For the use of LED as a light source to regulate the photosynthesis of chrysanthemum leaves under greenhouse conditions, the eff ects of diff erent red (660 ± 30 nm) to far-red (730 ± 30 nm) radiation ratios (R:FR) on the photosynthetic characteristics and chlorophyll fl uorescence parameters of chrysanthemum leaves were studied. Red and far-red LED light sources were combined in diff erent proportions to produce four R:FR ratio treatments: 0.5, 2.5, 4.5 and 6.5. Th e ch lorophyll a content, SPAD value, net photosynthetic rate, light-saturated maximum photosynthetic rate, CO 2 -saturated carboxylation rate, apparent quantum effi ciency and carboxylation effi ciency w ere all the highest under the R:FR ratio of 2.5, followed by the R:FR ratio of 4.5. Potential photochemical effi ciency of photosystem II, photochemical quenching and electron transport rate for the R:FR ratios of 2.5 and 4.5 were markedly higher than those for 0.5 and 6.5, however, those parameters did not diff er signifi cantly between th e R:FR ratios of 2.5 and 4.5.
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