Light response curve analysis of juvenile black locust clones: A case study from eastern Hungary

Assimilation (A) and photosystem II (PSII) efficiency value light response curves [A/PPFD and PSII/PPFD curves (PPFD – photosynthetic photon flux density)] of promising black locust clones (NK2 and PL251) and the registered Üllői clone were analysed to study the net assimilation rates and PSII efficiency within the function of PPFD levels. The natural logarithmic regression functions fitted well to the measured data points for A/PPFD, R² values varied between 0.9515–0.9884. For PSII/PPFD values, we used the exponential regression function with R² ranging from 0.9948 to 0.9989. Except for PL251, the A/PPFD curves of the tested clones increased steadily with increasing illumination levels but flattened at the 600 µmol·m–²·s–¹ PPFD level due to the effect of photorespiration on the assimilation rate. For PL251, the A/PPFD curve decreased at the 1 200 µmol·m–²·s–¹ PPFD level. Unlike A/PPFD results, the PSII/PPFD exponential curve decreased as the PPFD level increased. Europe is forecast to experience significant negative climate change factors, including increased drought, heat, and irregular precipitation. Under such conditions, relatively drought-tolerant tree species such as black locust will play a vital role in new afforestation and uninterrupted wood supply. Consequently, growing and improving newly bred black locust clones, including the ecophysiological studies of relatively drought-tolerant clones, is increasingly vital.