Combined effects of arbuscular mycorrhiza and drought stress on plant growth and mortality of forage sorghum

•Arbuscular mycorrhizal (AM) formation improved the performance of forage sorghum.•The AM alleviated drought-induced plant growth retardation and mortality.•Intensive drought stress hampered AM development and induced arbuscule collapse.•Mycelial networks most benefited plants that formed AM early under drought stress.

Drought stress affects the growth and mortality of forage sorghum cultivated in arid and semi-arid environments. Arbuscular mycorrhizal (AM) fungi can promote plant growth, protect host plants from various stresses, and mediate plant-plant interactions. In this study, a greenhouse experiment was conducted to investigate: (1) the function of an AM fungus (Funneliformis mosseae) in the alleviation of drought-induced growth retardation and mortality of forage sorghum plants, and (2) whether common mycorrhizal networks (CMNs) are more beneficial to plants that join CMNs earlier than to those that join later. AM formation largely promoted sorghum growth, and the biomass and the specific leaf area (SLA) of mycorrhizal plants were larger than those of non-mycorrhizal plants, regardless of whether they were grown under well-watered or drought stress conditions. Progressive drought stress suppressed plant growth and even induced plant mortality, while AM alleviated plant growth retardation and prolonged plant lifespan. Intriguingly, plants connected to the CMN and that formed an AM symbiosis earlier than other plants benefited more under intensive drought stress, with longer lifespans, more arbuscules and larger ratios of intact arbuscules. These findings may help to elucidate the seemingly random death of individual plants induced by drought stress in agricultural practices. The plants that formed AM earlier than other plants were not significantly superior in terms of biomass accumulation, SLA and AM colonization under well-watered conditions. Our results confirmed that the formation of an AM association could alleviate growth retardation and prolong the lifespan of plants under drought stress, demonstrating the prospect of utilizing AM fungi to increase the forage yield of sorghum plants cultivated under semi-arid and arid environments.