The Effects Of Enhanced UV-B Radiation On Growth And Photosynthesis In Mung Bean Seedlings At The Initial Stage Of Growth

The aim of this work was to study the effects of enhanced UV-B radiation (0.35w m-2) on the growth of seedlings and photosynthesis in mung bean's first pair of true leaves at the initial stage of germination and the involving mechanisms were studied also. It was shown that fresh weight, dry weight, thickness of leaves, height and area of leaves increased during experiment period in both control and UV-B treated seedlings. UV-B radiation remarkable inhibited fresh weight, dry weight, height and area of leaves since 2nd day and the magnitude increased with treatment time. At the same time, the thickness of leaves in UV-B treatment was promoted.The solar light is the energy source of photosynthesis. The chlorophyll-protein complexes on the thylakoid membrane regulate and control the absorption, transportation, distribution and transformation of light energy. There were little report on the effect of enhanced UV-B radiation on the chlorophyll-protein complexes on the thylakoid membrane. Our results showed firstly that enhanced UV-B radiation decreased the contents of different chlorophyll-protein complexes to different extent. Chlorophyll-protein complexes associated with PSII decreased severer than those associated with PSL which suggested that PSII was more sensitive to enhanced UV-B radiation than PSI. LHCII, the main component of chlorophyll - protein complexes on the thylakoid membrane,was remarkably decreased by enhanced UV-B.The photosynthentic electron transfer chain is a bridge between the primary photophysical and photochemical reactions and common reactions. The electron transfer from to DCPIP could reflect the activity of PSII electron chain, which is sensitive to UV-B radiation. We found that enhanced UV-B led remarkable inhibition in it. The differential recovery of DCPIP reduction by the addition of various electron donors indicated that enhanced UV-B radiation probably acts at two sites in PSII: one close to PSII reaction centre and the other near the donor sites of HQ, and the inhibitive effect on the latter site was larger than that on the former one.Ribulose bisphosphate carboxylase/oxygenase or Rubisco is a key enzyme in theC3 pathway. It was indicated that enhanced UV-B radiation led inhibition both in the contents of Rubisco and carboxylation activity of Rubisco. What's more, enhanced UV-B radiation led smaller inhibition in Rubisco large subunits than small subunits.A0 is the photosynthentic rate when Ls equals to zero, which reflectes the photosynthentic activity of plants, so it is called photosynthentic ability. Apparent quantum yield (AQY) can partly reflect the photochemistry activity of PSII. It was showed that enhanced UV-B led remarkable inhibition in AO and AQY, and the magnitude increased with treatment time.The contents of chlorophyll-protein complexes, dPn/dCi, A0, AQY, PSII electrons transfer activity and the amounts of Rubisco can all reflect photosynthetic activities of mesophyll cell. The results that enhanced UV-B radiation led inhibition in Pn and the parameters above showed that the inhibition of photosynthetic activities of mesophyll cell could probably be one of reasons why Pn decreased. But our results also indicated Gs decreased at the same time. So it is necessary to apply the stomatal analysis in order to judge the main reason why Pn decreased under enhanced UV-B radiation. It was shown that the UV-B treatment limited net photosynthetic rate (Pn), stomatal conduction (Gs) and stomatal limitation (Ls), whereas it promoted intercellular CO2 concentration (Ci). According to the theory of Farquhar and Sharkey (1982), enhanced UV-B radiation reduced Pn mainly by inhibiting photosynthetic activities of mesophyll cell.What's more, We concluded that main reason of enhancement of Pn in control and UV-B treatment during first 4 days was the promotion of photosynthetic activities of mesophyll cell because of the slight decreased Ci and the enhancements of chlorophyll a ,chlorophyll b ,carotenoid and chlorophyll-protein complexes, dPn/dCi, A0, AQY, PSII electron transfer...