Image-based 3D canopy reconstruction to determine potential productivity in complex multi-species crop systems

Abstract

Background and aims: Intercropping systems contain two or more species simultaneously in close proximity. Due to contrasting features of the component crops, quantification of the light environment and photosynthetic productivity is extremely difficult. However it is an essential component of productivity. Here, a low-tech but high-resolution method is presented that can be applied to single- and multi-species cropping systems to facilitate characterization of the light environment. Different row layouts of an intercrop consisting of Bambara groundnut ( Vigna subterranea ) and proso millet ( Panicum miliaceum ) have been used as an example and the new opportunities presented by this approach have been analysed.

Methods: Three-dimensional plant reconstruction, based on stereo cameras, combined with ray tracing was implemented to explore the light environment within the Bambara groundnut-proso millet intercropping system and associated monocrops. Gas exchange data were used to predict the total carbon gain of each component crop.

Key results: The shading influence of the tall proso millet on the shorter Bambara groundnut results in a reduction in total canopy light interception and carbon gain. However, the increased leaf area index (LAI) of proso millet, higher photosynthetic potential due to the C4 pathway and sub-optimal photosynthetic acclimation of Bambara groundnut to shade means that increasing the number of rows of millet will lead to greater light interception and carbon gain per unit ground area, despite Bambara groundnut intercepting more light per unit leaf area.

Conclusions: Three-dimensional reconstruction combined with ray tracing provides a novel, accurate method of exploring the light environment within an intercrop that does not require difficult measurements of light interception and data-intensive manual reconstruction, especially for such systems with inherently high spatial possibilities. It provides new opportunities for calculating potential productivity within multi-species cropping systems, enables the quantification of dynamic physiological differences between crops grown as monoculture and those within intercrops, and enables the prediction of new productive combinations of previously untested crops.

Keywords: 3D reconstruction; Bambara groundnut (Vigna subterranea (L.) Verdc.); canopy architecture; canopy productivity; intercropping; light interception; photosynthesis; proso millet (Panicum miliaceum); ray tracing.

Fig. 1.

Theoretical example of light transmission through a monocropped canopy (left) versus an intercrop canopy (right). The estimated leaf area index (LAI) as a function of depth is given for each canopy.

Fig. 2.

Validation of light interception in a sole Bambara groundnut canopy. Fractional interception was measured with a ceptometer (dots and bars, mean ± s.e.m.) and calculated from ray tracing (line) with distance along a row. Arrows indicate the location of the centre of the plants in a row.

Fig. 3.

Representative reconstructed canopies with the maximum PPFD ranges colour coded for 1200 h. (A) Sole Bambara groundnut. (B) Sole proso millet. (C–F) Rows of Bambara groundnut:proso millet 1:1 (C), 2:1 (D), 3:1 (E) and 4:1 (F).

Fig. 4.

Frequency of PPFD values according to the fraction of surface area received at the top layer within each canopy. (A, C) Bambara groundnut; (B, D) proso millet. (A, B) 1200 h, direct light from above. (C, D) 1500 h, direct light from the side.

Fig. 5.

Modelled total canopy light interception over the course of the day for different intercrop treatments and corresponding sole crops (A, C) per unit leaf area and (B, D, E) per unit ground area. (A, B) Bambara groundnut. (C, D). Proso millet. (E) Both component crops. The number of rows of BG are shown along the x axis and the rows of PM are shown in the key as either 1 row (grey) or 2 rows (blue).

Fig. 6.

Example light response curves. (A, B) Bambara groundnut layers: top (black) and bottom (grey). (A) Sole plot and (B) intercrop (3:1) treatment. (C, D) Proso millet layers: top (black), middle (dark grey) and bottom (light grey). (C) Sole plot and (D) intercrop (3:1) treatment.

Fig. 7.

Modelled predicted carbon gain over the course of the day for different intercrop treatments and respected sole crops (A, C) per unit leaf area and (B, D, E) per unit ground area. (A, B) Bambara groundnut. (C, D) Proso millet. (E) Both component crops. The number of rows of BG are shown along the x axis and the rows of PM are shown in the key as either 1 row (grey) or 2 rows (blue).

Fig. 8.

Reconstruction time course of a 3:1 (Bambara groundnut: proso millet) intercrop canopy development. (A) 21 DAS, (B) 30 DAS, (C) 39 DAS, (D) 48 DAS and (E) 57 DAS.