Photosynthetic responses of canola and wheat to elevated levels of CO₂ and O₃ in open-top chambers

PhD (Environmental Sciences), North-West University, Potchefstroom Campus

The concentrations of carbon dioxide (CO2) and ozone (O3) are increasing in the atmosphere. The effects of elevated CO2 (700 ppm), O3 (80 and 120 ppb) and the combination of these two gases on the photosynthetic performance of canola and wheat plants were studied under well-watered (WW) and water-stressed (WS) conditions in open-top chambers (OTCs). The plants were fumigated in OTCs for four weeks. The fast chlorophyll (Chl) a fluorescence transients, stomatal conductance and chlorophyll content index (CCI) were measured between Week 1 and 4 in fumigated plants as well as in control plants. Biomass measurements were done only after four weeks with and without fumigation. Analysis of the fluorescence transients by the JIP-test led to the calculation of several photosynthetic parameters and the total Performance Index (PItotal). Elevated CO2 resulted in a reduction of the PItotal in canola and wheat plants under well-watered conditions. In the absence of any other treatment, water stress caused a decrease of the PItotal, while it was partly eliminated by fumigation with elevated CO2 and the combination of elevated CO2 and O3. This indicates that elevated CO2 reduces the drought effect both in the absence and presence of O3. The effect of O3 was minor under water-stressed conditions in both crops. The absorption (ABS)/reaction centre (RC) increased as a result of elevated O3 levels, while the maximum quantum yield of primary photochemistry (φPo) underwent slight changes and trapping (TR0)/RC closely followed the increase in ABS/RC. This indicates that the changes of ABS/RC are changes of functional antenna size, meaning that the functional antenna size was affected by O3 and drought. The observed decline of the PItotal under the 80 ppb O3 treatment was due to a lower density of reaction centres (RC/ABS). The decline under the 120 ppb O3 was found to be due both to a further decline of RC/ABS and a pronounced lowering of the efficiency with which an electron can move from the reduced intersystem electron acceptors to the PSI end acceptors (δRo). The φPo indicated slight differences for all treatments, suggesting that this parameter is less sensitive to environmental stress. Elevated O3 levels resulted in a reduction of biomass in both crops. The reduction in biomass corresponded with the lowering of CCI and the photosynthetic efficiency parameters. These suggest that two simple, non-invasive and rapid methods, namely, the analysis of OJIP fluorescence transients and the measurement of CCI, can be used to screen the effect of elevated O3 levels on biomass. It can be concluded that 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 PSI acceptor side and the RC density on a chlorophyll basis.

Doctoral