Using modern plant trait relationships between observed and theoretical maximum stomatal conductance and vein density to examine patterns of plant macroevolution

<jats:title>Summary</jats:title><jats:p> <jats:list list-type="bullet"> <jats:list-item><jats:p>Understanding the drivers of geological‐scale patterns in plant macroevolution is limited by a hesitancy to use measurable traits of fossils to infer palaeoecophysiological function.</jats:p></jats:list-item> <jats:list-item><jats:p>Here, scaling relationships between morphological traits including maximum theoretical stomatal conductance (<jats:italic>g</jats:italic><jats:sub>max</jats:sub>) and leaf vein density (<jats:italic>D</jats:italic><jats:sub>v</jats:sub>) and physiological measurements including operational stomatal conductance (<jats:italic>g</jats:italic><jats:sub>op</jats:sub>), saturated (<jats:italic>A</jats:italic><jats:sub>sat</jats:sub><jats:italic>)</jats:italic> and maximum (<jats:italic>A</jats:italic><jats:sub>max</jats:sub>) assimilation rates were investigated for 18 extant taxa in order to improve understanding of angiosperm diversification in the Cretaceous.</jats:p></jats:list-item> <jats:list-item><jats:p>Our study demonstrated significant relationships between <jats:italic>g</jats:italic><jats:sub>op</jats:sub>, <jats:italic>g</jats:italic><jats:sub>max</jats:sub> and <jats:italic>D</jats:italic><jats:sub>v</jats:sub> that together can be used to estimate gas exchange and the photosynthetic capacities of fossils. We showed that acquisition of high <jats:italic>g</jats:italic><jats:sub>max</jats:sub> in angiosperms conferred a competitive advantage over gymnosperms by increasing the dynamic range (plasticity) of their gas exchange and expanding their ecophysiological niche space. We suggest that species with a high <jats:italic>g</jats:italic><jats:sub>max</jats:sub> (&gt; 1400 mmol m<jats:sup>−2</jats:sup> s<jats:sup>−1</jats:sup>) would have been capable of maintaining a high <jats:italic>A</jats:italic><jats:sub>max</jats:sub> as the atmospheric <jats:styled-content style="fixed-case">CO</jats:styled-content><jats:sub>2</jats:sub> declined through the Cretaceous, whereas gymnosperms with a low <jats:italic>g</jats:italic><jats:sub>max</jats:sub> would experience severe photosynthetic penalty.</jats:p></jats:list-item> <jats:list-item><jats:p>Expansion of the ecophysiological niche space in angiosperms, afforded by coordinated evolution of high <jats:italic>g</jats:italic><jats:sub>max</jats:sub><jats:italic>, D</jats:italic><jats:sub>v</jats:sub> and increased plasticity in <jats:italic>g</jats:italic><jats:sub>op</jats:sub><jats:italic>,</jats:italic> adds further functional insights into the mechanisms driving angiosperm speciation.</jats:p></jats:list-item> </jats:list> </jats:p>