PHOTOSYNTHETIC GAS EXCHANGE AND DISCRIMINATION AGAINST (CO2)-C-13 AND (COO)-O-18-O-16 IN TOBACCO PLANTS MODIFIED BY AN ANTISENSE CONSTRUCT TO HAVE LOW CHLOROPLASTIC CARBONIC ANHYDRASE

摘要

The physiological role of chloroplastic carbonic anhydrase (CA) was examined by antisense suppression of chloroplastic CA (on average 8% of wild type) in Nicotiana tabacum. Photosynthetic gas-exchange characteristics of low-CA and wild-type plants were measured concurrently with short-term, on-line stable isotope discrimination at varying vapor pressure deficit (VPD) and light intensity. Low-CA and wild-type plants were indistinguishable in the responses of assimilation, transpiration, stomatal conductance, and intercellular CO2 concentration to changing VPD or light intensity. At saturating light intensity, low-CA plants had lower discrimination against (CO2)-C-13 than wild-type plants by 1.2 to 1.8 parts per thousand. Consequently, tissue of the low-CA plants was higher in C-13 than the control plants. It was calculated that low-CA plants had chloroplast CO2 concentrations 13 to 22 mu mol mol(-1) lower than wild-type plants. Discrimination against (COO)-O-18-O-16 in low-CA plants was 20% of that of the wild type, confirming a role of chloroplastic CA in the mechanism of discrimination against (COO)-O-18-O-16 (Delta (COO)-O-18-O-16). As VPD increased, stomatal closure caused a reduction in chloroplastic CO2 concentration, and since VPD and chloroplastic CO2 concentration act in opposing directions on Delta (COO)-O-18-O-16, no effect of VPD was seen on Delta (COO)-O-18-O-16. [References: 22]