Tillering in spring wheat : a 3D virtual plant modelling study
ABSTRACT Keywords: wheat, tillering, phytomer, virtual plant, L-system, functional-structural modelling, plant population density, PAR, shading, red/far-red ratio, light scattering This thesis presents a 3D virtual plant modelling study to tillering (lateral branching) in spring wheat (Triticum aestivum L.). Tillering in wheat is influenced by light properties. Light intensity determines the amount of assimilates available for growth. Availability of assimilates is assumed to regulate the degree of tillering: a reduction in light intensity, or more specifically in the intensity of Photosynthetically Active Radiation (PAR), results in a lower number of tillers produced. Tillering is also affected by the spectral composition of the light (R :FR , the ratio between the intensities of red and far-red light). R :FR declines with depth in the canopy and with development of the canopy, due to the differential reflection and transmission of red and far-red light by the surrounding vegetation. A low R :FR triggers a shade avoidance response of the plants, which includes a reduction in tiller production. R :FR is highly related to the fraction of light intercepted. The relation between canopy architecture, light properties within the canopy, and tillering in spring wheat plants was studied using a 3D virtual plant modelling approach. The virtue of virtual plant models for this analysis is that each element in the architecture of the canopy is given an explicit 3D representation, which enables simulation of processes at the level of individual organs. The model used, called 'ADELwheat', was calibrated for spring wheat. The model was validated for ground cover and leaf area index, using an independent dataset. Experiments described in this thesis showed that new tillers ceased to appear when the fraction of light intercepted by the canopy exceeds 0.4. That threshold was independent of plant population density, shading, developmental stage of the plants and rank number of the tiller. When tillering ceased, also the red/far-red ratio was fairly similar across population densities and light treatments. Cessation of tillering in ADELwheat was made dependent on thresholds of light properties. A light model ('nested radiosity') was coupled to ADELwheat and was used to calculate both PAR interception and R :FR at the level of the individual organ. The simulation results showed that the virtual plant modelling approach is useful to simulate global effects of local stimuli. The simulated tillering patterns supported the hypothesis that tillering ceases at specific light conditions inside the canopy. The study demonstrated that the virtual plant modelling approach can provide insight into the factors that determine the developmental plasticity of wheat in terms of tillering.