The Relationship Between CO₂ Transfer Conductance and Leaf Anatomy in Transgenic Tobacco With a Reduced Content of Rubisco

Abstract

The CO₂ transfer conductance in leaves quantifies the ease with which CO₂ can diffuse from sub-stomatal cavities to sites of carboxylation within the chloroplast. The aim of this work was to test the hypothesis that the CO₂ transfer conductance is proportional to the surface area of chloroplasts exposed to intercellular airspaces. We compared two genotypes, wild-type and transgenic tobacco, that had been transformed with an antisense gene directed at the mRNA of the Rubisco small subunit. Transgenic tobacco had lower rates of CO₂ assimilation than wild-type but similar chlorophyll contents. Leaf anatomy was altered by growing plants in two different environments: a high daily irradiance in a growth cabinet (12 h photoperiod of 1 mmol quanta m^-2 s^-1) and a sunlit glasshouse. The growth cabinet gave at least twice the daily irradiance compared to the glasshouse. The CO₂ transfer conductance was calculated from combined measurements of gas exchange and carbon isotope discrimination measured in 2% oxygen.

Following gas exchange measurement, leaves were sampled for biochemical and anatomical measure- ment. In transgenic tobacco plants, Rubisco content was 35% of that found in the wild-type tobacco, the CO₂ assimilation rate was 50% of the wild-type rate and the chlorophyll content was unaltered. While leaf mass per unit leaf area of transgenic tobacco was 82% of that of the wild-type, differences in leaf thickness and surface area of mesophyll cells exposed to intercellular airspace per unit leaf area (S_mes) were small (92 and 87% of wild-type, respectively). Leaves grown in the growth cabinet under high daily irradiance were thicker (63%), had a greater S_mes (41%) due to the development of thicker palisade tissue, had higher photosynthetic capacity (27%) and contained more chlorophyll (58%) and Rubisco (77%), than leaves from plants grown in the glasshouse.

Irrespective of genotype or growth environment, CO₂ transfer conductance varied in proportion to surface area of chloroplasts exposed to intercellular airspaces. While the method for calculating CO₂ transfer conductance could not distinguish between limitations due to the gas or liquid phases, there was no reduction in CO₂ transfer conductance associated with more closely packed cells, thicker leaves, nor with increasing chloroplast thickness in tobacco.