The influence of the evolution of anatomical characters on leaf hydraulic capacity

Over geological time, changes in climatic factors have contributed to the evolution of plants. CO2 is one environmental factor that affects plants, and changes in CO2 concentration ([CO2]) in the atmosphere across geological time have been thought to have driven evolutionary changes in plant leaf anatomy, enabling plant leaves to adapt to declining atmospheric [CO2] over the last 120 million years. This thesis aimed to study the effect varying anatomical characters had on plant leaf function in different taxa. Atmospheric [CO2] at the crown group age of each species sampled made it possible to link anatomical and physiological variation to the different environmental conditions ([CO2]) each species (or taxa) evolved under. Several leaf gas exchange and hydraulics parameters were measured and leaves were subjected to light step-change and diurnal regimes to ascertain their gas exchange and hydraulic responses over prolonged periods. Results showed significant variation between species in leaf anatomical characteristics (stomatal and vein density). There was also a significant difference between species in their hydraulic responses to changes in light. No significant linear relationship s were found between leaf anatomical characteristics and gas exchange parameters and leaf hydraulics (hydraulic conductance or water flow into the leaf) . However, significant relationships were found among gas exchange parameters with leaf hydraulics, with species exhibiting higher photosynthetic capacities also displaying higher leaf water flow. T he general difference was between angiosperms and other taxa, with angiosperms showing higher values for anatomical and functional characters. Under dynamic light (step-change or diurnal change), angiosperms had more coordination among gas exchange parameters with leaf hydraulic flow. Even though no significant relationships were found between variables measured and [CO2] at the time of taxa divergence, the higher values shown by angiosperms, which evolved under declining atmospheric [CO2], still suggests towards a [CO2] effect on leaf evolution.