The effect of ozone and drought on the photosynthetic performance of canola

Canola plants were fumigated in open-top chambers with ozone (O₃) (120 ppb) under well-watered (WW) and water-stressed (WS) conditions for 4 weeks. Non-fumigated plants were also studied to facilitate comparison between treatments for the same week and over time. Therefore, the treatments were: WW, WW-O₃, WS and WS-O₃. The fast chlorophyll a fluorescence transients OJIP for the four treatments emitted upon illumination of dark-adapted leaves were measured after week 1, 2, 3, 4 and analysed by the JlP-test to evaluate the resulting changes in photosynthetic performance. Ozone fumigation led to a decline of total performance index (Pl_total) in well-watered plants. The effect of O₃ was minor under drought conditions, as revealed by a decrease of Pl_total by 3%. The Pl_total decreased as the treatment was prolonged, due to leaf ageing for all cases and the decline was more pronounced under WW-O₃. Taking the average of all weeks, WW had the highest Pl_total and the lowest WW-O₃ (decrease by 27%), while in WS and WS-O₃, it was lower than WW (14 and 17%, respectively). We found that the absorption (ABS)/reaction centre (RC) increases, while the maximum quantum yield of primary photochemistry (?_Po) undergoes slight changes, and trapping (TR₀)/RC closely followed the increase in ABS/RC. This indicates that O₃ and drought caused an increase in the functional antenna size. The maximum quantum yield of primary photochemistry showed slight differences for all treatments and over time, suggesting that this parameter is less sensitive to drought and O₃ stress. Therefore, the more sensitive components of the photosynthetic electron transport chain appeared to be the probability that an electron from the intersystem electron carriers is transferred to reduce end electron acceptors at the PSl acceptor side (δ_Ro) and the RC density on a chlorophyll basis (RC/ABS).