Exploiting heterogeneous environments: how do plants ‘decide‘ to acclimate to a highly variable light input?

Erik Murchie & Giles Johnson

Light is one of the most variable resources for plants and is capable of changes of large magnitude over quite short periods of time. Solar movement, canopy movement, climate and canopy architecture can combine to produce a complex pattern of light in time and space. This has profound consequences for photosynthetic CO2 assimilation of leaves, which is often slow to respond to the changes in light. In a matter of seconds light can shift from being limiting for photosynthesis to high levels that are sufficient to saturate photosynthesis and risk photooxidative stress. Over the short term the mechanisms that plants use to deal with these changes are relatively well understood. For example there is a balance required between photosynthesis and photoprotective processes.
One area that is not well understood is how photosynthesis in plants responds to changes in light over longer time periods (two or more days usually). ‘Acclimation’ is a change in the composition of the photosynthetic apparatus to light intensity which improves quantum efficiency under low light and increases the maximum capacity under high light. This optimizes light use in both cases. For example under high light, absorption is not limiting so the amount of light harvesting pigment-protein complex reduces while the level of Calvin cycle enzymes increases [1]. This allows photosynthetic rate to increase. Acclimation to light is a ubiquitous process in plants that enables the efficient exploitation of light and also maintains a high economy of form and function. Acclimation requires gene expression, protein synthesis and degradation and consumes energy and resources.
But light can be highly variable from day to day and hour to hour. Figure 1 shows typical daily variation in light intensity beneath a plant canopy. How do plants and leaves determine that the frequency and/or intensity of high light or low light events is sufficient to induce an acclimation process? This is important because of the metabolic cost involved. For short-term photosynthetic shifts (minutes, seconds) it is possible to invoke enzyme activation state, metabolite level and the state of energisation of the thylakoid membrane as a kind of ‘memory’ of past light history [3]. What about long term changes (days): do plants accumulate a ‘memory’ of past high and low light events and if so which features of the variation do they use before a threshold for acclimation is reached? Is it correct to assume that plants somehow determine the probability of future high / low light events?

Proceedings of the 6th Mathematics in the Plant Sciences Study Group