Comparison of Canopy and Flag Leaf Net Carbon Dioxide Exchange of 1920 and 1977 New York Winter Wheats 1

An increase in net carbon dioxide exchange (NCE) of the canopy or of flag leaves may have contributed to the historical trend toward increased yields of New York winter wheats ( Triticum aestivum L.). This hypothesis was tested by growing ‘Honor’, a tall, awnless cultivar released in 1920, and ‘Houser’, a semi‐dwarf, awned cultivar released in 1977, in a replicated field study as vernalized seedlings in 1982 and in normal biennial culture in 1983. Throughout development and simultaneously for both cultivars, small areas of canopy were placed within clear‐walled chambers and canopy NCE was measured in an open system for 24‐h periods. At anthesis and for several weeks thereafter NCE of individual flag leaves was determined by a carbon dioxide depletion method. In both years Honor grew more rapidly through stem elongation and maintained a greater stem weight from this stage to maturity. The total biomass at maturity was the same for the two cultivars and seed yield of Houser was significantly greater than that of Honor in 1983. In both years Honor had greater canopy NCE per unit land area than Houser until stem elongation but after this stage no difference was observed between cultivars. There were no consistent differences between clutivars in flag leaf NCE per unit leaf area. During grain filling, NCE of Houser was superior to Honor only at high irradiance extrapolated to full light interception but this advantage was offset by more dark respiration per unit dry weight. The yield advantage of the modern cultivar, Houser, over the older one, Honor, was due to a greater harvest index rather than increased photosynthesis.