Studies On The Physiological Effect And Mechanisms Of Arbuscular Mycorrhizal Fungi (AMF) In Dought Resistance Of Melon

Melon (Cucumis melo L.)-an important horticultural crop that is often cultivated in simply-equipped solar greenhouses in northwestern regions of China, usually suffers under poor water management. Arbuscular mycorrhizal (AM) symbiosis can play a great role in enhancing drought tolerance. In this thesis, bases on the selecting of arrbuscular mycorrhizal fungi (AMF) to drought in melon(Cucumis melo L. cv.'Zhongmi 3'), a series of experiments had been performed on physiological responses and molecular mechanisms of drought tolerance improved by arbuscular mycorrhizal fungi. The main results were as follows:1. The plant growth, physiological, and photosynthetic responses of melons inoculated with three Glomus species under two water conditions were investigated. Results show that inoculation with Glomus elevates the physiological and photosynthetic parameters of several seedlings compared with non-AM seedlings. Regardless of water conditions, plant height, root length, biomass production, antioxidant enzyme activity, soluble sugar content, net photosynthetic rate, and photosynthetic water use efficiency are elevated in AM seedlings compared to non-AM seedlings. Each Glomus species manifests unique effects under the two water conditions. We posit that arbuscular mycorrhizal symbiosis can protect melon plants against water deficiencies by improving their reactive oxygen activity, bi-directional transport, and photosynthetic capacity. In addition, regardless of water conditions, the most efficient fungus for melon (Cucumis melo L.) is Glomus mosseae.2. The root growth and nutrient acquisition of melons inoculated with Glomus mosseae under two water conditions were investigated. The results showed that: (1) inoculated with AMF can promote the melon seedling root growth, increased melon root active absorption area; (2) under well-watered conditions, inoculated with AMF can significantly improve melon leaves N, P, K and other elements of content. Under water stress conditions, this effect is highly efficient, which inoculated seedlings roots in the highest P content in comparison with non-AMF treatment increase 36.7%, significantly higher than other treatments. Melon seedlings inoculated with AMF increased N, P accumulation, increase of N, P absorption. Concentration of P content in the plant roots especially higher under water stress. The results showed that AMF inoculation significantly increased the host plant root absorption area, while the AM hyphae links formed a powerful highway further increase root growth and nutrient absorption, promote plant growth, improve the drought resistance of melon.3.The proline content, Ascorbat-glutahione cycle and endogenous hormones of melons inoculated with Glomus mosseae under two water conditions are investigated. The results showed that: (1) melon seedlings inoculated with AMF can adjust the Pro accumulation, increased APX and GR activities, to protect melon seedlings AsA-GSH cycle, to avoid injured from Reactive oxygen species; (2) under well-watered conditions, inoculated AMF can significantly improve the melon leaves and roots IAA, ZR, GA content, reducing the accumulation of ABA. Under water stress, except the ZR content of melon leaves decreased processing, melon seedlings IAA, ZR, GA and ABA content first increased and then decreased generally, which as IAA, ZR, GA content significantly higher in AM seedlings than in non-AM control, same as ABA accumulation was significantly lower. The results showed that, AM fungi can reduce injury on AsA-GSH cycle of melon caused by water stress, increased Pro accumulation and IAA, ZR, GA content, reduced ABA accumulation, to maintain antioxidant system activities and the balance of endogenous hormones relatively, to promote plant growth and improve the capacity of melon to drought.4. The light response, A/Ci curve analysis, daily variation of net photosynthetic rate (Pn), stomatal conductance(Gs), transpiration rate (Tr), intercellular CO2 concentration(Ci), carboxylation efficiency (CE) and assimilation at saturating CO2 (Amax.)were compared between plants inoculated with Glomus mosseae (AM plants)and those not infected (Non-AM plants) under different water treatments.The results showed that the Pn, Gs, Tr, CE of plants decreased significantly under drought stress. Inoculated AMF can significantly improve melon leaves Pn, Gs, Tr, CE and Amax. under water stress conditions; (2) The diurnal change of the Pn, Gs, Tr showed larger decline under the drought stress conditions. Drought stress could improve intercellular CO2 concentration (Ci), suggesting that although both Pn and Gs decreased under the stress condition, Gs was not a limiting factor for the decline of photosynthesis. The results show that AMF improved the Gs, CE and Amax of plants to compensate for the injury caused by drought.5. The mycorrhizal development, leaf plastochron index (PI), leaf relative water content (RWC), gas exchange characteristics, chlorophyll fluorescence parameters, and photosynthetic electron transport rate (ETR) of melons inoculated with Glomus mosseae under two water conditions were investigated. The results showed that: (1) the colonization of plants by arbuscular mycorrhizal fungi (AMF) was reduced under water stress. (2) under water stress, leaf relative water content (RWC), leaf growth index (PI), photosynthetic rate(Pn) and chlorophyll fluorescence decreased both in AM seedlings and non-AM control, but the down amplitude of AM seedlings was significantly smaller than non-AM control; only RWC, PI and Photosynthetic system of seedlings inoculated with AMF showed higher recovery capacity than non-AM control upon returning to normal conditions. Study shows that inoculation of AMF can improve the melon seedlings photosynthesis absorption and electron transport capacity under water stress, promote photosynthesis, mitigation damage from water stress on melon seedlings.6. Using designed primers based on the conserved amino acid sequences of known drought-related genes to amplify cDNA fragments from melon (Cucumis melo L.) by RT-PCR, a drought-related gene named MeP5CS was obtained. Bioinformatics analysis indicated that the full-length of cDNA sequence was 1000 bp, which contained an open reading frame of 753 bp and encoded a protein of 250 amino acid residues with a calculated molecular weight of 82.18 kD and isoelectric point of 4.90. The MeP5CS protein showed 94%, 82% and 73% similarity to the P5CS from Aegiceras corniculatum, Actinidia deliciosa and Vitis vinifera. The protein includeα-helix (40.2%),β-turn (25.2%), random coil(34.6%) and a cleavage site between nineteen and twenty amino acid residues. The protein is a hydrophobic protein and there is two transmembrane helix and thirteen Phosphorylation sites. The result of RT-PCR analysis indicated that MeP5CS expression levels were different in roots,stems and leaves, and it was highest in roots and middle in stems, lower in leaves . The expression of MeP5CS is induced by AMF under water stress, but is closely related to the processing time, increasing nitrogen fixation ability of host plants by AMF and signal transduction mechanisms in different tissues. The results show that the AMF can induce MeP5CS gene expression in melon under water stress, and enhance the drought resistance of melon; expression differences in tissues may be related to the duration of water stress.