Applications of the compensating pressure theory of water transport

Some predictions of the recently proposed theory of long-distance water transport in plants (the Compensating Pressure Theory) have been verified experimentally in sunflower leaves. The xylem sap cavitates early in the day under quite small water stress, and the compensating pressure P (applied as the tissue pressure of turgid cells) pushes water into embolized vessels, refilling them during active transpiration. The water potential, as measured by the pressure chamber or psychrometer, is not a measure of the pressure in the xylem, but (as predicted by the theory) a measure of the compensating pressure P. As transpiration increases, P is increased to provide more rapid embolism repair. In many leaf petioles this increase in P is achieved by the hydrolysis of starch in the starch sheath to soluble sugars. At night P falls as starch is reformed. A hypothesis is proposed to explain these observations by pressure-driven reverse osmosis of water from the ground parenchyma of the petiole. Similar processes occur in roots and are manifested as root pressure. The theory requires a pump to transfer water from the soil into the root xylem. A mechanism is proposed by which this pump may function, in which the endodermis acts as a one-way valve and a pressure-confining barrier. Rays and xylem parenchyma of wood act like the xylem parenchyma of petioles and roots to repair embolisms in trees. The postulated root pump permits a re-appraisal of the work done by evaporation during transpiration, leading to the proposal that in tall trees there is no hydrostatic gradient to be overcome in lifting water. Some published observations are re-interpreted in terms of the theory: doubt is cast on the validity of measurements of hydraulic conductance of wood; vulnerability curves are found not to measure the cavitation threshold of water in the xylem, but the osmotic pressure of the xylem parenchyma; if measures of xylem pressure and of hydraulic conductance are both suspect, the accepted view of the hydraulic architecture of trees needs drastic revision; observations that xylem feeding insects feed faster as the water potential becomes more negative are in accord with the theory; tyloses, which have been shown to form in vessels especially vulnerable to cavitation, are seen as necessary for the maintenance of P, and to conserve the supplementary refilling water. Far from being a metastable system on the edge of disaster, the water transport system of the xylem is ultrastable: robust and self-sustaining in response to many kinds of stress.     

Xylem feeding by spittlebug nymphs: some observations by optical and cryo-scanning electron microscopy

The feeding of spittlebug nymphs (Philaenus spumarius) from mature xylem vessels was studied by optical and cryo-analytical scanning electron microscopy. Feeding did not produce xylem embolisms and vessels remained liquid-filled during the day. Saliva secreted by the insect forms a hardened lining (salivary sheath) between the stylet bundle and the plant tissues. This sheath is continuous through the hole made by the stylets as they enter a vessel, and it extends into the vessel and along its periphery beyond the breach. The sheath is heterogeneous, with a thin outer layer adjoining the plant tissues and a thicker layer that contacts the stylet bundle. Both layers give positive histochemical reactions for proteins and, in fresh tissues, contain a red, strongly autofluorescent pigment, possibly condensed tannin derived from the plant (which is lost during tissue preparation), and other phenyl propanoid compounds, which are retained and which may produce the intense reaction of the periodic-acid-Schiff's-positive inner layer. It is concluded that the salivary sheath allows the insects to feed from functioning vessels without embolizing them or losing xylem fluid to the surrounding tissues. These findings and others in the entomological literature indicate low daytime tensions in the xylem conduits of the host plants.     

Plant feeding site selection on soybean by the facultatively phytophagous predator Orius insidiosus

Experiments were conducted to test whether the facultatively phytophagous predator Orius insidiosus (Say) (Heteroptera: Anthocoridae) ingested phloem, xylem or mesophyll contents from soybean plants (Glycine max L.). Potential uptake of phloem sap was examined by radiolabeling photosynthate with ¹⁴CO₂ and then measuring the accumulation of radiolabeled metabolites in feeding animals. Most O. insidiosus feeding on radiolabeled plants ingested no or very low levels of label; only 3% ingested small amounts of label, indicating the experimental insects fed very little, if at all, on the phloem. In contrast, well known phloem feeding insects used as positive controls accumulated substantial levels of labeled metabolites after feeding on known host plants. O. insidiosus did feed on xylem contents, as shown by ingestion of safranin-labeled xylem fluid. A few of the insects showed signs of feeding on the mesophyll, as indicated by the presence of chloroplasts in the gut. However, the small diameter of the food canal may cause limited passage of chloroplasts, which would contribute to an underestimation of the frequency of mesophyll feeding. Some radiolabeled metabolites remain in the mesophyll so those insects that ingested low levels of radiolabel probably ingested label from the mesophyll, which supports the notion that some level of mesophyll feeding occurred. Feeding site determines the nutrients ingested during phytophagy. These insects obtain water from the xylem, and may ingest small amounts of starches, sugars, and amino acids from the mesophyll. The results suggest that facultative phytophagy by this heteropteran predator primarily provides the insect with water, but also may provide some nutrients that supplement a prey diet and help the predator survive periods when prey are scarce.     

Cavitation induced by a surfactant leads to a transient release of water stress and subsequent 'run away' embolism in Scots pine (Pinus sylvestris) seedlings

Cavitation decreases the hydraulic conductance of the xylem and has, therefore, detrimental effects on plant water balance. However, cavitation is also hypothesized to relieve water stress temporarily by releasing water from embolizing conduits to the transpiration stream. Stomatal closure in response to decreasing water potentials in order to avoid excessive cavitation has been well documented in numerous previous studies. However, it has remained unclear whether the stomata sense cavitation events themselves or whether they act in response to a decrease in leaf water potential to a level at which cavitation is initiated. The effects of massive cavitation on leaf water potential, transpiration, and stomatal behaviour were studied by feeding a surfactant into the transpiration stream of Scots pine (Pinus sylvestris) seedlings. The stomatal response to cavitation in connection with the capacitive effect was also studied. A major transient increase in leaf water potential was found due to cavitation in the seedlings. As cavitation was induced by lowering the surface tension, the two mechanisms could be uncoupled, as the usual relation between xylem water potential and the onset of cavitation did not hold. Our results indicate that the seedlings responded more to leaf water potential and less to cavitation itself, as stomatal closure was insufficient to prevent the seedlings from being driven to 'run-away' cavitation in a manner of hours.     

Ion Transport and the Transpiration Stream

In a long-term experiment with maize grown at different humidities, Tanner and Beevers (1990) demonstrated that the amount of water lost by the plants in transpiration (plus guttation) could be reduced by a factor of three without any adverse effect on growth. As a consequence, the authors questioned the importance of the transpiration stream in supplying the shoot with minerals, arguing that there are other causes of mass flow in the xylem (such as Münch counterflow from phloem to xylem, and water consumed by growing sink tissues) that may, in the limit, be capable on their own of providing the shoot with minerals. This hypothesis is discussed here in the light of recent work on xylem water relations. It is shown to involve the incorrect premise that, if transpiration were required for long-distance ion transport, plants should grow less well at high humidity. Instead, solute flux to the shoot can be demonstrated by experiment to remain constant over a wide range of transpiration rates, since the concentration of solutes in the xylem sap varies inversely with transpiration rate. Independent evidence suggests that the non-transpirational component of mass flow in the xylem is small and is unlikely to be able to provide the shoot adequately with minerals in the absence of transpiration. A simple corollary of this view is that plant growth should be reduced at very low transpiration rates, a prediction that should be testable at sufficiently high humidities under carefully controlled conditions.     

Co-ordination of vapour and liquid phase water transport properties in plants

The pathway for water movement from the soil through plants to the atmosphere can be represented by a series of liquid and vapour phase resistances. Stomatal regulation of vapour phase resistance balances transpiration with the efficiency of water supply to the leaves, avoiding leaf desiccation at one extreme, and unnecessary restriction of carbon dioxide uptake at the other. In addition to maintaining a long-term balance between vapour and liquid phase water transport resistances in plants, stomata are exquisitely sensitive to short-term, dynamic perturbations of liquid water transport. In balancing vapour and liquid phase water transport, stomata do not seem to distinguish among potential sources of variation in the apparent efficiency of delivery of water per guard cell complex. Therefore, an apparent soil-to-leaf hydraulic conductance based on relationships between liquid water fluxes and driving forces in situ seems to be the most versatile for interpretation of stomatal regulatory behaviour that achieves relative homeostasis of leaf water status in intact plants. Components of dynamic variation in apparent hydraulic conductance in intact plants include, exchange of water between the transpiration stream and internal storage compartments via capacitive discharge and recharge, cavitation and its reversal, temperature-induced changes in the viscosity of water, direct effects of xylem sap composition on xylem hydraulic properties, and endogenous and environmentally induced variation in the activity of membrane water channels in the hydraulic pathway. Stomatal responses to humidity must also be considered in interpreting co-ordination of vapour and liquid phase water transport because homeostasis of bulk leaf water status can only be achieved through regulation of the actual transpirational flux. Results of studies conducted with multiple species point to considerable convergence with regard to co-ordination of stomatal and hydraulic properties. Because stomata apparently sense and respond to integrated and dynamic soil-to-leaf water transport properties, studies involving intact plants under both natural and controlled conditions are likely to yield the most useful new insights concerning stomatal co-ordination of transpiration with soil and plant hydraulic properties.     

Capacitive effect of cavitation in xylem conduits: results from a dynamic model

Embolisms decrease plant hydraulic conductance and therefore reduce the ability of the xylem to transport water to leaves provided that embolized conduits are not refilled. However, as a xylem conduit is filled with gas during cavitation, water is freed to the transpiration stream and this transiently increases xylem water potential. This capacitive effect of embolism formation on plant function has not been explicitly quantified in the past. A dynamic model is presented that models xylem water potential, xylem sap flow and cavitation, taking into account both the decreasing hydraulic conductance and the water release effect of xylem embolism. The significance of the capacitive effect increases in relation to the decreasing hydraulic conductance effect when transpiration rate is low in relation to the total amount of water in xylem conduits. This ratio is typically large in large trees and during drought.     

Dynamic changes in petiole specific conductivity in red maple (Acer rubrum L.), tulip tree (Liriodendron tulipifera L.) and northern fox grape (Vitis labrusca L.)

Diurnal variation in petiole specific hydraulic conductivity and simultaneous measurements of leaf water potential were recorded in red maple, tulip tree and fox grape. Petiole specific conductivity was determined from in situ measurements of water flow into the distal (leaf‐bearing) end of an attached petiole as a function of applied hydrostatic pressure and petiole dimensions. The hydraulic properties of the petiole dominated the measurements, indicating that this technique can be used for rapid estimates of petiole hydraulic conductivity. There was a significant decrease in petiole specific conductivity associated with increasingly more negative leaf water potentials in maple and tulip tree, but not in grape. Petiole specific conductivity increased during the afternoon while the plant was actively transpiring and the xylem sap was under tension. The recovery of petiole conductivity during the afternoon suggests that hydraulic conductivity reflects a dynamic balance between a loss of hydraulic conductivity with increasing water stress, and its restoration as tension within the xylem decreases. Three experimental manipulations were applied to red maple and tulip tree to examine the sensitivity of diurnal changes in petiole conductivity to various physiological perturbations. Both phloem girdling and application of HgCl₂ to the transpiration stream resulted in a marked decrease in the degree to which petiole specific conductivity recovered as xylem tension relaxed during the afternoon. Delivery of a surfactant to the xylem, however, did not significantly alter the relation between leaf water potential and petiole hydraulic conductivity.     

Relax and refill: xylem rehydration prior to hydraulic measurements favours embolism repair in stems and generates artificially low PLC values

Diurnal changes in percentage loss of hydraulic conductivity (PLC), with recorded values being higher at midday than on the following morning, have been interpreted as evidence for the occurrence of cycles of xylem conduits' embolism and repair. Recent reports have suggested that diurnal PLC changes might arise as a consequence of an experimental artefact, that is, air entry into xylem conduits upon cutting stems, even if under water, while under substantial tension generated by transpiration. Rehydration procedures prior to hydraulic measurements have been recommended to avoid this artefact. In the present study, we show that xylem rehydration prior to hydraulic measurements might favour xylem refilling and embolism repair, thus leading to PLC values erroneously lower than those actually experienced by transpiring plants. When xylem tension relaxation procedures were performed on stems where refilling mechanisms had been previously inhibited by mechanical (girdling) or chemical (orthovanadate) treatment, PLC values measured in stems cut under native tension were the same as those measured after sample rehydration/relaxation. Our data call for renewed attention to the procedures of sample collection in the field and transport to the laboratory, and suggest that girdling might be a recommendable treatment prior to sample collection for PLC measurements.     

Wide vessels sustain marginal transpiration flux and do not optimize inefficient gas exchange activity under impaired hydraulic control and salinity

Plants optimize water use and carbon assimilation via transient regulation of stomata resistance and by limiting hydraulic conductivity in a long-term response of xylem anatomy. We postulated that without effective hydraulic regulation plants would permanently restrain water loss and photosynthetic productivity under salt stress conditions. We compared wild-type tomatoes to a transgenic type (TT) with impaired stomatal control. Gas exchange activity, biomass, starch content, leaf area and root traits, mineral composition and main stems xylem anatomy and hydraulic conductivity were analyzed in plants exposed to salinities of 1 and 4 dS m⁻¹ over 60 days. As the xylem cannot easily readjust to different environmental conditions, shifts in its anatomy and the permanent effect on plant hydraulic conductivity kept transpiration at lower levels under unstressed conditions and maintained it under salt-stress, while sustaining higher but inefficient assimilation rates, leading to starch accumulation and decreased plant biomass, leaf and root area and root length. Narrow conduits in unstressed TT plants were related to permanent restrain of hydraulic conductivity and plant transpiration. Under salinity, TT plants followed the atmospheric water demand, sustained similar transpiration rate from unstressed to salt-stressed conditions and possibly maintained hydraulic integrity, due to likely impaired hydraulic regulation, wider conduits and higher hydraulic conductivity. The accumulation of salts and starch in the TT plants was a strong evidence of salinity tolerance via osmotic regulation, also thought to help to maintain the assimilation rates and transpiration flux under salinity, although it was not translated into higher growth.     

The Mechanism of Boron Immobility in Plants

A mechanism is postulated to explain the immobility of boron in plants, i.e., the absence of any significant retranslocation of this element from one leaf to another. It is shown that boron readily enters the bark and is translocated within the bark. Since it has also been shown that boron remains water-soluble in plants, the immobility cannot be explained through a chemical fixation, lack of entry into phloem, or absence of phloem transport. Rather, boron enters the phloem in leaf margins where concentrations are high, is transported in these conduits, is lost therefrom where the xylem concentration is low, i.e. in basal areas of the leaf and in petiole, is transported back in the xylem and accumulates in terminal place of the transpiration stream. A high local mobility of boron, together with the essentially unidirectional flow of the transpiration stream, thus cause a cyclic movement of B and prevent the efflux of this nutrient; this explains the immobility over long distances.     

Water transport properties of seven woody species from the semi-arid Mu Us Sandy Land,China

Maintenance of water transport is very important for plant growth and survival. We studied seven woody species that inhabit the semi-arid Mu Us Sandy Land, China, to understand their strategies for maintaining hydraulic function. We evaluated water transport properties, including cavitation resistance, hydraulic recovery, and water loss regulation by stomatal control, which are associated with xylem structural and leaf physiological traits. We also discussed the water-use characteristics of these species by comparing them with those of species in other regions. Species with tracheids had higher levels of xylem resistance to cavitation and a smaller midday transpiration rate than the other species studied. Although species with vessels were less resistant to cavitation, some recovered hydraulic conductivity within 12 h of rehydration. Species with xylem tracheids could maintain their hydraulic function through resistance to cavitation and by relaxing xylem tension. Although species with vessels had less resistant xylem, they could maintain hydraulic function through hydraulic recovery even when xylem dysfunction occurred. Additionally, the species studied here were less resistant to cavitation than species in semi-arid environments, and equally or less resistant than species in humid environments. Rather than allow hydraulic dysfunction due to drought-induced dehydration, they may develop water absorption systems to avoid or recover quickly from hydraulic dysfunction. Thus, not only stem cavitation resistance to drought but also stem–root coordination should be considered when selecting plants for the revegetation of arid regions.     

Continuous measurement of macronutrient ions in the transpiration stream of intact plants using the meadow spittlebug coupled with ion chromatography

A method is described for continuous, nondestructive analysis of xylem-borne mineral nutrients in intact transpiring plants. The method uses the xylem-feeding insect the meadow spittlebug (Philaenus spumarius L. [Homoptera: Cercopidae]). This insect will feed from a wide range of plant species and organs. Insect excreta can be collected at all times of the day and night, and its mineral ion content can be analyzed rapidly, and without purification, by ion chromatography. The excreta will have a mineral content virtually identical to that of xylem sap. Cages suitable for containing the insects and collecting excreta from any desired location on plants in both laboratory and greenhouse are described. Even in the greenhouse, evaporation had only a minor effect on the sample ion content. Example results are presented which illustrate dynamics, over several days, in the xylem concentrations of sodium (Na(+)), potassium (K(+)), NH(4)(+), magnesium (Mg(2+)), calcium (Ca(2+)), chloride (Cl(-)), NO(3)(-), PO(4)(3-), and SO(4)(2-). These data were collected from young plants growing in pots of compost in the laboratory and from fully mature pepper (Capsicum annuum L. cv Bellboy) plants growing in hydroponics (rockwool) in the greenhouse. This method should facilitate studies of macronutrient uptake and transport in a range of plants and environments.     

Transpiration inhibition by stored xylem sap from well-watered maize plants

There is increasing evidence that a chemical signal exists in xylem sap of plants subjected to water deficits which influences physiological responses in plant shoots. An important method of studying this signal is the transpiration response of excised leaves exposed to xylem sap collected from plants. However, Munns et al [Plant, Cell & Environment 16, 867–877] cautioned that transpiration inhibition is observed when xylem sap collected from wheat and barley is stored before determining physiological activity. The objective of the study reported here was to determine if transpiration inhibition develops in maize sap collected from well-watered plants when the sap is stored under various conditions. It was found that storage of maize sap collected from well-watered plants for only 1 d at -20°C resulted in the development of substantial transpiration inhibition in bioassay leaves. Storage of sap at 4°C resulted in the development of the effect after 2 weeks, while storage at ?86°C showed only small transpiration inhibition after 3 weeks. The major source of the transpiration inhibition was the development of a substance in the stored sap that resulted in physical blockage of the transpiration stream in bioassay leaves. However, a small signal component may also have developed in the stored sap. Because of the possibility of ionic activity under freezing conditions at ?20°C, calcium was studied for its potential involvement in the transpiration inhibition. However, the calcium concentrations found to inhibit transpiration were nearly an order of magnitude larger than the calcium concentrations observed in xylem sap.     

Phytochrome A increases tolerance to high evaporative demand

Stresses resulting from high transpiration demand induce adjustments in plants that lead to reductions of water loss. These adjustments, including changes in water absorption, transport and/or loss by transpiration, are crucial to normal plant development. Tomato wild type (WT) and phytochrome A (phyA)-mutant plants, fri1-1, were exposed to conditions of either low or high transpiration demand and several morphological and physiological changes were measured during stress conditions. Mutant plants rapidly wilted compared to WT plants after exposure to high evaporative demand. Root size and hydraulic conductivity did not show significant differences between genotypes, suggesting that water absorption and transport through this organ could not explain the observed phenotype. Moreover, stomatal density was similar between genotypes, whereas transpiration and stomatal conductance were both lower in mutant than in WT plants. This was accompanied by a lower stem-specific hydraulic conductivity in mutant plants, which was associated to lower xylem vessel number and transversal area in fri1-1 plants, producing a reduction in water supply to the leaves, which rapidly wilted under high evaporative demand. PhyA signaling might facilitate the adjustment to environments differing widely in water evaporative demand in part through the modulation of xylem dimensions.     

Control of Leaf Expansion Rate of Droughted Maize Plants under Fluctuating Evaporative Demand (A Superposition of Hydraulic and Chemical Messages?)

We have analyzed the possibility that chemical signaling does not entirely account for the effect of water deficit on the maize (Zea mays L.) leaf elongation rate (LER) under high evaporative demand. We followed time courses of LER (0.2-h interval) and spatial distribution of elongation rate in leaves of either water-deficient or abscisic acid (ABA)-fed plants subjected to varying transpiration rates in the field, in the greenhouse, and in the growth chamber. At low transpiration rates the effect of the soil water status on LER was related to the concentration of ABA in the xylem sap and could be mimicked by feeding artificial ABA. Transpiring plants experienced a further reduction in LER, directly linked to the transpiration rate or leaf water status. Leaf zones located at more than 20 mm from the ligule stopped expanding during the day and renewed expansion during the night. Neither ABA concentration in the xylem sap, which did not appreciably vary during the day, nor ABA flux into shoots could account for the effect of evaporative demand. In particular, maximum LER was observed simultaneously with a minimum ABA flux in the droughted plants, but with a maximum ABA flux in ABA-fed plants. All data were interpreted as the superposition of two additive effects: the first involved ABA signaling and was observed during the night and in ABA-fed plants, and the second involved the transpiration rate and was observed even in well-watered plants. We suggest that a hydraulic signal is the most likely candidate for this second effect.     

Quantification of the Daily Cytokinin Transport from the Root to the Shoot of Urtica dioica L.*

Cytokinins are predominantly root-born phytohormones which are distributed in the shoot via the xylem stream. In the hormone message concept they are considered as root signals mediating the transport of the photosynthates to the various sinks of a plant. In this paper the cytokinin relations of Urtica dioica L., the stinging nettle, are described, based on the daily flux from the roots to the shoot. Trans-zeatin-type cytokinins predominate in the various tissues of Urtica (Wagner and Beck, 1993), and accordingly trans-zeatin riboside and trans-zeatin are the forms transported by the xylem sap. The daily time-course of cytokinin concentration in root pressure exudates and in xylem sap collected from a petiole after pressurizing the root bed showed high concentrations in the morning, followed by a substantial drop to a level of 15–30% of the initial concentration which was then maintained during the afternoon. This time-course is interpreted as resulting from continuous synthesis and exudation of cytokinins into the xylem fluid of the roots whose cytokinin concentration is then modified by the dynamics of the transpiration stream. Loading of cytokinins into the xylem sap could be enhanced several times by increasing the flux rate of the xylem stream to the maximal transpiration rate when a maximum export rate was reached. The total daily cytokinin gain by the shoot depended on the nitrogen status of the plant. Roots of Urtica plants grown on a sufficient nitrogen supply had a significantly higher cytokinin content and exuded more cytokinins into the shoot than those of plants raised under nitrogen shortage. A positive correlation was found between the steady rates of cytokinin export measured during the afternoon and the shoot to root-ratios of biomass which, in turn, corresponded to the nitrogen status of the plants.     

Some Aspects of Translocation in Root Nodule Plants

By the use of ¹⁶N it is shown that the removal, over a shortzone at the base of the shoot of a typical root nodule plant(alder), of the tissues external to the xylem does not interferewith the upward movement of fixed nitrogen from the nodulesinto the shoot. It is concluded that the fixed nitrogen, whichis probably in organic form, can be translocated in the xylemin the transpiration stream, and that this is most likely itsnormal route. It is also shown that in unringed plants substantialenrichment in fixed ¹⁶N is detectable in the shoot within 6hours from the commencement of the exposure of the nodules toexcess free ¹⁶N. Structural and experimental evidence showsthat the alder nodule is not a water-absorbing organ, and themechanism of transfer of fixed nitrogen from the nodule intothe transpiration stream is not obvious.     

Zoophytophagous pentatomids feeding on plants and implications for biological control

Zoophytophagous insects can feed on a variety of prey, plants and plant products. By studying the interactions between predatory hemipterans and plants harbouring the prey of these insects, scientists have started to establish two potential outcomes: (1) positive effects like the enhancement of their life history characteristics by acquiring plant contents; and (2) negative effects mediated by plant resistance to herbivores or prey ingesting secondary plant metabolites. Despite this research, there is a lack of information about the feeding sites of predatory hemipterans on their host plants, what they ingest from plants, and whether they cause damage to their host plants. The results presented here indicate that the xylem is one of the feeding sites of predatory hemipterans on plants. The dissection of predators that fed on plants with marked vessels and testing insects for the presence of Cry protein constitutively expressed in the cytoplasm of plant cells revealed that bugs are not able to acquire cytoplasm contents from the plant cell. In addition, we demonstrate that systemic insecticide circulating inside plants from soil applications contaminates these predators. Our results are discussed in the context of zoophytophagous feeding behaviour exhibited by predatory hemipterans and the use of systemic insecticides for the conservation of natural enemies. This interaction contradicts the concept of ecological selectivity obtained for natural enemies through the placement of systemic insecticide in the soil as a selective method of deploying chemical control and predatory hemipteran conservation within the integrated pest management framework.     

Assimilate Transport in the Xylem Sap of Pedunculate Oak (Quercus robur) Saplings

Abstract: The rates of photosynthesis and transpiration, as well as the concentrations of organic compounds (total soluble non-protein N compounds [TSNN], soluble carbohydrates), in the xylem sap were determined during two growth seasons in one-year-old Quercus robur saplings. From the data, the total C gain of the leaves, by both photosynthesis and the transpiration stream, was calculated. Large amounts of C were allocated to the leaves by the transpiration stream; depending on the time of day and the environmental conditions the portion of C originating from xylem transport amounted to 8 to 91% of total C delivery to the leaves. Particularly under conditions of reduced photosynthesis, e.g., during midday depression of photosynthesis, a high percentage of the total C delivery was provided to the leaves by the transpiration stream (83 to 91 %). Apparently, attack by phloem-feeding aphids lowered the assimilate transport from roots to shoots; as a consequence the portion of C available to the leaves from xylem transport amounted to only 12 to 16 %. The most abundant organic compounds transported in the xylem sap were sugars (sucrose, glucose, fructose) with concentrations of ca. 50 to 500 μmol C ml^-1, whereas C from N compounds was of minor significance (3 to 20 μmol ml^-1 C). The results indicate a significant cycling of C in the plants because the daily transport of C with the transpiration stream exceeded the daily photosynthetic CO₂ fixation in several cases. This cycling pool of C may sustain delivery of photosynthate to heterotrophic tissues, independent of short time fluctuations in photosynthetic CO₂ fixation.