Perspectives on improving light distribution and light use efficiency in crop canopies

Abstract

Plant stands in nature differ markedly from most seen in modern agriculture. In a dense mixed stand, plants must vie for resources, including light, for greater survival and fitness. Competitive advantages over surrounding plants improve fitness of the individual, thus maintaining the competitive traits in the gene pool. In contrast, monoculture crop production strives to increase output at the stand level and thus benefits from cooperation to increase yield of the community. In choosing plants with higher yields to propagate and grow for food, humans may have inadvertently selected the best competitors rather than the best cooperators. Here, we discuss how this selection for competitiveness has led to overinvestment in characteristics that increase light interception and, consequently, sub-optimal light use efficiency in crop fields that constrains yield improvement. Decades of crop canopy modeling research have provided potential strategies for improving light distribution in crop canopies, and we review the current progress of these strategies, including balancing light distribution through reducing pigment concentration. Based on recent research revealing red-shifted photosynthetic pigments in algae and photosynthetic bacteria, we also discuss potential strategies for optimizing light interception and use through introducing alternative pigment types in crops. These strategies for improving light distribution and expanding the wavelengths of light beyond those traditionally defined for photosynthesis in plant canopies may have large implications for improving crop yield and closing the yield gap.

Figure 1

Wavelength-dependent light profiles in canopies. Modeled profiles of absolute photon availability (A), percent photon availability based on total photons within each layer (B), and photon ratios (C) in blue (B; 400–499 nm), green (G; 500–599 nm), red (R; 600–699 nm), and far-red (FR; 700–749 nm) wavelengths within a crop canopy illuminated by 1,000 µmol m⁻² s⁻¹. Absorbance of individual wavelengths in field-grown soybean leaves was used to determine light attenuation with cumulative leaf area index (LAI) from the top of the canopy using the Beer–Lambert equation (I=I₀e^{−k × LAI}).

Figure 2

Photoprotection of the photosynthetic membrane. In linear electron transport (blue lines), photosystem II (PSII)-associated light-harvesting complexes (LHCII) transfer excitation energy from an absorbed photon (solid orange line) to PSII, resulting in the splitting of water. Electrons from water flow from PSII to cytochrome b₆f (Cyt b₆f) via reduction of plastoquinone (PQ) to plastoquinol (PQH₂), translocating H⁺ from the stroma to the lumen via the Q cycle. Plastocyanin (PC) carries the electrons to photosystem I (PSI), where the excitation energy from a second absorbed photon (solid orange line) eventually leads to the reduction of NADP⁺ to NADPH via ferredoxin (Fd) and Fd-NADP⁺ oxidoreductase (FNR). Linear electron transport increases the ΔpH across the thylakoid membrane, which drives ATP synthesis through ATP synthase. A large ΔpH triggers dissipation of excitation energy in excess of PSII capacity (dashed orange line) as heat via zeaxanthin (Zea) in the energy-dependent component (termed “qE”) of non-photochemical quenching. P700⁺ quenches the excess excitation energy transferred from PSI-associated light-harvesting complexes (LHCI) to PSI (dashed orange line) and releases the energy as heat. Some electrons received by PSI flow through PGR5/PGRL1- or NDH-mediated cyclic electron transport (red lines), which further increases ΔpH and qE and prevents oxidation of PQH₂ to PQ at Cyt b₆f (termed “photosynthetic control”). This slows electron flow through linear electron transport and induces photoinhibition (qI) of PSII in extreme light stress to prevent irreversible damage to PSI. Excess electrons at PSI may also flow through pseudocyclic electron transport in the water-water cycle (magenta arrows). Figure adapted from Yamamoto and Shikanai (2019).

Figure 3

Wavelength-dependent light profiles in leaves. Profiles of relative absorbance (A) and the proportion of available light (B) in blue (B; 488 nm), green (G; 561 nm), and red (R; 638 nm) wavelengths within wild-type soybean leaves illuminated from the adaxial surface (data based on light-sheet microscopy analyses in Slattery et al. (2016)).

Figure 4

Solar energy spectrum and pigment absorption profiles. (A) Solar output versus wavelength in terms of number of photons (red) and amount of energy (black) from ASTM E-490 (https://www.nrel.gov/grid/solar-resource/spectra.html). (B) Relative absorbance spectra of chlorophylls (Chls) and bacteriochlorophyll (BChl) b in solvents. Chlorophyll absorbance data were obtained from http://vplapps.astro.washington.edu/pigments. BChl absorbance data were obtained from Frigaard et al. (1996).