In this study, Fig. 1 shows the time-course changes in the photosynthetic light induction period of ETR, NPQ, and both Fv/Fm and ΦPSII for 6-month (Figs. 1A, C, and E) and 2.4-year-old seedlings (Figs. 1B, D, and F) of Mahonia oiwakensis. Measurements were initially obtained under dark followed by exposure to 50, 100 (low LI), 300, 500, 1,000 (moderate LI), 1,500, and 2,000 (high LI) µmol m− 2 s− 1 PPFD light induction for 60 min at 25°C. When these overnight dark-adapted leaves were exposed to all light illuminations, the ETR of 6-month-old seedlings suddenly increased at the beginning of 2 min, but low LI treatments remained low (12 and 20 µmol m− 2 s− 1, respectively) as time passed, and no reduction of electron transport was detectable at longer illumination times compared to the other LI treatments that were gradually increased thereafter (> 30 µmol m− 2 s− 1) (Fig. 1A). In addition, ETR values under high LI conditions were significantly higher than in other LI treatments. A similar trend was observed in the ETR of 2.4-year-old seedlings, but the ETR value (> 50 µmol m− 2 s− 1) under L-1,000 was significantly higher than other LI treatments (< 50 µmol m− 2 s− 1) (Fig. 1B). Figures 1 C and D display NPQ being sharply increased in all leaves in high LI treatments. NPQ values for all seedlings under high LI conditions were significantly higher (> 2) than under other LI treatments (< 2), and continued to linearly increase until 60 min. Furthermore, all NPQ values in 6-month-old seedlings were higher than in 2.4-year-old seedlings under all treatments, indicating that high LI limited 6-month-old seedling leaf growth and development, but that 2.4-year-old seedlings can be grown under a specific and optimal light intensity. Notably, the NPQ of 6-month-old seedlings under L-100 peaked (= 1.8) at 2 min, and then dropped remarkably (= 0.4) thereafter during photo-inhibitory processes (Fig. 1C).
Fig. 2 presents the time-course changes in the relative values of ΔF/F’m and Fv/Fm (Fv/Fm value of the same leaves before illumination as 100%) for 6-month and 2.4-year-old seedlings of Mahonia oiwakensisre under 50, 100, 300, 500, 1,000, 1,500, and 2,000 μmol m−2 s−1 PPFD at 25 °C during 30 min darkness right after stopping illumination. Both ΔF/F’m and Fv/Fm (%) of all seedlings in all LI treatments rapidly increased in the dark right after stopping illumination, followed by gradually increasing and then remaining stable after 10 min of dark. The relative values of ΔF/F’m and Fv/Fm (%) of 6-month-old seedlings in high LI treatments recovered to 57.8% ~ 65.1% after 30 min of dark, which was significantly lower photo-protection compared to other LI treatments (79.6% ~ 99.8%). However, both ΔF/F’m and Fv/Fm (%) of 2.4-year-old seedlings in high LI treatments recovered to 70.8% ~ 82.7% after 30 min of darkness, but were still significantly lower than other LI treatments (98.7.6% ~ 99.9%), indicating that these seedlings can be grown under high LI conditions.
Fig. 3 illustrates the fractions of NPQ, qZ+qT, qE, and qI obtained for 6-month and 2.4-year-old seedlings of M. oiwakensis at 50, 100, 300, 500, 1,000, 1,500, and 2,000 μmol m−2 s−1 PPFD and after 30 min of dark. All seedlings at all LI substantially increased their NPQ, and the increase was not related to photo-protection (qE and qZ+ qT) but to photo-inhibition (qI), and qI values increased as LI increased from 50 to 2000 μmol m−2 s−1 PPFD. The values of qZ+ qT of all seedlings did not contribute remarkably to NPQ in LI treatments. All seedlings had significantly higher NPQ levels (ranged 3~5) in LI-1,000, 1,500, and 2,000 compared to other LI treatments (< 2.2). Furthermore, 300, 500, 1,000, 1,500, and 2,000 μmol m−2 s−1 treatments of 6-month-old seedlingsproduced a higher overall effect on NPQ than they did in 2.4-year-old seedlings, and their qI levels were also substantially increased.
Fig. 4 shows correlations between photo-inhibition% to qE and qI values from 6-month-old and 2.4-year-old seedlings under 50, 100, 300, 500, 1,000, 1,500, and 2,000 μmol m−2 s−1 PPFD at 25 °C for 60 min. Significant and highly negative relationships were detected in photo-inhibition% and qE values under moderate and high LI conditions in 6-month-old and 2.4-year-old seedlings with r2 values of 0.971 (p < 0.01) and 0.978 (p < 0.05), respectively (Figures 5A and C), implying that 6-month-old seedlings were more sensitive to high LI. Nevertheless, significant and highly positive correlations were observed between photo-inhibition% and qI values of 6-month-old and 2.4-year-old seedlings, with r2 values of 0.96 (p < 0.001) and 0.974 (p < 0.0001), respectively (Figures 4B and D). The increased photo-inhibition% could have led to increases in qI value and decreases in qE value due to greater energy dissipation when plants were exposed to moderate and high LI conditions where photo-inhibition% in 6-month-old seedlings was higher than in 2.4-year-old seedlings.