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The atmospheric CO2 concentration and the frequency and severity of heat waves are increasing, and the nitrogen - key to plant development - supplied to crops leaches easily, polluting ground water. In this experiment, these three elements were combined, in order to unravel the physiological mechanisms involved in the plant response to this future scenario.

Sweet pepper seedlings were exposed to distinct nitrate inputs (nutrient solutions containing 12 mM, 5 mM and 0 mM of N) and plant growth-promoting rhizobacteria, a high CO2 concentration (1000 mu mol mol(-1)), and a heat shock (43 degrees C), in a controlled environment. Physiological markers - such as photosynthesis rate, stomatal conductance (gs), water use efficiency (WUE), chlorophylls, chlorophyll fluorescence, lipid peroxidation, anions, and free amino acids - were measured.

Exposure to the high temperature did not lead to any measurable stress in the sweet pepper plants. In fact, it augmented photosynthesis and the nitrate concentration, particularly at the elevated CO2 concentration. Heat shock did not suppose any detriment for the plants beyond the expected increases in gs and WUE. On the contrary, heat triggered many processes that ended up favoring photosynthesis; when combined with elevated CO2, the result was even more beneficial for the plants.

On the other hand, nitrogen starvation produced serious damage - such as decreases in the photosynthesis rate and chlorophylls and increases in lipid peroxidation and the levels of anions, cysteine, leucine, and phenylalanine in the plant - which was ameliorated to a great extent by the action of high CO2. Finally, the plant growth-promoting rhizobacteria hardly enhanced plant growth and development, giving results similar to those of the nitrogen withdrawal treatment.