Xylem sap composition of beech (Fagus sylvatica L.) trees: seasonal changes in the axial distribution of sulfur compounds

During different phases of the annual growth cycle, xylem sap was collected from trunk segments of adult beech (Fagus sylvatica L.) trees by the water displacement technique. Irrespective of the height of the trunk, both sulfate and reduced sulfur compounds were detected in the xylem sap throughout the year. Sulfate was the predominant sulfur compound in all samples analyzed. Its concentration in the xylem sap varied between 10 and 350 micro mol l(-1), with highest concentrations in April, shortly before bud break. In contrast to other tree species, cysteine and not glutathione was the predominant thiol transported in the xylem sap of beech trees. The cysteine concentration ranged between 0.1 and 1 micro mol l(-1). As observed for sulfate, maximum cysteine concentrations were found in April. Apparently, both sulfate and cysteine transport contribute to the sulfur supply of the developing leaves. Seasonal changes in the axial distribution of cysteine and sulfate differed, indicating differences in the source-sink relations of these sulfur compounds. High, but uniform, xylem sap sulfate concentrations in April may originate from balanced sulfate uptake by the roots, whereas high cysteine concentrations in April, increasing with increasing height of the trunk, may originate in part from protein breakdown in the trunk. Reversal of the axial distribution of xylem sap cysteine in late summer-early fall to higher concentrations in the lower part of the trunk than in the upper part of the trunk suggests that the upper part of the trunk becomes a sink for cysteine as a result of the synthesis of storage proteins at this time of the year.     

Spatial and seasonal variation in amino compounds in the xylem sap of a mistletoe (Viscum album) and its hosts (Populus spp. and Abies alba)

In a field study, the composition and concentrations of amino compounds in the xylem sap of the mistletoe, Viscum album L., and in the xylem sap of two host species, an evergreen conifer (Abies alba Mill.) and a deciduous broad-leaved tree (Populus x euramericana), were analyzed. The xylem sap of both hosts and mistletoe contained large, but similar amounts of total organic nitrogen in low molecular weight amino compounds (TONLW). Nevertheless, individual amino compounds accumulated in the xylem sap of mistletoe relative to the host xylem sap, indicating selective uptake. In the xylem sap of Populus, major amino compounds (asparagine (Asn) and glutamine (Gln)) and the bulk parameters, TONLW and proteinogenic amino acids, showed significant seasonal variation. In Abies and in mistletoe on either host, variation of amino compounds in xylem sap was largely explained by inter-annual differences, not by seasonal variation. In both hosts, TONLW in the xylem sap was dominated by Gln. There was a steady decrease in relative abundance of Gln from the host xylem sap to the mistletoe xylem sap and to the stems and leaves of mistletoe. Simultaneously, the abundance of arginine (Arg) increased. Arginine was the predominant amino compound in the stems and leaves of mistletoe, occurring at concentrations previously observed only in leaves of trees exposed to excess nitrogen. We conclude that Gln (2 mol N mol(-1)) delivered by the host xylem sap is converted, in mistletoe, to Arg (4 mol N mol(-1)) and that the organic carbon liberated from Gln contributes significantly to the parasite's heterotrophic carbon gain. Statistical analyses of the data support this conclusion. Accumulation of Arg in mistletoe is an indication of excess N supply as a result of the uptake of amino compounds from the host xylem sap and a lack of phloem uploading.     

The effect of aqueous transport of CO(2) in xylem sap on gas exchange in woody plants

The influence of CO(2) transported in the transpiration stream on measurements of leaf photosynthesis and stem respiration was investigated. Measurements were made on trees in a temperate forest in Scotland and in a tropical rain forest in Cameroon, and on shrubs in the Sahelian zone in Niger. A chamber was designed to measure the CO(2) partial pressure in the gas phase within the woody stems of trees. High CO(2) partial pressures were found, ranging from 3000 to 9200 Pa. Henry's Law was used to estimate the CO(2) concentration of xylem sap, assuming that it was in equilibrium with the measured gas phase partial pressures. The transport of CO(2) in the xylem sap was calculated by multiplying sap CO(2) concentration by transpiration rate. The magnitude of aqueous transport in the studied species ranged from 0.03 to 0.35 &mgr;mol CO(2) m(-2) s(-1), representing 0.5 to 7.1% of typical leaf photosynthetic rates. These values strongly depend on sap pH. To examine the influence of aqueous transport of CO(2) on stem gas exchange, we made simultaneous measurements of stem CO(2) efflux and sap flow on the same stem. After removing the effect of temperature, stem CO(2) efflux was positively related to sap flow. The apparent effect on measurements of stem respiration was up to 0.7 &mgr;mol m(-2) s(-1), representing ~12% of peak stem respiration rates.     

Characterization of nitrogenous solutes in tissues and xylem sap of Leucaena leucocephala

Amino acid profiles of leaf, stem, and root tissues from nodulated and nonnodulated Leucaena leucocephala (Lam.) de Wit plants were determined by gas chromatography-mass spectrometry. High concentrations of mimosine and several other potentially toxic nonprotein amino acids, including pipecolic acid and two isomers of hydroxypipecolic acid, were identified in the tissues. Five metabolites remain unidentified. Of the foliar free amino acid nitrogen, 57-66% was associated with the potentially toxic amino acids. Major constituents in the leaf tissues of nonnodulated plants were mimosine and hydroxypipecolic acid (isomer 1). Mimosine was recovered in both the neutral plus basic and acidic amino acid fractions. Major differences between amino acid profiles of nodulated and nonnodulated roots were the low percentages of asparagine + aspartate (3.6% of the total pool compared to 33% in nodulated plants) and pipecolic acid in nonnodulated roots (1% of the total compared to 12.5% in nodulated plants). A novel plant betaine (dihydroxypipecolic acid betaine) was identified by fast-atom-bombardment mass spectrometry in leaf tissues, albeit at relatively low concentrations (< 1 micro mol per gram fresh weight). Analyses of the xylem sap collected from nodulated plants confirmed that Leucaena is an asparagine transporter, as suggested by the high concentrations of asparagine and the low concentrations of ureides in its root nodules. Amino acid profiles of xylem sap from nonnodulated plants showed extremely low concentrations of asparagine + aspartate (0.12 micro mol ml(-1)), whereas asparagine + aspartate was the major constituent (4.38 micro mol ml(-1)) in the xylem sap of nodulated plants. Two nonprotein amino acids, pipecolic acid and hydroxypipecolic acid, were major constituents of the xylem sap of nodulated and nonnodulated plants, respectively. Three unidentified compounds detected in xylem sap samples from both nodulated and nonnodulated plants did not correspond with any of the peaks characterized from tissue samples.     

Sulfate concentrations in Norway spruce needles in relation to atmospheric SO(2): a comparison of trees from various forests in Germany with trees fumigated with SO(2) in growth chambers

Concentrations of inorganic sulfur, organic sulfur and water-soluble cations and anions were determined in needles of young Norway spruce trees (Picea abies L. (Karst.)) that had been fumigated in growth chambers for weeks or months with different concentrations of SO(2), SO(2) plus ozone, or SO(2) plus NO(2). Measurements were also made on needles from older trees growing in forests in various regions of Germany with different mean annual atmospheric SO(2) emissions. In the fumigated young trees, sulfate accumulation in the needles was a linear function of atmospheric SO(2) concentration. Little or no sulfur was incorporated into the organic sulfur fraction. The mean accumulation rate of sulfate in needles of fumigated trees was about 0.4 nmol g(dw) (-1) (nl l(-1))(-1) h(-1), which is very similar to the estimated rate of uptake of atmospheric SO(2) calculated from mean stomatal conductances (15 mmol m(-2) s(-1)) and the external SO(2) concentration (the calculated rate of uptake was 0.37 nmol g(dw) (-1) (nl l(-1))(-1) h(-1)). Concentrations of organic acids and other inorganic ions did not change much in response to SO(2) fumigation. In needles collected from trees in south and southeast Germany, large differences in sulfate concentrations were observed that probably reflect SO(2) emissions in the different regions. The highest foliar sulfate concentrations, and the highest annual increase in sulfate concentration with needle age were observed in material collected from the heavily polluted Erzgebirge (up to 12 micro mol g(dw) (-1) year(-1)), followed by material from the Fichtelgebirge (up to 6 micro mol g(dw) (-1) year(-1)). If it is assumed that this annual increase is the result of uptake of SO(2) from the atmosphere, mean annual atmospheric SO(2) concentrations can be calculated. The calculated values were somewhat below the measured values in the Fichtelgebirge and in the heavily polluted Erzgebirge. Norway spruce trees can cope with high concentrations of atmospheric SO(2), provided that they can neutralize the sulfuric acid formed from SO(2). It appears that, in the field, the acid load is decreased by H(+) translocation to the roots and subsequent H(+)/K(+) exchange. However, this may be a very slow process, because it was not observed in the short-term fumigation experiments.     

Seasonal variation in xylem pressure of walnut trees: root and stem pressures

Measurements of air and soil temperatures and xylem pressure were made on 17-year-old orchard trees and on 5-year-old potted trees of walnut (Juglans regia L.). Cooling chambers were used to determine the relationships between temperature and sugar concentration ([glucose] + [fructose] + [sucrose], GFS) and seasonal changes in xylem pressure development. Pressure transducers were attached to twigs of intact plants, root stumps and excised shoots while the potted trees were subjected to various temperature regimes in autumn, winter and spring. Osmolarity and GFS of the xylem sap (apoplast) were measured before and after cooling or warming treatments. In autumn and spring, xylem pressures of up to 160 kPa were closely correlated with soil temperature but were not correlated with GFS in xylem sap. High root pressures were associated with uptake of mineral nutrients from soil, especially nitrate. In autumn and spring, xylem pressures were detected in root stumps as well as in intact plants, but not in excised stems. In contrast, in winter, 83% of the xylem sap osmolarity in both excised stems and intact plants could be accounted for by GFS, and both GFS and osmolarity were inversely proportional to temperature. Plants kept at 1.5 degrees C developed positive xylem pressures up to 35 kPa, xylem sap osmolarities up to 260 mosmol l(-1) and GFS concentrations up to 70 g l(-1). Autumn and spring xylem pressures, which appeared to be of root origin, were about 55% of the theoretical pressures predicted by osmolarity of the xylem sap. In contrast, winter pressures appeared to be of stem origin and were only 7% of the theoretical pressures, perhaps because of a lower stem water content during winter.     

Microelectrode technique for in situ measurement of carbon dioxide concentrations in xylem sap of trees

We developed a new microelectrode technique for measuring CO2 concentration ([CO2]) in xylem sap of trees. This technique enabled us to make rapid and continuous measurements of xylem sap [CO2] in situ. In this report, we discuss the methodology and establish the feasibility of the technique. We also describe calibration procedures, temperature sensitivity, field use and other characteristics of the microelectrodes. An example of data collected in the field is provided. Microelectrode calibration was accomplished at constant temperature in air of known [CO2]. When sampling temperature differed from calibration temperature, correction was necessary. We developed an equation to correct for temperatures between 15 and 35 degrees C when calibration was conducted at 25 degrees C. Equations based on Henry's Law were used to convert measured gas phase [CO2] (%) to concentration of all products of CO2 dissolved in sap (mmol l(-1)). We inserted microelectrodes into stems of three tree species to measure diurnal changes in [CO2] in the xylem sap. A diurnal pattern with depression during the day and elevation at night was observed. Mean daily [CO2] ranged from 1.6 to 10.3 mmol l(-1). Microelectrodes were suitable for making diurnal measurements for up to 7 days without recalibration. We also used the microelectrodes to measure [CO2] of soil in situ. Soil [CO2] ranged from 1 to 4% (gas phase), with little diurnal variation.     

Winter variation in xylem sap pH of walnut trees: involvement of plasma membrane H+-ATPase of vessel-associated cells

We studied seasonal variation in xylem sap pH of Juglans regia L. Our main objectives were to (1) test the effect of temperature on seasonal changes in xylem sap pH and (2) study the involvement of plasma membrane H+-ATPase of vessel-associated cells in the control of sap pH. For this purpose, orchard-grown trees were compared with trees grown in a heated (> or = 15 degrees C) greenhouse. During autumn, sap pH was not directly influenced by temperature. A seasonal change in H+-ATPase activity resulting from seasonal variation in the amount of protein was measured in orchard-grown trees, whereas no significant seasonal changes were recorded in greenhouse-grown trees. Our data suggest that H+-ATPase does not regulate xylem sap pH directly by donating protons to the xylem, but by facilitating secondary active H+/sugar transport, among other mechanisms.     

Stomatal and leaf growth responses to partial drying of root tips in willow

Root tips of intact willow (Salix dasyclados Wimm., Clone 81-090) plants were partially dried by exposure to ambient greenhouse air and then kept in water-vapor-saturated air for up to 3 days. The drying treatment increased abscisic acid (ABA) concentrations in both the root tips subjected to drying and in the xylem sap, while it reduced leaf stomatal conductance and leaf extension rate. Despite the decrease in stomatal conductance, leaf water potentials were unaffected by the root drying treatment, indicating that the treatment reduced hydraulic conductivity between roots and foliage. After roots subjected to drying were returned to a nutrient solution or excised, ABA concentrations in the remaining roots and in the xylem sap, stomatal conductance of mature leaves and extension rate of unfolding leaves all returned to values observed in control plants. The 4-fold increase in xylem sap ABA concentration following the root drying treatment was not solely the result of reduced sap flow, and thus may be considered a potential cause, not merely a consequence, of the observed reduction in stomatal conductance.     

Radial variation in sap velocity as a function of stem diameter and sapwood thickness in yellow-poplar trees

Canopy transpiration and forest water use are frequently estimated as the product of sap velocity and cross-sectional sapwood area. Few studies, however, have considered whether radial variation in sap velocity and the proportion of sapwood active in water transport are significant sources of uncertainty in the extrapolation process. Therefore, radial profiles of sap velocity were examined as a function of stem diameter and sapwood thickness for yellow-poplar (Liriodendron tulipifera L.) trees growing on two adjacent watersheds in eastern Tennessee. The compensation heat pulse velocity technique was used to quantify sap velocity at four equal-area depths in 20 trees that ranged in stem diameter from 15 to 69 cm, and in sapwood thickness from 2.1 to 14.8 cm. Sap velocity was highly dependent on the depth of probe insertion into the sapwood. Rates of sap velocity were greatest for probes located in the two outer sapwood annuli (P1 and P2) and lowest for probes in closest proximity to the heartwood (P3 and P4). Relative sap velocities averaged 0.98 at P1, 0.66 at P2, 0.41 at P3 and 0.35 at P4. Tree-specific sap velocities measured at each of the four probe positions, divided by the maximum sap velocity measured (usually at P1 or P2), indicated that the fraction of sapwood functional in water transport (f(S)) varied between 0.49 and 0.96. There was no relationship between f(S) and sapwood thickness, or between f(S) and stem diameter. The fraction of functional sapwood averaged 0.66 +/- 0.13 for trees on which radial profiles were determined. No significant depth-related differences were observed for sapwood density, which averaged 469 kg m(-3) across all four probe positions. There was, however, a significant decline in sapwood water content between the two outer probe positions (1.04 versus 0.89 kg kg(-1)). This difference was not sufficient to account for the observed radial variation in sap velocity. A Monte-Carlo analysis indicated that the standard error in estimated mean f(S) declined rapidly with increasing sample size. At n = 10, the coefficient of variation in mean f(S) was 7% and at n = 15 it was slightly less than 5%. These observations indicate that radial variation in sap velocity is an important, albeit often overlooked, source of uncertainty in the scaling process. Failure to recognize that not all sapwood is functional in water transport will introduce systematic bias into estimates of both tree and stand water use. Future studies should devise sampling strategies for assessing radial variation in sap velocity and such strategies should be used to identify the magnitude of this variation in a range of non-, diffuse- and ring-porous trees.     

Stomatal conductance,growth and root signaling in Betula pendula seedlings subjected to partial soil drying

Seedlings of Betula pendula Roth were grown with their root systems separated between two soil compartments. Four treatments were imposed: (i) adequate irrigation in both compartments (WW, controls); (ii) adequate irrigation in one compartment and drought in the other compartment (WD); (iii) drought in both compartments (DD); and (iv) half of the root system severed and the remainder kept well-watered (root excision, RE). Predawn leaf water potential, stomatal conductance, soil-to-leaf specific hydraulic conductance, and root and leaf growth decreased in DD-treated seedlings, which also displayed severe leaf shedding (30% loss in leaf area). The DD treatment also resulted in increased concentrations of abscisic acid (ABA) and its glucose ester in the xylem sap of roots and shoots compared to concentrations in control seedlings (about 200 versus 20 nM). Despite the difference in xylem sap concentrations, total ABA flux to the shoots was similar in the two treatments (1-2 pmol ABA m(-2) leaf area s(-1)) as a result of reduced transpiration in the DD-treated seedlings. Compared with root growth in control plants, root growth increased in the RE-treated plants and decreased in the drying compartment of the WD treatment; however, the RE and WD treatments only slightly reduced leaf expansion, and had no detectable effects on shoot water relations or ABA concentrations of the root and shoot xylem sap. We conclude that short-term soil water depletion affecting only 50% of the root system does not cause a measurable stress response in birch shoots, despite root growth cessation in the fraction of drying soil.     

Kinetics of nitrogen uptake by Populus tremuloides in relation to atmospheric CO(2) and soil nitrogen availability

Sustained increases in plant production in response to elevated atmospheric carbon dioxide (CO(2)) concentration may be constrained by the availability of soil nitrogen (N). However, it is possible that plants will respond to N limitation at elevated CO(2) concentration by increasing the specific N uptake capacity of their roots. To explore this possibility, we examined the kinetics of (15)NH(4) (+) and (15)NO(3) (-) uptake by excised roots of Populus tremuloides Michx. grown in ambient and twice-ambient CO(2) concentrations, and in soils of low- and high-N availability. Elevated CO(2) concentration had no effect on either NH(4) (+) or NO(3) (-) uptake, whereas high-N availability decreased the capacity of roots to take up both NH(4) (+) and NO(3) (-). The maximal rate of NH(4) (+) uptake decreased from 12 to 8 &mgr;mol g(-1) h(-1), and K(m) increased from 49 to 162 &mgr;mol l(-1), from low to high soil N availability.Because NO(3) (-) uptake exhibited mixedkinetics over the concentration range we used (10-500 &mgr;mol l( -1)), it was not possible to calculate V(max) and K(m). Instead, we used an uptake rate of 100 &mgr;mol g(-1) h(-1) as our metric of NO(3) (-) uptake capacity, which averaged 0.45 and 0.23 &mgr;mol g(-1) h(-1) at low- and high-N availability, respectively. The proximal mechanisms for decreased N uptake capacity at high-N availability appeared to be an increase in fine-root carbohydrate status and a decrease in fine-root N concentration. Both NH(4) (+) and NO(3) (-) uptake were inversely related to fine-root N concentration, and positively related to fine-root total nonstructural carbohydrate concentration. We conclude that soil N availability, through its effects on fine-root N and carbohydrate status, has a much greater influence on the specific uptake capacity of P. tremuloides fine roots than elevated atmospheric CO(2). In elevated atmospheric CO(2), changes in N acquisition by P. tremuloides appeared to be driven by changes in root architecture and biomass, rather than by changes in the amount or activity of N-uptake enzymes.     

Water content and bark thickness of Norway spruce (Picea abies) stems: phloem water capacitance and xylem sap flow

To determine the relationship between phloem transport and changes in phloem water content, we measured temporal and spatial variations in water content and sucrose, glucose and fructose concentrations in phloem samples and phloem exudates of 70- and 30-year-old Norway spruce trees (Picea abies (L.) Karst.). Large temporal and spatial variations in phloem water content (1.4-2.6 mg mg(dw)(-1)) and phloem total sugar concentration (31-70 mg g(dw)(-1)) paralleled each other (r(2) = 0.83, P < 0.0001 for the temporal profile and r(2) = 0.96, P < 0.008 for the spatial profile), indicating that phloem water content depends on the total amount of sugar to be transferred. Changes in phloem water content were unrelated to changes in bark thickness. Maximum changes in phloem water content calculated from dendrometer readings were only 8-11% of the maximum measured changes in phloem water content, indicating that reversible changes in bark thickness did not reflect changes in internal water relations. We also studied the relationship between xylem sap velocity and changes in bark thickness in 70-year-old trees during summer 1999 and winter 1999-2000. Sap flow occurred sporadically throughout the winter, but there was no relationship between bark shrinkage or swelling and sap velocity. In winter, mean daily xylem sap velocity was significantly correlated with mean daily vapor pressure deficit and air temperature (P < 0.0001, in both cases). Changes in bark thickness corresponded with both short- and long-term changes in relative humidity, in both winter and summer. Under controlled conditions at > 0 degrees C, changes in relative humidity alone caused changes in thickness of boiled bark samples. Because living bark of Norway spruce trees contains large areas with crushed and dead sieve cell zones-up to 24% of the bark is air-filled space-we suggest that this space can compensate for volume changes in living phloem cells independently of total tissue water content. We conclude that changes in bark thickness are not indicative of changes in either phloem water capacitance or xylem sap flow.     

Element concentrations in the xylem sap of Picea abies (L.) Karst. seedlings extracted by various methods under different environmental conditions

We used a Scholander pressure chamber to assess the effects of various extraction methods under different environmental conditions on element concentrations in xylem sap of 3-year-old Picea abies (L.) Karst. seedlings. Sap from excised shoots contained higher element concentrations when extracted at low than at high over-pressures. When comparing plants differing in water status, we found that a high extraction over-pressure introduced a systematic error into the data. For example, in well-watered non-transpiring plants relative to unwatered transpiring plants, potassium concentrations were 70% higher in sap extracted at 0.1 MPa over-pressure, but only 10% higher in sap extracted at 1.0 MPa over-pressure. Moreover, treatment effects depended on the time of day when the sap was extracted. Increased water flux in transpiring plants relative to non-transpiring plants resulted in reduced xylem sap element concentrations when samples were collected after 9 h of transpiration, but not after 4 to 6 h of transpiration. Drought had little effect on xylem sap element concentrations, indicating that rates of element release into xylem conduits, element depletion by growing tissues, and water flow maintained a balance that may prevent nutrient stress during short-term drought.     

Shoot and root vulnerability to xylem cavitation in four populations of Douglas-fir seedlings

The objectives of this study were to assess the range of genotypic variation in the vulnerability of the shoot and root xylem of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings to water-stress-induced cavitation, and to assess the trade-off between vulnerability to cavitation and conductivity per unit of stem cross-sectional area (k(s)), both within a species and within an individual tree. Douglas-fir occupies a broad range of environments and exhibits considerable genetic variation for growth, morphology, and drought hardiness. We chose two populations from each of two varieties (the coastal var. menziesii and the interior var. glauca) to represent environmental extremes of the species. Vulnerability curves were constructed for shoots and roots by plotting the percentage loss in conductivity versus water potential. Vulnerability in shoot and root xylem varied genetically with source climate. Stem xylem differed in vulnerability to cavitation between populations; the most mesic population, coastal wet (CW), was the most susceptible of the four populations. In the roots, the most vulnerable population was again CW; the interior wet (IW) population was moderately susceptible compared with the two dry populations, coastal dry (CD) and interior dry (ID). Root xylem was more susceptible to cavitation than stem xylem and had significantly greater k(s). The trade-off between vulnerability to cavitation and k(s), however, was not evident across populations. The most vulnerable population (CW) had a shoot k(s) of 0.534 +/- 0.067 &mgr;mol m(-2) s(-1) MPa(-1), compared with 0.734 +/- 0.067 &mgr;mol m(-2) s(-1) MPa(-1) for the less vulnerable CD stems. In the roots, IW was more vulnerable than ID, but had the same k(s).     

Drought tolerance,xylem sap abscisic acid and stomatal conductance during soil drying: a comparison of canopy trees of three temperate deciduous angiosperms

Patterns of water relations, xylem sap abscisic acid concentration ([ABA]) and stomatal aperture were characterized and compared in drought-sensitive black walnut (Juglans nigra L.), less drought-sensitive sugar maple (Acer saccharum Marsh.) and drought-tolerant white oak (Quercus alba L.) trees co-occurring in a second-growth forest in Missouri, USA. There were strong correlations among reduction in predawn leaf water potential, increased xylem sap [ABA] and stomatal closure in all species. Stomatal conductance was more closely correlated with xylem sap ABA concentration than with ABA flux or xylem sap pH and cation concentrations. In isohydric black walnut, increased concentrations of ABA in the xylem sap appeared to be primarily of root origin, causing stomatal closure in response to soil drying. In anisohydric sugar maple and white oak, however, there were reductions in midday leaf water potential associated with stomatal closure, making it uncertain whether drought-induced xylem sap ABA was of leaf or root origin. The role of root-originated xylem sap ABA in these species as a signal to the shoot of the water status of the roots is, therefore, less certain.     

Sap flow characteristics of neotropical mangroves in flooded and drained soils

Effects of flooding on water transport in mangroves have previously been investigated in a few studies, most of which were conducted on seedlings in controlled settings. In this study, we used heat-dissipation sap probes to determine if sap flow (J(s)) attenuates with radial depth into the xylem of mature trees of three south Florida mangrove species growing in Rookery Bay. This was accomplished by inserting sap probes at multiple depths and monitoring diurnal flow. For most species and diameter size class combinations tested, J(s) decreased dramatically beyond a radial depth of 2 or 4 cm, with little sap flow beyond a depth of 6 cm. Mean J(s) was reduced on average by 20% in Avicennia germinans (L.) Stearn, Laguncularia racemosa (L.) Gaertn. f. and Rhizophora mangle L. trees when soils were flooded. Species differences were highly significant, with L. racemosa having the greatest midday J(s) of about 26 g H(2)O m(-2) s(-1) at a radial depth of 2 cm compared with a mean for the other two species of about 15 g H(2)O m(-2) s(-1). Sap flow at a depth of 2 cm in mangroves was commensurate with rates reported for other forested wetland tree species. We conclude that: (1) early spring flooding of basin mangrove forests causes reductions in sap flow in mature mangrove trees; (2) the sharp attenuations in J(s) along the radial profile have implications for understanding whole-tree water use strategies by mangrove forests; and (3) regardless of flood state, individual mangrove tree water use follows leaf-level mechanisms in being conservative.     

Vascular tissue anatomy of Norway spruce needles and twigs in relation to magnesium deficiency

Vascular tissues of needles and twigs of Norway spruce with low foliar magnesium concentrations were examined by light microscopy. Observations were made on samples from (1) apparently healthy trees, (2) trees exhibiting progressive symptoms of decline, including the yellowing and fall of needles and the death of twigs and branches in the subapical canopy, and (3) trees recovering from symptoms of decline after fertilization with magnesium sulfate. Abnormalities in cambium and phloem anatomy, which were apparent in 2-year-old needles of declining trees, were seen only in needles at least 4 years old in healthy trees, suggesting that needles of declining trees are susceptible to precocious aging. Abnormalities in xylem anatomy were observed in needles 3 years or more in age in declining trees, but were never seen in needles of healthy trees. Over time, anatomical abnormalities in declining trees were seen in progressively younger twigs, which may explain the acropetal development of decline symptoms. After fertilization with magnesium sulfate, normal phloem and xylem were observed in the newly formed tissues.     

Photosynthetic gas exchange and accumulation of phytotoxins in mangrove seedlings in response to soil physico-chemical characteristics associated with waterlogging

We evaluated photosynthetic gas exchange and the accumulation of iron, manganese and sulfur in seedlings of five mangrove species (Aegiceras corniculatum (L.) Blanco, Avicennia marina (Forsk.) Vierh., Bruguiera gymnorrhiza (L.) Lamk., Hibiscus tiliaceus L. and Rhizophora stylosa Griff.) growing under anoxic soil conditions at low irradiance. Seedlings of the viviparous mangroves showed no significant responses to root anoxia. The presence of ferrous sulfate or manganous sulfate had a smaller effect on CO(2) assimilation, transpiration rate and stomatal behavior than the presence of sodium sulfide. Sodium sulfide inhibited photosynthetic gas exchange and caused complete stomatal closure in all species. Stomatal closure was probably the result of the damaging effect of sulfide ions on root cell membranes. Some leaf epinasty and wilting were also observed in response to the Na(2)S treatment in all species. A combination of FeSO(4), MnSO(4) and Na(2)S had a smaller effect on photosynthesis than Na(2)S alone, especially for Avicennia marina and Rhizophora stylosa seedlings, which maintained appreciable rates of CO(2) assimilation (2.49 and 3.84 &mgr;mol m(-2) s(-1), respectively) in the presence of all three phytotoxins. Roots of phytotoxin-treated seedlings of all species accumulated significant amounts of the corresponding toxin compared with roots of the control plants. The FeSO(4) or MnSO(4) treatments had no effect on foliar concentrations of iron or manganese, whereas the Na(2)S treatment resulted in an accumulation of S in the leaves of all species. Interactions between Fe(2+) and Mn(2+) and sulfide in the culture medium appeared to reduce their uptake by the seedlings. We conclude that high concentrations of sulfides in mangrove sediments may adversely affect both growth and survival of mangrove seedlings at low irradiances.     

Nitrate content, amino acid composition and growth of yellow birch seedlings in response to light and nitrogen source

Yellow birch (Betula alleghaniensis Britt.) seedlings were grown for three months in a greenhouse at two radiant flux densities-full light (FL) and 50% shade (LL)-and with three nitrogen sources- ammonium only (NH(4) (+)), nitrate only (NO(3) (-)) and a 1:1 mixture of ammonium and nitrate (NH(4) (+)/NO(3) (-))-in a completely randomized factorial design. The total biomass of seedlings grown under low light (LL) did not vary significantly with nitrogen source; although NO(3) (-)-treated seedlings were smaller and had a significantly lower (P 相似文献