Can improved canopy light transmission ameliorate loss of photosynthetic efficiency in the shade? An investigation of natural variation inSorghum bicolor

Abstract

Previous studies have found that maximum quantum yield of CO2 assimilation (ΦCO2,max,app) declines in lower canopies of maize and Miscanthus, a maladaptive response to self-shading. These observations were limited to single genotypes, leaving it unclear that the maladaptive shade response is a general property of this C4 grass tribe, the Andropogoneae. We explored the generality of this maladaptation by testing the hypothesis that erect leaf forms (erectophiles), which allow more light into the lower canopy, suffer less of a decline in photosynthetic efficiency than drooping leaf (planophile) forms. On average, ΦCO2,max,app declined 27% in lower canopy leaves across 35 accessions, but the decline was over twice as great in planophiles than in erectophiles. The loss of photosynthetic efficiency involved a decoupling between electron transport and assimilation. This was not associated with increased bundle sheath leakage, based on 13C measurements. In both planophiles and erectophiles, shaded leaves had greater leaf absorptivity and lower activities of key C4 enzymes than sun leaves. The erectophile form is considered more productive because it allows a more effective distribution of light through the canopy to support photosynthesis. We show that in sorghum, it provides a second benefit, maintenance of higher ΦCO2,max,app to support efficient use of that light resource.

Increased bundle‐sheath leakiness of CO2 during photosynthetic induction shows a lack of coordination between the C4 and C3 cycles

“…The gas‐exchange system was coupled to a TDL (model TGA 200A; Campbell Scientific Inc., Logan, UT, USA) to measure [ 12 CO 2 ], [ 13 CO 2 ] and δ 13 C (Bowling et al ., 2003; Pengelly et al ., 2010; Ubierna et al ., 2013; Jaikumar et al ., 2021). For sorghum, the reference line for the LI‐6400XT was split on the back of the sensor head so that a portion of the reference gas was diverted to the TDL.…”

“…A portion of this N 2 /O 2 air traveled to the gas‐exchange system while the remainder was used as CO 2 ‐free air in calibration to correct for drift in the TDL over the course of the measurements. The TDL was calibrated using the concentration series method by diluting a 10% CO 2 gas cylinder into the N 2 /O 2 stream to produce three different [CO 2 ] of the same isotopic composition (Pengelly et al ., 2010; Tazoe et al ., 2011; Ubierna et al ., 2013; Jaikumar et al ., 2021). The measurement sequence cycled through eight gas streams in the following sequence: CO 2 ‐free air, followed by three different CO 2 concentrations of the same isotopic signature, air from a calibration tank with a known [ 12 CO 2 ], [ 13 CO 2 ], and δ 13 C composition (NOAA Global Monitoring Laboratory, Boulder, CO, USA), the IRGA reference and leaf chamber air streams, and the IRGA reference again.…”

“…S1a). For maize, the TDL was connected to the LI-6800 reference air stream using the reference port on the back of sensor head while the port on the front of the head supplied air from the leaf chamber (Jaikumar et al, 2021;Fig. S1b).…”

A model-guided holistic review of exploiting natural variation of photosynthesis traits in crop improvement

“…Likewise, more prostrate leaves at the canopy bottom and more upright leaves at the top allow greater crop light interception. Erect orientation also reduces the risk of photoinhibition in top leaves under high irradiances at noon on sunny days ( Horton, 2000 ; Jaikumar et al , 2021 ).…”

Photosynthesis in sun and shade: the surprising importance of far‐red photons

“…This has particular importance for modeling photosynthesis in under‐canopy shade (a leaf shaded by neighboring or upper leaves), where the photon distribution has a high fraction of far‐red photons. Photosynthetically active radiation in such environments typically is measured using sensors that measure photons from 400 to 700 nm (Jaikumar et al ., 2021). Likewise, canopy photosynthesis is typically modeled as a function of the 400–700 nm photon flux at multiple canopy depths (Wang et al ., 2020).…”