Flag Leaf Variation and the Analysis of Diversity in Durum Wheat

Flag leaf in wheat (Triticum spp.) is a major source of photosynthate for the developing grain and its size contributes to interplant competition. Flag leaf length, width, and area in a sample of 778 entries of durum wheat (T. turgidum L. durum group) germplasm obtained from the U.S. National Small Grains Collection showed considerable phenotypic variation. Correlations between flag leaf and eight spike quantitative characters showed significant values greater than 0.30 for kernel weight per spike with all three flag leaf measurements and for awn length with flag leaf length and area. Larger flag leaf area appeared to be associated with accessions having a history of some deliberate plant breeding for high grain yield. A multivariate approach was used to assess how discrimination among 26 countries of origin obtained with spike characters in a previous study was affected by reducing the sample size from about 3,000 to 735 and by the addition of flag leaf measurements to the analysis. The reduced sample size had little impact on the results of the previous analysis on the whole collection of about 3,000 entries. The addition of flag leaf characters to the analysis caused a rearrangement in the clusters of countries, mostly from Asia, where less germplasm modified by plant breeding was included in the collection. Five clusters were identified: (1) 11 Mediterranean basin countries, (2) USA, UdSSR, and Yugoslavia, (3) four Near East Countries and Bulgaria, (4) three Middle East countries and Greece, India and Ethiopia, and (5) Afghanistan. Phenotypic differentiation of durum wheat, and probably many other crops, on a geographic basis is more pronounced in areas where plant breeding has had little impact.