimated by the model, especially for the hairless mutant in sand substrate because the water potential drop in the rhizosphere was not considered.ConclusionsA direct effect of root hairs on water uptake was not observed but root hairs might influence leaf water potential dependent growth.

Root hydraulic phenotypes impacting water uptake in drying soils.

Soil drying is a limiting factor for crop production worldwide. Yet, it is not clear how soil drying impacts water uptake across different soils, species, and root phenotypes. Here we ask (1) what root phenotypes improve the water use from drying soils? and (2) what root hydraulic properties impact water flow across the soil–plant continuum? The main objective is to propose a hydraulic framework to investigate the interplay between soil and root hydraulic properties on water uptake. We collected highly resolved data on transpiration, leaf and soil water potential across 11 crops and 10 contrasting soil textures. In drying soils, the drop in water potential at the soil–root interface resulted in a rapid decrease in soil hydraulic conductance, especially at higher transpiration rates. The analysis reveals that water uptake was limited by soil within a wide range of soil water potential (−6 to −1000 kPa), depending on both soil textures and root hydraulic phenotypes. We propose that a root phenotype with low root hydraulic conductance, long roots and/or long and dense root hairs postpones soil limitation in drying soils. The consequence of these root phenotypes on crop water use is discussed.

Quantifying seasonal and diurnal variation of stomatal behavior in a hydraulic-based stomatal optimization model

Plant responses to drought occur across many time-scales, with stomatal closure typically considered to be a critical short-term response. Recent theories of optimal stomatal conductance linked to plant hydraulic transport have shown promise, but it is not known if stomata update their hydraulic “shadow price” of water use (marginal increase in carbon cost with a marginal drop in water potential) over days, seasons, or in response to recent drought. Here, I estimate the hydraulic shadow price in five species – two semi-arid gymnosperms, one temperate and two tropical angiosperms – at daily timescales and in wet and dry periods. I tested whether the shadow prices varies predictably as a function of current and/or lagged drought conditions. Diurnal estimates of the hydraulic shadow price estimated from observed stomatal conductance, while variable, did not vary predictably with environmental variables. Seasonal variation in shadow price was observed in the gymnosperm species, but not the angiosperm species, and did not meaningfully influence prediction accuracy of stomatal conductance. The lack of systematic variation in shadow price and high predictive ability of stomatal conductance when using a single set of parameters further emphasizes the potential of hydraulic-based stomatal optimization theories.

Effects of root dehydration on biomechanical properties of woody roots of Ulex europaeus

AimsEffects of root water status on root tensile strength and Young’s modulus were studied in relation to root reinforcement of slopes.MethodsBiomechanical properties of woody roots, Ulex europaeus, were tested during progressive dehydration and after thirty-day moisture equilibration in soil with contrasting water contents. Root diameter, water content and water loss were recorded and root water potential versus water content relation was investigated. Tensile stresses induced by root contraction upon dehydration were measured.ResultsRoot tensile strength and Young’s modulus increased abruptly when root water content dropped below 0.5 g g−1. The strength increase was due to root radial and axial contraction induced by root water potential drop. Diameter decrease and strength gain were the largest for thin roots because of the relatively larger evaporative surface per volume of thin roots. Largely negative water potentials in dry soil induced root drying, affecting root biomechanical properties.ConclusionRoot water status is a factor that can cause (inappropriately) high strength values and the large variability reported in literature for thin roots. Therefore, all root diameter classes should have consistent moisture for fair comparison. Testing fully hydrated roots should be the routine protocol, given that slope instability occurs after heavy rainfall.

Interannual variations in needle and sapwood traits of Pinus edulis branches under an experimental drought.

In the southwestern USA, recent large‐scale die‐offs of conifers raise the question of their resilience and mortality under droughts. To date, little is known about the interannual structural response to droughts. We hypothesized that piñon pines (Pinus edulis) respond to drought by reducing the drop of leaf water potential in branches from year to year through needle morphological adjustments. We tested our hypothesis using a 7‐year experiment in central New Mexico with three watering treatments (irrigated, normal, and rain exclusion). We analyzed how variation in “evaporative structure” (needle length, stomatal diameter, stomatal density, stomatal conductance) responded to watering treatment and interannual climate variability. We further analyzed annual functional adjustments by comparing yearly addition of needle area (LA) with yearly addition of sapwood area (SA) and distance to tip (d), defining the yearly ratios SA:LA and SA:LA/d. Needle length (l) increased with increasing winter and monsoon water supply, and showed more interannual variability when the soil was drier. Stomatal density increased with dryness, while stomatal diameter was reduced. As a result, anatomical maximal stomatal conductance was relatively invariant across treatments. SA:LA and SA:LA/d showed significant differences across treatments and contrary to our expectation were lower with reduced water input. Within average precipitation ranges, the response of these ratios to soil moisture was similar across treatments. However, when extreme soil drought was combined with high VPD, needle length, SA:LA and SA:LA/d became highly nonlinear, emphasizing the existence of a response threshold of combined high VPD and dry soil conditions. In new branch tissues, the response of annual functional ratios to water stress was immediate (same year) and does not attempt to reduce the drop of water potential. We suggest that unfavorable evaporative structural response to drought is compensated by dynamic stomatal control to maximize photosynthesis rates.

Correlation between leaf size and hydraulic architecture in five compound-leaved tree species of a temperate forest in NE China

The divergence between simple and compound leaf form is a fundamental division in leaf architecture that has great impact on environmental adaptations of plants. Two hypotheses regarding the adaptive significance of compound leaf form have long been hypothesized: (1) it enables trees to have higher growth rates under favorable environmental conditions; (2) it contributes to better adaptation to seasonal and unpredictable drought stresses since dropping the whole leaf units could function as a protective mechanism of hydraulic segmentation and hence avoiding diebacks of the more carbon costly stems. These hypotheses, however, have not been firmly supported by mechanistic studies on the underlying physiology and more importantly the inter-specific variations within this functional group in relation to these two proposed hypotheses have largely been overlooked. In the present study, using a common garden setup we investigated the impact of leaf size, an important characteristic of leaf architecture, on xylem hydraulics and carbon economy of five commonly found sympatric compound-leaved tree species from a typical temperate forest of NE China. We specifically tested the hypotheses that larger compound leaf size would be associated with higher hydraulic conductance, increased efficiency of carbon assimilation and greater degree of hydraulic segmentation. Our results showed that the majority of the hydraulic resistance in shoots was allocated to leaf lamina (53–77% among the five species) and the compound leaf petiole only accounts for a small portion of the shoot hydraulic resistance (9–24%). Both stem hydraulic conductivity and whole-shoot hydraulic conductance showed strong positive correlations with compound leaf size contributing to significantly higher carbon assimilation efficiency in species with larger leaf sizes. The magnitude of water potential drop across transpiring leaves showed a strong positive correlation with leaf size resulting in less negative stem xylem water potential in species with larger leaf sizes, which supports our hypothesis that larger compound leaf enhances hydraulic segmentation. Our results also showed that the advantages associated with larger leaf size can be traded off by a greater susceptibility to freeze-thaw induced hydraulic dysfunction. Besides a deeper understanding of the environmental adaptation of compound-leaved tree species, these findings may contribute to a better utilization of this important type of trees in forestry.

Optimal stomatal behavior with competition for water and risk of hydraulic impairment

For over 40 y the dominant theory of stomatal behavior has been that plants should open stomates until the carbon gained by an infinitesimal additional opening balances the additional water lost times a water price that is constant at least over short periods. This theory has persisted because of its remarkable success in explaining strongly supported simple empirical models of stomatal conductance, even though we have also known for over 40 y that the theory is not consistent with competition among plants for water. We develop an alternative theory in which plants maximize carbon gain without pricing water loss and also add two features to both this and the classical theory, which are strongly supported by empirical evidence: (i) water flow through xylem that is progressively impaired as xylem water potential drops and (ii) fitness or carbon costs associated with low water potentials caused by a variety of mechanisms, including xylem damage repair. We show that our alternative carbon-maximization optimization is consistent with plant competition because it yields an evolutionary stable strategy (ESS)-species with the ESS stomatal behavior that will outcompete all others. We further show that, like the classical theory, the alternative theory also explains the functional forms of empirical stomatal models. We derive ways to test between the alternative optimization criteria by introducing a metric-the marginal xylem tension efficiency, which quantifies the amount of photosynthesis a plant will forego from opening stomatal an infinitesimal amount more to avoid a drop in water potential.

An allometry‐based model of the survival strategies of hydraulic failure and carbon starvation

AbstractA simplified soil–plant–atmosphere–continuum model of carbon starvation and hydraulic failure is developed and tested against observations from a drought‐manipulation experiment in a woodland dominated by piñon pine (Pinus edulis) and juniper (Juniperus monosperma) in New Mexico. The number of model parameters is reduced using allometric relationships. The model can represent more isohydric (piñon) and more anisohydric (juniper) responses. Analysis of the parameter space suggests four main controls on hydraulic failure and carbon starvation: xylem vulnerability curve, root:shoot area ratio, rooting depth and water use efficiency. For piñon, an intermediate optimal (1.5–2 m2 m−2) tree leaf area index reduces the risk of hydraulic failure. For both piñons and junipers, hydraulic failure was relatively insensitive to root:shoot ratio across a range of tree LAI. Higher root:shoot ratios however strongly decreased the time to carbon starvation. The hydraulic safety margin of piñons is strongly diminished by large diurnal variations in xylem/leaf water potential. Diurnal drops of water potential are mitigated by high maximum hydraulic conductivity, high root:shoot ratio and stomatal regulation (more isohydric). The safety margin of junipers is not very sensitive to diurnal drops in water potential so that there is little benefit in stomatal regulation (more anisohydric). Narrower tracheid diameter and a narrower distribution of tracheid diameters reduce the risk of hydraulic failure and carbon starvation by reducing diurnal xylem water potential drop. Simulated tree diameter‐dependent mortality varies between these two species, with piñon mortality decreasing with increasing tree size, whereas juniper mortality increases with tree size. Juvenile piñons might thus be overimpacted by water stress. Copyright © 2015 John Wiley & Sons, Ltd.

Daily dynamics of leaf and soil-to-branch hydraulic conductance in silver birch (Betula pendula) measured in situ

Daily dynamics of leaf (K(L)) and soil-to-branch hydraulic conductance (KS-B) was investigated in silver birch (Betula pendula Roth.) using evaporative flux method in situ: water potential drop was measured with a pressure chamber and evaporative flux was estimated as sap flux density measured with sap flow gauges. Canopy position had a significant (P < 0.001) effect on both K(L) and K(S-B). Upper-canopy leaves exhibited 1.7 and soil-to-branch pathway 2.3 times higher hydraulic efficiency than those for lower-canopy. K(L) varied significantly with time of day: K(L) for both upper- and lower-canopy leaves was lowest in the morning and rose gradually achieving maximal values in late afternoon (4.75 and 3.38 mmol m⁻² s⁻¹ MPa⁻¹, respectively). Relevant environmental factors affecting K(L) were photosynthetic photon flux density (Q(P)), air relative humidity (RH) and air temperature (T(A)). K(S-B) started rising in the morning and reached maximum in the lower canopy (1.44 mmol m⁻² s⁻¹ MPa⁻¹) at 1300 h and in the upper canopy (2.52 mmol m⁻² s⁻¹ MPa⁻¹) at 1500 h, decreasing afterwards. Environmental factors controlling K(S-B) were Ψ(S) and Q(P). The diurnal patterns of K(L) reflect a combination of environmental factors and endogenous rhythms. The temporal pattern of K(S-B) refers to daily up- and down-regulation of hydraulic conductance of water transport pathway from soil-root interface to leaves with respect to changing irradiance.

How the Rhizosphere May Favor Water Availability to Roots

Recent studies have shown that rhizosphere hydraulic properties may differ from those of the bulk soil. Specifically, mucilage at the root–soil interface may increase the rhizosphere water holding capacity and hydraulic conductivity during drying. The goal of this study was to point out the implications of such altered rhizosphere hydraulic properties for soil–plant water relations. We addressed this problem through modeling based on a steady‐rate approach. We calculated the water flow toward a single root assuming that the rhizosphere and bulk soil were two concentric cylinders having different hydraulic properties. Based on our previous experimental results, we assumed that the rhizosphere had higher water holding capacity and unsaturated conductivity than the bulk soil. The results showed that the water potential gradients in the rhizosphere were much smaller than in the bulk soil. The consequence is that the rhizosphere attenuated and delayed the drop in water potential in the vicinity of the root surface when the soil dried. This led to increased water availability to plants, as well as to higher effective conductivity under unsaturated conditions. The reasons were two: (i) thanks to the high unsaturated conductivity of the rhizosphere, the radius of water uptake was extended from the root to the rhizosphere surface; and (ii) thanks to the high soil water capacity of the rhizosphere, the water depletion in the bulk soil was compensated by water depletion in the rhizosphere. We conclude that under the assumed conditions, the rhizosphere works as an optimal hydraulic conductor and as a reservoir of water that can be taken up when water in the bulk soil becomes limiting.

Influence of evaporative demand on aquaporin expression and root hydraulics of hybrid poplar

When light levels and evaporative demand increase, dynamic physiological changes in roots may be required to restore the water balance at the whole plant level. We hypothesized that a dynamic increase in root hydraulic conductance (L(P)) and aquaporin (AQP) expression could moderate the transpiration-induced drop in water potential (Ψ), allowing continued gas exchange in hybrid poplar (Populus trichocarpa × deltoides) saplings. Fifty-six AQPs have been identified in poplar, but little information about their expression patterns in roots is available, especially from a whole-plant water relations perspective. We measured AQP expression and L(P) in plants subjected to different levels of light and evaporative demand. Shaded plants had only one-tenth the root area of plants growing at higher light levels. Shade-grown saplings experiencing a sudden increase in light exhibited a threefold higher L(P) than plants remaining in shade. This dynamic increase in L(P) corresponded with increased transcript abundance of 15 AQPs out of a total of 33 genes simultaneously assessed by quantitative RT-PCR. The tissue-level localization of transcripts of four AQPs was studied with in situ hybridization. Comprehensive expression profiling in conjunction with physiological and morphological measurements is a valuable reference for future studies on AQP function in poplar.

Dynamics of soil water content in the rhizosphere

Water flow from soil to plants depends on the properties of the soil next to roots, the rhizosphere. Although several studies showed that the rhizosphere has different properties than the bulk soil, effects of the rhizosphere on root water uptake are commonly neglected. To investigate the rhizosphere’s properties we used neutron radiography to image water content distributions in soil samples planted with lupins during drying and subsequent rewetting. During drying, the water content in the rhizosphere was 0.05 larger than in the bulk soil. Immediately after rewetting, the picture reversed and the rhizosphere remained markedly dry. During the following days the water content of the rhizosphere increased and after 60 h it exceeded that of the bulk soil. The rhizosphere’s thickness was approximately 1.5 mm. Based on the observed dynamics, we derived the distinct, hysteretic and time-dependent water retention curve of the rhizosphere. Our hypothesis is that the rhizosphere’s water retention curve was determined by mucilage exuded by roots. The rhizosphere properties reduce water depletion around roots and weaken the drop of water potential towards roots, therefore favoring water uptake under dry conditions, as demonstrated by means of analytical calculation of water flow to a single root.

A tracheomycosis as a tool for studying the impact of stem xylem dysfunction on leaf water status and gas exchange in Citrus aurantium L.

Permanent xylem blockage is a common result of attacks by herbivores and fungi. The mitosporic fungus Phoma tracheiphila (Petri) Kantschaveli et Gikachvili, is the agent of a Citrus tracheomycosis (“malsecco disease”) causing xylem impairment and leading to leaf shedding and plant dieback. In the present study, this pathogen was used for monitoring the effects of increasing levels of stem hydraulic resistance (Rstem) on leaf water status and gas exchange. In this view, measurements are reported of changes in the hydraulic resistance of infected stems (Rstem) of C. aurantium (sour orange) during progressive and irreversible xylem blockage with parallel measurements of leaf water potential and conductance to water vapour. Leaves were highly responsive to increasing Rstem as due to fungal infection, with substantial stomatal closure and drop in water potential.

Effects of light intensity and duration on leaf hydraulic conductance and distribution of resistance in shoots of silver birch (Betula pendula)

Variation in leaf hydraulic conductance (K(L)) and distribution of resistance in response to light intensity and duration were examined in shoots of silver birch (Betula pendula Roth). K(L) was determined on detached shoots using the evaporative flux method (transpiration was measured with a porometer and water potential drop with a pressure chamber). Although K(L) depended on light duration per se, its dynamics was largely determined by leaf temperature (T(L)). Both upper-crown leaves and branches developed in well-illuminated environment exhibited higher hydraulic efficiency compared with the lower crown, accounting for vertical trends of apparent soil-to-leaf hydraulic conductance in canopy of silver birch revealed in our previous studies. K(L) varied significantly with light intensity, the highest values for both shade and sun foliage were recorded at photosynthetic photon flux density of 330 micromol m(-2) s(-1). Light responses of K(L) were associated evidently with an irradiance-mediated effect on extravascular tissues involving regulation of cell membrane aquaporins. Effects of irradiance on K(L) resulted in changes of Psi(L), bringing about considerable alteration in partitioning of the resistance between leaves and branch (leafless shoot stem): the contribution of leaves to the shoot total resistance decreased from 94% at -1.0 MPa to 75% at -0.2 MPa. Treatment with HgCl2 decreased hydraulic conductance of both leaves and branches, implying that condition of bordered pit membranes or shoot living tissues may be involved in responses of xylem conductance to Hg2+.

The effect of vapor pressure deficit on leaf area and water transport in flower stems of soil-less culture rose

The effect of indoor air relative humidity on leaf area and water transport of flower stems in a greenhouse rose crop (Rosa X hybrida cv Mercedes) grown on a soil-less substrate was studied on 2-year-old plants, in a freely draining irrigated system ensuring a high leaching fraction. In one compartment of the greenhouse, the roof opened when the air temperature reached 28 °C. In the other, an evaporative wet pad and fans were operated at 28 °C. The wet pad treatment decreased vapor pressure deficit (VPD). A maximum VPD difference of 1.45 kPa between the two compartments occurred during the noon-hours on a warm day with high atmospheric evaporative demand. On days with moderate evaporative demand, the wet pad was either not operated or when operated, produced VPD differences smaller than 1.45 kPa. Wet pad treatment decreased the transpiration rate per unit leaf area of the flower stem. On a typical summer day, with high evaporative demand, mean maximum water loss per unit leaf area was 2.63 ± 0.13 and 1.79 ± 0.09 kg m −2 day −1 for the high and low VPD compartments, respectively. However, low VPD decreased only slightly mean daily water flow per stem: 104.3 ± 6.9 g at low VPD versus 112.4 ± 8.7 g at high VPD (not significant at p > 0.05), despite the significant differences ( p > 0.05) in VPD between the greenhouse compartments. The wet pad treatment mitigated leaf water potential drop at noon-time. The results suggest that rose flower stems adapt to high VPD by decreasing leaf area for maintaining high sap flow rate per unit area.

Water import in the young tomato fruit limited by pedicel resistance and calyx transpiration.

Approximately 3 weeks after pollination, the water import rate per unit tomato fruit area generally decreases linearly with fruit radius. In order to explain why, in the first 3 weeks, the water import rate of the fruit is lower than predicted by this regression, a model of water flow through the pedicel and a model of water import based on the potential of water entering the fruit and calyx transpiration were formulated. Using data sets available in the literature, these models predict a water potential drop along the pedicel that decreases sharply during the first 3 weeks, while the calculated hydraulic conductivity of the pedicel phloem, which is presumed to be the main pathway of the water imported in the tomato fruit, increases sharply in the lower range of values known for plant phloem conductivity. These models also predict an increase in water import into young fruit when calyx transpiration is decreased, which is consistent with data from the literature. In order to explain the increasing pedicel phloem conductivity, a model of water flow in the pedicel sieve tubes was formulated based on the literature data for the fruit stalk of Lupinus albus. It was suggested that the conductivity might increase because of the development of pores in the sieve plates. Using this hypothesis, the increase in pore radius values within an acceptable range was calculated. This study shows that, under a wide range of conditions, water import in young tomato fruit is limited by the low potential of the water entering the fruit due to pedicel resistance and calyx transpiration. It provides a model to predict young tomato fruit expansion and a testable hypothesis, which can be checked by measuring the size of the phloem component in the fruit pedicel.

Hydraulic architecture and water flow in growing grass tillers (Festuca arundinacea Schreb.)

ABSTRACTThe water relations and hydraulic architecture of growing grass tillers (Festuca arundinacea Schreb.) are reported. Evaporative flux density, E (mmol s−1 m−2), of individual leaf blades was measured gravimetrically by covering or excision of entire leaf blades. Values of E were similar for mature and elongating leaf blades, averaging 2·4 mmol s−1 m−2. Measured axial hydraulic conductivity, Kh (mmol s−1 mm MPa−1), of excised leaf segments was three times lower than theoretical hydraulic conductivity (Kt) calculated using the Poiseuille equation and measurements of vessel number and diameter. Kt was corrected (Kt*) to account for the discrepancy between Kh and Kt and for immature xylem in the basal expanding region of elongating leaves. From base to tip of mature leaves the pattern of Kt* was bell‐shaped with a maximum near the sheath–blade joint (≈ 19 mmol s−1 mm MPa−1). In elongating leaves, immature xylem in the basal growing region led to a much lower Kt*. As the first metaxylem matured, Kt* increased by 10‐fold. The hydraulic conductances of the whole root system, (mmol s−1 MPa−1) and leaf blades, (mmol s−1 MPa−1) were measured by a vacuum induced water flow technique. and were linearly related to the leaf area downstream. Approximately 65% of the resistance to water flow within the plant resided in the leaf blade. An electric‐analogue computer model was used to calculate the leaf blade area‐specific radial hydraulic conductivity, (mmol s−1 m−2 MPa−1), using , Kt* and water flux values. values decreased with leaf age, from 21·2 mmol s−1 m−2 MPa−1 in rapidly elongating leaf to 7·2 mmol s−1 m−2 MPa−1 in mature leaf. Comparison of and values showed that ≈ 90% of the resistance to water flow within the blades resided in the liquid extra‐vascular path. The same algorithm was then used to compute the xylem and extravascular water potential drop along the liquid water path in the plant under steady state conditions. Predicted and measured water potentials matched well. The hydraulic design of the mature leaf resulted in low and quite constant xylem water potential gradient (≈ 0·3 MPa m−1) throughout the plant. Much of the water potential drop within mature leaves occurred within a tenth of millimetre in the blade, between the xylem vessels and the site of water evaporation within the mesophyll. In elongating leaves, the low Kt* in the basal growth zone dramatically increased the local xylem water potential gradient (≈ 2·0 MPa m−1) there. In the leaf elongation zone the growth‐induced water potential difference was ≈ 0·2 MPa.

Factors controlling the decline of net primary production with stand age for balsam fir in Newfoundland assessed using an ecosystem simulation model

Net primary production (NPP) and growth efficiency (wood growth per leaf area) decline with stand age for forest ecosystems. Three hypotheses have been suggested: high respiration to photosynthesis ratios, hydraulic limitations on water transport in trees, and nutrient limitations of photosynthesis. Recent work indicates that hydraulic or nutrient limitations probably are the mechanism underlying the decline of growth efficiency in Douglas fir ( Pseudotsuga menziesii) and lodgepole pine ( Pinus contorta), both of which are native to western North America. However, balsam fir ( Abies balsamea) grows in the cool-humid boreal forest and has very high stem respiration rates, suggesting that the mechanism may be different. We predicted measured aboveground NPP of 24 out of 25 balsam fir stands reasonably well. These stands were selected for having large differences in leaf area index and wood mass. Simulations indicate that increased wood respiration is the major factor causing the decline of NPP and growth efficiency with stand age; a secondary factor for mature stands is nitrogen limitation. For simulations in which balsam fir was fertilized, final growth efficiency was less than that of unfertilized forest stands because of increased stem respiration. From values in the literature, we calculated the potential difference conductance to water flow in the xylem from the soil to the leaves for a mean tree in each stand. Twigs had the smallest conductance; however, because the twigs conductances are added in parallel, the bole was the smallest conductance in the series: bole, branch and twigs. The overall drop of water potential with very high transpiration flux densities was generally less than 2.5 MPa. Therefore, we conclude that, for balsam fir, the decline in NPP and growth efficiency with stand age may be caused by the high respiration to photosynthesis ratios. This conclusion differs from recent work, so there is not a single unifying mechanism of age-related decline for all forest ecosystems.

Developmental control of xylem hydraulic resistances and vulnerability to embolism in Fraxinus excelsior L.: impacts on water relations

The hydraulic properties and leaf gas exchanges of Fraxinus excelsior L. branches differing by their age and their vertical crown position, but in comparable ambient air conditions (vapour pressure deficit and global radiation) were compared. The variations in leaflet water potential ψleaflet, leaflet stomatal conductance and transpiration rate, E, were small between different branches of the same crown. Whole branch hydraulic resistances (r branch ), and partitioning between leaf (r leaf ) and xylem resistance (r xylem ) were assessed with a high pressure flowmeter. r leaf represented 90% and 10% of r branch for upper and lower crown branches, respectively. The changes resulted from increases in r xylem caused by the formation of short shoot internodes mostly located in secondary axes. However, leaf area-specific branch resistances (r branch =r branch x LA) were nearly constant throughout the crown. This was consistent with the vertical variations in ψ leaflet because r branch x E represents the water potential drop from the trunk to the leaves. Because r xylem was higher, lower ψxylem values were predicted in lower crown rachises. However, rachises from lower crown branches were less vulnerable to embolism than in upper branches ( ψxylem at onset of embolism, ψ cav, were -3 and -2MPa, respectively). It was concluded that r xylem increased with branch age, but r* branch remained constant because LA decreased. As a consequence, E was maximized and ψxylem remained above ψcav, This suggested that, in Fraxinus, leaf gas exchanges and leaf areas were coupled with xylem hydraulic capacities probably through a control of bud activity.