PHOTOSYNTHESIS DURING LEAF ONTOGENY IN FIELD-GROWN NICOTIANA-TABACUM-L LINES SELECTED BY SURVIVAL AT LOW CO2 CONCENTRATIONS

Leaf photosynthesis, stomatal conductance, internal CO2 concentration and leaf composition (photosynthetic pigments, total soluble protein a nd Rubisco content) during leaf ontogeny of field grown Nicotiana taba cum L. lines selected for survival at low atmospheric CO2 concentratio ns are described. Selection at low CO2 concentrations resulted in line s with higher total dry matter production than their parent cultivar, but this could not be related to improved photosynthesis which the sel ection method was designed to achieve (shown in previous work). In the present work we report higher rates of photosynthesis in the selected lines in mature and old leaves, not found in young leaves when the ca pacity for photosynthesis was maximum. Differences in the regulation o f the photosynthetic carbon reduction cycle as well as differences in the diffusive characteristics of the mesophyll, due to changes in the size of the cells, may be the cause for the higher rates of photosynth esis during leaf senescence in the selected lines.