A recent study points to novel ways of improving crop yields and food production by elucidating how long it takes for wheat to return to full photosynthesis when making the transition from shade to sun exposure.
The Green Revolution in the latter half of the 20th century, via combination of new plant varieties, more efficient farming methods, and fertilizer, increased the amount of grain harvested per acre of wheat, the world’s second largest direct source of human calories. Further improvements in wheat productivity, likely to be necessary to feed the planet’s growing population, need to come from increasing photosynthesis and the amount of the plant’s biomass, says senior author and University of Illinois plant biologist Stephen Long whose group published the work in Philosophical Transactions of the Royal Society B. “The changes from the Green Revolution are hitting their limit,” Long says. “We need a new way to move out of stagnation.”
Prior work attempting to tease out the molecular underpinnings of wheat photosynthesis came from plant biologist Elizabete Carmo-Silva at the University of Lancaster in the UK and other scientists who focused on ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the enzyme in all plants that starts the transformation of atmospheric carbon dioxide into glucose. “We’ve known for years that Rubisco is not a very efficient enzyme,” Carmo-Silva says. “It’s a very complex reaction and many things can go wrong.”
Other cellular molecules tend to bind to the active site of Rubisco, which requires energy in the form of ATP to be removed. At night or in the shade, the cell’s stores of ATP become depleted, which leaves Rubisco locked in an inactive state. With a return to sunlight, ATP stocks increase, which another enzyme, Rubisco activase, uses to clear out the debris. Experiments in tobacco and other agriculturally important plants showed that a plant can start harvesting energy from sunlight within seconds after a return to full sun. But Rubisco activity appeared to lag. Long and colleague Samuel Taylor, a plant physiologist at the University of Lancaster, wanted to know how this delay affected the wheat’s ability to turn inorganic carbon dioxide into organic carbohydrate.
Long and Taylor grew a variety of bread wheat called Highbury in a greenhouse and measured how much carbon dioxide its leaves could remove from the atmosphere. They found it took between 15 and 20 minutes for the plant to resume full carbon dioxide uptake after moving from shade to full sun. Over a season, these slow transitions could reduce the plant’s productivity by 21%.
“We’ve always studied photosynthesis in steady-state conditions, but that’s not what happens in the field,” says Gemma Molero, a wheat scientist at the International Maize and Wheat Improvement Center in Mexico. “This is the first time someone has quantified the effects of these changes and why this is a breakthrough discovery in improving wheat yields.”
Wheat geneticist Hikmet Budak of Montana State University, Bozeman, believes the study’s methods were sound, but he questions the generalizability of the results. “Every genotype of wheat will respond to shade or sun differently,” Budak says. “And what happens in a controlled environment is completely different from what happens in the field.”
That’s why Long says his team has begun testing responses of a variety of wheat cultivars and wheat relatives for their Rubisco responses in shade to sun shifts in the lab. In principle, innovative plant breeding programs could introduce these responses into bread and durum wheat. And in principle, the resultant wheat plant would be “extremely sustainable” says Long. “We would be getting more yield but not using more water or more nitrogen,” he says.