Sub-optimal temperature leads to tighter coupling between photosynthetic electron transport and CO2 assimilation...

The C4 carbon concentrating pathway promotes high CO2 assimilation rates. To keep C4 photosynthesis energetically efficient, electron transport reactions and downstream biochemistry need to be carefully balanced. Here we use a combination of noninvasive measurements and metabolic profiling to study the efficiency of C4 photosynthesis in maize (Zea mays) under 2 conditions that can lead to decoupling between electron transport and carbon assimilation: fluctuating light and suboptimal temperature. Measurements were performed under 3 fluctuating light regimes and at 3 temperatures, providing the most detailed study to date of the interaction between fluctuating light and suboptimal temperature on the photosynthetic performance of maize, an important global crop. At room temperature, CO2 assimilation rates were decoupled from photosynthetic electron transport under fluctuating light regimes, in contrast to tight coordination observed under constant light. This decoupling was underpinned by metabolic flexibility and buffering by large pools of C4 transfer metabolites. Surprisingly, at suboptimal temperatures, CO2 assimilation rates became more tightly coupled to photosynthetic electron transport rates under fluctuating light regimes. This appeared to be caused by strong feedback downregulation of electron transport and a greater degree of light saturation of CO2 assimilation at low temperature. Low temperature impacted carbon assimilation rates more strongly than metabolite pools or intercellular metabolite distribution, which could reflect negative effects on diffusional metabolite transfer through plasmodesmata. Altogether, these results show that maize is able to maintain energetic efficiency by buffering light transitions at room temperature, as well as avoid oxidative damage by strongly downregulating electron transfer under short-term exposure to low temperatures.