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 dou ble-skinned polyethylene tunnels in two consecutive seasons (1991-92 a nd 1992-93). Air containing ambient (350 ppm) or elevated (700 ppm) co ncentrations 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 temperat ure between seasons. Thousand grain weight and grain starch content de clined with increase in temperature (from 55 +/- 5 mg to 18 +/- 2 mg, and from 31 +/- 3 mg to 7 +/- 2 mg, respectively), the latter reflecti ng both decreases in granule sizes and fewer amyloplasts per endosperm . Contents of total amylose and lipid-free amylose increased with temp erature (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 temp erature. Starch gelatinisation temperatures ranged from 57.5 to 64.5 d egrees C in the first season, and from 58.0 to 61.9 degrees C in the s econd season, increasing with increase in temperature in both seasons, the data for the two seasons providing almost separate clusters. Gela tinisation enthalpy was constant in the first season (12.6 +/- 1 J/g a mylopectin) and in the second season (15.5 +/- 0.5 J/g amylopectin) wi th no effect of temperature. The differences in carbon dioxide concent ration 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 treatm ent combinations, specifically at warmer temperatures in the first sea son and at cooler temperatures in the second season, on thousand grain weight, non-starch solids and lipid-complexed amylose contents. (C) 1 995 Academic Press Limited