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Though eCO2 results in significant stimulation of photosynthetic rates, lower or lack of

stimulation of Anet during some time points observed in Chapter 2 (Pathare et al., 2017)

provides preliminary evidence for photosynthetic capacity down-regulation in these species. Also, the absence of an eCO2 effect on gs (Chapter 2) suggests that photosynthetic

capacity down-regulation must instead be related to the biochemistry of photosynthesis. Accordingly, in Chapter 3, I investigated the seasonal effects of eCO2 on photosynthetic

capacity of two dominant C3 species to determine if there was down-regulation and the

possible mechanisms involved. The two species, including a dominant C3 grass (M.

stipoides) and a dominant C3 forb (L. purpurascens), were measured for six seasons (two

years over each spring, summer and autumn seasons) in the second and third years of CO2

enrichment at EucFACE. Results from Chapter 3 demonstrate that eCO2 elicits down-

regulation of photosynthetic capacity in the dominant C3 herbaceous species, especially

during the peak growing season of spring. A decrease in Vcmax and Jmax along with a lack

of significant stimulation in Anet under eCO2 was evident during one spring season in the

C3 grass and two spring seasons in the C3 forb. For the summer and autumn periods,

photosynthetic capacities of both the species were maintained under eCO2 and there was

an average 30% stimulation of Anet across the species.

Chapter 4 involved a glasshouse study designed to simulate the soil conditions at the EucFACE facility with well-watered conditions. In Chapter 4, I demonstrated that growth at eCO2 significantly increases Anet in the C3 grasses, but not C3 forbs. Also, there was no

‘water-savings effect’ of eCO2 in the glasshouse experiment (Fig. S4.3). The lack of

significant stimulation of Anet under eCO2 in the C3 forbs coincided with biochemical

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photosynthetic capacity under eCO2 observed in C3 forbs was a result of inability to

maintain the fraction of N allocated to photosynthesis. In contrast to forbs, photosynthetic capacity, fraction of N allocated to Rubisco and stimulation of Anet under eCO2 was

maintained in the C3 grasses. Overall, based on key findings from Chapter 3 and 4 I

conclude that the down-regulation of photosynthetic capacity under eCO2 occurs in the C3

species from a grassy woodland, though the seasons or C3 grasses and C3 forbs may differ

in this regard.

It has been expected that photosynthetic capacity down-regulation under eCO2 will be

greater in the low N conditions compared to the high N conditions (Long et al., 2004, Moore et al., 1999) and some evidences support these expectations (Ellsworth et al., 2004). However, some studies conducted in the N sufficient ecosystems (Inauen et al., 2012) or N fertilized conditions (Crous et al., 2010, Lee et al., 2011, Ruiz-Vera et al., 2017) have also reported a down-regulation of photosynthetic capacity under eCO2. The

current study has been conducted in a relatively N sufficient ecosystem (Crous et al., 2015) compared to the cold-temperate ecosystem (Schulze et al., 1994) and there was evidence of photosynthetic acclimation. In the field experiment, photosynthetic down- regulation under eCO2 was evident during the peak growing season of spring (Chapter 3),

when photosynthetic acclimation under eCO2 is less expected due to higher growth sink

capacities of plants (Lewis et al., 1996). Also, in the glasshouse study, photosynthetic down-regulation under eCO2 was evident only in C3 forbs but not in the C3 grasses, in

spite of being grown under similar supply of soil nutrients and water (Chapter 4). Taken together, findings from the current study support the previous reports of photosynthetic down-regulation under eCO2 even under sufficient N supply (Inauen et al., 2012, Lee et

al., 2011). The ability of plants to maintain biomass enhancement under eCO2 largely

depends on their ability to maintain photosynthetic capacities (Long et al., 2004). If the photosynthetic capacity of plants is down-regulated under eCO2, the ecosystem may

become less responsive to eCO2 and consequently sequester less C than it would without

down-regulation (Luo et al., 2003).

Although the results from current study suggest limited evidence for photosynthetic capacity adjustment with long-term eCO2, the [CO2] of ambient +150 ppm (≈ 550 ppm)

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was used to addresses the effects of eCO2 on the understory herbaceous species. However,

with the rates of CO2 emissions steadily increasing, (Peters et al., 2012), values of CO2 up

to 1000 ppm have been considered as realistic experimental treatments for studying plant responses to higher [CO2] (Franks et al., 2013). However, at [CO2] greater than 550 ppm,

most of the C3 plant species will be CO2 saturated and operating in the asymptotic part of

the Anet-Ci response curve and hence will be largely limited by RuBP regeneration

capacity. This could result in a different pattern of acclimation of photosynthetic capacity compared to that observed at ambient +150 ppm [CO2]. Thus, any further increase in

atmospheric [CO2] (> 550 ppm) may not result in increases in photosynthetic rates.

Furthermore, though rise in atmospheric [CO2] may cause competitive inhibition of

oxygenation of Rubisco, this may not always be beneficial to the plants in terms of increase in photosynthetic rates. For instance, recent studies suggest photorespiration is important for nitrate assimilation (Bloom et al., 2014) as well as P recycling in P-limited conditions (Ellsworth et al., 2015). Thus, increase in atmospheric [CO2] (> 550 ppm) will

result in decrease in photorespiration and may further exacerbate N and P-limitation of photosynthesis and growth.

5.2.3 Elevated CO2 does not increase biomass of the understory herbaceous species