Crop management decisions such as sowing density, row distance and orientation, choice of cultivar, and weed control define the architecture of the canopy, which in turn affects the light environment experienced by crop plants. Phytochromes, cryptochromes, phototropins, and the UV-B photoreceptor UVR8 are sensory photoreceptors able to perceive specific light signals that provide information about the dynamic status of canopy architecture. These signals include the low irradiance (indicating that not all the effects of irradiance occur via photosynthesis) and low red/far-red ratio typical of dense stands. The simulation of selected signals of canopy shade light and/or the analysis of photoreceptor mutants have revealed that canopy light signals exert significant influence on plant performance. The main effects of the photoreceptors include the control of (a) the number and position of the leaves and their consequent capacity to intercept light, via changes in stem height, leaf orientation, and branching; (b) the photosynthetic capacity of green tissues, via stomatic and nonstomatic actions; (c) the investment of captured resources into harvestable organs; and (d) the plant defences against herbivores and pathogens. Several of the effects of canopy shade-light signals appear to be negative for yield and pose the question of whether breeding and selection have optimised the magnitude of these responses in crops. 1. Light Signals in Crops 1.1. Light as a Source of Energy and Light as a Signal The biomass produced by a crop can be accounted for by the product of three variables: the incident radiation, the efficiency to intercept the incident radiation, and radiation use efficiency (the relationship between plant dry matter and radiation intercepted), integrated for the duration of the growth cycle [1]. In turn, the yield of grain crops can be accounted for by the product of the biomass by the harvest index. Light has fundamental importance in crop yield due to its function in photosynthesis. The aim of this paper is to present an overview of the experimental evidence that supports the often neglected contribution of light as a signal (i.e., as a source of information) in crop yield. A light signal is a variable aspect of the light environment, perceived by specific sensory receptors, which affects selected plant traits. In this sense, while the action of light as a source of energy is explicit among the aforementioned components involved in biomass and yield generation, implicit in the other components there are effects of light as a source of information
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