| 5-aminolevulinic Acid (ALA) promoted yield and stress tolerance of plants, which was assoiated with increased photosynthetic efficiency. Photosynthesis is the basis of growth and organic substance and energy for all horticultural plants activities. But the physiological mechanism of ALA promotion is unclear. During protected culture of the horticultural plants, low light and chilling affect the normal growth. So the research of ALA provides academic gist for the horticultural plants and is of practical significance for protected culture in the future. In the paper, by CIRAS-1 photosynthesis system and FMS-2 portable pulse modulation fluoremeter, we study the effect of the foliar application with 5-aminolevulinic Acid (ALA) on photosynthesis, chlorophyll fluorescence and interrelated photosynthetic physiological. The main results are as follows:1 Effect of 5-aminolevulinic Acid on Photosynthesis of Strawberry Plants and its Chilling tolerance improving The characteristics of strawberry under shading and the effect of the foliar application with 5-aminolevulinic Acid (ALA) on photosynthesis, chlorophyll fluorescence, antioxidative enzymes activity and MDA content were studied. The results showed that Pn of strawberry leaves under shading was lower, although higher chlorophyll and carotenoid content, and greater leaf area. ALA treatment promoted the photosynthesis of strawberry leaves, the concentration of 100 mg·L-1 ALA was better than 300mg\L"'and 600mg·L-1. The leaves of the plants treated with 100 mg·L-1ALA solutions showed higher net photosynthetic rate (Pn), apparent quantum yield(AQY), carboxylation efficiency(CE), stomatal conductance(Gs),Chlorophyll b,Carotenoid and lower light compepensation point(LCP), CO2 compepensation point(CCP), which promote the normal growth of strawberry plants under low light. The analysis of chlorophyll fluorescence kinetics showed that ALA application not only depressed the minimal fluorescence(Fo) of the base leaves under shading, but also the maximal fluorescence(Fm) and the variable fluorescence(Fv) of the base and the middle leaves, however, it did not affect PS IImaximal photochemical effiency(Fv/Fm) but promoted the PS II actual photochemical efficiency in the light (ΦpSII) for the base leaves. Furthermore, 100 mg-L''ALA treatment promoted the activities of superoxide dismutase (SOD), peroxidase (POD). The data presented in the paper suggested that ALA treatment might promote low light tolerance and increased photosynthesis induced by ALA treatment in strawberry plants which might be associated with the Mehler reaction in the protective mechanism of photoinhibition. The analysis also showed that ALA treatment improved chilling tolerance of strawberry plants.2 Effects of 5-aminolevuIinic Acid on Photosynthesis of RadishSeedlings The foliar application with 100 300 mg·L-1 5-aminolevulinic Acid(ALA) promoted the growth and increased soluble sugars and cellulose content in rootof radish plants, which was assoiated with increased photosynthetic rate anddecreased dark respiration.Additionally, ALA treatment increased the chlorophyllcontent in the base leaves and the carotenoid content in the base and middle leaves ofradish plants. The analysis of chlorophyll fluorescence kinetics showed that ALAapplication depressed fluorescence quantum yield, including the minimal fluorescence(Fo), the maximal fluorescence (Fm) and the variable fluorescence(Fv) of leaves atdark, and the fluorescence in the stable(Fs) of leaves in light, however, it did not affectPS II maximal photochemical effiency(Fv/Fm) but promoted the actual photochemicalefficiency of PS II in the light (ΦpsII) for the middle and the base leaves.The acyclicelectron transfer rate (ETR) was increased by exogenous ALA treatment with thedecreased relative limit of photosynthesis (L(pfd))- Furthermore, ALA treatment led toboth the increase of photochemical fluorescence quenching and nonphotochemicalfluorescence quenching, where the former increased the leaf photochemistry rate, andthe latter was ass...
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