Autotoxicity effect of water extracts from rhizosphere soil of Elymus sibiricus in different planting years on seed germination, physiological characteristics and phytohormones of seedlings

Elymus sibiricus is a highly valuable perennial forage that is widely planted in the Qinghai-Tibet Plateau (QTP) region. However, E. sibiricus artificial grasslands have a short utilization lifespan, and reach the highest yield in the 2nd and 3rd year of plantation, then rapidly drop its productivity. We hypothesized that autotoxicity is one of the mechanisms for the reduction of the productivity. To test this hypothesis, we prepared the water extract from rhizosphere soils of E. sibiricus planted for 3, 4, 5, and 8 years and examined the effects of the extract concentrations at 0.05, 0.1, 0.2, and 0.5 g/mL on seed germination, seedling growth, physiological characteristics and phytohormones in the aboveground and roots of E. sibiricus. The results showed that the soil extract concentration, planting years, and their interaction had significant influences on the most of these indices. The soil extract inhibited the seed germination and growth of seedlings, and the inhibitory effects appeared to be stronger at the 0.5 g/mL rhizosphere soil extract for 5 and 8 years. The superoxide dismultase and peroxide activities, the free proline concentration, soluble sugar concentration were altered. The malondialdehyde concentration was, in general, increased, especially in 8 years soil extract. The indole acetic acid and gibberellic acids concentrations were lowered, while the abscisic acid concentration varied. These changes were depending on the extract concentration and the years of planting, without clear patterns in some of them in response to the extract concentration and planting years. In summary, autotoxicity can be a contributor to the retardation of the growth and development of artificial E. sibiricus grasslands. The inhibitory effects could be attribute to impaired antioxidant capacity and disturbance of osmortic-regulatory substances and plant hormones, and are more profound on the root than on the aboveground part of the seedlings.