The role of environment, sink capacity, and carbon translocation in determining C3 plant responses to elevated [CO2]
Over the past century, CO2 concentration ([CO2]) in the atmosphere has been steadily increasing, leading to global climate change. Elevated [CO2] increases yield, biomass, and photosynthesis in most C3 plants, but the degree to which elevated [CO2] stimulates crop yields can depend upon climatic factors and plant physiological attributes, including sink strength and sugar transport capacity. This thesis uses field, laboratory, and meta-analytic techniques to investigate factors that influence the responsiveness of plants to elevated [CO2], with the ultimate aim of understanding variation in and improving future crop production. Photosynthesis is typically stimulated in C3 crops exposed to elevated [CO2], while stomatal conductance is typically decreased. Theory predicts that the magnitude of stimulation of photosynthesis at elevated [CO2] is greater at higher temperatures. In Chapter 2, the degree to which these physiological responses of C3 crops to elevated [CO2] would translate to yield responses was tested. Using a global dataset of published yield data from Free Air CO2 Enrichment (FACE) and Open Top Chamber (OTC) experiments, there was greater yield response to elevated [CO2] in C3 crops under dryer conditions, but there was no correlation between yield response to elevated [CO2] and growing season temperature. Thus, the theoretical response of photosynthesis to elevated [CO2] and temperature was not observed in seed yield, perhaps due to direct effects of temperature on respiration or reproductive processes. In Chapter 3, intraspecific variation in soybean (Glycine max) response to elevated [CO2] and the agronomic traits associated with greater yield responsiveness to elevated [CO2] were analyzed. Eighteen soybean cultivars, varying in maturity group, year of release date, and agronomic traits, were grown at SoyFACE from 2003 to 2008. There was significant intraspecific variation in yield response to elevated [CO2], with shorter cultivars and those with high harvest index showing greater response to elevated [CO¬2]. Harvest index is an indicator of sink strength, which may be important for CO2 response, because it can relieve the accumulation of carbohydrates in the photosynthesizing leaves, which at high concentrations in elevated [CO2] can signal down-regulation of photosynthetic capacity. In Chapter 4, the hypothesis that different mechanisms of phloem loading can lead to a change in the photosynthetic response to elevated [CO2] was tested. Plants have evolved different strategies to load phloem with sugars to send to sink tissue. One method, apoplastic loading, uses active sugar transporters to load phloem, while another method, symplastic loading, uses passive diffusion along a sucrose gradient from leaf mesophyll cells to phloem. The hypothesis was that passive loaders, adapted to high mesophyll sucrose concentrations, would experience less sugar-mediated feedback of photosynthesis at elevated [CO2] compared to apoplastic loaders. To test this, Pisum sativum (pea) and Beta vulgaris (beet; apoplastic phloem loaders) and Fragaria x ananassa (strawberry) and Paeonia lactiflora (peony; passive phloem loaders) were grown at elevated [CO2] in the field in 2013 and 2014, testing their biochemical, photosynthetic, and growth responses. All species responded to elevated [CO2] with increased photosynthesis and little down-regulation of capacity. There was a strong stimulation in leaf starch but little increase in leaf soluble sugar content at elevated [CO2], suggesting little sugar mediated downregulation of photosynthesis in any species. Thus, phloem loading strategy does not appear to be a strong determinant of plant response to elevated [CO2]. In Chapter 5, the impact of phloem loading on response to elevated [CO2] was studied further in two transgenic lines of Arabidopsis thaliana with altered sucrose transporter expression. In the HvSUT1 genotype, the primary sucrose transporter used for phloem loading in Arabidopsis (AtSUC2), was replaced with a barley sucrose transporter (HvSUT1), driven by the native AtSUC2 promoter since in vitro, HvSUT1 is more active than AtSUC2. In the AtSUC1 genotype, AtSUC1 was overexpressed in a wild-type background using the viral companion cell-specific promoter CoYMV to increase sucrose transporter expression. Neither transgenic line showed improved growth at ambient or elevated [CO2] compared to wild-type. The AtSUC1 genotype had a much greater response to elevated [CO2] than the other two genotypes, but only because growth at ambient [CO2] was significantly reduced. The reasons for stunted growth at ambient [CO2] in AtSUC1 are not clear, but do not appear to be related to phosphate limitation. This dissertation research provides insight into the physiological mechanisms behind the response of plants to elevated [CO2]. Across field experiments, water availability significantly alters response to elevated [CO2], with drier experiments showing a greater response. Within the soybean germplasm, height and partitioning coefficient both correlate to response to elevated [CO2]. There did not, however, appear to be a link between phloem loading strategy and response to elevated [CO2] and phloem loading capacity had mixed effects on response to elevated [CO2]. This research will be important for better estimating and maximizing response to elevated [CO2].
Climate change, food security, crop yield, phloem
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign