Effect of high temperature during grain filling on starch synthesis in the developing barley grain

Plants of malting barley variety Schooner were exposed to 5 days of high temperatures (up to 35 degrees C) during mid grain filling under controlled environment conditions, Grains from heat treated plants accumulated c. 30% less starch than grains from control plants (21/16 degrees C; 14 h day). Reduced starch deposition was not limited by assimilate levels in heat treated grains, but high temperature reduced the conversion of sucrose to starch. The reduction in starch synthesis appeared to result from the effects of diminished catalytic activity of a number of enzymes in the committed pathway of starch synthesis, and/or delayed recovery of enzyme activity in the cooler recovery conditions, Reductions of 11-75% in the activity of the enzymes under investigation followed high temperature exposure. In addition, ADPglucose pyrophosphorylase, branching enzyme and granule bound starch synthase showed increased activity during exposure to moderate temperatures (28-32 degrees C), but reduced activity at high temperatures, while soluble starch synthase showed an immediate loss of activity, even at moderate temperatures, Sucrose synthase and UDPglucose pyrophosphorylase showed the greatest reduction in catalytic activity after plants were returned to cooler conditions, Individual enzymes showed variation in the level of recovery under the cooler temperature conditions which followed the heating period.