Chlorophyll Can Be Reduced in Crop Canopies with Little Penalty to Photosynthesis

Abstract

The hypothesis that reducing chlorophyll content (Chl) can increase canopy photosynthesis in soybeans was tested using an advanced model of canopy photosynthesis. The relationship among leaf Chl, leaf optical properties, and photosynthetic biochemical capacity was measured in 67 soybean (Glycine max) accessions showing large variation in leaf Chl. These relationships were integrated into a biophysical model of canopy-scale photosynthesis to simulate the intercanopy light environment and carbon assimilation capacity of canopies with wild type, a Chl-deficient mutant (Y11y11), and 67 other mutants spanning the extremes of Chl to quantify the impact of variation in leaf-level Chl on canopy-scale photosynthetic assimilation and identify possible opportunities for improving canopy photosynthesis through Chl reduction. These simulations demonstrate that canopy photosynthesis should not increase with Chl reduction due to increases in leaf reflectance and nonoptimal distribution of canopy nitrogen. However, similar rates of canopy photosynthesis can be maintained with a 9% savings in leaf nitrogen resulting from decreased Chl. Additionally, analysis of these simulations indicate that the inability of Chl reductions to increase photosynthesis arises primarily from the connection between Chl and leaf reflectance and secondarily from the mismatch between the vertical distribution of leaf nitrogen and the light absorption profile. These simulations suggest that future work should explore the possibility of using reduced Chl to improve canopy performance by adapting the distribution of the "saved" nitrogen within the canopy to take greater advantage of the more deeply penetrating light.

Leaf and canopy photosynthesis of a chlorophyll deficient soybean mutant

“…Concerning the nitrogen demands, our results are in line with the simulations of Walker et al () who demonstrated that similar rates of canopy photosynthesis can be maintained with 9% savings in leaf nitrogen resulting from decreased Chl. In our study, higher nitrogen savings were observed, as MinnGold had 24% less N per unit leaf area than Eiko (Table ).…”

“…The increased photochemical efficiency of individual leaves of the mutant (Figure ) would suggest that for a given amount of APPFD CANOPY , GPP should have been higher in MinnGold than in Eiko. However, this was not the case (Figure ), suggesting that the higher photochemical efficiency was enough to only offset the lower light absorption (Walker et al, ).…”

Converging phenomics and genomics to study natural variation in plant photosynthetic efficiency

“…Decreasing leaf chlorophyll content to allow more radiation into the canopy is a possible route to improving canopy photosynthetic efficiency (Ort et al ., , ). This should result in minor reductions in overall photosynthetic efficiency while significantly saving on nitrogen input (Slattery et al ., ; Walker et al ., ). Therefore, this trait has received considerable attention in GWAS (Dhanapal et al ., ; Hao et al ., ; Lin et al ., ; Wang et al ., ).…”

“…In the past 15 years, attempts to improve plant photosynthetic efficiency have focused on identifying the efficiency bottlenecks in photosynthesis and then bio‐engineering photosynthetic pathways to overcome them (Long et al ., ). This approach has led to suggestions, amongst others, to modify specific subunits of photosystem II (PSII) to optimise light‐harvesting capacity (Long et al ., ; Ort et al ., ; Walker et al ., ), improve the catalytic properties of Rubisco (Carmo‐Silva et al ., ) and transplant the C4 photosynthesis machinery into C3 plants (Crovshoff and Hibberd, ; Schüler et al ., ). Application of this strategy has been successful.…”

The Dynamic Plant: Capture, Transformation, and Management of Energy

“…On the flipside, Walker et al (2018) address the longheld hypothesis that photosynthesis can be increased by limiting the chlorophyll content of upper leaves of the canopy, where photosynthetic activity is readily maximized at low levels of solar radiation. Their simulations for soybean (Glycine max) predict that reductions in chlorophyll can provide a savings of nearly 10% of leaf nitrogen without compromising photosynthetic assimilation at the canopy scale.…”