Figure 2: Intensity of the canopy shade-light signals and competition for PAR as affected by leaf area index. Three stages are distinguished: at low leaf area indexes (typically lower than 1), stage 1 is characterised by the presence of light signals involving spectral changes (mainly a reduction in the red/far-red ratio). At stage 2, the leaves shade the stem of neighbours, which receive low red/far-red ratios and low irradiance, but leaves do not shade mutually. At stage 3, mutual shading among leaves is established, and therefore the plants receive light signals (both spectral and irradiance signals) and compete for PAR.