Ecophysiological Response to Increased Photosynthetic Photon Flux Density (PPFD) in Handroanthus chrysanthus (Jacq.) S.O. Grose and Swietenia macrophylla King Seedlings Under Amazonian Conditions

Handroanthus chrysanthus and Swietenia macrophylla are ecologically and economically significant tree species whose populations have declined due to high demand for their timber, adversely affecting their natural regeneration. This study aimed to evaluate the ecophysiological response to increased photosynthetic photon flux density (PPFD) in H. chrysanthus and S. macrophylla seedlings under Amazonian conditions. Assimilation measurements (A) were performed using a portable iFL-LCpro-SD system. The evaluation of A in response to increased PPFD ranged from 25 to 1800 μmol m⁻²s⁻¹. Water use efficiency (WUE) was calculated as the ratio between A and the transpiration rate (E). S. macrophylla exhibited a significantly higher maximum photosynthetic assimilation rate (Amax, 9.4 ± 0.52 µmol CO₂ m⁻² s⁻¹) compared to H. chrysanthus (6.18 ± 0.17 µmol CO₂ m⁻² s⁻¹, p < 0.05), indicating greater carbon fixation efficiency. S. macrophylla showed a maximum WUE (WUEmax) of 13.17 ± 0.24 µmol mmol⁻¹ at 600 μmol m⁻² s⁻¹ PPFD, while H. chrysanthus reached its WUEmax (6.16 ± 0.23 µmol mmol⁻¹) at 750 μmol m⁻² s⁻¹ PPFD. These results suggest that S. macrophylla exhibits higher WUE under high irradiance conditions, potentially due to more efficient stomatal regulation and an optimized balance between carbon fixation and water loss via transpiration.