Leaf Anatomy and Gas Exchange in Nearly Isogenic Semidwarf and Tall Winter Wheat

Although leaf area and photosynthetic cell size have been negatively correlated with leaf CO 2 exchange rate (CER) in many studies, the use of diverse genotypes has often confounded interpretation of this relationship. We found leaves of a semidwarf (SD) ( Rht1 Rht1 Rht2 Rht2 ) winter wheat ( Triticum aestivum L.) genotype were smaller compared to its nearly isogenic standard height (Tall) ( rht1 rht1 rht2 rht2 ) line, suggesting an interesting system in which to investigate this relationship. Samples from newly expanded leaves of growth chamber‐grown plants were fixed, then embedded in resin for anatomical study using stereological methods. Steady state gas exchange measurements at ambient CO 2 concentration were performed on equivalent leaves and, in a follow‐up experiment, at five levels of CO 2 for constructing intercellular CO 2 ( C i )response curves of CER. Tangential sections revealed that dwarfing genes reduced lamina cell elongation, resulting in smaller and more numerous cells than in the Tall line. A consequence of this response was that total mesophyll surface area/leaf area ( Ames/A ) was increased by 10% in SD compared to Tall leaves. Leaf CER and conductance to water vapor ( g ) were also higher in SD plants. Greater stomatal density in the SD line was associated with their higher g , but ( C i )i response curves of CER indicated that higher CER in the SD line was due to greater mesophyll photosynthetic capacity, not to higher g . We conclude that the greater photosynthetic capacity conferred by dwarfing genes results from their anatomical effect on Ames/A .