Abstract
Drought stress has become an increasingly serious environmental issue that influences the growth and production of watermelon. Previous studies found that arbuscular mycorrhizal (AM) colonization improved the fruit yield and water use efficiency (WUE) of watermelon grown under water stress; however, the exact mechanisms remain unknown. In this study, the effects of Glomus versiforme symbiosis on the growth, physio-biochemical attributes, and stress-responsive gene expressions of watermelon seedlings grown under well-watered and drought conditions were investigated. The results showed that AM colonization did not significantly influence the shoot growth of watermelon seedlings under well-watered conditions but did promote root development irrespective of water treatment. Drought stress decreased the leaf relative water content and chlorophyll concentration, but to a lesser extent in the AM plants. Compared with the non-mycorrhizal seedlings, mycorrhizal plants had higher non-photochemical quenching values, which reduced the chloroplast ultrastructural damage in the mesophyll cells and thus maintained higher photosynthetic efficiency. Moreover, AM inoculation led to significant enhancements in the enzyme activities and gene expressions of superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, and monodehydroascorbate reductase in watermelon leaves upon drought imposition. Consequently, AM plants exhibited lower accumulation of MDA, H2O2 and compared with non-mycorrhizal plants. Under drought stress, the soluble sugar and proline contents were significantly increased, and further enhancements were observed by pre-treating the drought-stressed plants with AM. Taken together, our findings indicate that mycorrhizal colonization enhances watermelon drought tolerance through a stronger root system, greater protection of photosynthetic apparatus, a more efficient antioxidant system and improved osmoregulation. This study contributes to advances in the knowledge of AM-induced drought tolerance.
Highlights
Drought is well known as a significant environmental problem that restricts plant growth and crop yield worldwide
No arbuscular mycorrhizal (AM) root colonization was detected in the non-inoculated watermelon plants, while good symbiosis was established in the inoculated plants, with a colonization rate above 75% under both WW and DS conditions (Figures 1A,B; Table 1)
A decrease in the chlorophyll content may reflect pigment destruction in the chloroplast; we examined the chloroplast ultrastructure of the watermelon leaves to determine the impairment of the photosynthetic system
Summary
Drought is well known as a significant environmental problem that restricts plant growth and crop yield worldwide. Symbiosis with AM fungi can positively enhance plant nutrient acquisition (Cartmill et al, 2012), promoting plant growth and favoring survival under both biotic and abiotic stress conditions without harming the environment (Tian et al, 2013; Song et al, 2015) Such an eco-friendly and effective biological technique to enhance plant resistance to adverse environmental conditions, drought stress, has received increasing attention from crop scientists in recent years (Maya and Matsubara, 2013). The precise mechanisms involved are still under debate and depend on the plant species involved (Zhang et al, 2015)
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