Sunfleck utilisation and shade tolerance

Abstract

Fluctuating light is the major source of energy for understorey plants, yet relatively few studies have focused on variation in characteristics that influence its utilisation. To test the hypothesis that acclimation to a light environment allows plants to fix more carbon in their native light environment than plants from other environments, I compared steady-state and dynamic photosynthetic responses of four native tree species growing in a range of natural light environments. Despite large increases in photosynthetic capacity in sun acclimated plants, steady-state photosynthetic performance under low irradiance was similar between plants acclimated to different light environments. Larger differences in performance under low irradiance were seen when photosynthesis was expressed per unit mass. I found no effect of growth irradiance on induction rates. While rates of induction and photosynthetic capacity were negatively correlated, plants with increased photosynthetic capacity had higher absolute photosynthetic rates during induction. Thus, there was mixed evidence for a trade-off between photosynthetic performance under high light and rates of induction. I found a strong relationship between increased stomatal conductance and rates of induction, suggesting that stomata play a large role in controlling induction rates. To further explore the relationships between induction rates, photosynthetic capacity and stomatal conductance, I developed a method for partitioning limitations during induction into stomatal and biochemical components. Applying this method to my data showed that stomata were strongly limiting to photosynthesis during induction. Plants acclimated to high growth irradiances were more limited by stomata than were shade acclimated plants, duo to decreased stomatal conductance under low light. This suggests that shade plants likely reduce their water use efficiency for increased potential carbon gain, while sun plants have reduce their potential carbon gain in exchange for increased water use efficiency.