Modeling soil–root water transport and competition for single and mixed crops

A knowledge of above and below ground plant interactions for water is essential to understand the performance of intercropped systems. In this work, root water potential dynamics and water uptake partitioning were compared between single crops and intercrops, using a simulation model. Four root maps having 498, 364, 431 and 431 soil-root contacts were used. In the first and second cases, single crops with deep and surface roots were considered, whereas in the third and fourth cases, roots of two mixed crops were simultaneously considered with different row spacing (40 cm and 60 cm). Two soils corresponding to a clay and a silty clay loam were used in the calculations. A total maximum evapotranspiration of 6 mm d^-1 for both single or mixed crops was considered, for the mixed crops however, two transpiration distributions between the crops were analyzed (3:3 mm d^-1, or 4:2 mm d^-1 for each crop, respectively). The model was based on a previous theoretical framework applied to single or intercropped plants having spatially distributed roots in a two-dimensional domain. Although water stress occurred more rapidly in the loam than in the clay, due to the rapid decrease of the soil water reserve in the loam, the role of the root arrangement appeared to be crucial for water availability. Interactions between the distribution of transpiration among mixed crops and the architecture of the root systems which were in competition led to water movements from zones with one plant to another, or vice versa, which corresponded to specific competition or facilitation effects. Decreasing the distances between roots may increase competition for water, although it may determine greater water potential gradients in the soil that increase lateral or vertical water fluxes in the soil profile. The effects of the root competition on water uptake were quite complicated, depending on both environmental conditions, soil hydrodynamic properties, and time scales. Although some biological adaptive mechanisms were disregarded in the analysis, the physically 2-D based model may be considered as a tool to study the exploitation of environmental heterogeneity at microsite scales.     

Modelling competition for water in intercrops: theory and comparison with field experiments

A knowledge of plant interactions above and below ground with respect to water is essential to understand the performance of intercrop systems. In this study, a physically based framework is proposed to analyse the competition for soil water in the case of intercropped plants. A radiative transfer model, associated with a transpiration-partitioning model based on a modified form of the Penman-Monteith equation, was used to estimate the evaporative demand of maize (Zea mays L.) and sorghum ( Sorghum vulgare R.) intercrops. In order to model soil–root water transport, the root water potential of each species was calculated so as to minimise the difference between the evaporative demand and the amount of water taken up by each species. A characterisation of the micrometeorological conditions (net radiation, photosynthetically active radiation, air temperature and humidity, rain), plant water relations (leaf area index, leaf water potential, stomatal conductance, sap flow measurements), as well as the two-component root systems and water balance (soil–root impacts, soil evaporation) was carried out during a 7-day experiment with densities of about 4.2 plant m-2 for both maize and sorghum. Comparison of the measured and calculated transpiration values shows that the slopes of the measured versus predicted regression lines for hourly transpiration were 0.823 and 0.778 for maize and sorghum, respectively. Overall trends in the variation of volumetric water content profiles are also reasonably well described. This model could be useful for analysing competition where several root systems are present under various environmental conditions.     

Adaptation properties of the ERG in the grasshopper,Romalea microptera

The grasshopper ERG displays a rapid recovery of responsivity following the onset of a background light. Although observed earlier in skate and frog, this phenomenon has not previously been seen in an invertebrate. Furthermore, a period of hyperresponsivity exists in early dark adaptation and resembles that found in skate and frog. Thus, recovery in the light and hyperresponsivity in the dark seem to be corollaries of each other. Finally, spectral sensitivity of the ERG is determined and two peaks are found: one at 510 nm and the other at 360 nm. The former appears to be a rhodopsin-mediated sensitivity but the latter does not and they are not clearly separated by chromatic adaptation.     

Modeling soil-root water transport with non-uniform water supply and heterogeneous root distribution

Plant and Soil - A 2D physically based framework is proposed to analyze the effect of a non-uniform water supply at the soil surface generated by rainfall interception and stemflow on soil-root...     

Root clumping may affect the root water potential and the resistance to soil-root water transport

We have appraised for clumped root systems the widely-accepted view that the resistance to water flux from soil to roots (‘soil resistance’) is low under most field conditions, so that root water potential would closely follow the mean soil water potential. Three root spatial arrangements were studied, simulating either the regular pattern generally assumed in models, or two degrees of root clumping frequently observed in the field. We used a numerical 2-dimensional model of water transfer which assumes a control of evapotranspiration by root signalling. Calculations were carried out at two evaporative demands and for two contrasting soil hydraulic properties. The rate of soil depletion, the timing of the reduction in evapotranspiration and the difference between root water potential and mean soil water potential were all affected by the root spatial arrangement, with a greater effect at high evaporative demand and low soil hydraulic conductivity. Almost all the soil water reserve was available to plants without reduction in evapotranspiration in the regular case, while only a part of it was available in clumped cases. In the regular case, calculated ‘soil resistances’ were similar to those calculated using Newman's (1969) method. Conversely they were higher by up to two orders of magnitude in clumped root spatial arrangements. These results place doubt on the generality of the view that ‘soil resistance’ is low under common field conditions. They are consistent with the results of field experiments, especially with recent data dealing with root-to-shoot communication.