Using the rapid A-Ci response (RACiR) in the Li-Cor 6400 to measure developmental gradients of photosynthetic capacity in poplar

The rapid A-C-i response (RACiR) technique alleviates limitations of measuring photosynthetic capacity by reducing the time needed to determine the maximum carboxylation rate (V-cmax) and electron transport rate (J(max)) in leaves. Photosynthetic capacity and its relationships with leaf development are important for understanding ecological and agricultural productivity; however, our current understanding is incomplete. Here, we show that RACiR can be used in previous generation gas exchange systems (i.e., the LI-6400) and apply this method to rapidly investigate developmental gradients of photosynthetic capacity in poplar. We compared RACiR-determined V-cmax and J(max) as well as respiration and stomatal conductance (g(s)) across four stages of leaf expansion in Populus deltoides and the poplar hybrid 717-1B4 (Populus tremula x Populus alba). These physiological data were paired with leaf traits including nitrogen concentration, chlorophyll concentrations, and specific leaf area. Several traits displayed developmental trends that differed between the poplar species, demonstrating the utility of RACiR approaches to rapidly generate accurate measures of photosynthetic capacity. By using both new and old machines, we have shown how more investigators will be able to incorporate measurements of important photosynthetic traits in future studies and further our understanding of relationships between development and leaf-level physiology.