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The uptake of the organophosphates tris(2-chloroethyl) phosphate (TCEP), tris(1-chloro-2-propyl) phosphate (TCPP), tributyl phosphate (TBP), the insect repellant N,N-diethyl toluamide (DEET), and the plasticizer n-butyl benzenesulfonamide (NBBS) into plants was studied in greenhouse experiments and simulated with a dynamic physiological plant uptake model. The calibrated model was coupled to a tipping buckets soil transport model and a field scenario with sewage sludge application was simulated. High uptake of the polar, low-volatile compounds TCEP, TCPP, and DEET into plants was found, with highest concentrations in straw (leaves and stem). Uptake into carrot roots was high for TCPP and TBP. NBBS showed no high uptake but was rapidly degraded. Uptake into barley seeds was small. The pattern and levels of uptake could be reproduced by the model simulations, which indicates mainly passive uptake and transport (i.e., by the transpiration stream, with the water) into and within the plants. Also the field simulations predicted a high uptake from soil into plants of TCEP, TCPP, and DEET, while TBP is more likely taken up from air. The BCF values measured and calculated in the greenhouse study are in most cases comparable to the calculated values of the field scenario, which demonstrates that greenhouse studies can be suitable for predicting the behavior of chemicals in the field. Organophosphates have a high potential for bioaccumulation in crops and reach agricultural fields both via sewage sludge and by atmospheric deposition.

It is now acknowledged that food-borne pathogens present in the irrigation water or soil can become associated with crop plants in the field, penetrate internal plant tissues via the root, translocate and survive inside plants. Only little information is available concerning interaction between enteric pathogens and plants. The present study evaluated the potential for contamination of the aromatic plant, sweet basil during cultivation, by Salmonella enterica serovar Newport. Root internalization was plant-age-dependent, with the highest susceptibility occurring at the beginning of the rapid growth phase of the root. Higher incidence of internalization was detected in vegetative than reproductive plant organs, pointing at bacterial transport in the transpiration stream. Internalized Salmonella survived only

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 Munch 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.

Since its inception, the Farquhar et al. (1980) model of photosynthesis has been a mainstay for relating biochemistry to environmental conditions from chloroplast to global levels in terrestrial plants. Many variables could be assigned from basic enzyme kinetics, but the model also required measurements of maximum rates of photosynthetic electron transport (J max ), carbon assimilation (Vcmax ), conductance of CO2 into (g s ) and through (g m ) the leaf, and the rate of respiration during the day (R d ). This review focuses on improving the accuracy of these measurements, especially fluxes from photorespiratory CO2, CO2 in the transpiration stream, and through the leaf epidermis and cuticle. These fluxes, though small, affect the accuracy of all methods of estimating mesophyll conductance and several other photosynthetic parameters because they all require knowledge of CO2 concentrations in the intercellular spaces. This review highlights modified methods that may help to reduce some of the uncertainties. The approaches are increasingly important when leaves are stressed or when fluxes are inferred at scales larger than the leaf.

Humic substances (HS) play important roles in the biotic-abiotic interactions of the root plant and soil contributing to plant adaptation to external environments. However, their mode of action on plants remains largely unknown. In this study the HS distribution in tissues of wheat seedlings was examined using tritium-labeled humic acid (HA) derived from leonardite (a variety of lignites) and microautoradiography (MAR). Preferential accumulation of labeled products from tritiated HA was found in the roots as compared to the shoots and endodermis was shown to be the major control point for radial transport of label into vascular system of plant. Tritium was also found in the stele and xylem tissues indicating that labeled products from tritiated HA could be transported to shoot tissues via the transpiration stream. Treatment with HA lead to an increase in the content of polar lipids of photosynthetic membranes. The observed accumulation of labeled HA products in root endodermis and positive impact on lipid synthesis are consistent with prior reported observations on physiological effects of HS on plants such as enhanced growth and development of lateral roots and improvement/repairs of the photosynthetic status of plants under stress conditions.

Salinity and drought adversely affect rice production globally. Here we have examined physiological responses to drought and salinity across four rice cultivars with varying sensitivity to these stresses. The salt tolerant Pokkali restricts fluid entry to limit Na+ uptake under saline stress, while the drought-tolerant ARB6 needs to enhance fluid uptake under drought. Surprisingly, Pokkali does reasonably well when subjected to drought as does ARB6 under saline stress-in contrast to the stress-sensitive but high yielding varieties IR-20 and Jaya. Both tolerant varieties use long roots to mine water under deficit conditions, increasing aerenchyma and suberization of the exodermis to provide oxygen to deep-reaching roots. Major alterations in patterns of suberization in both exodermis and endodermis are undertaken, the patterns being dramatically different under the two stresses. Genes implicated in suberin biosynthesis also showed variation in transcript levels under stress, corresponding with the observed suberization patterns. Osmolyte accumulation drives uptake of water under deficit conditions, while restricting fluid flow to symplastic routes minimizes Na+ entry. Overall, the morphological and physiological responses of the tolerant varieties ensure adequate fluid flow through the transpiration stream without excessive salt uptake, thereby promoting growth under both drought and salinity.

The gradient of leaf traits in a canopy from sunlit upper regions to shaded lower ones is regulated in response to the density of its leaf area. The gradients of environmental factors act as signals for the regulation. The result is improved resource use efficiency for carbon gain at the whole plant level. Herbaceous species with relatively fast leaf turnover typically grow new leaves at the top in high light that are subsequently progressively shaded in developing dense canopies. Export of resources associated with photosynthetic capacity accompanies the progressive shading, later on followed by degradation of light harvesting components when senescence is induced. The red:far-red ratio of the light gradient is involved in the reallocation of resources and the induction of leaf senescence, but the irradiance component of the light gradient dominates the canopy effect. It impacts a multitude of physiological processes. Their effect can operate locally such as perception by photoreceptors and excitation pressure implicated in chloroplast organization. Other effects impact processes operating at the whole plant level such as the distribution of signaling compounds in the transpiration stream and the supply of assimilates to developing young leaves. These systemically operating pathways are at the basis of a coordinated response of plants to the shading effect in a canopy gradient, which is different from whole plant shading. The available evidence for mechanisms involved in the regulation of leaf traits in canopies is discussed.

Tipburn is necrosis at the leaf apex of young developing leaves and the major cause is often considered to be Ca2 + deficiency. However, tipburn is caused not only by Ca2 + deficiency in the plant, but also the interaction of various factors. The inhibition of Ca2 + absorption is affected by pH, root temperature, and water stress. Ca2 + is transferred from root to shoot by the transpiration stream, so an environment in which transpiration is suppressed prevents this transfer. Moreover, there is competition for Ca2 + distribution among different tissues of the plants. Thus, in order to prevent tipburn, it is important to understand the various factors that cause it.

Upward transport of CO₂ via the transpiration stream from belowground to aboveground tissues occurs in tree stems. Despite potentially important implications for our understanding of plant physiology, the fate of internally transported CO₂ derived from autotrophic respiratory processes remains unclear. We infused a ¹³CO₂-labeled aqueous solution into the base of 7-yr-old field-grown eastern cottonwood (Populus deltoides) trees to investigate the effect of xylem-transported CO₂ derived from the root system on aboveground carbon assimilation and CO₂ efflux. The ¹³C label was transported internally and detected throughout the tree. Up to 17% of the infused label was assimilated, while the remainder diffused to the atmosphere via stem and branch efflux. The largest amount of assimilated ¹³C was found in branch woody tissues, while only a small quantity was assimilated in the foliage. Petioles were more highly enriched in ¹³C than other leaf tissues. Our results confirm a recycling pathway for respired CO₂ and indicate that internal transport of CO₂ from the root system may confound the interpretation of efflux-based estimates of woody tissue respiration and patterns of carbohydrate allocation.

Boron (B) is known to accumulate in the leaf margins of different plant species, arguably a passive consequence of enhanced transpiration at the ends of the vascular system. However, transpiration rate is not the only factor affecting ion distribution. We examine an alternative hypothesis, suggesting the participation of the leaf bundle sheath in controlling radial water and solute transport from the xylem to the mesophyll in analogy to the root endodermis. In banana, excess B that remains confined to the vascular system is effectively disposed of via dissolution in the guttation fluid; therefore, impairing guttation should aggravate B damage to the leaf margins. Banana plants were subjected to increasing B concentrations. Guttation rates were manipulated by imposing a moderate osmotic stress. Guttation fluid was collected and analysed continuously. The distribution of ions across the lamina was determined. Impairing guttation indeed led to increased B damage to the leaf margins. The kinetics of ion concentration in guttation samples revealed major differences between ion species, corresponding to their distribution in the lamina dry matter. We provide evidence that the distribution pattern of B and other ions across banana leaves depends on active filtration of the transpiration stream and on guttation.

The DO3SE (Deposition of O3 for Stomatal Exchange) model is an established tool for estimating ozone (O3) deposition, stomatal flux and impacts to a variety of vegetation types across Europe. It has been embedded within the EMEP (European Monitoring and Evaluation Programme) photochemical model to provide a policy tool capable of relating the flux-based risk of vegetation damage to O3 precursor emission scenarios for use in policy formulation. A key limitation of regional flux-based risk assessments has been the assumption that soil water deficits are not limiting O3 flux due to the unavailability of evaluated methods for modelling soil water deficits and their influence on stomatal conductance (gsto), and subsequent O3 flux. This paper describes the development and evaluation of a method to estimate soil moisture status and its influence on gsto for a variety of forest tree species. This DO3SE soil moisture module uses the Penman-Monteith energy balance method to drive water cycling through the soil-plant-atmosphere system and empirical data describing gsto relationships with pre-dawn leaf water status to estimate the biological control of transpiration. We trial four different methods to estimate this biological control of the transpiration stream, which vary from simple methods that relate soil water content or potential directly to gsto, to more complex methods that incorporate hydraulic resistance and plant capacitance that control water flow through the plant system. These methods are evaluated against field data describing a variety of soil water variables, gsto and transpiration data for Norway spruce (Picea abies), Scots pine (Pinus sylvestris), birch (Betula pendula), aspen (Populus tremuloides), beech (Fagus sylvatica) and holm oak (Quercus ilex) collected from ten sites across Europe and North America. Modelled estimates of these variables show consistency with observed data when applying the simple empirical methods, with the timing and magnitude of soil drying events being captured well across all sites and reductions in transpiration with the onset of drought being predicted with reasonable accuracy. The more complex methods, which incorporate hydraulic resistance and plant capacitance, perform less well, with predicted drying cycles consistently underestimating the rate and magnitude of water loss from the soil. A sensitivity analysis showed that model performance was strongly dependent upon the local parameterisation of key model drivers such as the maximum gsto, soil texture, root depth and leaf area index. The results suggest that the simple modelling methods that relate gsto directly to soil water content and potential provide adequate estimates of soil moisture and influence on gsto such that they are suitable to be used to assess the potential risk posed by O3 to forest trees across Europe.

Despite a long history of observations of the hemiparasitic plant, mistletoe, the mechanism of mineral movement from the host to the mistletoe is still not fully understood. In this article, we focused on the leaf development and nutrient dynamics of Loranthus europaeus and the host tree, Quercus pubescens. The nitrogen, potassium and calcium contents of leaves, current-year twigs and 1-year-old twigs were analysed. The timing of the leaf development differed between species. Leaf expansion occurred in the first 23 and 136 days, and leaf senescence took 78 and 24 days for Quercus and Loranthus, respectively. The similar nitrogen concentrations per unit leaf area may indicate that both species have the same assimilation rate. The differences in nutrient accumulation seem to support the hypothesis that nitrogen is the limiting nutrient in the transpiration stream. Larger differences in the nutrient dynamics between species were revealed in the accumulation potassium and calcium. Nutrients seemed to be transferred passively through the xylem sap between Loranthus and Quercus as we found a strong correlation between the calcium and potassium concentrations within the species and between the species. There was no correlation in the case of 1-year-old twigs, possibly due to the relatively small amount of nutrients incorporated into 1-year-old twigs and the fact that nutrient translocation occurs according the needs of the physiologically more active leaves and current-year-old twigs.

Trees,especially big ones,have huge and complicated vascular systems which make the long-distance water transport difficult to study.We used the heat dissipation method and deuterated water as tracers to characterize water transport and storage properties of Quercus variabilis trees under the different water stress treatment.The transit time for D2O transport from the trunk base to the upper branches and the tracer residence time were determined by measuring hydrogen isotope ratios in water extracted from branches sampled in regular intervals.The result indicated that the maxima δD of full irrigation(FI),mild water stress(LWS) and severe water stress(SWS) respectively increased to 586.67‰、997.33‰ and 1364.89‰ after injecting deuterium tracer.There were significant differences in maxima δD among the three treatments.The tracer velocities with LWS and SWS were significantly lower than FI treatment but the tracer half-life and residence time were longer than FI treatment.The branch PLC,sapflow rate,water potential and transpiration rate were also observed.The branch PLC treated with LWS and SWS was significantly higher than FI treatment and the sapflow rate,water potential and transpiration rate were lower.There significantly positive correlations between tracer velocity and leaf transpiration rate(P0.01,n=27),and between tracer velocity and sapflow rate(P0.01,n=9).The results indicated that water transport was determined by transpiration rate.The PLC and branch water potential were inversely correlated with tracer velocity(P0.01,n=27),suggesting that drought stress-induced embolism and lower water potential should increase resistance to water transport.Tracer half-life and residence time increased as water stress increased.Tracer half-life and residence time were significant exponential functions with the branch PLC(P0.01,n=27),but power functions with the leaf transpiration rate.The times needed for transporting the same volume tracer were increased,indicating that water transport efficiencies were reduced because of PLC,transpiration and storage water variations under water-stress condition.Analyses of sapflow showed that the calculative fluxes of drought treatments within tracer half-life and residence time were significantly higher than FI treatment.The amounts of water carrying the same volume tracer were increased as stress degree increased.The results indicated that the increased water was possibly stored in the sapwood and re-exchanged to transpiration stream when transpiration was strong in the daytime.Quercus variabilis trees should reduce the water transport velocity and efficiency to adapt water stress environment by xylem embolism and storage-water exchange.

In this Special Issue, a focus is also placed on the role of the xylem as an essential conduit for the long-distance delivery of water and mineral nutrients from the soil to the vegetative (above-ground) regions of the plant. Xylem cells destined to form tracheids or vessel members, which will make up the conduit for this water and mineral transport from the roots to the shoots, undergo apoptosis, a process of programmed cell death. In their review, Venturas et al. (2017) provide an in-depth analysis of the processes underlying the passage of water through these mature, and thus, dead, xylem conducting cells. They elegantly describe the physics associated with this transport of water from the soil to the above-ground tissues and organs of the plant. A tensional gradient, within the water column, pulls the transpiration stream through the plant and Venturas et al. (2017) address the challenges that this presents to the plant, including the unthinkable, in that the water column can rupture, a process termed cavitation. The authors provide an up-to-date analysis of the debate as to how plants might refill such cavitated xylem cells, an important topic with respect to tissue hydraulics. This review closes with an insightful section on the impact of climate change on xylem function.

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.

Respiration in forest tree stems is an important component of the global carbon cycle. This respiration is traditionally estimated by measurements of the CO(2) efflux from the stem. However, recent studies have suggested that movement of CO(2) in the transpiration stream causes large errors in the respiration estimated by the CO(2) efflux. Here we demonstrate a new approach for determining the ratio of respiration to CO(2) efflux, which is based on specially designed chambers, and combined CO(2) and O(2) measurements. The high accuracy O(2) measurement is performed by mass spectrometric measurement of the O(2)/Ar ratio. Testing the method gave repeatable results which point that in some conditions up to 40% of the respired CO(2) can be carried away from the site of respiration.

We studied water use in a common bamboo species (Bambusa blumeana J.A. and J.H. Schultes) in the Philippines with the aim to (1) estimate bamboo water use and its dependence on environmental factors, (2) evaluate internal water storage and water dynamics and (3) compare water use characteristics of this bamboo species with those of co-occurring tree species. Two thermal sap flow methods were applied and complemented with a deuterium tracing experiment. Sap flow measured using the stem heat balance method (SHB) was in agreement with simultaneous flow measurements from thermal dissipation probes (TDP) which were used for long term measurements in this study. Maximal sap flux densities measured at the culm base using the TDP method were up to 25.7 g cm−2 h−1, but can be 2–3 times higher at other positions along the culm due to changes in the culm wall cross-section. Maximal water use rates of bamboo culms were on average 12 kg d−1, corresponding to a maximal transpiration rate of 1.4 mm d−1 at the clump level. These values are in line with those of co-occurring tree species, but bamboo tends to limit water use more under reduced soil water availability than most co-occurring tree species. Deuterium added to the transpiration stream at the culm base travelled upwards more slowly than in trees, leading to maximal deuterium levels in the canopy at the 3rd or 6th day after labelling, whereas this was the 1st or 2nd day for trees. This may indicate higher water storage capacities in bamboo relative to its water use rates, although diurnal patterns of sap flux density did not support this interpretation.

There are large gaps in our understanding of parasite–host nutrient relationships. Our goal was to evaluate transdermal water loss, parasite–host mineral relationships, and heterotrophy in the holoparasitic genus Hydnora. We estimated in situ transdermal water loss in Hydnora and measured nutrient profiles and δ 13 C and δ 15 N signatures for Hydnora and hosts in southern Africa and Madagascar. For comparison we also measured δ 13 C and δ 15 N for aerial hemiparasites at the same sites. Transdermal water loss in Hydnora ranged from 0.14 ± 0.02 to 0.38 ± 0.04 mg cm −2 h −1 and was comparable to transpiration rates for water conservative xerophytes. Concentrations of P and K were higher in Hydnora relative to CAM hosts; other mineral concentrations were significantly lower in the parasite or were not different. δ 13 C signatures of holoparasites and hemiparasites relative to their hosts reflected host metabolism and differences in commitment to heterotrophic C gain. Holoparasite δ 13 C values were significantly enriched (by 0.55‰ ± 0.23) compared to host shoot and depleted compared to host root tissues (by −0.97‰ ± 0.12). Holoparasite δ 13 C values were not significantly different compared to the estimated whole host δ 13 C value. δ 15 N values for holoparasites and hemiparasites were significantly correlated with hosts. The water conservative nature of Hydnora spp. combined with parasite–host mineral nutrition profiles are suggestive of active processes of solute uptake. Stable isotope fractionation in host tissues dictated significant differences between parasite and host (shoot and root) δ 13 C signatures. The confirmation of complete heterotrophy and the lack of a confounding transpiration stream may make Hydnora a promising model organism for the examination of parasite solute uptake.

Hyperaccumulators store accumulated metals in the vacuoles of large leaf epidermal cells (storage cells). For investigating cadmium uptake, we incubated protoplasts obtained from leaves of Thlaspi caerulescens (Ganges ecotype) with a Cd-specific fluorescent dye. A fluorescence kinetic microscope was used for selectively measuring Cd-uptake and photosynthesis in different cell types, so that physical separation of cell types was not necessary. Few minutes after its addition, cadmium accumulated in the cytoplasm before its transport into the vacuole. This demonstrated that vacuolar sequestration is the rate-limiting step in cadmium uptake into protoplasts of all leaf cell types. During accumulation in the cytoplasm, Cd-rich vesicle-like structures were observed. Cd uptake rates into epidermal storage cells were higher than into standard-sized epidermal cells and mesophyll cells. This shows that the preferential heavy metal accumulation in epidermal storage cells, previously observed for several metals in intact leaves of various hyperaccumulator species, is due to differences in active metal transport and not differences in passive mechanisms like transpiration stream transport or cell wall adhesion. Combining this with previous studies, it seems likely that the transport steps over the plasma and tonoplast membranes of leaf epidermal storage cells are driving forces behind the hyperaccumulation phenotype.

The terrestrial hydrological cycle is strongly influenced by transpiration—water loss through the stomatal pores of leaves. In this report we present studies showing that the energy content of radiation absorbed by the leaf influences stomatal control of transpiration. This observation is at odds with current concepts of how stomata sense and control transpiration, and we suggest an alternative model. Specifically, we argue that the steady-state water potential of the epidermis in the intact leaf is controlled by the difference between the radiation-controlled rate of water vapor production in the leaf interior and the rate of transpiration. Any difference between these two potentially large fluxes is made up by evaporation from (or condensation on) the epidermis, causing its water potential to pivot around this balance point. Previous work established that stomata in isolated epidermal strips respond by opening with increasing (and closing with decreasing) water potential. Thus, stomatal conductance and transpiration rate should increase when there is condensation on (and decrease when there is evaporation from) the epidermis, thus tending to maintain homeostasis of epidermal water potential. We use a model to show that such a mechanism would have control properties similar to those observed with leaves. This hypothesis provides a plausible explanation for the regulation of leaf and canopy transpiration by the radiation load and provides a unique framework for studies of the regulation of stomatal conductance by CO 2 and other factors.

Two common sorrel (Rumex acetosa) accessions, one from a Zn–Pb contaminated site (CS accession) and the other from an uncontaminated site (UCS accession), were hydroponically exposed to a mixture of heavy metals (Pb2+ + Zn2+ + Cd2+) with and without EDTA at an equimolar rate. The metallicolous CS accession showed a higher tolerance to metal treatment in the absence of the chelating agent, whereas the UCS accession was especially tolerant to EDTA treatment alone. Combination of metal and EDTA treatment resulted in a higher Pb accumulation in shoots of both accessions although plants hardly showed phytotoxic symptoms. Cd and Zn uptake was not augmented by EDTA addition to the polymetallic medium. Chelant-assisted Pb accumulation was 70% higher in the CS accession than in the UCS accession, despite the fact that the former accession evapotranspired less water than the UCS accession. These results support the existence of a non-selective apoplastic transport of metal chelates by R. acetosa roots, not related to transpiration stream.

To investigate the intrinsic bioavailability of heptachlor to various plant species, the root of each test plant was treated with hydroponic solution containing 14C-heptachlor for 72 hr. The levels and nature of 14C-residues in roots and shoots of treated plants were determined to estimate the values of the root concentration factor (RCF) and the transpiration stream concentration factor (TSCF). RCF and TSCF values ranged from 126 (corn) to 4086 (lettuce) and 0.052 (Chinese cabbage) to 0.494 (zucchini), respectively. In pumpkin and zucchini, unmetabolized heptachlor itself was translocated from the root to the shoot via a transpiration stream and no further extensive metabolism was observed. cis-Heptachlor epoxide was the predominant metabolite in the shoots of tomato, cabbage, Chinese cabbage and lettuce. Less metabolism of cis-heptachlor epoxide was suggested in the shoots of these plants.

Fluxes of mineral nutrients in the xylem are strongly influenced by interactions with the surrounding stem tissues and are probably regulated by them. Toward a mechanistic understanding of these interactions, we applied stable isotope tracers of magnesium, potassium, and calcium continuously to the transpiration stream of cut bean (Phaseolus vulgaris) shoots to study their radial exchange at the cell and tissue level with stem tissues between pith and phloem. For isotope localization, we combined sample preparation with secondary ion mass spectrometry in a completely cryogenic workflow. After 20 min of application, tracers were readily detectable to various degrees in all tissues. The xylem parenchyma near the vessels exchanged freely with the vessels, its nutrient elements reaching a steady state of strong exchange with elements in the vessels within 20 min, mainly via apoplastic pathways. A slow exchange between vessels and cambium and phloem suggested that they are separated from the xylem, parenchyma, and pith, possibly by an apoplastic barrier to diffusion for nutrients (as for carbohydrates). There was little difference in these distributions when tracers were applied directly to intact xylem via a microcapillary, suggesting that xylem tension had little effect on radial exchange of these nutrients and that their movement was mainly diffusive.

Stem respiration rates are often quantified by measuring the CO(2) efflux from stems into chambers. It has been suggested that these measurements underestimate respiration because some of the respired CO(2) can be either retained or transported upwards in the transpiration stream. If the stem CO(2) efflux does not represent all respired CO(2), then the interpretation of its isotopic signal may be compromised as well. The C-isotope composition of the respired CO(2) and the measured efflux could differ due to (i) the release of CO(2) produced elsewhere into the stem and transported upwards in xylem water (soil CO(2) or root respired CO(2)); (ii) the retention or release of CO(2) storage pools within the tree stem and (iii) the removal of CO(2) by the transpiration stream. We investigated the effects of these processes in large conifer trees using two manipulative experiments: a labelling experiment and a crown removal experiment. The labelling experiment used an extreme enrichment of dissolved CO(2) in soil water to assess the C uptake by the roots. In this experiment, we found no contamination of the stem CO(2) pool despite clear evidence that the water itself had been taken up. The crown removal experiment tested for vertical CO(2) flux in xylem water by eliminating transpiration. Here, we found no change in the delta(13)C of stem CO(2) efflux (delta(EA); P > 0.05). We concluded that for these large conifers, sap-flow influenced neither delta(13)C of stem efflux nor that of the stem CO(2) pool. By parameterizing Henry's Law for conditions inside the stem, we estimated the transport flux to represent 1-3% of the stem CO(2) efflux to the atmosphere. Finally, assuming a 2 per thousand difference between delta(13)C of root and stem respiration, we estimated that potential contamination of delta(EA) by root respired CO(2) would be < 0.1 per thousand. Thus, neither the release of soil or root CO(2), nor storage in the stem, nor vertical transport of CO(2) in the xylem sap had any detectable influence on delta(13)C of the CO(2) measured in stem efflux.

Embolism results in a dramatic loss of xylem hydraulic transport capacity that can lead to decreased plant productivity and even death. The ability to refill embolized conduits despite the presence of tension in the xylem seems to be widespread, but how this occurs is not known. To promote discussion and future research on this topic, we describe how we believe refilling under tension might take place. Our scenario includes: (i) an osmotic role for low-molecular weight sugars; (ii) an apoplastic sugar-sensing mechanism to activate refilling; (iii) the contribution of vapor transport in both the influx of water and removal of entrapped gases; and (iv) the need for a mechanism that can synchronize reconnection to the transpiration stream through multiple bordered pits.

Root uptake and subsequent translocation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in 12 agricultural crops were comparatively investigated. All crop plants were exposed hydroponically to a mixture of three kinds of dioxin congeners over 4 d. The root concentration factor (RCF) of dioxin showed a logarithmic correlation with extractable lipid content in plant root. On the assumption that the dioxin escaping via gas phase from nutrient solution in the closed container can evenly diffuse in the air and equally absorb onto the shoot tissues of the dioxin-exposed plant and their nearby blank control plant, the amount of translocated dioxin was estimated by subtracting dioxin content in the shoot tissues of the blank control plant from that of the dioxin-exposed plant, and then the transpiration stream concentration factor (TSCF) of dioxin was calculated. The TSCF values of PCDD/Fs largely varied according to the plant species, and the TSCF values of 2,4,8-TrCDF were a little higher than those for 1,3,6,8-TeCDD expect for zucchini. For 1,3,6,8-TeCDD, zucchini had the highest TSCF value of 0.0089, followed by pumpkin (0.0064) towel gourd (0.0027), and cucumber (0.0010), verifying plants of the genus Cucurbita have the higher abilities of dioxin translocation. The TSCF values of 1,3,6,8-TeCDD for wheat and sorghum were 0.0013 and 0.0012, respectively. For maize, soybean, rice, Chinese cabbage, tomato and garland chrysanthemum, translocation was an insignificant mechanism of dioxin contamination in shoot tissues.

The amount of water stored in the stem introduces uncertainty when estimating diurnal whole-tree transpiration (ET) and canopy stomatal conductance (GC) using sap flow measured at the base of the stem (Q). Thus, to examine how ET can be calculated from Q, we obtained ET using sap flow and stem water content measurements and a whole-tree water balance equation, and compared it with Q. In this study, we measured sap flows in 33-year-old individual trees of Cryptomeria japonica D. Don and Chamaecyparis obtusa Endl. using constant-heat sap flow probes. Sap flows were measured at several depths at the base of the stem, and at the upper trunk as a surrogate of ET. Stem water contents were measured at three vertical positions on the trunk using amplitude-domain reflectometry (ADR) sensors. We also measured sapwood volumes of the study trees. Using simultaneous sap flow and stem water content measurements along the tree stem, we confirmed that stem water storage has impacts on the transpiration stream. These include sap flow lags along the tree heights and an enhanced peak of transpiration from stem sap flow. These results enabled us to calculate the correct ET by multiplying Q by 1·18 and shifting its time series forward by 30 min. The ET value was then used to calculate GC for both tree species. The factor of 1·18 is based on the fact that at noon, the value of ET was higher than that of Q, due to the prolonged Q during the evening. Establishing the time lag was relatively simple and was determined by comparing Q and vapor pressure deficit. The multiplier is more challenging to ascertain due to the difficulty in obtaining ET correctly. Copyright © 2009 John Wiley & Sons, Ltd.

Salicylic acid (SA) and its glucoside (SAG) were detected in xylem sap of Brassica napus by HPLC-MS. Concentrations of SA and SAG in xylem sap from the root and hypocotyl of the plant, and in extracts of shoots above the hypocotyl, increased after infection with the vascular pathogen Verticillium longisporum. Both concentrations were correlated with disease severity assessed as the reduction in shoot length. Furthermore, SAG levels in shoot extracts were correlated with the amount of V. longisporum DNA in the hypocotyls. Although the concentration of SAG (but not SA) in xylem sap of infected plants gradually declined from 14 to 35 days post infection, SAG levels remained significantly higher than in uninfected plants during the whole experiment. Jasmonic acid (JA) and abscisic acid (ABA) levels in xylem sap were not affected by infection with V. longisporum. SA and SAG extend the list of phytohormones potentially transported from root to shoot with the transpiration stream. The physiological relevance of this transport and its contribution to the distribution of SA in plants remain to be elucidated. peerReviewed

Water deficit and salt accumulation in soil presents serious problems to crop production in semi-arid regions. These problems depend on the active transpiration stream and the selective absorption of ions by crop roots. In this study, a large sized soil column system was used to examine the dynamics of water and ion transport and salt accumulation in soil layers. Special reference was placed on the effects of the active and selective absorption by roots of different crops (i.e., corn plants, sunflower plants and no plants). The column system was equipped with on-line systems for the control of groundwater level. Soil water content sensors enabled time-course evaluations of the volumetric water content and hence upward flux of the groundwater in the soils at different depths. Furthermore, the distribution and accumulation of ions in soil layers, plant organs and xylem sap were analyzed using ion chromatography. In this column experiment, diurnal and longer term changes in water movement and ion accumulation in soil, affected by root absorption characteristics of plants, were evaluated quantitatively. The results demonstrated that the column system was applicable for the quantitative analysis of the effects of root absorption by different crops on water deficit and salinization in soils.

In order to analyze the salt transport affected by roots and its effects on soil salinity in an experimental irrigated field newly established in an alluvial valley of the Yellow River in China, spatial distribution of ions contained in waters, soils and crops relevant to these phenomena were evaluated there. During the intensive surveys conducted in year 2007–2008, the Yellow River water, irrigation canal water, groundwater, field soils and crops, etc. were sampled and their chemical characteristics such as electrical conductivity, concentrations of ions Na+, Ca2+, Mg2+, K+, Cl−, SO 4 2− and NO 3 − were measured. Irrigation seemed to cause increases in the concentrations of ions Na+, Cl− and SO 4 2− in the groundwater. Although those were also major ions contained in the field soil, the soil was classed as saline but not sodic according to the standard classification. On the other hand, K+, which is one of the major essential nutrients for plant growth, was highly concentrated in the crops, while Na+ was not concentrated because of crop’s poor ability to absorb it. The ion concentration within the plant body seemed to be reflected by the active and selective ion uptake by roots and the transpiration stream. Furthermore, salt accumulation in the surface-irrigated field largely depended on the upward transport of water and ions in the soil profile affected by root absorption capacity. The information obtained in this study will contribute to the development of scientific methods for sustainable and effective plant production in irrigated fields.

The thermal dissipation probe was described in the early 1930s for the demonstration of a volume and mass flow of sap in the conductive elements of the xylem in trees. It was subsequently developed further and is now widely used in physiological ecology including measurements in the field. Thermal dissipation demonstrates the occurrence of sap flow and allows determination of its velocity. Here we report simultaneous continuous measurements of sap flow using the thermal dissipation technique and of transpiration by infrared gas analysis for diurnal and annual cycles in a deciduous and an evergreen oak tree, Quercus robur L. and Quercus turneri Willd., respectively, in a deciduous and an evergreen conifer, Larix decidua Mill. and Pinus griffithii McClell., respectively, and the host/mistletoe consortium of the deciduous linden Tilia mandschurica Rupr. & Max. and the evergreen Viscum album L. We show (1) that in diurnal cycles sap flow closely follows dynamic changes of the rate of transpiration elicited by daily fluctuations of weather parameters (sunshine, cloudiness, air temperature and humidity), (2) that in annual cycles sap flow reflects autumnal yellowing and shedding of leaves of the deciduous trees. We report for the first time comparative measurements of sap flow towards mistletoe shoots and host branches in a parasite/host consortium. This demonstrates (3) that mistletoes maintain considerably larger sap flow rates in their xylem conduits than the adjacent host branches dragging the transpiration stream of their host towards their own shoots. We also show (4) that even after the deciduous host has shed its leaves and itself does not transpire any more the evergreen mistletoe towards its shoots can maintain the persistence of a continuous sap flow via the stem and branches of the host as long as frost does not prevent that. The work presented underlines the contention that transpiration is the driving force for sap flow with continuous files of water in the xylem. It shows for the first time that mistletoes direct the flow of water through host roots and stems towards its own shoots by not only performing stronger transpiration as it is known from the literature but also by maintaining larger sap flow rates in the xylem conduits of its stems.

Cadmium is a toxic pollutant, but the relationships between morphological modifications and Cd accumulation in different organs, as well as the physiological impact of accumulated Cd in leaves of different ages, remain an open question. We compared Cd concentration in the primary branches and main stems of the perennial shrub Zygophyllum fabago L. Six weeks of treatment with 10 mM Cd inhibited stem elongation and axillary bud development, and Cd accumulated to a higher level in the oldest leaves on the main stem than in ramification leaves. Although ramifications always contained lower amounts of Cd than main-stem leaves, ramification leaves were more sensitive to the pollutant. While Cd accumulation induced stomatal closure, a decrease in carbon isotope discrimination, and an increase in the water use efficiency of main-stem leaves, an inverse trend was recorded for primary-branch leaves. Cadmium induced an increase in chlorophyll concentration in the main stem but decreased it in lateral branches. Bypass flow, quantified by the 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt fluorescent dye, correlated significantly with Cd accumulation in leaves, suggesting that it contributes to Cd translocation through the transpiration stream. Our study demonstrates that Cd absorbed by the roots is unevenly distributed in the shoot leaves. Accumulated toxic ion, however, is not the only factor conditioning the organ response; for example, the physiological status of the tissues in relation to the leaf ontogeny may directly influence the extent of metabolic disorders resulting from Cd accumulation.

Sodium exclusion from leaves is an important mechanism for salt tolerance in durum wheat. To characterize possible control points for Na(+) exclusion, quantitative cryo-analytical scanning electron microscopy was used to determine cell-specific ion profiles across roots of two durum wheat genotypes with contrasting rates of Na(+) transport from root to shoot grown in 50 mm NaCl. The Na(+) concentration in Line 149 (low transport genotype) declined across the cortex, being highest in the epidermal and sub-epidermal cells (48 mm) and lowest in the inner cortical cells (22 mm). Na(+) was high in the pericycle (85 mm) and low in the xylem parenchyma (34 mm). The Na(+) profile in Tamaroi (high transport genotype) had a similar trend but with a high concentration (130 mm) in the xylem parenchyma. The K(+) profiles were generally inverse to those of Na(+). Chloride was only detected in the epidermis. These data suggest that the epidermal and cortical cells removed most of the Na(+) and Cl(-) from the transpiration stream before it reached the endodermis, and that the endodermis is not the control point for salt uptake by the plant. The pericycle as well as the xylem parenchyma may be important in the control of net Na(+) loading of the xylem.

To investigate the effect of carbohydrate on carotenoid accumulation in leaves, excised plants of tomato (Lycopersicum esculentum var. cerasiformae, wva 106) were supplied with glucose through the transpiration stream for 48 h. We report here that sugar accumulation in leaves led to a decrease of carotenoid content, which was related to the reduction of Chl. The decrease in carotenoid amount correlated with a sugar-induced repression of genes encoding enzymes of the carotenoid and of the Rohmer pathways. The lower 1-deoxy-d-xylulose-5-phosphate synthase transcript level probably leads to a decreased metabolic flux through the methylerythritol pathway and subsequently to a lower amount of substrate available for plastidic isoprenoid synthesis. Differences between responses of young (sink) and mature (source) leaves to carbohydrate accumulation are discussed.

Improved understanding of flow and radionuclide transport in vadose zone sediments is fundamental to future planning involving radioactive materials. To that end, long-term experiments were conducted at the Savannah River Site in South Carolina, where a series of lysimeters containing sources of plutonium (Pu) in different oxidation states was placed in the shallow subsurface and exposed to the environment for 2 to 11 yr. Then, sediment Pu activity concentrations were measured along vertical cores taken from the lysimeters. Results showed anomalous activity distributions below the source, with significant migration of Pu above the source. A previously developed reactive transport model with surface-mediated redox reactions successfully simulated the lysimeter results below the source, assuming a steady, net downward flow. To simulate more realistic field conditions, a transient variably saturated flow model with root water uptake is developed and coupled to the reactive transport model. Overall, the fully transient analysis shows results nearly identical to the much simpler steady flow analysis. Thus, the surface-mediated redox hypothesis remains consistent with the below-source experimental data, the most important variable affecting transport being the oxidation state of the source material. However, none of the models studied could produce the upward Pu transport observed in the data. The hydraulic and chemical mechanisms tested as potential causes for upward migration—a modified root distribution, hysteresis, and air-content dependent oxidation—yielded little or no enhancement of the upward Pu movement. This suggests another transport mechanism such as root Pu uptake and translocation in the transpiration stream.

The effect of root inundation on the leaf emissions of ethanol, acetaldehyde and acetic acid in relation to assimilation and transpiration was investigated with 2–3 years old tree seedlings of four Amazonian floodplain species by applying dynamic cuvette systems under greenhouse conditions. Emissions were monitored over a period of several days of inundation using a combination of Proton Transfer Reaction Mass Spectrometry (PTR-MS) and conventional techniques (HPLC, ion chromatography). Under non-flooded conditions, none of the species exhibited measurable emissions of any of the compounds, but rather low deposition of acetaldehyde and acetic acid was observed instead. Tree species specific variations in deposition velocities were largely due to variations in stomatal conductance. Flooding of the roots resulted in leaf emissions of ethanol and acetaldehyde by all species, while emissions of acetic acid were only observed from the species exhibiting the highest ethanol and acetaldehyde emission rates. All three compounds showed a similar diurnal emission profile, each displaying an emission burst in the morning, followed by a decline in the evening. This concurrent behavior supports the conclusion, that all three compounds emitted by the leaves are derived from ethanol produced in the roots by alcoholic fermentation, transported to the leaves with the transpiration stream and finally partly converted to acetaldehyde and acetic acid by enzymatic processes. Co-emissions and peaking in the early morning suggest that root ethanol, after transportation with the transpiration stream to the leaves and enzymatic oxidation to acetaldehyde and acetate, is the metabolic precursor for all compounds emitted, though we can not totally exclude other production pathways. Emission rates substantially varied among tree species, with maxima differing by up to two orders of magnitude (25–1700 nmol m−2 min−1 for ethanol and 5–500 nmol m−2 min−1 for acetaldehyde). Acetic acid emissions reached 12 nmol m−2 min−1. The observed differences in emission rates between the tree species are discussed with respect to their root adaptive strategies to tolerate long term flooding, providing an indirect line of evidence that the root ethanol production is a major factor determining the foliar emissions. Species which develop morphological root structures allowing for enhanced root aeration produced less ethanol and showed much lower emissions compared to species which lack gas transporting systems, and respond to flooding with substantially enhanced fermentation rates and a non-trivial loss of carbon to the atmosphere. The pronounced differences in the relative emissions of ethanol to acetaldehyde and acetic acid between the tree species indicate that not only the ethanol production in the roots but also the metabolic conversion in the leaf is an important factor determining the release of these compounds to the atmosphere.

In a two-year (2005–2006) study conducted at three sites in central (Larissa) and northern (Alexandroupolis, Thessaloniki) Greece, we aimed to investigate the relationships between seedcotton yield and lint quality with leaf physiological traits (carbon isotope discrimination-Δ, ash content and K concentration). Eighteen lines with their original cultivars (Christina, Flora, Corona) were tested under the ultra-low density of 1.2 plants m−2. In combined data over years, a significant, positive correlation between seedcotton yield and Δ or ash content was found only in the driest and lowest-yielding site (Larissa), indicating that genotypes that keep their stomata open and in turn exhibit the highest Δ values, had an advantage in such environments. In pooled data from the two most productive sites (Thessaloniki and Alexandroupolis), which had the highest Δ and ash content values, evidenced a negative correlation between seedcotton yield and both physiological traits. Seedcotton yield was negatively related with leaf K concentration in Larissa and Alexandroupolis but no significant relationship was found in Thessaloniki where leaf K concentration was below adequacy limit. Δ was positively related with ash content which suggests that the latter could be a putative surrogate of Δ. Negative correlation between Δ and leaf K concentration was found in two out of three sites (Larissa and Thessaloniki) as well as between ash content and K in one site (Larissa). These findings suggest that K accumulation in leaves is not just a passive procedure via transpiration stream. Significant, linear relationships of each physiological trait between sites showed that genotypic ranking was constant in the three sites, an indication of heritability. Ash content had the highest significance levels and correlation coefficients. Even though significant genotypic differentiation was observed for the three physiological and two of the lint quality traits (i.e. fibre length, micronaire) determined in Alexandroupolis, only a weak, negative relationship between fibre length and leaf K concentration was evident. In sum, leaf physiological traits (Δ, ash content and K concentration) could not be reliably used for yield selection in cotton owing to site-specific effects, which prejudice the yield–physiological traits relationship. Ash content–Δ relationship merits further research in order the former to be established as a putative surrogate of Δ.

The calcium (Ca) isotope system is potentially of great use for understanding biogeochemical processes at multiple scales in forest ecosystems, yet remains largely unexplored for this purpose. In order to further our understanding of Ca behavior in forests, we examined two nearly adjacent hardwood-dominated catchments with differing soil Ca concentrations, developed from crystalline bedrock, to determine the variability of 44Ca/40Ca ratios (expressed as δ44Ca) within soil and vegetation pools. For both sugar maple and American beech, the Ca isotope compositions of the measured roots and calculated bulk trees were considerably lighter than those of soil pools at these sites, suggesting that the trees were able to preferentially take up light Ca at the root–soil interface. The Ca isotope compositions of three of four root samples were among the lightest values yet reported for terrestrial materials (δ44Ca ≤−3.95‰). Our results further indicate that Ca isotopes were fractionated along the transpiration streams of both tree species with roots having the least δ44Ca values and leaf litter the greatest. An approximately 2‰ difference in δ44Ca values between roots and leaf litter of both tree species suggests a persistent fractionation mechanism along the transpiration stream, likely related to Ca binding in wood tissue coupled with internal ion exchange. Finally, our data indicate that differing tree species demand for Ca and soil Ca concentrations together may influence Ca isotope distribution within the trees. Inter-catchment differences in Ca isotope distributions in soils and trees were minor, indicating that the results of our study may have broad transferability to studies of forest ecosystems in catchments developed on crystalline substrates elsewhere.

Methanol emissions from several deciduous tree species with predominantly mature leaves were measured under laboratory and field conditions. The emissions were modulated by temperature and light. Under constant light conditions in the laboratory, methanol emissions increased with leaf temperature, by up to 12% per degree. At constant temperatures, emissions doubled when light intensity (PAR) increased from darkness to 800 micromol x m(-2) x s(-1). A phenomenological description of light and temperature dependencies was derived from the laboratory measurements. This description was successfully applied to reproduce the diel cycle of methanol emissions from an English oak measured in the field. Labelling experiments with (13)CO(2) provided evidence that less than 10% of the emitted methanol was produced de novo by photosynthesis directly prior to emission. Hence, the light dependence of the emissions cannot be explained by instantaneous production from CO(2) fixation. Additional experiments with selective cooling of plant roots indicated that a substantial fraction of the emitted methanol may be produced in the roots or stem and transported to stomata by the transpiration stream. However, the transpiration stream cannot be considered as the main factor that determines methanol emissions by the investigated plants.

Recent discoveries in the phytoremediation of volatile organic compounds (VOCs) show that vapor-phase transport into roots leads to VOC removal from the vadose zone and diffusion and volatilization out of plants is an important fate following uptake. Volatilization to the atmosphere constitutes one fundamental terminal fate processes for VOCs that have been translocated from contaminated soil or groundwater, and diffusion constitutes the mass transfer mechanism to the plant-atmosphere interface. Therefore, VOC diffusion through woody plant tissues, that is, xylem, has a direct impact on contaminant fate in numerous vegetation-VOC interactions, including the phytoremediation of soil vapors and dissolved aqueous-phase contaminants. The diffusion of VOCs through freshly excised tree tissue was directly measured for common groundwater contaminants, chlorinated compounds such as trichloroethylene, perchloroethene, and tetrachloroethane and aromatic hydrocarbons such as benzene, toluene, and methyl tert-butyl ether. All compounds tested are currently being treated at full scale with tree-based phytoremediation. Diffusivities were determined by modeling the diffusive transport data with a one-dimensional diffusive flux model, developed to mimic the experimental arrangement. Wood-water partition coefficients were also determined as needed for the model application. Diffusivities in xylem tissues were found to be inversely related to molecular weight, and values determined herein were compared to previous modeling on the basis of a tortuous diffusion path in woody tissues. The comparison validates the predictive model for the first time and allows prediction for other compounds on the basis of chemical molecular weight and specific plant properties such as water, lignin, and gas contents. This research provides new insight into phytoremediation efforts and into potential fruit contamination for fruit-bearing trees, specifically establishing diffusion rates from the transpiration stream and modeling volatilization along the transpiration path, including the trunk and branches. This work also has importance in other plant-VOC interactions, such as potential uptake from the atmosphere for hydrophobic compounds and also uptake from vapor-phase soil contaminants.

Salinity damage in rice and other salt-sensitive species is due to excessive transport of NaCl through the root system to the leaves and consequently low salt transport to the shoot can be a major trait determining salt resistance. Since the rapid uptake of sodium ions is such a crucial part of the response of rice to salinity, physiological experiments were carried out to compare bypass flow in two genotypes of rice (IR4630 and IR15324) differing in salt tolerance, because it has been suggested that an apoplastic pathway, bypass flow, is a major contributory pathway for Na+ entrance into rice plants. Experiments on the youngest fully expanded photosynthetic leaf (the third from the base), using PTS as a tracer for apoplastic movement and Philaenus spumarius (a xylem-feeding insect) as a means to sample the xylem sap, did not demonstrate any apparent difference in bypass flow between the two lines. The similarity of Na+ concentration in the xylem sap of both genotypes paralleled the results of PTS (a fluorescent dye used as an apoplastic tracer for the transpiration stream) measurements. Despite the similarity of Na+ concentration in the xylem sap of the third leaves, the Na+ concentration in the bulk of these leaves of IR15324 plants (the sensitive line) was about twice that of IR4630 (the tolerant line). Measurements of transpiration over 8 d of salinisation showed the similarity of rates in both lines providing evidence that the greater accumulation of NaCl in IR15324 than in IR4630 plants was unlikely to be due to a difference in the delivery of salt to the leaves by an apoplastic route. Results of the current work suggest that the difference in salt tolerance might be a consequence of damage to leaves 1 and 2 of IR15324 that allowed Na+ to leak into the phloem - and consequently move to leaf 3. Danos por salinidade em arroz e em outras espécies sensíveis ao estresse salino são devidos ao transporte excessivo de NaCl, desde o sistema radicular até as folhas; assim, baixo transporte de sais para os ramos pode ser uma característica importante na determinação da resistência salina. Uma vez que a absorção rápida dos íons sódio é uma parte crucial da resposta do arroz à salinidade, conduziram-se experimentos para comparar-se o fluxo (apoplástico) pelo desvio (bypass flow) de dois genótipos de arroz (IR4630 e IR15324) diferindo na tolerância salina, visto ser esse fluxo uma das principais rotas pelas quais o Na+ entra nas raízes de arroz. Experimentos com as folhas mais jovens completamente expandidas (terceira a partir do ápice), usando-se o PTS (um corante fluorescente como um traçador apoplástico para a corrente transpiratória) e o inseto Philaenus spumarius para amostragens da seiva do xilema, não demonstraram qualquer diferença aparente no chamado fluxo pelo desvio entre os dois genótipos. A similaridade da concentração de Na+ na seiva do xilema de ambos genótipos foi respaldada pelos resultados de medições de PTS. Apesar dessa similaridade, a concentração de Na+ nos tecidos da terceira folha, como um todo, em IR15324 (linhagem sensível) foi cerca de duas vezes maior que em IR4630 (linhagem tolerante). As taxas de transpiração ao longo 8 d de salinização foram similares entre os dois genótipos, evidenciando que o maior acúmulo de NaCl nas plantas de IR15324 não foi, provavelmente, devido a diferenças de suprimento de sal às folhas, via uma rota apoplástica. Os resultados deste trabalho sugerem que a diferença na tolerância salina pode ser uma conseqüência de danos às folhas 1 e 2 em IR15234, que permitiram maior vazamento de Na+ no floema - e conseqüentemente transporte de Na+ para a folha 3.

Additional index words. apical dominance, epinasty, photoperiod, root anaerobiosis, soil moisture status, stomatal conductance, tuberization Abstract. Plant growth regulators (PGRs) play important roles in the way plants grow and develop. Myriad processes important to horticultural crops are regulated by PGRs. Changes in the presence, balance, and distribution of PGRs communicate developmental, stress-related, or environmental cues that alter growth. Short-distance communication involves changes in biosynthesis or metabolic conversion, whereas longer-distance communication may also require export and translocation of PGRs, their precursors or metabolites. Examples are presented that demonstrate PGR communication between roots and shoots in horticultural commodities. For example, increased duration and intensity of flooding stress can result in synthesis of 1-aminocyclopropane-1-carboxylic acid (ACC), precursor of the PGR ethylene, in roots. ACC transported to the shoot through the transpiration stream is converted to ethylene and causes leaf epinasty. Roots sense the onset of water stress and can communicate the need to close leaf stomata by altering abscisic acid (ABA) levels in the shoot. Daylength and temperature regulate synthesis and transport of gibberellins, which promote stem elongation and stolon formationandinhibit tuberization inpotato.Outgrowth ofaxillarybuds followingthedecapitation ofthe apicalmeristem is dependent on synthesis and transport of cytokinin from root to the axillary buds as well as the balance of indole-3-acetic acid (IAA) cytokinin, and additional messengers. Current research in the field of long-distance communication within plants is uncovering novel messengers and altering our view of the central roles for PGRs in such signaling.

Plants growing in dense canopies are exposed to vertical light gradients and show photosynthetic acclimation at the whole-plant level, resulting in efficient photosynthetic carbon gain. We studied the role of cytokinins transported through the transpiration stream as one of probably multiple signals for photosynthetic acclimation to light gradients using both tobacco (Nicotiana tabacum) and Arabidopsis (Arabidopsis thaliana). We show that substantial variation in leaf transpiration parallels the light gradient in tobacco canopies and experimental reduction of the transpiration rate of a leaf, independent of light, is sufficient to reduce photosynthetic capacity in both species, as well as transcript levels of the small subunit of Rubisco (rbcS) gene in Arabidopsis. Mass spectrometric analysis of xylem sap collected from intact, transpiring tobacco plants revealed that shaded leaves import less cytokinin than leaves exposed to high light. In Arabidopsis, reduced transpiration rate of a leaf in the light is associated with lower cytokinin concentrations, including the bioactive trans-zeatin and trans-zeatin riboside, as well as reduced expression of the cytokinin-responsive genes ARR7 and ARR16. External application of cytokinin to shaded leaves rescued multiple shade effects, including rbcS transcript levels in both species, as did locally induced cytokinin overproduction in transgenic tobacco plants. From these data, we conclude that light gradients over the foliage of a plant result in reduced cytokinin activity in shaded leaves as a consequence of reduced import through the xylem and that cytokinin is involved in the regulation of whole-plant photosynthetic acclimation to light gradients in canopies.

The plant nutrient phosphorus (P) is spread throughout the plant within the transpiration stream after uptake in the form of phosphate via the roots. Short-term distribution is therefore strongly dependent on transpiration rather than on sinks, so that P mainly enters adult leaves. The objective of this work was to investigate the transport of phosphate in leaf veins and its distribution within the leaf. Experiments were performed with broad bean (Vicia faba L. var. Scirocco) using radioactively labeled phosphate. In a relatively new approach, the tracer was monitored by a sensitive imaging plate using Fujifilm's Bioimaging Analyzer System. Radioactive label could be monitored in fresh leaves (without fixing) using exposure times of only 5–20 min. For this reason, the method offered the possibility to obtain several subsequent autoradiographic images of the same sample after different feeding times. Phosphate tracer quickly reached small veins, which were then more intensely labeled than the tissue between them. Within the first 15 min, intercostal fields were only slightly stained. After application of phosphate tracer onto the leaf blade, export from the leaf was pronounced and started within 30 min. Phosphorus is effectively redistributed from adult leaves to sinks such as the terminal bud or growing root tips. Using the imaging method, an impression of the high velocity and effectiveness of this process was obtained. The results furthermore imply that leaf veins may be one of the most important locations where exchange between xylem and phloem transport takes place, although exchange between xylem and phloem was also observed in the shoot axis and in the petioles.

Transpiration streams are induced by evaporative demand from the environment and distribute water solution from roots to tissues and cells in the respective organs through the complicated pathways. In these processes, physiological functions of plants such as stomatal movement, photosynthesis, phloem and xylem transport, nutrient uptake and expansive growth are affected by transpiration streams. Therefore, many kinds of techniques for measuring evaporative demand and transpiration streams in leaf boundary layer, leaves, fruits, stem, roots and plant canopy have been developed. The information about transpiration streams provided by the developed techniques must be more informative with cooperative measurements of plant physiological functions and must be more effective for the speaking plant approach to the environment control.

Silica is distributed in rice plant by the transpiration stream. However, it is unknown the effects of atmospheric carbon dioxide concentration ([CO2]) on silica deposition in the rice plants. In the present study, we investigated the effects of elevated [CO2] on silica contents of the rice plant and focused on the relationships between transpiration rate and silica content of the panicle and leaves under elevated [CO2] at the flowering stage. The rice seedlings were grown hydroponically in two chambers until the flowering stage. The [CO2] in the chambers was maintained at 350 µmol mol-1 (ambient [CO2]) or 700 µmol mol-1 (elevated [CO2]). Silica contents of the panicle and the flag leaf under elevated [CO2] significantly decreased compared with those under ambient [CO2] when silica was applied to the culture solution. The transpiration rate of the panicle decreased with increasing panicle silica content under ambient [CO2]. No correlations between transpiration rate and silica content of the panicles and leaves were observed under elevated [CO2].