The timescale of photosynthetic acclimation in Glycine max

Plants acclimate to fluctuating light by adjusting traits to optimize photosynthesis, a costly process requiring resource acquisition or remobilization over a timescale that remains uncertain. Potential disruptions to carbon assimilation during this period can affect carbon cycling predictions in terrestrial ecosystem models, which are sensitive to photosynthetic acclimation. The timescale may be influenced by resource investment trends: high-light conditions require more resources to sustain a higher photosynthetic capacity, leading to potentially longer acclimation rates, while low-light conditions reduce resource investment and may allow for faster rates. Leaf lifespan may also influence the timescale, where short-lived leaves may prioritize producing new leaves over acclimating mature ones, due to mature leaves being less capable of adjusting traits. To test whether reduced investment leads to faster acclimation and if new leaves fully acclimate to new light conditions, a fully factorial light-swap experiment was conducted on soybean (Glycine max), where plants were swapped from low to high light and high to low light conditions with measurements of gas exchange and fluorescence. Results were inconsistent with typical light acclimation responses, with high-light plants showing reduced photosynthetic capacity compared to low-light plants, likely due to increased investment in non-photochemical quenching. This adjustment may have contributed to faster acclimation in plants transferred from high to low light, supporting hypothesis one; however, it may not reliably predict acclimation timescales, as the acclimation of traits was inconsistent, with simultaneous up- and downregulation occurring. New leaves showed a limited acclimation response under high-light transfers, as several traits did not acclimate. Overall, while reduced investment may shorten acclimation time, investment trends may not correspond to acclimate rates, which may instead correspond with leaf lifespan. However, short-lived species may still exhibit incomplete acclimation under high-light transfers, especially when non-photochemical quenching is present.