| Banana is one of the most important economic crops in tropical and subtropical regions worldwide, and is also an important fruit tree in Southern China. Banana needs large amounts of water during its growth process,because of its large,fast-growing and strong-transpirating leaves. At the same time, it is difficult for banana to absorb water from deep soil due to its shallow root system. Therefore, drought stress affects the yield and quality of banana production severely.5-aminolevulinic acid(ALA) is the essential precursor of porphyrin compounds in all living bodies. It is considered to be a novel non-toxic growth regulating substance with low concentration, and is closely related to the resistance of plants. However, Up to now, little study has been made on ALA on improving plant drought resistance, and the study on physiological effect and regulatory mechanism of ALA to drought resistance in banana has not been carried out. The experiment was conducted with the seedlings of ’Baxi’banana, which is one of main cultivars in Southern China. After being sprayed with ALA on the leaves for several days, banana seedling roots were grown in Hogland nutrient solution with 10% PEG-6000 to study the changes of photosynthetic physiology, water physiology, cell membrane permeability, activity of antioxidant enzymes, osmoregulation substance and structure dynamic in banana seedlings under drought stress. The main results indicated as follows:1. ALA could effectively enhance the photosynthesis of banana seedlings under drought stress. At 0 d to 5 d under drought stress, chlorophyll content, net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), stomatal limitation (Ls), intercellar CO2 concentration (Ci) and water use efficiency (WUE) showed a trend of gradual decline; The pretreatment with different concentrations of exogenous ALA improved the chlorophyll content, Pn, Tr, Gs, Ci and Ls in leaves of banana seedlings under drought stress, enhanced the drought resistance of banana seedlings; The treatment of 10 mg/L ALA showed the best effect, all indexes were significantly higher than single drought stress control on the fifth day.2. ALA could effectively increase the efficiency of the photochemical reaction of banana leaves under drought stress. Through analyzing the chlorophyll fluorescence parameters of different stress periods, the results showed that 1-100 mg/L ALA treatments could efficiently alleviate the decrease of photochemical maximum efficiency of PSII(Fv/Fm), photochemical efficiency of PS Ⅱ(Fv’/Fm’), actual photochemical efficiency of PS Ⅱ(φ<DPS Ⅱ), photochemical quenching coefficient(Qp), the electron transport rate (ETR), photochemistry rate(PCR) and the ration of absorbed light in photochemistry(P)as well as the increase of minimal fluorescence(Fo), nonphotochemical quenching coefficient(NPQ), the ration of thermal dissipation(D)and excess energy(Ex)in banana seedlings in duced by drought stress.3. ALA could effectively maintain the stability of the plasma membranes of banana leaves under drought stress. With the extension of the stress time, SOD and POD activity in leaves and roots of banana seedlings first increased and then decreased, while the MDA content and membrane permeability both increased significantly. The pretreatment with 10 mg/L ALA improved SOD and POD activity in leaves and roots of banana seedlings under drought stress, decreased the membrane permeability and the MDA content, enhancing the antioxidation ability of banana seedlings, alleviating the membrane injury degree of roots and leaves in banana seedlings under drought stress.4. ALA could effectively maintain the water balance of banana leaves under drought stress. The relative water content of banana leaves and root activity significantly decreased under single drought stress, but the proline, soluble sugar and soluble protein content in leaves and roots significantly increased; The pretreatment with 10 mg/L ALA improved the relative water content in leaves and root activity, increased the content of proline, soluble sugar and soluble protein. These showd that ALA could reduce water loss of banana leaves, while improving the osmotic adjustment ability of banana seedlings under drought stress.5. ALA could promote the production of new adversity proteins in banana leaves under drought stress.176,114,106.82,55.51,47,43,40.35 and 27 kilodaltons proteins were expressed in banana seedling leaves whether it had been under stress, while 100 and 57 kilodaltons proteins were induced for single drought stress treatment on the third day, but 57 kilodaltons disappeared in the next time while 100 kilodaltons remained higher activity. The pretreatment with 10 mg/L ALA could not only increase the number of new adversity proteins(such as 53,57,100 and 29 kilodaltons), but also advance the time of the induced proteins’ process, such as 57 and 100 kilodaltons protein sharply increased on the first day. The generation of these specific proteins might enhance the drought resistance ability and the adaptability of banana seedlings.6. ALA could effectively maintain the structural integrity of banana leaves under drought stress. Drought stress could significantly decreased stomatal conductance and increased the stomatal density of the banana leaves. At the same time, it could cause leaf thinning, the obvious deformation of upper and lower epidermis cells, loose and shorter palisade tissue, larger pore of sponge tissue, and the aerenchyma was long and narrow oval. Compared with single drought stress control, the pretreatment with 10 mg/L ALA reduced the stomatal density of banana leaves effectively, and increased stomatal length and width of stomata; at the same time the ALA treatment had thicker stratum corneum, intact upper and lower epidermis cells, neat rows of palisade tissue and little pore of sponge tissue. Leaf thickness and palisade tissue thickness were higher than single drought stress control, while spongy tissue thickness had the opposite result. CTR and ration of palisade/spongy tissue were thicker than single drought stress control and close to conventional cultured control. Positively correlation were detected between palisade tissue thickness and leaf thickness; ration of palisade/spongy tissue, which was negative correlated with SR; Significantly negative correlations were detected between SR and CTR, ration of palisade/spongy tissue, leaf tissue structure showed a significant correlation with the opening of stomata. |
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