A Maize Leaf Starch Knockout Has Reduced Productivity Under Field Conditions.

Modification of enzymes involved in sink or source strength has been one common method used in attempts to improve cereal yield.  In this regard, ADP-glucose pyrophosphorylase (AGPase) has received considerable attention as it controls the rate limiting step in starch biosynthesis in both leaves and seeds.  Previous studies, focused on increasing AGPase activity in seeds, indicate that productivity increases occur only when environmental resources are non-limiting.  The objective of this study was to determine the importance of leaf starch on the productivity and growth of maize.  Field trials of BC₄F_2:3 plants segregating for the presence or absence of the agps-m1 mutation and leaf starch were conducted in Citra, Florida.   The results demonstrate the importance of normal leaf starch levels to maize productivity.  The leaf starch mutant agps-m1 was significantly shorter and later to flower than plants with wild type leaf starch levels.  The agps-m1 plants lacking leaf starch were 6 to 15 cm shorter, flowered 2 to 6 days later, and were 30 percent lower yielding than their wild type sister lines.  Photosynthetic rates were not significantly different between wild type and agps-m1 plants.  However, agps-m1 leaves were significantly lower in the levels of major photosynthetic proteins such as PEPCase, PPDK, and RBCL until the end of the photoperiod.  The results demonstrate a link between leaf starch and whole plant productivity under agronomic conditions.  Further study would help to define how leaf starch regulates the production of leaf photosynthetic proteins.