Experimental support for optimization of photosynthetic biochemistry and leaf gas exchange in response to combinations of rising CO2, drought stress and phosphorous deficit.
Authors: de Boer Hugo , van Dijk Jerry , van der Ploeg Martine , Wagner-Cremer Friederike 
Published: March 2022

&lt;p&gt;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 CO&lt;sub&gt;2&lt;/sub&gt;, 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 (&lt;em&gt;g&lt;/em&gt;&lt;sub&gt;s&lt;/sub&gt;) and the maximum capacities of carboxylation (&lt;em&gt;V&lt;/em&gt;&lt;sub&gt;cmax&lt;/sub&gt;) and electron transport (&lt;em&gt;J&lt;/em&gt;&lt;sub&gt;max&lt;/sub&gt;) (&lt;em&gt;e.g.&lt;/em&gt; Prentice &lt;em&gt;et al.&lt;/em&gt;, 2014; Smith &lt;em&gt;et al.&lt;/em&gt;, 2019; Harrison &lt;em&gt;et al.&lt;/em&gt;, 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 CO&lt;sub&gt;2&lt;/sub&gt;-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 CO&lt;sub&gt;2&lt;/sub&gt; 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 CO&lt;sub&gt;2&lt;/sub&gt;-nutrient treatment showed significant effects of growth CO&lt;sub&gt;2&lt;/sub&gt; and P supply on &lt;em&gt;V&lt;/em&gt;&lt;sub&gt;cmax&lt;/sub&gt; and &lt;em&gt;J&lt;/em&gt;&lt;sub&gt;max&lt;/sub&gt;, as well as the whole-plant biomass at the point of harvest. Interaction effects between growth CO&lt;sub&gt;2&lt;/sub&gt; and P supply were observed for &lt;em&gt;g&lt;/em&gt;&lt;sub&gt;s&lt;/sub&gt;, the light-saturated photosynthesis rate, leaf P content, and the N:P ratio of the leaf. Results of the combined CO&lt;sub&gt;2&lt;/sub&gt;-drought experiment showed that g&lt;sub&gt;s&lt;/sub&gt;, &lt;em&gt;V&lt;/em&gt;&lt;sub&gt;cmax&lt;/sub&gt; and &lt;em&gt;J&lt;/em&gt;&lt;sub&gt;max&lt;/sub&gt; decreased significantly under rising CO&lt;sub&gt;2&lt;/sub&gt; treatments, whereas whole-plant biomass at the point of harvest increased significantly. When scaled with non-stressed conditions, &lt;em&gt;g&lt;/em&gt;&lt;sub&gt;s&lt;/sub&gt; and light-saturated photosynthesis declined consistently across CO&lt;sub&gt;2&lt;/sub&gt; treatments. These experimental results align with quantitative predictions of g&lt;sub&gt;s&lt;/sub&gt;, &lt;em&gt;V&lt;/em&gt;&lt;sub&gt;cmax&lt;/sub&gt; and &lt;em&gt;J&lt;/em&gt;&lt;sub&gt;max&lt;/sub&gt; based on optimality theory. However, additional formulations are required to predict whole-plants growth responses as well as changes in plant nutrient-stoichiometry.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Harrison SP, Cramer W, Franklin O, Prentice IC, Wang H, Br&amp;#228;nnstr&amp;#246;m &amp;#197;, de Boer H, Dieckmann U, Joshi J, Keenan TF, &lt;em&gt;et al.&lt;/em&gt;&lt;/strong&gt; &lt;strong&gt;2021&lt;/strong&gt;. Eco-evolutionary optimality as a means to improve vegetation and land-surface models. &lt;em&gt;New Phytologist&lt;/em&gt; &lt;strong&gt;231&lt;/strong&gt;: 2125&amp;#8211;2141.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Prentice IC, Dong N, Gleason SM, Maire V, Wright IJ&lt;/strong&gt;. &lt;strong&gt;2014&lt;/strong&gt;. Balancing the costs of carbon gain and water transport: testing a new theoretical framework for plant functional ecology. &lt;em&gt;Ecology Letters&lt;/em&gt; &lt;strong&gt;17&lt;/strong&gt;: 82&amp;#8211;91.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Smith NG, Keenan TF, Prentice IC, Wang H, Wright IJ, Niinemets &amp;#220;, Crous KY, Domingues TF, Guerrieri R, Ishida FY, &lt;em&gt;et al.&lt;/em&gt;&lt;/strong&gt; &lt;strong&gt;2019&lt;/strong&gt;. Global photosynthetic capacity is optimized to the environment. &lt;em&gt;Ecology Letters&lt;/em&gt; &lt;strong&gt;22&lt;/strong&gt;: 506&amp;#8211;517.&lt;/p&gt;