Abstract:

Wheat grain quality is affected by the environment. Temperature is a major component in environmental variation and has a marked effect on grain filling in wheat. Short period of heat shock, such as over 30℃ for a few days, happens frequently during grain filling in China and many wheat-growing areas in the world, and has become an important factor limiting wheat quality. Starch is the major storage substance in grains of wheat, comprising 65%–70% of dry weight. Investigating the effects of transient high temperature during grain filling period on starch formation and its context is useful for understanding the mechanism of heat damage and improving wheat quality under high temperature stress.
Four temperature levels, i.e. daily average temperature 25℃, 30℃, 35℃ and 40℃, respectively, were set in a phytotron, and wheat plants during different grain filling stages, including the 15th–17th, 19th–21st, 25th-27th and 33rd-35th day after anthesis (DAA), were treated. The wheat cultivars tested were Yangmai 9 with weak-gluten and Yangmai 12 with medium-gluten. Results showed that grain starch content was highest in the treatment of 30℃ and was lowest in the treatment of 40℃ in two cultivars .Under the same temperature, the effects of heat shock during the 25th–27th DAA were greatest, while that during the 33rd–35th DAA were lowest (Table 1). For the treatments of the 25th–27th DAA, the higher the temperature, the lower the soluble sugar and sucrose contents and sucrose phosphate synthase (SPS) activity in flag leaves. Sucrose synthase (SS), starch branching enzyme (SBE) activities were lowest in 40℃ treatment, and SS and SBE activities reached maximum in 25–30℃ treatments (Fig. 2–4). As for starch morphology, the granules in grains were in ellipsoid shape and combined loosely with protein sheath for 40℃ treatment of Yangmai 9 (PlateⅠ­3, 4), but were damaged and in flat ball shape with a crack for that of Yangmai 12 (PlateⅠ­7, 8). Starch granules of Yangmai 9 were slightly damaged for 30℃ treatment (PlateⅠ­1, 2). At the 1st day after 30℃ heat shock treatment, the cracked granules were observed for Yangmai 12, but disappeared at maturity (PlateⅠ­5, 6). While in 40℃ treatment, the crack could still be observed at maturity. It indicated that the heat damage at 30℃ for 3d could be considerably compensated other than at 40℃. It was probable that a kind of protective mechanism for enzymes existed in the endosperm, which had the critical temperature of 30℃. Resistance to high temperature varied with cultivars, and Yangmai 9 was stronger than Yangmai 12.

Key words: Wheat, Transient high temperature, Grain filling period, Starch formation