Effects Of Shading On Photosynthetic And Flowering Characteristics Of Osmanthus Fragrans

Osmanthus fragrans is a good garden plant species in recent years, It was more and more widely used as hedge plant in the southwest area due to lower plant type, pruning resistance, long flowering period, good flora color, beautiful flowers and leaves when it was planted in semi shade environment. Three-year-old Osmanthus fragrans cutting seeding were used as materials, pot experiment, the changes in photosynthetic physiological and bloom characteristics and the reason for the change of color were studied under four kinds of shadings [CK(full sunlight), C1(25% shading rate), C2(55% shading rate)and C3 (80% shading rate)] in Chengdu in summer. The results showed:(1) In shading treatment, Osmanthus fragrans branches grew thicker and leaves became greater, and specific leaf weight(SLW) decreased significantly; the content of chlorophyll increased and chlorophyll b (Chlb) content also increased significantly, but the ratio of chlorophyll a/b (Chla/b) reduced significantly, soluble sugar and soluble protein content decreased, it showed the most significant effect while 80% shading. When the sunlight was strong, Malondialdehyde (MDA) content in leaf reduced, catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) activity weakened with the increase of shading rate, MDA content decreased significantly under 55%、80% shading treatments.(2) In July, light saturation point (LSP) of Osmanthus fragrans was 1238.6μmol·m-2·s-1, light compensation point (LCP) was 38.1μmol·m-2·s-1, maximum net photosynthetic rate (Pnmax) was 8.33μmol·m-2·s-1, apparent quantum efficiency (AQE) was 0.027, dark respiration rate (Rd) was 1.02μmol·m-2·s-1 under the natural light. The LSP, LCP, Pnmax and Rd all decreased, but the AQE increased under shading treatment. Otherwise, instantaneous light energy utilization increased, net photosynthetic rate (Pn), stomatal conductance (Cond) and stomatal limitation value (Ls) decreased, inversely intercellular CO2 concentration (Ci) increased with the shading rate increase, according to this, it is inferred that non stomatal factors to cause Pn decreased.(3) The Pn, Tr, Cond and Ci curves of the trend under the shading are the consistent as full sunlight for Osmanthus fragrans, but before thel 1:00 and after 14:00, the first three parameters were decreased, while the Ci increased.12:00-13:00 in summer, Osmanthus fragrans midday depression of photosynthesis. The Pn showed two-peak curve obviously, the first peak was at about 11:00, the second peak appeared at about 15:00, the phenomenon was significantly improved by shading. Under 55% and 80% shading treatment, Pn curve showed a single peak curve, however the peak appeared at about 14:00 under 80% shading treatment, two hours later than the control. Noon hour (12:00-14:00) in hot summer, Osmanthus fragrans seeding should be taken intense shading measures treatment (80% shading rate) to ensure the normal growth of seedlings.(4)In moderate shading treatment(25%、55% shading rate), Osmanthus fragrans florescence advanced for 2-3 days and the amount of flowers increased significantly. Florescence delayed three days on average under heavy shade (80% shading rate), the amount of flowers reduced significantly. With increasing of shading rate, total flavonoids content of petal reduced, carotenoids content first increased and then decreased, color first became deep and then became shallow. It was inferred that the petals color was mainly affected by the carotenoid content. On the whole, Osmanthus fragrans was the most popular in the market with a longer florescence, more flowers and deeper color under 55% shading rate.(5) Osmanthus fragrans adapted to a wide range of light, both grew and bloomed normally from full light to 80% shading rate. When the natural light was strong, it reduced glare hurt by increasing the Pn, Tr, Cond and antioxidant enzyme; On the contrary, when the light was too weak, it improved the ability of weak light utilization by decreasing the SLW, LSP, LCP and Rd, increasing chlorophyll,Ci content and AQE, LUE. In summer, Setting up 55% shading rate, Osmanthus fragrans not only had a good foliage, but also the midday depression of photosynthesis was broken, which improved the ornamental of plants group due to early flowering, early blooming,more flowers, and enhanced color. In practical use, Osmanthus fragrans should be planted in half shade environment, such as under the sparse forest, in the edge of the forest or between tall building blocks, which will help to greening and beautify.