The Wageningen Rhizolab — a facility to study soil-root-shoot-atmosphere interactions in crops

A research facility is described for the integrated study of soil-root-shoot-atmosphere relationships in crops. The Wageningen Rhizolab has been in use since 1990, and consists of two rows, each with eight below-ground compartments aligned along a corridor. A rain shelter automatically covers the experimental area at the start of rainfall. Compartments are 125 cm × 125 cm and 200 cm deep. Each compartment has a separate drip irrigation system. Crop canopy photosynthesis, respiration, and transpiration can be measured simultaneously and continuously on four out of eight compartments at a time. Each compartment can be filled with a selected soil material (repacked soil) and is accessible from the corridor over its full depth. Multiple sensors for measuring soil moisture status, electrical conductivity, temperature, soil respiration, trace gases and oxygen are installed in spatial patterns in accordance with the requirements of the experiments. Sensors are connected to control and data-acquisition devices. Likewise, provisions have been made to sample manually the soil solution and soil atmosphere. Root observation tubes (minirhizotrons) are installed horizontally at depth intervals ranging from 5 cm (upper soil layers) to 25 cm (below 1 m). The facility is at present in use to study growth and development of vegetation (crops) in relation to drought, nutrient status, soil-borne diseases, and underground root competition. One important application is the study of elevated CO₂ concentration and climate change and the way they affect crops and their carbon economy. Growth and development of field grown vegetables and winter cover crops are also evaluated. The common aspect of those studies is to gain a better understanding of crop growth under varying environmental conditions, and to collect datasets that may help to improve mechanistic crop growth simulation models that can address suboptimal growth conditions.     

Physical and morphological constraints on transport in nodules

For active nodule nitrogen fixation, O₂, N₂, and carbohydrate must be transported throughout the nodule. No quantitative analysis of these transport processes in the nodules has been presented. By invoking several simplifying assumptions, a second-order differential equation for the various gradients and concentrations in the nodule was solved. Even though the nodule can only be approximated in this analysis, it indicates clearly that intercellular gas spaces must exist in nodules for adequate O₂ distribution. To preserve low O₂ concentrations and protect the nitrogenase, these gas spaces cannot be in direct contact with the ambient atmosphere. It is hypothesized that a gas barrier exists in the cortical region of the nodule to limit O₂ diffusion. This barrier would not substantially inhibit N₂ transport. Carbohydrate transport from the vascular tissue via diffusion in the liquid phase can adequately accommodate the requirements within the nodule.     

CD36 deficiency increases insulin sensitivity in muscle,but induces insulin resistance in the liver in mice

CD36 (fatty acid translocase) is involved in high-affinity peripheral fatty acid uptake. Mice lacking CD36 exhibit increased plasma free fatty acid and triglyceride (TG) levels and decreased glucose levels. Studies in spontaneous hypertensive rats lacking functional CD36 link CD36 to the insulin-resistance syndrome. To clarify the relationship between CD36 and insulin sensitivity in more detail, we determined insulin-mediated whole-body and tissue-specific glucose uptake in CD36-deficient (CD36-/-) mice. Insulin-mediated whole-body and tissue-specific glucose uptake was measured by d-[3H]glucose and 2-deoxy-d-[1-3H]glucose during hyperinsulinemic clamp in CD36-/- and wild-type control littermates (CD36+/+) mice. Whole-body and muscle-specific insulin-mediated glucose uptake was significantly higher in CD36-/- compared with CD36+/+ mice. In contrast, insulin completely failed to suppress endogenous glucose production in CD36-/- mice compared with a 40% reduction in CD36+/+ mice. This insulin-resistant state of the liver was associated with increased hepatic TG content in CD36-/- mice compared with CD36+/+ mice (110.9 +/- 12.0 and 68.9 +/- 13.6 microg TG/mg protein, respectively). Moreover, hepatic activation of protein kinase B by insulin, measured by Western blot, was reduced by 54%. Our results show a dissociation between increased muscle and decreased liver insulin sensitivity in CD36-/- mice.     

Modelling Diurnal Courses of Photosynthesis and Transpiration of Leaves on the Basis of Stomatal and Non-Stomatal Responses,Including Photoinhibition

A mathematical model for photoinhibition of leaf photosynthesis was developed by formalising the assumptions that (1) the rate of photoinhibition is proportional to irradiance; and (2) the rate of recovery, derived from the formulae for a pseudo first-order process, is proportional to the extent of inhibition. The photoinhibition model to calculate initial photo yield is integrated into a photosynthesis-stomatal conductance (g _s) model that combines net photosynthetic rate (P _N), transpiration rate (E), and g _s, and also the leaf energy balance. The model was run to simulate the diurnal courses of P _N, E, g _s, photochemical efficiency, i.e., ratio of intercellular CO₂ concentration and CO₂ concentration over leaf surface (C _i/C _s), and leaf temperature (T ₁) under different irradiances, air temperature, and humidity separately with fixed time courses of others. When midday depression occurred under high temperature, g _s decreased the most and E the least. The duration of midday depression of g _s was the longest and that in E the shortest. E increased with increasing vapour pressure deficit (VPD) initially, but when VPD exceeded a certain value, it decreased with increasing VPD; this was caused by a rapid decrease in g _s. When air temperature exceeded a certain value, an increase in solar irradiance raised T ₁ and the degree of midday depression. High solar radiation caused large decrease in initial photon efficiency (). P _N, E, and g _s showed reasonable decreases under conditions causing photoinhibition compared with non-photoinhibition condition under high irradiance. The T ₁ under photoinhibition was higher than that under non-photoinhibition conditions, which was evident under high solar irradiance around noon. The decrease in C _i/C _s at midday implies that stomatal closure is a factor causing midday depression of photosynthesis.     

Plant leaf area expansion and assimilate production in wheat (Triticum aestivum L.) growing under low phosphorus conditions

Under phosphorus deficiency reductions in plant leaf area have been attributed to both direct effects of P on the individual leaf expansion rate and to a reduced availability of assimilates for leaf growth. In this work we use experimental and simulation techniques to identify and quantify these processes in wheat plants growing under P-deficient conditions. In a glasshouse experiment we studied the effects of soil P addition (0–138 kg P₂O₅ ha^-1) on tillering, leaf emergence, leaf expansion, plant growth, and leaf photosynthesis of wheat plants (cv. INTA Oasis) that were not water stressed. Plants were grown in pots containing a P-deficient (3 mg P g^-1 soil) sandy soil. Sowing and pots were arranged to simulate a crop stand of 173 plants m^-2. Experimental results were integrated in a simulation model to study the relative importance of each process in determining the plant leaf area during vegetative stages of wheat. Phosphorus deficiency significantly reduced plant leaf area and dry weight production. Under P-deficient conditions the phyllochron (PHY) was increased up to a 32%, compared to that of high-P plants. In low-P plants the rate of individual leaf area expansion during the quasi-linear phase of leaf expansion (LER) was significantly reduced. The effect of P deficiency on LER was the main determinant of the final size of the individual leaves. In recently expanded leaves phosphorus deficiency reduced the photosynthesis rate per unit leaf area at high radiation (AMAX), up to 57%. Relative values of AMAX showed an hyperbolic relationship with leaf P% saturating at 0.27%. Relative values of the tillering rate showed an hyperbolic relationship with the shoot P% saturating at values above 0.38%. The value of LER was not related to the concentration of P in leaves or shoots. A morphogenetic model of leaf area development and growth was developed to quantify the effect of assimilate supply at canopy level on total leaf area expansion, and to study the sensitivity of different model variables to changes in model parameters. Simulation results indicated that under mild P stress conditions up to 80% of the observed reduction in plant leaf area was due to the effects of P deficiency on leaf emergence and tillering. Under extreme P-deficient conditions the simulation model failed to explain the experimental results indicating that other factors not taken into account by the model, i.e. direct effects of P on leaf expansion, must have been active. Possible mechanisms of action of the direct effects of P on individual leaf expansion are discussed in this work.     

CD36 deficiency in mice impairs lipoprotein lipase-mediated triglyceride clearance

CD36 is involved in high-affinity peripheral FFA uptake. CD36-deficient (cd36(-)(/)(-)) mice exhibit increased plasma FFA and triglyceride (TG) levels. The aim of the present study was to elucidate the cause of the increased plasma TG levels in cd36(-)(/)(-) mice. cd36(-)(/)(-) mice showed no differences in hepatic VLDL-TG production or intestinal [(3)H]TG uptake compared with wild-type littermates. cd36(-)(/)(-) mice showed a 2-fold enhanced postprandial TG response upon an intragastric fat load (P < 0.05), with a concomitant 2.5-fold increased FFA response (P < 0.05), suggesting that the increased FFA in cd36(-/-) mice may impair LPL-mediated TG hydrolysis. Postheparin LPL levels were not affected. However, the in vitro LPL-mediated TG hydrolysis rate as induced by postheparin plasma of cd36(-)(/)(-) mice in the absence of excess FFA-free BSA was reduced 2-fold compared with wild-type plasma (P < 0.05). This inhibition was relieved upon the addition of excess FFA-free BSA. Likewise, increasing plasma FFA in wild-type mice to the levels observed in cd36(-)(/)(-) mice by infusion prolonged the plasma half-life of glycerol tri[(3)H]oleate-labeled VLDL-like emulsion particles by 2.5-fold (P < 0.05). We conclude that the increased plasma TG levels observed in cd36(-)(/)(-) mice are caused by decreased LPL-mediated hydrolysis of TG-rich lipoproteins resulting from FFA-induced product inhibition of LPL.     

Approach-bias predicts development of cannabis problem severity in heavy cannabis users: results from a prospective FMRI study

A potentially powerful predictor for the course of drug (ab)use is the approach-bias, that is, the pre-reflective tendency to approach rather than avoid drug-related stimuli. Here we investigated the neural underpinnings of cannabis approach and avoidance tendencies. By elucidating the predictive power of neural approach-bias activations for future cannabis use and problem severity, we aimed at identifying new intervention targets. Using functional Magnetic Resonance Imaging (fMRI), neural approach-bias activations were measured with a Stimulus Response Compatibility task (SRC) and compared between 33 heavy cannabis users and 36 matched controls. In addition, associations were examined between approach-bias activations and cannabis use and problem severity at baseline and at six-month follow-up. Approach-bias activations did not differ between heavy cannabis users and controls. However, within the group of heavy cannabis users, a positive relation was observed between total lifetime cannabis use and approach-bias activations in various fronto-limbic areas. Moreover, approach-bias activations in the dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex (ACC) independently predicted cannabis problem severity after six months over and beyond session-induced subjective measures of craving. Higher DLPFC/ACC activity during cannabis approach trials, but lower activity during cannabis avoidance trials were associated with decreases in cannabis problem severity. These findings suggest that cannabis users with deficient control over cannabis action tendencies are more likely to develop cannabis related problems. Moreover, the balance between cannabis approach and avoidance responses in the DLPFC and ACC may help identify individuals at-risk for cannabis use disorders and may be new targets for prevention and treatment.     

A mechanistic model on methane oxidation in a rice rhizosphere

A mechanistic model is presented onthe processes leading to methane oxidation inrice rhizosphere. The model is driven byoxygen release from a rice root into anaerobicrice soil. Oxygen is consumed by heterotrophicand methanotrophic respiration, described bydouble Monod kinetics, and by iron oxidation,described by a second order reaction.Substrates for these reactions – ferrous iron,acetate and methane – are produced by anexponential time dependent organic mattermineralisation in combination with modifiedMichaelis Menten kinetics for competition foracetate and hydrogen. Compounds diffusebetween rhizosphere, root and atmosphere. Adiffusion resistance between the rice root andshoot is included. Active transport across theroot surface occurs for root exudation andplant nutrient uptake. Iron adsorption isdescribed dependent on pH. The model predictswell root oxygen release, compound gradientsand compound concentrations in a ricerhizosphere. Methane oxidation estimates arecomparable to experimental estimates. Asensitivity analysis showed however thatmethane oxidation is highly dependent on modelinitialisation and parameterisation, which ishighly dependent on the history of therhizosphere and root growth dynamics.Equilibrium is not obtained within the periodthat a single root influences a soil micrositeand results in a large change in methanestorage. Equilibrium is moreover dependentupon the diffusion resistance across the rootsurface. These factors make methane oxidationdynamics highly variable in space and time anddependent on root dynamics. The increasedunderstanding of methane oxidation does notdirectly lead to increased predictiveabilities, given this high variability and theuncertainties involved in rhizospheredynamics.     

Leaf area expansion and assimilate production in sunflower (Helianthus annuus L.) growing under low phosphorus conditions

Reductions in leaf area and plant growth as a consequence of phosphorus (P) limitations have been attributed both to direct effects of P shortage on leaf expansion rate and to a reduced production of assimilates required for growth. Canopy assimilation and leaf area expansion are closely interrelated processes. In this work we used experimental and simulation techniques to identify and study their importance in determining leaf area on sunflower (Helianthus annuus L.) growing under P-deficient conditions. Experiment 1 was done outdoors, in Buenos Aires, Argentina, and Experiment 2 in a glasshouse in Wageningen, The Netherlands. In both experiments we studied the effects of soil P addition on leaf appearance, leaf expansion, dry matter accumulation, and leaf photosynthesis of non-water stressed plants grown in pots containing a P-deficient soil. Before sowing the equivalent amounts of 0–600 kg of super phosphate ha^-1 were added to the pots. Phosphorus deficiency delayed leaf appearance increasing the value of the phyllochron (PHY) up to 76%, the rate of leaf area expansion during the quasi-linear phase of leaf expansion (LER) was reduced by up to 74%, with respect to high P plants. Phosphorus deficiency reduced by up to 50% the rate of light saturated photosynthesis per unit of leaf area (AMAX) in recently expanded leaves, while at low levels of leaf insertion in the canopy, AMAX was reduced by up to 85%, when compared to that in high P plants. Phosphorus deficiency also reduced the duration of the quasi-linear phase of leaf expansion by up to eight days. The values of LER were related (r = 0.56, P < 0.05) to the mean concentration of P in all the leaves (Leaves P%) and not to the concentration of P in the individual leaf where LER was determined (r = 0.22, P < 0.4) suggesting that under P deficiency individual leaf expansion was not likely to be regulated by the total P concentration at leaf level. The values of AMAX of individual leaves were related (r = 0.79, P < 0.01) to the concentration of total P in the corresponding leaf (Leaf P%). LER showed a hyperbolic relationship with Leaves P% (R² = 0.94, P < 0.01, n = 13) that saturate at 0.14%. AMAX showed a hyperbolic relationship with Leaf P% (R² = 0.73, P < 0.01, n = 53) that saturated with values of Leaf P% higher than 0.22. A morphogenetic model of leaf area development and growth was developed to quantify the effect of assimilate supply at canopy level on total leaf area expansion, and to study the effects of model parameters on the growth of sunflower plants under P-deficient conditions. With this model we identified the existence of direct effects of P deficiency on individual leaf area expansion. However, we calculated that under mild P stress conditions up to 83% of the reduction in the observed leaf area was explained by the particular effects of P% on the rate of leaf appearance, on the duration of the linear period of leaf expansion, and on the value of AMAX. We also calculated that the effects of P deficiency on the value of AMAX alone, explained up to 41% of the observed reductions in total leaf area between the highest and the intermediate P level in Experiment 2. Possible mechanisms of action of the direct effects of P on individual leaf expansion are discussed in this paper.