Researchers have discovered an important clue in the quest to make more biofuel from switchgrass—one which could (speculatively) boost its biomass by over 50% and help accelerate the departure from fossil fuels.
Switchgrass is becoming one of the most important ethanol biofuel crops globally, owing to its ability to produce five times the energy of corn-based biofuels. But the Michigan State University researchers on the new paper think it could accomplish even more, if we could figure out what’s behind its unusual habit of halting photosynthesis in summertime.
Every year, this plant stops photosynthesizing in the middle to late part of the growing season—precisely when it could be building up more biomass and making stronger, bigger plants. The key, the researchers think, could lie in its tangled network of roots.
As in many grass species, switchgrass roots are punctuated by rhizomes, little capsules that accumulate energy-rich starch that’s produced from the sugars made during the plant’s photosynthesis. These rhizomes act like an emergency underground pantry that helps grasses survive unfavorable weather conditions in hard times. But in other ways they may limit the plant, the researchers believe.
To test their hunch, they ran a series of experiments over the switchgrass growing season of 2020 in Michigan, USA: these compared plots of naturally rain-fed switchgrass, with plots grown under a cover to deprive them of rain. This comparison was important because greater precipitation is associated with higher rates of CO2 assimilation (a measure of photosynthesis)—while the reverse is true for drought conditions, when CO2-absorption typically declines.
The researchers also analyzed plant tissues across these two sets of crops.
First of all, their experiments showed that while increasingly dry conditions in the open-air crops did coincide with lower rates of CO2 intake, a sudden heavy shower late in the growing season after photosynthesis had tapered off wasn’t enough to restore it to early-summer levels. That suggested the declining photosynthesis must be explained by something other than the amount of rainfall that the switchgrass did or didn’t receive.
The tissue analysis revealed clues about what this might be. Just as photosynthesis was declining in plants, the researchers found evidence of increasing starch levels in the rhizomes. In fact, during the mid- to late- growing season, a 50% decline in photosynthesis corresponded with a fourfold increase in rhizome starch.
Rhizome starch levels seem to be inversely related to photosynthesis decline. The researchers don’t know the precise mechanisms behind this yet, but their hypothesis is that carbohydrates are swiftly accumulated in the rhizomes in the earlier summer phase, as photosynthesis ramps up. But their space is limited, and so as these little pantries fill up, they act as a constraint on the plant’s production. Once full, the rhizomes seem to signal to the switchgrass that there’s nowhere to put the sugars it makes, flipping the switch on photosynthesis and bringing the process grinding to a halt.
The implication is that plants miss out on weeks of sunshine that could be turned into biomass. And this isn’t an insignificant amount: the researchers made a provisional estimate and it includes many complicating factors—but the 50% of carbon accumulation forfeited through declining photosynthesis represents about 1.2 metric tonnes of extra biomass per hectare of switchgrass. Having this additional production could be a game-changer for biofuels.
And that’s what researchers will be working on next. Identifying rhizomes as potential gatekeepers on photosynthesis has at least given them a starting point from which to try and solve the puzzle of declining switchgrass production, the study authors say.
“Now we can start looking for breeding solutions. We can start looking for plants that have an insatiable appetite for photosynthesis.”
Tejera-Nieves et. al. “Seasonal decline in leaf photosynthesis in perennial switchgrass explained by sink limitations and water deficit.” Frontiers in Plant Science. 2023.
Image: Root system of switchgrass grown at the Land Institute