Nitric oxide production, sugar metabolism, and antioxidant capacity related to differential cold tolerance and recovery between two Dichondra repens genotypes

As an important herb, warm-season dichondra (Dichondra repens) is traditionally used to treat multiple diseases and also added into herbal tea beverage. However, chilling stress (CS) and freezing stress (FS) limit its cultivation and yield. The objectives of current study were to reveal differential cold tolerance and recovery capacity between cold-tolerant Dr5 and cold-sensitive Dr17 associated with changes in photosynthesis, enzymatic and non-enzymatic antioxidant systems, sugar metabolism, and nitric oxide (NO) production. Two genotypes were firstly subjected to CS (8/4°C) for 25 days and FS (2/-3°C) for 3 days and then recovered from cold stress (30/25°C) for 15 days. The results showed that CS and FS significantly reduced chlorophyll (Chl) content, the Chl a/b ratio, photochemical efficiency, and net photosynthetic rate (Pn), but Dr5 exhibited significantly higher Chl content, Fv/Fo, Fv/Fm, PIabs, Pn, and water use efficiency as well as lower Fo/Fm than Dr17 under CS and FS. Both Dr5 and Dr17 significantly up-regulated antioxidant enzyme activities (SOD, POD, CAT, APX, MR, GR, and DR) and the accumulation of non-enzymatic antioxidants (AsA and GSH) to mitigate cold-induced oxidative damage in response to CS and FS. Dr5 not only maintained higher these antioxidant enzyme activities and contents of AsA and GSH, but also had higher the AsA/DHA and GSH/GSSG ratios compared with Dr17 under CS and FS, indicating better cellular redox state in Dr5. In addition, Dr5 improved the accumulation of sucrose for osmotic adjustment and osmoprotectant, whereas Dr17 enhanced fructose and glucose metabolisms for energy supply and reactive oxygen species scavenging under CS and FS. Dr5 also showed higher NO content and nitrate reductase activity than Dr17 under cold and recovery conditions. These findings indicated that better cold tolerance and recovery capacity of Dr5 could be related to stable photosynthetic systems, antioxidant defense, sucrose biosynthesis, and NO production. Current study provides an in-depth understanding of cold tolerance and recovery capacity in warm-season crops and also establishes a theoretical foundation for developing cold-resistant dichondra cultivars.