Plants are integrated organisms in which all parts must work well together in order to maximize fitness of the whole, termed trait integration. This is particularly evident when we consider photosynthetic traits. Plant leaves must balance CO2 supply and demand because it is costly to provide a leaf with high photosynthetic capacity, especially if there is limited CO2 supply. Stomatal anatomy, especially density and size, regulates gas exchange with the atmosphere and is a significant determinant of CO2 supply to the leaf. The resulting stomatal size and density of a fully expanded leaf however do not develop in isolation, rather they develop in the context of whole leaf cell differentiation and expansion. The genus Limonium, commonly known as sea-lavender, is adapted to stressful environments in the Mediterranean basin. Importantly there are differences among Limonium species in the biochemical properties of the key CO2 fixing enzyme, Rubisco, and also in leaf size making it an ideal system to study photosynthetic trait integration.
A new study by Conesa et al. and published in AoBP tests the hypothesis that trait integration arises because of both (i) functional coordination between stomatal and Rubisco kinetics and (ii) shared development between leaf size and stomatal density. They used a common garden experiment with ten Limonium species to demonstrate that variation in Rubisco kinetics (kccat and Sc/o) and leaf size are coordinated through stomatal traits (density, size, gsmax, operational gs). Specifically, they show (i) that species with low stomatal conductance have evolved Rubisco enzymes better adapted to low operational chloroplastic CO2 concentrations, and (ii) that lower stomatal density was associated with greater leaf size, which can be explained by a greater proportion of pavement cells in large-leaved species. They also found that trait relationships seen within Limonium are in some cases opposite to those described previously. Considering the diversity of plant species worldwide, trait coordination within particular species’ groups may be highly variable. The authors conclude by stating the importance of studying plants as integrated organisms in order to understand patterns of trait covariation across species.