Early acclimation changes in the photosynthetic apparatus of bean plants during short-term exposure to elevated CO2 concentration under high temperature and light intensity

To follow the early response of the photosynthetic apparatus to elevated [CO2] applied in combination with other stress factors we exposed 19-day-old bean plants (Phaseolus vulgaris L. cv. Cheren Starozagorski) for relatively short period (4 days, 8 h per day) to different temperature, light and [CO2] regimes: 23 or 39 °C air temperature, low (100 μmol m−2 s−1 - LL) or high (1000 μmol m−2 s−1 - HL) photosynthetic photon flux density (PPFD) and ambient (350 μmol mol−1) or elevated (1300 μmol mol−1) CO2 concentration. Chlorophyll fluorescence kinetics and the response of assimilation rate to intercellular CO2 concentration were registered during the 4-day period. At growth temperature (23 °C), we observed some stimulation of photosynthesis by elevated [CO2], expressed by higher maximal net photosynthetic rate, carboxylation capacity and electron transport capacity, while at high temperature (39 °C) the effect of elevated [CO2] on photosynthesis was negative, i.e. HT induced downward acclimation. The negative effect of the CO2 enrichment at HT, expressed as diminished Rubisco activity and limitations in RuBP and P regeneration, may be due to starch accumulation as it was seen from the electron micrographs. Our results show that the ratio of intercellular to ambient [CO2], which reflects the coupling between stomata and photosynthetic CO2 assimilation, was unaffected by elevated [CO2] as well as by changes in light and temperature. In the 4-day period, the PSII efficiency was not affected by the elevated [CO2]. The fluorescence parameters were noticeably reduced at HL already on the first day and decreased further with time. We concluded that 4-day growth of plants in atmosphere with CO2 concentration more than three times above the natural modifies the response of plants to light and temperature, which was connected with functional and structural changes.