Flag leaf physiological traits in two high-yielding Canada Western Red Spring wheat cultivars

Because of the common negative correlation between grain yield and grain protein (nitrogen) concentration, it is difficult to improve yield in the Canada Western Red Spring (CWRS) wheat (Triticum aestivum L.) class, which characteristically possesses high quantity and quality of protein. Previous studies found that the increased yield of some new CWRS cultivars relative to old cultivars was mainly attributed to higher yield per spike and spike filling rate and more efficient carbohydrate and N partitioning to the grain. The objective of this study was to investigate if the yield increase of two new CWRS cultivars, AC Barrie (registered in 1996) and AC Elsa (registered in 1997), is associated with a change in photosynthetic rate or other physiological traits of flag leaves compared with two old cultivars, Neepawa (registered in 1969) and Marquis (introduced in 1909). No difference was found between new and old cultivars in flag leaf photosynthesis rate or nitrogen concentration. Both new cultivars tended to have lower specific leaf area (SLA) and higher N accumulation per leaf when flag leaves were young and more N remobilization during grain filling compared with old cultivars. Over the 2 yr, flag leaf N remobilizations of AC Elsa and AC Barrie averaged 69 and 32% higher than Marquis and 50 and 17% higher than Neepawa, respectively. The enhanced flag leaf N remobilization along with increased N remobilization from other organs of new cultivars shifted the undesirable negative relationship between grain yield and grain protein concentration. AC Elsa had greater total photosynthesis per flag leaf than other three cultivars due to its larger leaf area and longer green leaf duration. Further research is required to determine if low SLA, large flag leaf and long green leaf duration are effective selection criteria for high yielding CWRS cultivars.