Diurnal pattern of acetaldehyde emission by flooded poplar trees

The emission of the tropospheric trace gas acetaldehyde was determined in leaves of 4-month-old poplar trees ( Populus tremula × P. alba ) grown under controlled environmental conditions in a greenhouse. Using a dynamic cuvette system together with a high sensitivity laser-based photoacoustic detection unit, rates of acetaldehyde emission were measured with the high time resolution of about 15 min. Submergence of the roots resulted in the emission of acetaldehyde by the leaves. The emission increased linearly before reaching more or less steady-state values (ca 350 nmol m⁻² min⁻¹; ca 470 ng g⁻¹ dry weight min⁻¹) after approximately 6 h. Prolonged flooding of poplar trees resulted in a clear diurnal rhythm of acetaldehyde emission. The emission rates decreased when the light was switched off in the evening and peaked in the morning after the light was turned on again. This pattern significantly correlated with diurnal rhythms of stomatal conductance, photosynthesis, transpiration and with the concentrations of ethanol, the assumed precursor of acetaldehyde, in the xylem sap of flooded poplar trees. It may be concluded that under conditions of diminished stomatal conductance, acetaldehyde emission declines because its diffusive flux is reduced. Alternatively, reduced transpiration may decrease ethanol transport from the roots to the shoots and appreciable amounts of the acetaldehyde precursor ethanol are lacking in the leaves. The present results support the view that acetaldehyde emitted by the leaves of plants is derived from ethanol produced by alcoholic fermentation in submerged roots and transported to the leaves with the transpiration stream.     

Metabolic origin of acetaldehyde emitted by poplar (Populus tremula x (P. alba) trees

The metabolic origin and emission by the leaves of the tropospheric trace gas acetaldehyde were examined in 4-month-old poplar trees (Populus tremula x P. alba) cultivated under controlled environmental conditions in a greenhouse. Treatments which resulted in increased ethanol concentration of the xylem sap caused significantly enhanced rates of acetaldehyde and ethanol emission by the leaves. Leaves fed [¹⁴C]-ethanol via the transpiration stream emitted [^<14C]-acetaldehyde. These findings suggest that acetaldehyde in the leaves is synthesized by a metabolic pathway that operates in the opposite direction of alcoholic fermentation and results in oxidation of ethanol. Enzymatic studies showed that this pathway is mediated either by alcohol dehydrogenase (ADH; EC 1.1.1.1) or catalase (CAT; EC 1.11.1.6), both constitutively present in the leaves of poplar trees. Labelling experiments with [¹⁴C]-glucose indicated that the ethanol delivered to the leaves by the transpiration stream is produced in anaerobic zones of submersed roots by alcoholic fermentation. Anoxic conditions in the rhizosphere caused by flooding of the root system resulted in an activation of alcoholic fermentation and led to significantly increased ethanol concentrations in the xylem sap. These results support the hypothesis that acetaldehyde emitted by the leaves of trees is derived from xylem transported ethanol which is synthesized during alcoholic fermentation in the roots.Keywords: Acetaldehyde, emission, ethanol, anaerobiosis, Populus tremula x P. alba      

Waterlogging responses in dune,swale and marsh populations of Spartina patens under field conditions

Summary Soil waterlogging responses were examined in three Spartina patens populations along a steep flooding gradient in coastal Louisiana. Root anatomy and physiological indicators of anaerobic metabolism were examined to identify and compare flooding responses in dune, swale and marsh populations, while soil physicochemical factors were measured to characterize the three habitats. Soil waterlogging increased along the gradient from dune to marsh habitats and was accompanied by increases in root porosity (aerenchyma). Aerenchyma in marsh roots was apparently insufficient to provide enough oxygen for aerobic respiratory demand, as indicated by high root alcohol dehydrogenase activities and low energy charge ratios. Patterns of root metabolic indicators suggest that dune and swale roots generally respired aerobically, while anaerobic metabolism was important in marsh roots. However, in each population, relatively greater soil waterloging was accompanied by differences in enzyme activities leading to malate accumulation. In dune and swale roots under these circumstances, depressed adenylate energy charge ratios may have been the result of an absence of increased ethanol fermentation. These trends suggest that: 1) Aerenchyma formation was an important, albeit incomplete, long-term adaptation to the prevalent degree of soil waterlogging. 2) All populations adjusted root metabolism in response to a relative (short-term) increase in soil waterlogging.     

长期水淹对‘中山杉118’幼苗呼吸代谢的影响

中山杉(Taxodium ‘Zhongshansha’)具有极强的耐淹性, 但其耐淹机理仍没有明确。该研究以‘中山杉118’ (Taxodium ‘Zhongshansha 118’)幼苗为对象, 在经过93天不同水淹处理(对照、水浸、浅淹、深淹)后测定中山杉叶片和根系的无氧呼吸酶活性、淀粉及可溶性糖含量、生物量以及根系活力, 从能量消耗的角度初步探索了中山杉的耐淹性。结果表明: 长期水淹使中山杉叶片与根系中3种无氧呼吸酶(乙醇脱氢酶、丙酮酸脱羧酶、乳酸脱氢酶)活性显著增加, 且叶片与根系的乙醇脱氢酶活性均高于乳酸脱氢酶活性, 中山杉的根系和叶片是通过加强以酒精发酵为主的无氧呼吸适应长期缺氧环境; 不同水淹处理的叶片中3种无氧呼吸酶活性均高于根系, 叶片对缺氧环境更加敏感; 中山杉叶片和根系淀粉、可溶性糖含量均随水淹深度的增加显著增加, 根系淀粉含量显著高于叶片, 可溶性糖含量低于叶片; 中山杉根系淀粉含量高是其能够长期忍受水淹的重要原因, 且中山杉适应长期水淹的策略为忍耐型; 经受长期水淹后中山杉根茎结合部长出气生根及茎基部膨大, 同时根系外壁的木质化能将根系与外部水淹环境隔离, 具有很强的耐淹性, 可作为湿地生态修复、消落带生物治理的优良植物材料。     

涝渍对3个树种生长、组织孔隙度和渗漏氧的影响

为了了解落羽杉(Taxodium distichum)、乌桕(Sapium sebiferum)和美国山核桃(Carya illinoensis)等树种的耐涝机制, 采用盆栽模拟涝渍环境的试验方法, 设置了淹水、渍水和对照3个处理, 测定了一年生落羽杉、乌桕和美国山核桃实生苗的生长、组织孔隙度、根氧消耗等指标。结果表明, 涝渍处理抑制了落羽杉、乌桕和美国山核桃的生物量和生物量增量(渍水处理下落羽杉的生长得到了促进), 增加了3树种的根冠比, 从生物量和生物量增量下降幅度来评价, 落羽杉的耐涝性最强, 其次为美国山核桃。淹水和渍水处理下, 落羽杉、乌桕和美国山核桃的根、茎和叶中的组织孔隙度显著增加, 且随着处理时间的延长, 各器官的组织孔隙度有增加的趋势, 3个树种中, 落羽杉的根、茎和叶中的组织孔隙度均较其他2个树种高。淹水和渍水处理下, 移除茎明显增加了落羽杉、美国山核桃和乌桕的根的氧消耗, 表明涝渍处理增强了O₂在3个树种体内的运输并通过根系扩散到涝渍土壤中的能力, 并且随着处理时间的延长, 3个树种体内运输O₂并扩散到土壤中的能力有逐渐增强的趋势。因此, 涝渍环境总体上抑制了落羽杉、乌桕和美国山核桃等树种的生长, 但各树种为了适应这种生长环境, 形成了大量的通气组织, 从而导致各器官组织孔隙度的增加, 增强了O₂通过植物体运输到根系并扩散到土壤中的能力, 解决了根系及根际缺氧的矛盾。     

Transport Exchange of Carbon,Nitrogen and Water in the Context of Whole Plant Growth and Functioning — Case History of a Nodulated Annual Legume

The economy of carbon, nitrogen and water during growth of nodulated, nitrogen-fixing plants of white lupin (Lupinus albus L.) was studied by measuring C, N and H₂O content of plant parts, concentrations of C and N in bleeding sap of xylem and phloem, transpirational losses of whole shoots and shoot parts, and daily exchanges of CO₂ between shoot and root parts and the surrounding atmosphere. Relationships were studied between water use and dry matter accumulation of shoot and fruits, and between net photosynthesis rate and leaf area, transpiration rate and nitrogen fixation. Conversion efficiencies were computed for utilization of net photosynthate for nitrogen fixation and for production of dry matter and protein in seeds. Partitioning of the plant's intake of C, N and H₂O was described in terms of growth, transpiration, and respiration of plant parts. An empirically-based model was developed to describe transport exchanges in xylem and phloem for a 10-day interval of growth. The model depicted quantitatively the mixtures of xylem and phloem streams which matched precisely the recorded amounts of C, N and H₂O assimilated, absorbed or consumed by the various parts of the plant. The model provided information on phloem translocation of carbon and nitrogen to roots from shoots, the cycling of carbon and nitrogen through leaves, the relationship between transpiration and nitrogen partitioning to shoot organs through the xylem, the relative amount of the plant's water budget committed to phloem translocation, and the significance of xylem to phloem transfer of nitrogen in stems as a means of supplying nitrogen to apical regions of the shoot.     

渍害胁迫下小麦生长的响应机理及调控措施研究进展

全球气候变化导致近年来渍害频发,而旱地作物小麦对渍害敏感。受气候、土壤、轮作制度等因素的影响,我国长江中下游小麦主产区的渍害灾情严重。渍害引起的土壤溶氧量降低可以导致小麦根系生长受到抑制,进而限制植株生长,最终降低小麦产量和品质。本文基于国内外相关研究,从根系呼吸代谢、水分传导、矿质养分吸收、光合作用、氧化还原代谢等方面概述了渍害胁迫抑制小麦生长的生理机理;讨论了小麦通过无氧呼吸维持能量供应和改变根系形态维持氧气供应等渍害适应机制;总结了肥料调控、生长调节剂调控和胁迫记忆等栽培措施在小麦抗渍上的应用及其机理,并对未来小麦抗渍研究进行了展望,以期为小麦的耐渍栽培和稳产高产管理提供理论支撑。     

Alcohol dehydrogenase and ethanol in the stems of trees : evidence for anaerobic metabolism in the vascular cambium

Anaerobic fermentation in plants is usually thought to be a transient phenomenon, brought about by environmental limitations to oxygen availability, or by structural constraints to oxygen transport. The vascular cambium of trees is separated from the air by the outer bark and secondary phloem, and we hypothesized that the cambium may experience sufficient hypoxia to induce anaerobic fermentation. We found high alcohol dehydrogenase activity in the cambium of several tree species. Mean activity of alcohol dehydrogenase in Populus deltoides was 165 micromoles NADH oxidized per minute per gram fresh weight in May. Pyruvate decarboxylase activity was also present in the cambium of P. deltoides, with mean activity of 26 micromoles NADH oxidized per minute per gram fresh weight in May. Lactate dehydrogenase activity was not present in any tree species we examined. Contrary to our expectation, alcohol dehydrogenase activity was inversely related to bark thickness in Acer saccharum and unrelated to bark thickness in two Populus species. Bark thickness may be less important in limiting oxygen availability to the cambium than is oxygen consumption by rapidly respiring phloem and cambium in actively growing trees. Ethanol was present in the vascular cambium of all species examined, with mean concentrations of 35 to 143 nanomoles per gram fresh weight, depending on species. Ethanol was also present in xylem sap and may have been released from the cambium into the transpiration stream. The presence in the cambium of the enzymes necessary for fermentation as well as the products of fermentation is evidence that respiration in the vascular cambium of trees may be oxygen-limited, but other biosynthetic origins of ethanol have not been ruled out.     

Stem photosynthesis not pressurized ventilation is responsible for light-enhanced oxygen supply to submerged roots of alder (Alnus glutinosa)

BACKGROUND AND AIMS: Claims that submerged roots of alder and other wetland trees are aerated by pressurized gas flow generated in the stem by a light-induced thermo-osmosis have seemed inconsistent with root anatomy. Our aim was to seek a verification using physical root-stem models, stem segments with or without artificial roots, and rooted saplings. METHODS: Radial O2 loss (ROL) from roots was monitored polarographically as the gas space system of the models, and stems were pressurized artificially. ROL and internal pressurization were also measured when stems were irradiated and the xylem stream was either CO2 enriched or not. Stem photosynthesis and respiration were measured polarographically. Stem and root anatomy were examined by light and fluorescence microscopy. KEY RESULTS: Pressurizing the models and stems to 相似文献   

Flooding tolerance of Tabebuia cassinoides: Metabolic, morphological and growth responses

Tabebuia cassinoides (Lam.) DC (Bignoniaceae) is a tree species that occurs in swampy areas of the coastal “restinga” in SE Brazil (a coastal sandy plains scrub and forest formation). To elucidate possible adaptive strategies that enable this species to occupy areas subjected to seasonal or perennial waterlogging, metabolic, morphological and growth responses of plants under flooding conditions were studied. The root system of T. cassinoides plants presented elevated amounts of ethanol (10.6 μmol g⁻¹ fresh wt) only in the first 5 d of soil water saturation. The two-fold increase in ethanol production under flooding was corroborated by an increase in ADH activity in the same period. Lactic acid concentrations did not change significantly during four months of flooding treatment. The decrease of alcoholic fermentation under hypoxia was associated with the appearing of new roots. The induction of aerenchyma formation in roots developed under flooding conditions, allowed oxygen transport from the shoot to these organs, thus maintaining an aerobic respiration. We conclude that this characteristic and the capacity to oxidize the rhizosphere are probably responsible for the survival and growth of plants while flooded and for their success in an environment, which restricts the presence of the majority of competing tree species.     

Phloem flow and sugar transport in Ricinus communis L. is inhibited under anoxic conditions of shoot or roots

Anoxic conditions should hamper the transport of sugar in the phloem, as this is an active process. The canopy is a carbohydrate source and the roots are carbohydrate sinks. By fumigating the shoot with N₂ or flooding the rhizosphere, anoxic conditions in the source or sink, respectively, were induced. Volume flow, velocity, conducting area and stationary water of the phloem were assessed by non‐invasive magnetic resonance imaging (MRI) flowmetry. Carbohydrates and δ¹³C in leaves, roots and phloem saps were determined. Following flooding, volume flow and conducting area of the phloem declined and sugar concentrations in leaves and in phloem saps slightly increased. Oligosaccharides appeared in phloem saps and after 3 d, carbon transport was reduced to 77%. Additionally, the xylem flow declined and showed finally no daily rhythm. Anoxia of the shoot resulted within minutes in a reduction of volume flow, conductive area and sucrose in the phloem sap decreased. Sugar transport dropped to below 40% by the end of the N₂ treatment. However, volume flow and phloem sap sugar tended to recover during the N₂ treatment. Both anoxia treatments hampered sugar transport. The flow velocity remained about constant, although phloem sap sugar concentration changed during treatments. Apparently, stored starch was remobilized under anoxia.     

Supply and partitioning of assimilates to roots of Medicago sativa L. and Lotus corniculatus L. under anoxia

Abstract A current explanation of the mechanism of flooding injury to roots suggests that oxygen deficiency depresses the supply of respirable carbohydrates sufficiently to inhibit fermentation. However, even though it has been shown that phloem transport of assimilate is sharply reduced to anaerobic roots, inhibition of assimilate metabolism has also been suggested to be an important factor. This study examines these hypotheses by relating assimilate supply and metabolic activity in anoxic roots of alfalfa (Medicago sativa L.), a flood-intolerant species, and birdsfoot trefoil (Lotus corniculatus L.), a flood-tolerant plant. Roots were made anoxic (severe O₂ deficiency) for 2, 4 or 6 d and shoots were labelled with ¹⁴CO₂. Assimilate transport to the roots and metabolism to structural components were significantly decreased in both species in response to anoxia. Trefoil exhibited significantly greater ¹⁴C incorporation into the residue fraction at 4 d anoxia than did alfalfa, and this was consistent with the greater flooding tolerance of trefoil. When assimilate supply to O₂-deficient roots was decreased by shoot shading, shoot fresh weight was reduced by both anoxia and light treatments. Root-soluble sugars were significantly decreased by shading but were greatly increased in response to anoxia. Root starch concentration also increased under anoxia. Root K⁺ concentration was reduced by anoxia only. The energy status (ATP/ADP) of roots was significantly decreased by shading; however, anoxia reduced the energy status only in unshaded plants. The data indicate that carbohydrate supply to anaerobic roots does not appear to be a limiting factor in the metabolic response of alfalfa roots. Alternatively, metabolism of assimilate in anoxic roots may be an important determinant of survival.     

Growth responses of <Emphasis Type="Italic">Salix gracilistyla</Emphasis> cuttings to a range of substrate moisture and oxygen availability

The objective of this study is to determine the effects of substrate moisture and oxygen availability on growth traits of Salix gracilistyla Miquel, which colonizes gravel bars along rivers, the shoot growth schedule, biomass production, and resource allocation were examined under greenhouse conditions. We used four treatments representing a range of substrate moisture and oxygen availability: drought (D), flooding with standing water (FS), flooding with running water (FR), and control without drought or flooding (C). Cuttings in D stopped flushing and had low biomass production, reduced total leaf mass, and small leaves. Under anaerobic conditions, cuttings in FS stopped flushing and had low biomass production, small root biomass, low biomass allocation to roots, shallow roots, high biomass allocation to hypertrophied lenticels, and a few small, thick leaves. Under aerobic conditions, cuttings in FR showed continuous branch elongation and flushing, large biomass production, and large leaf biomass, similar to cuttings in C, in addition to low allocation to hypertrophied lenticels and many large leaves. The growth of cuttings was not inhibited by flooding of the roots throughout the experiment unless the conditions were anaerobic. Thus, cuttings respond to water stress under low moisture conditions by reducing the transpiration area and respond to flooding under low oxygen conditions by high allocation to hypertrophied lenticels and reduced transpiration area. Plasticity in the shoot growth schedule, biomass production, and resource allocation according to moisture conditions and the ability to develop hypertrophied lenticels upon flooding allow S. gracilistyla to colonize sites in which both desiccation and flooding occur.     

The role of nitrate reduction in the anoxic metabolism of roots I. Characterization of root morphology and normoxic metabolism of wild type tobacco and a transformant lacking root nitrate reductase

Two tobacco lines with (Nicotiana tabacum cv. Gatersleben, WT) or without (transformant LNR-H) nitrate reductase in roots were chosen as model systems to re-evaluate the role of root nitrate reduction for survival of anoxia. In this first paper, the two hydroponically grown lines were compared with respect to their root morphology, root respiration and the root content of inorganic cations, anions, and metabolites. Leaf transpiration in relation to root morphology was also determined. In comparison to WT roots containing NR, the NR-free LNR-H transformants had slightly shorter and thicker roots with a lower root surface area per g leaf FW. Consistent with that, LNR-H leaves had lower transpiration rates than WT. LNR-H-roots also showed consistently higher respiration and higher contents of ATP, starch and hexose monophosphates than WT roots. Concentrations of free sugars were only slightly higher in LNR-H roots. Total soluble protein content was identical in both lines, whereas amino acids were higher in LNR-H. Contents of major inorganic cations and anions were also almost identical in both lines. We conclude that WT versus LNR-H plants are a suitable tool to re-evaluate the role of nitrate reduction in flooding tolerance.     

Root Respiration and Carbohydrate Status of Two Wheat Genotypes in Response to Hypoxia

To investigate root respiration and carbohydrate status in relationto waterlogging or hypoxia tolerance, root respiration rateand concentrations of soluble sugars in leaves and roots weredetermined for two wheat (Triticum aestivum L.) genotypes differingin waterlogging-tolerance under hypoxia (5% O₂) and subsequentresumption of full aeration. Root and shoot growth were reducedby hypoxia to a larger extent for waterlogging-sensitive Coker9835. Root respiration or oxygen consumption rate declined withhypoxia, but recovered after 7 d of resumption of aeration.Respiration rate was greater for sensitive Coker 9835 than fortolerant Jackson within 8 d after hypoxia. The concentrationsof sucrose, glucose and fructose decreased in leaves for bothgenotypes under hypoxia. The concentration of these sugars inroots, however, increased under hypoxia, to a greater degreefor Jackson. An increase in the ratio of root sugar concentrationto shoot sugar concentration was found for Jackson under hypoxicconditions, suggesting that a large amount of carbohydrate waspartitioned to roots under hypoxia. The results indicated thatroot carbohydrate supply was not a limiting factor for rootgrowth and respiration under hypoxia. Plant tolerance to waterloggingof hypoxia appeared to be associated with low root respirationor oxygen consumption rate and high sugar accumulation underhypoxic conditions.Copyright 1995, 1999 Academic Press Oxygen consumption rate, sugar accumulation, Triticum aestivum L., waterlogging tolerance     

Rapid flood-induced stomatal closure accompanies xylem sap transportation of root-derived acetaldehyde and ethanol in Forsythia

Plants grown in containers frequently suffer from difficulties in managing their water status due to either insufficient or too much water. In the case of the latter, little information is available regarding how container-grown woody plants respond to anaerobic media. The aims of this work were therefore to use Forsythia as a model woody plant system to provide a mechanistic understanding of the physiological events and their timing during soil flooding. Exposure of pot-grown Forsythia to root hypoxia had a dramatic effect on leaf growth and stomatal conductance. Within 24 h of flooding a decline in leaf growth rate was detected along with a reduction in stomatal conductance. The effects of hypoxia appear initially with older leaves, but if flooding is prolonged (>2 days) younger expanding leaves are affected. These responses and their timing have not been described for woody perennial plants but appear comparable to those described for herbaceous plants such as tomato and castor bean. Measurements of stem and leaf tissue and during flooding showed large time dependent increases in the concentrations of acetaldehyde and ethanol; products associated with anaerobic respiration. Both of these chemicals were shown to be root-derived and are produced in a significant amount only as flooding time increases and the decline in leaf growth and conductance become apparent. Xylem sap was collected at a range of flow rates to measure whole root system conductance and determine changes in delivery of sap flux constituents. Calculated delivery rates of acetaldehyde and ethanol changed little with sap flow, particularly in well-drained control plants, while root hydraulic conductance declined when measured, 4 days after flooding. However, neither acetaldehyde nor ethanol, when used in a detached leaf transpiration bioassay, at physiologically realist concentrations (as determined from sap collection) failed to induce a dramatic reduction in leaf transpiration rates. The reasons for this discrepancy are discussed.     

Phloem import and storage metabolism are highly coordinated by the low oxygen concentrations within developing wheat seeds

We studied the influence of the internal oxygen concentration in seeds of wheat (Triticum aestivum) on storage metabolism and its relation to phloem import of nutrients. Wheat seeds that were developing at ambient oxygen (21%) were found to be hypoxic (2.1%). Altering the oxygen supply by decreasing or increasing the external oxygen concentration induced parallel changes in the internal oxygen tension. However, the decrease in internal concentration was proportionally less than the reduction in external oxygen. This indicates that decreasing the oxygen supply induces short-term adaptive responses to reduce oxygen consumption of the seeds. When external oxygen was decreased to 8%, internal oxygen decreased to approximately 0.5% leading to a decrease in energy production via respiration. Conversely, increasing the external oxygen concentration above ambient levels increased the oxygen content as well as the energy status of the seeds, indicating that under normal conditions the oxygen supply is strongly limiting for energy metabolism in developing wheat seeds. The intermediate metabolites of seed storage metabolism were not substantially affected when oxygen was either increased or decreased. However, at subambient external oxygen concentrations (8%) the metabolic flux of carbon into starch and protein, measured by injecting (14)C-Suc into the seeds, was reduced by 17% and 32%, respectively, whereas no significant effect was observed at superambient (40%) oxygen. The observed decrease in biosynthetic fluxes to storage compounds is suggested to be part of an adaptive response to reduce energy consumption preventing excessive oxygen consumption when oxygen supply is limited. Phloem transport toward ears exposed to low (8%) oxygen was significantly reduced within 1 h, whereas exposing ears to elevated oxygen (40%) had no significant effect. This contrasts with the situation where the distribution of assimilates has been modified by removing the lower source leaves from the plant, resulting in less assimilates transported to the ear in favor of transport to the lower parts of the plant. Under these conditions, with two strongly competing sinks, elevated oxygen (40%) did lead to a strong increase in phloem transport to the ear. The results show that sink metabolism is affected by the prevailing low oxygen concentrations in developing wheat seeds, determining the import rate of assimilates via the phloem.     

Carbon balance in leaves of young poplar trees

In the present study, important components of carbon metabolism of mature leaves of young poplar trees (Populus x canescens) were determined. Carbohydrate concentrations in leaves and xylem sap were quantified at five different times during the day and compared with photosynthetic gas exchange measurements (net assimilation, transpiration and rates of isoprene emission). Continuously measured xylem sap flow rates, with a time resolution of 15 min, were used to calculate diurnal balances of carbon metabolism of whole mature poplar leaves on different days. Loss of photosynthetically fixed carbon by isoprene emission and dark respiration amounted to 1% and 20%. The most abundant soluble carbohydrates in leaves and xylem sap were glucose, fructose and sucrose, with amounts of approx. 2 to 12 mmol m(-2) leaf area in leaves and about 0.2 to 15 mM in xylem sap. Clear diurnal patterns of carbohydrate concentration in xylem sap and leaves, however, were not observed. Calculations of the carbon transport rates in the xylem to the leaves were based on carbohydrate concentrations in xylem sap and xylem sap flow rates. This carbon delivery amounted to about 3 micromol C m(-2) s(-1) during the day and approx. 1 micromol C m(-2) s(-1) at night. The data demonstrated that between 9 and 28 % of total carbon delivered to poplar leaves during 24 h resulted from xylem transport and, hence, provide a strong indication for a significant rate of carbon cycling within young trees.     

Carbon allocation to shoots and roots in relation to nitrogen supply is mediated by cytokinins and sucrose: Opinion

In this paper we firstly show some general responses of biomass partitioning upon nitrogen deprivation. Secondly, these responses are explained in terms of allocation of carbon and nitrogen, photosynthesis and respiration, using a simulation model. Thirdly, we present a hypothesis for the regulation of biomass partitioning to shoots and roots.Shortly after nitrogen deprivation, the relative growth rate (RGR) of the roots generally increases and thereafter decreases, whereas that of the shoot decreases immediately. The increased RGR of the root and decreased RGR of the shoot shortly after a reduction in the nitrogen supply, cause the root weight ratio (root weight per unit plant weight) to increase rapidly.We showed previously that allocation of carbon and nitrogen to shoots and roots can satisfactorily be described as a function of the internal organic plant nitrogen concentration. Using these functions in a simulation model, we analyzed why the relative growth rate of the roots increases shortly after a reduction in nitrogen supply. The model predicts that upon nitrogen deprivation, the plant nitrogen concentration and the rate of photosynthesis per unit plant weight rapidly decrease, and the allocation of recently assimilated carbon and nitrogen to roots rapidly increases. Simulations show that the increased relative growth rate of the root upon nitrogen deprivation is explained by decreased use of carbon for root respiration, due to decreased carbon costs for nitrogen uptake. The stimulation of the relative growth rate of the root is further amplified by the increased allocation of carbon and nitrogen to roots. Using the simple relation between the plant nitrogen concentration and allocation, the model describes plant responses quite realistically.Based on information in the literature and on our own experiments we hypothesize that allocation of carbon is mediated by sucrose and cytokinins. We propose that nitrogen deprivation leads to a reduced cytokinin production, a decreased rate of cytokinin export from the roots to the shoot, and decreased cytokinin concentrations. A reduced cytokinin concentration in the shoot represses cell division in leaves, whereas a low cytokinin concentration in roots neutralizes the inhibitory effect of cytokinins on cell division. A reduced rate of cell division in the leaves leads to a reduced unloading of sucrose from the phloem into the expanding cells. Consequently, the sucrose concentration in the phloem nearby the expanding cells increases, leading to an increase in turgor pressure in the phloem nearby the leaf's division zone. In the roots, cell division continues and no accumulation of sugars occurs in dividing cells, leading to only marginal changes in osmotic potential and turgor pressure in the phloem nearby the root's cell division zone. These changes in turgor pressure in the phloem of roots and sink leaves affect the turgor pressure gradients between source leaf-sink leaf and source leaf-root in such a way that relatively more carbohydrates are exported to the roots. As a consequence RWR increases after nitrogen deprivation. This hypothesis also explains the strong relationship between allocation and the plant nitrogen status.