Effects of elevated growth temperature and carbon dioxide levels on some physicochemical properties of wheat starch

Crops of winter wheat (cv. Hereward) were grown in the field under double-skinned polyethylene tunnels in two consecutive seasons (1991-92 and 1992-93). Air containing ambient (350 ppm) or elevated (700 ppm) concentrations of carbon dioxide was circulated through the tunnels, and temperature gradients, typically from 1 degrees C below ambient to 4-7 degrees C above ambient, were maintained within each tunnel. Despite a shorter crop duration and warmer temperatures in the first season, most grain and starch properties showed a similar response to temperature between seasons. Thousand grain weight and grain starch content declined with increase in temperature (from 55 +/- 5 mg to 18 +/- 2 mg, and from 31 +/- 3 mg to 7 +/- 2 mg, respectively), the latter reflecting both decreases in granule sizes and fewer amyloplasts per endosperm. Contents of total amylose and lipid-free amylose increased with temperature (from 26 +/- 1% to 31 +/- 1%, and from 21 +/- 1% to 25 +/- 1%, respectively), but the contents of lipid-complexed amylose (5.2 +/- 1.5%) and lysophospholipids (0.9 +/- 0.2%) varied independently of temperature. Starch gelatinisation temperatures ranged from 57.5 to 64.5 degrees C in the first season, and from 58.0 to 61.9 degrees C in the second season, increasing with increase in temperature in both seasons, the data for the two seasons providing almost separate clusters. Gelatinisation enthalpy was constant in the first season (12.6 +/- 1 J/g amylopectin) and in the second season (15.5 +/- 0.5 J/g amylopectin) with no effect of temperature. The differences in carbon dioxide concentration had no consistent effects on the parameters measured, but small effects were discernible on thousand grain weight, starch content and lipid-free amylose content. There were also effects in certain treatment combinations, specifically at warmer temperatures in the first season and at cooler temperatures in the second season, on thousand grain weight, non-starch solids and lipid-complexed amylose contents.