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Figure 4: Canopy light signals impact on crop yield. Incident radiation is intercepted by the canopy with an efficiency that depends on canopy architecture; intercepted radiation is transformed into biomass with an efficiency that depends on the processes that fix (photosynthesis) and release (mitochondrial respiration, photorespiration) carbon dioxide; part of the biomass is allocated to harvestable organs. The light signals of dense canopies have both positive and negative effects on these processes, and selected examples of the processes that impact on PAR interception, radiation use efficiency, and harvest index are included.