Experimental support for optimization of photosynthetic biochemistry and leaf gas exchange in response to combinations of rising CO2, drought stress and phosphorous deficit.

Current land surface models hold large uncertainties in the predictions of how key biosphere processes such as photosynthesis, respiration and transpiration will respond to the combined effects of rising atmospheric CO₂, nutrient enrichment and changes in water availability. Recent developments in optimality theory provide new approaches to explicitly predict coordinated changes in leaf photosynthetic traits, specifically stomatal conductance (g_s) and the maximum capacities of carboxylation (V_cmax) and electron transport (J_max) (e.g. Prentice et al., 2014; Smith et al., 2019; Harrison et al., 2021). These novel formulations show promising results when tested with meta-analyses and global data sets. However, support from manipulative experiments that include changes in CO₂-growth conditions remains scarce. Here we summarize the results from two manipulative experiments using walk-in growth chambers in which a variety of species were exposed to sub-ambient, ambient and elevated growth CO₂ in combination with either a Phosphorous (P) treatment or a drought treatment, and compare the experimental results with predictions from optimality theory. The P treatment exposed plants to either severe P limitation at an N:P ratio of 45:1 or severe Nitrogen (N) limitation at an N:P ratio of 1:1, with a similar supply rate of N. The drought treatment consisted of a continuous dry-down after an initial period of unstressed establishment and growth. Results of the combined CO₂-nutrient treatment showed significant effects of growth CO₂ and P supply on V_cmax and J_max, as well as the whole-plant biomass at the point of harvest. Interaction effects between growth CO₂ and P supply were observed for g_s, the light-saturated photosynthesis rate, leaf P content, and the N:P ratio of the leaf. Results of the combined CO₂-drought experiment showed that g_s, V_cmax and J_max decreased significantly under rising CO₂ treatments, whereas whole-plant biomass at the point of harvest increased significantly. When scaled with non-stressed conditions, g_s and light-saturated photosynthesis declined consistently across CO₂ treatments. These experimental results align with quantitative predictions of g_s, V_cmax and J_max based on optimality theory. However, additional formulations are required to predict whole-plants growth responses as well as changes in plant nutrient-stoichiometry.

 

References

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