Photosynthesis, Light Use Efficiency, and Yield of Reduced-Chlorophyll Soybean Mutants in Field Conditions.

Rebecca A Slattery,Andy Vanloocke,Carl J Bernacchi,Donald R Ort,Xin-Guang Zhu

doi:10.3389/fpls.2017.00549

Abstract

Reducing chlorophyll (chl) content may improve the conversion efficiency of absorbed photosynthetically active radiation into biomass and therefore yield in dense monoculture crops by improving light penetration and distribution within the canopy. The effects of reduced chl on leaf and canopy photosynthesis and photosynthetic efficiency were studied in two reportedly robust reduced-chl soybean mutants, Y11y11 and y9y9, in comparison to the wild-type (WT) “Clark” cultivar. Both mutants were characterized during the 2012 growing season whereas only the Y11y11 mutant was characterized during the 2013 growing season. Chl deficiency led to greater rates of leaf-level photosynthesis per absorbed photon early in the growing season when mutant chl content was ∼35% of the WT, but there was no effect on photosynthesis later in the season when mutant leaf chl approached 50% of the WT. Transient benefits of reduced chl at the leaf level did not translate to improvements in canopy-level processes. Reduced pigmentation in these mutants was linked to lower water use efficiency, which may have dampened any photosynthetic benefits of reduced chl, especially since both growing seasons experienced significant drought conditions. These results, while not confirming our hypothesis or an earlier published study in which the Y11y11 mutant significantly outyielded the WT, do demonstrate that soybean significantly overinvests in chl. Despite a >50% chl reduction, there was little negative impact on biomass accumulation or yield, and the small negative effects present were likely due to pleiotropic effects of the mutation. This outcome points to an opportunity to reinvest nitrogen and energy resources that would otherwise be used in pigment-proteins into increasing biochemical photosynthetic capacity, thereby improving canopy photosynthesis and biomass production.

Highlights

Increasing the yield potential (Yp) of important agronomic crops is imperative for meeting predicted future production needs
Yp for a given crop during a growing season is the product of several components: the incident solar radiation across the growing season (St), the proportion of St that is photosynthetically active radiation (PAR; estimated as 0.487), the radiation interception efficiency, the conversion efficiency of intercepted radiation into biomass, and the partition efficiency of biomass into harvestable product
Reducing chl content in soybean was hypothesized to lead to an improved distribution of light in the canopy, resulting in benefits to leaf and canopy photosynthesis and yield (Zhu et al, 2010; Ort et al, 2011; Drewry et al, 2014)

Summary

Introduction

Increasing the yield potential (Yp) of important agronomic crops is imperative for meeting predicted future production needs. Εp has reached values of 0.60 for soybean (Dermody et al, 2008; Koester et al, 2014), which is at or near the estimated theoretical maximum of ∼0.55–0.67 for major food crops (Austin et al, 1980; Bugbee and Monje, 1992; Khush, 1995; Smil, 1999; Hay and Porter, 2006; Prasad et al, 2006). These advancements in εi and εp leave only modest potential for further improvement of Yp (Zhu et al, 2010). Εc operates substantially below the theoretical maxima for C3 (0.094) and C4 (0.123; Zhu et al, 2010) crop canopies and limits yield potential (Zhu et al, 2008, 2010), especially in soybean where the maximum realized εc (0.028) is estimated at less than a third of the C3 theoretical potential (Slattery and Ort, 2015)

Methods

Results

Discussion

Conclusion