Study On The Mechanism Of Promotion By 5-Aminolevulinic Acid (ALA) On Photosynthesis And Photoinhibition Of Watermelon Leaves Under Stresses

Photosynthesis is a very important biochemical reaction in world which is the process of green plant and algae using light energy to biosynthesize organic compounds by cheap carbon dioxide and water evolving oxygen in large scale.5-Aminolevulin acid (ALA) is the essential precursor of porphyrine compounds biosynthesis such as chlorophyll, protoheme and so on. ALA treatment could improve photosynthetic efficiency of plant leaves. We studied effects of ALA on photosynthesis of watermelon seedlings in greenhouse under shade, low temperature, high light intensity and heat conditions in order to elucidate the mechanism of ALA regulating on photosynthesis and photoinhibition of plant under stress condition and study the physiological mechanism of promotion effect by ALA on photosynthesis efficiency. The main results as follows,1. The photosynthetic rate and maximum photochemical efficiency of watermelon leaves decreased greatly grown under shade condition. ALA treatment could improve the net photosynthetic rate (Pn) of watermelon leaves under shade condition. The results of measuring the chlorophyll fluorescence showed that the photochemical efficiency of watermelon leaves under shade condition decreased greatly, and ALA treatment improved the PS II actual photochemical efficiency (ΦPSⅡ), the electron transport rate (ETR), the photochemical quenching (qP) and the percentage of light intensity distributed in photochemistry of PS II reaction center (Pc) of watermelon leaves under shade condition. The primary photochemical reaction of watermelon leaves measured by a Plant Efficiency Analyzer (PEA) showed that shade decreased performance index on an absorption basis (PⅠABS) quantum yield of electron transport (φEo), possibility of a trapped exciton moves an electron into the electron transport chain beyond QA- (Ψo) and so on, while ALA treatment tended to improve PⅠABS,φEo andΨo, but decreased the approximate initial slope of the fluorescence transient (Mo), suggesting that ALA treatment might alleviate the degree of photoinhibition of watermelon seedlings leaves by low light stress.2. Measurements of the fast chlorophyll fluorescence transient showed that 4℃chilling stress decreased J, I, P phases of watermelon seedlings gradually; however, ALA treatment preserved higher fluorescence intensities of watermelon under chilling stress. Chilling stress decreased the performance index on a basis of absorption (PⅠABS), the performance due to trapping probability (PTR) and the performance due to electron transport probability (PET) greatly with the time going on, while ALA-pretreated watermelon leaves had higher PⅠABS, PTR and PET. Under 4℃low temperature stress for 96 h, the watermelon leaves with ALA treatment had higher maximal photochemical efficiency (φpo), quantum yield of electron transport (cpEo) and possibility of a trapped exciton moves an electron into the electron transport chain beyond QA- of photosynstemⅡ(Ψo), while the fluorescence parameters related to closure of reaction centers Mo and Vj were lower than that of the controls. ALA treatment improves the amount of active PSⅡreaction centers per excited cross section (RC/CS) of watermelon leaves under chilling stress. Furthermore, Fk-j,Fj-I and Fi-p, which are performance index related to the receiving side of photosynstemⅡelectron transport, were increased by ALA treatment, suggesting that promotion effect of ALA on photosynthesis of watermelon leaves under chilling stress was related with the increased ability of receiving side of PSⅡfor electron transport.3. The photosynthesis of watermelon leaves shows a "middy rest" phenomenon. The diurnal variation of fast chlorophyll fluorescence transient showed that,φEo,Ψo, ETo/CS and ETo/RC which correlated to photochemical efficiency of watermelon leaves decreased greatly under middy high light intensity. The results measured by a modulated chlorophyll fluoremeter PAM-2100 showed that, under high light intensity conditions, the actual photochemical efficiency (ΦPSⅡ), photochemical quenching (qP) and energy distribution in photochemistry of photosystemⅡreaction center (Pc) of watermelon leaves decreased greatly, while ALA pretreatment improvedΦPSⅡ, qP and Pc suggesting that ALA treatment could improve photochemical efficiency of watermelon seedling leaves under high light condition.4. Measurement of the fast chlorophyll transient of watermelon seedling leaves under heat stress showed that the J, I, P phases decreased greatly with the temperatures increasing from 30℃to 49℃. Under 46℃high temperature or above for 0.5 h the photochemical efficiency of watermelon leaves significantly damaged. ALA pretreatment protected the photosynthetic apparatus of watermelon leaves under heat stress. Under 49℃heat stress the watermelon leaves with ALA treatment had higher fluorescence parameters such as PⅠABS, PTR, PET,φEo andΨo.At the same time Wk and Fo were lower, suggesting that ALA treatment protected the activity of OEC and thylakoid membranes of photosystemⅡunder heat stress.5. The inhibitors experiment showed the protection effect of ALA on photosynthesis and photoinhibition under stress conditions in watermelon seedlings leaves might be related with ALA-induced anti-oixdative enzyme activity, but not with xanthophyll cycle and so on. Dimethyldithiocarbamate (DDC), an inhibitor of superoxide dismutase, significantly inhibited photochemical efficiency of watermelon under stresses, including low light, chilling, high light and heat stress, while ALA treatment could partly reverse the inhibition of DDC. Measurement of anti-oxidant enzyme activities showed that ALA could induce increase of SOD, POD and APX activities. Thus, it can be deduced that exogenous ALA can improve the activities of anti-oxidant enzymes, which are located near PSⅠreaction center, which can promote H2O-H2O cycle with more heat dissipation and higher electron transfer rate, and alleviate photoinhibition induced by stresses. Therefore, ALA treatment can protect leaf photosynthesis of watermelon seedlings grown under stresses such as chilling, low light, heat and high light.