Abstract
Previous studies have found that maximum quantum yield of CO2 assimilation (Φ CO2,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 whether 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, Φ CO2,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 Φ CO2,max,app to support efficient use of that light resource.
Highlights
A major component of crop yield increases over the last century has been increased planting density (Duvick,2005;Lee and Tolenaar, 2007)
The objectives of this study were 3-fold: (i) to test the hypothesis that quantum yields would decrease in the lower canopy of a wide range of sorghum genotypes; (ii) to test the hypothesis that erectophile genotypes, that allow improved light penetration through the canopy, would show a smaller loss of photosynthetic efficiency in the lower canopy leaves; and (iii) to test whether bundle sheath leakiness would increase and concentrations of C4 enzymes needed to support the light-saturated rate of photosynthesis would decline in the lower canopy leaves
Similar effects were evident in 2017: in planophile accessions, Φ CO2,max,abs was 30% lower in lower canopy leaves than in the upper canopy, while the corresponding decrease was only 12% in erectophiles (t=2.25, P=0.037, df=18 for the type×position contrast)
Summary
A major component of crop yield increases over the last century has been increased planting density (Duvick,2005;Lee and Tolenaar, 2007). The ability to tolerate high planting density depends on a number of morphological traits, including leaf inclination angle (Williams et al, 1968; Tian et al, 2011; ). For example, erectophile plants, with small inclination angles relative to the vertical (i.e. highly erect leaves), are associated with higher grain yield than planophiles (i.e. with drooping leaves and high leaf inclination angles: Pendleton et al, 1968;Lambert and Johnson, 1978; Lee andTolenaar, 2007; Lauer et al, 2012). In sorghum [Sorghum bicolor (L.) Moench], the world’s fifth most important cereal crop (Rai et al, 1999), considerable variation exists for leaf inclination angle (Xin et al, 2009) and for further increases in planting density (Xin et al, 2015). The causes of the association between erect leaf angle and yield improvement may not yet be fully understood, in canopies of C4 crops of the Andropogoneae.This tribe includes the highly productive crops: sorghum, sugarcane (Saccharum officinarum L.), maize, and the key bioenergy crop giant miscanthus (Miscanthus×giganteus Greef et Deuter). there is considerable scope for further work both to improve leaf erectness through selection and to understand better the mechanistic basis for why leaf erectness is linked to improved carbon accumulation by sorghum and related crops
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