Herbicidal effects of harmaline from Peganum harmala on photosynthesis of Chlorella pyrenoidosa: Probed by chlorophyll fluorescence and thermoluminescence

The herbicidal effects of harmaline extracted from Peganum harrnala seed on cell growth and photosynthesis of green algae Chlorella pyrenoidosa were investigated using chlorophyll a fluorescence and thermoluminescence techniques. Exposure to harmaline inhibited cell growth, pigments contents and oxygen evolution of C pyrenoidosa. Oxygen evolution was more sensitive to harmaline toxicity than cell growth or the whole photosystem II (PSII) activity, maybe it was the first target site of harmaline. The JIPtest parameters showed that harmaline inhibited the donor side of PSII. Harmaline decreased photochemical efficiency and electron transport flow of PSII but increased the energy dissipation. The charge recombination was also affected by harmaline. Amplitude of the fast phase decreased and the slow phase increased at the highest level of harmaline. Electron transfer from Q(A)(-) to Q(B) was inhibited and backward electron transport flow from Q(A)(-) to oxygen evolution complex was enhanced at 10 mu g mL(-1) harmaline. Exposure to 10 mu g mL(-1) harmaline caused appearance of C band in thermoluminescence. Exposure to 5 mu g mL(-1) harmaline inhibited the formation of proton gradient. The highest concentration of harmaline treatment inhibited S(3)Q(B)(-), charge recombination but promoted formation of Q(A)(-)Y(D)(+) charge pairs. P. harmala harmaline may be a promising herbicide because of its inhibition of cell growth, pigments synthesis, oxygen evolution and PSII activities. (C) 2014 Elsevier Inc. All rights reserved.