The variable light environment within complex 3D canopies

With an expanding population and uncertain consequences of climate change, the need to both stabilise and increase crop yields is important. The relationship between biomass production and radiation interception suggests one target for improvement. Under optimal growing conditions, biomass production is determined by the amount of light intercepted and the efficiency with which this is converted into dry matter. The amount of light at a given photosynthetic surface is dependent upon solar movement, weather patterns and the structure of the plant, amongst others. Optimising canopy structure provides a method by which we can improve and optimise both radiation interception and also the distribution of light among canopy layers that contribute to net photosynthesis. This requires knowledge of how canopy structure determines light distribution and therefore photosynthetic capacity of a given crop species. The aim of this thesis was to assess the relationships between canopy architecture, the light environment and photosynthesis. This focused on two core areas: the effect of varietal selection and management practices on canopy structure and the light environment and; the effect of variable light on select photosynthetic processes (photoinhibition and acclimation). An image-based reconstruction method based on stereocameras was employed with a forward ray tracing algorithm in order to model canopy structure and light distributions in high-resolution. Empirical models were then applied using parameterisation from manually measured data to predict the effects of variable light on photosynthesis. The plasticity of plants means that the physical structure of the canopy is dependent upon many different factors. Detailed descriptions of canopy architecture are integral to predicting whole canopy photosynthesis due to the spatial and temporal differences in light profiles between canopies. This inherent complexity of the canopy means that previous methods for calculating light interception are often not suitable. 3-dimensional modelling can provide a quick and easy method to retain this complexity by preserving small variations. This provides a means to more accurately quantify light interception and enable the scaling of cellular level processes up to the whole canopy. Results indicate that a canopy with more upright leaves enables greater light penetration to lower canopy layers, and thus higher photosynthetic productivity. This structural characteristic can also limit radiation-induced damage by preventing exposure to high light, particularly around midday. Whilst these features may lead to higher photosynthetic rates per unit leaf area, per unit ground area, photosynthesis is usually determined by total leaf area of the canopies, and within this study, the erect canopies tended to have lower total leaf areas than the more horizontal canopies. The structural arrangement of plant material often led to low levels of light within the lower canopy layers which were punctuated by infrequent, high light events. However, the slow response of photosynthesis to a change in light levels meant that these sun flecks cannot be used by the plant and thus the optimal strategy should be geared towards light harvesting and efficient photosynthesis under low light conditions. The results of this study contribute to our understanding of photosynthetic processes within the whole canopy and provide a foundation for future work in this area.