Gas-exchange analysis of chloroplastic fructose-1,6-bisphosphatase antisense potatoes at different air humidities and at elevated CO2

Gas-exchange measurements were performed to analyze the leaf conductances a nd assimilation rates of potato (Solanum tuberosum L. cv. Desiree) plants e xpressing an antisense construct against chloroplastic fructose-1,6-bisphos phatase (FBPase, EC 3.1.3.11) in response to increasing photon flux densiti es, different relative air humidities and elevated CO2 concentrations. Assi milation rates (A) and transpiration rates (E) were observed during a stepw ise increase of photon flux density. These experiments were carried out und er atmospheric conditions and in air containing 500 mu mol mol(-1) CO2. In both gas atmospheres, two levels of relative air humidity (60-70% and 70-80 %) were applied in different sets of measurements. Intercellular CO2 concen tration, leaf conductance, air-to-leaf vapour pressure deficit, and instant aneous water-use efficiency (A/E) were determined. As expected, assimilatio n rates of the FBPase antisense plants were significantly reduced as compar ed to the wild type. Saturation of assimilation rates in transgenic plants occurred at a photon flux density of 200 mu mol m(-2) s(-1), whereas satura tion in wild type plants was observed at 600 mu mol m(-2) s(-1). Elevated a mbient CO2 levels did not effect assimilation rates of transgenic plants. A t 70-80% relative humidity and atmospheric CO2 concentration the FBPase ant isense plants had significantly higher leaf conductances than wild-type pla nts while no difference. emerged at 60-70%. These differences in leaf condu ctance vanished at elevated levels of ambient CO2. Stomatal response to dif ferent relative air humidities was not affected by mesophyll photosynthetic activity. It is suggested that the regulation of stomatal opening upon cha nges in photon flux density is merely mediated by a signal transmitted from mesophyll cells, whereas the intercellular CO2 concentration plays a minor role in this kind of stomatal response. The results are discussed with res pect to stomatal control by environmental parameters and mesophyll photosyn thesis.