Exudation Rate Research Articles

Organic Anion Exudation by Lowland Rice (Oryza sativa L.) at Zinc and Phosphorus Deficiency

The objectives of this paper were to determine (1) if lowland rice (Oryza sativa L.) plants respond similarly to low zinc (Zn) and phosphorus (P) availability by increased root exudation of low-molecular weight organic anions (LMWOAs) and (2) if genotypic variation in tolerance to low soil supply of either Zn or P is related to LMWOA exudation rates. Exudation of LMWOAs can increase bioavailability of both Zn and P to the plant, through partly similar chemical mechanisms. We used seven lowland rice genotypes and showed in two experiments that genotypes that grow relatively well on a soil with low Zn availability also grow well on a sparingly soluble Ca-phosphate (r = 0.80, P = 0.03). We measured exudation rates of LMWOAs on nutrient solution and found that both Zn and P deficiency induced significant increases. Among the LMWOAs detected oxalate was quantitatively the most important, but citrate is considered more effective in mobilizing Zn. Citrate exudation rates correlated with tolerance to low soil levels of Zn (P=0.05) and P (P = 0.07). In a low-Zn-field we found an increased biomass production at higher plant density, which is supportive for a concentration-dependent rhizosphere effect on Zn bioavailability such as LMWOA exudation. We, for the first time, showed that tolerance to low Zn availability is related to the capacity of a plant to exude LMWOAs and confirmed that exudation of LMWOAs must be regarded a multiple stress response.

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 microM 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.

Microbial production, utilization, and enzymatic degradation of organic matter in the upper trophogenic layer in the pelagial zone of lakes along a eutrophication gradient

The major aim of this study was to evaluate the relationships between the rates of microbial activities (phytoplankton primary production, bacterial secondary production, bacterial utilization of organic matter, enzymatic activities, protozoan grazing on bacteria), bacterial numbers, and dissolved organic carbon concentrations and the trophic state index (TSI) of lakes in the upper trophogenic water layer in the pelagial zone along the trophic gradient (from oligo/mesotrophy to hypereutrophy) in 19 lakes of the Mazurian Lake District (northeastern Poland). Multiple regression analysis (analysis of variance—ANOVA) on all collected data and the TSI along eutrophication gradient showed that all studied microbial processes and parameters were very tightly coupled to the trophic conditions of the studied lakes. All studied microbial processes involved in utilization and enzymatic degradation of organic matter were strongly positively dependent on the intensity and rates of photosynthetic organic matter production and exudation that markedly increased along the eutrophication gradient of lakes. Vmax of alkaline phosphatase, aminopeptidase, and nonspecific esterase showed significant correlation with the TSI of the studied lakes. Protozoans removed a significant portion of bacterial production, i.e., from ~20% to 75‐85% of newly produced bacterial biomass was simultaneously consumed by protozoans along the eutrophication gradient. These observations suggest that the importance of protozoan grazing on bacteria on regulation of bacterial production depends on lake productivity. The general working hypothesis that the intensity of microbial processes of organic matter can be tightly coupled to increasing eutrophication was proven in these studies.

A Novel Function of Abscisic Acid in the Regulation of Rice (Oryza sativa L.) Root Growth and Development

Plant roots retain developmental plasticity and respond to environmental stresses or exogenous plant growth regulators by undergoing profound morphological and physiological alteration. In this study, we investigated the effects of exogenous ABA on root growth and development in Taichung native 1 (TN1) rice. Exogenous application of 10 microM ABA leads to swelling, root hair formation and initiation of lateral root primodia in the tips of young, seminal rice roots. Cortex cells increased in size and were irregularly shaped. ABA treatment significantly increased 2, 3, 5-triphenyl tetrazolium chloride (TTC) reductase ability in the root tips and the exudation rate of xylem sap. In addition, the K(+) ion content in xylem sap increased nearly 2-fold, but not that of Ca(2+) or Mg(2+). Analysis of proteins expressed in the root tips identified several ABA-induced or -repressed proteins, including actin depolymerization factor (ADF), late embryo abundant protein (LEA), putative steroid membrane-binding protein, ferredoxin thionine reductase and calcium-binding protein. The effects of ABA on root morphogenesis change were Ca(2+) dependent and required the participation of calmodulin and de novo protein synthesis. A model is presented that illustrates how ABA acts through a potential cellular and signal transduction mechanism to induce morphological and physiological changes in rice roots.

Oxalate and ferricrocin exudation by the extramatrical mycelium of an ectomycorrhizal fungus in symbiosis with Pinus sylvestris

Accurate estimates of mycelial exudation in time and space are crucial for the assessment of ectomycorrhizal involvement in biogeochemical processes. Knowledge of exudation from mycelia of ectomycorrhizal fungi is still limited, especially for fungi in symbiosis with a host. Pinus sylvestris seedlings colonized by Hebeloma crustuliniforme were grown in aseptic multicompartment dishes. This novel system enabled identification of exudates originating only from extramatrical mycelium. At harvest, hyphal density and numbers were estimated using microscopic imaging. A fractal geometric approach was adopted for calculation of exudation rates. The main compounds identified were oxalate and ferricrocin. The exudation rate for oxalate was 19 +/- 3 fmol per hyphal tip h(-1) (mean +/- standard error of the mean) or 488 +/- 95 fmol hyphal mm(-2) h(-1). Ferricrocin rates were approx. 10 000 times lower. The fractal dimension (D) of the mycelia was 1.4 +/- 0.1, suggesting an explorative growth. Potassium nutrition was a significant regulatory factor for ferricrocin but not oxalate. The results suggest that hyphal exudation may alter the chemical conditions of soil microsites and affect mineral dissolution. Calculations also indicated that oxalate exudation may be a significant carbon sink.

Evidence That Sulfur Deficiency Enhances Molybdenum Transport in Xylem Sap of Tomato Plants

ABSTRACT The findings of an experiment using serially harvested young tomato plants grown in water culture are reported, showing the dramatic influence of sulfur status of the nutrient medium on transport of molybdenum in xylem sap. On average for the five harvests taken over a 14 d growth period, the molybdenum (Mo) concentration in the sap was approximately 11 times greater in the absence of sulfate in the nutrient medium. Restoring sulfate to the nutrient medium without sulfur (S) on day four of the experiment depressed the Mo concentration of the sap at the next harvest (taken three days later) to a value similar to that in plants receiving sulfate from the onset of the growth period and, similarly, raised the S concentration as well. Rates of transport of Mo as measured by root pressure exudates were slightly less spectacular, as S deficiency depressed the rate of exudation. The results support the concept that sulfate and molybdate compete for the same carrier and transport sites in uptake, and that sulfate deficiency leads to excess Mo uptake. The findings are of little consequence for plant cultivation, as plants are tolerant to elevated Mo concentrations, but are relevant to animal nutrition—particularly that of ruminants, which are susceptible to excess Mo and Mo-induced copper (Cu) deficiency.

Ecophysiological characterization of carnivorous plant roots: Oxygen fluxes, respiration, and water exudation

Various ecophysiological investigations on carnivorous plants in wet soils are presented. Radial oxygen loss from roots of Droseraceae to an anoxic medium was relatively low 0.02 - 0.07 µmol(O2) m -2 s -1 in the apical zone, while values of about one order of magnitude greater were found in both Sarracenia rubra roots and Genlisea violacea traps. Aerobic respiration rates were in the range of 1.6 - 5.6 µmol kg -1 (f.m.) s -1 for apical root segments of seven carnivorous plant species and 0.4 - 1.1 µmol kg -1 (f.m.) s -1 for Genlisea traps. The rate of anaerobic fermentation in roots of two Drosera species was only 5 - 14 % of the aerobic respiration. Neither 0.2 mM NaN3 nor 0.5 mM KCN influenced respiration rate of roots and traps. In all species, the proportion of cyanide-resistant respiration was high and amounted to 65 - 89 % of the total value. Mean rates of water exudation from excised roots of 12 species ranged between 0.4 - 336 mm 3 kg -1 (f.m.) s -1 with the highest values being found in the Droseraceae. Exudation from roots was insensitive to respiration inhibitors. No significant difference was found between exudation rates from roots growing in situ in anoxic soil and those kept in an aerated aquatic medium. Carnivorous plant roots appear to be physiologically very active and well adapted to endure permanent soil anoxia. Additional key words: aerobic respiration, anaerobic CO2 release, CN - -resistant respiration, Dionea, Drosera, Genlisea, Sarracenia, soil anoxia.

The prevalence and production of turf-forming algae on a temperate subtidal coast

M. Copertino, S.D. Connell and A. Cheshire. 2005. The prevalence and production of turf-forming algae on a temperate subtidal coast. Phycologia 44: 241–248.This study shows that canopy-forming algae, composed mainly of fucoids and the kelp Ecklonia radiata (both Phaeophycea), dominated space on South Australian coasts relative to turf-forming algae. However, where canopy-forming algae are absent turf-forming algae are the primary occupiers of space (~ 70%). On some reefs where canopy-forming algae are restricted in spatial extent, turfs can occupy as much as 40% of reefs. Turf-forming algae are an abundant component of algal assemblages, but relatively little is known about their contribution to the primary productivity on temperate reefs, relative to canopy-forming species. This study reveals that net productivity rates of turfs at one South Australian location were very high across depths (1.3–2.9 g C m−2 day−1 or 23.2–88.0 mg C g ash-free dry weight−1 day−1), comparable to the values discovered on tropical reefs. Although turf-forming algae are much more productive than canopy-forming algae on a biomass basis, the annual net production per area is two to seven times lower for turfs than for canopy-forming algae. However, if negligible exudation rates are assumed for turfs (up to 1%), the biomass of carbon produced by turf algae represents 44–71% of the carbon incorporated into biomass of kelps. Taken together, these results suggest that while canopy-forming algae can be correctly assumed to be the major source of total carbon produced on temperate reefs, the contribution of turf-forming algae may be substantial to the biomass production and turnover on South Australian reefs.

Root Cluster Formation and Citrate Exudation of White Lupin (Lupinus albus L.) as Related to Phosphorus Availability

: A split-root system was used to investigate whether the external or internal P concentration controls root cluster formation and citrate exudation in white lupin (Lupinus albus L.) grown under controlled conditions. In spite of low P concentrations in the shoots and roots of the -P plant, its dry weight was not reduced compared with the +P plant. Supplying external P (0.25 mmol/L) to one root half resulted in an increase in P concentration not only in the shoot, but also in the P-deprived root half, indicating P cycling within the plants. Omitting P from both split-root pots stimulated root cluster formation in both root halves, whereas P supply to one root half stimulated root cluster formation at the beginning of the treatment. Neither P supply to just one root half continuously nor resupply of P to one root half after 19 d of P starvation inhibited root cluster formation on the P-deprived side, although the concentration of P in this root half and shoot increased markedly. The results indicate that root cluster formation in L. albus is controlled by both shoot and root P concentrations. The rates of citrate exudation by both root halves with P deficiency were higher than those of the one root half supplied with P only. In the treatment with one root half supplied with P, the rates of citrate exudation by either the P-supplied or -deprived root halves were almost the same, regardless of P concentration in the roots. The results suggest that internal P concentration controls root cluster formation and citrate exudation in white lupin, but these processes may be regulated by different mechanisms. ( Managing editor: Ping HE)

Plant phosphorus status has a limited influence on the concentration of phosphorus-mobilising carboxylates in the rhizosphere of chickpea

Two experiments were conducted to investigate whether carboxylate exudation by chickpea (Cicer arietinum L.) is a response to phosphorus (P) deficiency or a constitutive trait. The effect of P supply on carboxylate concentrations in the plant and in the rhizosphere of chickpea cultivar Heera was studied in a sand culture. Plants were grown in pots supplied with 200 mL of solution containing 0-500 μm P every 3 d. Malonate was the main carboxylate exuded, and the main carboxylate in roots; shoots contained mainly citrate and malate. Contrary to what has been reported for other species, carboxylate concentrations in the rhizosphere decreased only slightly at high P supply, but they were still substantial. The effect of P supply on the rate of exudation was studied in a split-root sand culture. Root systems were split into two pots, one root half received no P and the other half received 200 mL of solution containing 0-500 μm P. The rhizosphere of both root halves contained similar concentrations of carboxylates, even when the plants received a different supply of P. Our results indicate that carboxylate exudation is determined by internal P rather than external factors. The fact that chickpea roots always exude carboxylates indicates that exudation in this species is largely constitutive.

Sources and sinks of dissolved phytochelatin in natural seawater

We modeled dissolved phytochelatin concentrations by studying its exudation from phytoplankton and its removal from seawater. Exudation rates were obtained from experiments in which a marine diatom, Thalassiosira pseudonana, was grown in Cd-containing medium. Dissolved phytochelatin in the culture medium was measured as a function of time, and a first-order model was fit to the data to derive exudation rate constants. A synthetic phytochelatin standard was used to study phytochelatin removal from natural seawater. The addition of Cd or Cu, as well as decreasing temperature, significantly decreased phytochelatin removal rates. An acclimation period followed by rapid removal was observed; subsequent incubations showed no acclimation. Using rate constants derived from these experiments and some assumptions regarding the particulate phytochelatin pool, we used a model to calculate the steady state concentration of dissolved phytochelatin in the field and obtained values that are close to field measurements of dissolved phytochelatin in the Elizabeth River estuary, Norfolk, Virginia, where metal concentrations are high.

Contents of soluble, cell-wall-bound and exuded phlorotannins in the brown alga Fucus vesiculosus, with implications on their ecological functions.

Phlorotannins are ubiquitous secondary metabolites in brown algae that are phenotypically plastic and suggested to have multiple ecological roles. Traditionally, phlorotannins have been quantified as total soluble phlorotannins. Here, we modify a quantification procedure to measure, for the first time, the amount of cell-wall-bound phlorotannins. We also optimize the quantification of soluble phlorotannins. We use these methods to study the responses of soluble and cell-wall-bound phlorotannin to nutrient enrichment in growing and nongrowing parts of the brown alga Fucus vesiculosus. We also examine the effects of nutrient shortage and herbivory on the rate of phlorotannin exudation. Concentrations of cell-wall-bound phlorotannins were much lower than concentrations of soluble phlorotannins; we also found that nutrient treatment over a period of 41 days affected only soluble phlorotannins. Concentrations of each phlorotannin type correlated positively between growing and nongrowing parts of individual seaweeds. However, within nongrowing thalli, soluble and cell-wall-bound phlorotannins were negatively correlated, whereas within growing thalli there was no correlation. Phlorotannins were exuded from the thallus in all treatments. Herbivory increased exudation, while a lack of nutrients had no effect on exudation. Because the amount of cell-wall-bound phlorotannins is much smaller than the amount of soluble phlorotannins, the major function of phlorotannins appears to be a secondary one.

Nutrient uptake, cluster root formation and exudation of protons and citrate in Lupinus albus as affected by localized supply of phosphorus in a split-root system

White lupin ( Lupinus albus L. cv. Amiga) plants were grown in nutrient solution, using a split-root system with two compartments to examine the effect of localized phosphorus (P) supply on nutrient uptake, cluster root formation and root exudation. Phosphorus was supplied at 0 and 250 μM P in the −P and +P compartments, respectively, with three treatments of −P+P, −P−P and +P+P. Shoot dry weight in −P+P was higher than that in −P−P, but not significantly different from that in +P+P. Shoot P concentration [P] in −P+P was lower than that in +P+P, but higher than that in −P−P. In the −P+P treatment, root [P] was lower in the −P than in the +P root half and the similar trend was observed for root dry weight. Compared with −P−P, 19% of P uptake in the +P root half in −P+P was translocated to the −P root half. Phosphorus deficiency decreased K, Mg and S uptake in −P−P, and localized P deficiency reduced S uptake in the −P root half in −P+P. The number of cluster roots and exudation rates of protons, citrate and acid phosphatase (APase) in both root halves were higher in −P−P than in +P+P. The localized P supply increased the total number of cluster roots in the +P root half compared with the −P root half in −P+P. Compared with −P−P, the localized P supply significantly increased the shoot [P] and decreased the exudation rate of citrate and APase in both root halves in −P+P. Exudation rate of citrate in the −P root half in −P+P was higher than that of the +P root half in +P+P. Localized P supply reduced proton release in the +P and even −P root halves in −P+P, but the pH was lower in the −P than +P compartments for the individual plant, indicating a significant effect of the localized P supply on proton release. The results suggest that cluster root formation and citrate exudation are regulated by the shoot [P] and affected by localized supply of external P, and that proton release is inhibited by localized P supply by altering the balance of anion and cation uptake.

Production, respiration and exudation of dissolved organic matter by the kelp Laminaria hyperborea along the west coast of Norway

Changes in O2 concentration were measured during in situ incubation of whole, young Laminaria hyperborea and used to evaluate primary production and respiration over a 3-y period (1995–1998). The exudation of dissolved organic carbon (DOC) during incubation was also measured. Total carbohydrates and polyphenols in the exudates and the regional seawater were determined and fractionated into low molecular weight (LMW) and high molecular weight (HMW) species. Net production rates varied significantly, being highest during the growth phase (March), while respiration rates did not vary significantly. The exudation rates correlated with the plant strategy of growth and production in which high requirements for growth led to a lower proportion of fixed carbon being exuded. More fixed carbon was exuded during the non-growth phase. The annual exudation of DOC is estimated to be ∼26% of fixed carbon and amounts to ∼1·29 kgC m−2 y−1. The annual amount of carbon retained by the kelp (the difference between fixed and exuded carbon), assumed to contribute to growth, corresponds to a biomass increase of 3·0 kgC m−2 y−1, a value similar to those reported using direct measurements of biomass increase in the region. Carbohydrate concentrations in the exudates were lower during March than in other months, in accordance with the plant growth strategy. Phenols, on the other hand, which are assumed to function as predator deterrent, showed no significant variations in the exudate. The fraction is dominated by carbohydrates while phenols occurred mainly in the LMW fractions.

Exudation of organic anions by roots of cabbage, carrot, and potato as influenced by environmental factors and plant age

AbstractA previous study demonstrated that cabbage was P efficient compared to carrot and potato. However, calculating plant P uptake by a mechanistic simulation model based on P transport by diffusion and mass flow, P uptake of roots according to the Michaelis‐Menten kinetics, and morphological root characteristics including root hairs, revealed that these parameters could explain only 2/5 of the total P uptake of cabbage, but 4/5 of that of carrot and potato (Dechassa et al., 2003). Therefore, it was hypothesized that a higher root exudation of organic anions may enhance P mobilization and hence P uptake of cabbage. The objective of this research was to determine root exudation of organic anions by the three species, and to investigate the influence of plant age and dark/light period on organic‐anion exudation by cabbage. Experiments were conducted in a growth chamber in nutrient solution with or without P. Organic anions were determined in root exudates and in root tissue. With cabbage and potato, P deficiency induced exudation of citrate and succinate, respectively. Citrate‐exudation rate of P‐deficient cabbage plants was correlated with accumulation of citrate in root tissue. In contrast, high succinate‐exudation rates in potato were not correlated with an increased concentration in root tissue. For carrot, no change was observed in the exudation of any of the organic anions in response to P deficiency. The results also showed that succinate‐ and citrate‐exudation rates of cabbage roots increased with increased plant age. There was also a significant increase in exudation rates of organic anions of cabbage roots during the light period of the day. It was concluded that cabbage had the ability to exude large amounts of citrate in response to P deficiency by which it can additionally enhance its P‐uptake efficiency, whereas carrot and potato showed little evidence of possessing such a mechanism.

Rhizosphere carboxylate concentrations of chickpea are affected by genotype and soil type

We investigated whether concentrations of carboxylates in the rhizosphere of chickpea (Cicer arietinum L.) roots were related to soil phosphorus levels. In a field experiment, cultivar Sona was grown at two P levels on eight soil types at three locations. There were large differences in extractable (0.2 mM CaCl2) rhizosphere carboxylate concentrations amongst the locations. The effect of P fertiliser was variable and carboxylate concentrations depended on soil type. To examine the effect of soil P in more detail, a glasshouse experiment was carried out, in which three cultivars (Heera, Sona and Tyson) were grown at four P levels on one soil type. The biomass of chickpea plants increased with increasing P level of the soil, and the root mass ratio decreased at the highest soil P level. However, rhizosphere concentrations of the carboxylates malonate, malate and citrate did not differ significantly between P treatments. This implied that there was no simple relation between available P and root exudation rates, in contrast to earlier results in studies using hydroponics. Cultivars differed in carboxylate concentration pattern: Sona and Tyson showed a tendency towards increased rhizosphere carboxylate concentrations at the second harvest, whereas the carboxylate concentration of Heera tended to decrease. It is hypothesised that chickpea roots always exude a basal level of carboxylates into the rhizosphere. They only increase carboxylate exudation considerably when the P availability is extremely low, which may occur in soils that strongly bind P.

Rhizosphere effect and root growth of two maize ( Zea mays L.) genotypes with contrasting P efficiency at low P availability

Exploring the genetic resource of crops is one alternative way to utilize the less available phosphorus (P) in soils, and copy with the incoming shortage of rock phosphate (Rock P). Genotypic differences in low-P tolerance exist in many crop species and result from various physiological and morphological mechanisms. In this study, low-P tolerance of two maize genotypes that had been identified to have contrasting P efficiency (grain yield) in a calcareous soil was investigated. Parameters measured were biomass accumulation, root growth and root exudation of organic acids, root acid phosphatase (APase) activity, and rhizosphere pH under P-deficient (−P) and P-sufficient (+P) conditions in solution culture. The results showed that −P treatment increased root biomass (from 6th to the 15th day), root to shoot ratio, lateral root length, and APase activity in roots and on the root surface, but reduced root exudation of organic acids and pH in rhizosphere for both genotypes. The P-efficient line 181 had a larger root system in terms of root weight and lateral root length than the P-inefficient line 197 in both P treatments, indicating root morphology of line 181 is an advantageous but non-specific trait in adaptation to low P stress. Genotype 181, when grown with −P markedly reduced the pH of the solution and rizhosphere and increased the APase activity in the roots and on the root surfaces. Surprisingly, root exudation of organic acids was reduced by −P in both genotypes. Exudation rate of organic acids in 181 was lower than that of 197 under both P treatments. It was concluded that efficient use of P in the calcareous soil by 181 is related to its large root system, greater ability to acidify the rhizosphere, and positive response of APase production and excretion to low P conditions.

Mathematical Simulation of the Dynamics of Interacting Populations of Rhizosphere Microorganisms

A quantitative model is proposed to describe the population dynamics of associative nitrogen-fixing microorganisms in the plant rhizosphere as dependent on the rate of carbon substrate exudation by plant roots. By changing the values of the basic model parameters, the effect of various factors on the behavior of two competing populations of rhizosphere microorganisms can be studied.

Roots regulate ion transport in the rhizosphere to counteract reduced mobility in dry soil

Diffusion of ions in the soil depends on soil moisture content. In a dry soil, transport of nutrients towards the root and the concomitant uptake could be reduced. However, pot and field experiments showed that this is not always the case. The objective of this paper was to investigate possible mechanisms of plants to counteract reduced nutrient supply due to water shortage. A split root system was used to investigate P and K inflow of oat and sugar beet at different soil moisture contents (Θ) without water shortage for the plant. The measured average P and K inflows were compared to model calculations considering diffusion, mass-flow, sorption and uptake processes. In the calculations, soil dryness impeded diffusion and decreased nutrient inflow as expected. Measured K inflow was decreased in a similar way indicating that Θ influences K diffusion. In contrast to this, measured P inflow was not influenced by Θ and under-estimated by the model. Low and high molecular exudates were collected at different water supply levels showing that exudation rate of both compounds was increased at water shortage. Especially the high molecular exudates (i.e. mainly mucilage) from water-stressed plants increased P concentration in soil solution under dry conditions in an incubation experiment. Calculated inflow considering this increased P concentration agreed well with measured P inflow indicating that exudation of mucilage could be a mechanism to overcome nutrient transport problems due to soil dryness.

Temporal dynamics of carbon partitioning and rhizodeposition in wheat.

The temporal dynamics of partitioning and rhizodeposition of recent photosynthate in wheat (Triticum aestivum) roots were quantified in situ in solution culture. After a 30-min pulse of (14)CO(2) to a single intact leaf, (14)C activities of individual carbon fluxes in the root, including exudation, respiration, and root content, were measured continuously over the next 20 h concurrently with (14)C efflux from the leaf. Immediately after the end of the (14)CO(2) pulse, (14)C activity was detected in the root, the hydroponic solution, and in root respiration. The rate of (14)C exudation from the root was maximal after 2 to 3 h, and declined to one-third of maximum after a further 5 h. Completion of the rapid phase of (14)C efflux from the leaf coincided with peak (14)C exudation rate. Thus, exudation flux is much more rapidly and dynamically coupled to current photosynthesis than has been appreciated. Careful cross-calibration of (14)C counting methods allowed a dynamic (14)C budget to be constructed for the root. Cumulative (14)C exudation after 20 h was around 3% of (14)C fixed in photosynthesis. Partitioning of photosynthate between shoot and root was manipulated by partial defoliation before applying the (14)CO(2) pulse to the remaining intact leaf. Although the rate of photosynthesis was largely unaffected by partial defoliation, the proportion of new photosynthate subsequently partitioned to and exuded from the root was substantially reduced. This clearly indicates that exudation depends more on the rate of carbon import into the root than on the rate of photosynthesis.