Abstract
Arbuscular mycorrhizal fungi (AMF) are known to improve the resistance of host plants against various heavy metal stresses. However, the arsenic (As) resistance mechanism of AMF-inoculated woody legumes remains unclear. In this study, black locust (Robinia pseudoacacia L.) seedlings were cultivated in potted soils inoculated with or without AMF Rhizoglomus intraradices under three different levels of As stress (0, 100, and 200 mg As kg–1 soil) over 4 months. The objective of this paper was to investigate the effects of AMF on plant growth, root morphology, and the content and ratio of endogenous phytohormones and soil glomalin under As stress condition. As stress toxicity suppressed the AM spore germination and colonization, plant growth, and the content of soil glomalin and changed the morphological characteristics of the roots and the balance of endogenous hormone levels in plants. However, R. intraradices inoculation improved the shoot and root dry weights, total root length, root surface area, root volume, and the number of root forks and tips across all As treatments. R. intraradices inoculation obviously decreased the percentage of root length in the 0- to 0.2-mm diameter class and increased those in the 0.5- to 1.0-mm and >1.0-mm diameter classes; the percentages in the 0.2- to 0.5-mm diameter class were less affected by R. intraradices inoculation. The concentrations of the easily extractable glomalin-related (EE-GRSP) and total glomalin-related soil protein (T-GRSP) were higher in the of R. intraradices-inoculated seedlings than those in the non-inoculated seedlings. Furthermore, R. intraradices inoculation increased the concentrations of indole-3-acetic acid (IAA) and abscisic acid (ABA), but decreased the concentrations of gibberellic acid (GA) and zeatin riboside (ZR). The phytohormone ratios of IAA/ABA, GA/ABA, ZR/ABA, and (IAA + GA + ZR)/IAA in the R. intraradices-inoculated seedlings were lower than those in the non-inoculated seedlings. These results indicated that R. intraradices alleviated As toxicity in R. pseudoacacia seedlings by improving their plant growth, altering root morphology, regulating the concentrations and ratios of phytohormones, and increasing the concentration of soil glomalin. The results suggested that AMF-inoculated R. pseudoacacia seedlings would be a critical factor in successful vegetation restoration and soil development in As-contaminated soils.
Highlights
Arsenic (As) is a poisonous metalloid and exists naturally in all types of the crust rocks, especially orpiment, realgar, and other ores (Li et al, 2018)
Culture time had a noticeable effect on the germination of R. intraradices spores (P < 0.01), and the germination rate was gradually enhanced with the increase of culture time at the same As levels (Figure 1A)
Our results showed that R. intraradices symbiosis with R. pseudoacacia seedlings coordinated the balance of phytohormones through adjusting the hormone ratios under As stress, as a result slowing down the plant growth and development, reducing plant respiration, maintaining normal water balance, and improving osmotic pressure in plant cells
Summary
Arsenic (As) is a poisonous metalloid and exists naturally in all types of the crust rocks, especially orpiment, realgar, and other ores (Li et al, 2018). As-based compounds are often reported to be widely released in groundwater and soils in Asia, Europe, and North and South America (Andrade et al, 2015; Kalita et al, 2018). They can enter the food chain and cause many pathologies, including carcinogenesis (Li et al, 2016; Kalita et al, 2018). The physiological reactions to As toxicity in plants include inhibited seed germination, reduced growth and yield, decreased chlorophyll content and photosynthesis rate, senescence, and even death, which lead to vegetation degradation in high As-contaminated areas (Srivastava et al, 2009; Spagnoletti and Lavado, 2015; Spagnoletti et al, 2017). Some remedial measures need to be taken to prevent even further environmental degradation
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