The Effect of Hydrostatic Pressure Gradients on the Movement of Potassium across the Root Cortex

Transpiration reduces the hydrostatic pressure in the xylemand water moves into the root and across the root cortex inresponse to the pressure gradient so produced. This effect wassimulated by raising the pressure on the external medium surroundinga detopped root system. The flux of sap from the stem stumpand the concentration of potassium in the sap were measuredand the flux of potassium into xylem obtained as (sap flux Xconcentration of potassium). The application of a pressure of 2 atm. caused an approximatelyfourfold increase in potassium flux. This increase was independentof the presence of potassium in the external medium and wasdue, therefore, to an efflux of ions already stored in the roottissues. The effect was specifically due to hydrostatic pressureand not to the D.P.D. difference across the cortex, and wascaused, in part at least, by an increase in the permeabilityof the tissues to iona. This is evident since the increasedpotassium flux occurred in response to pressure, even when theconcentration of potassium in the xylem increased at the sametime, thus precluding any ‘dilution effects‘. It was confirmed that metabolic inhibitors and low temperaturereduce the sap flux but leave the concentration of the sap unchanged.Under a pressure gradient, however, there was an even greaterporportionate reduction of sap flux by equivalent concentrationsof inhibitor or low temperature and the concentration of potassiumin the sap gradually increased. Adding calcium or magnesium ions to the medium caused an increaseof potassium concentration in the sap, demonstrating exchangeprocesses in the transpiration pathway. The difficulty of framing a hypothesis to cover all the factsis discussed and it is tentatively suggested that movement acrossthe cortex to the xylem is a catenary process in which soluteand water move independently at one stage and together as asolution at another stage.     

The concentration of xylem sap constituents in root exudate, and in sap from intact, transpiring castor bean plants (Ricinus communis L.)

Root exudates were sampled from detopped root systems of castor bean (Ricinus communis). Different volume flux rates were imposed by changing the pneumatic pressure around the root system using a Passioura-type pressure chamber. The concentrations of cations, anions, amino acids, organic acids and abscisic acid decreased hyperbolically when flux rates increased from pure root exudation up to values typical for transpiring plants. Concentrations at low and high fluxes differed by up to 40 times (phosphate) and the ratio of substances changed by factors of up to 10. During the subsequent reduction of flux produced by lowering the pneumatic pressure in the root pressure chamber, the concentrations and ratios of substances deviated (at a given flux rate) from those found when flux was increased. The flux dependence of exudate composition cannot therefore be explained by a simple dilution mechanism. Xylem sap samples from intact, transpiring plants were collected using a Passioura-type root pressure chamber. The concentrations of the xylem sap changed diurnally. Substances could be separated into three groups: (1) calcium, magnesium and amino acid concentrations correlated well with the values expected from their concentration-flux relationships, whereas (2) the concentrations of sulphate and phosphate deviated from the expected relationships during the light phase, and (3) nitrate and potassium concentrations in intact plants varied in completely the opposite manner from those in isolated root systems. Abscisic acid concentrations in the root exudate were dependent on the extent of water use and showed strong diurnal variations in the xylem sap of intact plants even in droughtstressed plants. Calculations using root exudates overestimated export from the root system in intact plants, with the largest deviation found for proton flux (a factor of 10). We conclude that root exudate studies cannot be used as the sole basis for estimating fluxes of substances in the xylem of intact plants. Consequences for studying and modelling xylem transport in whole plants are discussed.     

Relationships between sap flow and water potential in woody or perennial plants on islands of the Great Barrier Reef

Abstract. This paper describes studies on trees of Pisonia grandis , bushes of Argusia argentea , and the perennial herb Melanthera biflora , growing on One Tree Island, a coral cay of the Great Barrier Reef with 'soil' of coarse coral rubble. Water potential (Ψ_b, measured on small shoots with a pressure chamber), sap flow, stomatal conductance, vapour pressure deficit and photon flux density were monitored over day/night cycles. Sap flow and Ψ_b responded to changes in light and humidity. From these experiments good linear correlations were found between sap flow in a shoot and Ψ_b of similar adjacent shoots. The linearity suggests that the resistance to sap flow is constant as Ψ_b varies. The correlation, however, does not indicate a causal relationship between Ψ_b of an individual shoot on the plant and its sap flow. Ψ_b was only slightly different in shaded shoots from those in sunshine, although sap flow would be expected to differ between them. Enclosing shoots and so reducing their transpiration and sap flow to very low rates resulted in only small changes in Ψ_b of the enclosed shoots; Tb of such enclosed shoots should closely approximate that of the xylem at the point of shoot attachment. From these results it is suggested that the resistance to water flow in shoot and leaf xylem is small compared to the resistance further down the plant, in the root or at the root/soil interface. Shoot xylem water potential would be similar for all parts of the plant, and in such plants the water potential of shoots in the shade would be determined by the overall water use of the plant.     

Calcium in Xylem Sap and the Regulation of its Delivery to the Shoot

Amounts of total and free calcium in root and shoot xylem sapwere quantified for a number of species grown in comparableenvironments and in a rooting medium not deficient in calcium.The potential for the shoot to sequester calcium was also examined,along with the ability for ABA to regulate calcium flux to theleaf. Xylem sap calcium showed considerable interspecific and diurnalvariation, even though the plants were grown with similar rhizosphericcalcium concentrations. The potential for the shoot to sequesterxylem sap calcium was also highly variable between species andimplied an ability, at least in some species, to regulate thecalcium reaching the shoot in the transpiration stream. Long distance transport of calcium in the xylem was not primarilyby mass flow, because neither calcium uptake nor distributionwere closely related to water uptake or transpiration. The diurnalchanges in xylem sap total ion concentration appeared to benegatively correlated with transpiration while, in contrast,the calcium ion concentration showed two peaks, one occurringin the dark and the other in the light period. The application of ABA to roots caused an increase in the rateof exudation from the xylem of detopped well-watered plants.These experiments suggest that changes in root water relationsdriven by ionic fluxes were the likely cause for enhanced sapexudation from ABA-treated roots. The steady-state concentrationof calcium in the xylem sap was unaffected by ABA when exudationrate increased and, consequently, the flux of calcium must alsohave increased. Key words: Abscisic acid, calcium, xylem sap, ionic fluxes     

Studies on the Movement of Solutes between the Sieve Tubes and Surrounding Tissues3: I. INTERFERENCE BETWEEN SOLUTES AND RATE OF TRANSLOCATION MEASUREMENTS

Certain aspects of the secretion of solutes into, and removalfrom, the sieve tubes of isolated stem segments and rooted cuttingsof Salix viminalis have been studied. Sieve-tube sap was obtainedeither as honeydew from whole individuals or via the severedstylets of the aphid Tuberolachnus salignus (Gmelin). It was shown that interference occurred between the chemicallyunrelated solutes, sucrose and the cations potassium and rubidium.On raising the potassium concentration in the sieve-tube sapby passing a solution of this ion through the xylem, the sucroseconcentration declined. When the sucrose concentration fellover a period of days due to respiratory loss of carbohydratesfrom an isolated stem segment, a concomitant rise in eitherthe potassium or rubidium level in the sap occurred. When a solution of sodium was passed through the xylem, theconcentration of this ion in the sieve-tube sap rose, whilstthat of potassium fell at first, but later rose higher thanits initial value, indicating that both antagonism and synergycan occur between these ions. On introducing both these cationsinto the xylem simultaneously, more sodium than potassium wastaken up by the segment, though the increase in the sodium concentrationin the sieve-tube sap was less than that of the potassium. Perfusingthe xylem with a calcium solution had no effect upon the concentrationof potassium in the sieve tube. It has been shown that the rate of translocation of a solutealong the sieve tube, as measured by the two colony technique,depends upon the rate of removal of this solute from the sievetube. The amount of such lateral loss from the sieve tube isrelated to the potential gradient for a solute between the sievetube and surrounding cells.     

转Bt 基因抗虫棉(Gossyposium)伤流中Bt 毒蛋白的运输

本文以常规棉品种‘石远321’和抗虫棉品种‘SGK321’与‘99B’为材料, 通过考马斯亮蓝和ELISA方法分析了伤流中可溶性蛋白和Bt(苏云金芽孢杆菌)毒蛋白含量。结果发现, 转基因抗虫棉伤流中可溶性蛋白含量显著高于常规棉, 并在抗虫棉伤流中检测到了Bt毒蛋白, 而常规棉伤流中没有Bt毒蛋白的存在。通过嫁接实验研究发现, 将常规棉嫁接到转Bt基因抗虫棉砧木上, 叶片中也有Bt毒蛋白的积累。这些结果说明, Bt毒蛋白可以通过木质部伤流液向地上部运输, 并在叶片中积累, Bt毒蛋白由根系向地上部的运输对棉花的抗虫性是有益的。     

Effects of drought on nutrient and ABA transport in Ricinus communis

We studied the effects of variations of water flux through the plant, of diurnal variation of water flux, and of variation of vapour pressure deficit at the leaf on compensation pressure in the Passioura-type pressure chamber, the composition of the xylem sap and leaf conductance in Ricinus communis. The diurnal pattern of compensation pressure showed stress relaxation during the night hours, while stress increased during the day, when water limitation increased. Thus compensation pressure was a good measure of the momentary water status of the root throughout the day and during drought. The bulk soil water content at which predawn compensation pressure and abscisic acid concentration in the xylem sap increased and leaf conductance decreased, was high when the water usage of the plant was high. For all xylem sap constituents analysed, variations in concentrations during the day were larger than changes in mean concentrations with drought. Mean concentrations of phosphate and the pH of the xylem sap declined with drought, while nitrate concentration remained constant. When the measurement leaf was exposed to a different VPD from the rest of the plant, leaf conductance declined by 400mmol m^?2 s^?1 when compensation pressure increased by 1 MPa in all treatments. The compensation pressure needed to keep the shoot turgid, leaf conductance and the abscisic acid concentration in the xylem were linearly related. This was also the case when the highly dynamic development of stress was taken into account.     

Xylem sap composition: A tool for investigating mineral uptake and cycling in adult spruce

Xylem sap composition of spruce is influenced by several factors, such as the sampled organ, the sampling period, the availability of soil nutrients, and the soil water potential. Based on literature data and ongoing investigations carried out with adult trees, we present an overview on the main factors influencing xylem sap concentrations of Norway spruce. Direct measurements of nutrient fluxes in the xylem sap are then used to suggest a general scheme of mineral element cycling within adult trees. In Norway spruce (Picea abies Karst.), nutrient concentration in the xylem sap was higher in twigs and fine roots compared to the bottom of the trunk, the highest concentrations beeing observed in spring during the shoot elongation. Xylem sap concentrations were higher in spruce growing at nutrient rich sites than at poor sites. The combination of twig and trunk xylem sap analysis, together with xylem flow measurements in the trunk during the course of a vegetation period allowed the quantification of mineral fluxes via xylem sap flow in the trunk and twigs. These results were compared to gross mineral uptake measurements at the same site. Ca flux in the trunk xylem sap was lower than the gross uptake of Ca. Mg flux in trunk sap was approximately equivalent to Mg gross uptake whereas P and K fluxes in trunk sap were much higher than the gross uptake. Fluxes of Ca, Mg, K and P in the twig sap were much higher than that in trunk sap. Data suggest that internal cycling is responsible for a large part of the nutrient fluxes in the xylem sap of the crown. Xylem sap composition thus appears to be a tool which can complement other sources of information on mineral uptake and cycling in adult spruce     

Nutrient translocation in the xylem of poplar — diurnal variations and spatial distribution along the shoot axis

This investigation shows diurnal variations in the xylem sap composition of poplar (Populus tremula x P. alba). All major macronutrients reached a maximum concentration in the first half of the light period and decreased to the middle of the night. The relative abundance of the nutrients did not change during the day. The sap flow, which responded very fast to the environmental changes (2.2-fold increase within 10-20 min of illumination), reached a maximum value in the second half of the light period. Transpiration (and photosynthesis) was constant throughout the light phase. The calculated translocation rates displayed a maximum in the first half of the light period and therefore did not fit the time course of sap flow. During the night, translocation rates were 63-69% lower than the maximum. The regulation of nutrient translocation is discussed taking the active xylem loading into account. The axial distribution located the nitrate assimilation in younger leaves and storage of nitrate (and other macronutrients) in older leaves. Nitrate and potassium concentrations in the xylem sap did not change along the plant axis. However, the sap flow was greater in younger shoot sections than in older sections. We assume that the greater demand for nitrate in the younger shoot section was satisfied via an increased volume flow rather an increased nitrate concentration.     

Dynamics of concentrations and nutrient fluxes in the xylem of Ricinus communis– diurnal course, impact of nutrient availability and nutrient uptake

Diurnal courses of nutrient transport in the xylem and their response to external availability of nutrients were studied. In soil culture, maximal concentrations in all analysed substances were observed during night‐time. Over experimental periods of up to 20 d, concentrations of some ions increased, most by accumulation in the soil. Stringent nutrient conditions were established in a novel pressure chamber. An aeroponic nutrient delivery system inside allows the sampling of xylem sap from intact plants under full control of the nutrient conditions at the root. Analysis of xylem transport under these highly defined conditions established that (1) diurnal variations in concentrations and fluxes in the xylem are dominated by plant‐internal processes; (2) concentrations of nutrients in the xylem sap are highly but specifically correlated with each other; (3) nitrate uptake and nitrate flux to the shoot are largely uncoupled; and (4) in continuous light, diurnal variations of xylem sap concentrations vanish. Step changes in nitrate concentrations of the nutrient solution established that (5) the concomitant increase in nitrate concentration and flux in the xylem is delayed by 2–3 h and is only transient. Diurnal variations of xylem sap composition and use of the new technique to elucidate xylem‐transport mechanisms are discussed.     

黄土丘陵区辽东栎和刺槐树干液流时滞效应与蒸腾特征的关联性

运用Granier热扩散探针法,于2016年7-9月对半干旱黄土丘陵区天然次生林树种辽东栎和人工林树种刺槐的树干液流进行连续测定,并同步监测气象因子和土壤含水量,用错位相关法分析液流通量密度与空气水汽压亏缺日变化的时滞长度,研究2个树种不同径级个体在不同土壤水分条件下液流通量密度与蒸腾驱动因子之间的时滞效应.结果表明:辽东栎和刺槐液流通量密度的日变化节律与气象因子显著相关,空气水汽压亏缺峰值的出现较辽东栎树干液流通量密度滞后118.2 min,较刺槐树干液流通量密度滞后39.5 min;而光合有效辐射的峰值通常滞后于辽东栎12.4 min,提前于刺槐68.5 min.液流通量密度和空气水汽压亏缺的时滞长度与树种和土壤含水量显著相关,辽东栎、刺槐在土壤含水量较高时段的时滞长度分别大于土壤含水量较低时段32.2和68.2 min.时滞长度与径级的相关性整体上未达到显著水平,但在土壤含水量较低时段小径级刺槐的时滞长度大于大径级21.4 min,差异达到了显著水平.两树种液流通量密度与空气水汽压亏缺之间的时滞效应反映了对蒸腾驱动因子的敏感性,较好的土壤水分条件有利于液流通量密度提早达到峰值,较低土壤水分会导致树干液流对气象环境因子响应的敏感性降低;刺槐树干液流受土壤水分的影响更显著.     

Differences in the influx of glutamine and nitrate into Viscum album from the xylem sap of its hosts.

The flux of inorganic and organic nitrogen into the mistletoe Viscum album L. from the xylem sap of a deciduous (Populus x euamericana) and a coniferous host (Abies alba Mill.) was analyzed. For this purpose, a perfusion system was developed in which the xylem sap of the host was replaced by an artificial perfusion solution. With this system flux rates into the mistletoe were determined in feeding experiments either with the organic nitrogen source [1,2-13C2]glutamine at high and the inorganic nitrogen source 15NO3- at low concentration or vice versa. Glutamine influx was already saturated at the low concentration in the xylem sap and was--different from nitrate--not enhanced, when a 250-fold higher concentration was applied. Nitrate influx matched glutamine influx only at high inorganic/organic nitrogen ratios in the perfusion solution. This result indicates a preferential influx of glutamine over nitrate from the host xylem into the mistletoe at the concentrations found in the xylem sap of trees. Surprisingly, a high percentage of both N sources were accumulated in the mistletoe stem, indicating excessive N nutrition of the mistletoe leaves. Since 13C isotope signature was significantly reduced in the outflowing perfusion solution, either an upload of organic compounds from the phloem into the xylem, or an efflux of organic compounds from haustorium of mistletoe into the xylem has to be assumed. 15N isotope signatures enriched in the outflowing perfusion solution support the idea of a nitrate uptake system at the host xylem-haustorium interface, which favors the light N isotope of nitrate.     

Gas exchange and water balance of a mistletoe species and its mangrove hosts

Summary The gas exchange and water relations of the hemiparasite Pthirusa maritima and two its mangrove host species, Conocarpus erectus and Coccoloba uvifera, were studied in an intertidal zone of the Venezuelan coast. Carbon uptake and transpiration, leaf osmotic and total water potential, as well as nutrient content in the xylem sap and leaves of mistletoes and hosts were followed through the dry and wet season. In addition, carbon isotope ratios of leaf tissue were measured to further evaluate water use efficiency. Under similar light and humidity conditions, mistletoes had higher transpiration rates, lower leaf water potentials, and lower water use efficiencies than their hosts. Potassium content was much higher in mistletoes than in host leaves, but mineral nutrient content in the xylem sap of mistletoes was relatively low. The resistance of the liquid pathway from the soil to the leaf surface of mistletoes was larger than the total liquid flow resistance of host plants. Differences in the daily cycles of osmotic potential of the xylem sap also indicate the existence of a high resistance pathway along the vascular connection between the parasite pathway along the vascular connection between the parasite and its host. P. maritima mistletoes adjust to the different physiological characteristics of the host species which it parasitizes, thus ensuring an adequate water and carbon balance.     

Seasonal and spatial variation of carbohydrates in mistletoes (Viscum album) and the xylem sap of its hosts (Populus x euamericana and Abies alba)

In the present field study we analysed the seasonal pattern of carbohydrate composition and contents in the xylem sap of Viscum album and the xylem sap of a deciduous ( Populus × euramericana ) and a coniferous ( Abies alba ) host tree species. The results were compared with the soluble carbohydrate composition and contents of mistletoe tissues. On both hosts significant amounts of glucose, fructose, and sucrose were found in the xylem sap of Viscum throughout the seasons. The general seasonal pattern of sugar contents, i.e. high concentrations in spring and lower concentrations in other seasons on Populus , and intermediate concentrations throughout the year on Abies , largely reflected the xylem sap carbohydrate composition and contents of the respective host. These observations provide indirect evidence for carbohydrate flux from the xylem sap of the host into the mistletoe. However, in both hosts xylem sap seems to be deviated into the mistletoe without specific control of carbohydrate flux. Differences observed between the seasonal pattern of xylem sap carbohydrate concentrations in Viscum on Populus and Abies may originate from the different time of leaf development of these species. A clear-cut seasonal pattern of soluble carbohydrates was not observed in the leaves of Viscum on both hosts. Still soluble carbohydrates seem to be reallocated from the senescing to the newly developed leaves of Viscum indicating that the seasonal requirement of carbohydrate for growth and development can only completely be met by carbohydrate acquisition from the host and their own photosynthesis.     

Enhanced salt tolerance mediated by AtHKT1 transporter-induced Na unloading from xylem vessels to xylem parenchyma cells

AtHKT1 is a sodium (Na+) transporter that functions in mediating tolerance to salt stress. To investigate the membrane targeting of AtHKT1 and its expression at the translational level, antibodies were generated against peptides corresponding to the first pore of AtHKT1. Immunoelectron microscopy studies using anti-AtHKT1 antibodies demonstrate that AtHKT1 is targeted to the plasma membrane in xylem parenchyma cells in leaves. AtHKT1 expression in xylem parenchyma cells was also confirmed by AtHKT1 promoter-GUS reporter gene analyses. Interestingly, AtHKT1 disruption alleles caused large increases in the Na+ content of the xylem sap and conversely reduced the Na+ content of the phloem sap. The athkt1 mutant alleles had a smaller and inverse influence on the potassium (K+) content compared with the Na+ content of the xylem, suggesting that K+ transport may be indirectly affected. The expression of AtHKT1 was modulated not only by the concentrations of Na+ and K+ but also by the osmolality of non-ionic compounds. These findings show that AtHKT1 selectively unloads sodium directly from xylem vessels to xylem parenchyma cells. AtHKT1 mediates osmolality balance between xylem vessels and xylem parenchyma cells under saline conditions. Thus AtHKT1 reduces the sodium content in xylem vessels and leaves, thereby playing a central role in protecting plant leaves from salinity stress.     

Root hypoxia reduces leaf growth : role of factors in the transpiration stream

This study examined the potential role of restricted phloem export, or import of substances from the roots in the leaf growth response to root hypoxia. In addition, the effects of root hypoxia on abscisic acid (ABA) and zeatin riboside (ZR) levels were measured and their effects on in vitro growth determined. Imposition of root hypoxia in the dark when transpirational water flux was minimal delayed the reduction in leaf growth until the following light period. Restriction of phloem transport by stem girdling did not eliminate the hypoxia-induced reduction in leaf growth. In vitro growth of leaf discs was inhibited in the presence of xylem sap collected from hypoxic roots, and also by millimolar ABA. Disc growth was promoted by sap from aerated roots and by 0.1 micromolar ZR. The flux of both ABA and ZR was reduced in xylem sap from hypoxic roots. Leaf ABA transiently increased twofold after 24 hours of hypoxia exposure but there were no changes in leaf cytokinin levels.     

不同光强下单叶蔓荆的光合蒸腾与离子累积的关系

同步分析了生长在滨海滩涂沙地的灌木单叶蔓荆钠和钾离子的累积量与植物的水分累积消耗量与光合产物累积量的日变化特点和累积的关系,并比较了单叶蔓荆在晴天和阴天的木质部溶液离子浓度与植物水势的关系和水分利用效率的差别,结果表明:单叶蔓荆在光强度较高的晴天的水分利用效率高于阴天,在晴天的光合产物累积达到了阴天的约4倍,却只消耗了约为阴天3倍的水。随着植物体水势的降低,单叶蔓荆木质部溶液的钠离子的浓度和钾离子浓度呈降低趋势但不明显。木质部溶液的钾离子浓度和钾离子累积量无论在晴天和阴天都明显低于钠离子的浓度和累积量。单叶蔓荆的高浓度的钠离子吸收有可能在液泡累积并降低细胞的渗透势,增加细胞的吸水能力和植物的抗旱抗盐能力。     

Mineral uptake and transport in xylem and phloem of the proteaceous tree,Banksia prionotes

Xylem sap of sinker (tap) root, cluster feeding roots, lateral roots and from an age series of main stem extensions of 6-year trees of Banksia prionotes was collected and analyzed for principal organic and inorganic solutes. During the phase of root uptake activity in winter and spring, cluster roots were principal xylem donors of malate, phosphate, chloride, sodium, potassium and amino acid N whereas other parts of the root served as major sources to the shoot of other cations, nitrate and sulphate. Sinker root xylem sap was at all times less concentrated in solutes than that of lateral roots into which cluster roots were voiding exported solutes. Phosphate was abstracted from xylem by stem tissue during winter and it and a range of other solutes released back to xylem immediately prior to extension growth of the shoot in summer. Phloem sap collected from mid regions of stems was unusually low in potassium and phosphate relative to chloride and sulphate in comparison with phloem sap of other species, and its low potassium: sodium ratio relative to xylem indicated poor discrimination against sodium during phloem loading. Data are discussed in relation to the asynchronous seasonal cycles of nutrient uptake and shoot growth.     

Seasonal dynamics of major cations in xylem sap and needles of Norway spruce (Picea abies L. Karst.) in stands with different soil solution chemistry

Soil solution, xylem sap and needles of mature trees were sampled in three spruce stands over one vegetation period and analysed for major cations. Investigations of nutrient distribution between these three pools and evaluations of seasonal dynamics give the following results: The highest nutrient concentrations in the xylem sap occur at the time of bud break and become gradually lower during the vegetation period. The trees show similar trends of xylem sap concentrations with time for potassium, calcium and magnesium regardless of the nutritional status of the plots. No coupling of xylem sap to soil solution composition can be observed in spite of a high variability of soil solution chemistry in time. The major cations in the current needles exhibit a significantly different trend with time. No time-based correlations for nutrient contents could be found for the needles from the previous year.Despite mobilisation of storage pools in the deficient stand, trees are not able to increase the Ca and Mg contents in the needles up to the level of the other stands. Potassium could be retranslocated in sufficient extent for nutrition of current needles. Due to seasonal variability and dependence upon internal processes, such as retranslocation and mobilisation of nutrients, xylem sap does not seem to be a good tool for the estimation of the nutritional status of forest sites.It was concluded that only minor transport into new foliage via xylem sap will proceed after nutrient flush during the bud break and the nutrient content in the new biomass will be governed by dilution due to biomass growth or by nutrient transport by other means than xylem sap.     

Radial patterns of xylem sap flow in non-, diffuse- and ring-porous tree species

We investigated radial patterns of sap flux density and wood properties in the sapwood of young loblolly pine (Finns taeda L.), mature white oak (Quercus alba L.) and sweetgum (Liquidambar styraciflua L.), which represent three major classes of wood anatomy: non-porous (coniferous), ring-porous and diffuse-porous. Radial measurements of xylem sap flux density were made in sections of xylem extending to 20 mm and 20–40 mm from the cambium. These measurements were compared with measurements of the relative water content (R_s) and sapwood specific gravity (ρ_r) of corresponding radial sections. In both hardwood species, sap flow differences were rarely significant between the two depth intervals. In pine, a 59% reduction in daily sap flux density from outer to inner sapwood was found. This could not be accounted for by a 3% drop in R_s; rather, an accompanying 9% reduction in ρ_r indicated a transition between the depth intervals from mature to juvenile sapwood, and is the probable cause of the lower flux rate in the inner xylem of pine.