Drought Stress Alters Gas Exchange, Chlorophyll Fluorescence, and Antioxidant Enzyme Activities in Glycyrrhiza uralensis in the Hexi Corridor, China

Prolonged drought can negatively impact a variety of plant physiological processes. We conducted a drought experiment on one-year-old plants of Glycyrrhiza uralensis to characterize how gas exchange, chlorophyll fluorescence, the activities of antioxidant enzymes, and growth responded to droughts of varying magnitudes. Plants grown under control (no drought stress) and three droughted conditions, differing in their duration (7, 14 and 21 days). Net photosynthetic rate ( P n ), transpiration rate ( E ), and stomatal conductance ( g s ) were significantly lower with increasing water stress. The moderate (14 days) and severe (21 days) water stress treatments resulted in lower P n , as well as stomatal limitation of photosynthesis during the morning but nonstomatal limitation during the afternoon. Meanwhile, the increase in F 0 and decrease in F m , Φ PSII , and F v / F m further suggested that photosynthetic capacity of G. uralensis leaves was reduced and PSII partially inactivated during the day under drought. These patterns indicated that drought damaged PSII of G. uralensis leaves. However, part of the damage to PSII might be alleviated under mild or moderate drought, but it was not reparable under severe drought. Water stress decreased the P n -PAR curves of G. uralensis and led to higher LCP and R D and lower P max and LSP . Together, these results suggest that drought stress weakened the light adaptability of leaves and reduced the efficiency of light transformation. In addition, the activities of antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)) were elevated in G. uralensis under moderate (14 days) and severe (21 days) water stress during first 14 days but were significantly reduced after that time. These results suggest that protective enzymes were activated early to resist the damage caused by drought stress but that prolonged stress caused accumulation of reactive oxygen species that exceeded the capacity of the antioxidant system, resulting in cellular damage. Water stress also reduced plant growth (lower plant height, basal diameter and crown area) and yield (smaller roots, stems, leaves, and total biomass). Moreover, water stress led to yellowing, wilting, and senescence of leaves and suppressed growth of main roots. We found that the drought treatment lasting 21 days is the moisture threshold of G. uralensis , and that longer droughts in the Hexi Corridor can cause G. uralensis to wither and die.