Effect of O3 fumigation on photosynthesis and growth of quinoa and its interaction with drought and elevated CO2
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South Africa is a water scarce country frequently experiencing drought and has an extremely energy intensive economy contributing to the escalating levels of air pollutants which are threatening food production and agriculture. The effects of water stress and elevated levels of O3 and CO2 were investigated on Chenopodium quinoa Willd. The physiological response of quinoa was evaluated by subjecting quinoa to severe drought (10% field capacity), moderate drought (20% field capacity), watered (75% field capacity) and well-watered (90% field capacity) conditions in the ambient environment. In a separate set of experiments well-watered and drought-induced quinoa plants were fumigated with 80 ppb O3, 120 ppb O3, 700 ppm CO2 and 700 ppm CO2 + 80 ppb O3 in open-top chambers. Ozone exposure-thresholds for damage, elevated CO2 and water stress effects were assessed by prompt chlorophyll a fluorescence induction kinetics. Chlorophyll a fluorescence data for both drought-induced and ozone treated plants exhibited a marked decrease in photochemical efficiency, active photosystem II (PSII) reaction centres per leaf cross section and increase in non-photochemical dissipation. Drought-induced plants had higher fluorescence intensity at the J-phase compared to the well-watered plants, indicating a decrease in the electron transport further than reduced plastoquinone (QA-). The exhibition of a positive ?VK-band and ?VL-band by the plants under severe and moderate drought stress indicate that PSII was susceptible to drought stress. The functional antenna size of absorption (ABS/RC) and heat dissipation (DIo/RC) was increased and energetic grouping of PSII units was decreased as drought stress progressed. Severe drought stress also decreased the amount of active PSII reaction centres (RC) per excited cross section (RC/CS). Exposure to elevated levels of CO2 resulted in a significant increase in PItotal of the well-watered plants compared to drought-induced plants, which is an indication of the potential increase in photosynthesis. Drought-induced plants exposed to elevated CO2, 80 ppb O3 and CO2 + O3 had a higher energetic grouping and stability of PSII compared to well-watered plants from 14 to 21 days fumigation. The exhibition of a positive ?VL-band from the start in plants treated with 120 ppb O3 indicates a strong concentration-dependent O3-induced inhibition and decrease in parameters like reduction of end electron acceptors per reaction centre (RE/RC) and photosynthetic performance index (PIABS and PItotal). O3 fumigation increased the stomatal conductance (gH2O) under well-watered and drought-induced conditions indicating that the stomata were possibly severely damaged by O3. Exposure to severe drought stress and 120 ppb O3 resulted in delayed flowering date, abortion of flowers and potential decrease in photosynthesis, biomass accumulation, total leaf area, plant height, and grain yield. Elevated CO2 potentially increase photosynthesis and ameliorated the negative effects of O3 in all variables.