Pivotal Role of Mesophyll Conductance in Shaping Photosynthetic Performance across 67 Structurally Diverse Gymnosperm Species

Premise of research. Recent research on evolutionarily old species has uncovered mesophyll conductance (gm) as a key player in realized assimilation, but considerable variation was found. However, in gymnosperms, photosynthetic limitations, particularly how limitation combinations contribute to realized photosynthetic rates, have been studied in very few species, precluding any broad conclusions about the share of different controls and hindering reliable global modeling of the gymnosperm forest’s contributions to the global carbon cycle and responses to climate change. Methodology. Using combined gas exchange and fluorescence measurements, we studied photosynthetic limitations in 67 species of gymnosperms (7% of all extant species) to uncover the share of photosynthetic limitations and the consequences of the variation in gm. These studied species were grown in a common garden but originated from globally contrasting habitats and evolutionary histories and represent species with flat leaves, scales, and needle-shaped foliage. Pivotal results. Remarkably, gm varied 21-fold, extending almost across the whole range of values observed in vascular plants to date, and this variation was associated with 6- and 12-fold differences in the net assimilation rate per area and mass and with more than 3-fold differences in the CO2 drawdown from the intercellular airspace to chloroplasts. Photosynthetic limitation analysis revealed that mesophyll and stomatal limitations were dominant, correspondingly spanning 8%–72% and 5%–57% of the total photosynthetic limitation. Both limitations were correlated with foliage structural characteristics, challenging the hypothesis that the phylogenetic background of a species directly determines the partitioning of photosynthetic limitations. Conclusions. The results of the current study emphasize that highly variable gymnosperm leaf structure results in major variations in mesophyll diffusion conductance that needs to be included in carbon gain estimations, especially in evolutionarily old species.