Quantification of OJIP Fluorescence Transient in Tomato Plants Under Acute Ozone Stress

Abstract

Ozone is one of the environmental stresses that limit crop photosynthesis. To determine structural and functional alterations of photosynthetic machinery, tomato plants aged 30 d were exposed to ozone (500 µg.m-3) for 4 hr. The plants were grown in a net house at the Department of Horticulture, Kasetsart University, Thailand from May to June 2012. Crop responses to ozone were evaluated by a chlorophyll a fluorescence OJIP test using a FluorPen FP 100 fluorometer, where minimum fluorescence was measured at 50 µs when all PSII reaction centers are open and is defined as the O step, followed by the J step (at 2 ms), the I step (at 60 ms) and at maximum fluorescence (FM) when all PSII reaction centers are closed, known as the P step. Measurements were done three times (before ozone exposure, 20 min and 20 hr after ozone treatment). Results indicated that the shape of the OJIP fluorescence transient alters under ozone stress. FM significantly reduced after ozone exposure while other fluorescence levels (from O to J) did not change significantly. The percentage of relative variable fluorescence and the net rate of photosystem II closure following ozone exposure were greater in ozone-treated plants. However, specific energy fluxes per reaction center and the performance index were significantly decreased under ozone stress. Reduction of the maximum quantum yield of primary photochemistry (the maximal variable fluorescence divided by the maximal fluorescence intensity) and efficiency following ozone exposure depicted that ozone impaired photosynthetic systems by deactivating reaction centers. It can be concluded that the OJIP test can easily detect photosynthetic activity induced by ozone.