Exploring the spatial distribution of light interception and photosynthesis of canopies by means of a functional-structural plant model

Abstract

Background and aims: At present most process-based models and the majority of three-dimensional models include simplifications of plant architecture that can compromise the accuracy of light interception simulations and, accordingly, canopy photosynthesis. The aim of this paper is to analyse canopy heterogeneity of an explicitly described tomato canopy in relation to temporal dynamics of horizontal and vertical light distribution and photosynthesis under direct- and diffuse-light conditions.

Methods: Detailed measurements of canopy architecture, light interception and leaf photosynthesis were carried out on a tomato crop. These data were used for the development and calibration of a functional-structural tomato model. The model consisted of an architectural static virtual plant coupled with a nested radiosity model for light calculations and a leaf photosynthesis module. Different scenarios of horizontal and vertical distribution of light interception, incident light and photosynthesis were investigated under diffuse and direct light conditions.

Key results: Simulated light interception showed a good correspondence to the measured values. Explicitly described leaf angles resulted in higher light interception in the middle of the plant canopy compared with fixed and ellipsoidal leaf-angle distribution models, although the total light interception remained the same. The fraction of light intercepted at a north-south orientation of rows differed from east-west orientation by 10 % on winter and 23 % on summer days. The horizontal distribution of photosynthesis differed significantly between the top, middle and lower canopy layer. Taking into account the vertical variation of leaf photosynthetic parameters in the canopy, led to approx. 8 % increase on simulated canopy photosynthesis.

Conclusions: Leaf angles of heterogeneous canopies should be explicitly described as they have a big impact both on light distribution and photosynthesis. Especially, the vertical variation of photosynthesis in canopy is such that the experimental approach of photosynthesis measurements for model parameterization should be revised.

Fig. 1.

Model flow chart.

Fig. 2.

Example of the visual output of the 3-D tomato model. The basic unit of the model is two plant rows of five plants each. Lines along the path and the plant canopy represent the visual sensors used for the model calibration.

Fig. 3.

Relationship between (A) leaf angle to the horizontal plane, (B) leaf length, (C) leaf area in relation to phytomer number starting from the top of the plant and (D) leaf azimuth angle distribution. In (A–C) each symbol represents a specific week. In (D) the numbers represent the number of leaves per leaf orientation class.

Fig. 4.

(A) Leaflet angle with respect to the horizontal in relation to the leaflet position on the leaf petiole. The position counting starts from the leaflets nearer to the stem and ends with the terminal leaflet. Every two leaflets form a pair positioned opposite to each other on the leaf petiole. Each column represents the average of ten leaves ± standard error of the mean. (B) Relationship of leaflet area to leaflet length (y = 0·15x + 0·25, R² = 0·81). n = 10 leaves.

Fig. 5.

Measured vs. simulated values of light interception. Values are from 6 weeks of measurements in eight different canopy heights. The continuous line is 1 : 1.

Fig. 6.

Measured and simulated horizontal light distribution in a tomato crop row. The light intensity is plotted against the plant row length at three different plant canopy heights (0·5, 1 or 1·75 m, as indicated). The lines represent simulated values while symbols represent measured values ± standard error of the mean. Plant rows are located at 20 cm and 140 cm while the middle of the path is located at 80 cm.

Fig. 7.

Seasonal variation in light interception for 21 December and 21 June for a north–south and east–west row orientation, as indicated. LAI was 3·1. Calculations were performed for exactly the same canopy structure on both dates.

Fig. 8.

Horizontal distribution of photosynthesis in the crop path. In this graph only leaves positioned towards the path at three different canopy heights (0·5, 1 or 1·75 m, as indicated) are used. Each data point is the average of a pair of leaflets and includes the result from two or three leaves per plant from 20 plants depending on the leaves position. Simulation was performed under diffuse light conditions.