In modern intensive systems, crops form dense canopies where both sun and shade leaves contribute to photosynthetic carbon assimilation and productivity. Shaded leaves are estimated to contribute about 50% of total canopy carbon gain and therefore the efficiency with which shade leaves use light is a critical factor determining crop yield potential.

… With the perceived need to increase crop production, given forecasts of future demand, it becomes increasingly important to understand leaf photosynthetic shade response of major C4 crops and in turn whether this could affect canopy photosynthesis and productivity.

—Pignon et al.

The study found that when top and bottom leaves are placed in the same low light, the lower canopy leaves showed lower rates of photosynthesis. Shaded corn leaves are 15% less efficient than top leaves—and worse, lower leaves are 30% less efficient than the top leaves of Miscanthus, a perennial bioenergy crop that is 60% more productive than corn in Illinois.

Considering the crop as a whole, this loss of efficiency in lower leaves may costs farmers about 10% of potential yield—a cost that will increase as planting density increases. This likely applies to other C4 relatives, such as sugarcane and sorghum.

The wild ancestors of C4 crops are thought to have grown as individuals in open habitats where the number of leaves that they produced would have been limited by water and nitrogen and most leaves would be exposed to full sunlight.

Today we grow these crops in ever denser stands, and provide them with nitrogen and water so that they can produce many more layers of leaves. But as a result, the proportion of leaves that are shaded has increased, and the production of grain will depend more and more on the contribution of this increasing proportion of shaded leaves.

—principal investigator Steve Long

First author Charles Pignon, a doctoral candidate in the crop sciences and at the Carl R. Woese Institute for Genomic Biology, explained that the loss in efficiency in the lower canopy was not due to the leaf senescing and dying off. The leaves were still perfectly healthy when examined.

Next, it will be important to find out why this loss in efficiency occurs and if there’s any way that we can fix it, since overcoming this and gaining a 10% increase in production would be very significant.

—Charles Pignon

This work was supported by the Energy Biosciences Institute (EBI). The EBI is a public-private collaboration supported from BP.

Resources

• Charles P. Pignon, Deepak Jaiswal, Justin M. McGrath, Stephen P. Long (2017) “Loss of photosynthetic efficiency in the shade. An Achilles heel for the dense modern stands of our most productive C4 crops?” J Exp Bot 68 (2): 335-345. doi: 10.1093/jxb/erw456