Changes in photosynthesis of alpine plant Saussurea superba during leaf expansion

Abstract

The native alpine plant Saussurea superba is widely distributed in Qinghai–Tibetan Plateau regions. The leaves of S. superba grow in whorled rosettes, and are horizontally oriented to maximize sunlight exposure. Experiments were conducted in an alpine Kobresia humilis meadow near Haibei Alpine Meadow Ecosystem Research Station (37°29′–37°45′N, 101°12′–101°33′E; alt. 3200 m). Leaf growth, photosynthetic pigments and chlorophyll fluorescence parameters were measured in expanding leaves of S. superba. The results indicate that leaf area increased progressively from inner younger leaves to outside fully expanded ones, and then slightly decreased in nearly senescent leaves, due to early unfavorable environmental conditions, deviating from the ordinary growth pattern. The specific leaf area decreased before leaves were fully expanded, and the leaf thickness was largest in mature leaves. There were no significant changes in the content of chlorophylls (Chl) and carotenoids (Car), but the ratios of Chl a/b and Car/Chl declined after full expansion of the leaves. The variation of Chl a/b coincided well with changes in photochemical quenching (qP) and the fraction of open PSII reaction centers (qL). The maximum quantum efficiency of PSII photochemistry after 5 min dark relaxation (F(v)/F(m)) continuously increased from younger leaves to fully mature leaves, suggesting that mature leaves could recover more quickly from photoinhibition than younger leaves. The light-harvesting capacity was relatively steady during leaf expansion, as indicated by the maximum quantum efficiency of open PSII centers (\(F_{\text{v}}^{{\prime }}\)/\(F_{\text{m}}^{{\prime }}\)). UV-absorbing compounds could effectively screen harmful solar radiation, and are a main protection way on the photosynthetic apparatus. The decline of qP and qL during maturation, together with limitation of quantum efficiency of PSII reaction centers (L(PFD)), shows a decrease of oxidation state of QA in PSII reaction centers under natural sunlight. Furthermore, light-induced (ΦNPQ) and non-light-induced quenching (ΦNO) were consistent with variation of L(PFD). It is concluded that the leaves of S. superba could be classified into four functional groups: young, fully expanded, mature, and senescent. Quick recovery from photoinhibition was correlated with protection by screening pigments, and high level of light energy trapping was correlated with preservation of photosynthetic pigments. Increasing of ΦNPQ and ΦNO during leaves maturation indicates that both thermal dissipation of excessive excitation energy in safety and potential threat to photosynthetic apparatus were strengthened due to the declination of qP and qL, and enhancement of L(PFD).