Physiological basis of genotypic variation in plant response to elevated CO2
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2013
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Melbourne Authors
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Forest and Ecosystem Science
Agriculture and Food Systems
Agriculture and Food Systems
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Masters Research thesis
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© 2013 Chamindathee Lakmini Thilakarathne
Abstract
The CO2 concentration [CO2] in the atmosphere is increasing and is predicted to double by the end of the 21st century which is likely to have a profound effect on plant growth and yield, especially for C3 plants. Rising [CO2] has direct effect on photosynthesis (A), thus its potential stimulation drives the plant growth and development. However, these responses varied among species but, little is known about the underlying mechanism of genotypic variation in plant response to elevated [CO2]. In this study, wheat (Triticum aestivum L.) was used to test the hypotheses genotypic variation in response to elevated [CO2] exists within the wheat germplasm and if genotypic variability exists, whether it is related to leaf level A and related other traits. Based on above hypotheses, one experiment under glass house and two experiments under Australian Grains Free Air CO2 Enrichment (AGFACE) condition were conducted in the year 2010 and 2011.
In experiment 1, seven wheat genotypes were grown at either ambient [CO2] (~384 µmol mol–1) or elevated [CO2] (700 µmol mol–1) in temperature controlled glass houses. Grain yield increased under elevated [CO2] by an average of 38% across all seven genotypes, and this was correlated with increases in both spike number (r = 0.868) and above ground biomass (AGB)(r = 0.942). Across all the genotypes, flag leaf A increased by an average of 57% at elevated [CO2]. The response of A to elevated [CO2] ranged from 31% (in H45) to 75% (in Silverstar). Only H45 showed A acclimation to elevated [CO2] which was characterised by lower leaf maximum RuBisCO carboxylation efficiency (Vc.max), maximum electron transport rate (Jmax) and leaf nitrogen (N) concentration. Leaf level traits responsible for plant growth, such as leaf mass per unit area (LMA), carbon (C), N concentration on dry mass basis (NFLAG LEAF), N content on an area basis (NLA) and the C:N increased at elevated [CO2]. LMA stimulation ranged from 0% to 85% and was clearly associated with increased NLA. Both of these traits were positively correlated with grain yield, suggesting that differences in LMA play an important role in determining the grain yield response to elevated [CO2]. Thus increased LMA can be used as new trait to select genotypes for a future [CO2]-rich atmosphere.
In the second experiment, leaf blade elongation rate (LER) and leaf C and N dynamics in expanding leaf blade (ELB, sink) and A, C and N status in the last fully expanded leaf blade (LFELB, source) were determined using two wheat genotypes, Yitpi and H45. Plants were grown at either ambient [CO2] (~384 µmol mol–1) or elevated [CO2] (~550 µmol mol–1) within the AGFACE facility. Elevated [CO2] increased the leaf area and plant total above ground dry mass by 42% and 53% respectively for Yitpi compared to 2% and 13% for H45. Both genotypes showed increased LER at elevated [CO2] which was 36% for Yitpi compared to 5% for H45. In contrast, A increased by 99% for Yitpi at elevated [CO2] whereas H45 showed no change. For both genotypes, a strong correlation (r=0.807) was observed between LER of ELB and soluble carbohydrate concentration in LFELB over diurnal cycle. In ELB, the highest spatial N concentration was observed in the cell division zone, where N concentration was 67.3 and 60.6 mg g-1 for Yitpi compared to 51.1 and 39.2 mg g-1 for H45 at ambient and elevated [CO2] respectively. In contrast, C concentration in the cell division and cell expansion zone of the ELB increased only in Yitpi suggesting that C supply from the LFELB is genotypes dependent and well associated with LER, leaf area expansion and whole plant growth response to elevated [CO2].
In the third experiment, variation in A acclimation to elevated [CO2] at different leaf ontogeny was investigated. Seven wheat genotypes with contrasting growth and yield response to elevated [CO2], Janz, Yitpi, Silverstar, H45, Drysdale, Hartog, and Zebu were grown under rain fed condition at either ambient [CO2] (~384 µmol mol–1) or elevated [CO2] (~550 µmol mol–1) in the AGFACE facility. Leaf gas exchange measurements were conducted at two stages of flag leaf blade development: pre and post-anthesis. Immediately after the gas exchange measurements, flag leaf blade was sampled for biochemical analysis. Vc.max was calculated from gas exchange data to estimate the A acclimation to elevated [CO2]. Genetic variation in A acclimation to elevated [CO2] was observed. H45 showed strong acclimation to elevated [CO2] where Silverstar showed no acclimation. A acclimation to elevated [CO2] was observed only at post-anthesis, which was characterized by both reductions of Vc.max and Jmax. In both stages of the flag leaf, soluble protein concentration was significantly decreased at elevated [CO2]. Nevertheless, association of A acclimation and soluble protein concentration was not significant. In particular, Silverstar showed a large reduction in soluble protein concentration at elevated [CO2] at post-anthesis, but still showed no significant A acclimation (no reduction in Vc.max) to elevated [CO2]. The reductions in total soluble protein content at elevated [CO2] were not similar either between growth stages or genotypes. These findings suggest that A acclimation to elevated [CO2] varied between wheat genotypes but it is not fully associated with changes in soluble protein concentration.
In summary, elevated [CO2] stimulated A, growth and grain yield and found large genetic variability of CO2 response. Variation in growth and yield response to elevated [CO2] was closely associated with leaf level traits such as LMA, leaf N and A capacity suggesting that plasticity of leaf level A related traits play a key role in response to elevated [CO2]. Particularly, genotypes showed higher A response to elevated [CO2], also showed increase LER which later translate to biomass and greater grain yield under elevated [CO2].