Quantitative estimation of root exudation of maize plants

Summary The rate of root exudation of maize plants was estimated by measuring the rate of denitrification in a hermetically sealed root system. While CO₂ production measured in the rhizosphere results both from root respiration and microbial respiration N₂O production during nitrate respiration is solely related to the amount of root exudates available for bacterial degradation. With 4 week old plants growing in quartz sand or soil root exudation amounted to 7% of the net photosynthates. Calculations revealed that about 25% of the organic matter flowing into the root system was excreted into the rhizosphere.     

The effect of abscisic acid and K+ on xylem exudation from excised roots of Lupinus luteus

External application of abscisic acid (ABA) induces a relatively high rate of xylem exudation in excised roots of Lupinus luteus L. cv. Weiko III. The response is relatively slow with a lag period of ca 1 h. It is also slowly, but reversibly, abolished by application of 3.6 or 36 μ M cycloheximide (CHX). Contrary to expectation, K⁺ is not a significant factor in maintaining flow rates in lupin roots as no response was measured after adding K⁺ to root systems, from which K⁺ had been withheld for periods ranging from 3 h to several days. In fact, excised roots obtained from seedlings raised in the absence of K⁺ failed to respond to added K⁺. Total depletion of K⁺ is difficult to achieve, because of initial seed reserves, and prolonged exudation in lupins can be maintained utilising only a small proportion of the K⁺ originally present in the root tissue. Nevertheless, the data cast doubt on the general applicability of the concept that volume flow is maintained by an osmotic gradient with K⁺ as the principal mineral ion.     

Effects of pressures inside Monterey pine trees

Summary The diurnal change in diameter of trees, aside from effects due to temperature, must be due to changes of pressure inside the tree. Monterey pines (Pinus radiata), as well as other pines, have two liquid systems, the sap system and the resin system. The sap is under a negative pressure, while the resin is under a positive pressure. Each system shows a diurnal change and, for these Monterey pines, the changes are in phase. The close agreement indicates that the changes in the one produce the changes in the other. These changes have been linked with the change in diameter of these trees by means of the modulus of elasticity of the wood. To gain further information on the effect of internal pressure on the diameter of the stem of a tree, caps were clamped on the ends of a section of a stem and the change of diameter was measured as water pressure was applied. Again, satisfactory agreement was obtained with that expected from the measured elasticity of the wood. The instruments developed to make these measurements, a dendrograph and a pressure recorder, are briefly described.     

Bark-carving behavior of the Japanese horned beetle Trypoxylus dichotomus septentrionalis (Coleoptera: Scarabaeidae)

The Japanese horned beetle, Trypoxylus dichotomus septentrionalis, usually aggregates at the wounds caused by boring insects on Quercus oak trees to feed on exuding sap. Here, a unique behavior of the beetle, which enables it to independently obtain sap without any activities of other insects, is reported. Beetles carved intact bark of Fraxinus griffithii, where no sap was exuding, using clypeus. As a result of carving behavior, bark was stripped, and sap exuded from these scars temporarily, which beetles immediately began to lick. A digital video image relating to the article is available at ().     

Enhanced root exudation stimulates soil nitrogen transformations in a subalpine coniferous forest under experimental warming

Despite the perceived importance of exudation to forest ecosystem function, few studies have attempted to examine the effects of elevated temperature and nutrition availability on the rates of root exudation and associated microbial processes. In this study, we performed an experiment in which in situ exudates were collected from Picea asperata seedlings that were transplanted in disturbed soils exposed to two levels of temperature (ambient temperature and infrared heater warming) and two nitrogen levels (unfertilized and 25 g N m^?2 a^?1). Here, we show that the trees exposed to an elevated temperature increased their exudation rates I (μg C g^?1 root biomass h^?1), II (μg C cm^?1 root length h^?1) and III (μg C cm^?2 root area h^?1) in the unfertilized plots. The altered morphological and physiological traits of the roots exposed to experimental warming could be responsible for this variation in root exudation. Moreover, these increases in root‐derived C were positively correlated with the microbial release of extracellular enzymes involved in the breakdown of organic N (R² = 0.790; P = 0.038), which was coupled with stimulated microbial activity and accelerated N transformations in the unfertilized soils. In contrast, the trees exposed to both experimental warming and N fertilization did not show increased exudation rates or soil enzyme activity, indicating that the stimulatory effects of experimental warming on root exudation depend on soil fertility. Collectively, our results provide preliminary evidence that an increase in the release of root exudates into the soil may be an important physiological adjustment by which the sustained growth responses of plants to experimental warming may be maintained via enhanced soil microbial activity and soil N transformation. Accordingly, the underlying mechanisms by which plant root‐microbe interactions influence soil organic matter decomposition and N cycling should be incorporated into climate‐carbon cycle models to determine reliable estimates of long‐term C storage in forests.     

Time and substrate dependent exudation of carboxylates by Lupinus albus L. and Brassica napus L.

Root exudates influence significantly physical, chemical and biological characteristics of rhizosphere soil. Their qualitative and quantitative composition is affected by environmental factors such as pH, soil type, oxygen status, light intensity, soil temperature, plant growth, nutrient availability and microorganisms. The aim of the present study was to assess the influence of growth substrate and plant age on the release of carboxylates from Lupinus albus L. and Brassica napus L.Both plant species were studied in continuously percolated microcosms filled with either sand, soil or sand + soil (1:1) mixture. Soil solution was collected every week at 7, 14, 21, 28 and 35 days after planting (DAP). Carboxylate concentrations were determined by reversed-phase liquid chromatography - electrospray ionization - time of flight mass spectrometry (LC-ESI-TOFMS).Oxalate, citrate, succinate, malate and maleate were detected in soil solutions of both plant species. Their concentrations were correlated with the physiological status of the plant and the growth substrate. Oxalate was the predominant carboxylate detected within the soil solution of B. napus plants while oxalate and citrate were the predominant ones found in the soil solutions of L. albus plants.The sampling determination of carboxylates released by plant roots with continuous percolation systems seems to be promising as it is a non-destructive method and allows sampling and determination of soluble low molecular weight organic compounds derived from root exudation as well as the concentration of soluble nutrients, which both might reflect the nutritional status of plants.     

利用热扩式边材液流探针（TDP）测定树木整株蒸腾耗水量的研究

 介绍了热扩散式液流探针的工作原理及利用液流探针测定树木边材液流速率的方法。利用边材液流探针和多种气象因子传感器及数据采集系统组成的微型气象站,通过对北京西山地区油松(Pinus tabulaeformis)、栓皮栎(Quercus variabilis)混交林林分平均木树干边材液流速率及风速、有效辐射和空气温度、空气相对湿度的日变化和连日变化的测定和分析,揭示了5月干旱季节两树种蒸腾耗水的日变化和连日变化规律,以及栓皮栎树干基部和树冠大枝边材液流的差异,并进行了理论推导,同时分析了液流速率的波动规律与主要气象因素波动的相关性。     

Low-level herbivory by root-knot nematodes (Meloidogyne incognita) modifies root hair morphology and rhizodeposition in host plants (Hordeum vulgare)

Low amounts of root infestation by plant parasitic nematodes are suggested to increase nutrient supply and in turn enhance microbial activity and net mineralization rate in the rhizosphere. These effects are generally related to “leakage” of plant-derived metabolites from damaged roots. Besides leakage, the present study examines other nematode–host interactions such as alterations in root exudation and morphology, which were almost not considered yet. This includes undamaged root parts in order to assess systemic plant response. The root-knot nematode Meloidogyne incognita (Kofoid and White 1919; Chitwood 1949) and barley (Hordeum vulgare L. cv. Europa) was used as model system. Host plants were grown in mini-rhizotrons inoculated with 0, 2,000, 4,000 or 8,000 M. incognita for 4 weeks. Root morphology, rhizodeposition (sugars, carboxylates, amino acids), and rhizosphere microbial communities (PLFAs) were assessed. In treatments with 4,000 nematodes, shoot biomass, total N and P content increased by the end of the experiment. Generally, an enhanced release of plant metabolites (sugars, carboxylates, amino acids) from the apical root zone occurred 1 week after inoculation with 4,000 and 8,000 M. incognita, indicating root leakage. Low levels of root herbivory stimulated root hair elongation in both infected and uninfected roots. These systemic changes in root morphology likely contributed to the increased sugar exudation in uninfected roots in all nematode treatments at 3 weeks after inoculation. Root-knots formed a separate microhabitat within the root-system. They were characterised by decreased rhizodeposition and increased fungal to bacterial ratio in the adhering rhizosphere soil. The present study provides the first evidence that, apart from leakage, nematode root herbivory at background levels induces local and systemic effects on root morphology and exudation, which in turn may affect plant performance.     

Aluminum resistance of cowpea as affected by phosphorus-deficiency stress

Plants growing in acid soils suffer both phosphorus (P) deficiency and aluminum (Al) toxicity stresses. Selection of genotypes for adaptation to either P deficiency or Al toxicity has sometimes been unsuccessful because these two soil factors often interact. Two experiments were conducted to evaluate eight cowpea genotypes for Al resistance and to study the combined effect of P deficiency and Al toxicity stress on growth, P uptake, and organic acid anion exudation of two genotypes of contrasting Al resistance selected from the first experiment. Relative root inhibition by 30 μM Al ranged from 14% to 60% and differed significantly among the genotypes. Al significantly induced callose formation, particularly in Al-sensitive genotypes. P accumulation was significantly reduced (28% and 95%) by Al application for both the Al-resistant and the Al-sensitive genotypes. Al supply significantly enhanced malate release of root apices of both genotypes. However, the exudation rate was significantly higher in the Al-resistant genotype. P deprivation induced an enhanced malate exudation in the presence of Al only in the Al-resistant genotype IT89KD-391. Citrate exudation rate of the root apices was lower than malate exudation by a factor of about 10, and was primarily enhanced by P deficiency in both genotypes. Al treatment further enhanced citrate exudation in P-sufficient, but not in P-deficient plants. The level of citrate exudation was consistently higher in the Al-resistant genotype IT89KD-391 particularly in presence of Al.It is concluded that the Al-resistant genotype is better adapted to acid Al-toxic and P-deficient soils than the Al-sensitive genotype since both malate and citrate exudation were more enhanced by combined Al and P-deficiency stresses.