Abstract:

【Objective】 The objectives of this study were to elucidate the relationship between starch accumulation and grain sink strength in superior and inferior grains of wheat, and to characterize the variance between the compact and loose spike cultivars in the starch accumulation and grain sink strength in superior and inferior grains. 【Method】 The field experiments were carried out at Tai’an Experimental Station of Shandong Agricultural University in two growing seasons from October 2005 to June 2006 and from October 2006 to June 2007. Two loose spike wheat cultivars, SN1391 and SN12, and two compact spike wheat cultivars, LM21and JM20, were chosen in this study. Heads flowering on the same date were labeled with red thread after full heading. Spikes were sampled at 5-day intervals after anthesis till maturity, and partitioned into two groups, superior grains (the 1st and 2nd grains from the basal part of middle spikelets) and inferior grains (the 3st and 4nd grains from the basal part of middle spikelets). The starch composition accumulation, endosperm cell numbers and activities of enzymes involved in starch synthesis were examined during grain development of winter wheat. 【Result】 The results showed that the accumulation of amylose and amylopectin in superior grains were significantly higher than those in inferior grains in both spike type cultivars. But the magnitude of difference in amylose and amylopectin accumulation between superior and inferior grains in compact spike wheat cultivars was large as compared with that in loose spike wheat cultivars. The accumulation courses of starch were well fitted to the logistic equation by relating starch contents against days after anthesis. Logistic simulation revealed that the higher starch accumulation in superior grains resulted from earlier initiating accumulation time and greater accumulation rate. There was a larger difference in starch accumulation rates between superior and inferior grains in compact spike wheat cultivars as compared with that in loose spike wheat cultivars, which may be one of the reasons that caused larger difference in starch accumulation between superior and inferior grains in compact spike wheat cultivars. Endosperm cell numbers in superior grains were significantly higher than those in inferior grains. The magnitude of difference in endosperm cell numbers between superior and inferior grains varied with cultivars, the greater difference was detected in compact spike cultivars, whereas smaller in loose spike cultivars. The sucrose content in superior grains was lower than those in inferior grains during grain filling, which indicated that the sucrose content (substrate of starch synthesis) may not be a main limiting factor for starch synthesis in inferior grain. The activities of enzymes involved in starch synthesis including sucrose synthase (SS), adenosine diphosphorate glucose pyrophosphorylase (AGPP), soluble starch synthase (SSS) and granule-bound starch synthase (GBSS) in superior grain were significantly higher than those in inferior grains. The magnitude of difference in the above indexes between superior and inferior grains was larger in compact spike cultivars than that in loose spike cultivars. 【Conclusion】 The results here suggested that grain sink strength, determined by both endosperm cell numbers and activities of enzymes involved in starch synthesis, is the main factor responsible for starch accumulation in wheat grain. The larger difference in the grain sink strength was found to result in larger difference in starch accumulation between superior and inferior grain in compact spike cultivars as compared with loose spike cultivars.

Key words: winter wheat (Triticum aestivum L.), spike type, superior grain, inferior grain, starch, sink strength