Shade avoidance is a set of responses that plants display when they are subjected to the shade of another plant. It often includes elongation, altered flowering time, increased apical dominance and altered partitioning of resources. This set of responses is collectively called the shade-avoidance syndrome (SAS).
Shade responses display varying strength along a continuum. Most plants are neither extreme shade avoiders or tolerators, but possess a combination of the two strategies; this helps adapt them to their environment. However, the ability to perceive and respond to shade plays a very important role in all plants: they are sessile by nature and access to photosynthetically active radiation is essential for plant nutrition and growth. In the model organism Arabidopsis thaliana, the shade avoidance response varies at different points in the life cycle. Dry, dormant seeds will not germinate if they are in the shade. Once dormancy has been broken and they have imbibed water, the seeds are committed to germination. Water-imbibed seeds display hypocotyl elongation; if the shade were caused by excessive soil depth, this would help the seedling grow vertically very quickly and push up and out of the ground. If an Arabidopsis seedling becomes shaded, its petioles and internodes elongate. It may even lose rosette morphology. If a mature plant becomes shaded, this often prompts early flowering, as it is unlikely that growing more structures will result in profitable nutrient gain.
Plants can tell the difference between the shade of an inanimate object (e.g. a rock) and the shade of another plant. In the shade of a plant, far red light is present in a higher irradiance than red light, as a result of the absorption of the red light by the pigments involved in photosynthesis. This is known as far red enrichment. Phytochrome can be used to measure the ratio of far red to red light, and thus to detect whether the plant is in the shade of another plant, so it can alter its growth strategy accordingly (photomorphogenesis). In Arabidopsis, phytochrome B is the predominant photoreceptor that regulates SAS. Phytochromes exist in two forms: PR and PFR. It is synthesised as PR, but red light triggers a conformational change, producing PFR. Far red light causes the phytochrome to be converted back into PR. For a given red:far red light ratio, there will be a dynamic equilibrium in the relative quantities of PR and PFR present. Far red enrichment causes a build-up of PR. If PR is present above a species-specific threshold, shade avoidance signal transduction pathways will be activated.
Over the past few decades, major increases in grain yield have come largely through increasing planting densities. As planting densities increase so does the proportion of far red light in the canopy. Thus, it is likely that plant breeders have selected for lines with reduced SAS in their efforts to produce high yields at high density.