Root Front Research Articles

Do newer maize hybrids grow roots faster and deeper?

AbstractUnderstanding historical changes in root depth attributes is needed for crop productivity and sustainability assessments, but such information is rare. We explored whether newer maize (Zea mays L.) hybrids grow roots faster and deeper than older hybrids and quantified the role of management and environment on root trait expression. We measured root front velocity (RFV) and maximum root depth in 11 Bayer Crop Science legacy hybrids released from 1983 to 2017 across five environments in the US Corn Belt during 2021 and 2022. Root depth was measured weekly during vegetative stages with manual probes and the maximum root depth at crop harvest with a Giddings probe. Results indicated that the RFV and maximum root depth slightly increased with the year of hybrid release (0.13% per year, p = 0.1) at 8.7 plants m−2. Historical increases in plant density from 4.7 to 8.7 plants m−2 lowered RFV and maximum root depth, but the new hybrids compensated for this loss, resulting in 4% higher RFV and 3% higher maximum depth when comparing systems from 1983 to 2017. The environment strongly influenced root trait expression (>41%). Rain anomaly and soil bulk density explained a portion of this variation. We found a linear relationship between root depth and leaf number (R2 = 0.95) and a nonlinear relationship between RFV and maximum root depth (R2 = 0.77), which can stimulate crop model improvements. Faster and deeper roots were not correlated with maize yields in our environments. This study enhances our understanding of maize breeding impacts on root traits.

Effects of Slope Aspect and Rainfall on Belowground Deep Fine Root Traits and Aboveground Tree Height.

The vertical root distribution and rooting depth are the main belowground plant functional traits used to indicate drought resistance in arid and semiarid regions. The effects of the slope aspect on the aboveground traits are visible but not the belowground deep root traits. We aimed to investigate the fine root traits of the locust tree (Robinia pseudoacacia L.) planted on southerly and northerly aspects, and the variations in the rooting depth in regions with different rainfall, as well as assessing how deep rooting, might affect the response to drought in a loess region. We selected three study sites with different rainfall amounts, with six sampling plots at each site (three each with southerly and northerly aspects). Soil core samples were collected down to the depth where no roots were present. The locust trees tended to develop deeper fine roots rather than greater heights. The tree height and diameter were greater for locust trees on northerly aspects, whereas trees on southerly aspects had significantly deeper rooting depths. Fine root traits (root length, root area, and root dry weight density) were higher in the southerly aspect for both Changwu and Ansai, but lower in Suide. The ratio of the root front depth tree height ranged from 1.04 to 3.17, which was higher on southerly than northerly aspects, and it increased as the rainfall decreased. Locust tree growth traits (belowground fine root and aboveground tree height) were positively correlated with the mean annual rainfall. The soil moisture content of the topsoil decreased as the rainfall decreased, but the pattern varied in the deep layer. Our results suggest that the variations in the belowground rooting depth under different slope aspects may be related to plant survival strategies. The vertical extension of the rooting depth and tree height may be key functional traits that determine plant growth in drought-prone regions.

Comparative analysis of the fracture splitting deformation of the big end for C70S6, 36MnVS4 and 46MnVS5 connecting rods

During the fracture splitting process of the connecting rod, the plastic deformation, especially the plastic deformation before cracking, not only causes the big end hole of the connecting rod to be out of roundness, but also increases the residual stress, affecting the assembly precision and even the service life of the connecting rod. In this paper, the mechanical properties of microalloyed steel C70S6, 36MnVS4 and 46MnVS5 connecting rod were determined by tensile test. The displacement field, plastic strain and the deformation principle of the big end before the crack initiation of the fracture splitting of the connecting rod were numerically simulated. The results show that the yield strength, tensile strength and plasticity index of the new medium carbon microalloyed steels 46MnVS5 and 36MnVS4 are significantly higher than those of C70S6. However, the C70S6 connecting rod has a long loading time before crack initiation, and the plastic deformation area of the groove root front is large and the strain is high. The plastic deformation of 36MnVS4 and 46MnVS5 connecting rods is small and the area is concentrated. Among the three, the 46MnVS5 connecting rod has the smallest deformation and the C70S6 connecting rod has the largest deformation.

Influences of edaphoclimatic conditions on deep rooting and soil water availability in Brazilian Eucalyptus plantations

Brazilian roundwood industry is one of the most important and productive in the world, with Eucalyptus plantations alone representing 73% of the total planted forests. Deep rooting in these plantations represents a more common phenomena than generally expected. However, there is still a lack of information on environmental factors that drive root growth in deep soil layers, with particular emphasis on edaphoclimatic conditions, and related consequence in terms of soil water behavior. As a part of a larger project, this research aimed to investigate soil water and fine root system distribution in deep tropical soils under a commercial Eucalyptus plantation chronosequence. Along a 2800-km gradient (from south- to north-east Brazil), 14 experimental areas were planted with a “plastic” clone (E. urophylla) and investigated in terms of climatic conditions, soil and water features, and plant/stand development for an entire 6-years rotation period. Fine roots distribution were investigated in one site (in Brazil) till to 20 m deep at 3, 9, 24, 48, and 65 months after planting. Results showed a fast displacement of the root front down to 75, 325, 825, 1250, and 1575 cm at month 3, 9, 24, 48, and 65 after planting, respectively. Fine root densities (g cm−3) and proportional water capture exponentially decreased with soil depth. Deep fine roots showed a relativelly higher efficiency in acquiring water than the shallower, denser roots. The relationship between stand height vs root front depth followed an exponential trend, suggesting that these stands developed relatively faster in height rather than in depth during the first 48 months, with the opposite characterizing plantation afterwards. Regardless of stand age, E. urophylla trees rapidly explored a considerable volume of soil at a relatively limited carbon cost. Multivariate statistics showed that edaphoclimatic conditions play a major role in Eucalyptus plant/stand development. This study outlined the major role played by soil development. From poorly developed sandy Entisols, to medium developed Inceptisols, and to most developed fine textured Oxisols, both plant growth and stand productivity greatly improved accordingly. This study suggests that soil type, together with other environmental factors, are likely to influence both the development and behavior of Eucalyptus plantations for an extent greater than commonly anticipated.

Field estimation of water extraction coefficients with APSIM-Slurp for water uptake assessments in perennial forages

The assessment of water uptake in agricultural systems commonly relies on the use of process-based biophysical models. To accurately represent crop-soil relationships, these models require local calibration which is often limited by the availability of site-specific data. This is the case for the water extraction coefficient (kl) in perennial forage cropping systems. The kl is critical to control water uptake by roots influencing soil moisture dynamics at different soil depths. This crop-soil parameter is particularly challenging to assess in species that establish deep root systems during long periods of regrowth, such as perennial forages. Using three years of detailed field data, we test a method to estimate kl for two perennial crops of broad socio-economic significance (lucerne and perennial ryegrass). The method is based on three physically meaningful parameters: Root Front Velocity (RFV, mm/day), surface kl (kl0,/day) and the rate of kl decay with soil depth (λkl, dimensionless). Our analysis showed that soil volumetric water content dynamics was most sensitive to kl0 values. A model fitting procedure showed that the highest accuracy of soil volumetric water content (θ) simulations was obtained with RFV of 10 mm/day and a kl0 of 0.11/day for both forages. In contrast, λkl estimates differed among species, being higher for shallow fibrous ryegrass roots than for the deep lucerne taproots. The analysis also highlighted that multiple parameter set combinations were found to give simulations of acceptable accuracy. The proposed method can be used as a first approximation to parameterise kl when local calibration data is unavailable. However, even for the fitted parameter sets, the accuracy of θ estimates was low at soil depths where there was a substantial transition of soil texture. These insights highlight important aspects to be considered in future development and parameterisation of water uptake models for perennial forages.

Maize and soybean root front velocity and maximum depth in Iowa, USA

Quantitative measurements of root traits can improve our understanding of how crops respond to soil and weather conditions, but such data are rare. Our objective was to quantify maximum root depth and root front velocity (RFV) for maize (Zea mays) and soybean (Glycine max) crops across a range of growing conditions in the Midwest USA. Two sets of root measurements were taken every 10–15days: in the crop row (in-row) and between two crop rows (center-row) across six Iowa sites having different management practices such as planting dates and drainage systems, totaling 20 replicated experimental treatments. Temporal root data were best described by linear segmental functions. Maize RFV was 0.62±0.2cmd−1 until the 5th leaf stage when it increased to 3.12±0.03cmd−1 until maximum depth occurred at the 18th leaf stage (860°Cd after planting). Similar to maize, soybean RFV was 1.19±0.4cmd−1 until the 3rd node when it increased to 3.31±0.5cmd−1 until maximum root depth occurred at the 13th node (813.6°C d after planting). The maximum root depth was similar between crops (P>0.05) and ranged from 120 to 157cm across 18 experimental treatments, and 89–90cm in two experimental treatments. Root depth did not exceed the average water table (two weeks prior to start grain filling) and there was a significant relationship between maximum root depth and water table depth (R2=0.61; P=0.001). Current models of root dynamics rely on temperature as the main control on root growth; our results provide strong support for this relationship (R2>0.76; P<0.001), but suggest that water table depth should also be considered, particularly in conditions such as the Midwest USA where excess water routinely limits crop production. These results can assist crop model calibration and improvements as well as agronomic assessments and plant breeding efforts in this region.

Deep rooting of rainfed and irrigated orange trees in Brazil

Fine roots of orange trees reached a depth of approx. 6 m in a sandy tropical soil. Root area index was more responsive to irrigation than leaf area index. Although the crucial role of deep rooting on the hydraulic functioning of tropical trees has been pointed out, studies dealing with root development below 2 m are still scarce. Our study aimed to gain insight into the fine root traits of rainfed and irrigated orange trees down to the root front in deep tropical soils. Irrigation was applied during dry periods, only 3–15% more than the annual amounts of water supplied by rain. Fine roots were sampled down to a depth of 8 m on four dates in a randomized block design. The effects of soil depth and irrigation on major fine root traits, total fine root length and the relationship between leaf area index (LAI) and root area index (RAI) were studied. The total fine root mass was 728 g m−2 in rainfed plots and 536 g m−2 in irrigated plots during the driest period (Sept/Oct 2012). Across the four sampling dates, the mean depth of the root front was 6.1 m in rainfed plots and 5.5 m in irrigated plots close to the trees, and approximately 4.5 m in the inter-row covered by Brachiaria decumbens plants. LAI was little influenced by irrigation (about 4.5 m2 m−2), but mean RAI was 19.1 m2 m−2 in rainfed plots and 13.7 m2 m−2 in irrigated plots. Small irrigation rates reduced root development in very deep soil layers, and increased fruit production by 9% during the dry year. Deep rooting provides access to water stored in deep soil layers during the rainy season, and thus might have an important functional role during dry periods in tropical orange orchards.

A fast exploration of very deep soil layers by Eucalyptus seedlings and clones in Brazil

Although pioneer studies showed several decades ago that deep rooting is common in tropical forests, direct measurements of fine root distributions over the entire soil profile explored by the roots are still scarce. Our study aimed to compare, 2years after planting, fine root traits of Eucalyptus trees planted from cuttings and from seedlings in order to assess whether the propagation mode has an influence on the capacity of the trees to explore very deep soils. Soils cores were sampled down to a depth of 13.5m at the peak of leaf area index (LAI), 2years after planting, under three Eucalyptus clones (belonging to species E. saligna, E. grandis×E. urophylla, E. grandis×E. camaldulensis) and under E. grandis seedlings in the same Ferralsol soil. LAI was estimated using allometric equations based on destructive sampling of eight trees per genotype.All the genotypes exhibited fine root densities roughly constant between the depths of 0.25 and 6.00m. Changes in fine root traits (diameter, specific root length and specific root area) were low between the topsoil and the root front. The ratios between mean tree height and root front depth ranged from 0.8 to 1.2 for the four genotypes. Although tree vertical extension was roughly symmetric above and belowground for all the genotypes, the depth of the root front ranged from 8.0m for the seedlings and the E. grandis×E. urophylla clone to 11.5m for the E. saligna clone. Soil water content profiles suggested that the four genotypes had the capacity to withdraw water down to a depth of 8–10m over the first 2years after planting. Total fine root length ranged from 3.3 to 6.0km per m2 of soil depending on the genotype. The root area/leaf area ratio ranged from 1.3 to 3.2 and was negatively correlated with LAI across the four genotypes. This pattern suggests that the genotypes more conservative for water use (with a low LAI) invest more in fine root area relative to leaf area than genotypes adapted to wet regions (with a high LAI). The velocity of downward movement of the root front might be a relevant criterion in the last stage of the breeding programs to select clones with a fast exploration of deep soil layers in drought prone regions.

Almost symmetrical vertical growth rates above and below ground in one of the world's most productive forests

Whilst the relationships between growth strategies and leaf traits are well established in functional plant ecology, little attention has been paid to root traits in very deep soil layers. The objective of our study was to compare the vertical velocity of the above- and belowground exploration of the environment for one of the fastest-growing tree species. Fine roots were studied in a chronosequence of intensively-managed Eucalyptus plantations established on highly weathered soils. Here we show that the root front depth was accurately predicted at 85% of mean tree height for stands <20 m in height, in the absence of any physical or chemical barrier. Tree height and root front growth velocities peaked at 0.59 and 0.55 m month?1 respectively 9-10 months after planting, and decreased steadily thereafter. Fast root front displacement might provide a competitive advantage to fast-growing species in forests established on deep soils. Our study may contribute to the debate on the environmental impact of short-rotation plantation forests in the Tropics. (Resume d'auteur)

Growth of the whole root system for a plant crop of sugarcane under rainfed and irrigated environments in Brazil

Sugarcane crops are managed over 8 million hectares in Brazil and future extensions might occur on less favorable lands where irrigation would be necessary to increase and stabilize yields. Root growth was studied by sequential soil coring under rainfed and irrigated conditions for one cultivar widely planted in Brazil. Root length densities (RLD) were measured 34, 49, 125, 179, 241 and 322 days after planting (DAP) down to a depth of 1 m. At the harvest (332 DAP), root intersects (a proxy for RLD) were counted on two vertical trench walls in each water supply regime, down to a depth of 6.0 m. The highest RLD in deep layers (below a depth of 0.6 m) were observed in the rainfed crop from 125 DAP onwards. By contrast, the highest RLD in the upper layers during dry periods were found in the irrigated crop. The maximum depth reached by roots at the harvest was little affected by irrigation: 4.70 m and 4.25 m in the rainfed and irrigated crop, respectively. About 50% of root intersects were observed below the depth of 1 m in the two water supply regimes. This pattern suggested a strong genetic control of root growth in deep soil layers. The total amount of root intersects 332 DAP was 49% higher in the rainfed crop than in the irrigated crop, and root distribution was more homogeneous. Mean root front velocity was about 0.5 cm day −1 the first 4 months after planting and increased thereafter up to the end of the harvest (1.86 cm day −1 and 1.75 cm day −1 on average in the rainfed and the irrigated crops, respectively). Our study pointed out the necessity to take into account the development of sugarcane roots in deep soil layers to improve our understanding of net primary production control by water availability.

Root characteristics of selected field crops: Data from the Wageningen Rhizolab (1990–2002)

Since being built in 1990, the rhizotron facility in Wageningen, the Wageningen Rhizolab, has been used for experiments on crops (e.g. Alfalfa, Brussels sprouts, common velvet grass, field bean, fodder radish, leeks, lupins, maize, potato, beetroot, ryegrass, spinach, spring wheat, winter rye and winter wheat). In the experiments, horizontal glass minirhizotron tubes combined with auger sampling were used to assess rooting characteristics. For this paper we took the root data from these experiments and looked for a general relationship between thermal time/time after planting and rooting depth, the velocity of the root front and root proliferation. For certain depths (fixed by the depth at which the horizontal minirhizotrons were installed) a simple linear regression was established between the average root number per cm2 minirhizotron surface area and thermal time after planting. The compartments selected for each crop were those in which there had been a control treatment and/or in which conditions for rooting were considered to be optimal. We performed regression analyses per compartment and per depth, but only for the period after planting in which a linear increase of root numbers vs. thermal time was observed. After averaging the results, the regression procedure yielded two parameters of rooting for each crop: (a) the actual or thermal time at which the first root appeared at a certain depth and (b) the root proliferation rate after the first root had appeared. In this way, inherent crop differences in rooting behaviour (rooting depth and root proliferation) became apparent. For each crop, the ‘velocity of the root front’ after planting could be established (calculated in cm (°C day)−1). This parameter differed greatly between crops. Some crops (such as leeks and common velvet grass) explored the soil profile slowly: the root front moved at a velocity of only 0.07 cm (°C day)−1. Among the crops whose roots grew down much faster (0.18–0.26 cm (°C day)−1) were cereals and fodder radish. For a day with an average temperature of 15 °C these rates would have corresponded with the root front travelling approximately 1–4 cm per day. In the crops studied the root front velocity did not correlate with the root proliferation rate.

Comparison of CERES, WOFOST and SWAP models in simulating soil water content during growing season under different soil conditions

A lysimeter experiment conducted on three soil types in a main agricultural production region of Austria in Marchfeld (latitude 48°12′N, longitude 16°34′E and altitude 150 m above sea level), was used to test the performance of the three widely used crop models, CERES, SWAP and WOFOST. The soils included chernozem, sandy chernozem and fluvisol with a 2.0 m profile depth. Daily measurements of the soil water content were taken using TDR probes (one per 0.3 m of depth) in six replicates for each soil type. The analysis was carried out for winter wheat and spring barley grown on the site during seasons 2000 and 2001 and included a detailed comparison of the simulated and measured soil water contents as well as an analysis of seasonal soil water balances, root front velocities and an evaluation of the modeled crop yields. CERES and SWAP, in contrast to WOFOST, simulated the grain yield of barley and wheat well. All three models simulated soil water content in the profile with similar results. The root mean square error (RMSE) range of soil water content was 0.71–4.67% for barley and 2.32–6.77% for wheat, depending on the model and soil type. None of the models simulated total soil water content in the profile significantly better, but there was a general tendency for the models to overestimate soil water depletion. Both CERES and SWAP mimicked the soil water content dynamics well in the top 0.3 m of the soil. The study shows that the multiple layer approach models (SWAP or CERES) including more sophisticated estimation methods for root growth and soil water extraction should be preferred in comparable environments. Further adjustments of evapotranspiration subroutines to the local conditions should be considered prior to the model use for drought impact assessment, yield forecasting or climate change impact studies.

Response of the root system of a winter wheat crop to waterlogging

This study was aimed at analysing and quantifying the response of the root system dynamics of a wheat crop to waterlogging. Two experiments were carried out in parallel: one under controlled conditions with semi-permanent water tables using lysimeters equipped with oxygen measurers, and the other under conditions of artificially drained plots by continually monitoring their hydraulic functioning. The root system was observed frequently using the root mapping method, and this made it possible to measure the growth of the root front, to estimate root densities, and infer growth indices from them. The results showed that the anoxic medium for wheat roots consisted of a water-saturated soil with an oxygen concentration of below a critical threshold estimated to be 0.12 mol m−3 water. The results also showed that the area which was unfavourable to root growth corresponded to the water table topped with a capillary zone of approximately 6 cm. Once the critical threshold had been reached, it was the water-table duration that explained root behaviour and the first effects were perceptible after approximately 48 h. On the basis of these results, two stress variables were analysed: water-table duration in the root zone (WTD) and proportion of roots in the water table (RPWT). The RPWT variable gave the best results within the two experimental contexts. In the case of the permanent regime, this variable made it possible to consider the root proliferation observations made above the saturated zone. Equations linking the stress variable RPWT to the growth indices are proposed that offer new perspectives to modelling waterlogging effects.

Development and validation of a model to describe root length density of maize from root counts on soil profiles

Root length density (RLD) is an important determinant of crop water and nutrient acquisition, but is difficult to measure in the field. On a soil profile, in-situ counts of root impacts per unit surface on soil profiles (NI) can be used to calculate RLD if crop-specific parameters for preferential root orientation (anisotropy) are known. An improved method for field determinations of RLD was developed and validated for maize at sites in Cote d'Ivoire and Burkina Faso. Root anisotropy was measured with cubes of undisturbed soil with 0.1 m sidelength, based on NI observed on three planes oriented perpendicularly to each other. RLD was also measured for the enclosed volume. Repetition of such measurements enabled estimation of the robustness across sites of empirical and geometric models for the relationship between RLD and NI:RLD = NI CO, with CO being the coefficient of root orientation, theoretically equals 2 for an isotropic distribution. Root systems were found to be nearly isotropic, except near the root front (0.3 to 0.5 m), where roots had a preferentially orthotropic orientation. Measured RLD was generally about 50% larger than RLD calculated from observed NI and CO, indicating that at least one of the measurement techniques had a systematic error. The ratio between measured and calculated RLD (CE), which ranged from 0.8 to 2, increased with the age of the plants and decreased with soil depth. CE was therefore introduced as an additional coefficient, resulting in RLD = NI CO CE. The empirical value for CO CE was between 2 and 5. The empirical coefficients CO and CE were the same for the sites in Cote d'Ivoire (oxisol with an iron pan at 0.6 to 0.9 m) and Burkina Faso (alfisol with an iron pan at 0.4 to 0.8 m). The model was validated with independent data sets at both sites, and gave satisfactory predictions of RLD on the basis of NI obtained from single soil planes, which can be easily measured in the field.

Modelling the effect of varying soil water on root growth dynamics of annual crops

We present a simple framework for modelling root growth and distribution with depth under varying soil water conditions. The framework considers the lateral growth of roots (proliferation) and the vertical extension of roots (root front velocity). The root front velocity is assumed to be constant when the roots descend into an initially wet soil profile. The lateral growth of roots is governed by two factors: (1) the current root mass or root length density at a given depth, and (2) soil water availability at that depth.

Comparison of water extraction models for grain sorghum under continuous soil drying

The aim of this study was to compare three contrasting models of water extraction, by comparing predictions of the models with water extraction observed under continuous soil drying from seven crops of grain sorghum in the field. Sensitivity of the model predictions to variation in evaporative demand, and soil and root characteristics were also assessed. The datasets covered soil types ranging from 68 to 252 mm of total extractable soil water (TESW), and levels of mean evaporative demand ranging from 1.8 to 4.9 mm d −1 . The first model was based on the simple function relating daily relative transpiration rate (RTR), the ratio of transpiration to demand, to the fraction of extractable water (FESW) in the maximum root zone of 200 cm, where RTR declined from 1.0 to zero as FESW fell from 0.3 to zero. The second model was the same as the first, except that RTR depended on FESW in the current root zone, where the current root zone was assumed to increase by 3 cm d −1 from emergence to reach 200 cm at anthesis. The third, more complicated model calculated daily transpiration as the lesser of potential extraction and transpirational demand, with potential uptake calculated in each soil layer from root-soil diffusivity and simulated root length density. Inputs of transpirational demand and extractable soil water contents were the same for all models. All models gave good predictions of the observed pattern of RTR and cumulative extraction in the seven crops studied, despite the differences in the level of detail specified within the models. The results indicated that in most situations, RTR will decline once FESW in the maximum rooting depth reaches around 0.3. Sensitivity analysis revealed that this threshold will change substantially where crops are growing on a low TESW soil under high demand, or on a high TESW soil under low demand. Of the soil and root characteristics assessed, the threshold FESW at which RTR declined was most sensitive to the downward rate of root front penetration, rather than the density of rooting or the root-soil diffusivity. This study confirms the robustness and validity of the simple relationship between RTR and extractable soil water currently used in many crop growth models. Use of the more detailed model showed under which situations the form of this relationship is likely to change.

Water extraction by grain sorghum in a sub-humid environment. II. Extraction in relation to root growth

An experiment was conducted under continuous drying in the field to examine the relationship between soil water extraction and root growth of grain sorghum. Extraction was analysed in terms of two components: (1) the time when the extraction front reached a depth, defined as the moment when soil water content (θ) begins to decline exponentially with time and, (2) the decline of θ with time at each depth after the extraction front arrives. A second aim was to test if severe assimilate limitation, caused by shading the crop during late vegetative growth, would influence the rate of root front penetration and root length accumulation. The root front penetrated from sowing at a constant downward rate of 2.72 cm day −1. Down to 100 cm depth, the extraction front lagged behind the root front; thereafter the extraction and root fronts descended together, to reach the same maximum depth of 190 cm shortly after anthesis. Total root length also reached a maximum at that time. Shading had no effect on the rate of root front penetration and length accumulation, despite reducing above-ground growth by 70%. Once the root front arrived at a given depth, root proliferation continued until about 40-20% of the extractable water in the layer remained. As a consequence of the roots proliferating ahead of the extraction front, a small amount of water was taken up before the extraction front arrived and the extraction rate at each depth reached a maxium when half of the extractable water remained. As a consequence, the decline in θ was better described by a sigmoidal, than an exponential curve, particularly in the middle layers of the profile. The applicability of the sigmoidal curve to describe the decline in θ with time is limited, however, to the special situation where root length increases during water extraction.

Modelling root growth of grain sorghum using the CERES approach

A simple model is described, based on the approach used in the CERES crop growth models, which simulates the depth of rooting and root length density in each soil layer for grain sorghum growing under soil drying. The model has five main components: (1) daily accumulation of root length is proportional to above-ground biomass growth, (2) the root front descends at a constant rate from sowing until early grain-filling, (3) daily accumulation of root length in water non-limiting conditions is partitioned among the occupied soil layers in an exponential pattern with depth, (4) proliferation of root length is restricted in any layer if the extractable soil water in that layer declines below a threshold, and (5) a fixed proportion of existing root length is lost due to senescence each day. The parameter values for the relationships were derived from analysis of measured depth distributions of root length from crops of grain sorghum grown in the sub-humid subtropics of Australia, on oxisol and vertisol soil types. The soils had no physical or chemical restrictions to root growth. The model was validated using four independent data sets. Overall, the model simulated the root distribution with depth well, but predictions of accumulated root length were less reliable. One of the most sensitive parameters affecting the modelled distribution of root length with depth was the factor used to partition daily accumulation of root length among the occupied layers, and the value of this parameter varied between well-watered and water-limited crops. The study shows that it is possible to model root growth of field crops using only five simple relationships, with inputs that are already used in most crop growth models.

How do crops manipulate water supply and demand?

The supply of water provided by the root system of a crop stand is defined in terms of the rate at which water is extracted by a root front moving downwards with a constant velocity, the available water per unit soil volume, and a time constant that is inversely proportional to root density. The demand for water, often identified with a potential transpiration rate, is defined in terms of a maximum crop growth rate multiplied by the conservative ratio of transpiration to dry-matter production. From experimental evidence, supported by theory, this ratio is proportional to the mean saturation vapour-pressure deficit. As hypothesized, the root front accelerates during seedling establishment to keep demand and supply in balance. Once a maximum root velocity is reached (ca. 2-4 cm d-1) transpiration is limited by water supply, except when the soil behind the root front is wetted by rain or irrigation, when it is limited by demand. Irrigation amounts and timing can both be estimated from this scheme.