2005 Annual Report 4d.Progress report. This report serves to document research conducted under a specific cooperative agreement between ARS and Iowa State University. Additional details of research can be found in the report of the parent CRIS project 3625-21000-047-00D, "Germplasm Enhancement of Maize Project (GEM)." This project relates to the primary objectives of the parent project which includes the development and evaluation of genotypes for value-added traits such as starch digestibility, and “resistant” starch. The objectives of this project include (i) determining the mechanism of starch granule initiation and growth; and (ii) to develop value added products from GEM germplasm. Starch was isolated from endosperm and pericarp of the inbred B73 grown in Ames, IA in 2004. Starch was first observed in the endosperm 6 days after pollination (DAP), and granule size increased with maturation. Starch content increases rapidly in the endosperm from 10.7% (14 DAP) to 88.9% (30DAP). Amylose content also increased from 12.8% (12 DAP) to 30.8% (30 DAP). Average branch chain length of amylopectin increased in degree of polymerization (DP) from23.6 (10 DAP) to DP26.7 (14 DAP), and then decreased to DP25.4 (30 DAP). This is accompanied by an increase in starch gelatinization temperature up to 14 DAP, and then a decrease; and also with an increase in % retrogradation up to 14 DAP, and then a decrease between 14- 30 DAP. The results indicate that biosynthesis of branch chains of amylopectin reach a maximum at 14 DAP, and then decreases which impacts starch functional properties. The results also suggest that the mechanism for starch biosynthesis in the endosperm is different from that in the pericarp. For the second objective, samples were selected that displayed a high percent retrogradation and low gelatinization temperatures. Enzymatic digestibility tests were run using porcine alpha amylase. Two promising GEM genotypes were found with 65% percent digestibility(AR17056:N2025-574 and DKB844:S1601-289) and both lines are being increased in the GEM Ames nursery, and crossed together for future trials. These lines are candidates for highly digestible starch desired for animal feed. GEM line starch from the following genotypes, 02GEM00880, 02GEM00403, 04GEM00242, 04GEM00243, 02GEM00325, 03GEM00401, 04GEM00234, 04GEM00241, and 02GEM00412 display enzyme digestibility (%ED) of 65.6%, 48.6%, 56.5%, 65.3%, 60.2%, 52.1%, 55.9%, 59.8%, and 61.4%, respectively. The amylopectin branch chain-lengths of these starches were analyzed to understand how structure affects the digestibility of starch. The correlation coefficients (R) of the %ED/the average branch chain length was -0.394; R%ED/% content of DP13-24=0.39; R %ED/%retrogradation=0.331; R%ED/onset gelatinization temp= 0.0041. The results showed effects of amylopectin branch chain length on the digestibility and also showed a relationship between the rate of starch retrogradation and digestibility, but there was no correlation found between the onset of gelatinization and starch digestibility (R=0.0041). Generally, genotypes with higher % digestibility had shorter branch chain lengths. Our future plans are to identify GEM lines including those with high amylose content as candidates for the manufacture of resistant starch.