Short-term effects of potassium fertilization on the hydraulic conductance of Laurus nobilis L

This study reports experimental evidence on the effect of short-term potassium fertilization on potassium uptake, tissue concentration and hydraulic conductance of pot-grown laurel plants. Potassium uptake and loading into the xylem of laurel seedlings increased within 24 h after fertilization. Potassium was not accumulated in roots and leaves, but the [K(+)] of xylem sap was 80% higher in fertilized plants (+K) than in potassium-starved plants (-K), as a likely result of recirculation between xylem and phloem. Increased xylem sap [K(+)] resulted in a 45% increase in transpiration rate, a 30% increase in plant hydraulic conductance (K(plant)) and a 120% increase in leaf-specific conductivity of the shoot (k(shoot)). We suggest that this increase was due to ion-mediated up-regulation of xylem hydraulics, possibly caused by the interaction of potassium ions with the pectic matrix of intervessel pits. The enhancement of hydraulic conductance following short-term potassium fertilization is a phenomenon that can be of advantage to plants for maintaining cell turgor, stomatal aperture and gas exchange rates under moderate drought stress. Our data provide additional support for the important role of potassium nutrition in agriculture and forestry.     

A potential role for xylem-phloem interactions in the hydraulic architecture of trees: effects of phloem girdling on xylem hydraulic conductance

We investigated phloem-xylem interactions in Acer rubrum L. and Acer saccharum Marsh. Our experimental method allowed us to determine xylem conductance of an intact branch by measuring the flow rate of water supplied at two delivery pressures to the cut end of a small side branch. We found that removal of bark tissue (phloem girdling) upstream of the point at which deionized water was delivered to the branch resulted in a decrease (24% for A. rubrum and 15% for A. saccharum) in branch xylem hydraulic conductance. Declines in hydraulic conductance with girdling were accompanied by a decrease in the osmotic concentration of xylem sap. The decrease in xylem sap concentration following phloem girdling suggests that ion redistribution from the phloem was responsible for the observed decline in hydraulic conductance. When the same measurements were made on branches perfused with KCl solution (approximately 140 mOsm kg(-1)), phloem girdling had no effect on xylem hydraulic conductance. These results suggest a functional link between phloem and xylem hydraulic systems that is mediated by changes in the ionic content of the cell sap.     

Incorporation of transfer resistance between tracheary elements into hydraulic resistance models for tapered conduits

The model of West, Brown and Enquist (1999) shows that hydraulic resistance in trees can be independent of path length, provided that vascular conduits widen sufficiently from tree top to base. We demonstrate that this result does not depend theoretically on branching architecture or cross-sectional conductive area of the stem. Previous studies have shown that pit membrane resistance, encountered when water moves between either tracheids or vessels, accounts for up to 60% of the total resistance in stem segments. When pit membrane resistance, which is neglected by most whole-tree hydraulic models, was incorporated in hydraulic models in three different ways, the near invariance of hydraulic resistance was preserved. If relative pit resistance was independent of tracheid size or if tracheid dimensions were scaled to minimize wood resistivity, the minimum conduit taper required for path length independence equaled that in the original model of West et al. (1999). Under the most realistic model, in which relative pit resistance increased with tracheid radius, this value was doubled. Such taper is not possible within the typical size range of tracheids over the entire length of moderately tall trees, but it might be possible for vessel-bearing trees. Preliminary results indicated that although tracheid radius in the outer growth ring initially increased basipetally from the top of an 18-m tall Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), it stabilized at mid-trunk. Also, conduit taper was not constant in this species, violating a key assumption of the model of West et al. (1999), on which the invariance of hydraulic resistance depends.     

Fertilization effects on mean stomatal conductance are mediated through changes in the hydraulic attributes of mature Norway spruce trees

Stomatal conductance was quantified with sap flux sensors and whole-tree chambers in mature Norway spruce (Picea abies (L.) Karst.) trees after 3 years of exposure to elevated CO(2) concentration ([CO(2)]) in a 13-year nutrient optimization experiment. The long-term nutrient optimization treatment increased tree height by 3.7 m (67%) and basal diameter by 8 cm (68%); the short-term elevated [CO(2)] exposure had no effect on tree size or allometry. Nighttime transpiration was estimated as approximately 7% of daily transpiration in unchambered trees; accounting for the effect of nighttime flux on the processing of sap flux signals increased estimated daily water uptake by approximately 30%. Crown averaged stomatal conductance (g(s)) was described by a Jarvis-type model. The addition of a stomatal response time constant (tau) and total capacitance of stored water (C(tot)) improved the fit of the model. Model estimates for C(tot) scaled with sapwood volume of the bole in fertilized trees. Hydraulic support-defined as a lumped variable of leaf-specific hydraulic conductivity and water potential gradient (K(l)DeltaPsi) -was estimated from height, sapwood-to-leaf area ratio (A(s):A(l)) and changes in tracheid dimensions. Hydraulic support explained 55% of the variation in g(s) at reference conditions for trees across nutrient and [CO(2)] treatments. Removal of approximately 50% of A(l) from three trees yielded results suggesting that stomatal compensation (i.e., an increase in g(s)) after pruning scales inversely with K(l)DeltaPsi, indicating that the higher the potential hydraulic support after pruning, the less complete the stomatal compensation for the increase in A(s):A(l).     

Co-optimal distribution of leaf nitrogen and hydraulic conductance in plant canopies

Leaf properties vary significantly within plant canopies, due to the strong gradient in light availability through the canopy, and the need for plants to use resources efficiently. At high light, photosynthesis is maximized when leaves have a high nitrogen content and water supply, whereas at low light leaves have a lower requirement for both nitrogen and water. Studies of the distribution of leaf nitrogen (N) within canopies have shown that, if water supply is ignored, the optimal distribution is that where N is proportional to light, but that the gradient of N in real canopies is shallower than the optimal distribution. We extend this work by considering the optimal co-allocation of nitrogen and water supply within plant canopies. We developed a simple 'toy' two-leaf canopy model and optimized the distribution of N and hydraulic conductance (K) between the two leaves. We asked whether hydraulic constraints to water supply can explain shallow N gradients in canopies. We found that the optimal N distribution within plant canopies is proportional to the light distribution only if hydraulic conductance, K, is also optimally distributed. The optimal distribution of K is that where K and N are both proportional to incident light, such that optimal K is highest to the upper canopy. If the plant is constrained in its ability to construct higher K to sun-exposed leaves, the optimal N distribution does not follow the gradient in light within canopies, but instead follows a shallower gradient. We therefore hypothesize that measured deviations from the predicted optimal distribution of N could be explained by constraints on the distribution of K within canopies. Further empirical research is required on the extent to which plants can construct optimal K distributions, and whether shallow within-canopy N distributions can be explained by sub-optimal K distributions.     

Evidence that hydraulic conductance limits photosynthesis in old Pinus ponderosa trees

We tested the hypotheses that hydraulic conductance is lower in old (about 250 years old and 30 m tall) compared to young (about 40 years old and 10 m tall) Pinus ponderosa Dougl. ex Laws. trees and that lower hydraulic conductance of old trees limits their photosynthesis. Hydraulic conductance at the end of summer 1995, calculated from leaf water potential and leaf gas exchange measurements on one-year-old needles, was 44% lower in old trees compared to young trees growing in a mixed age-class stand on the east slope of the Oregon Cascades. Whole-tree sapflow per unit leaf area averaged 53% lower in old trees compared to young trees and mean hydraulic conductance calculated from sapflow and water potential data was 63% lower in old trees than in young trees. For the entire summer, stomatal conductance (g(s)) and assimilation (A) declined more steeply with air saturation deficit (D) in old trees than in young trees. For both old and young trees, mean g(s) and A were approximately 32 and 21% lower, respectively, at typical midday D values (2.5-3.0 kPa). We hypothesized that if hydraulic conductance limits g(s) and A, then increasing or decreasing the leaf specific conductance of a branch will result in proportional changes in the responses of g(s) and A with D. Removal of 50% of the foliage from a set of experimental branches on old trees caused g(s) and A to decline less steeply with D in early summer, but values were not significantly different from control values in late summer. Cutting transverse notches in branches on young trees had no effect on the responses of g(s) and A with D. Leaf nitrogen content and photosynthetic capacity were similar suggesting that differences in g(s) and A between old and young trees were not caused by differences in photosynthetic capacity.     

Canopy and hydraulic conductance in young, mature and old Douglas-fir trees

We tested for reductions in water transport with increasing tree size, a key component in determining whether gas exchange and growth are hydraulically limited in tall trees. During the summers of 1998 and 1999, we measured water transport with Granier-type, constant-heat sap flow probes, vapor pressure deficit, and leaf and soil water potentials in overstory Pseudotsuga menziesii (Mirb.) Franco trees in three stands differing in size and age (15, 32 and 60 m in height and about 20, 40 and 450 years in age, respectively) in a P. menziesii-dominated forest in the Pacific Northwest, USA. A total of 24 trees were equipped with sap flow sensors--six 60-m trees, nine 32-m trees and nine 15-m trees. Based on the sap flow measurements and leaf area information estimated from leaf area-sapwood area relationships, we estimated crown-averaged stomatal conductance (GS) and leaf-specific hydraulic conductance (KL). We tested the hypothesis that GS and KL vary inversely with tree height (15 > 32 > 60 m). Analysis of variance of GS ranked as 15 = 60 > 32 m during the early summer and 15 > 60 > 32 m during late season drought. Over the growing season, mean daily GS (+/- SE) was 29.2 +/- 4.4, 24.0 +/- 6.8 and 17.7 +/- 7.2 mmol m-2 s-1 for the 15-, 60- and 32-m trees, respectively. The value of K(L) differed among tree heights only during late season drought and ranked 15 > 32 = 60 m. A hydraulic mass balance suggests that greater sapwood conductivity in 60-m trees compared with 32- and 15-m trees is a likely cause for the departure of the above rankings from those predicted by height and leaf-to-sapwood area ratio.     

Experimental evidence supporting the concept of light-mediated modulation of stem hydraulic conductance

It is a well-described phenomenon that plant leaves respond to changes in light intensity and duration by adjusting leaf hydraulic efficiency, and there is current consensus that up- or down-regulation of water channels (aquaporins) in the plasma membrane of the bundle sheath and mesophyll cells play a central role in the underlying mechanisms. Recently, experimental evidence has been provided also for light-mediated changes of stem hydraulic conductance (K(stem)) in field-grown laurel plants. This effect was attributed to differences in potassium ion concentration of xylem sap as a function of light conditions. In the present article, we report evidence obtained in silver birch (Betula pendula Roth), supporting the concept of light-mediated modulation of K(stem). Both canopy position (long-term effect) and current photosynthetic photon flux density (PPFD; short-term effect) had a significant impact (P < 0.001) on K(stem) measured in shoots taken from the lower (shade shoots) and upper (sun shoots) third of the crowns of ～25-year-old trees growing in a natural forest stand. The shade shoots responded more sensitively to light manipulation: K(stem) increased by 51% in shade shoots and 26% in sun shoots when PPFD increased from 70 to 330 μmol m?2 s?1. In 4-year-old trees growing in a dense experimental plantation, K(stem), specific conductivity of branch-wood (k(bw)) and potassium ion concentration ([K(+)]) in xylem sap varied in accordance with canopy position (P < 0.001). Both K(stem) and k(bw) increased considerably with light availability, increasing within the tree crowns from bottom to top; there was a strong relationship between mean values of K(stem) and [K(+)] in hydraulically sampled branches.     

Xylem cavitation and loss of hydraulic conductance in western hemlock following planting

Following planting, western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings experience water stress and declining xylem pressure potential (Psi(x)). Low Psi(x) can result in xylem cavitation and embolism formation, causing a decline in hydraulic conductance. This study focused on the relationship between Psi(x), xylem cavitation and transpiration (E) of newly planted seedlings. Leaf specific hydraulic conductance (k(AB)) declined from 0.56 to 0.09 mmol m(-2) s(-1) MPa(-1) over a 9-day period. Stomatal conductance (g(s)) declined from 143.5 to 39.15 mmol m(-2) s(-1) over the same period without an associated change in environmental conditions. A vulnerability profile indicated a 30% loss in hydraulic conductivity when seedlings experienced a Psi(x) between -2.5 and -3.0 MPa. A Psi(x) of -4.0 MPa led to a complete loss of conductivity. We conclude that following planting, western hemlock seedlings often experience Psi(x) values that are low enough to cause xylem cavitation and a decline in k(AB).     

Stomatal control and hydraulic conductance, with special reference to tall trees

A better understanding of the mechanistic basis of stomatal control is necessary to understand why modes of stomatal response differ among individual trees, and to improve the theoretical foundation for predictive models and manipulative experiments. Current understanding of the mechanistic basis of stomatal control is reviewed here and discussed in relation to the plant hydraulic system. Analysis focused on: (1) the relative role of hydraulic conductance in the vicinity of the stomatal apparatus versus whole-plant hydraulic conductance; (2) the influence of guard cell inflation characteristics and the mechanical interaction between guard cells and epidermal cells; and (3) the system requirements for moderate versus dramatic reductions in stomatal conductance with increasing evaporation potential. Special consideration was given to the potential effect of changes in hydraulic properties as trees grow taller. Stomatal control of leaf gas exchange is coupled to the entire plant hydraulic system and the basis of this coupling is the interdependence of guard cell water potential and transpiration rate. This hydraulic feedback loop is always present, but its dynamic properties may be altered by growth or cavitation-induced changes in hydraulic conductance, and may vary with genetically related differences in hydraulic conductances. Mechanistic models should include this feedback loop. Plants vary in their ability to control transpiration rate sufficiently to maintain constant leaf water potential. Limited control may be achieved through the hydraulic feedback loop alone, but for tighter control, an additional element linking transpiration rate to guard cell osmotic pressure may be needed.     

Diurnal and seasonal variations in leaf hydraulic conductance in evergreen and deciduous trees

We studied changes in the hydraulic conductance of leaves (K(leaf)) between dawn and dusk during the growth period (July) and at midday at the beginning of autumn in four tree species. The main objectives of the study were to check the extent of diurnal and seasonal changes in K(leaf) and the relationships between K(leaf), irradiance and leaf gas exchange. Two evergreen (Aleurites moluccana and Persea americana) and two deciduous trees (Platanus orientalis and Quercus rubra) were studied. Leaf hydraulic conductance was measured every 2 h between 0700 and 1900 h in July and compared with values measured between 0900 and 1300 h in October. Other variables measured were photosynthetically active radiation (PAR), leaf conductance to water vapor (gL) and water potential (psiL). In July, K(leaf) varied by up to 75% in Pe. americana on a diurnal basis and by at least 44% in Q. rubra. The diurnal time course of K(leaf) showed a distinct increase between dawn and late morning (1100 h) and a subsequent decrease in the evening in A. moluccana and Pl. orientalis, whereas in the other two species, K(leaf) was highest just after dawn and lowest in the evening. In October, K(leaf) of all the species studied was lower than in July, with differences of 20 to 28% for A. moluccana and Pl. orientalis and of 66 to over 70% in Pe. americana and Q. rubra, respectively. Significant correlations were found between PAR and K(leaf) (in all species) as well as between gL and K(leaf) (in three out of four species). Leaf habit (evergreen or deciduous) did not influence absolute values of K(leaf) or its diurnal variation.     

Light response of hydraulic conductance in bur oak (Quercus macrocarpa) leaves

A four- to seven-fold enhancement of leaf hydraulic conductance by light has been reported in three temperate tree species. The enhancement occurs in the liquid-flow pathway between the petiole and the site of water evaporation. The enhancement occurs within 1 h, and dissipates in darkness over a period of 1 to 10 h depending on species. Here we report light-induced enhancement of leaf hydraulic conductance in a fourth species, bur oak (Quercus macrocarpa Michx.), the dependence of the effect on light flux and color, its absence in leaves of seedlings, and the impact on the response of leaf vein severance and several metabolic inhibitors. The light response of leaf hydraulic conductance approached saturation at a photosynthetic photon flux of 150 mumol m(-2) s(-1). Hydraulic enhancement was greater in response to blue and green light than to visible radiation of longer wavelengths, although at the same irradiance, the response to white light was greater than to light of any single color. Atrazine (a photosystem II inhibitor), fusicoccin (which stimulates plasma membrane-bound H(+)-ATPase) and HgCl(2) (an aquaporin blocker) reduced the light response of leaf lamina hydraulic conductance. When 2-mercaptoethanol was added following mercury treatment, the light response was totally suppressed. Our results are consistent with the notion that the effect of light on leaf lamina hydraulic conductance is controlled by factors acting outside the leaf veins, possibly through light-induced changes in membrane permeability of either mesophyll or bundle sheath cells, or both.     

The response of Pinus sylvestris to drought: stomatal control of transpiration and hydraulic conductance

We investigated the impact of drought on the physiology of 41-year-old Scots pine (Pinus sylvestris L.) in central Scotland. Measurements were made of the seasonal course of transpiration, canopy stomatal conductance, needle water potential, xylem water content, soil-to-needle hydraulic resistance, and growth. Comparison was made between drought-treated plots and those receiving average precipitation. In response to drought, transpiration rate declined once volumetric water content (VWC) over the top 20 cm of soil reached a threshold value of 12%. Thereafter, transpiration was a near linear function of soil water content. As the soil water deficit developed, the hydraulic resistance between soil and needles increased by a factor of three as predawn needle water potential declined from -0.54 to -0.71 MPa. A small but significant increase in xylem embolism was detected in 1-year-old shoots. Stomatal control of transpiration prevented needle water potential from declining below -1.5 MPa. Basal area, and shoot and needle growth were significantly reduced in the drought treatment. In the year following the drought, canopy stomatal conductance and soil-to-needle hydraulic resistance recovered. Current-year needle extension recovered, but a significant reduction in basal area increment was evident one year after the drought. The results suggest that, in response to soil water deficit, mature Scots pine closes its stomata sufficiently to prevent the development of substantial xylem embolism. Reduced growth in the year after a severe soil water deficit is most likely to be the result of reduced assimilation in the year of the drought, rather than to any residual embolism carried over from one year to the next.     

Stomatal sensitivity to vapor pressure deficit and its relationship to hydraulic conductance in Pinus palustris

We studied the response of stomatal conductance at leaf (gS) and canopy (GS) scales to increasing vapor pressure deficit (D) in mature Pinus palustris Mill. (longleaf pine) growing in a sandhill habitat in the coastal plain of the southeastern USA. Specifically, we determined if variation in the stomatal response to D was related to variation in hydraulic conductance along the soil-to-leaf pathway (KL) over the course of a growing season. Reductions in KL were associated with a severe growing season drought that significantly reduced soil water content (theta) in the upper 90-cm soil profile. Although KL recovered partially following the drought, it never reached pre-drought values. Stomatal sensitivity to D was well correlated with maximum gS at low D at both leaf and canopy scales, and KL appeared to influence this response by controlling maximum gS. Our results are consistent with the hypothesis that stomatal response to D occurs to regulate minimum leaf water potential, and that the sensitivity of this response is related to changes in whole-plant hydraulics.     

Leaf hydraulic conductance in relation to anatomical and functional traits during Populus tremula leaf ontogeny

Leaf hydraulic conductance (K(leaf)) and several characteristics of hydraulic architecture and physiology were measured during the first 10 weeks of leaf ontogeny in Populus tremula L. saplings growing under control, mild water deficit or elevated temperature conditions. During the initial 3 weeks of leaf ontogeny, most measured characteristics rapidly increased. Thereafter, a gradual decrease in K(leaf) was correlated with a decrease in leaf osmotic potential under all conditions, and with increases in leaf dry mass per area and bulk modulus of elasticity under mild water deficit and control conditions. From about Week 3 onward, K(leaf) was 33% lower in trees subjected to mild water deficit and 33% higher in trees held at an elevated temperature relative to control trees. Mild water deficit and elevated temperature treatment had significant and opposite effects on most of the other characteristics measured. The ontogenetic maximum in K(leaf) was correlated positively with the width of xylem conduits in the midrib, but negatively with the overall width of the midrib xylem, number of lateral ribs, leaf dry mass per area and bulk modulus of elasticity. The ontogenetic maximum in K(leaf) was also correlated positively with the proportion of intercellular spaces and leaf osmotic potential, but negatively with leaf thickness, volume of mesophyll cells and epidermis and number of cells per total mesophyll cell volume, the closest relationships being between leaf osmotic potential and number of cells per total mesophyll cell volume. It was concluded that differences in protoplast traits are more important than differences in xylem or parenchymal cell wall traits in determining the variability in K(leaf) among leaves growing under different environmental conditions.     

Spatiotemporal variation of crown-scale stomatal conductance in an arid Vitis vinifera L. cv. Merlot vineyard: direct effects of hydraulic properties and indirect effects of canopy leaf area

Vineyards were planted in the arid region of northwest China to meet the local economic strategy while reducing agricultural water use. Sap flow, environmental variables, a plant characteristic (sapwood-to-leaf area ratio, A(s)/A(l)) and a canopy characteristic (leaf area index, L) were measured in a vineyard in the region during the growing season of 2009, and hourly canopy stomatal conductance (G(si)) was estimated for individual vines to quantify the relationships between G(si) and these variables. After accounting for the effects of vapor pressure deficit (D) and solar radiation (R(s)) on G(si), much of the remaining variation of reference G(si) (G(siR)) was driven by that of leaf-specific hydraulic conductivity, which in turn was driven by that of A(s)/A(l). After accounting for that effect on G(siR), appreciable temporal variation remained in the decline rate of G(siR) with decreasing vineyard-averaged relative extractable soil water (θ(E)). This variation was related to the differential decline ofθ(E) near each monitored vine, decreasing faster between irrigation events near vines where L was greater, thus adding to the spatiotemporal variation of G(siR) observed in the vineyard. We also found that the vines showed isohydric-like behavior whenθ(E) was low, but switched to anisohydric-like behavior with increasingθ(E). Modeledθ(E) and associated G(s) of a canopy with even L (1.9 m(2) m(-2)) were greater than that of the same average L but split between the lowest and highest L observed along sections of rows in the vineyard (1.2 and 2.6 m(2) m(-2)) by 6 and 12%, respectively. Our results suggest that managing sectional L near the average, rather than allowing a wide variation, can reduce soil water depletion, maintaining G(s) higher, thus potentially enhancing yield.     

Stomatal behavior of four woody species in relation to leaf-specific hydraulic conductance and threshold water potential

Midday stomatal closure is mediated by the availability of water in the soil, leaf and atmosphere, but the response to these environmental and internal variables is highly species specific. We tested the hypothesis that species differences in stomatal response to humidity and soil water availability can be explained by two parameters: leaf-specific hydraulic conductance (K(L)) and a threshold leaf water potential (Psi(threshold)). We used a combination of original and published data to estimate characteristic values of K(L) and Psi(threshold) for four common tree species that have distinctly different stomatal behaviors: black cottonwood (Populus trichocarpa Torr. & Gray.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), red alder (Alnus rubra Bong.) and western hemlock (Tsuga heterophylla (Raf.) Sarg.). We used the values to parameterize a simple, nonelastic model that predicts stomatal conductance by linking hydraulic flux to transpirational flux and maintaining Psi(leaf) above Psi(threshold). The model successfully predicted fundamental features of stomatal behavior that have been reported in the literature for these species. We conclude that much of the variation among the species in stomatal response to soil and atmospheric water deficits can be explained by K(L) and Psi(threshold). The relationship between Psi(threshold) and xylem vulnerability to cavitation differed among these species.     

Pit membrane chemistry influences the magnitude of ion-mediated enhancement of xylem hydraulic conductance in four Lauraceae species

The ion-mediated enhancement of xylem hydraulic conductivity in angiosperms is thought to be controlled by the pectin chemistry of intervessel pit membranes. However, there is little or no direct evidence on the ultrastructure and chemical nature of pit membranes in species that show an 'ionic effect'. The potential link between the magnitude of the ionic effect and pectin composition in intervessel pit membranes of four Lauraceae species (Laurus nobilis, Lindera megaphylla, Litsea sericea and Umbellularia californica) that show rather similar vessel and pit dimensions was studied using transmission electron microscopy (TEM). The TEM observations confirmed the presence of a pectic matrix associated with intervessel pit membranes, indicating that the relative abundance of acidic versus methylesterified pectins was closely related to the ionic effect. The two species examined with a high ionic effect ~20%, i.e. Laurus nobilis and Umbellularia californica) showed relatively high levels of acidic pectins, whereas methylesterified pectins were abundant in Lindera megaphylla and Litsea sericea, which showed a low ionic effect (~10%). Variation in the ionic effect is strongly associated with the chemical nature of pit membrane pectins in the species studied. Our findings support the current interpretation of the ionic effect due to dynamic swelling and shrinking behaviour of pit membrane pectins.     

Agroforestry and grass buffer effects on water quality in grazed pastures

Conservation practices including agroforestry and grass buffers are believed to reduce nonpoint source pollution (NPSP) from pastured watersheds. Agroforestry, a land management practice that intersperses agricultural crops with trees, has recently received increased attention in the temperate zone due to its environmental and economic benefits. However, studies are limited that have examined buffer effects on the quality of water from grazed pastures. Six treatment areas, two with agroforestry buffers, two with grass buffers, and two control treatments were used to test the hypothesis that agroforestry and grass buffers can be used to effectively reduce NPSP from pastured watersheds. Vegetation in grass buffer and pasture areas includes red clover (Trifolium pretense L.) and lespedeza (Kummerowia stipulacea Maxim.) planted into fescue (Festuca arundinacea Schreb.). Eastern cottonwood trees (Populus deltoides Bortr. ex Marsh.) were planted into fescue in agroforestry buffers. Soils at the site are mostly Menfro silt loam (fine-silty, mixed, superactive, mesic Typic Hapludalfs). Treatments were instrumented with two-foot H flumes, water samplers, and flow measuring devices in 2001. Composite water samples were analyzed for sediment and total nitrogen after each runoff event to compare treatment differences. Treatments with agroforestry and grass buffers had significantly lower runoff volumes as compared to the control. The loss of sediment and total nitrogen were smaller for the buffered treatments. The results of this study suggest that establishment of agroforestry and grass buffers help reduce NPSP pollution from pastured watersheds. It is anticipated as trees grow and roots occupy more soil volume, the reduction in N in runoff will increase on the agroforestry watershed.     

Variations of root hydraulic conductance of Fraxinus mandshurica seedlings in different concentrations of NH4NO3 solution

Absorbing water from soil by roots in vascular plants is an important physiological function and plays an essential role on their water balance. The root hydraulic conductance (L _P) determined by radical water transport inside the root is a major influence on the shoot water status, plant growth, and development. However, a few studies have focused on the effect of different substances on L _P of roots, and the role of radical water transport was poorly understood. Based on the pressure-flux approach, this study used the roots of Fraxinus mandshurica seedlings with different treatments, i.e., distilled water, NH₄NO₃ solution, and HgCl₂ to determine the effect of various substances on L _P of roots. The objectives are: 1) to evaluate the difference in L _P occurred between distilled water and NH₄NO₃ solution with various concentrations; and 2) to examine the changes of L _P under distilled water and NH₄NO₃ solution with various concentrations after HgCl₂ treatment. The results showed that L _P of roots were 18.85×10⁻⁸ m/(s·MPa) in distilled water, 31.25–34.15×10⁻⁸ m/(s·MPa) in four NH₄NO₃ solutions (2, 4, 8 and 16 mmol/L), 14.69×10⁻⁸ m/(s·MPa) in distilled water after HgCl₂-treated, and 9.63–13.57×10⁻⁸ m/(s·MPa) in four NH₄NO₃ solutions after HgCl₂-treated, respectively. Aquaporins play an important role in regulating water uptake and transport in roots. NH₄ ⁺ and NO₃ ⁻ could stimulate activity of aquaporins, and L _P of roots in NH₄NO₃ solution was distinctly 77% higher than in distilled water. Nevertheless, Hg²⁺ can inhibit activity of aquaporins, and and L _P of roots decreased 22% in distilled water and 68% in NH₄NO₃ solution after treatment by HgCl₂ respectively. These evidences suggested that both Hg²⁺-sensitive aquaporins and ion channels existing in the protoplasm and vacuole membranes could regulate root water uptake, transport, and integral plant water balance. __________ Translated from Acta Phytoecologica Sinica, 2005, 29(5): 706–712 [译自: 植物生态学报, 2005, 29(5): 706–712]