Complex interplay of responses when plants experience canopy shading

Plants are able, as they grow, to change their physical form by altering their growth patterns and physiology in response to environmental conditions. Fully understanding the effects of the environment on plant form is complicated, because there are often changes in multiple traits, leading to responses interacting with one another in complex ways. An important example of this is in plants’ reactions to shading by an overhead canopy, which comprises both a reduction in photosynthetically active radiation (PAR) and a reduction in the ratio of red to far-red light (R:FR). These two factors occur simultaneously, but at different magnitudes depending on the degree of shading, and elicit different responses in plants. Broadly speaking, reduced PAR leads to changes in leaf photosynthesis and respiration, while reduced R:FR leads to alterations in plant architecture, though it has thus far been difficult to analyze these phenomena separately in order to fully understand them.

In a recent article published in Annals of Botany, Ningyi Zhang and colleagues attempted to model and separately quantify the reactions of woody perennial roses to canopy shading. The researchers first conducted a glasshouse experiment with different light treatments, then used this as a basis for the creation of a functional-structural plant model with which shading responses could be evaluated individually.

Under light canopy shading, response to reduced R:FR had the larger effect, while under heavier shading, responses to reduced PAR became dominant. Modelling revealed that the overall shading reaction was not simply the sum of these two parts, suggesting interactions between individual components of the plant’s response, a balance that shifted with the degree of shading. “In the early stages of canopy development, plants could immediately experience significant reductions in R:FR when reductions in PAR are still absent or relatively minor,” the authors write. “Therefore, low R:FR is widely considered as an early warning signal for plants regarding the proximity of neighbours, and the subsequent shade avoidance responses are considered to improve plant performance by preventing plants from becoming shaded.”

The authors point out that their modelling addresses only the specific situation of shading by overhead canopy, such as in an understorey or agro-forestry system. Plants also commonly experience shading by neighbours of the same height, a scenario not examined in this work. “In those situations, responses to low R:FR may be relatively more important than those to low PAR. We did not simulate those situations because our experiment did not allow us to make reasonable assumptions to create reliable virtual phenotypes in crowding populations. However, if combined with appropriate experiments, the modelling approach presented in our study could account for those situations too.”