Raymond Bonhomme

Bases and limits to using ‘degree.day’ units

Degree.day units, for which there are also several synonymous terms, are often used in agronomy essentially to estimate or predict the lengths of the different phases of development. The physiological and mathematical bases upon which they are founded are, however, sometimes forgotten, resulting in questionable interpretations. Such is particularly the case for anything relating to variations in the temperature thresholds which enter into the calculation of these degree.day sums. Without seeking to draw up a synthesis of the extremely numerous works published in the field, this review article sets out to present the basic principles of the degree.day unit notion as well as the limits of its use. On this last point, we will particularly emphasise the influence of the non-linearity of the temperature response of the method used in determining the threshold temperature as well as the pertinence of the temperature taken into account in studying the phenomenon. Several practical conclusions are drawn from this review article.

Grain yield components in maize: I. Ear growth and kernel set

Abstract Maize (Zea mays, L) grain yield is correlated with kernel number (KN), but uncertainty exists about the extension of the critical period for kernel set. In this work, the actual period of active ear elongation was determined and defined in thermal units (°C day=degree days). Kernel number was related to the amount of intercepted photosynthetically active radiation (IPAR) during this period. Field experiments were conducted in France (48°51′N) and Argentina (32 to 37°S). Different sowing dates and plant populations were used to vary the amount of IPAR plant−1. Hybrids of different cycle length (FAO maturity ranking between 300 and 630) were tested. Ear length was measured weekly after axillary bud differentiation, and values were normalized to final ear length. A significant linear model (r2=0.92, n=51) was fitted, which indicated that: (i) Active ear elongation took place between −227 and 100°C day from silking; (ii) ca. 41% of final ear length was reached at silking; (iii) Ear elongation rate increased after silking; and (iv) The duration of this period varied among sites and treatments. Kernel number plant−1 was significantly related to the IPAR plant−1 during the critical period (r2=0.70, n=81), but a significant (P

Modeling radiative transfer in mixed and row intercropping systems

Abstract A radiative transfer model for two-dimensional intercropping systems is presented. Based on the turbid medium analogy, it considers the canopy as a set of contiguous cells characterized by leaf area densities and angle distributions for each species. Radiation interception within a cell is inferred from relationships accounting for light partitioning between species. Because of horizontal heterogeneity, mean exchange coefficients between radiation sources and receivers are computed from the course of several elementary beams within the canopy. Radiative exchange coefficients are defined for both the direct and diffuse components of incident radiation, and for scattered radiation. Finally, the radiative balance of the canopy is solved by using the radiosities method. As a partial validation, the model is tested with reflected and transmitted radiation measurements made on a row intercrop. This consists of two rows of tall maize alternating with two rows of small maize sown 40 days later. Agreement between reflected radiation measurements and simulations is satisfactory. For locally transmitted radiation (i.e. between rows of tall maize and between rows of small maize), discrepancies are seen but no estimation bias is detected. As an example, the model is used to investigate the effect of canopy structure on light partitioning for the case of a simulated row intercropping in tropical conditions. Simulations show that light partitioning is most influenced by the vertical stratification of the species. If the whole canopy is horizontally homogeneous, the row effect is small and vertical leaf distribution dominates. Numerical differences between cloudy- and sunny-day simulations are small, except at sun elevations

Beware of comparing RUE values calculated from PAR vs solar radiation or absorbed vs intercepted radiation

Abstract The concept of radiation-use efficiency (RUE: biomass produced/energy intercepted by a crop) is commonly used for analysis and modelling of crop growth. The various values found in the literature have not, however, been calculated in the same way. This paper focuses on two aspects that account for differences in the published values. First, the differential effect of canopy development on radiation interception and hence on RUE calculated from photosynthetically active radiation (PAR) and overall solar radiation, respectively. Second, the differences in RUE values calculated from absorbed as compared with intercepted radiation. The analyses reveal why it is difficult to make precise comparisons of published values of RUE and why the biggest differences are found for crops with small LAI or with long LAI establishment phase.

Estimating the three-dimensional geometry of a maize crop as an input of radiation models: comparison between three-dimensional digitizing and plant profiles

Abstract The radiation regime of the plant canopies is largely influenced by the geometrical structure, i.e. the angular and spatial distributions of the leaf area. In this paper, two methods for investigating the structure of a maize row crop are compared. The first is a new method which consists of a three-dimensional (3D) digitizing of the foliage using a sonic 3D digitizer. The second one is the plant profile method in which a picture of the plant is taken and then digitized in two dimensions. The latter assumes that the leaves of a plant are in a single vertical plane called the ‘azimuthal plane’. The 3D digitizer has the ability to locate a given point with an accuracy of within ±1 cm: the representation and the repeatability of a leaf description are satisfactory and not largely influenced by the digitizing density. The azimuthal plane assumption of the plant profile method was then tested using the results of 3D digitizing as the reference. Regarding the individual plant description, the leaf azimuth function is bimodal as expected for the maize phyllotaxy. The theoretical azimuthal plane was computed by a principal component analysis. The plant leaf area projection onto this plane represents about 96% of the 3D spatial distribution. Moreover, this plane is close to the azimuthal plane estimated by eye. That is why the spatial distributions of the leaf area in the azimuthal plane are quite similar for the two methods. However, the leaf area with an azimuth within ±15° around the plant azimuth is less than 40% of the total area and the foliage spreads out in a slice 40 cm thick, around the azimuthal plane. Regarding the canopy structure description, the two methods gave similar results for both the leaf inclination and the vertical leaf area density function. In contrast, the distributions in the horizontal plane were different: the plant profile method overestimates the row effect, increasing the foliage clumping around the stem because of the plant azimuth plane assumption. Such discrepancies have repercussions on the modelled radiative balance of a row crop, so the plant profile method should not be used in estimating the spatial distribution of the leaf area in the horizontal plane.

Grain yield components in maize: II. Postsilking growth and kernel weight

Maize kernel weight (KW) results from kernel growth during two stages of grain filling, the lag phase (formative period) and the effective grain-filling phase. Environmental conditions may affect kernel biomass accumulation in each phase. This work analyzed: (1) changes in duration and rate of kernel growth on a thermal time (°C day) basis; and (2) KW response to postsilking biomass production kernel−1 (source:sink ratio). Sowing date, plant population, and nitrogen fertilization experiments were conducted in France and Argentina to induce changes in assimilate availability per kernel. Hybrids of different KW were tested. Hybrids differed in the duration of the lag phase, which determined kernel growth rate during the effective grain-filling period for hybrids with similar grain-filling duration (ca. 745°C day). Environments with low air temperature ( 300 mg) with reduced kernel number (2800 to 4000 kernels m−2). For the former, grain yield increments should not be based on increased kernel number but on increased biomass production.

Estimating the temperature of a maize apex during early growth stages

Abstract When the leaf area index is low at the early stages of growth, the temperature of a maize apex can be much higher than the air temperature measured at screen level. In order to account for temperature effects in plant growth simulation models, it would be better to use plant temperature rather than air temperature. We propose a model to estimate the apex temperature for both day-time and night-time averages from a small number of readily available meteorological data: solar radiation, wind speed, air temperature and humidity. It is based on an energy balance of a maize apex under field conditions. It performs a radiation balance that separates diffuse and direct components, and assumes a similarity between the apex and soil surface temperature evolutions. In the absence of any references, the apex stomatal resistance was parameterized as a simple linear function of water vapour deficit, deduced from experimental data. The calculated temperatures were compared with those measured for two sets of experimental data collected in 1989 and 1990. The agreement was quite satisfactory, the average absolute error being in all cases less than 1.0°C. Furthermore, the empirical relationship between stomatal resistance and water vapour deficit was shown to be valid for both sets of data. We should now confirm this relation under different soil or climatic conditions, as well as the similarity between the apex and soil surface temperatures.

A theoretical analysis of radiation interception in a two-species plant canopy

Classical radiation interception laws for monospecific canopies cannot be used directly for bispecific canopies. They are always based on the gap frequency concept (i.e., the probability of no interception), which does not provide any information about the sharing of intercepted radiation between species. A theoretical analysis is reported that relates the radiation interception probabilities to the geometrical structure of the crop (i.e., the leaf area density and the leaf angle distribution of each component) and the foliage dispersion. The leaf dispersion globally describes the spatial relations between the leaf elements; it may be regular if the leaves avoid mutual shading, random, or clumped if they tend to overlap. For such two-species canopies, the leaf dispersions within each component (WSLD: within-species leaf dispersion) and between two species (BSLD: between-species leaf dispersion) are distinguished. Using bivariate multinomial distributions, general expressions for the gap frequency and the interception probabilities of a homogeneous vegetation layer were set as exponential functions of the foliage thickness, taking into account a number of dispersion parameters as small as possible. First, one WSLD for each species describes the rate of foliage overlap between the leaves of this species; it is quite similar to the leaf dispersion of single-species canopies. Second, the rate of foliage overlap between species is characterized by one BSLD. As in monospecific canopies, this parameter is positive, zero, or negative, respectively, for regular, random, or clumped BSLD. Third, another BSLD parameter has to be used if the foliage overlap between species is more than random (i.e., in the case of clumped BSLD); the latter shows the direction of overlap between species and may be taken as the probability of finding a leaf element of the first species in the case of marked overlapping. Suggestions for estimating the leaf dispersion parameters and possible uses of such relations are also discussed.

Variations in the vegetative and reproductive systems in individual plants of an heterogeneous maize crop

Abstract Two field experiments were carried out in the Parisian basin, in which realistic within-row and between-row variations in maize plant spacing were created. Modifications of incident radiation in each situation were characterized by independent measures, using a plant canopy analyser which measures diffuse non-interceptance; resulting changes in plant performance characteristics were observed. Considering the appearance of significant differences in foliar stages between treatments, it appears that competition for light started during the last week of June, about 400°C after sowing, before the 11 visible leaf stage. During the vegetative phase, the widths of the leaves from the 10th to the 13th were the measured plant characteristics most closely associated with available energy per plant; during the reproductive phase, grain number per ear row and mean weight of a kernel were sensitive to competition, so that grain weight per plant was the most significant response to available energy per plant. In our conditions (plant population of 130 000 ha −1 ), the ears lost corresponding to missing plants are poorly compensated by increased yield of surrounding plants because of additional light intercepted: when two or three adjacent plants were missing, compensation for missing plants was only 16% and 34%, respectively.

DALI: An automated laser distance system for measuring profiles of vegetation

An automated device for measuring directional radiation interception (DALI, Device for Automated measurement of Laser beam Interception) is described. It is based on a laser distance meter (Dr Johannes Riegl GmbH, Austria) moving within a horizontal frame of 5 m × 5 m above the canopy. The distance meter emits a low power helium-neon laser beam which simulates a sunbeam. Backscattered radiation hits a photodiode via the receiver optics. The distance to the first beam-vegetation contact is inferred from the time interval between emitted and received beams measured by a quartz-stabilized clock. Moving the distance meter above the canopy enables many beams to be sampled for an estimation of the spatial distribution of radiation interception. The distance meter is moved in two perpendicular horizontal directions by means of two stepping motors. A microcomputer manages both data acquisition from the distance meter and control of the motors. Before application to vegetation canopies, the distance meter was first tested by using fixed targets in order to analyze its response in the special case of a vegetation canopy made of a set of discontinuous elements. Because of the finite laser diameter, beams may simultaneously hit several targets. In that case, the estimated distance is a value averaged over the multiple targets. The whole system was then tested by comparing vertical profiles of radiation interception, gap frequency and projected leaf area density of potted maize plant canopies with those obtained from the silhouette method. Agreement between variables estimated by 0.1 m thick layers was satisfactory because linear regression analysis showed slopes close to 1 and r2 greater than 0.8. However, the behavior of the distance meter with multiple targets was likely to lead to overestimation of interception in the middle layers. Finally, comparison of the proposed system with other existing devices providing similar data is discussed. The major improvement required for multiple targets appears to be improved processing of signals from backscattered beams to determine the distribution of distances rather than the average value.