Studies on the Physiological Response of Hemerocallis middendorffii to Two Types of Drought Stresses

Drought is a major environmental factor limiting plant growth and development. <i>Hemerocallis middendorffii</i> is a perennial herbaceous plant with high drought resistance, and high ornamental and application values. Understanding the mechanism of drought stress resistance in <i>H. middendorffii</i> is helpful for better utilization of plant resources and selection of excellent germplasms. In this study, the phenological and physiological traits of <i>H. middendorffii</i> were comprehensively analyzed under natural drought stress (ND) and PEG-simulated drought stress (PD), and the resistance of <i>H. middendorffii</i> to different levels of drought stress was evaluated. ND was treated using a natural water loss method. PD was treated under drought stress by using PEG-6000. <i>H. middendorffii</i> were able to grow within 15 d of ND and 4 d of 20% PD. Beyond this drought time, <i>H. middendorffii</i> will wilt and lose their ornamental value. Further study showed that <i>H. middendorffii</i> protect themselves from damage and enhance drought resistance mainly by increasing the content of osmoregulatory substances, enhancing the activity of antioxidant enzymes, and inhibiting photosynthesis. Malondialdehyde (MDA) content accumulated rapidly at 15 d of ND and 7 d of PD. Antioxidant enzyme activities peaked at 15 d of ND and 4 d of PD. Photosynthetic parameters decreased at 15 d of ND and 4 d of 20% PD, respectively. Moreover, we identified that the <i>HmWRKY9</i> gene was up-regulated for expression in the leaves after ND and PD. <i>HmWRKY9</i> may be involved in regulating the response of <i>H. middendorffii</i> to drought stress.